Polymer microneedles for controlled-release drug delivery.
ABSTRACT As an alternative to hypodermic injection or implantation of controlled-release systems, this study designed and evaluated biodegradable polymer microneedles that encapsulate drug for controlled release in skin and are suitable for self-administration by patients.
Arrays of microneedles were fabricated out of poly-lactide-co-glycolide using a mold-based technique to encapsulate model drugs--calcein and bovine serum albumin (BSA)--either as a single encapsulation within the needle matrix or as a double encapsulation, by first encapsulating the drug within carboxymethylcellulose or poly-L: -lactide microparticles and then encapsulating drug-loaded microparticles within needles.
By measuring failure force over a range of conditions, poly-lactide-co-glycolide microneedles were shown to exhibit sufficient mechanical strength to insert into human skin. Microneedles were also shown to encapsulate drug at mass fractions up to 10% and to release encapsulated compounds within human cadaver skin. In vitro release of calcein and BSA from three different encapsulation formulations was measured over time and was shown to be controlled by the encapsulation method to achieve release kinetics ranging from hours to months. Release was modeled using the Higuchi equation with good agreement (r2 > or = 0.90). After microneedle fabrication at elevated temperature, up to 90% of encapsulated BSA remained in its native state, as determined by measuring effects on primary, secondary, and tertiary protein structure.
Biodegradable polymer microneedles can encapsulate drug to provide controlled-release delivery in skin for hours to months.
- SourceAvailable from: Ciprian Iliescu[Show abstract] [Hide abstract]
ABSTRACT: The paper presents a review on transdermal drug delivery using mechanical enhancer – microneedles. The paper overviews the delivery mechanism, the main delivery methods utilizing the microneedle array, the frequently used materials for the fabrication process, geometrical and shape considerations as well as the current pre-clinical and clinical applications of the microneedles array. Finally, we express our point of view regarding the perspectives in the field of transdermal drug delivery using mechanical enhancer.Annals of Academy of Romanian Scientists Series on Science and Technology of Information. 07/2014; 7(1):7-34.
Technical Report: Fabrication and Characterization of Biodegradable Polymer Microneedles
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ABSTRACT: The transdermal route is an excellent site for drug delivery due to the avoidance of gastric degradation and hepatic metabolism, in addition to easy accessibility. Although offering numerous attractive advantages, many available transdermal systems are not able to deliver drugs and other compounds as desired. The use of hypodermic needles, associated with phobia, pain and accidental needle-sticks has been used to overcome the delivery limitation of macromolecular compounds. The means to overcome the disadvantages of hypodermic needles has led to the development of microneedles for transdermal delivery. However, since the initial stages of microneedle fabrication, recent research has been conducted integrating various fabrication techniques for generating sophisticated microneedle devices for transdermal delivery including progress on their commercialisation. A concerted effort has been made within this review to highlight the current advances of microneedles, and to provide an update of pharmaceutical research in the field of microneedle-assisted transdermal drug delivery systems.Journal of Controlled Release 05/2014; · 7.63 Impact Factor
Polymer Microneedles for Controlled-Release Drug Delivery
Jung-Hwan Park,1Mark G. Allen,2and Mark R. Prausnitz1,3,4
Received December 2, 2005; accepted January 11, 2006
Purpose. As an alternative to hypodermic injection or implantation of controlled-release systems, this
study designed and evaluated biodegradable polymer microneedles that encapsulate drug for controlled
release in skin and are suitable for self-administration by patients.
Methods. Arrays of microneedles were fabricated out of poly-lactide-co-glycolide using a mold-based
technique to encapsulate model drugsVcalcein and bovine serum albumin (BSA)Veither as a single
encapsulation within the needle matrix or as a double encapsulation, by first encapsulating the drug
within carboxymethylcellulose or poly-L-lactide microparticles and then encapsulating drug-loaded
microparticles within needles.
Results. By measuring failure force over a range of conditions, poly-lactide-co-glycolide microneedles
were shown to exhibit sufficient mechanical strength to insert into human skin. Microneedles were also
shown to encapsulate drug at mass fractions up to 10% and to release encapsulated compounds within
human cadaver skin. In vitro release of calcein and BSA from three different encapsulation formulations
was measured over time and was shown to be controlled by the encapsulation method to achieve release
kinetics ranging from hours to months. Release was modeled using the Higuchi equation with good
agreement (r2Q 0.90). After microneedle fabrication at elevated temperature, up to 90% of
encapsulated BSA remained in its native state, as determined by measuring effects on primary,
secondary, and tertiary protein structure.
Conclusions. Biodegradable polymer microneedles can encapsulate drug to provide controlled-release
delivery in skin for hours to months.
KEY WORDS: controlled-release drug delivery; microneedles; protein stability; transdermal drug
Conventional drug delivery using pills or injection is
often not suitable for new protein, DNA, and other therapies
(1,2). Devices for controlled release of such compounds have
been developed, which enable slow delivery over hours to
years. Controlled release is often achieved by encapsulating
drugs within biodegradable polymer matrices, from which
release is governed by drug diffusion and polymer erosion.
Decades of research on this topic have yielded clinical
products, such as the Lupron Depot, which delivers leupro-
lide acetate systemically for months (3), and the Gliadel
wafer, which administers carmustine locally to the brain for
days to weeks (4).
A limitation, however, of controlled-release systems is
that they typically require hypodermic needle injection of
polymeric microparticles or possibly surgical implantation of
macroscopic devices within the body. These painful and
invasive procedures are generally not suitable for self-
administration by patients and therefore are limited to use in
hospitals or clinics.
The goal of this study was to develop a minimally
invasive polymeric controlled-release system suitable for self-
administration without the pain or complexity of current
controlled-release devices. Rather than using a hypodermic
needle to introduce polymeric microparticles into the body,
we propose redesigning the microparticles to have the shape
of microneedles and thereby give these polymeric particles
the functionality of both needles and drug matrices for
controlled release (Fig. 1). By integrally forming these
microscopic needles onto a patch substrate, arrays of drug-
loaded microneedles could be inserted into the skin and worn
like a transdermal patch for slow release over time. An al-
ternative approach would involve intentionally separating the
patch base from the needles after insertion into the skin,
thereby leaving the drug-filled needles invisibly buried in the
skin for slow release. Because these microneedles are made
of FDA-approved, biodegradable polymer, they should safely
disappear after drug delivery is complete. Previous studies
have shown that microneedles are painless (5,6).
0724-8741/06/0500-1008/0#2006 Springer Science + Business Media, Inc.
Pharmaceutical Research, Vol. 23, No. 5, May 2006 (#2006)
1Wallace H. Coulter Department of Biomedical Engineering at
Georgia Tech and Emory University, Georgia Institute of Technol-
ogy, Atlanta, Georgia 30332, USA.
2School of Electrical and Computer Engineering, Georgia Institute
of Technology, Atlanta, Georgia 30332, USA.
3School of Chemical and Biomolecular Engineering, Georgia In-
stitute of Technology, Atlanta, Georgia 30332, USA.
4To whom correspondence should be addressed. (e-mail: prausnitz@
Microneedles have previously been proposed and devel-
oped for related applications. Solid microneedles have been
used to pierce the skin for increased permeability (7) as well
as to provide a substrate on which drug can be coated (8) or
encapsulated (9) for rapid release. Using this approach, a
range of compounds has been delivered to the skin, including
proteins, such as insulin and human growth hormone; genetic
material, including plasmid DNA and oligonucleotides; and
vaccines directed against hepatitis B and anthrax (10,11).
Hollow microneedles have also been developed for infusion
of drug solutions into the skin (12Y14). However, we believe
that this is the first study to address the use of microneedles
to encapsulate drug for controlled-release delivery (15).
Guided by previous microneedle studies, controlled-
release microneedles should measure hundreds of microns in
length and have a radius of curvature less than 10 mm at the tip
to ensure easy penetration into skin by manual insertion (16).
Microneedles of this size can penetrate past the skin’s outer
barrier of stratum corneum and deliver drug to the epidermis
and superficial dermis, where drug can diffuse rapidly for
local delivery to skin or systemic distribution via uptake by
dermal capillaries. Through the use of biodegradable poly-
mers such as poly-lactide-co-glycolide (PLGA), well-estab-
lished controlled-release mechanisms can be exploited to
control release from microneedles (17,18).
Microneedles of the proposed dimensions can be made
by adapting microfabrication technology (19). Although
microfabrication often involves lithography and etching of
silicon, the field is being expanded to include laser cutting,
molding, and other fabrication techniques to produce micro-
devices made of other materials, including metals and
polymers. By leveraging these technologies of the microelec-
tronics industry, methods to make microneedles should
provide reproducible mass production at disposable cost.
MATERIALS AND METHODS
Fabrication of Biodegradable Microneedles
Fabrication of Microneedle Master Structures and Molds
Microneedles were fabricated by first making master
structures using lithography-based methods, then creating
inverse molds of these master structures, and finally prepar-
ing replicate microneedles by melting biodegradable polymer
formulations into the molds. In this way, one master structure
could be used to make multiple molds, which could each be
used to make multiple replicates. Microneedles were fabri-
cated using two different geometries: beveled tip and tapered
cone. Methods to fabricate these master structures and molds
have been described in detail previously (20) and are sum-
Beveled-tip microneedle master structures were fabri-
cated out of SU-8 epoxy using standard UV-lithographic
techniques (20). SU-8 epoxy (SU-8 100; MicroChem, New-
ton, MA, USA) was coated onto a silicon wafer and
lithographically patterned into cylinders in the shape of the
desired needles. The space between the cylinders was filled
with a sacrificial polymer (PLGA 85/15, Sigma-Aldrich, St.
Louis, MO, USA) and a copper mask was patterned to
asymmetrically cover the tops of the epoxy cylinders and
some of the sacrificial polymer on one side of each cylinder.
Reactive ion etching (RIE; Plasma Therm, St. Petersburg,
FL, USA) partially removed the uncovered sacrificial layer
and asymmetrically etched the tips of the adjacent epoxy
cylinders. All remaining sacrificial polymer was removed by
ethyl acetate, leaving an array of epoxy cylinders with
asymmetrically beveled tips. This array of needles was coated
with poly(dimethylsiloxane) (PDMS; Sylgard 184, Dow
Corning, Midland, MI, USA), which was subsequently peeled
off to make an inverse mold.
Tapered-cone microneedles were fabricated using a
novel microlens technique (Y.-K. Yoon, J.-H. Park, and M.
G. Allen. Multidirectional UV lithography for complex 3-D
MEMS structures. J MEMS, in press). A chromium layer was
first deposited and patterned on a glass substrate to form an
array of circular dots of exposed glass. Isotropic wet etching
of the exposed glass was then performed to create concave
wells, which were filled with SU-8 epoxy cast on the surface.
The refractive index mismatch between glass and SU-8 epoxy
created an array of integrated microlenses. After soft-baking,
the SU-8 film was exposed from the bottom (i.e., through the
glass) to UV light, which passed through the microlenses to
form latent images in the SU-8 epoxy as ray traces from the
lenses. After development of the SU-8 epoxy, the resulting
tapered-cone microneedle master structure was used to make
an inverse PDMS mold.
Fabrication of Microneedles Encapsulating Drug
To prepare microneedles encapsulating drug for con-
trolled release, PDMS microneedle molds were first filled
with a model drug formulation and then filled with a PLGA
melt, which was allowed to cool and solidify. Three for-
mulations were used to achieve different timescales of
controlled release. For rapid release, the model drug was
Fig. 1. Controlled-release drug delivery using polymer microneedles.
Polymeric controlled release is often achieved by encapsulating drug
within microparticles, which are then injected into the body using a
hypodermic needle (shown on left). Polymer microneedles can
similarly be designed to encapsulate drug for controlled release, but
can be directly inserted into the skin without the need for
hypodermic injection (shown on right).
1009Polymer Microneedles for Controlled Release
directly encapsulated within the microneedles. For slower
release, drug was first encapsulated either within carboxy-
methylcellulose (CMC) or poly-L-lactide (PLA), which was
then encapsulated within microneedles. The process is
summarized in Fig. 2.
For the first formulation, calcein or Texas-Red-labeled
bovine serum albumin (BSA) powder (used as received from
Sigma-Aldrich or Molecular Probes, Eugene, OR, USA,
respectively) was suspended in acetonitrile (Sigma-Aldrich)
at a solids content of 10% (w/v) and then homogenized for
5 min at 10,000 rpm (PowerGen 700 homogenizer, Fisher
Scientific, Pittsburgh, PA, USA) to make drug microparticles.
The homogenized particles, with a broad size distribution over
the approximate range of 1Y100 mm, were filtered first through
a 30-mm filter, and then the filtrate was passed through a 1-mm
filter (nylon net filter, Millipore, Billerica, MA, USA). The
final solids cake containing particles 1Y30 mm in size was
redispersed in acetonitrile at a solids content >20% (w/v). The
resulting suspension was poured onto a PDMS microneedle
mold and placed in a vacuum chamber at j20 kPa for õ5 min.
This filled the mold with drug particles by first allowing the
then evaporate off the organic solvent. Residual particles
remaining on the surface of the mold were removed using
adhesive tape (Blenderm, 3M, St. Paul, MN, USA). As
described previously (20), the mold was then filled with
melted PLGA (PLGA 50/50, 1.2 dL/g, Birmingham Polymer,
Birmingham, AL, USA) in a vacuum oven at 135-C and
j70 kPa for 10Y20 min. After cooling, the resulting micro-
needles with encapsulated drug were manually removed from
To retard release from microneedles using a double-
encapsulation formulation approach (21), calcein was first
encapsulated within CMC microparticles, which were then
encapsulated within microneedles. A CMC solution was
prepared by dissolving 0.25 g of CMC sodium salt (reference
viscosity of 400Y800 cP @ 2% aqueous solution; Sigma-
Aldrich) in 9.6 mL of deionized (DI) water for 12 h on a 50-C
hot plate with stirring at 300 rpm. Then, 25 mg calcein
(Sigma-Aldrich) was dissolved in the CMC solution at a
calcein: CMC ratio of 1:10 (w/w). The resulting clear
solution was poured onto aluminum foil and dried to remove
water for 6 h under j50 kPa of vacuum. The resulting film
containing calcein dispersed in a solid CMC matrix was
pulverized by an agate mortar and pestle to form particles
measuring a few hundred microns to a few millimeters in size.
These large particles were dispersed in acetonitrile, homoge-
nized, and filtered to yield particles of 1Y30 mm in size, as
described above. The average diameter of the particles was 9.6
mm, with a standard deviation of 6.2 mm, determined by
analyzing scanning electron microscope (SEM) images. The
small CMC particles encapsulating calcein were finally loaded
into a PDMS mold, which was subsequently filled with PLGA,
as described above, to form PLGA microneedles, which
encapsulated CMC particles that further encapsulated calcein.
To slow release still more, a similar approach was used,
where calcein was first encapsulated within PLA micropar-
ticles, which were then encapsulated within microneedles.
Using the well-known double-emulsion technique to make
PLA microparticles (22), 50 mg of calcein was dissolved in 15
mL DI water and 0.2 g of PLA (L-PLA, 1.0 dL/g; Birming-
ham Polymer) was separately dissolved in 2 mL methylene
chloride (Sigma-Aldrich). Then, 200 ml of the calcein solution
was homogenized in 2 mL of PLA solution for 2 min at
15,000 rpm. The resulting water-in-oil emulsion was homoge-
nized in 50 mL of an aqueous solution of 0.1% polyvinyl
alcohol (Sigma-Aldrich) for 2 min at 10,000 rpm, which
produced a water-in-oil-in-water emulsion. After mixing for
3 h at 300 rpm, the methylene chloride was extracted into the
Fig. 2. Method to fabricate polymer microneedles that encapsulate
drug for controlled release. First, a suspension of drug particles is
filled into a microneedle mold. Evaporation of the solvent leaves
solid drug particles partially filling the mold. Pellets of biodegradable
polymer are then melted into the mold under vacuum. Cooling and
solidification of the polymer yields biodegradable polymer micro-
needles with encapsulated drug particles.
1010Park, Allen, and Prausnitz
continuous phase, which solidified the discontinuous phase
into PLA microparticles encapsulating calcein. Micropar-
ticles of 1Y30 mm in size were isolated by filtration, loaded
into a PDMS mold, and encapsulated in PLGA micro-
needles, as described above.
Characterization of Microneedles Containing Drug
In Vitro Release Test in Saline
To measure release rates from microneedles, an array
containing 100Y200 needles encapsulating one of the formula-
tions of calcein or BSA was attached to the bottom or side of a
30-mL glass vial (All-Pak, Bridgeville, PA, USA) containing 5
or 10 mL of phosphate-buffered saline (PBS, pH 7.4, Sigma-
Aldrich) filtered using a 0.2-mm filter (Millipore). Glass vials,
PBS, and magnetic stir bars were autoclaved prior to use. The
vials were magnetically stirred at 300 rpm and incubated in a
37-C water bath. Periodically, a 100-ml aliquot of PBS was
sampled fromeach vial, replaced with fresh PBS,and analyzed
to determine the concentration of calcein or Texas Red-
labeled BSA by calibrated spectrofluorometry (QM-1, Photon
Technology International, South Brunswick, NJ, USA). Mea-
sured concentrations were converted into cumulative drug
released (Mt) by accounting for PBS volume. Total drug
content (M0) was determined by placing microneedles in 1 N
NaOH overnight at the end of each experiment to fully
degrade remaining PLGA and PLA and thereby release all
encapsulated drug (23). The resulting solution was returned
to pH 7.4 using HCl before analysis. M0was typically õ1 mg
for calcein only,BSAonly,andcalceininCMCmicroparticles
andõ0.1mg for calcein in PLA microparticles.
Drug release from microneedle formulations was mod-
eled using the Higuchi equation (24), which indicates that dif-
fusion-mediated release should be proportional to the square
root of time.
In this expression, D is the apparent diffusion coefficient
of the drug in the polymer matrix, t is time, and r is the radius
of the microneedle (50 mm) (25). This equation was fitted to
experimental data to yield D, which is the only unknown
(26). The fitting procedure used a least-squares method that
minimized the differences between experimental and theo-
In Vitro Release Test in Skin
To study the dissolution and release of drug in skin,
microneedles encapsulating calcein were inserted into full-
4-C. Refrigeration was used to avoid dehydration and degra-
dation of the skin. Recognizing that skin properties and drug
of 4-C and the body temperature of 37-C, we conducted this
experiment to qualitatively verify that our microneedles insert
and that encapsulated drug is released in the skin.
After 8 h, the needles were removed and any residual
calcein on the skin surface was cleaned off with wet tissue
paper. The spatial profile of calcein released in the skin was
then imaged by confocal microscopy (LSM 510; Zeiss,
Thornwood, NY, USA). Human cadaver skin was obtained
from the Emory University Body Donor Program with
approval from the Georgia Tech and Emory University
Institutional Review Boards.
Microneedle Failure Force Measurement
To determine the effect of calcein encapsulation on
microneedle mechanical properties, the microneedle failure
force was measured as described previously (16,20). Briefly,
stressYstrain curves were generated using a displacement-
force test station (Model 921A, Tricor Systems, Elgin, IL,
USA) while pressing an array of 35 microneedles against a
stainless steel surface at a rate of 1.1 mm/s until a preset
maximum load (19.6 N) was reached. Microneedles had a
base radius of 100 mm, tip radius of 12 mm, and height of 1
mm. Microneedle failure was indicated by a sudden drop in
applied force. After each test, microneedles were visually
inspected by microscopy to confirm that all microneedles had
deformed and failed uniformly. Failure force was determined
at calcein contents of 0, 2, and 10% prepared using a single-
Because encapsulation of drugs within microneedles
involves a brief exposure to a high-temperature polymer
melt, the encapsulation process could be damaging to drugs,
especially proteins. To assess possible damage, protein
stability was tested by measuring protein solubility, dynamic
light scattering, and circular dichroism (CD), using BSA as a
model protein (27). Because protein content in microneedles
is small, larger samples were generated by dispersing 500 mg
of homogenized BSA particles in a 27.5 g solution of 10%
(w/w) PLGA in acetonitrile and then pouring the suspen-
sion onto aluminum foil. A thin polymer film encapsulating
BSA particles was formed by evaporating off the acetoni-
trile for 5 h under j67 kPa vacuum. The film was cut into
3 ? 3-cm squares and placed on a 1 cm-thick PDMS film
to simulate the molding process. Samples prepared in this
way were placed in the oven at 135-C for predetermined
times. After cooling, PLGA samples were dissolved again in
acetonitrile, and the BSA particles were recovered by filtra-
tion, washed with methylene chloride, and dried for 6Y8 h
under j30 kPa vacuum. Each condition was tested in
As a first measure of stability, BSA samples were
dissolved in PBS to determine the fraction of BSA remaining
soluble after thermal exposure, which includes both native
BSA and reversibly denatured BSA. Insoluble aggregates
were removed by centrifugation at 23,000 ? g for 20 min (28).
Protein concentration in the supernatant was determined by
the Lowry protein assay (29).
The presence of soluble aggregates of BSA was detected
by dynamic light scattering (30). BSA particles were dis-
solved in PBS at a concentration of 800 mg/mL and filtered to
removed insoluble aggregates, and 45 ml of solution was
placed in a quartz cuvette (Proterion, now Wyatt Technolo-
gy, Santa Barbara, CA, USA) at room temperature for
1011 Polymer Microneedles for Controlled Release
dynamic light scattering measurements (DynaPro-MS/X,
Proterion) using CONTIN analysis.
To identify possible changes in the ratio of a-helix/b-
sheet components of BSA structure, CD polarimetry mea-
surements were performed using BSA particles dissolved in
PBS at a concentration of 70 mg/mL (J700, Jasco, Easton,
MD, USA) (31). CD spectra were obtained over a wave-
length range of 400Y190 nm with a sensitivity of 20 mdeg and
a response time of 2 s.
Fabrication of Microneedles for Transdermal Drug Delivery
Fabrication of Microneedle Master Structures
The first step to make polymer microneedles for con-
trolled-release drug delivery involved fabricating master
structures using microelectromechanical systems (MEMS)
techniques. These master structures were then used to make
molds, which were in turn used to make replicate micro-
needles out of biodegradable polymers. Two different geom-
etries of microneedle master structures were fabricated out
of SU-8 epoxy using lithography-based methods. Representa-
tive beveled-tip microneedles are shown in Fig. 3A and have a
base radius of 50 mm, a tip radius of 5 mm, and a height of
600 mm. The needles are positioned in a 20 ? 6 array with a
center-to-center spacing between needles of 400 and 1400 mm.
The entire array occupies an area of 9 ? 9 mm. Geometric
parameters of beveled-tip microneedle arrays, such as needle-
to-needle spacing, needle base radius, and base shape, were
controlled by adjusting the size, shape, and spacing of the
lithography mask. The needle height was controlled by the
thickness of SU-8 photoresist casting and etching parame-
ters. The tip sharpness was controlled by the etching
Fig. 3. Microscopy images of microneedles. A section of an array of (A) bevel-tip microneedles
and (B) tapered-cone microneedles used as master structures (imaged by SEM). Making a mold
and using it to prepare polymer microneedles as described in Fig. 2 yielded (C) bevel-tip and
(D) tapered-cone microneedles made of PLGA and encapsulating calcein within their tips
(imaged by fluorescence and bright-field microscopy, respectively). Using a double-encapsula-
tion method produced microneedles that encapsulate microparticles that, in turn, encapsulate
calcein. (E) Cutting off the tip of a PLGA microneedle reveals the PLA microparticles within
(imaged by SEM). (F) A complete 20 ? 10 array of PLGA microneedles is shown (imaged by
1012Park, Allen, and Prausnitz