«UNIVERSITY OF CALIFORNIA Santa Barbara Design and Characterization of Fibrillar Adhesives A Dissertation submitted in partial satisfaction of the ...»
Revisions to the microfabrication processes are another option for creating better adhesives. A sticky default state was present with the semicircular ﬁbers because the mold was smooth on the top of the ﬁber. Using a thicker polymer layer for the angled lithography, such as polymethylglutarimide (PMGI) SF-19, would eliminate reaching the glass wafer underneath. Taller ﬁbers could also create higher shear and adhesion forces by making contact with the opposing surface over greater lengths, as well as oﬀer greater compliance to surface roughness. The maximum angle from vertical achievable with the angled lithography was found to be 20◦. Dry etching techniques using a Faraday cage have been used to create angled ﬁbers angled 30–60◦ from vertical . Such a process would allow for a greater tilt angle and a larger range of angles for better understanding of the inﬂuence of tilt. An addition beneﬁt of the angled dry etching technique is the mold material would be made of silicon which has been show to have greater reusability than the polymer molds.
Chapter 7. Conclusions and Future Work The pressure values of the developed adhesives have yet to surpass those achieved by the gecko.
To create higher forces, the ﬁber density should be able to be increased for larger areas of contact. No ﬁber-to-ﬁber adhesion was seen for the rectangular ﬂap or semicircular adhesives, suggesting that the ﬁbers can be placed closer to one another. An alternative approach to higher force values involves optimizing the ﬁber. The use of materials other than PDMS allows for greater freedom when selecting geometric features such as aspect ratio and minimum ﬁber size. A diﬀerent material will also allow speciﬁc selection of tensile strength, elongation to break, toughness, and Young’s modulus. Material selection will be limited by the ease of which it can be integrated with the fabrication process. Materials which can’t be separated from the mold or have a tendency to break can’t be used. Depending on the selections made, anisotropy could be further increased from the present values.
7.2.2 Integration with Climbing Robots An active area of research and potential application for the gecko-like adhesives being developed is integration with climbing robots. Climbing robots can be used for remote sensing applications or exploration of dangerous environments without risking human life. Current robots have successfully climbed a variety of surfaces, but are not yet practical to use in their envisioned environments. For Chapter 7. Conclusions and Future Work the adhesives presented here to be integrated with climbing robotic platforms, the microfabrication techniques must be expanded to larger areas, testing must be increased to centimeter dimensions, and environmental characterization must be performed.
The current microfabrication processes can reliably create adhesive patches on the centimeter scale. However, climbing robots can weigh up to 4 kg  and will require large patch sizes to support the robot during inverted movement and vertical climbing. A step-and-repeat approach using a small area of the photolithography mask was chosen in the past for characterization of multiple ﬁber designs using a single mask. Future designs will need to use modeling to optimize ﬁber shape, dimensions, and density so that single designs can be patterned over an entire 4-inch wafer. An alternative to full wafer patterning involves designing processes to join smaller samples together over a large area. The microfabrication of the angled ﬂaps and semicircular ﬁbers was only possible over a maximum area of 2 cm2. When larger areas were attempted, delamination was seen to occur between the the PMGI and glass wafer due to stress mismatches between the mold material and wafer. A repeatable method to join these smaller pieces would allow for sizes relevant to climbing robots to be created.
Although the microfabrication process currently creates patch areas greater than those needed by the 4 mm diameter glass puck, there is not a method to Chapter 7. Conclusions and Future Work characterize them. Modiﬁcations to the Bio-F test apparatus would allow for large test areas by replacing the glass puck with a glass slide or wafer. In this manner, the size of the adhesive would limit the test area. With larger patch sizes, the glass leveling attachment would likely need to be modiﬁed. Replacing the screws with micrometers would reduce the smallest incremental motion and improve alignment when leveling the two surfaces. A second option for less precise measurements would be to test the adhesives without precise alignment using load-pull and load-drag tests on the macroscale test station. The macroscale test station requires placing the adhesive by hand and uses weights to load the adhesive sample. The lack of alignment procedures leveling the two surfaces is similar to adhesive placement when used with a climbing robot and oﬀers insight into the scalability of the adhesive.
The surfaces that a climbing robot would encounter are much diﬀerent than those used in testing the adhesive. Surfaces are likely to be rough on multiple length scales and dirty. The self-cleaning property has been discussed in Section 7.2.1, but the results have great relevance to climbing robots. Tests of synthetic adhesives have been performed against a variety of rough surfaces. The rough substrate has been patterned in a regular manner  and in some cases, the pattern wavelength of the rough surface has matched the wavelength of the adhesive structures . Other testing has used common surfaces with large gaps in roughChapter 7. Conclusions and Future Work ness  or randomly rough surfaces with variations in roughness only extending over slightly more than an order of magnitude . Adhesive characterization over all relevant roughness scales, as well as the ability to adapt to these variations in roughness, will be needed for truly mobile robots.
A ﬁnal area of work will be to characterize the adhesive in diﬀerent environmental conditions. The velocities used for the adhesive testing of rectangular ﬂaps and semicircular ﬁbers has been very low to minimize any rate-dependent eﬀects.
However, the retraction speed has been shown to inﬂuence the adhesion of ﬁbrillar structures  and viscoelastic eﬀects have been used for pick-and-place operations of microplatelets and glass cover slips . Similar characterization of rate eﬀects on future adhesives will need to be performed on the Bio-F tester. The Bio-F test apparatus does not have the ability to control the humidity, which has been shown to inﬂuence the adhesive properties of gecko spatulae at the nanoscale .
Existing collaboration with Jacob Israelachvili’s lab would allow the adhesives to be tested in diﬀerent humidities using the surface forces apparatus (SFA).
Clearly, there is much work needed to fully develop a gecko-inspired adhesive.
Adhesive ﬁber hierarchy, self-cleaning properties, and higher force values all must be introduced into the design of future adhesives to have gecko-like behavior.
Increasing the adhesive area, characterizing these larger patches on a variety of surfaces, and characterization of other eﬀects such as testing speed and humidity Chapter 7. Conclusions and Future Work all must be performed before the adhesives can be integrated on climbing robots.
The potential applications of a true gecko-like adhesive are limited only by one’s imagination which makes future research in this area an exciting endeavor.
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