Contact Self-Cleaning of Synthetic Gecko Adhesive from Polymer
Microfibers
Jongho Lee*
,†
and Ronald S. Fearing
‡
Department of Mechanical Engineering and Department of Electrical Engineering and
Computer Sciences, UniVersity of California, Berkeley, California 94720
ReceiVed July 7, 2008. ReVised Manuscript ReceiVed August 21, 2008
Natural gecko toes covered by nanomicro structures can repeatedly adhere to surfaces without collecting dirt.
Inspired by geckos, we fabricated a high-aspect-ratio fibrillar adhesive from a stiff polymer and demonstrated self-
cleaning of the adhesive during contact with a surface. In contrast to a conventional pressure-sensitive adhesive (PSA),
the contaminated synthetic fibrillar adhesive recovered about 33% of the shear adhesion of clean samples after multiple
contacts with a clean, dry surface.
Conventional pressure-sensitive adhesives (PSA) use soft
viscoelastic polymers (Young’s modulus <100 kPa measured at
1Hz
1-3
) to make intimate contact with surfaces to achieve high
adhesion. However, soft polymers tend to collect dirt and lose
adhesion with repeated use. In contrast, a gecko uses millions
of keratinous nano and microhairs (Young’s modulus E≈1.5
GPa
3,4
) to cling to and walk on virtually any surface. These hairs
shed dirt particles during contact with a surface, keeping its
natural adhesive sufficiently clean to support the gecko’s body
weight.
5
A key factor in the self-cleaning ability of gecko structures
is the nonadhesive default state exhibited by the gecko fibers.
6
To adhere, the fibers need to be dragged to expose the spatula
tips, increasing the contact fraction by approximately 7.5-fold.
6
In contrast to the well-known lotus effect
7
in which particles are
removed from a nonadhesive and highly hydrophobic surface by
water droplets, gecko setae self-clean particles during use, even
on dry surfaces. We restrict our discussion here to the self-
cleaning of adhesives on dry surfaces during use. Natural gecko
setae are the only previously reported self-cleaning adhesive on
dry surfaces.
Recently, gecko-inspired synthetic adhesives (GSAs)
8
have
been fabricated using soft polymers (Young’s modulus e10
MPa)
9-14
or hard polymers
15-18
(Young’s modulus g1.5 GPa).
Also, arrays of carbon nanotubes (CNT) have been used to achieve
adhesion.
19-21
Fibrillar adhesive cleaning has been demonstrated
using water
16,22
and mechanical vibration.
22
Superhydrophobicity
may lead to the cleaning of fibrillar adhesive by water.
23
However,
no synthetic adhesive has demonstrated self-cleaning on dry
surfaces during use, one of the important advantages of a gecko-
inspired adhesive over conventional pressure-sensitive adhesives.
Autumn
24
has identified seven benchmark properties that are
characteristic of geckolike adhesives, which are (1) anisotropic
attachment, (2) a high adhesion coefficient, (3) a low detachment
force, (4) material-independent adhesion, (5) self-cleaning, (6)
anti-self-adhesion, and (7) a nonsticky default state. Although
properties 1-4 and 7 have been previously demonstrated
25,26
in
a single material, in this letter we report the first geckolike
microfibrillar material that also demonstrates self-cleaning during
contact.
To create a self-cleanable adhesive, we fabricated high-aspect-
ratio fibrillar arrays from polypropylene (Young’s modulus E≈
1.5 GPa, measured with Sintech tensile tester 2/S, MTS Systems).
In previous work, these hard-polymer-based fibrillar materials
have shown unique adhesion properties, similar to those of gecko
setae, including sliding enhanced shear adhesion
25
with low
peeling force and frictional adhesion
27
with a spherical indenter.
28
In this letter, we use a contact “step” protocol similar to that used
for natural gecko setal arrays
5
to demonstrate self-cleaning of
the synthetic fibrillar adhesive. The self-cleaning synthetic
* To whom correspondence should be addressed. E-mail: jongho@
eecs.berkeley.edu.
†
Department of Mechanical Engineering.
‡
Department of Electrical Engineering and Computer Sciences.
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10.1021/la8021485 CCC: $40.75 2008 American Chemical Society
Published on Web 09/10/2008
