A ‘‘Self-Pinning’’ Adhesive Based on Responsive Surface Wrinkles
Edwin P. Chan,1* Jeffrey M. Karp,1,2,3 Robert S. Langer1,4
1Harvard-MIT Division of Health Sciences and Technology, Massachusetts Institute of Technology,
Cambridge, Massachusetts 02139
2Center for Regenerative Therapeutics, Department of Medicine, Brigham and Women’s Hospital,
Harvard Medical School, Cambridge, Massachusetts 02319
3Harvard Stem Cell Institute, Harvard University, Cambridge, Massachusetts 02138
4Department of Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139
Correspondence to: E. Chan (E-mail: edwin.chan@nist.gov) or J. M. Karp (E-mail: jkarp@rics.bwh.harvard.edu) or R. Langer
(E-mail: rlanger@mit.edu)
Received 5 August 2010; revised 26 August 2010; accepted 15 September 2010; published online 22 October 2010
DOI: 10.1002/polb.22165
ABSTRACT: Surface wrinkles are interesting since they form
spontaneously into well-defined patterns. The mechanism of
formation is well-studied and is associated with the develop-
ment of a critical compressive stress that induces the elastic
instability. In this work, we demonstrate surface wrinkles that
dynamically change in response to a stimulus can improve
interfacial adhesion with a hydrogel surface through the
dynamic evolution of the wrinkle morphology. We observe that
this control is related to the local pinning of the crack separa-
tion pathway facilitated by the surface wrinkles during debond-
ing, which is dependent on the contact time with the hydrogel.
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C 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys
49: 40–44, 2011
KEYWORDS: adhesion; adhesives; biomaterials; hydrogels; stim-
uli-sensitive polymers; surfaces; swelling
INTRODUCTION Interests in stimuli-responsive materials
stem from their amazing ability to develop sudden macro-
scopic property changes in response to an external stimu-
lus.1 These materials are potentially useful in many applica-
tions that require the dynamic control of interfacial
properties, such as adhesion,2,3 friction,4 and wettability.5,6
One interesting concept involves utilizing surface wrinkles as
stimuli-responsive materials. Surface wrinkles are a class of
elastic instability that develops in response to a critical com-
pressive stress.3,6–14 Because the resultant topographic wrin-
kle patterns are well-defined, several groups have utilized
stable surface wrinkles to tune the adhesion of ‘‘dry’’ poly-
mer interfaces. The results are very interesting since soft
polymer adhesion can be enhanced or suppressed, and this
control is related to the surface wrinkles feature size-scale
in relationship to a materials-defined length-scale. More
recently, Hayward and coworkers have studied the dynamic
response of wrinkled hydrogel materials.7,15 The key result
of their work is the demonstration of the dynamic change of
the surface wrinkles via an external stimulus such as pH.
Based on these inspirations, we present a new strategy to
tailor soft polymer adhesion via responsive surface wrinkles
that takes advantage of a polymer’s natural propensity to
swell in a solvent as a stimulus to drive wrinkle formation.
Specifically, we develop a Responsive Surface-wrinkled Adhe-
sive (RSA) that changes wrinkle morphology as it swells in
water. By interfacing this RSA material with a gelatin-based
hydrogel, we show that interfacial adhesion can be con-
trolled with contact time to the hydrogel surface. We illus-
trate that the primary mechanism of adhesion is associated
with the topographical evolution of the wrinkled surface
with contact time, which locally pins the crack propagation
pathway as the RSA releases from the hydrogel. This
approach is interesting as it takes advantage of a dynamic
topographic surface to control adhesion.
EXPERIMENTAL
Materials
RSA films and probes were synthesized via photopolymeriza-
tion by using a formulation of 74 wt % polyethylene glycol
methyl ether acrylate, 24 wt % acrylic acid, and 2 wt %
polyethylene glycol dimethacrylate (Sigma-Aldrich, St. Louis,
MO), and 1 wt % IrgacureTM 819 photoinitator (Ciba Spe-
cialty Chemicals, Tarrytown, NY). To fabricate the RSA hemi-
spheres, the solution was deposited into a crosslinked di-
methyl siloxane hemispherical mold and then covered with a
glass substrate. The sample was then irradiated with UV
(k ¼ 365 nm, intensity ¼ 20 mW/cm2) for 4 min, demolded
*Present address: Polymers Division, National Institute of Standards and Technology, 100 Bureau Drive, Gaithersburg, Maryland 20899.
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and then used without additional processing. Gelatin was
used as the hydrogel material. The gelatin films were fabri-
cated by mixing 12.5 wt % Bloom 225 (Sigma-Aldrich, St.
Louis, MO) with 81.5 wt % hot deionized water. A 1 mL so-
lution was deposited onto 2.54 cm2 glass substrates and
used immediately to minimize drying of the films.
Optical Profilometry
The wrinkle amplitude at the maximum wavelength was
determined using a Zygo NewVIEW 6000 3D optical inter-
ferrometer (Mirau 50X objective, Zygo Corporation, Middle-
field, CT). Due to the difficulty in measuring the amplitude
through a water medium, we measured the long time (at 10
min. swelling) amplitude by removing the excess water on
the wrinkled surface and immediately measuring the
amplitude.
Contact Adhesion Testing
Contact adhesion testing was used to measure the adhesion
between the RSA and gelatin interface. Briefly, the test
involves forming an interface between the RSA and gelatin,
followed by subsequent separation of the interface. The test
begins by bringing the RSA probe, with a 5 mm radius of
curvature, into contact with the gelatin surface at a fixed dis-
placement rate of 1.5 lm/s. After reaching the maximum
compressive load of 2 mN, the probe is retracted from the
gelatin to separate the interface at the same displacement
rate. For the contact time studies, the probe was compressed
at this load for the defined contact time. Then, the probe
was retracted to cause separation at the same displacement
rate. The contact times investigated were 0, 60, 120, 300,
and 600 s. The load is monitored by a force transducer
(Honeywell Sensotec, Columbus, OH) while the displacement
is controlled by a nanopositioner (Burleigh Instruments IW-
820, Exfo Burleigh Products Group, Victor, NY). The contact
areas were recorded by an inverted optical microscope (2.5X
objective, Nikon Instruments, Melville, NY). All the data were
recorded via a custom-developed National Instruments Lab-
VIEWVR software.
RESULTS
Figure 1 demonstrates the dynamic change of the RSA when
exposed to water. Within 5 sec of water exposure, the sur-
face of the initially smooth RSA respond immediately by
developing well-defined surface wrinkles [Fig. 1(a)]. These
patterns are stable, as long as the surface remains hydrated,
and persist over the entire swelling period. Additionally, the
wavelength evolves with swelling time [Fig. 1(a,b)], with an
initial wavelength of
80 lm and a final value of
540 lm.
The wavelength measurements were determined by averag-
ing the center-to-center distance between two adjacent
wrinkles from the optical micrographs for five wrinkles to
determine an average value. From optical profilometry, the
amplitude is
42 lm when the wavelength reaches its upper
value. Beyond 5 min, the wavelength stabilizes to this upper
value [Fig. 1(b)]. Upon drying, we observe the disappearance
of the wrinkles and reappearance of the original smooth
surface. This response can be cycled simply by the swelling/
de-swelling process.
Solvent-induced wrinkling has been observed in a variety
of materials and is associated with the competition be-
tween osmotic pressure and lateral confinement [Fig.
1(c,d)].7–9,13,14,16,17 Water absorption causes swelling of the
free surface of the RSA. However, lateral expansion is con-
strained by the rigid substrate. This nonuniform swelling
leads to a net compressive force developing at the free sur-
face of the RSA and subsequent formation of surface wrin-
kles. Similar to other materials, the wavelength grows with
swelling time and is determined by the relative thicknesses
of the swollen and unswollen layers of the film.
Next, we measure the adhesion of the RSA in contact with
the gelatin surface [Fig. 2(a)]. A representative force-time
history of the contact adhesion test is presented in Figure
2(b). The contact area grows laterally as the RSA probe is
compressed into the gelatin until the maximum compressive
force is reached. At this point, the interface is allowed to
dwell for a predetermined contact time. Then, the probe
retracts from the gelatin surface until the entire interface
separates. We use the maximum tensile force (Ps) to com-
pare the different testing conditions [Fig. 2(c)].
Figure 2(b) shows that the RSA-gelatin interface develops
surface wrinkles with a similar morphology as shown in Fig-
ure 1. This result is observed in all the materials and sug-
gests that the RSA is wrinkling primarily due to water
absorption from the gelatin. Additionally, the wrinkle mor-
phology evolves during the testing time, which again is
consistent with results in Figure 1. To understand the
FIGURE 1 (a) Formation of surface wrinkles and their dynamic
evolution with swelling time in water. (b) Increase in wave-
length (k) versus swelling time. (c) Schematic of the increase in
wavelength and amplitude with swelling time. (d) Mechanism
of the wrinkle formation.
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