First synthetic gecko adhesive which cleans itself during use, as the natural gecko does
POC: Prof. Ronald Fearing
Dept. of
Electrical Engineering
University of California, Berkeley
(510)
642-9193
ronf@eecs.berkeley.edu
Gecko-inspired synthetic adhesive showing self-cleaning. Left : microfibrillar adhesive contaminated by microspheres, Right : after repeated contacts, microspheres are shed by microfibrillar adhesive. (The fibers are 18 micrometers long with diameter of 0.6 micrometer. The spherical particles are 2 micrometer in diameter.) Higher-quality image available for download
Self-cleaning synthetic adhesive
The gecko inspired synthetic
adhesive (GSA) self-cleans and recovers shear adhesion after multiple
contacts with a clean dry surface. Conventional adhesive tapes use a soft
polymer adhesive material which is ``sticky''. As is well known, these
tapes tend to collect dust and lose adhesion with repeated use. Mimicking
the gecko's unique adhesive will make it possible to use synthetic
self-cleaning adhesives in daily life, especially where conventional tapes
can be easily contaminated. The self-cleaning adhesive is a key enabler
for diverse applications such as medical equipment, climbing
robots,�apparel, etc. Similar to the natural gecko, our gecko-inspired
synthetic adhesive consists of millions of stiff polymer micro-fibers
which exhibit mechanically switchable adhesion.
Key features
Bio-inspiration
As previously reported in Hansen and Autumn ``Evidence for self-cleaning in gecko setae,’’ (PNAS 102, 385-389) natural gecko setae are the only known adhesive materials which can self-clean particle contamination solely by contact with a dry clean surface. A gecko uses millions of keratinous nano and micro hairs to cling to and walk on virtually any surface. These hairs shed dirt particles during contact with a surface, maintaining its natural adhesive sufficiently clean to support the gecko’s body weight between molts.
Lotus effect
Other well-known self-cleaning surfaces which use the ``lotus effect'' require water droplets to remove particles. Lotus removes dirt particles from a non-adhesive and waxy surface by water droplets. Gecko setae self-clean particles during use, even on dry surfaces. The self-cleaning synthetic adhesive does not require water droplets.
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The gecko-inspired self-cleaning synthetic adhesive is described in a paper appearing online in Langmuir (Sep. 2008). The paper demonstrates for the first time a synthetic adhesive material which self-cleans as the gecko does, by contact with a dry clean surface, and without requiring water or another liquid.
Jongho Lee and Ronald S. Fearing, ``Contact Self-cleaning of Synthetic Gecko Adhesive from Polymer Microfibers'', Langmuir.
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How the self-cleaning synthetic adhesive works
Due to the fiber structure of the gecko adhesive, the fibers tend to push off dirt particles when the adhesive is not in contact. When touching sa mooth surface, such as glass, the fibers have less contact area with the particle than the glass does. Since the adhesion strength is proportional to contact area, the particles will prefer to adhere to the glass rather than the sythetic gecko fibers.
Illustration of dry self-cleaning. (A) Initially, the microsphere is in contact with microfibers. (B) When the fibrillar adhesive is pressed against the glass and slides, the microsphere makes contact with the flat glass. During a simulated step, the microsphere may roll or slide, but displacement is quite small compared to fiber array size, and hence the microsphere maintains contact with the glass and fibers before detachment. (C) During detachment, the microsphere is in tension between fibers and the substrate. (D) At detachment, the microsphere is deposited on the glass due to a greater attraction of the microsphere for the glass than for the fibers. Thus, more fiber tips are exposed to the glass at the next step, increasing adhesion. Higher-quality image available for download
Optical images showing whole contact area after simulated step (scale bars : 1 cm). MS: microspheres deposited on glass substrate by the micro fibrillar adhesive at each step. The quantity of microspheres deposited on the glass decreases with increasing step number. Initial contact steps left many microspheres on the clean glass substrate, with diminishing particle removal after further steps. Higher-quality image available for download
Conventional tape
Conventional soft polymer tape shows greater contamination after contacting a surface. Left : Conventional tape contaminated by microspheres. Right : Conventional tape after contacts on clean glass substrate. The conventional tape has greater contamination after steps in contrast to gecko inspired micro fibrillar adhesive in which microspheres are removed by simulated steps. Higher-quality image available for download
(A) Initially, the soft polymer of the conventional tape is contaminated with microspheres. (B) When the fibrillar adhesive is preloaded, the microsphere makes contact with the flat glass. During the step, some microspheres not in direct contact with the soft polymer may be taken off and recaptured in the exposed area of the soft polymer. The microspheres sink into the soft polymer further. (C) At detachment, the microspheres do not lose contact with the soft polymer of the tape. Higher-quality image available for download
Links
Biologically Inspired Synthetic Gecko Adhesives Project
Previous work on sliding induced adhesion (2008)
Previous work on high friction (2006)
Background information on gecko adhesion