SU-8 has become the favourite photoresist for high-aspect-ratio (HAR) and three-dimensional (3D) lithographic patterning due to its excellent coating, planarization and processing properties as well as its mechanical and chemical stability. However, as feature sizes get smaller and pattern complexity increases, particular difficulties and a number of material-related issues arise and need to be carefully considered. This review presents a detailed description of these effects and describes reported strategies and achieved SU-8 HAR and 3D structures up to August 2006.

Inspired by biological attachment systems, micropatterned elastomeric surfaces with pillars of different heights (between 2.5 and 80 microm) and radii (between 2.5 and 25 microm) were fabricated. Their adhesion properties were systematically tested and compared with flat controls. Micropatterned surfaces with aspect ratios above 0.5 were found to be more compliant than flat surfaces. The adhesion significantly increases with decreasing pillar radius and increasing aspect ratio of the patterned features. A preload dependence of the adhesion force has been identified and demonstrated to be crucial for obtaining adhesives with tunable adherence.

Following a recent bioinspired paradigm, patterned surfaces can exhibit better adhesion than flat contacts. Previous studies have verified that finer contact structures give rise to higher adhesion forces. In this study, we report on the effect of the tip shape, which was varied systematically in fibrillar PDMS surfaces, produced by lithographic and soft-molding methods. For fiber radii between 2.5 and 25 microm, it is found that shape exerts a stronger effect on adhesion than size. The highest adhesion is measured for mushroom-like and spatular terminals, which attain adhesion values 30 times in excess of the flat controls and similar to a gecko toe. These results explain the shapes commonly found in biological systems, and help in the exploration of the parameter space for artificial attachment systems.

The amazing adhesion of gecko pads to almost any kind of surfaces has inspired a very active research direction over the last decade: the investigation of how geckos achieve this feat and how this knowledge can be turned into new strategies to reversibly join surfaces. This article reviews the fabrication approaches used so far for the creation of micro- and nanostructured fibrillar surfaces with adhesive properties. In the light of the pertinent contact mechanics, the adhesive properties are presented and discussed. The decisive design parameters are fiber radius and aspect ratio, tilt angle, hierarchical arrangement and the effect of the backing layer. Also first responsive systems that allow thermal switching between nonadhesive and adhesive states are described. These structures show a high potential of application, providing the remaining issues of robustness, reliability, and large-area manufacture can be solved.

A new method for the fabrication of structured polymer surfaces possessing pillars with controlled 3D tip geometries resembling those found in biological attachment devices is reported. The fabrication strategy exploits the filling mechanism of 2D lithographic templates, combined with inking and printing steps using elastomeric precursors with various viscosities and crosslinking kinetics. Homogeneously structured areas were obtained that allow reproducible and reliable testing of adhesion behavior and can be scaled up to prototypes. The results presented here provide the first experimental evidence of the influence of the contact shape in adhesion of structured surfaces and pave the road to a better understanding of biological-attachment systems and to optimum designs of artificial analogues.The ability of some insects and geckos to firmly attach to and rapidly detach from surfaces of almost any kind has intrigued biologists for many centuries. [1][2][3][4][5][6][7] Microscopic examination of their attachment pads has revealed a complex surface hyperstructure consisting on setae organized in a hierarchical arrangement spanning the millimeter to nanometer range. This split contact area has been shown to enhance adhesion performance, [8,9] but the detailed mechanisms and critical parameters behind nature's hierarchical design are still under debate. Relevant theoretical works have been published on this topic in recent years, [10][11][12][13][14][15][16][17][18] together with measurements on biological systems. [19][20][21][22][23][24][25] Design maps have been developed, which point out the interplay of several geometrical and material parameters, [26] but a clear picture of how they contribute to the final adhesion performance is still missing. Modest artificial mimics have also been published, although reported adhesion performances-when measured with reliable methods-are far from those of natural systems. [27][28][29][30][31][32] A possible reason for lack of adhesion is the oversimplification of the geometrical design of the artificial mimics. These mainly consist of arrays of low-to high-aspect-ratio pillars made of elastomeric materials (mainly poly(dimethylsiloxane)), carbon nanotubes, or hard polymers (poly(styrene) or poly(methylmethacrylate)). These pillars terminate in flat tips. In contrast, the tips of the fine hairs in biological systems show different geometries: spherical, conical, filamentlike, bandlike, suckerlike, flat, and toroidal shapes have been observed in different animals.[33] Although recent theoretical work already points out the importance of the tip geometry (in particular the importance of a compliant tip, as in the gecko spatulae), this factor has not been considered in the artificial systems up to now. The main reason is the intrinsic difficulty and scarce availability of 3D micro-and nanofabrication methods to obtain such structures over medium size areas (a couple of centimeters squared) required for adhesion testing. This fact presently limits further achievements in this area ...