Optimization And Fabrication Of High-Aspect-Ratio Micro/Nano Hierarchical Structures Mimicking Gecko Foot-Hairs

It is very urgent to develop a kind of powerful and repeatable adhesive, due to its wide applications in deployment and disassembly of micro electro mechanical system (MEMS) devices such as wall-climbing robots, micro-manipulation, micro-assembly, etc. The commonly used wet adhesives are easy to be contaminated and fall off, and can not be reused, which have unfavorable effect on adhesion. As a result, the developing of a kind of dry adhesive without the above shortcomings would have important significance and wide application prospect. In nature, geckos have developed complex foot-hair structures capable of smart adhesion. They can crawl quickly or keep sill on horizontal cell, vertical wall, rough bark and smooth glass, even in a wet environment. The powerful adhesive characteristics of geckos inspire human for design and fabrication of a new kind of biomimetic dry adhesive. The multiscale hierarchical structure of the gecko foot-hair, especially the high-aspect-ratio structure of its micro-scale seta and nano-scale spatulae is the critical factor of gecko’s ability to adopt and stick to any different surfaces with powerful adhesion force. Accordingly, the design and fabrication method mimicking gecko foot-hairs will be investigated in this paper. Starting from the adhesion mechanism of gecko foot-hairs, a bottom-up optimal design method based on various restraining conditions is proposed. Subsequently, a simple and low-cost way to make dual-level microfiber arrays based on inductively coupled plasma (ICP) etching and thick film photolithography is developed. The main contens of this paper are shown in the following.Firstly, the adhesion mechanism of gecko foot-hairs has been studied. We analyze the force of a single spatula in top contact and side contact through JKR model and minimum potential energy principle. The adhesion force and the detachment angle are calculated, which can explain why geckos both have stable attachment and easy detachment. By establishing the models of rough surfaces and hierarchical structures, we simulate the processes of attachment and detachment of gecko foot-hairs. The relationships among adhesion force, surface roughness and layer of structures are discussed, which can explain why geckos have excellent adaptability on different surfaces. We analyze the hydrophobicity of spatulae through Cassie model, and compare the adhesion between dirt particle and spatulae through JKR model, which can explain how geckos manage to keep their feet clean while walking both in watery and dry environment.Secondly, an optimal design mimicking gecko foot-hairs has been finished. We discuss and select the layer of hierarchical structure. In order to achieve goals of robust adhesion, well stability and excellent adaptability, we propose constraints such as the optimal adhesion and stability of a single fiber, the anti-bunching between adjacent fibers, the adaptability on rough surface, the hydrophobicity and self-cleaning of fiber arrays, and the maximal adhesion work of hierarchical structures. A bottom-up optimal design method for geometrical parameters is proposed based on these constraints, and a procedure is also developed here to find out optimum parameters mimicking gecko foot-hairs with a discussion of the results.Thirdly, investigations have been made on the fabrication methods of structures mimicking gecko foot-hairs. The thick film photolithography based on SU-8photoresist is studied, and single-level fiber arrays of SU-8with an aspect ratio of6are fabricated. The ICP etching process based on Bosch technology is also studied, and single-level fiber arrays of Polydimethylsiloxane (PDMS) with an aspect ratio of5are fabricated. Then we propose a process of micromodeling from a composite mold of SU-8photoresist and silicon by using ICP etching and thick film photolithography, and fabricate the dual-level fiber arrays of PDMS with a top aspect-ratio of5and a bottom aspect-ratio of2.Finally, the properties of structures mimicking gecko foot hairs have been tested. We compare the hydrophobic properties, the normal and shear stresses among unpatterned, single-level and dual-level structures, and explaine the experimental results with previous theories. The average contact angle of single-level arrays is143.6°, while the dual-level arrays’ is150.4°. The maximum normal stress of patterned structures is1.52N/cm2(single-level array with5μm diameter,5μm spacing and20μm height), while the maximum shear stress is0.79N/cm2(single-level array with3μm diameter,3μm spacing and15μm height). It is found that dual-level structures have better hydrophobicity and adaptability on rough surface than single-level and unpatterned ones.