«UNIVERSITY OF CALIFORNIA Santa Barbara A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive A Dissertation submitted in partial satisfaction of ...»
The first major difference is that the MICS adhesion was measured in a normal direction to the surface. While in the gecko testing, adhesion was measured in a direction parallel to the surface, likely creating a composite adhesion and friction force. Autumn et al. did show that adhesion in the transverse direction was over a factor of 30 times greater than in the normal direction. By measuring the force of a single 20 μm seta, on an atomically smooth surface, normal adhesion strengths of 3 kPa were measured(6). Additionally, based on transverse adhesion measurements, a 10 fold reduction in adhesion can be expected for macro-scale testing (or whole gecko pad tests) (6, 11). Taking this into account, a normal adhesion measurement between a smooth surface and an entire gecko pad may yield adhesion values around 300 Pa. This leaves only an order of magnitude difference in adhesion without taking onto account the reduced adhesion expected on a rough surface.
Diminished performance of the artificial adhesive can be attributed to the incomplete organorod coverage on the platform fingers. The ends of the fingers offer the highest compliance of the structure, providing the potential for improved surface contact. However, without the organorod coating van der Waals interactions are greatly reduced, along with the subsequent adhesion strength. Improved adhesion is expected with refined processing to produce complete coverage.
The difficulty comparing values here clearly presents the need for a standardized test technique for adhesion on the millimeter size scale. For now it may be best to perform experiments on standardized samples to determine relative adhesion strengths. In this work, the photoresist surface was used as a baseline and relative improvements were shown with each additional level of compliance: first with the organorods and then with combination of organorods and flexible platforms.
Future work will focus on enhancing adhesion through modification of the three key design components. These components are the silicon support pillar, the silicon dioxide platform, and the organorod surface. The aspect ratio of the silicon pillar can be modified to assess an optimum height and shape for adhesion. The pattern of the silicon dioxide platform can be modified to further enhance surface compliance while maintaining structural stability. Also the spacing between individual platforms can be reduced to offer larger surface contact area. Perhaps, most importantly, the height, size, spacing, and material of the organorods can be modified to optimize adhesion.
E. Conclusion Multi-scale Integrated Compliant Structures (MICS) have been fabricated with aligned vertical photoresist organorods coating lithographically defined flexible silicon dioxide fingers supported by a single silicon pillar. The MICS have been produced using batch fabrication techniques creating 10 mm x 10 mm arrays across an entire 100 mm wafer.
To measure adhesion a new test technique has been developed. The technique employs nanoindenter instrumentation to measure the pull-off force between a 5 mm diameter aluminum flat punch and test surfaces. Appreciable increases in adhesion are observed with the integration of the multiple scales of compliant structures. Additionally, the MICS offer enhanced wear characteristics with sustainable adhesion over repeated testing.
Acknowledgements This work is funded by AFOSR FA9550-05-1-0045.
F. Figures Figure VII-1 Multiscale integrated compliant structures (MICS). Portion of a 2,500 array of MICS (top), scale bar 500 μm. Individual MICS (middle), scale bar 50 μm. Central portion of a platform showing organorod integration (bottom), scale bar 10 μm.
Figure VII-2 Scanning electron micrograph of organic looking polymeric nanorods (“organorods”) on the central portion of a platform, scale bars 5 μm and 2 μm, top and bottom respectively.
Figure VII-3 Adhesion strength versus applied normal load between a 5 mm diameter aluminum flat punch and a smooth photoresist surface (triangles); a surface of vertically aligned photoresist organorods (diamonds);
and a surface of arrayed multiscale integrated compliant structures consisting of silicon dioxide platforms coated with organorods (squares).
surface on solid substrate (diamonds) and a organorod surface on compliant structures (squares).
VIII. A bio-inspired reversible adhesive Michael T. Northen†, Christian Greiner*, Eduard Arzt* & Kimberly L. Turner‡ Geckos, as well as many insects, have evolved a robust reversible adhesion mechanism, enabling them to traverse rough, smooth, vertical or inverted surfaces. Here we report the fabrication and demonstration of a synthetic reversible adhesive composed of flexible nickel paddles coated by aligned vertical polymeric nanorods. When subjected to a magnetic field, the nickel paddles undergo a reversible conformational change, greatly reducing the contact area, and decreasing adhesion by a factor of 40. Such controllable adhesion may impact technologies ranging from ubiquitous latching systems to high-tech applications such as microrobotics.
The mechanism of adhesion in the gecko has been of scientific interest since Aristotle (74). Since then scientific investigations have revealed much about the construction of the pad in the gecko’s foot (5, 9, 75-77). Most recently there has been an intensifying scientific investigation into the fundamental physics of the adhesive, isolating van der Waals as the primary source of adhesion (6, 10), with additional evidence that humidity may also play an important role (25, 78). Van der Waals interactions produce weak and short-range forces, therefore the gecko must create a large amount of intimate surface contact to have enough adhesion to hang from a vertical or inverted surface. The gecko accomplishes this with a highly compliant pad structure, which allows it to conform to surfaces, without creating a large amount of elastic repulsive force (29). This ability to comply to a wide range of surfaces, from the curvature of a tree branch to micro- and nano-scale roughness of bark, is a result of a multi-scale compliant structure (26, 28, 79, 80). The hierarchical structure consists of 200 nm wide, 5 nm thick spatulas at the ends of ~100 μm long, ~5 μm diameter setae (5, 75, 76, 81). The fine and thin spatulas conform to nanoscale roughness of a surface, enhancing the van der Waals forces and increasing adhesion through a contact splitting phenomenon (27). The setae provide the next level of surface compliance by bending to conform to micro- and millimeter scale roughness. Without the compliance of the setal stalks, the spatulae would not come into contact with even the most moderately rough surface, greatly affecting the adhesive properties. There is evidence that the hierarchical structure may serve another purpose than enhancing adhesion – to reduce adhesion (6, 14, 82).
As interesting as the gecko adhesion mechanism is, if the gecko were unable to release a surface, it would not be possible to take the next step. The nano/microscale components integrated into a hierarchical attachment/detachment structure allows the gecko to control adhesion at the nano-scale through macroscopic muscle movements (10, 14, 29).
Previously, a bio-inspired synthetic system enhancing adhesion utilizing a hierarchical structure was fabricated and tested (83). The system consisted of aligned vertical nanorods coating thin silicon dioxide platforms. The nanorods provided sufficient short-distance interactions to provide adhesion and the platform provided the bulk scale conformity necessary to adhere to rough or contoured surfaces. The combination of both structures provided increased adhesion over either isolated component. However, unlike the gecko, the system did not provide a mechanism for decreasing adhesion. This attribute is critical to any application of such a biomimetic system.
Here we report a new biomimetic system which provides a mechanism for decreasing adhesion using a magnetic field to actuate nickel cantilevers. The nickel beams, when placed in a magnetic field, reorient themselves so that the terminal pad of the structure, responsible for adhesion, rotates to face away from an adhering surface, Fig. 2. This conformational change effectively switches off the structure’s ability to adhere by drastically reducing the available adhesive area.
Further development of reversible adhesive systems will lead to a new class of materials, able to stick and unstick controllably. These controllable adhesives may find applications ranging from everyday consumer products; such as a nonmechanical car door latching and sealing system; to improving manufacturing techniques with the ability to grip just about any surface; to high-tech niche applications, e.g. microrobotics. Just as this adhesive motif enables creatures such as beetles, spiders, and geckos to climb up and over objects, controlled adhesion will enable small scale robots to surmount obstacles of all sizes – allowing for the exploration of environments inhospitable or inaccessible to man, e.g. the surface of mars or the inside of a burning building.
Fabrication of the multi-scale structures required the integration of two different processing modalities. The nickel platform microstructures were photolithographically defined and etched using standard microfabrication reactive ion etching. The vertically aligned polymeric nanostructures were created through a stochastic growth method. Both methods employ batch fabrication techniques and are scalable to production quantities.
Released 150 nm thick and 130 μm long nickel structures, coated with aligned vertical arrays of stiff polymeric nanorods ~200 nm in diameter and ~3 μm tall, were fabricated using a combination of compatible massively parallel fabrication techniques. The fabrication process began by coating blank 4-inch (100) silicon wafers with a 1.4 μm thick layer of image reversal photoresist (AZ 5214). The negative image of the desired platforms was then transferred into the resist across the entire wafer using a Karl Suss MA6 contact aligner. After developing, a 150 nm thick nickel layer was electron beam evaporated onto the entire wafer. The photoresist was then removed, via an ultrasonic acetone bath, lifting off the excess nickel. The wafer was cleaned and dried and a 7 μm layer of photoresist was spun onto the wafer surface (Shipley SPR 220-7). The positive pattern of the platforms was then transferred into the resist, aligned with the nickel platforms below. The resist and nickel pattern was transferred into the exposed silicon alternating between a highly reactive mostly isotropic SF6 etch and a C4F8 passivation deposition (the Bosch process) effectively etching vertically into the silicon. After etching approximately 30 μm into the silicon, a sustained SF6 etch was performed to undercut the nickel/photoresist platforms. The released platforms were then placed in oxygen plasma with an applied bias between wafer and plasma, creating ~200 nm diameter nanorods, orthogonally to the surface, with an aspect ratio of ~15, Fig. 1.
The structures were characterized using a home-built adhesion test apparatus (Basalt II), Fig. 3 (84). The basic operating principle of the system is similar to an atomic force microscope, but implemented on a larger scale: the deflection of a glass spring is monitored, using laser interferometry, to determine the forces applied to the spring tip. This tip was a glass flat punch of 5 mm diameter. In order to ensure proper alignment between the tip and the sample, the tip was attached to the cantilever with high-strength glue while in intimate contact with the sample stage.
Test samples were placed on the micropositioning stage and moved to near contact with the spring tip. The tip was then lowered using a piezo electric actuator, and proper alignment was ensured through a horizontally oriented stereomicroscope.
Actuation of the probe and data collection was performed using an automated National Instruments LabView™ program. Through calibration of the cantilever (spring constant, k=137.1 N/m) it was possible to determine the interaction forces between the flat punch tip and the test surface. Upon withdrawal from the surface, adhesion produced a characteristic pull-off event, evident in a negative dip of the force-displacement curve. The reversible adhesive was tested with and without Neodymium Iron Boron (Nd2Fe14B) rare earth metal magnet below the silicon chip.
While the gecko setae and spatulae are composed of β-keratin, here a combination of photoresist, silicon and nickel was used to create a 3-dimensional structure actuated through the application of a magnetic field. The photoresist (E = 6.2 +/- 0.2 GPa) is transformed into 200 nm diameter 3 μm tall nanorods, analogous to the β-keratin [E = 1-15 GPa (13)], spatulae of the gecko. These nanorods coat the thin nickel beams and act to enhance adhesion through contact splitting and nanoscale roughness conformation – thus acting as the active portion of the adhesive. The 150 nm thick nickel beams aid in surface conformation (just as the setae in the gecko) and as a deactivation mechanism for the adhesive. The stress mismatch between the photoresist and nickel causes the cantilevers to bend away from the surface. The upwards bend of these beams gives added compliance to a rough test surface by allowing individual cantilevers to bend and conform long before the test surface makes contact with the rigid adhesive substrate. In addition, the upwards bending of the beams isolates the active portion of the adhesive from the substrate.
With the active portion of the adhesive isolated, the properties of the adhesive could then be controlled by actuating the platforms. High-aspect-ratio ferromagnetic structures have been shown to rotate within a magnetic field to align their long axis with the magnetic field vector (85). When the structures were placed on top of a permanent magnet the paddles were observed to rotate about their long axis, Fig. 2.
This rotation is attributed to the preferential alignment of the long axis of the width of the pad in the magnetic field. In order to rotate the paddles in given direction, the stress inducing photoresist was offset on the paddles causing a slight pre-rotation, Fig. 4. The large rotation induced by the magnetic field causes the paddles to turn sideways, concealing the active portion of the adhesive from the test surface, Fig. 2.