Translation from Research to Applications
ERNST HUNZIKER,1MYRON SPECTOR,2JEANETTE LIBERA,3ARTHUR GERTZMAN,4
SAVIO L.-Y. WOO,5ANTHONY RATCLIFFE,6MICHAEL LYSAGHT,7ARTHUR COURY,8
DAVID KAPLAN,9and GORDANA VUNJAK-NOVAKOVIC10
The article summarizes the collective views expressed at the fourth session of
Engineering—the Next Generation, which was devoted to the translation of results of tissue engineering re-
that tissue engineering should be guided by the dimensions and physiological setting of the bodily com-
partment to be repaired. Myron Spector discussed collagen-glycosaminoglycan (GAG) scaffolds for mus-
engineering, and described a completely autologous procedure for engineering cartilage using the patient’s
pedic surgery, and outlined the potential of allograft tissues as models for biological and medical studies.
Savio Woo discussed a list of functional tissue engineering approaches designed to restore the biochemi-
cal and biomechanical properties of injured ligaments and tendons to be closer to that of the normal tissues.
Specific examples of using biological scaffolds that have chemoattractants as well as growth factors with
the translation of the results of research into products that are profitable and meet regulatory requirements.
Michael Lysaght challenged the proposition that commercial and clinical failures of early tissue engineering
engineering products based on the example of Genzyme, and how various definitions of success and failure
can affect perceptions and policies relative to the status and advancement of the field of tissue engineering.
the workshop Tissue
and there is potential for the development of new products
HE TECHNOLOGY OF TISSUE ENGINEERING has been shown
to be feasible; some products are already on the market
with significantclinicalimpact.Translationofresearch from
conditions, particularly in cases where the main clinical out-
1ITI Research Institute for Dental and Skeletal Biology, University of Bern, Bern, Switzerland.
2The VA Boston Healthcare System and Orthopaedic Research Laboratory, Brigham and Women’s Hospital and Harvard Medical
School, Boston, Massachusetts.
3Co.don, Molecular Medicine, Biotechnology, and Tissue Engineering, Teltow, Germany.
4Musculoskeletal Transplant Foundation, Edison, New Jersey.
5Department of Bioengineering, Musculoskeletal Research Center, University of Pittsburgh, Pittsburgh, Pennsylvania.
6Synthasome, San Diego, California.
7Center for Biomedical Engineering, Brown University, Providence, Rhode Island.
8Genzyme Corporation, Cambridge, Massachusetts.
9Department of Biomedical Engineering, Tufts University, Medford, Massachusetts.
10Department of Biomedical Engineering, Columbia University, New York, New York.
Volume 12, Number 12, 2006
# Mary Ann Liebert, Inc.
investment money, business plans geared to realistic cost/
benefit trade-offs, less hype, more sophisticated personnel
skilled at product development and manufacturing scale-up
are needed to move the field toward the clinic. Along with
these, continued progress on the fundamental side is needed
to provide support for the translational advancements.
FUNCTIONAL TISSUE ENGINEERING:
PARADIGM OF A DUAL
Ernst Hunziker from the University of Bern, Bern,
Switzerland, described the concept of functional tissue engi-
neering (FTE) as an applied science, and proposed the para-
digm of a dual translational approach. He made the case that
practical tissue engineering should be approached with a
clear view of the bodily compartment to be repaired. Not
only the dimensions of the defect and its etiology, but also
the physiological setting and its influences must be known.
To gain an insight into the biological factors at play within
an implanted construct, the ‘‘raw’’ product must be sub-
jected to a short- and midterm troubleshooting investigation
in vivo. The level of adverse foreign-body reactivity, the
ingress of undesired cell populations from neighboring tis-
sue compartments, and the influence of local signaling ac-
tivity must be evaluated. Ways must be found to deal with
these problems along a course that cooperates rather than
contends with the desired tissue-compartment-specific re-
pair response. After having made the necessary theoretical
adjustments to the engineering principle, the predictors of
success identified in the initial in vivo experiments can be
optimized in a first translational phase in vitro. The quality,
functionality, and durability of the in vitro–optimized tissue
constructs can then be evaluated in a second, and final,
translationphase invivo,initiallyinthe short- andmidterms,
Tissue engineering is an interdisciplinary science. For the
promise of success, advice of specialists in all disciplines
related to construct development should be sought—from
the initial concept, through experimentaltesting, to potential
industrial manufacture for clinical use—so as to anticipate
and circumvent as many difficulties as possible before these
converge to become a Gordian knot. Tissue engineering is
also an applied science, which aims to restore tissue activi-
ties that have been undermined by destruction or degener-
ation, and which either cannot be reestablished by available
therapeutic means, or, if they can, depend upon the trans-
plantation of cellular material. To improve our knowledge
and understanding of biological systems, we tend to design
experiments that will reveal the fundamental processes and
the underlying mechanisms. The insight thereby gained is
then applied to develop tools that will selectively inhibit or
enhance system components.
Successful tissue engineering depends on a thorough
knowledge and understanding of the biological system in
hand. But instead of experimentally stripping the system to
reveal its components, the tissue engineering approach takes
these components, or suitable substitutes, with a view of
reestablishing the system structurally and functionally. In-
evitably, therefore, most tissue engineering approaches are
initially of an essentially empirical nature, the concept being
tested first in vitro and then in vivo. However, many strate-
gies that appear to be promising in vitro yield disappointing
resultswhentranslated toliving organisms, thereby bringing
home to us the complexity of the natural system and our
incomplete grasp of its intricacies. The impact of one bio-
logical phenomenon in particular, namely, immunoreacti-
vity and tissue rejection, cannot be fully appreciated in vitro.
Engineered tissue will be treated by the body as a foreign
object. Hence, it is not surprising that many new concepts
fail due to inadequate testing in vivo.
Identification of the medical problem and
of the therapeutic needs
The medical problem to be solved must be defined not
only symptomatically but also structurally. By way of il-
lustration, Dr. Hunziker will refer to his own studies on the
repair of structural lesions in the articular cartilage layer of
synovial joints.1,2Such lesions can arise either traumati-
cally, or pathologically during the course of osteoarthritis.
They may be confined to the avascular layer of articular
cartilage (partial-thickness defects) or may penetrate the
sanguinous subchondral bony compartment (full-thickness
defects). The biology of these two types of defects differs
Partial-thickness defects manifest no spontaneous repair
subchondral bone is involved, two different types of tissue
need to be repaired, one of which is vascularized and the
other is not, and this distinction must be preserved if joint
function is to be fully restored. Hence, it does not suffice
of the structural damage incurred must be established.
Tissue engineering systems
Once the nature of the medical problem to be tackled has
been established, the type of system that we wish to elabo-
rate must be selected from the three existing alternatives: an
acellular approach, a cell-based approach, and a combined
approach. Acellular approaches aim to facilitate the repair
activityofnative cellpopulations. They may consist justofa
nude matrix, or of a matrix containing biological signaling
agents that will recruit and trigger the differentiation of local
stem cell populations, thereby resulting in the formation of
a functionally competent type of repair tissue. Cell-based
approaches can involve adult stem cells, embryonic stem
cells,ordifferentiated cells. Ifimmature cells are used, these
HUNZIKER ET AL.
may either be left to differentiate in vivo or be induced to do
so in vitro, prior to implantation. The cells can be applied
either alone or in conjunction with a matrix. The combined
approach is the most popular of the three alternatives. In
such systems, cell populations are trapped within a matrix
that is functionalized with a biological signaling agent.
Likewise with this approach, cell maturation can be effected
either prior to or after implantation.
At this conceptual stage, investigators should also decide
on the intended mode of application of their engineered
product—whether it is to be introduced surgically or ar-
throscopically, or as a solid entity or in an injectable form—
more than one is required, whether these are likely to be
acceptable and well tolerated.
Clinical product considerations
An awareness of the likely costs of manufacture of a
construct for clinical use is also essential nowadays. If a
construct is too costly to produce, it is unlikely to have any
market value. Investigators with a purely scientific back-
ground are not accustomed to think along these lines, which
is why tissue engineering should, ideally, be an interdisci-
plinary enterprise. The advice of persons with industrial and
marketing experience should be sought early, at the con-
ceptual stage of the project. Clinical input relating to the
importance of the medical problem to be tackled and a
practicable and convenient mode of applying the engineered
product is also essential. Not only clinical practitioners but
also public health authorities should be consulted, in or-
der to assess the social impact of the medical problem and
to gain some idea of the size of the patient population that
is likely to benefit from the envisioned tissue engineer-
ing strategy. Further, it should be borne in mind that the
end product destined for medical use will be subject to
the stringent evaluation process that is imposed on all po-
tential therapeutic tools prior to their approval. To facilitate
and expedite this screening at a later stage, all experi-
ments conducted should be meticulously documented. If
due and timely consideration is given to these various is-
sues, the tissue engineering project will be highly focused,
with clear aims in view, and will proceed in a direction that
is most likely to yield a simple, effective, and marketable
Initial evaluation of an engineered construct in vivo
In principle, tissue engineering approaches deal with the
control of wound-healing processes and aim to boost or
redirect the body’s own capacity for repair. If a therapeutic
interventionistobeefficient,then we musttake intoaccount
implantation, and thoroughly understand the biological
conditions operating physiologically and in a pathological
state. These issues, and the relevant factors at play, can be
assessed only when the engineered construct is implanted in
When a construct is implanted, the initial local reaction
or trauma that involves hemorrhaging, the formation of a
hematoma, and blood clotting (Table 1). Blood-borne com-
ponents released from a lesioned vessel are recognized as
foreign material and they trigger an inflammatory response,
which is mediated by macrophages (and, to a lesser extent,
by foreign-body giant cells). This reaction leads to the re-
sorption of the fibrinous clot and its substitution, first by
granulation tissue and then by a primitive type of vascular-
ized scar tissue laid down by the invading population of
fibroblasts. Although this repair tissue is unspecific and does
not correspond in type to the compartment within which it is
deposited, it is nevertheless biocompatible and thus persists
(Table 1). An implanted construct will trigger a similar in-
flammatory reaction, which will lead first to its resorption by
foreign-body giant cells and macrophages, and then, fol-
lowing vascular invasion, to its replacement with unspecific
scar tissue (Table 2).
Tissue biocompatibility is a tissue-compartment-specific
phenomenon. The only type of tissue that is universally
biocompatible with all five mammalian tissue types is the
unspecific scar tissue. Hence, the art of tissue engineering
lies in our ability to control and to redirect the remodeling
process that accompanies the resorption of a construct, in
such a manner as to lead to its replacement, not by an
unspecific type of scar tissue but by a compartment-specific
type of functional parenchymal tissue.
A true conception of the type of tissue that needs to be
engineered can be gained only if the microenvironment of
the destined implantation site is thoroughly understood in
terms of its biology, mechanical characteristics, and the
immunological backdrop. The topographical location of the
Table 1.LOCAL TISSUE RESPONSE TO INJURY AND TRAUMA
Hemorrhaging from lesioned vessels and blood clotting
Blood-bome physiological components, leaking from the usually
closed sanguinous compartment, are recognized as foreign
material by the surrounding tissue, since they are not
Inflammatory response mediated by macrophages (and more
rarely by foreign-body giant cells)
Resorption of blood clot
Ingrowth of blood vessels and fibroblasts, and the formation of
Deposition of an unspecific but biocompatible type of repair (scar)
tissue by fibroblasts
TRANSLATION FROM RESEARCH TO APPLICATIONS
be defined, since the volume of a construct will have an
important bearing on its chances of survival in the long run.
Hence, in the initial animal studies, the influence of a le-
sion’s dimensions on repair must be analyzed. The assess-
ment of newly formed tissue should also include an
evaluation of its integration with the surroundings, which
is essential for its structural and functional competence,
and of its connectivity with the existing blood, lymphatic,
and nervous systems (if relevant). Investigators should also
ascertain whether local adult stem cell pools are avail-
able for recruitment, and whether these can be efficiently
stimulated to migrate and proliferate, and to repair small as
well as large structural defects (for examples, see Hunziker
A problem that is frequently overlooked by investigators
is the local presence of unwanted cells with a high prolif-
erative capacity, such as fibroblasts, which could invade the
construct and remodel it into unspecific scar tissue. The
migration of such cells into the defect site can be prevented
by applying the principles of structural or functional guided
tissue regeneration.3,4A classical example of this problem is
the repair of atrophic bony defects in the mandible or
maxilla. In this case, structural barriers are introduced to
prevent epithelial cells and fibroblasts from invading the
The importance of the mechanical microenvironment is
frequently underestimated or even ignored in situations that
do not involve components of the musculoskeletal appara-
tus. The cardiovascular system, for example, has a very
important mechanical function of pumping blood around the
entire body. The mechanical forces operating within the
microenvironment of the lesion during tissue remodeling
will have an important bearing on repair.7If these me-
chanical considerations are neglected, the remodeling pro-
cess is unlikely to yield a tissue with the native structural
characteristics, which depend partially on the mechanical
stress fields operating during the terminal differentiation of
cells in the repair tissue, and which are required for the
mechanical competence of the tissue.
One of the most important issues to be tackled is the
adverse reactivity of immunocomponent cells, such as mac-
rophages, foreign-body giant cells, and lymphocytes. Suc-
cess in tissue engineering depends greatly on our ability to
control the foreign-body reaction and to steer it along a
course that will cooperate rather than contend with the di-
rected remodeling process. This point is best illustrated by
an example drawn from Dr. Hunziker’s studies on bone re-
pair (implant osseointegration). Titanium alloy discs coated
biomimetically with a functionalized layer of calcium phos-
During the early postoperative phase, foreign-body giant
cells were observed to invade the implantation site and to
actively degrade the calcium phosphate coatings. The oste-
ogenic agent incorporated into the inorganic lattice work
was consequently freed by the activity of these foreign-body
genitor cells and their transformation into osteoblasts, which
then dominated the scene.8
It is also worthwhile to test for the species-specificity of
the engineered construct. If this limitation exists, then the
concept must be adapted to render it universally applicable.
Hence, the construct should be evaluated in several animal
species at an early stage.
To recapitulate, these initial short- and midterm experi-
ments with living animals should aim at identifying all the
variables operating at the prospective implantation site. In
the light of the information thereby gained, it will then be
necessary to redefine and fine-tune the initial concept, and to
In Dr. Hunziker’s studies relating to the repair of partial-
thickness defects in articular cartilage, exploratory animal
experiments of the kind described above revealed many of
the local biological factors that can undermine the healing
response1(Fig. 1). Having identified the intrinsic stumbling
blocks, it was possible to overcome these by an informed,
rational, and systematic approach.2After having made the
necessary adjustments to the engineering principle, investi-
first translation phase, the purpose of which is to optimize in
vitro the predictors of success identified in the initial in vivo
This experimental fine-tuning phase is conducted in vitro,
testing in living animals would be too great. In this first
translation phase, the predictors of success identified in the
initial animal experiments must be systematically investi-
gated to optimize the repair results achieved with the con-
struct after implantation. First and foremost, the in vitro
system should simulate as closely as possible the local
‘‘bioreactor’’ conditions operating in vivo. For example, the
dimensions of the construct should correspond to those of
the lesion, and the mechanical microenvironment, as well
as the immunological scenery, pertaining locally in vivo
should be imitated. The quality of the repair tissue can be
optimized by testing the effects of various signaling agents
on cell differentiation and the production of a tissue-specific
extracellular matrix (ECM). Aspects relating to the nutri-
tional limitations of the construct and to the permeation of
signaling agents can also be investigated in relation to its
Table 2.LOCAL TISSUE RESPONSETOANIMPLANTEDCONSTRUCT
Vascular invasion and scar-tissue formation
HUNZIKER ET AL.
size. And, to a limited extent, the problems of tissue inte-
gration can be addressed in vitro. The in vitro process of
optimization and refinement helps to perfect the product,
thereby lessening the expense and improving the efficiency
of the second, and final, phase of testing in vivo.
Second translation: In vivo adaptation of the
system, and optimization of its efficacy, safety,
and long-term functionality
In this second, and final, in vivo phase, the principle of the
system is put to the test. In order to acquire a full picture of
what is taking place in the living organism, the fate of the
implanted construct should be monitored at three time
points, which reflect crucial events in the usual course of
repair. As a rule of thumb, suitable sampling intervals would
be 6days, 6weeks, and 6months.
During the initial 6 to 12 postoperative days, the magni-
tude of the inflammatory response, the level of immuno-
logical activity, and the extent of implant resorption will be
revealed. During the intermediate 6- to 10-week interval, it
will be possible to ascertain whether a structurally and
remodeling process. To establish this fact, the tissue must be
thoroughly characterized. If the evaluation yields promising
preferably, the 1-year stage, at which juncture the long-term
structural and functional competence of the remodeled tis-
sue should be reevaluated (Table 3). Obviously, the sug-
gested time points should be taken cum grano salis, and will
need to be adapted to the individual case.
Probing during the first week of implantation will yield
information respecting the severity of the native tissue’s
response to the construct, and will indicate whether the en-
gineering principle is operating successfully to redirect the
‘‘adverse’’ reactivity in a ‘‘positive’’ direction, namely, to-
ward the formation of a site-specific type of repair tissue. At
this stage, and together with the surgeon’s input respecting
the ease and practicability of the implant-application mode,
it will be possible to assess whether the surgical technique is
an appropriate one to elicit the desired course of events. The
quality of implant integration with native tissue should also
be evaluated, since a good cohesion result at this stage will
bode well (and vice versa) for an optimal bonding of the
remodeled construct with the defect borders. The suitability
of the method chosen for implant fixation will also be re-
vealed at this early time point. Data gleaned from the first
in vivo phase of the project will help complete the pool of
information garnered at the short-term juncture.
An analysis performed at the midterm, 6- to 10-week
juncture will reveal whether the construct has been re-
modeled according to our hypothetical expectations into a
structurally and functionally relevant type of repair tissue.
The 6-month or 1-year evaluation will indicate whether the
spontaneously. Further, they cannot be induced to repair simply by
the local application of a chemotactic/mitogenic growth factor;
such an agent triggers the migration of mesenchymal stem cells
from the synovium into the lesion, along the floor of which they
form mono- or bilayers. Hence, the proliferative response of these
cells is abortive. For these cells to fill the defect volume, this space
must be defined by a matrix containing the chemotactic/mitogenic
agent (A). Under these conditions, the cells lay down a primitive
type of repair tissue, which does not transform into cartilage. To
effect chondrogenesis and the formation of cartilage-like tissue
(B), a chondrogenic differentiation factor must be introduced into
the matrix in a liposome-encapsulated form (for delayed release at
a timely juncture), together with the free chemotactic/mitogenic
agent. Bars in (A) and (B)¼125mm. Color images available on-
line at www.liebertpub.com/ten.
Partial-thickness articular cartilage defects do not heal
FINAL IN VIVO PHASE OF THE INVESTIGATION: 6DAYS,
6WEEKS, AND 6MONTHS
TIME POINTS FOR MONITORING IN THE SECOND AND
Short term, 6-day, juncture:Evaluation of the inflammatory
response, of immunoreactivity,
and of implant resorption.
Has a structurally and
functionally relevant tissue
been remodeled from the
Has the remodeled tissue
persisted in a structurally and
functionally competent form?
Mid-term, 6-week, juncture:
TRANSLATION FROM RESEARCH TO APPLICATIONS
structural and functional competence of this tissue is likely
to persist in the long run.
Animal experiments must be thoroughly thought out with
respect to the variables at play in the system. It is indispens-
able that the appropriate negative and positive control
individually. Further, each group should be established in at
least two different animal types, in order to check out the
species-specificity of the construct. The more complex the
system, the more complex will be the experimental design
(and the lesser the chance of yielding a marketable product).
The tissue characterization to be undertaken at the mid- and
long-term junctures should include a rigorous quantitative
evaluation of well-defined structural parameters, quantita-
tive immunohistochemistry, and the biochemical analysis of
appropriate tissue-specific markers (Table 4).
It cannot be too emphatically stressed that the histological
analysis of repair-tissue quality should not be based on
subjective scoring schemes, which are all but useless. Well-
defined and biologically meaningful structural parameters,
which can be readily quantified, are much more powerful.
The morphometric analysis of these structural parameters
must be based on an appropriate and well-defined tissue-
The functional characterization will of course depend
upon the tissue or organ under investigation. Forexample, in
the case of articular cartilage the mechanical properties of
the tissue should be quantified, whereas in the case of the
liver the biochemical and secretory competence should be
If the midterm analysis yields promising results, then the
long-term experiments can be executed. The tissue should
not be analyzed untilat least 6months have elapsed from the
time ofsurgery,and preferably notbefore 1year.Ifpossible,
the implantation site should be periodically surveyed during
the postoperative course using in vivo monitoring tech-
niques, such as X-radiography or MRI. In parallel with the
characterization of the repair tissue, adjacent tissues should
be subjected to a toxicological analysis. For example, in
articular-cartilage-repair studies, not only normal articular
cartilage in the defect vicinity, but also vicinal subchondral
bone, synovial tissue, synovial fluid, joint ligaments, and
joint-associated lymph nodes, as well as regional lymph
nodes and serum, should be sampled and analyzed for ad-
verse or even deleterious changes. Toxicological evaluation
is important in assessing the physiological impact of the
engineered construct, and will be required when the product
is submitted for approval by federal authorities.
The ultimate aim oftissue engineeringistodevelopmeth-
odologies whereby structural and functional bodily defects
can be healed. Tissues and organs that do not heal sponta-
neously are induced to repair by attempting to overcome the
biological limitations that undermine this process in nature.
At the stage of its conception, a tissue engineering principle
must be screened to ascertain the size of the patient popu-
lation that will be targeted, its socioeconomic impact, the
feasibility and likely cost of its manufacture, and its surgical
practicability. Hence, tissue engineering is necessarily an
for the engineering of a product that will be universally ap-
plicable to all bodily tissues. An engineered construct must
be tailor-made on a tissue-specific basis. Ernst Hunziker
concluded that in order to yield a product that will be bene-
ficial to the targeted patient population, a tissue engineering
principle should be experimentally developed along system-
atic and, whenever possible, rationallines, and in the light of
a thorough understanding of the biological system in hand.
COLLAGEN-GAG SCAFFOLDS FOR
MUSCULOSKELETAL TISSUE ENGINEERING
Myron Spector from Harvard Medical School, Boston,
MA, discussed the challenges of developing biomaterial
scaffolds for tissue engineering and regenerative medicine.
The growth of cells inthree-dimensional scaffolds continues
to provide unique opportunities to observe selected cell
behavior as well as to inform changes in scaffold composi-
tion and structure that may improve clinical performance.
Investigations of cell-scaffold interactions in vitro thus
continue to offer the opportunity for discoveries of cell bi-
ology. In this regard, tissue engineering promises to provide
critical new knowledge that will deepen our understanding
of the cell phenotypes and enable meaningful advances in
tissue engineering and regenerative medicine. These en-
deavors are notable particularly because there is a growing
consensus that the challenges of developing biomaterial
scaffolds for tissue engineering and regenerative medicine
exceed the challenges that were faced in the cell biological
work that led to the phenotypic proliferation of cells in vitro,
and in genetic engineering that has led to the production of
growth factors and cloning of their genes, the other pillars of
tissue engineering and regenerative medicine.
Table 4.CHARACTERIZATION OF THE REPAIR TISSUE
Structural organization:Histological and ultrastructural
Choice of biologically
meaningful and well-defined
instigation of well-defined
sampling strategy; avoidance
of subjective scoring schemes.
Assessment of mechanical or
Persistence of tissue-specific
characteristics at the
Evaluation in several different
of the repair-tissue result:
HUNZIKER ET AL.
Tissue engineering and regenerative medicine can now be
related to the tissue engineering triad of cells, matrices, and
regulators. One of the more important technological ad-
vances enabling tissue engineering relates to the means of
production of the porous, absorbable scaffolds that are re-
quired to contain the cells in vitro and/or in vivo. Control of
the pore characteristics, including pore volume fraction,
pore diameter, and pore orientation, as well as the chemical
composition of the matrix, has played a critical role in the
advance of tissue engineering. In order to further advance
and facilitate translation of research to clinical implemen-
tation, numerous questions will have to be answered. The
following provides a framework for posing such questions.
Scaffolds for tissue engineering
and regenerative medicine
Scaffolds for engineering bone and soft tissues have been
synthesized from an array of synthetic and natural calcium
phosphates and myriad synthetic (e.g., polylactic acid
[PLA] and polyglycolic acid [PGA]) and natural (e.g.,
collagen and fibrin) polymers. The underlying concepts
guiding the development of scaffolds are predicated on the
selected biomaterial or on the method of production of the
scaffold. Examples of biomaterials-based approaches in-
clude the use of (i) biomaterials that have been frequently
employed in other implant applications (e.g., PLA-PGA);10
(ii) treated natural ECM materials (e.g., inorganic bone );11
(iii) biomimetics and analogs of ECM (e.g., collagen-GAG12
and collagen-hydroxyapatite scaffolds);13(iv) biopolymers
for nanoscale matrix (e.g., self-assembling peptides);14and
(v) new types of biomaterials designed specifically for tissue
engineering scaffolds. Alternatively, the driving force for the
design of scaffolds may be the precision (computer) multi-
scale control of material, architecture, and cells; solid free-
form fabrication technologies. This has become possible
with the introduction of a wide array of solid free-form
fabrication techniques and apparatus.15While it is likely that
several scaffolds will be suitable for certain clinical appli-
cations, it is becoming increasingly important to determine
the unique features of certain biomaterials and the methods
of production that best favor clinical outcome. Do certain
biomaterials offer a greater number of ligands for adhesion
proteins and thus offer an advantage of cell attachment or
otherwise regulate cell function? Will vascular networks be
able to be introduced into large constructs produced by free-
form fabrication methods and thus advance organ printing?
What is the effect of the mechanical behavior, and change of
such behavior with degradation of the scaffold, on tissue
formation in vitro and in vivo?
There are many roles that a scaffold can play in the tissue
regeneration process. The potential role of the scaffold as a
important in a wide variety of tissues and organs in the light
of recent advances in the investigation of cell therapy for
local repair. Injection of exogenous cells, expanded in num-
ber in monolayer culture, is being studied for the treatment
of defects and degenerative conditions in many tissues, ex-
amples being (i) chondrocytes for the repair of defects in
articular cartilage on the surface of joints,16(ii) interverte-
bral disc cells for herniated disc,17(iii) stem cells into spinal
cord lesions,18(iv) myoblasts and stem cells for myocardial
infarction,19and (v) cells into the retina.20
An alternative to the injection of cells is implantation of
a cell-seeded scaffold. As noted above, the large surface
area of porous scaffolds allows for the delivery of an ex-
ceedingly large number of attached cells, and facilitates the
retention of the cells at the implant site. Questions related
to the use of scaffolds for these applications include the
specific pore structures that will be necessary to accom-
modate cell function. Moreover, that regulatory molecules
may be required to stimulate certain cell functions raises
questions about the best way to have the scaffold deliver
recombinant proteins or their genes.
Investigations of cell-scaffold interactions in vitro
Investigations of cell-scaffold interactions in vitro can
inform the rational formulation of scaffold composition and
structure for improved performance in tissue engineering
and regenerative medicine applications. These investiga-
tions of cells in three-dimensional scaffolds that may mimic
certain aspects of the natural ECM in vivo can also provide
insights into, and discoveries of, cell biology. For example,
in the course of investigations of the behavior of articular
chondrocytes in collagen-GAG scaffolds, it was observed
that the disc-shaped scaffolds were decreasing in size.21
Subsequent histological studies demonstrated a reduction in
the pore diameter of the matrices and suggested a cell-me-
diated process. This led to the finding that chondrocytes
were expressing the muscle actin isoform, a-smooth muscle
actin (SMA). Following were findings that adult canine and
human articular chondrocytes, and many other connective
tissue cells and their mesenchymal stem cell progenitors
express SMA and can contract.22These findings have sug-
gested roles for the contractile behavior of connective tissue
cells in the control of the architecture of the ECM and in the
response of the tissue to injury. Many new insights into cell
biology may be gained from the continued study of the be-
havior of cells in three-dimensional scaffolds.
Chondrocyte-seeded collagen-GAG scaffolds
for cartilage repair
Translating knowledge about the cell-scaffold interac-
tions acquired from research in vitro to clinical applications
has led to investigations of chondrocyte-seeded scaffolds for
cartilage repair procedures. Studies demonstrating the po-
tential benefit of injection of culture-expandedchondrocytes
TRANSLATION FROM RESEARCH TO APPLICATIONS
forcartilagerepairdateback torabbit studiesfirst performed
in the mid-1980s.23,24While subsequent experiments of
autologous chondrocyte implantation (ACI) in a canine
model25,26were less promising, the procedure has been in-
troduced into widespread clinical use16with promising
symptomatic relief in many patients.16,27
Current efforts in many laboratories around the world are
being directed toward determining whether the results of
ACI can be improved if the cells are implanted as a cell-
seeded scaffold rather than delivered by injection. As noted
earlier, one design approach has been to employ scaffolds
that can serve as analogs of the ECM of the tissue to be en-
gineered.12This concept recognizes that the molecular com-
position and architecture of the ECM display chemical and
mechanical properties required by the parenchymal cells and
the physiological demands of the tissue. For a wide array of
employed,12and for articular cartilage, type II collagen—
the principal constituent of the tissue—has been commended
by prior studies.
in chondral defects in adult dogs implanted with cultured
autologous chondrocytes (CACs) alone, that is, ACI26and
CAC-seeded type II collagen-GAG scaffolds cultured for
24hours28and 4weeks29prior to implantation. The cell-
than the sites implanted with the CACs alone. The cell-
seeded scaffolds cultured for 24hours induced more repar-
ative tissue formation than the injection of cells alone, but
this tissue was made up of fibrocartilage and fibrous tissue
with virtually no hyaline cartilage. The question remains as
to the relative importance of amount versus composition of
the reparative tissue with respect to providing symptomatic
relief for individuals with focal cartilage defects. Related to
this point is the fact that the hyaline cartilage found at sites
treated by CACs alone and in the collagen scaffolds did not
display the architecture of articular cartilage.
Of note was that the greatest amount of reparative tissue
prior to implantation, and that this group demonstrated the
same amount of hyaline and articular cartilage as defects
implanted with the cells alone.29How developed should the
cartilaginous construct be before it is implanted? While
these studies demonstrate the promise of implementing
scaffolds for cartilage repair, there are potential problems
and significant expense associated with the culturing a cell-
seeded scaffold for 4weeks prior to implantation. This fo-
cuses attention on the implementation of growth factors to
accelerate cell proliferation and matrix synthesis in the
scaffolds prior to implantation.30
Myron Spector concluded that these and other animal
studies underscore the importance of in vivo investigation.
Translation of research from the laboratory to the clinic will
understandably require animal studies. A host of questions
revolve around the most suitable animal models for human
conditions. While certain tissue engineered constructs may
not yet meet the initial expectations for the reproduction of
the composition and structure of the target tissues, even in
animal models, several should be advanced to human trial,
appreciating the inadequacy of any animal experiment to
fully model the human condition. This is especially true of
problems with the main clinical outcome variable of pain
AND CLINICAL ASPECTS
Jeanette Libera from co.don, a company based in Berlin,
Germany, focused on tissue engineering of completely au-
tologous cartilage based on the patient’s own chondrocytes
and blood serum. She argued that the cell-based therapies
already available on the market provide insight into the re-
quirements of therapy development, such as the definition of
clinical strategy, in vitro engineering and functional tests,
and animal and clinical studies. Already during formulation
of repair strategies a balance between medical (e.g., avail-
ability of cell source, pathology), biological (e.g., regener-
ative capability of selected cell, scaffold resorption rate),
and technical (e.g., intraoperative, technical possibilities for
transplantation) practicability and potential has to be found.
The long-standing experience with autologous cell-based
repair demonstrated that chondrocytes have high potential
for tissue engineering even without the use of scaffolds.
However, for safe application, stem cells need to be char-
acterized for their carcinogenic risk. Regardless of the cell
source and of the use of scaffolds, commercially successful
strategies are likely those that are simple with regard to
characterization of the transplant, and operative techniques.
Chondrogenesis and growth factors
It is well established that chondrogenesis is regulated by
growth factors.31–33Stimulation of tissue regeneration by
application of a single growth factor was already done
clinically for bone repair.34However, in related studies,
counter effects were observed that might be related to the
undefined dose of the used growth factor.35Therefore, also
for cartilage, future research should focus on the use of
understanding of the in vivo differentiation-dependent ex-
pression of growth factors and of their receptors. This will
provide the basis to selectively influence cell differentiation
by regulating expression of growth factors and receptors
and/or by adding respective factors. In principle, the latter
strategy is more appealing circumventing any problem
HUNZIKER ET AL.
associated with transfection of cells for a differentiation-
dependent expression of growth factors.
Chondrocyte spheroid culture
Remarkably, differentiation-dependent expression of
several growth factors has been observed in the autologous
three-dimensional chondrocyte spheroid culture system.37
This again underlines the high potential of isolated and
cultured human chondrocytes to regulate their chondro-
genesis. The growth factors released during spheroid re-
modeling, aggregation, and integration into native cartilage
could explain the observed stimulation of chondrocyte dif-
ferentiation and matrix maturation during spheroid re-
modeling.37Spheroid adhesion and subsequent remodeling
are likely the crucial steps in structural and functional inte-
gration of engineered constructs into native cartilage.
Integration with host tissues. Structural and functional
integration with host tissues is essential for long-term
functionality of implants.38,39For quantitative analysis of
integration of tissue engineered constructs, there are already
available invitro models.40Forimplant integration, which is
a cell-regulated process, ultrastructural integration of se-
creted matrixcomponentsisnecessary.41–43Further analysis
of adhesion processes and mediators will be necessary for
the identification of therapeutic strategies to enhance or se-
cure integration of tissue engineered constructs.32,44–47Im-
proved arthroscopic and imaging techniques should permit
characterization of tissue integration and maturation during
postoperative follow-up. Biomarker imaging technologies,
and arthroscopic and mechanical test systems are among the
technologies that would support and advance functional
characterization of tissue regeneration.
To retain first in vivo data on safety and efficacy of cell-
based therapies, animal models were used. These models
were selected to mimic the human situation, for example,
with regard to mechanical loading and self-repair processes.
dependent chondrocyte characteristics have to be taken into
account.48Culturing articular chondrocytes of the human,
sheep, horse, dog, cow, and mini-pig species, we observed
differences in monolayer morphology, proliferation, and
migration as well as a different potential to form an in vitro
autologous tissue construct. Compared to human chondro-
cytes, we found for mini-pig chondrocytes a similar cell
differentiation and matrix formation after transferring
chondrocytes into the spheroid culture system. Based on this
observation, we used this model to analyze the capacity of
chondrocyte spheroids to heal cartilage defects.
Jeanette Libera concluded that tissue engineering will
certainly require new animal models for developing novel
technologies to treat specific indications, for example, large
animal model of osteoarthritis without destabilization of
joints by meniscectomy or anterior cruciate ligament (ACL)
ALLOGRAFT TISSUES FOR
Arthur Gertzman from Musculoskeletal Transplant Foun-
dation (MTF) discussed successful applications of allograft
tissues in orthopedic surgery and outlined the potential of
allograft tissues as models for elucidating cell-matrix in-
teractions in orthopedic applications. He reminded that tis-
sue engineering requires three key components, all of which
are necessary for success. First, cells are required to pro-
vide the genetic instructions and energy system to create the
needed new cells. Second, growth factor and cytokines me-
diate the cell activities. Third, a three-dimensional matrix
is required to provide a scaffold upon which the cells and
cytokines can act. The need for the three-dimensional nature
of the required structure should be self-evident, as all natu-
ral biological structures are three-dimensional. Much of
historical research has been in a two-dimensional mode of
monolayers, a ‘‘flat biology’’ approach that may not be pre-
dictive of the natural, biological milieu.
Human allograft tissues
Human allograft tissues are widely used in orthopedic
surgical procedures in the United States. Allografts are
recovered and processed with conventional metallic or poly-
well-developed protocols that have proven to be as safe
as surgery. Both the US Food and Drug Administration
(FDA) and the American Association of Tissue Banks have
published and enforced extensive rules related to donor
selection, processing, storage, and transport. Tissues from
complying tissue banks have an unparalleled record of
patient safety and successful outcome over the past two
decades. Allograft tissues are bone or soft tissues removed
from one human and transplanted to another for repair of
desired or traumatized tissues.
Load-bearing tissues. Cortical bone is a dense tissue with
a specific microstructure based on the osteon and inter-
connecting channels: Haversian, Volkmann, and cell-sized
canaliculi (Fig. 2A). Cancellous bone is open and ‘‘webby,’’
with interconnecting trabeculae enclosed in a thin cortical
shell. The condyles of long bone and such special bones as
the patella are cancellous in nature (Fig. 2B). As determined
empirically, allograft bone integrates by a mechanism of
‘‘creeping substitution.’’49Cancellous bone, with its open
structure that is osteoconductive and somewhat osteo-
inductive, will biointegrate into a host within a few months.
slowly; a significant portion of the originally implanted
TRANSLATION FROM RESEARCH TO APPLICATIONS
cortical bone may be present several years after surgical
implantation. Cortical bone has had extensive application in
spinal fusion. ‘‘Rings’’ from long bones have been used in
anterior and posterior lumbar spinal fusion. The cortical
bone has excellent compressive strength. If processed asep-
gamma irradiation, cortical bone has compressive strength
exceeding 25kN. The use of cortical bone as a load-bearing
spacer allows the adjacent vertebrae to fuse under the in-
fluence of osteoinductive substances placed adjacent to or
within the implant.
A recent design combines the load-bearing properties of
cortical bone with the osteoconductive and osteoinductive
characteristics of cancellous bone. The design places the
load-bearing cortical component in the anterior aspect of the
implant, while the cancellous component is placed poster-
iorally. The fusion occurs first in the posterior zone through
the creeping substitution mechanism. This design has been
used for 2years in cervical spinal fusion. Another modifi-
cation has been devised to create some degree of surface
osteoinduction by partially demineralizing a 50mm thick
zone on the cortical surface.
Soft tissues—tendons and ligaments. Several human al-
lograft soft tissues have been adopted for surgical repair,
particularly in sports medicine. Several long tendons of the
legs, knee, and feet are widely used for repair of the ACL of
the knee. The gracilus and semi-tendinosus ligaments (Fig.
2C, D) are used to replace torn ACL. They may be doubled
Fixation in the tibial and femoral tunnel is accomplished
with fixation screws, ‘‘buttons,’’ or staples.50Similarly, the
Achilles tendon (Fig. 2E) with attached talus bone is a
popular choice for ACL repair because of its high strength,
The most widely used allograft in sports medicine is the
bone-patellar tendon-bone (BPTB). It is recovered from the
donor tissue by excising a suitable bone block with its ten-
doninsertion fromthe tibialcondyle(cancellous). Theintact
contiguous patella and patellar tendon (PT) are also col-
prepared tunnel with metallic or polymeric fixation screws.
The bone blocks are primarily cancellous bone and rapidly
incorporate. The tendon portion is very strong and also in-
tegrates by serving as a scaffold, which is slowly remodeled
by the host tissue. The clinical result restores functionally
useful strength over a period of 12–24 months.
Soft tissue—fascia. Fascial tissue is usually recovered
from the fascia lata, a primarily collagenous tissue. Com-
mercial processes have been developed to decellularize the
tissue in a manner that retains the gross morphology and
precludes loss of the tensile properties of the tissue. Decel-
lularization removes cells and collagen and the tissue is
rendered stable, both biomechanically and biochemically
(Fig. 2G, H). Such stabilized fascia has found utility in
urethral slings and shoulder rotator cuff repair.
Soft tissue—skin. Human allograft skin has been used for
many decades as a temporary dressing to stabilize third-
degree burns. This form of skin is the split-thickness tissue
recovered with, and retaining at use, the epidermal layer. A
newer form of skin is more thoroughly processed to remove
the epidermis, resulting in a dermal layer that can be further
treated to remove cells and achieve sterilization by chemical
means (Fig. 2I, J). The preservation of natural channels fa-
cilitates cell and tissue in-growth and revascularization.
Decellularized dermis is an excellent matrix for new tissue
growth and has been successfully used in repair of abdom-
inal ventral hernia.
Overall, bone and soft-tissue allografts described above
have been very successful as surgical implants. They per-
form well because of their inherent biomechanical proper-
ties. However, the mechanisms of creeping substitution are
only empirically understood. Allografts with their three-
dimensional matrix framework can help in further studies of
cellous bone. (C) Gracilus and (D) Semi-tendinosus for anterior
cruciate ligament (ACL) repair. (E, F) Achilles tendon. (G, H)
Fascia before and after decellularization. (I, J) Dermis before
and after decellularization. Color images available online at www
Human tissue allografts. (A) Cortical bone. (B) Can-
HUNZIKER ET AL.
cell-matrix interactions. For example, cancellous bone can
be utilized as a matrix for studying osteoclast-osteoblast
interaction to assist in elucidating the osteoporotic mecha-
nisms. Skin grafts with primarily type I collagen, and ar-
ticular cartilage grafts with primarily type II collagen could
conceivably be experimental matrices to study the interac-
tion of fibroblasts and chondrocytes, respectively. Further,
the allograft matrices can be used to study the cell mecha-
nisms underlying cell-matrix interaction. The details of the
osteoclast-osteoblast mechanisms as a function of different
matrices could be used to elucidate growth factor sequenc-
ing. Cortical, cancellous, long and flat bone, all may have
significantly different forming mechanisms.
Arthur Gertzman concluded that human allografts re-
present a precious biomaterial made available by the gen-
erous gift of tissue by a donor family immediately after the
unexpected death of a family member. These families are
asked to donate tissue and, ifthey so choose,to designatethe
tissues for use in research. Human allograft tissue is avail-
able from MTF in limited quantities at no cost to accredited
academic institutions and hospitals with research interests.
The US tissue-banking industry is largely not-for-profit and
has as part of its mission the support of research to honor the
requests made by the donors and their families.
FUNCTIONAL TISSUE ENGINEERING
OF LIGAMENTS AND TENDONS
Savio Woo from the University of Pittsburgh, Pittsburgh,
PA, discussed the functional tissue engineering (FTE) of
ligaments and tendons, with reference to the structural and
mechanical requirements derived from healing.
Ligaments and tendons are bands of dense connective
tissue that stabilize joint movement by transmitting forces.
The biochemical and biomechanical properties of ligaments
and tendons make them well suited to function in the unique
environment ofeach particularjoint. Unfortunately,boththe
medical collateral ligament (MCL) and the ACL of the knee
joint are frequently injured during sports and work-related
activities, with 100,000 to 200,000 tears occurring in the
United States each year.51The MCL ruptures can heal
spontaneously, while midsubstance tears of the ACL do not
heal. Thus, to improve knee stability and allow patients an
earlier return to preoperative levels of activities, surgical
reconstructions using BPTB or hamstring tendons as auto-
grafts to replace the ruptured ACL are performed.52,53
However, there are issues related to the autograft harvest.
Defects in the PT are not completely healed for months.54–56
Lessening the severity of these complications by enhancing
healing of the PT should lead to improved patient out-
come.57–60The MCL, on the other hand, can heal but with a
significantly different biochemical composition and matrix
organization, resulting in inferior biomechanical properties
even 2years after injury.61–64Changes in collagen content,
cross-links, and organization have been identified as three of
the major contributors to the inferior biomechanical prop-
erties of the healing MCL. Therefore, efforts involving the
use of FTE technology to enhance the healing of the liga-
ment and tendons are of great interest.
Structure and composition of ligaments and tendons
densely packed ECM.65The ECM is predominately a net-
work of fibrillar collagen structures arranged in parallel
along the long axis of the tissue. Between 65% and 70% of
the total weight of ligaments and tendons is water. Type I
and is primarily responsible for the tensile strength of the
tissue.66–70Type III and V collagen make up to 8% and 12%
of the dry weight, respectively.70–72To determine the bio-
mechanical properties of ligaments and tendons, uniaxial
tensile testing of bone-ligament-bone and bone/muscle-
tendon-bone complexes is usually performed because the
human MCL has an elastic modulus of 332.2?58.3MPa
and a tensile strength of 38.6?4.8MPa.73Those for the
central PT are 305 to 660MPa and 57 to 64MPa, respec-
The healing process
The healing of ligaments and tendons can be described as
Phase II is the inflammatory response and granulation of the
tissue,Phase III isfibroblast proliferationand blood clotting,
and Phase IV is clot replacement by disorganized collage-
nous tissue, which remodels with time (months to years). In
the early healing phase, the collagen content, especially
types III and V collagen, increased.70After weeks of heal-
ing, the totalcollagen content and hydroxypyridinium cross-
link density became lower than that of control ligaments.77
Thecollagenfiberdiameter ofthe healingMCL at60–70nm
is small when compared to the bimodal distribution of the
The mechanical properties of the healing ligament’s
midsubstance remained substantially inferior to those of the
intact ligament up to 1year after injury.80,81To gain func-
tion, the cross-sectional area of the healing MCL continues
to increase so as to compensate for the lack of improvement
in the mechanical properties. Similarly, the healing tissue in
the PT, after the central third is harvested for ACL repair, is
also abnormal, with disorganized collagen alignment, al-
tered ECM composition, and inferior tensile properties
compared to normal PT tissue.82-–84Further, the remaining
PT becomes abnormal due to hypertrophy to a tissue of
lesser quality (inferior mechanical properties).82,84,85
Novel treatment strategies based on FTE have shown
some promise in restoring the normal function of injured
ligaments and tendons.86–91These approaches include in-
novative biological and bioengineering techniques using
TRANSLATION FROM RESEARCH TO APPLICATIONS
growth factors, gene transfer therapy, cell therapy, scaf-
folding materials, and mechanical stimuli.92–94
Cell function and matrix synthesis. In vitro, growth fac-
tors increased cell proliferation, cell migration, as well as
ECM synthesis and production. In particular, fibroblast
growth factor, epidermal growth factor, platelet-derived
growth factor-BB, and transforming growth factors (TGF-
the in vitro findings to in vivo experiments, many contradic-
tory results were found, suggesting that in vivo conditions
are much more complex. Further investigations are needed
to determine optimal dosage, carrier vehicles, and timing of
applications for these exogenous growth factors.
Gene therapy has been used to mediate the production of
specific matrix proteins, directly or indirectly. Ex vivo and
in vivo gene transfer are often performed, in which viral
vectors or liposomes can be used as carriers for genes.100In
an ex vivo approach, a marker gene (LacZ) was successfully
introduced and expressed in the rabbit MCL and ACL using
an adenovirus as a vector.101To date, there have been var-
ious successes in the delivery of therapeutic genes to the
PT.100,102Another technique, antisense gene therapy, is the
delivery of genetic material that binds to signaling RNA and
reduces the expression of an undesired gene. Currently, li-
posomes are also being used in our research center to deliver
antisense oligonucleotides that reduce the expression of
collagen types III and V, which, as discussed earlier, are
elevated during the early stages of ligament healing.103
Cell and scaffold based approaches. Cell therapy is an-
other potential method to enhance ligament and tendon
healing. For example, bone marrow derived cells (BMDCs)
have the potential to improve the healing process of large
defects in the Achilles’ tendon and PT.82,88The BMDCs
have been shown to play an important role in wound
healing104–106and can be obtained in high numbers with
relative ease.82,107Recently, new methods geared toward
PT healing have tried to fill the central third PT defect with
collagen gels of different BMDC seeding densities. Im-
proved mechanical properties were seen when compared to
nontreated defects.82,107,108This particular cell therapy is
attractive because the use of autogenous cells would min-
imize the immune response at the injury site.
The use of biological scaffolds, such as the porcine small
intestine submucosa (SIS), offers distinct promise in accel-
eratingligament andtendon healing andregeneration.108–112
The SIS possesses a structural hierarchy that is naturally
arranged, and it is mostly composed of collagentype I. Forty
by-products have been shown to be chemoattractants for
and causes a limited inflammatory reaction.120
The potential of SIS to guide and support soft-tissue
regeneration, promote ECM organization, and eventually
improve the quality of the healing tissue has been demon-
strated in the rabbit MCL model.91,111A single layer of SIS
applied to a 6mm gap injury improved healing at 12 and 26
weeks. At 12 weeks, the collagen content, as represented by
hydroxyproline, in SIS-treated groups was 36% higher than
that in the nontreated group. Moreover, the collagen type V/
type I ratio, measured by sodium dodecyl sulphate poly-
acrylamide gel electrophoresis (SDS-PAGE), was reduced
sion electron microscopy demonstrated that larger diameter
collagen fibrils started to appear with increasing numbers at
26 weeks due to SIS treatment. The SIS also improved
mechanical properties of the treated MCL as compared to
nontreated MCL: the tangent modulus and the tensile
strength were 33% and 50% higher, respectively, at 26
weeks postinjury (p<0.05).
Chemoattractant degradation products and bioactive
agents of SIS could enhance healing.114The accelerated
healing of the defect in response to SIS treatment allows
better maintenance of stress- and motion-dependent ho-
meostasis. Also, the SIS can be modified in vitro by seeding
BMDCs on the scaffold and applying cyclic stretching in
order to increase the alignment of cells, as well as improve
the production and orientation of collagen. Hence, when
applied in vivo, the scaffold could accelerate the initiation of
the healing process, ultimately helping to make a better
neoligament or tendon.
Mechanical stimuli. Differenttypesofmechanicalstimuli
have been attempted to improve the reparative process of
ligaments and tendons.121–126Cyclic stretching of cells from
ligaments and tendons in vitro has been shown to cause
increase in collagen synthesis125,127and changes in intracel-
lular processes (i.e., different regulation of metabolic and
inflammatory genes and calcium signaling).126,128–132In-
terestingly, fibroblasts align themselves via contact guid-
ance from a substrate with microgrooves and along the
direction of the maximum principal strain in collagen
gels.122Similarly, in vitro studies have shown that multidi-
mensional mechanical strains applied to BMDCs embedded
types I and III and tenascin-C, which are typically expressed
in fibroblasts.123When a bioscaffold is applied to a healing
ligament or tendon in vivo, it will likely serve as a substrate
that will provide contact guidance for cells. As a result, the
healing neotissue will have more aligned collagen fibers
with a concomitant improvement in mechanical and vis-
coelastic properties when compared to nontreated controls.
Savio Woo summarized that biotechnology has seen
many recent exciting developments such as the sequencing
of the human genome, stem cell based therapies, and the
promise of tissue engineering. Still, however, these oppor-
tunities also present many challenges as the knowledge
gained about a particular gene, protein, or cell is eventually
issues will require that experts from different disciplines
HUNZIKER ET AL.
work together in unison. The role of biomedical engineers
within this framework can help link interactions at various
levels of scale: molecules to cells, cells to tissues, tissues to
organs, and organs to body function. Such a seamless in-
teraction of talented biologists, biomedical engineers, and
clinicians, as well as professionals from many other disci-
plines, willleadtotherapies thatallow injuredligamentsand
tendons to heal closer to that of normal. With the help of
funding agencies aware of the need for this research, a team-
based approach, and the new developments in FTE, the fu-
ture appears to be very bright.
ISSUES IN THE TRANSLATION OF
RESEARCH INTO PRODUCTS
Anthony Ratcliffe from Synthasome, a company based in
San Diego, CA, discussed the challenges related to the
conversion of the results of research into products that are
profitable and meet regulatory requirements. Tissue engi-
potentialtoaddress unmet clinical needs and tosubstantially
improve patient treatment. The technical areas to be con-
centrated on will be repair and replacement therapies, while
may also have an impact. The conversion of interesting
technologies and experimental results into products that
meet regulatory requirements are challenging. Perhaps the
greatest challenge is in the commercialization of the pro-
duct. If a tissue engineered technology and concept are to be
successful in the long term, there is no doubt that the clinical
value must be accompanied by financial profit.
Tissue engineered products
The initial thrust of tissue engineering was driven by
the envisioned commercial opportunities, and this has seen
development of some new products, particularly in wound
care and orthopedics. In wound care, TransCyte?, a dermal
fibroblast-derived ECM on a sheet biomaterial, was intro-
duced in 1997, for the treatment of third- and second-degree
burns. Apligraf?and Dermagraft?, introduced in 1999 and
2001, respectively, contain allogeneic live cells and human
venous and diabetic foot ulcers. In orthopedics, Carticel?
(autologous cultured chondrocytes) was introduced in 1997
for the treatment of focal articular defects. In 2004, Infuse?
was approved for spinal fusion, and consisted of the growth
factor bone morphogenic protein 2 (BMP-2) within a colla-
gen sponge (to regulate growth factor release) placed within
a metal cage.
A commonality of these products is that they have been
expensive to develop and produce, and once introduced
to the market require substantial commercial commitment
by their manufacturing companies to allow the products to
become established within the clinical marketplace. All the
tissue engineered products other than Infuse have struggled
to approach profitability, in spite of their demonstrated
clinical value to certain patient populations. Infuse now has
robust sales and demonstrates that profitability in the fields
of regenerative medicine and tissue engineering is possible.
Level of difficulty in developing new products
The technical challenges faced in the development of a
new tissue engineered product must be of prime consider-
ation, given that the field is complex even in its simplest
forms. The more difficult the clinical challenge, the more
technically complex the product is likely to be, and more
difficult and expensive will be the product development
pathway. It is useful to consider the mechanisms of tissue
engineered products to be in one of the three general cate-
gories: (a) facilitate repair, (b) induce repair, and (c) re-
placement. The simplest mechanism by which a tissue
engineered product can function is by facilitating repair,
where the product participates in a passive manner to en-
hance the ongoing repair process. An example of this would
be TransCyte, where the human ECM appears to enhance an
ongoing repair process.
A more difficult mechanism for products is when they
must induce a repair process. The Dermagraft and Apligraf
products achieve this when they are placed into a chronic
wound site where the normal repair activities are stalled, no
longer able to move a repair process forward. These prod-
ucts, with live cells secreting a variety of growth factors that
can participate in the wound-repair cascade, induce the
wound-repair process to reinitiate. Carticel and Infuse also
fall into this general category. The most difficult mechanism
by which tissue engineered products can function is in the
replacement of tissues or organs, where functionality of the
implanted product is required at or near the time of im-
plantation. There are no products on the market that fit this
category, but envisioned products include articular cartilage
that repairs large articular areas, blood vessel replacements,
heart valves, and others.
As can be seen, the more difficult the functional task re-
nical challenges will be associated with increased develop-
ment time cost, and probably increased risk of the product
being successful clinically. The development of new prod-
ucts should therefore consider the level of difficulty when
determining whether or not to proceed with a product de-
velopment program. Technically simple products are likely
to be introduced sooner. The more technically difficult
products, where the clinical need is likely to be highest, will
the interests of the health care community that work be done
also on the development of these products since their devel-
opment pathway is almost certain to be substantially longer.
Technical challenges. The product development pathway
should be initiated with an assessment of what is necessary
TRANSLATION FROM RESEARCH TO APPLICATIONS
in a product, its manufacture, and clinical performance to
achieve the clinical objective. This process, sometimes re-
ferred to as ‘‘Quality Functional Design,’’ can prove to be
invaluable in determining the vital characteristics that the
product should have, and distinguishing these from the fea-
tures that are less important. Three categories that may be
considered are (i) what must you have, (ii) what should you
have, and (iii) what would you like to have. This process
should emphasize moving as much as possible out of the
‘‘must’’ category. If this is done effectively, then the product
to be developed will efficiently address the clinical and
The work done so far in tissue engineering has clearly
demonstrated how products might be developed, and what
their limitations and challenges are. There are many key
technical hurdles in the field of tissue engineering, and in
the development of any new tissue engineered products,
and one or more of these are likely to be hurdles that must
be overcome. These include
? Cell source
? Mechanical properties
? Immune response
? Scale-up for manufacture
? Storage/shelf life
New technical advances and innovative manufacturing
may be necessary to allow product manufacture to meet the
envisioned clinical needs and be cost effective.
The development of industry standards will enhance the
efficiency of product development and regulation. The stan-
dards organizations, including ASTM and ISO, are in the
process of developing these standards, and when they are
established using consensus and with input from regulatory
authorities, they can enhance the product development path-
Regulatory pathways, associated costs, and timelines.
Tissue engineered products must be regulated to ensure
safety and effectiveness. In the United States, the FDA will
most probably regulate a tissue engineered product either as
a device, biologic, or drug. The quickest and least costly
regulatory pathway is as a device with a 510(k) designation.
Outside of this, once products have undergone development
and are ready for their key preclinical animal studies, it is
likely to take 5years and $30–$200million as a device, and
8years and $50–$300million as a biologic or drug.
While product development time may be shorter and
costs less, it is most likely that the time will be longer and
the costs higher. These timelines and approximate costs are
key to considering if and how to develop a concept into a
product. The technical risks and predicted financial returns
are in a continual balance, and the higher predicted finan-
cial returns can motivate taking higher technical risks.
However, it is important that realistic financial predictions
are made, otherwise profitability will not be reached and
the product will not have longevity.
Potential market sizes for tissue engineered products
The general markets that tissue engineered products are
likely to be placed are usually extremely large, multibillion-
dollar markets. However, realistic assessment of the true
market size for tissue engineered products usually shows that
the products will be used in niche areas where the total rev-
enue is predicted to be approximately $20–$200 million per
year. These are modest revenues, and they emphasize the
importance of realistic business assessments. When these
levels of revenue are compared to the potentially long de-
velopment timelines and associated costs, the return on in-
vestment is sometimes not particularly convincing. This in
turn argues for minimizing technical challenges that new
for the current products, and for most of the envisioned
when a robust business plan is developed and followed.
Key administrative hurdles for tissue
The administrative hurdles, like the technical hurdles, are
substantial, and include the regulatory system, the intellec-
tual property rights, and funding of product development
through to profitability. The regulatory pathways in the
United States and other major markets worldwide require a
and efficacy of the product. The manufacturing of a product
must undergo strict and thorough development, validation,
and monitoring. This rigorous regulation of products is crit-
ical for the industry. Limitations include the uncertainty in
regulatory requirements for each individual product and the
nonuniformity of regulatory systems between the major in-
ternational regulatory agencies.
Patent protection for an individual product is a critical
feature of product development. Developing new patents is
costly, the outcome is uncertain, and it must occur near the
beginning of product development. The limited time for
patent protection (usually 20years from initial submission)
requires that the product development pathway be followed
in an efficient manner; otherwise patent protection will be
lost by the time profitability arrives.
The funding of new product development, particularly at
the early, high-risk stages, is always going to be difficult.
This problem has increased recently, with the usual venture
capital funding being concentrated on the late-stage product
The federally funded Small Business Innovation Research
programs have been successful at initiating product devel-
opment; however, the funding of midstage product develop-
This funding gap represents a major administrative hurdle
HUNZIKER ET AL.
that must be planned for using a robust business model.
Recommendations for product development
The development of engineered products should incor-
porate the following in the product development pathway:
1. Maximize the knowledge base and infrastructure.
This requires that academic and industry groups form
collaborative teams working for the same coordinated
goals. The academic groups can provide detailed
technical knowledge and input; the industry groups
can more readily assess product development issues.
A functional team approach will maximize progress.
2. Use good science and engineering. The early phases
of any new technology require risk taking and leaps in
technical understanding, rather than incremental ad-
vances. However, whatever stage the technology is in,
it should be based on high-quality science and engi-
neering. Without this, the product development ef-
forts will not be robust, and will be prone to failure, at
early, middle, or late stages.
3. Involve, listen, and respond to regulatory agencies. A
collaborative approach with the regulatory agencies
will lead to good product development. It is unfortu-
nate that these agencies are limited in resources, and
new technologies such as tissue engineering present
difficulties that they must address for safety. However,
their input must be maximized. Ultimately, their re-
quirements must be met for a product to get to market.
4. Have robust and realistic business plans. Generate
revenue early, create value, and minimize the need for
tic market projections, anticipate hurdles, and develop
contingency plans. Technical success but commercial
failure does not generate a product. The best interests
of the R&D, investors, and patients will all be served
by using a realistic business plan.
Anthony Ratcliffe summarized that the technology of
tissue engineering has been shown to be feasible, products
are already on the market, and there is the potential for new
products to be developed that have significant clinical im-
pact. The technical and administrative hurdles are substan-
tial, but with realistic technical, regulatory, and business
planning, success should be assured.
THE PUTATIVE NEED FOR INCREASED
BASIC RESEARCH IN THE FIELD
OF TISSUE ENGINEERING
Michael J. Lysaght from Brown University, Providence,
RI, challenged the widely floated proposition that com-
mercial and clinical failures of early tissue engineering
products demonstrate a need for more focus on basic re-
search, and a clearer understanding of interactions between
living cells and biomaterials. He argued that such a per-
spective is largely a priori and based upon the miscon-
ception that the starting difficulties of tissue engineering
represented scientific failures. In fact, early trials and
tribulations in this field have been the result of under-
powered clinical trials, flawed business plans, ineffective
marketing, and a Procrustean regulatory stance by the FDA.
Basic research and scientific understanding are eminently
worthwhile in their own right, but they cannot, and will not,
in the perspective elaborated here, contribute much to the
realization of tissue engineering and regenerative medicine.
The early experiences of tissue engineering represent a
unique, and still unfinished, chapter in the history of con-
temporary biotechnology. The field had its beginnings in the
seventies with early research into artificial skin and the
biohybrid pancreas, two initial therapeutic constructs that
combined biomaterials and living cells. Isolated investiga-
tions continued throughout the eighties, but the field ac-
quired its own identity and significant momentum during the
nineties. Commercial failure of early tissue engineering
products at the turn of the century and the demise of the
flagship tissue engineering firms led to turbulent upheaval,
restructuring, and reassessment. The implicit leitmotif of the
NIH workshop Tissue Engineering—The Next Generation
was that more and better fundamental science is needed to
rescue tissue engineering from a premature demise. The
leading thinkers in the field convened to outline the intel-
lectual pathways (aka roadmaps) describing the structure of
this new scientific knowledge. Dr. Lysaght offered a con-
trary viewpoint to this premise.
The growth of tissue engineering and its macroeconomic
structure are well documented. During the period from 1990
to 2003 (latest available data), the private sector invested
nearly $4.5 billion in the field. Most of the money went into
small start-up firms with anywhere from a handful of em-
ployees to a workforce numbering in the several hundreds.
At the end of 2000, the tissue engineering industry boasted
73 companies with over 3,000 employees. The capital value
of the six largest public companies, none of which yet had
significant products, was $2.5 billion. Even by its most
generous estimates, government and foundation support
constituted less than 10% of the total funding. Smaller firms
spent their money on product development; larger organi-
zations on activities related to regulatory approval. The goal
was rapid progression to the clinic, and there was very little
in the way of resources and enthusiasm for basic research or
fundamental inquiry. Lack of federal support was consid-
ered unfortunate but was not regarded as limiting. Industry
worked closely with academia and supported work at uni-
versity, but very pragmatic focus attended even these col-
Starting in 2001, things began to go wrong for the field.
In short succession, Organogenesis and Advanced Tissue
TRANSLATION FROM RESEARCH TO APPLICATIONS
Sciences launched their ‘‘Living Skin Equivalents,’’ the
market reception and sales of which were sufficiently dis-
appointing to drive both companies to bankruptcy; over 500
workers in the field became redundant. CytoTherapeutics
andCircecompleted phase IIIclinicaltrials oftheir products
without the statistically sufficient efficacy data needed to
win FDA approval. Diacrin and Vitagen abandoned clinical
trials in phase II because of, respectively,clinical difficulties
and financial constraints. These four companies directly
or indirectly ceased operations, leading to a further loss of
about 400 positions. And then came the stock market crash,
which reduced the capital value of publicly traded tissue
engineering companies by 90%. The investing public lost its
appetite for these high-risk ventures, forcing the remaining
companies to downsize their workforce and stretch out their
business plans. About half of all employees in tissue engi-
neering found their jobs eliminated. Very fortunately, stem
cells had now entered the picture and much of the dislocated
workforce, sadder but wiser, was easily assimilated.
The field has changed rapidly since 2003. Venture capital
plays a much smaller role. Large firms, for example, Gen-
zyme, Medtronics, and J&J, have begun devoting serious
resources to focused projects. Low-risk products, for exam-
ple, textured matrices to foster wound healing but which
contain no living cells, are increasingly attractive. Interest in
stem cells, which may or may not be related to tissue engi-
product meeting the classical definition of tissue engineering
is in FDA efficacy trials. Tissue engineering still seems
emergence islikelytotake longerthanoriginallyenvisioned.
Meanwhile, the academic community has responded with
relish to the challenge of defining what went wrong with
tissue engineering first time around, and to reappraising the
strategy for advancing this technology in the future. This is
certainly understandable. Academia played a subordinate
rather than a leading role in tissue engineering in the nine-
ties. And the fact is that the ‘‘waste’’ of 5 billion dollars in
the first generation of tissue engineering—in the sense that
no successful clinical product has emerged—is certainly
grating within a community where success or failure
is measured in terms of research grants usually valued at
$250,000 per investigator per year. The prescription, cer-
tainly explicit in other contributions to this workshop, is
clear: more basic research, more understanding of the in-
teractions between living cells and synthetic materials, more
and better biomaterials, better cell biology, and the like. All
such pursuits are worthwhile, even laudable, and all are
worthy of support in their own rights. But, by themselves,
none would have been likely to either change the fate of the
first generation of tissue engineering or alter the course of
future efforts. Rather, the need is for improved product de-
velopment, better management and sounder business plans,
and a somewhat more coherent regulatory environment.
The view that more basic research would enable the
clinical realization of tissue engineering is generally floated
as an a priori assertion. Proponents do not provide an in-
ventory of missing knowledge and then elaborate how its
availability would have impacted the fate of Dermagraft or
to support simply because the failure of these products was
far more grounded in the business side than in the scientific
side. And until such a reckoning is made, the call for more
basic research represents more of a faith-based initiative
than one grounded in solid evidence.
The history of first-generation artificial organs (hemo-
devices first demonstrated the possibility of replacing dete-
riorated natural organs with man-made substitutes and ar-
guably breached a more significant development chasm than
will be requiredfortissue engineering. But the emergence of
first-generation devices was largely a matter of trial and
error. Very little basic and clinical science was involved;
engineering was primitive; and biomaterials science was
highly Edisonian. Moreover, once organ replacement got
past its start-up problems, which are not very different from
those now being experienced by tissue engineered products,
its further growth was fueled more by skill at medical
management than by advances in basic science. For exam-
ple, to this day, nephrologists are just not sure what a he-
modialyzer has to remove.
Dr. Lysaght argued that, in any event, the real reason why
more basic scientific research is not going to ‘‘fix’’ tissue
engineering is that the field’s start-up problems have mostly
not been scientific but rather related to business and regu-
lation. Consider living-skin equivalents, itself a telltale
misnomer. These products failed, and led to the bankruptcy
of the parent companies, for a variety of reasons none of
which could have been remediated by a more extensive
science base. First, the firm’s sales forecasts were vastly
inflated. Ramp-up times were hugely underestimated. In a
classic case of technology push rather than market pull, the
companies simply did not appreciate the difficulty in getting
practicing dermatologists to change their treatment methods
for slightly improved outcomes. They also failed to create a
suitable reimbursement environment, one that provided a
positive-sum proposition to the practicing dermatologist.
The products were likely mispositioned as skin substitutes
when they were really wound-healing agents. The basic
calculus of the agreements between the producing firms and
the marketing partners was sufficiently off-kilter to provide
neither party with appropriate incentives. It is encouraging
to note that most of these issues have been resolved and the
sales of these products are currently both profitable and
growing—without more basic research.
Several metabolic devices also failed to win FDA ap-
not due to a lack of basic science but rather due to a lack of
funds to field trials large enough to reach statistical signifi-
cance of benefit. Enrolment for efficacy trials of tissue en-
gineered products was typically 1 to 300 patients; drug trials
usually run in the several thousands. In the case of Circe’s
HUNZIKER ET AL.
bioartificial liver, a clearly responding and statistically sig-
nificant benefit was identified for a subset of patients, but
FDA approval was denied since this group had not been
identified in advance.
Perhaps at the root of these problems is the mentality of
the venture capital groups, who supplied much of capital for
the industry and who maintained effective board-level
control well into the period of greatest difficulty. The ven-
ture capitalists (VCs) implicitly imposed the business model
that was proven lucrative and successful for earlier bio-
technology start-ups based upon recombinant molecular
because of the offsetting benefits of the occasional success.
Values of a start-up increased in formulaic fashion with
cess in small animal models, publication in a high impact-
factor journal, corporate partnership, first clinical exposure,
and regulatory-level clinical trials. These milestones be-
came ends in themselves; their achievement was crucial to
firms raising the capital needed to subsist and, often, to
providing an exit strategy for the VCs. Speed was of the
essence, and a prevailing belief was that firms with much
fewer than 150–250 employees were simply too constrained
High-risk ‘‘bet-the-farm’’ strategies often took prece-
dence over the more methodical development and iterative
experimentation suitable for medical devices and implants.
Firms had to target very large markets from the outset to
justify the high cost of rapid development and thereby
missed the opportunity to gain initial clinical exposure in
cumulative effect was a tissue engineering industry with a
Ponzi-like ethos, until things went south in 2001–2003 with
The encouraging news is that these mistakes are unlikely to
be repeated anytime soon; firms leading the recovery of
tissue engineering are restrained in their enthusiasm for
success. Therelevantconclusion isthatallthe basicresearch
in the world would not have let the industry avoid the
inevitable consequences of its flawed business models.
Finally, there is the impact of regulation on the field.
the Center for Devices and Radiological Health fared rea-
sonably well, those regulated as drugs (by the Center for
Drug Evaluation and Research) or as biologics (by the
Center for Biologics Evaluation and Research) did not, and,
in fact, these bureaus have yet to approve a single tissue
engineered product. The reason for this relates to the eco-
approval for a life-sustaining or life-supporting medical
device averages to $50 million, and the corresponding cost
for even a me-too drug is around $800 million. Drug re-
double blind trials with populations in the thousands, and a
‘‘cost-be-damned’’ approach by applicants. The relatively
modest efforts of tissue engineered products simply failed to
meet these expectations. The FDA has a genuine desire to
promote new medical technology, and considerable dialog
has taken place between agency management and advocates
for tissue engineering. But events such as the Vioxx episode
tend to reinforce a culture of extreme caution.
Michael Lysaght summarized what tissue engineering
needs to bring its promising concepts to clinical practice:
long-term rather than short-term investment money, busi-
ness plans geared to realistic cost/benefit trade-offs, less
hype, more sophisticated regulatory staff, and engineers
skilled at product development and manufacturing scale-up.
Basic research will not help much. Is this an argument
against support of basic research? Absolutely not. Such re-
search has an almost unlimited capacity to advance the
health and well-being of mankind. It should be supported
as lavishly as societal resources permit, but for the right
TISSUE ENGINEERING PRODUCTS:
SUCCEEDING OR FAILING DEPENDS ON
INTERPRETATION OF THE DEFINITION
Arthur Coury from Genzyme Corporation, Cambridge,
MA, presented a talk on tissue engineering initiatives at this
organization, which was of great interest because of Gen-
zyme’s long-term efforts in the field, its set of cell-based
products, and its commitment to developing advanced tissue
engineering products. Genzyme is one of just a few corpo-
rations that have been able to maintain their commitment to
cell therapies for well over a decade, ‘‘toughing’’ it through
the difficult development process and unprofitable times
until profitability was achieved. The Carticel autologous
cartilage implant became profitable in 2005, and Epicel#
autologous keratinocyte graft is maintained as a humani-
tarianproducttosave thelives ofserious burnvictims.Some
would interpret definitions of tissue engineering in the nar-
row sense of applying cells; cells and scaffolds; or cells,
scaffolds, and factors for therapeutic purposes and would
conclude that tissue engineering has, with few exceptions,
been a failure. Dr. Coury addressed tissue engineering in the
context of proffered definitions, and how their interpretation
can affect perceptions and policies relative to its status and
advancement, using Genzyme initiatives as examples em-
bodying the proposed scope of the field.
Scope of tissue engineering
Most published definitions of tissue engineering suggest a
broad scope. For example, Gordana Vunjak-Novakovic and
David Kaplan, for this conference, stated: ‘‘Tissue Engi-
neering has been defined as the application of the principles
and methods ofengineering and the life sciencestowardsthe
improve functions.’’133Langer and Vacanti’s pioneering
definition was very similar, defining tissue engineering as
TRANSLATION FROM RESEARCH TO APPLICATIONS
‘‘[a]n interdisciplinary field that applies the principles of
engineering and life sciences toward the development of
biological substitutes that restore, maintain, or improve tis-
sue function.’’10David Williams, in his Dictionary of Bio-
materials, provides a detailed, yet still broad definition:
‘‘Tissue Engineering is the persuasion of the body to heal
itself, through the delivery to the appropriate sites of mo-
lecular signals, cells and supporting structures.’’134In 2005,
in detail: ‘‘The generation, regeneration, augmentation or
limitation of the structure and function of living tissues by
the application of scientific and engineering principles.’’135
Any of these definitions covers a broad range of therapeutic
approaches leading, basically, to the control of cells and
their matrices (Table 5).
Wound dressings and tissue sealants
The choice of a wound dressing (occlusive, hydrogel, or
gauze) would have tissue engineering implications, as each
dressing induces a different rate of healing and potential
outcome. Many engineering and scientific principles, such
as adherence, permeability, leaching, and lamination, are
manifest in wound dressings. The dressings are intended to
effect guided tissue regeneration, a component of tissue
wound-care products are tissue engineering products, then
this segment of the field is a very large, profitable, and
multibillion-dollar one136(Table 6). Therapies such as spi-
nal fusion using demineralized bone, skin expansion using
expandable implants, or bone-lengthening using mechanical
prostheses and surgical techniques would fit Dr. Coury’s
interpretation of tissue engineering.
Surgical sealants (Table 6) are not normally considered
tissue engineering products. They are poly(ethylene oxide)-
based, tissue adherent hydrogel coating, resorbable over 3–
4months, and formed from a liquid photopolymerizable
macromer formulation applied to dural lesions (FocalSeal).
Inarabbit study,a4mm2duralexcisionwasmadeabove the
brain, and the opening was sealed with an experimental
sealant, using a priming technique that only adhered the
hydrogel to the dura, not the brain (Fig. 3A). After 10days,
the dural site was examined and found to have formed a
contiguous, fibrovascular film under the hydrogel, which
sealedthe excision andwould maturetoa strong,autologous
tissue seal (Fig. 3B). Without the sealant, the dural leak was
sealed by the brain forming adhesions to the soft tissue
above the brain (Fig.3C). The effect produced by the sealant
and should thus be considered tissue engineering. Guided
healing has been observed for hydrogels used as dermal
control by tissue engineering products for all of the ap-
proaches listed in Table 6, advocating for an expanded
perception of tissue engineering.
Replacement parts and electronic devices
Some would say that the broadened definition dilutes
the impact of a focused concept, and sounds like it is all-
inclusive. However, many therapies would not be included.
Replacement parts such as prosthetic hips and knees, or-
thopedic plates and screws, dental implants, crowns and
fillings, intraocular lens, vascular grafts, circulatory assist
devices, and heart valves would be excluded. Implantable
delivery devices such as central venous catheters, drug
pumps, and percutaneous access devices do not meet the cri-
teria. Free-standing diagnostic devices would be excluded.
Electronic devices such as cardiac pacemakers and neu-
rological stimulators would fall into a gray area. They def-
initely affect the function of responsive cells, but they also
replace the function of defective cells and matrices. To the
extent that the function of electronic devices is preserved or
enhanced by cell and matrix (e.g., fibrosis) control (e.g.,
drug-eluting pacemaker leads), tissue engineering is in-
volved. Thus, some segments of this $10 billion market137
could be categorized as tissue engineering. Drug-eluting
coronary stents form another family of products (device-
drug combination products) that can either be placed in the
tissue engineering category or be excluded. On the con side,
Table 5.TISSUE ENGINEERING THERAPEUTIC APPROACHES
? Guided Tissue Regeneration
? Organ Generation/Regeneration
? Tissue Reinforcement
? Tissue Augmentation/Space Filling
? Healing Modulation
? Hypertrophy Inhibition
? Fouling Prevention
? Tissue Adhesion/Sealing
? Adhesion Prevention
? Enhanced Mitosis/Matrix Deposition
? Organ Function Modulation
? Expression of Bio-Actives
? Gene Therapy
? Protein Therapy
Table 6.WOUND CARE MARKETS, 2003
Surgery and Trauma: $4.3 Billion
? Sutures, Staples, Adhesives, Sealants
? Anti-Infective, Cleansing, Debridement Products
? Tapes, Dry Dressings
? Moist Dressings (Hydrogels, Hydrocolloids, etc.)
? Biologicals (Bio-Artificial Skin, Collagen, Growth Factors,
HUNZIKER ET AL.
their functionistoreplacethe lumenofstenosed oroccluded
vessels. On the pro side, drug-eluting coronary stents, in
particular, which dominate the $6 billion stent market,138
function to control the cellular response and matrix pro-
duction locally, and it could be argued with much justifica-
tion that this is true tissue engineering.
Discussion and recommendations
Taken together, the exemptions from tissue engineering
therapeutic products constitute a majority of the therapeutic
device and combination device markets, which exceed $100
billion worldwide.139Dr. Coury estimated that a very re-
spectable 20–30% of the total may be considered tissue
engineering products, much higher than the few tens of mil-
lions of dollars attributed under limited interpretations.140
Any discussion of pharmacologic therapy alone, which can,
indeed, exert control over cells and matrices, is beyond the
scope of this discussion.
Another argument against the expanded interpretation of
tissue engineering is that the narrow concept is well estab-
lished and entrenched, and it would be hard to reverse this
perception. Dr. Coury argued that, in spite of the similarities
of most definitions, there is no widespread agreement on the
limits of tissue engineering. Some would argue that isolated
cells are always involved. Disagreements could ensue if
scaffolds were not included. Others would say that scaffolds
alone could constitute the tissue engineering therapy if tissue
regeneration occurs. Some, not all, would insist that the de-
livery of growth factors to enhance tissue generation along
with a device is tissue engineering. The debate, even over the
most likely candidates for tissue engineering therapeutics, at-
tests to the lack of general agreement on the specifics of
the concept. There is room for influencing the interpretation.
Afinalargumenttobeconsideredisbased ontheimage of
tissue engineering, which fails to achieve its promise in its
projected time frame. Of some 20 plus companies listed by
Business Week as tissue engineering start-ups in 1998,141
only three exist today as intact companies, and they are not
profitable from sales of tissue engineered products. Invest-
ment in narrowly interpreted tissue engineering in the Uni-
2000.140One side would believe that expanding the scope of
tissue engineering to successful products would somehow
reflect negatively on the latter in terms of public image and
growth potential. The opposite effect could be the one to
materialize. Successful products have made it because of
medical needs being satisfied. The products are efficacious
and demand is generally growing nicely, based on their ef-
fectiveness with a compound annual growth rate of 9%.139
The reputation earned by successful therapeutic products
over decades, included in the expanded concept of tissue
engineering, should enhance the image of the field and bring
many benefits. There is the public relations benefit of a
successful image. Success and continuing optimistic growth
projections attract public and private investment funding for
research, development, and education. The interpretation
that major successful therapies come under the umbrella of
tissue engineering will attract students for careers in this
field, and enhance their optimism for its robust growth.
The arguments for inclusiveness of therapies in tissue
engineering todemonstrate successesare in no way meant to
minimizethe challenges tobefacedinachievingthegoalsof
regenerative therapy embodied in the narrower interpreta-
tion of the field. We are years away from off-the-shelf or
even autologous cultured or internally generated commer-
cial structures such as blood vessels, bladders, corneas,
nerves, and menisci, even though clinical prototypes exist.
resorbable hydrogel. (A) Dural excision of 4mm2was made above the brain, and the opening was sealed with an experimental sealant,
using a priming technique that only adhered the hydrogel to the dura, not the brain. (B) After 10days, the dural site was examined and
found to have formed a contiguous, fibrovascular film under the hydrogel, which sealed the excision and would mature to a strong,
autologous tissue seal. (C) Without the sealant, the dural leak was sealed by the brain adhesions to the soft tissue above the brain. Color
images available online at www.liebertpub.com/ten.
Rabbit study used in the development of Genzyme’s dural sealant product, a poly(ethylene oxide)-based, tissue adherent
TRANSLATION FROM RESEARCH TO APPLICATIONS
We are decades away from more complex structures (kid-
ney, liver, heart, pancreas tissues).142But most of the prin-
ciples required to achieve even the loftiest of our tissue
engineering goals are known and applicable. Utilization of
these principles with resources, persistence, and dedication
will eventually but inevitably lead to the envisioned tissue
regeneration. But this will require an environment that
promotes the necessary research and development in an at-
mosphere of optimism and commitment.
Genzyme took the visionary position of executing the
long-term development of its cartilage regeneration product
using retained earnings from other products and supported it
during its slow adoption. The company’s commitment ex-
tends to the development of improved cartilage regeneration
therapies, surgical techniques, and multiple uses for its cell
therapies, as well as to other devices such as adhesion pre-
vention products that are included in the broad tissue engi-
neering interpretation. Its position for further exploiting
tissue engineering therapy is excellent. Other formulas for
tissue engineering product development, whether through
venture capital, public funding, or private investment, will
require vision, optimism, and persistence.
Arthur Coury concluded that the environment for gener-
ating the necessary support to advance tissue engineer-
ing wouldbe promoted by the expanded interpretation of the
optimism, which could lead to increased funding and human
resource commitment to the field. We are on stable ground
here—there is robust justification that many more therapeu-
tic products really do exert control over cells and matrices
Myron Spector thanks the US Department of Veterans
Affairs for financial support. Savio Woo acknowledges the
contributions of Alex Almarza and Steve Abramowitch to
the preparation of his section of the manuscript.
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