, 1603 (2010);
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Materials and Mechanics for Stretchable Electronics
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Materials and Mechanics
for Stretchable Electronics
John A. Rogers,1,2,3* Takao Someya,4Yonggang Huang5
Recent advances in mechanics and materials provide routes to integrated circuits that can offer the
electrical properties of conventional, rigid wafer-based technologies but with the ability to be stretched,
compressed, twisted, bent, and deformed into arbitrary shapes. Inorganic and organic electronic
materials in microstructured and nanostructured forms, intimately integrated with elastomeric substrates,
offer particularly attractive characteristics, with realistic pathways to sophisticated embodiments. Here,
we review these strategies and describe applications of them in systems ranging from electronic eyeball
cameras to deformable light-emitting displays. We conclude with some perspectives on routes to
commercialization, new device opportunities, and remaining challenges for research.
are impossible to achieve with hard, planar inte-
surgical and diagnostic implements that naturally
integrate with the human body to provide ad-
biologically inspired designs to achieve superior
performance. Sensory skins for robotics, structural
and other systems that require lightweight, rugged
possible. Establishing the foundations for this fu-
ture in electronics represents an emerging direction
for research, much different from the one dictated
by the ongoing push toward smaller and faster
devices that are still confined to the planar surfaces
of silicon wafers.
Work toward mechanically unconventional
forms of electronics began, in earnest, ~15 years
ago with polymer transistors formed on bendable
sheets of plastic (1, 2), where paperlike displays
(3, 4) represented the main target application.
Advances in printing and related patterning tech-
niques (5) and in organic semiconductors (6) were
key to much of the initial progress in this field.
Research in the past few years on ultrathin inor-
ganics, nanotubes, and nanowires promises fur-
iology is soft, elastic, and curved; silicon
wafers are not. An electronics technology
ther improvements, as described in recent review
articles (7–9). Many successes, in the form of
working demonstration devices with hundreds to
thousands of active components, have already
been achieved; several, including displays, are
nearing commercial reality.
More recently, the scope of research has ex-
panded dramatically to include more compelling,
and more technically challenging, opportunities in
soft, biointegrated devices; in curved, bioinspired
designs; and in other areas. Here, the electronics
maintaining levels of performance, reliability, and
integration that approach those of well-developed
mentary, ways. One relies on the use of new struc-
tural layouts in conventional materials (10), the
We focus first on approaches that have yielded the
mostsophisticateddevicesandthen present amore
comprehensive summary of options.
Structures That Stretch
Two simple ideas underlie the strategy based on
structure. The first exploits an elementary result in
flexible, by virtue of bending strains that decrease
rigid, but nanoscale ribbons, wires, or membranes
of silicon are flexible. For example, ribbons with
thicknesses of 100 nm experience peak strains of
bend radii remain well below the fracture limits
by movingthesiliconaway fromthe surface ofthe
where these strains are zero.
Ultrathin circuits that use silicon
mechanical plane design can be
bent to radii of ~150 mm, with
strains in the silicon that are less
than ~0.1% (12, 13).
Configuring such structures
into “wavy” shapes and bonding
them to elastomeric substrates
yields systems that can not only
flex but also stretch and com-
press, with a mechanics similar
to that of an accordion bellows.
Figure 1, A and B, shows two
possibilities, illustrated with ul-
trathin sheets of silicon inte-
grated on slabs of the elastomer
to a sheet formatted into waves
with a herringbone configuration
by a controlled buckling process
(14), as a two-dimensional ana-
in ribbons (15). The resulting
and compressed reversibly, with
lengths of the waves change in
1Departments of Materials Science and Engineering, Mechanical
Science and Engineering, and Electrical and Computer Engineer-
ing and Chemistry, University of Illinois at Urbana-Champaign,
1304 West Green Street, Urbana, IL 61801, USA.2Beckman
Institute for Advanced Science and Technology, University of
Illinois at Urbana-Champaign, 405 North Mathews Avenue,
Urbana, IL 61801, USA.3Materials Research Laboratory, Univer-
Urbana, IL 61801, USA.4Department of Electrical and Electronic
Engineering and Information Systems, University of Tokyo, 7-3-1
Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.5Departments of
Civil and Environmental Engineering and Mechanical Engineer-
ing, Northwestern University, Evanston, IL 60208, USA.
*To whom correspondence should be addressed. E-mail:
Fig. 1. Concepts for stretchable electronic materials. (A) Stretchable
silicon membrane (~100-nm thickness) configured in a wavy shape
force (bottom) microscope images. (B) Extremely stretchable silicon
membrane (~100-nm thickness) patterned into a mesh geometry and
shaped bridge structures, presented in moderate (top) and high
(bottom) magnification scanning electron microscope (SEM) images.
that provide electrical pathways in these composites.
VOL 327 26 MARCH 2010
on March 26, 2010
in a way that involves considerable
strains in the PDMS, but not in the
eling reveals that the peak strains in
the silicon can be 10 to 20 times as
designs represent, then, a stretchable
form of silicon with a strain range of
10 to 20%, that is, 10 to 20 times as
large as the intrinsic fracture limits of
A related strategy improves this
range by structuring the sheet into a
mesh and bonding it to the PDMS
only at the nodes. The buckled, arc-
shaped interconnecting structures
(Fig. 1B) can move freely out of the
approach the fracture limits of the
PDMS (17). Related approaches that
use mesh layouts with planar leaf-
arm (18, 19), coiled spring (20), and
noncopolanar serpentine (17) inter-
vide stretchability in certain direc-
figurations (17), have been used to
achieve integrated systems, as dis-
cussed in a following section.
Materials That Stretch
New materials provide an alternative
route to stretchable electronics. The
most successful approaches use elas-
tic conductors as electrical intercon-
nects between active devices that are
ductive rubbers based on elastomers
loaded with carbon black have been
known for decades, the resistances
and their dependence on strain are
long, single-walled carbon nanotubes
(SWNTs) serve as conductive dop-
SWNTs processed by grinding in an
ionic liquid and then mixing with a
fluorinated copolymer yield a black,
pastelike conductive substance, referred to as a
matsinthesegels, with the capacity to reconfigure
in response to applied strain in a manner that
preserves highly conductive pathways for charge
transport. This material can be printed onto sheets
of PDMS to yield elastic conducting traces with
stretchablility in the range of 100%. Figure 1C
shows a picture of such a sample and a micro-
graph of the associated network of SWNTs. Al-
ternative, related approaches use SWNTs in thin
film networks formed by solution casting or other
Stretchable Electronics, Optoelectronics,
and Integrated Systems
able, integrated systems. Figure 2A presents an
example of the design approach of Fig. 1A applied
to an ultrathin, neutral mechanical circuit sheet that
supports an array of silicon transistors, logic gates,
and ring oscillators (13). A pair of transistors in an
inverter appears in the upper inset.
The waves are influenced by varia-
positions across the area of the circuit
to yield layouts that are much more
complex than those in Fig. 1A. The
mechanical responses that enable
stretchability, however, are the same:
The wavy shapes change to accom-
modate applied strains, and the un-
derlying PDMS substrate provides
an elastic restoring force. The main
image shows the circuit deformed in
its center with a glass pipette to il-
system. Figure 2B presents an exam-
1B. Here, microscale arc-shaped “rib-
bon cables” of metal and plastic in
neutral mechanical layouts intercon-
nect silicon devices located at the
nodes of the mesh (17). The circuit is
conformallyintegrated onto a model
surface whose nonzero Gaussian
curvature would be impossible to
wrap with a system that is only flex-
ible. Figure 2C shows a similar out-
come achieved with stretchable
conductors (22). This circuit consists
of arrays of organic transistors inter-
1C) and supported by a thin sheet of
PDMS in stretched and curvilinear
configurations. During deformation,
only the interconnection lines stretch,
such that negligible changes in tran-
sistor characteristics occur even for
strains up to 70%.
The left frame of Fig. 3A shows
an image of an integrated system that
exploits the concepts of Fig. 1B and
on an array of silicon photodetectors
in the approximate size and curved
layout of the human retina (28). This
design offers enhanced field of view
and uniformity in illumination com-
when simple imaging optics are used
(28, 29, 19). Fabrication of such an
“eyeball” camera starts with an array
of silicon photodiodes and blocking diodes formed
wrapping onto a concave, hemispherical glass
substrate, followed by integration with an imaging
lens and a printed circuit board interface to a com-
puter for data acquisition, completes the device. A
picture of an eye, captured with a camera whose
detector curvature (i.e., elliptical paraboloid)
matches the image surface formed with a plano-
convex lens, appears on the right in Fig. 3A (29).
The top and bottom frames correspond to the
300 µ µm
500 µ µm
Fig. 2. Examples of stretchable electronics. (A) Stretchable silicon circuit in a
wavy geometry, compressed in its center by a glass capillary tube (main) and
wavy logic gate built with two transistors (top right inset). (B) Stretchable silicon
circuit with a mesh design, wrapped onto a model of a fingertip, shown at low
(left), moderate (center) and high (right) magnification. The red (left) and blue
(center) boxes indicate the regions of magnified views in the center and right,
respectively. The image on the right was collected with an automated camera
field. (C) Array of organic transistors interconnected by elastic conductors on a
sheet of PDMS in a stretched (left) and curvilinear (right) configuration.
26 MARCH 2010VOL 327
Materials for Electronics
on March 26, 2010
image rendered in the curved format of the camera
and a planar projection, respectively; the inset
illustrates the picture that was imaged. These ideas
where the geometry of the detector array can be
most promising initial application opportunities are
considerations of cost, size, and/or weight can be
addressed by introducing curvature in the detector
to enable dramatic reductions in the complexity of
the optics.The same concepts have the potential to
allow other, more complex biologically inspired
designs. Ultimately, such devices might be used as
to restore or enhance vision.
Light-emitting devices are also possible using
the same approaches. Figure 3B shows an image
of a stretchable inorganic light-emitting diode
(LED) display that uses ultrathin, microscale
AlInGaP LEDs interconnected in a mesh layout
and bonded to a PDMS substrate (30). Figure 3C
provides an example of a related demonstrator
produced with stretchable conductors and organic
LEDs (23). Both types of display can be stretched
by 30 to 50% and wrapped onto curvilinear sup-
ports without any mechanical damage or change
in operating characteristics. This class of technol-
ogy could, of course, be useful as a flexible
lighting or display system, with extreme levels
of bendability and mechanical robustness. More
interesting opportunities in the future might lie in
optogenetics, where programmable light sources
wrapped onto the convoluted surface of the brain
could provide insights into brain function. Light-
based therapies and diagnostics in other parts of
the body might also be achievable in similar
Work in stretchable electrodes, as opposed to elec-
tronics,hasa comparatively longhistory and broad
range of materials and design options. In fact, the
field of stretchable electronics owes its origins to
observations that films of gold formed by physical
ly adopt microstructured or nanostructured forms
(31) and that these structures (Fig. 4A) provide
electrodes that can accommodate large applied
strains without fracture (32). Detailed studies sug-
gest that stretchability in this case derives from a
physics similar to that of the silicon structures of
Fig. 1A but with additional contributions from the
during fabrication and subsequent deformation
(33). Figure 4B shows a different, but related, ex-
ample in which direct writing with a silver nano-
particle ink yields a wavy metallic microwire (34),
wavy shapes can be achieved in conducting and
atomic force microscope measurements on an in-
dividual SWNT (35), formed in a wavy configu-
ration on PDMS using the techniques that yielded
the structure of Fig. 1A. Newtonian mechanics
models like those that describe similar deforma-
tions in silicon nanoribbons (15) can capture the
physics even at the molecular scale of the nano-
tube (35). With nanowires, related types of wavy
deformations form in the plane of the substrate.
scale metal wires, as illustrated with a stretchable,
grated circuit chips as a route to stretchable devices
(37) with a type of spatially discrete functionality
that can complement the capabilities of the dis-
tributed systems of Figs. 2 and 3.
Finally, in addition to structured wires of Fig.
4, A to D, similar levels of stretchability can be
achieved directly in electrodes of suitable mate-
Figure 4E shows, as an example, a low-melting-
in PDMS (38). Here, the metal can plastically
deform and flow in response to large strain de-
formations, while the PDMS provides an elastic
restoring force. More recently, work shows that
graphene on PDMS exhibits reversible responses
to large applied strain (39), perhaps involving a
mechanics similar to that of the gold electrodes of
Fig. 4A. An image appears in Fig. 4E.
Paths to Commercialization
Although nearly all current activities in stretchable
electronics are centered in academic laboratories,
there is growing interest at small and large com-
panies. A path to commercialization might begin
with stretchable electrodes, followed by devices
with discrete configurations, and culminating in
highly functional, distributed systems. An impor-
tant perspective is that many of the underlying
concepts are aligned well with the incremental, but
collectively substantial, developments in silicon
packaging. Three examples are noteworthy. First,
boards, and related components continues to moti-
vate the development of sophisticated, multilayer
flexible interconnection cables and printed circuit
boards that can accommodate bending to small
silicon chips to thicknesses in the range of tens of
microns is of increasing importance for advanced
to evolve chips into forms that seamlessly integrate
withprinted circuitboards,ina mannerthatblurs
Fig. 3. Examples of stretchable electronic systems. (A) Electronic eyeball camera (left) that uses a
hemispherically curved array of silicon photodetectors and picture collected with a similar camera that
uses a paraboloid design (right). The image in the top is rendered in a form consistent with the curvature
(B and C) Stretchable LED display devices that use mesh designs with microscale inorganic LEDs (B) and
stretchable interconnects with organic LEDs (C).
VOL 32726 MARCH 2010
on March 26, 2010
the distinction between the two. Interest in such
design and manufacturing strategies associated
relevant to certain approaches in stretchable elec-
tronics, and the reverse is also true. This situation
time scales that have the potential to be much
shorter than those typically associated with new
technologies derived from academic research.
Challenges and Outlook
Collectively, the advances in materials and me-
chanics described here provide several promising
integrated systems in discrete,
distributed, or hybrid forms. The
underlying science encompasses
many research topics of funda-
mental interest, from materials
growth, processing, and heter-
ogeneous integration, to micro-
mechanics and nanomechanics,
to charge transport and its cou-
pling to strain and geometry, to
adhesion and interface science.
of silicon is ~100,000 times as
high as a typical elastomer; the
thermal conductivity is ~1000
times as great, and the thermal
expansion coefficient is ~100
times as small. Such extreme
mismatches in properties lead to
interesting, and similarly ex-
treme, behavior in systems that
intimately integrate these dis-
research on nonlinear behavior
viding new insights into the
mechanics of their deformation
nonlinear behavior in buckling
modes, with explicit relevance
to stretchable electronics. The
challenges are even more pro-
nounced for SWNTs (22–26)
and graphene (39), where larger
mismatches in properties occur
and little is known about even
the basic foundational mechan-
Other areas for study in these
and other heterogeneous sys-
mal management, and the ma-
terials science of interfaces, to
ensure mechanical reliability. From an engineer-
ing standpoint, these and related issues must be
understood clearly before levels of integration in
stretchable electronics, which currently range, in
distributed forms, from hundreds to thousands of
transistors, can begin to reach those of estab-
electronics, from hard, rigid, planar chips to soft,
stretchable, curvilinear sheets. Here, mechanics
design will be as important as electrical design in
tial areas of application, some of the most compel-
problems in human health. Soft, elastic mechanical
properties and curvilinear layouts can provide both
ical tissues. Such “tissue-like” devices, particularly
when implemented with biocompatible materials,
will facilitate solutions to long-standing challenges
in the establishment of viable, intimate biotic-
abiotic interfaces with high levels of functionality.
able electrodes for research applications (42) and
with modest degrees of deformability, will rapidly
expand into fully distributed, biointegrated, multi-
functional devices for clinical use.
Beyond biology, stretchable electronics will
components for communications, perhaps even in
cellular telephones of the future, as envisioned
recently by a large company in this industry. In
other possibilities, stretchable sensor tapes will
provide thin, conformal monitors of the structural
health in the wings of aircraft or the blades of
windmills, as examples. The basic ideas can also
be exploited in other semiconductor technologies,
latter case, devices for scavenging power will be
available in the form of sheets that can wrap the
mal coupling. These and related opportunities for
engineering in areas of application with important
motivation for continued and expanded efforts in
this emerging field.
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Fig. 4. Structures and materials for stretchable electrodes. (A) Thin
gold film formed by physical vapor deposition onto a PDMS substrate,
nanocracks. (B) Wavy silver microwire formed by direct write printing
onto the surface of a metal spring. The inset shows the structure in its
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Opportunity for Electronics
J. Mannhart1* and D. G. Schlom2*
Extraordinary electron systems can be generated at well-defined interfaces between complex
oxides. In recent years, progress has been achieved in exploring and making use of the
fundamental properties of such interfaces, and it has become clear that these electron systems
offer the potential for possible future devices. We trace the state of the art of this emerging field of
electronics and discuss some of the challenges and pitfalls that may lie ahead.
be said that the interface is the
device” (1). Transistors, lasers,
and solar cells all exploit inter-
facial phenomena. Interfaces
enable data processing, mem-
ory, and electronic communi-
cation. Moreover, interfaces in
semiconductor structures are
the birthplace of a multitude of
fascinating discoveries in fun-
damental science. Curiously,
away from interfaces, in the
bulk of the material, the be-
havior of electrons in semi-
conductors such as silicon is
through the crystal lattice es-
sentially as independent, free
particles, barely interacting
with one another. In contrast,
there are other materials (e.g.,
many oxides) in which electron
interactions in the bulk of the
material give rise to spectacular
phenomena, including colossal
magnetoresistance and high-
erbert Kroemer began
his Nobel lecture by
stating, “Often, it may
These phenomena arise in oxides from regularly
spaced ions interacting with the electrons, from
the unique electronic character of oxygen ions,
and from the electronic correlations—interactions
among the electrons, which make them deviate
from free-particle behavior.
The introduction of interfaces into semiconductor
structures spawned numerous semiconductor de-
analogy,itistantalizing to incorporate well-defined
interfaces into oxides to generate novel phenome-
to grow than semiconductor heterostructures,
progress in this direction was thwarted for many
grow oxide superconductors, however, led to de-
cisive progress in the growth of oxide multilayers
(Fig. 1). Key steps were the ability to terminate
oxide substrates at well-defined ionic planes (2),
the application of pulsed-laser deposition (PLD)
(3) and molecular-beam epitaxy
(MBE) (4) to the growth of mul-
ticomponent oxides containing
and the development of high-
pressure reflection high-energy
electron diffraction (5) to monitor
the deposition of individual atom-
erostructures of oxides can now be
grown with atomic-layer preci-
sion. The chemical abruptness
and crystalline perfection of ox-
semiconductor multilayers; it is
possible to change from one ma-
a single unit cell (6, 7) (Fig. 1B).
Such oxide heterostructures can
also be patterned laterally (8)
(Fig. 1D), even with nanometer
resolution (9). The ability to pre-
cisely create interfaces connecting
a wealth of new possibilities to
generate novel electronic phases.
trol interfaces in standard semi-
conductors, such as the formation
of charge carriers have reached
values so high that the quantum Hall effect (QHE)
graphene sheets. As a result of improvements in
1Center for Electronic Correlations and Magnetism, University
of Augsburg, 86135 Augsburg, Germany.
Materials Science and Engineering, Cornell University, Ithaca,
NY 14853, USA.
*To whom correspondence should be addressed. E-mail:
Fig. 1. MicrographsofLaAlO3-SrTiO3heterostructures.(A)TopviewofaLaAlO3-SrTiO3
bilayer containing eight monolayers of LaAlO3, taken by scanning force microscopy
(figure courtesy of S. Paetel). (B) Cross-sectional view of a corresponding sample con-
taining five monolayers of LaAlO3(figure courtesy of L. Fitting Kourkoutis and
and K. Wiedenmann). (D) Scanning force microscopy image of a conducting ring
patterned by electron beam lithography into a LaAlO3-SrTiO3structure [from (8)].
VOL 327 26 MARCH 2010
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