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Breakthrough gecko-inspired adhesives use array of polypropylene microfibers, multi-walled CNTs |
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Marking a remarkable breakthrough in field of adhesives, researchers from University of California, Berkeley, used as many as 42 mln hard polypropylene (PP) microfibers on a square centimeter of a substrate material to biomimic gecko lizard's gravity defying surface traction. The gecko lizard's ability to grip and scurry-up the vertical surfaces and wall-ceilings owing to the innumerable protein microhairs found at its toe has been a subject of study for many researchers. The man-made gecko-like traction achieved using millions of hard plastic fibers has two very important characteristics which make it superior and effective relative to the conventional tape- and glue-like adhesives. Firstly, the unique material is capable of partly self-releasing off dirt and dust particles when repeatedly used. Secondly, the reptile-inspired adhesive mechanism becomes stronger with use while maintaining its easy-grip, easy-detach system as against tapes which turn weaker and shabby with use.
As per the research team, this novel and fascinating development can be employed to all terrain-robots which will scale up the walls and ceilings to salvage disaster-ridden people and objects in the hour of emergency. The study was carried out by a research team led by Ron Fearing, a U.C. Berkeley professor of electrical engineering and supported by National Science Foundation. Besides providing a gravity-defying grip and traction for the all-terrain robots, this remarkable adhesion mechanism made possible by millions of PP fibers, may increasingly find applications in clothing and medical devices to everyday uses like office stationary and slip-proof shoes.
Since long, scientists have made innumerable efforts to mimic the toe hairs that make the lizard's acrobatic feats possible. One crucial related revelation came in 2005 by way of a study where it was found that the gecko keeps its padded feet sticky yet clean by shedding dirt particles with every step. In 2008, the team developed an adhesive using polymer microfibers that could easily attach to and detach from clean surfaces. The real insight into the study came from an exhaustive study of adhesion bio-mechanism that makes clinging to vertical surfaces possible. The research team says each of the five gecko toes possess protein microhairs 100 microns long and 5 microns in diameter. These hairs, roughly 5,00,000/foot�further split at ends into spatulae, cup-like suction structures that maximize the contact surface area. As per the scientists in the field, each gecko has approximately billion spatulae as it gains traction on the wall ceilings and vertical surfaces. Unlike the adhesive tapes, the microhairs touch, drag against and then firmly grip the surface when moved in one direction thereby establishing adhesion. This unidirectional movement provides an easy and effortless surface adhesion while the same when moved in the exact opposite direction, the reptile is freed from the surface traction. The gecko has a highly well-developed and stratified structure comprising tractable toes, which have microfibres, which in turn possess nanofibres and nanoattachment plates that make possible the effortless attachment and release.
Taking cues from the same 'easy-stick, easy free' mechanism, the research team deviced PP microfibers�having length one-fifth of the thickness of a sheet of paper (20 microns) and diameter 1/100th the diameter of a human hair (0.6 microns). The same microfibers were bundled on a 2 sq.cm. piece of white plastic to mimic the gecko toe adhesion. The same plastic strip was later attached to a vertically positioned piece of smooth glass while clinging a small load weight to the plastic patch. The team noted the maximum load weight the plastic patch could withstand before slipping off the glass surface and than augmented the load. As per the findings, there was a touch increase in the weight which can be sustained indicating that the dust particles were being shed with every attempt. The team has set goal to develop the adhesive which can hold up a force of about 10 Newtons/sq.cm., a similar adhesion to that found on gecko toes. Furthering the study, the research team dispersed microspheres on the same piece of glass to simulate the dust and dirt particles to gauge how effectively did the PP microfibers-based adhesive freed itself of foreign particles while attaching and detaching with the surface. The researchers were successful in showing that the microfibers freed itself of the 'dirt' microspheres particles towards the tips while the material was detached form the surface. As per the researchers, the fibers shed the microspheres while detaching from the glass piece because the microspheres made greater contact with glass surface than with the fibers as adhesive strength is directly proportional to contact area. With each mock step, more and more microspheres were shed from the fibers. Finally, after thirty steps, the adhesive shed about 60% of the microspheres onto the glass substrate. The adhesion achieved by the team using array of PP microfibers is also dubbed 'smart' as the traction gains strength with use. In this mechanism, the polypropylene microfibers drag, bend and grip as the weight is increased. As the weight reduces, the fibers detach themselves thereby allowing for clean, controlled adhesion and release. Further, polypropylene been a hard polymer, the microfibers are more effective in shedding contaminants than other softer plastic fibers. However, there is more scope for study as the large-size contaminants fastened to the fibers because the surface area of contact was larger with them than with the glass surface. This is the additional sphere of focus and the next phase of the research will be to develop similar adhesive traction on the uneven surfaces.
Tackling this challenge, researchers at the University of Dayton, the Georgia Institute of Technology, the Air Force Research Laboratory and the University of Akron devised a multi-walled carbon nanotube-based adhesive material which they say has 10 times more stronger adhesive force than the gecko feet. The research team has described the material which creates directionally-varied traction while on vertical surfaces. The key to the new material is two layer carbon nanotube array. The lower layer comprises of vertically-aligned carbon nanotubes whereas the upper layer is curly �entangled� spaghetti like tubes. Researchers believes the top layer resembling jungle of wines, closely imitate the hierarchical structure of real gecko feet, which include branching hairs having varied diameters. When forced down onto a vertical surface--glass, a polymer sheet, Teflon and even rough sandpaper--the curly top layer of the nanotubes get aligned in contact with the surface. This significantly increases the amount of contact between the nanotubes and the surface, boosting the atomic-level van der Waals forces. When detaching from surface in a direction parallel to the main body of the nanotubes and perpendicular to the surface, only the lower layer vertically aligned nanotubes remain in contact and lowers the attraction forces. The measured adhesive forces clocked 100 Newtons per square centimetre in the shear direction and the 10 Newtons per square centimeter in the normal direction. The resistance to shear increased with the length of the nanotubes, while the resistance to normal force was independent of the same, the team noted. The quintessence, according to the researchers, of this material is the directional variation in the traction force caused by the two differently designed CNT layers.
Knowing that the carbon nanotubes conduct heat and electrical current, the dry adhesive arrays could be used to connect electronic devices and components, thereby eliminating the need for soldering. The van der Waals forces generated by the adhesive material will be enough to hold electronic devices together since heat management is the key for such applications. As far as the prospects and scope of study is concerned, increasing the adhesive forces on uneven and non-uniform surfaces, as well as long-term durability are two most important points to further the research.
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