News- Sep. 2008

Self Cleaning Gecko Adhesive (Sep. 2008) First synthetic gecko adhesive which cleans itself during use, as the natural gecko does. After contamination by microspheres, the microfiber array loses all adhesion strength. After repeated contacts with clean glass, the microspheres are shed, and the fibers recover 30% of their original adhesion. The fibers have a non-adhesive default state, which encourages particle removal during contact. Contact Self-Cleaning of Synthetic Gecko Adhesive, Langmuir 2008

Before and after repeated contacts.

Overview of Gecko Adhesion Project

Geckos have the remarkable ability to run at any orientation on just about any smooth or rough, wet or dry, clean or dirty surface. The basis for geckos' adhesive properties is in the millions of micron-scale setae on each toe of the gecko form a self-cleaning dry adhesive. The tip of each seta consists of 100 to 1000 spatulae only 100 nanometers in diameter. Our interdisciplinary team of biologists and engineers has been working since 1998 developing models for how the natural nanostructures function in a hierachical combination of spatulae, spatular stalks, setal stalks, setal arrays, and toe mechanics, and developing nanofabrication processes which allow large arrays of hair patches to be economically fabricated. Keywords: synthetic gecko adhesion, gecko adhesive, gecko tape

Synthetic Gecko Nano Hair Properties

Using insights from biological models, our work in the gecko hair project aims to develop mechanical models for gecko hair adhesion and then to design and fabricate a synthetic gecko hair array. Work by Autumn (MRS Bulletin 2007) has identified seven benchmark functional properties of the gecko adhesive system: anisotropic attachment, high pulloff to preload ratio, low detachment force, material independence / van der Waals adhesion, self-cleaning, anti-self matting, and non-sticky default state. The low detachment force, self-cleaning, and non-sticky default state suggest hard polymers, rather than the soft polymers typically used in pressure sensitive adhesives. As reported in 2002, [Sitti and Fearing 2002] and Autumn et al [2002] we have made synthetic spatulae, which have shown adhesion similar to natural spatulae in the range of 100-300 nN. These patches of bumps lacked the setal stalks, and achieved adhesion forces on the order of a few milliNewtons on an area of a square centimeter. In 2003, we fabricated high density arrays of spatular stalks [Campolo et al 2003] which showed adhesion in shear on the order of 0.5 Newton per sq. cm. In 2006, we demonstrated a novel high friction array of 0.6 micron fibers which showed shear resistance of 4 Newton per sq. cm. with only 0.8 Newton per sq. cm. of normal load. In 2007, we showed how the polypropylene fiber arrays can provide shear force without a normal load being present. The final goal is to build arrays incorporating the necessary geometrical features which have similar adhesion to geckos, about 10 Newtons per square centimeter.

(left) Anolis Equistris, length 23 micron, diameter 0.5 micron (right) polyimide synthetic gecko adhesive, length 22 micron, diameter 0.6 micron

Interesting Facts about Gecko Adhesion

1. Gecko toes are not ``sticky'' like tape.

   If you touch a gecko toe it feels soft and smooth, and not sticky at all. If you pressed a gecko toe onto a hard surface it would not stick. The toe will only adhere when the microfibers (setae) are engaged, by dragging or sliding the toe parallel to the surface. (If toes were sticky like tape, it would be difficult for a gecko to walk or run, as it would be too hard to pull its feet up.)

Progress in Gecko-inspired Synthetic Adhesion
Self Cleaning Gecko Adhesive (Sep. 2008) First synthetic gecko adhesive which cleans itself during use, as the natural gecko does. After contamination by microspheres, the microfiber array loses all adhesion strength. After repeated contacts with clean glass, the microspheres are shed, and the fibers recover 30% of their original adhesion. Contact Self-Cleaning of Synthetic Gecko Adhesive, Langmuir 2008
Adhesion of Elastic Plate to a Sphere (Feb. 2008) Adhesion of even a thin membrane to a non-developable surface, such as a sphere, requires  stretching as well as bending. Dividing contacts into small plates reduces membrane strain and increases adhesion. Adhesion of an elastic plate to a sphere, Proc. Royal Soc. A 2008
Directional gecko adhesive (Jan. 2008) First easy attach, easy release, and directional synthetic gecko adhesive using hard polymer microfibers. Microfiber array using 42 million polypropylene microfibers per square centimeter. Patches can support 9 N/sq.cm. in estimated contact region with preload of just 0.1N/sq.cm. Sliding-induced adhesion of stiff polymer, Interface 2008
Shear adhesion from polypropylene microfibers (2007) Shear adhesion of 0.1 N/sq.cm. using 0.6 micron polypropylene fiber array. Normal preload required was less than 0.05 N/sq. cm [Schubert et al. Jnl. of Adhesion Sci. and Tech. 2007].
High friction from polypropylene microfibers (2006) Using an array of vertically oriented polypropylene microfibers, high friction is demonstrated without using a soft/sticky material. At 0.8 N/sq. cm, coefficient of friction is greater than 5. [Majidi et al PRL 2006 ]. Copyright (2006) by the American Physical Society High friction from a stiff polymer using micro-fiber arrays, Phys. Rev. Letters, 2006
Side contact model for fiber adhesion (2005) Analysis of fiber adhesion in side contact. [Majidi, Groff, Fearing  J. Appl. Phys 2005] shows that sufficiently long fibers, e.g. carbon nanotubes can stably make side contact. This side contact can give 10-20 times greater adhesion force than a hemispherical tip contact.
Verification of side-side contact clumping (2004) Fiber density is limited by clumping- both tip-tip at larger gaps between fibers and side-side for closer fibers. A square lattice is predicted to have better clumping resistance. Polyimide fibers (0.6 micron diameter) showing clumping behavior. [Majidi, Groff, Fearing 2004].
Effect of surface roughness on adhesion strength (2003) Surface roughness significantly reduces adhesion strength of fibrillar adhesives. A cantilever fiber model predicts a drop in adhesion by a factor of 5 when surface roughness increased from 1 to 15 um. [Campolo, Jones, Fearing IEEE Nano 2003]
High density nanofibers (2003) Using a casting process in a template, polyurethane hairs 200 nm diameter and 60 micron long were fabricated. Due to clumping, adhesion force was limited.
Synthetic spatula array (2002) Synthetic spatula array from nano-indenting and casting. Approximately 200-300 nN adhesion force was measured per spatula. Total area of array was less than 100x100 sq. um. [Sitti and Fearing IEEE Nano 2002]
Rubber micro stalk array (2002) Array of silicone rubber stalks, each 6 um in diameter and 6 um height demonstrated 0.003 N/sq. cm. adhesion and 60 nN adhesion per stalk.[Sitti and Fearing IEEE Nano 2002]
Synthetic spatula (2002) Single spatula were constructed from silicone rubber (E ~ 0.5 MPa) and polyester (E ~ 1000 MPa) by using nano-indentation and casting. Using atomic force microscope, 290 nN pulloff force was measured from a single polyester spatula with tip radius of 350 nN, and 180 nN for silicone rubber. The similarity of pulloff forces supports the hypothesis of material independence for gecko adhesives. [Autumn et al. PNAS 2002]