Main Points

In light of the new publications (see References below) I think that our main claim is still that we have a directional and controllable dry adhesive system -- which is inherently more suited for climbing. This point is close to the arguments of the IEEE ICRA07 "adhesion principles" paper (accepted Dec. 30, 2006 -- Follow the link to access the current PDF, reviewers' comments and responses).

Regarding the refs below, we can say that recently [Sitti06, Gorb06] synthetic dry adhesive patches have been created that use arrays of elastomeric (~3 MPa Young's modulus) features with "spatula-like" tips. These flattened tip features provide a more uniform stress concentration over each stalk/substrate contact region and prevent the stress concentrations that would otherwise produce premature peeling at the periphery of the contact areas. Gorb et al. [Gorb06] argue that the adhesion for such structures is essentially similar to that first described by Kendall [Kendallxx] for the peeling of an elastomeric tape, and is function primarily of the periphery of each contact rather than the area. Kim and Sitti [Sitti06] use similar but smaller features, with a stalk length of approximately 20 micrometers and tip diameter of 9 micrometers. They measure the work involved in a loading/unloading cycle and the maximum pull-off force when pressing arrays of the features against a glass sphere. The maximum pull-off force is 18 N/cm^2 for a relatively high preload of 12N/cm^2. These

References

Gorb et al in Prof. Royal Society -- mushroom shaped fibrillar adhesives

• Material is polyvinylsiloxane ~3 MPa (same E as Sitti, below) and features are 100 micrometers tall, 60 micrometer base diameter, 35 micrometer middle diameter, 40 micrometer "plate" diameter.
• Cites Chrysomelidae beetles for inspiration
• Passive self-aligning system for tests.
• Cites Kendall peel model for the tips. Also did their own tape peeling experiments for structured tape. Procedure: Attach for 20 seconds. Then apply a weight between 70 and 300 mN force and adjust the pull-off angle until it had a peeling crack propagation speed of 100 micrometers/second crack.
• Pull-off is done as F/b (per Kendall model) where b is proportional to the periphery of the patches. Maximum of almost 40 N/m is obtained. With 90 second preload time and preloads of 50 to 120 mN per 2.9mm diameter patch. Pull-off speed = 700 micrometers/second.

S. Gorb, M. Varenberg, A. Peressadko, J. Tuma, 
Biomimetic mushroom-shaped fibrillar adhesive microstructure, 
Journal of The Royal Society Interface, 17 Oct 2006, 
DOI 10.1098/rsif.2006.0164, 
URL http://dx.doi.org/10.1098/rsif.2006.0164

Kim and Sitti, Biologically Inspired ... spatulate tips

• Similar design to the mushroom-shaped features in the paper by Gorb et al., but with smaller features 4.5 micrometer stalk diameter x 20 micrometer long and 9 micrometer tip diameter. Material is polyurethane with E of about 3 MPa.
• Pull-off work is computed by taking the area of the hysteresis loop for load/unload cycle and dividing by area. Citation for method: Controlling Polymer Adhesion with "Pancakes" Alfred J. Crosby, Mark Hageman, and Andrew Duncan, Langmuir; 2005; 21(25) pp 11738 - 11743; DOI: 10.1021/la051721k
  • This article does a good job of describing the deltaW procedure and contrasting with thermodynamic work of pull-off.
• They use a process with a sacrificial mold (like micro SDM). They also arrange the sacrificial material removal process such that the sidewalls of the fibers get coated with a "teflon like" coating to reduce clumping.
• They do their pull-off experiment using a polished glass sphere to avoid alignment issues. The rate of motion is very slow (1 micrometer/second).
• Quoted levels of adhesion are very high: 18 N/cm^2 (higher than anything previously reported) for maximum preload 12N/cm^2. It looks like the maximum adhesion grows more or less linearly with preload (Fig. 3) so you can estimate what mu' would be.

@article{kim:261911,
author = {Seok Kim and Metin Sitti},
collaboration = {},
title = {Biologically inspired polymer microfibers with spatulate tips as repeatable fibrillar adhesives},
publisher = {AIP},
year = {2006},
journal = {Applied Physics Letters},
volume = {89},
number = {26},
eid = {261911},
numpages = {3},
pages = {261911},
keywords = {polymer fibres; adhesives; moulding; sputter etching; adhesion},
url = {http://link.aip.org/link/?APL/89/261911/1},
}