News
Low-modulus micro fiber arrays (May 2014) We produced high-aspect-ratio, angled polymer microfiber arrays by directional pulling of polyethylene from low-aspect-ratio, microporous polycarbonate templates. These arrays represent a novel substrate for electrostatic sensors and actuators because they are characterized by low stiffness (<24 kPa effective elastic modulus), low Poisson's ratio (effectively zero at low strains), and very low density (<1% of solid polyethylene). Angled microfiber arrays as low-modulus, low Poisson's ratio compliant substrates, J. Micromech. Microeng. 24 065016, 2014.
GSA shearing on rough surfaces (April 2014) A model for the relationship between surface feature size and the adhesive terminal feature shape was developed. On sinusoidal surfaces with amplitudes much larger than the nanoscale features, spatula-shaped features can increase adhesive forces by 2.5 times on smooth surfaces and 10 times on rough surfaces. Simulation of synthetic gecko arrays shearing on rough surfaces, J. R. Soc. Interface, April 2014. movie
Gecko Toe on Rough Surfaces (Oct. 2013) We measured the maximum shear force a gecko foot can attain on sinusoidal patterns in sizes similar to the dimensions of the toes and lamellae structures. We found shear adhesion significantly decreased on surfaces that had amplitudes and wavelengths approaching the lamella length and inter-lamella spacing, losing 95% of shear adhesion over the range tested. Gecko toe and lamellar shear adhesion on macroscopic, engineered rough surfaces, J Exp Biol, October 2013.
Hard vs Soft Fibrillar Self-cleaning (June 2013) For certain particle sizes and fiber diameters, both hard (polypropylene) and soft (silicone rubber) fibrillar adhesives can recover adhesive capabilities after fouling with spherical particles. An analysis of the contact strength between fibers, particles and substrates reveals that dry self-cleaning is more effective for smaller fiber diameters and smaller loss functions, such as hard thermoplastics. Dry Self-Cleaning Properties of Hard and Soft Fibrillar Structures, ACS Applied Materials and Interfaces, June 2013 (just accepted)
Controllable Particle Adhesion (Feb. 2013) Controllable adhesion to glass spheres with a magnetically actuated synthetic gecko adhesive is demonstrated. Results show sphere pull-off forces can be increased 10-fold by changing the ridge orientation via the external magnetic field, and that the effective elastic modulus can be changed from 65 kPa to 1.5 MPa. Controllable particle adhesion with a magnetically actuated synthetic gecko adhesive, Advanced Functional Materials, 2013
Wet Cleaning of HDPE GSA (Oct. 2012) Hard-polymer-based GSA formed with high-aspect-ratio microfibers from high-density polyethylene (HDPE) self-cleans with water. The microfiber adhesive shows almost complete wet self-cleaning of dirt particles with water droplets, recovering 98% of the adhesion of the pristine microfiber adhesives. Wet Self-Cleaning of Superhydrophobic Microfiber Adhesives Formed from High Density Polyethylene, Langmuir, 2012
GSA Adhesive Material Limits (Aug. 2011) HDPE and PP fibrillar arrays have shear adhesion stress (0.3 MPa) sufficient to deform the fibers. Hence the fiber material strength is a limit to greater adhesion strength. Surprisingly, the GSA maintained 54% of original stress in spite or marked deformation over 10,000 cycles. Shear Adhesion Strength of Thermoplastic Gecko-Inspired Synthetic Adhesive Exceeds Material Limits, Langmuir, 2011
Combined Lamellar Nanofibrillar Array  (Oct. 2009) Lamellar structures act as base support planes for high-aspect ratio HDPE fiber arrays. Nanofiber arrays on lamella can adhere to a smooth grating with 5 times greater shear strength than flat nanofiber array. Langmuir, Oct 2009
Gecko Tire for Model Car (Nov. 2008) Microfiber array wrapped on model car tire demonstates high friction. (Note: so far, tire only works on smooth surfaces.) High friction from a stiff polymer using micro-fiber arrays, Phys. Rev. Letters, 2006	1/18 scale model car with gecko tires on acrylic sheet.
Directional Adhesion of Angled Microfibers (Nov. 2008) Angled polypropylene microfibers show strong directional adhesion effects, with shear strength in direction of fibers 45 times larger than sliding against fiber directions. A 1 sq. cm. patch supported a load of 450 grams in shear. Directional adhesion of gecko inspired angled microfiber arrays, Applied Physics Letters, 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 hierarchical 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 biology, we develop mechanical models for gecko hair adhesion and then design, fabricate, and test micro and nanofibrillar structures. We aim to achieve the seven benchmark functional properties of the gecko adhesive system identified by Autumn (MRS Bulletin 2007): 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.	(left) Anolis Equistris, length 23 micron, diameter 0.5 micron. (right) polyimide synthetic gecko adhesive, length 22 micron, diameter 0.6 micron
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 [Majidi et al. PRL 2006]. In 2007, we showed how the polypropylene fiber arrays can provide shear force without a normal load being present [Schubert et al. JAST 2007]. In 2008, we made easy-attach easy-release hard polymer gecko adhesives which have a non-adhesive default state [Lee et al. JRSI 2008], and can self-clean during contact [Lee and Fearing Langmuir 2008].  The final goal is to build arrays incorporating the necessary geometrical features which have the same adhesion as geckos to rough and smooth surfaces.

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
Combined Lamellar Nanofibrillar Array  (Oct. 2009) Lamellar structures act as base support planes for high-aspect ratio HDPE fiber arrays. Nanofiber arrays on lamella can adhere to a smooth grating with 5 times greater shear strength than flat nanofiber array. Langmuir, Oct 2009
Hybrid CoreShell Nanowire Connectors (April 2009) Arrays of parylene coated Ge nanowires connect with themselves to form a reusable connector. Uniquely, NW chemical connectors exhibit high macroscopic shear adhesion strength (1.6 MPa) with minimal binding to non-self-similar surfaces, anisotropic adhesion behavior (shear to normal strength ratio 25), low preloading, reusability, and efficient binding for both micro- and macroscale dimensions. Nanoletters, April 2009
Reduced Resistance Nanowire Contacts (Dec. 2008) Nickel nanowires can increase the true area of contact and reduce contact resistance. Resistance between a probe and an array of 200 nm by 10 micron nickel nanowires was reduced by an order of magnitude compared to contacting a flat sheet. Reducing contact resistance using compliant nickel nanowire arrays, IEEE Trans. on Comp. and Pack. Tech. 2008.
Directional Adhesion of Angled Microfibers (Nov. 2008) Angled polypropylene microfibers show strong directional adhesion effects, with shear strength in direction of fibers 45 times larger than sliding against fiber directions. Angled fibers also show normal adhesion without shear load, unlike vertical fibers. Directional adhesion of gecko inspired angled microfiber arrays, Applied Physics Letters, 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. 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]