«UNIVERSITY OF CALIFORNIA Santa Barbara A Micro/Nano-Fabricated Gecko-Inspired Reversible Adhesive A Dissertation submitted in partial satisfaction of ...»
Adhesion testing of the structures, without an applied magnetic field, produced unloading curves with a characteristic pull-off event shown in figure 4 (upper inset).
The pull-off force was observed to vary with the maximum applied normal load (due to slight misalignments between the flat punch and the test surface) until a saturation adhesion strength of ~14 Pa was observed (obtained by dividing the adhesion force by the projected area of all pad surfaces), Fig. 4. It should be noted that this is a purely adhesive measurement testing in the normal pull-off direction, whereas reported values for the gecko test in the transverse frictional direction (86), making comparisons between the two systems tenuous.
Alignment issues, surface inconsistencies and unknown probe geometries have presented difficulties in quantification of this new class of bio-inspired non-pressuresensitive-adhesives. One suggested metric is to simply divide the adhesion force by the maximum preload force, μ’= Fadhesion/Fpreload (86). In this system the maximum μ’ value was found to be 1.47 +/- 0.4, occurring at the minimum pre-load with an observable pull-off event (limited by the noise level of the instrumentation). This value offers a substantial increase from previous synthetic work with μ’ values of 0.125 (83) and 0.06 (11), but still falls short of the gecko with μ’ = 8 to 16 (86).
In contrast to the adhesion seen in a rest state, the application of a magnetic field to the structures produced a catastrophic loss of adhesion, Fig. 4. The minimum negative force detected was 0.37 +/- 0.28 Pa (compared with 14 Pa without a magnetic field). For no tests on the structures with an applied magnetic field was there an observable pull-off incident. This complete reduction in adhesion is attributed to the concealing of the nanorod-coated platforms from the test probe.
Subjected to a magnetic field, the platforms rotate to align themselves with the magnetic field lines. The rotation leaves the edge of the platforms facing in the normal direction and the “sticky” face to the side. Thus when a surface approaches from the normal direction it only contacts the edges of the platforms. Since the edges of the platforms provide very little surface area, and have no nanorod coating, very little adhesion is produced – less than the noise in the instrumentation.
Additionally, a decrease in surface compliance was seen in the structures with an applied magnetic field. The twisting of the cantilevers increases the second moment of area of the structures, relative to the indenting tip, increasing the stiffness and consequently reducing the compliance of the system. Ultimately, the sideways turned paddles will contact the underlying substrate and statically block an adhering surface from contacting the support substrate – completely turning off adhesion.
In this paper, a novel approach has been presented for creating a synthetic analogue to the gecko adhesive system. The hierarchical system is composed of aligned vertical nanorods coating flexible micron scale cantilever paddles. The paddles, composed of nickel, rotate when subjected to a magnetic field. This rotation conceals the nanostructures on the paddle surface and greatly reduces the available surface area for adhesion. Testing of the system showed reversible adhesion behavior switching from a μ’ value (Fadhesion/Fpreload) of 1.47 +/- 0.4 (largest reported value for a biomimetic system to date (86)) to less than the noise level in the instrumentation. Thus an active hierarchical structure has been fabricated and demonstrated to display controlled and reversible adhesion. Further development of switchable adhesives will find applications ranging from everyday consumer products such as latching and fastening systems; to high-tech applications, such as enabling microrobotics to explore extraterrestrial surfaces or harsh climates otherwise not accessible to man.
Figure VIII-1 – Electron micrographs of synthetic structures (left) and the analogous gecko structures (right), samples from a Tokay Gecko (Gekko Gecko). (A) Paddle surface coated with evenly spaced uncondensed aligned vertical polymer nanorods (left) and the branched terminus of a seta into spatulae (right), same magnification and scale bar 10 μm. (B) Freestanding nickel cantilevers and paddles coated with nanorods (left) and an array of setae (right), same magnification and scale bar 50 μm. (C) Low angle view of cantilevers showing upwards bending of the structures relative to the solid substrate (left) and a profile view of curving setal stalks (right), same magnification and scale bar 50 μm. (D) Lower magnification view of a portion of the synthetic array (left) and the setal array (right), scale bars 500 μm (left) and 200 μm (right).
Figure VIII-2 - Stereomicrographs of the adhesive: (A) in the ‘ON’ state, no applied magnetic field, with the adhesive paddles facing vertically; and (B) in the ‘OFF’ state, with an applied magnetic field rotating the paddles sideways, concealing the adhesive faces. Scale bars, 100 μm.
interferometer monitors the deflection of a glass cantilever spring as a piezo actuator moves a 5 mm glass flat punch into and away from the test surface.
The interaction forces are calculated by relating the stiffness and deflection of the cantilever upon contact with the surface.
Figure VIII-4 - Adhesion results showing the on/off behavior of the structures without and with an applied magnetic field, respectively. The insets represent actual adhesion data, where in the ‘ON’ state distinctive pull-off events were observed (top) and in the ‘OFF’ state no pull-off events were observed (bottom). Strength values were obtained by dividing the interaction force by the contact area of the paddles. In the ‘ON’ state, the devices showed an initial increase in adhesion with preload force, characteristic of increased surface contact with applied load (likely a result of slight misalignment between the 5 mm flat punch and test surface). Error bars represent 10 data sets at a specified displacement with no emission of outliers.
IX. Concluding Remarks and Future Work In this thesis, novel fabrication strategies have been presented and implemented to create a bio-inspired adhesive system. Characterization techniques developed to qualify the biomimetic structures, indicate that the hierarchical system found in nature may serve two purposes. One, the hierarchical structure enhances surface compliance, increasing the surface contact area, and subsequently increasing adhesion. Second, the hierarchical structure may be necessary for a detachment mechanism.
In the first instance, a hierarchical system (nanostructures atop microstructures) was shown to enhance adhesion, over a one component system (nanostructures or microstructures). This enhancement is attributed to the microstructures bending to allow for more surface contact, while the nanostructures provided the short range interactions necessary for adhesion. In addition, the platforms, when bent, change the orientation of the pull-off force, translating some of the normal pull-off force into a shear frictional force.
While the dry sticking properties of these biological systems are of great interest, it is the combination of sticking and unsticking that may provide the greatest technological impact. Here again it was shown that mimicking the hierarchical structure of gecko is beneficial. To illustrate this point, compare the force required to separate two blocks, with a piece of double sided Scotch™ tape between them;
with the force required to peel a piece of tape off of a flat surface. Clearly peeling facilitates the removal of the tape. The peeling action deforms the tape backing, which, for the confines of this discussion, represents another level of hierarchy in that system. In this work, unlike with tape, a surface was created that is controlled to switch adhesive properties. This was accomplished through creating flexible micrometer scale paddles that can undergo a conformational change. The paddles, made of nickel, change orientation when a magnetic field is applied. In the ‘on’ state, the paddles face up, presenting a sticky nanorod surface. In the presence of a magnetic field, the paddles rotate to face to the side, presenting a single thin edge.
The adhesion change was found to be some 40 fold. While this is not the exact mechanism of the gecko, it does demonstrate the use of hierarchy to create a release mechanism.
While a proof of principle has been developed for a reversible adhesive, further work will need to be done to create a working prototype. Future work will need to focus on extending the underlying design principles presented here. While this work has rapidly developed a new research area, there is now much work to be done to optimize individual components of the system and intelligently integrate them into an overall system.
The obvious place to start optimizing is at the beginning, in this case that would be where the adhesive first makes contact – the nanorods. While the nanorods are the right overall length and width, have an ideal modulus (6 GPa compared with 1GPa for the gecko), and are spaced without condensation, refinement of the tip geometry will significantly enhance adhesion. The work of Arzt et al. has shown that tip geometry can play a significant role in the adhesive properties (87). The gecko, as well as other creatures using this adhesive motif, has flat spatulas at the end of the nanorods. The spatulas are approximately 200 nm in width and 5 nm thick. The ultra-thin spatulas are able to easily deform to the contact surface, making intimate contact and greatly enhancing the van der Waals interactions – remember the attraction scales with the inverse square distance. Further development of the nanorod structure to include flat thin tips will greatly enhance the adhesion of the surface.
Further enhancing the adhesion in the spatulas is the direction of applied force.
The gecko achieves maximum adhesion when the pull-off force is at a 30° angle from the setal stalk to the surface. It is at this angle that both the frictional component of adhesion and the normal component are at a maximum (88). The prior is due to optimum combination of applied normal load maximizing the frictional component, and the latter due to a compressive force at the base of the spatula arresting crack initiation. The important lesson is that the appropriate force must be applied to the macro structure to cause the microstructure to exert the optimum force on the nanostructure. This will require a systems engineering approach to design microstructures capable of transmitting loads intelligently to the nanostructures.
Preliminary work has been done to create 3-dimensional microstructures with built in curvature, Figs. IX-1. When the structures are contacted, the load causes the structures to bend, creating a pre-stress similar to the pre-loaded setae in the gecko.
This pre-stress induces a resolved lateral force as well as a normal force. While the gecko uses a squeezing action to enhance adhesion through the incorporation of a frictional force, these structures now induce a frictional force, passively and at the microscale. In figure IX-1 two sets of opposing paddles bend towards each other. In this configuration, when a surface makes contact with the paddles, the paddles will bend down and away from each other—creating a component of lateral force between the surface and the paddle face.
Figure IX-1 Electron micrographs of a double paddle configuration. The paddles are bending out of the base plane. By opposing each other the two paddles create a squeezing action, transforming a pure normal force into a combined normal and lateral force, enhancing a frictional component.
While the paddles presented in figure IX-1 induce a lateral force component and should aid in adhesion, they clearly are not an optimized surface for adhesion. To maximize adhesion, the contact area between two surfaces should be maximized. In the case of the hierarchical structures this requires increasing the active area of the adhesive, the presented nanorod coated surface. While in chapter VIII a reversible adhesive was presented, the overall adhesion strength is quite low. The primary method of improving this adhesion is to create structures with a greater contact area.
Again, with the lithographic processing employed here, this should be no problem.
It is simply a matter of better designing an optimal microstructure to increase contact area. In figure IX-2 an example of this presented. Instead of having a long beam supporting a single paddle, a thin beam extends with paddles radiating along the length. In this manner, the beam maintains its flexibility while the presented contact area is increased. While two opposing paddles produce a bilateral squeezing force, more complex shapes may produce alternative adhesion forces. Again using the robustness of photolithography creative designs may be implemented to further enhance adhesion, Fig. IX-3.
Figure IX-2 Electron micrograph of a branched fingers bending out of plane. The branch fingers enhance the beams bending while providing surface
There may be many other approaches to improving adhesion. These approaches will be enabled through the extension of a simple and robust fabrication process.
The process based on photolithography will enable improved design of the microstructure. At the same time additional processing steps may be added to incorporate new levels of sophistication. One obvious place for the next level of innovation is creating the next level of hierarchy, moving from the micro-scale to the milli-scale. This will be the last gap to close between the state of art in robotics and this new class of bio-inspired adhesives.
Figure IX-3 Electron micrographs of eight fingered branched platform structures coated with aligned vertical nanorods. The fingers are bending up out of the plane of the center support. Scale bars, in descending order, of 20, 100, 200 and 1000 μm.
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