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. 2009 Dec 6;6(41):1223-32.
doi: 10.1098/rsif.2009.0048. Epub 2009 Mar 18.

A Microfabricated Wedge-Shaped Adhesive Array Displaying Gecko-Like Dynamic Adhesion, Directionality and Long Lifetime

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A Microfabricated Wedge-Shaped Adhesive Array Displaying Gecko-Like Dynamic Adhesion, Directionality and Long Lifetime

Aaron Parness et al. J R Soc Interface. .
Free PMC article

Abstract

Gecko adhesion has become a paradigmatic example of bio-inspired engineering, yet among the many gecko-like synthetic adhesives (GSAs), truly gecko-like performance remains elusive. Many GSAs have previously demonstrated one or two features of the gecko adhesive. We present a new wedge-shaped GSA that exhibits several gecko-like properties simultaneously: directional features; zero force at detachment; high ratio of detachment force to preload force; non-adhesive default state; and the ability to maintain performance while sliding, even after thousands of cycles. Individual wedges independently detach and reattach during sliding, resulting in high levels of shear and normal adhesion during drag. This behaviour provides a non-catastrophic failure mechanism that is desirable for applications such as climbing robots where sudden contact failure would result in serious falls. The effects of scaling patch sizes up to tens of square centimetres are also presented and discussed. Patches of 1 cm(2) had an adhesive pressure of 5.1 kPa while simultaneously supporting 17.0 kPa of shear. After 30 000 attachment/detachment cycles, a patch retained 67 per cent of its initial adhesion and 76 per cent of its initial shear without cleaning. Square-based wedges of 20 mum and 50 mum are manufactured in a moulding process where moulds are fabricated using a dual-side, dual-angle lithography process on quartz wafers with SU-8 photoresist as the mould material and polydimethylsiloxane as the cast material.

Figures

Figure 1
Figure 1
Scanning electron microscope images of microfabricated wedge-shaped adhesive array. (a) Side view of 20 μm base width by 80 μm height wedges and (b) diagonal view of a large array of 50 μm base width by 200 μm height wedges.
Figure 2
Figure 2
Fabrication sequence. (1) Deposit aluminium on UV transparent quartz wafer, (2) pattern aluminium to create self-aligned mask, (3) deposit SU-8 on top of aluminium, (4) angled self-aligned UV exposure from backside, (5) align mask to topside and UV expose, (6) develop, (7) cast and spin polydimethylsiloxane (PDMS) and (8) peel out cast adhesive structure and backing layer. Black, PDMS; light grey, quartz wafer; dark grey, exposed SU-8; grey, unexposed SU-8.
Figure 3
Figure 3
Typical clumping behaviour observed between adjacent stalks for very soft materials such as Silicones Inc. P-20. Stiffer materials did not exhibit extensive clumping.
Figure 4
Figure 4
Wedge-shaped adhesive load–drag–pull (LDP) data from a single trial, 1 cm2 patch size. Preload occurs from point 0 to 1 consisting of a 45° approach angle to a depth of 80 μm. This was followed by a 1 mm drag (points 2 and 3) and a vertical pull-off (points 3 and 4). The substrate moved at a constant 1 mm s−1 over the course of the trial. Note the sustained dynamic adhesion (solid curve) and shear (dashed curve) between points 2 and 3, indicating independent detachment and reattachment of single wedges.
Figure 5
Figure 5
Gecko setae LDP data from a single trial. Comparison with figure 4 shows the strong behavioural similarity between the microfabricated wedge-shaped adhesive and gecko setae, especially the dynamic adhesion observed between points 2 and 3 (solid curve, adhesion; dashed curve, shear).
Figure 6
Figure 6
Limit surface of a Sylgard 170 sample of 50 μm base width wedges on a 225 μm backing layer, 1 cm2 patch size. Points indicate contact failures either through slipping or detachment from the surface. Important to note: adhesion is achieved only in the presence of shear loading, following the frictional adhesion model (Autumn et al. 2006a). Also, the limit surface intersects the origin, indicating that, when no shear is present, the adhesive can be detached with zero force.
Figure 7
Figure 7
Limit surface of a flat sample of Sylgard 170. This set of data follows the embedded friction cone model exhibiting the strongest adhesion when no shear loads are present.
Figure 8
Figure 8
Comparison of arrays of 20 μm base width (squares) and 50 μm base width (diamonds) wedges. Patch size of 1 cm2 and backing layer thickness of 600 μm. Trajectories were scaled to wedge size.
Figure 9
Figure 9
Lifetime data for a single sample of 50 μm base width wedges made from Sylgard 170, patch size 1 cm2. The wedge-shaped adhesive retained over 67% of their initial adhesion (solid line) and 76% of their initial shear (dashed line) after 30 000 trials.
Figure 10
Figure 10
Adhesion pressures for various patch sizes. All samples were taken from the same mould and have a 300 μm backing layer thickness. Identical trajectories consisting of a 45° preload to a depth of 120 μm followed by a 150° pull-off were used for the fixed preload (pluses) dataset. The maximum adhesive pressures (crosses) are also presented. These data occurred at higher preload depths ranging from 140 to 180 μm. Using a higher preload helps to overcome misalignments in the contact between wedges and the glass substrate. This tactic, however, is impractical for climbing applications where the ability to apply preload forces is limited.

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