Mother Nature has proven to be an inspiration for the development of adhesives, particularly because of the lizard known as a gecko. This animal has shown an extraordinary ability to attach to any surface no matter whether the orientation is vertical or even upside down.

In a previous TLT article,¹ a discussion was held (complete with a cover picture of a gecko) about how a gecko attaches itself to surfaces. The authors indicated that a gecko has a dry attachment system that consists of micro/nanostructures known as small hairs on their pads. These tiny hairs are able to adjust to the shape of surface irregularities enabling the gecko to adhere to the substrate no matter the orientation. At the end of the hair are multiple branched, flattened structures known as spatulae that interact through van der Waals interactions with the surface.

A second TLT article² describes a macroscopic model that was developed to show how a gecko releases from a specific surface. The model involved the use of small magnets attached to a piece of spring steel. The magnet emulated the van der Waals interactions while the spring steel represented a spatula. In pulling the magnet from a bearing surface, a researcher developed a relationship between normalized loads and normalized cantilever lengths to predict how the gecko detached from the surface.

Previous efforts to prepare dry reversible adhesives have focused on mimicking two types of structures: passive attachment hairs specific for certain male leaf beetles and muscle-activated attachment hairs that evolved independently in many insects, spiders and some lizards. STLE-member Michael Varenberg, assistant professor in Georgia Tech’s George W. Woodruff School of Mechanical Engineering, says, “The mushroom-shaped geometry of passive adhesive hairs in beetles was found to be relatively easily reproduced, and it served as a model for a first generation of working dry adhesives. The spatula-shaped geometry of active adhesive hairs in insects, spiders and lizards was more difficult to replicate, so it took longer until working shear-activated adhesives were reported. However, regardless of their geometry, all synthetic adhesive microstructures have been created through the use of molding.”

Techniques such as photolithograph, laser micromachining and ultraprecision cutting are used to produce molding templates for these adhesives. Varenberg says, “These techniques are expensive, time consuming and complex.”

To find an alternative, Varenberg considered examining drawing after finding that this technique can be used to prepare thin filaments from polymers. Varenberg says, “We noted that the epoxy-based polymer, SU-8, can be drawn to form thin and long fibers that have micron scale diameters. We assumed that this process also might work in polymers we use in making our bio-inspired microstructures.”

Varenberg and his associate have now employed drawing to produce reversible adhesives.

Razor blades
The researchers produced reversible adhesives through a four-step process. Initially, the drawing array and a polymer layer with uniform thickness are prepared. Uniformly spaced and aligned elements consisting of thick or thin common laboratory blades are used in the drawing process where sharp or blunt edges of the blades can be employed. Office adhesive tape was used to evenly space the U-shaped (blunt) drawing elements while the V-shaped (sharp) drawing elements were set up without tape. Microstructures were prepared on an area at least 2.5 millimeters in width with a gap between thick and thin drawing elements being between 240 and 60 microns.

Two polymers, polyvinylsiloxane (PVS) and polyurethane (PU), were used to prepare adhesive surfaces. Each polymer was partially cured before being applied to the glass slide substrate to ensure sufficient viscosity was established so it did not flow out. Varenberg says, “We have used PVS for several years because this material cures within about five minutes, which makes it very convenient in operation. PU is used because it is resistant to wear, fatigue, cutting and chemicals, which makes it attractive for industrial applications.”

The drawing array is then dipped into the curing polymer taking distance, speed and time into consideration. Optimizing these parameters is important to minimize possible necking, capillary rise and wrinkle formation on the polymer surface.

The actual drawing process is the third step where the blade array is withdrawn from the polymer layer. Varenberg indicates there are three critical parameters that must be considered. He says, “The distance the razors need to be moved, the speed of the process and the starting time will all impact the geometry of the microstructured surface.”

The fourth step concerns holding the drawing array motionless to enable the polymer to completely cure, leading to the release of a fully polymerized structured surface (see Figure 3, which shows a schematic of the four-step process).


Figure 3. A four-step process was started by placing razor blades into a setting polymer, which leads to the production of reversible adhesives through a drawing process when the razor blades are pulled out (see nearest image on the right). The reversible adhesives perform in a similar manner to a lizard known as a gecko (see image on the left). Figure courtesy of Georgia Tech/Varenberg lab.

The researchers prepared microstructures with different geometries and compared their pull-off force/adhesive strength to the molded adhesive surfaces. Varenberg says, “We found several of our best microstructures produced a shear-activated driven pull-off force with an amplification factor (force ratio of activated over disactivated stated) of 40 that is superior to the value of 15 achieved with a molded wedge-shaped adhesive. This result demonstrates that reversible adhesives can be successfully produced using drawing, which is a much simpler and less expensive method than molding.”

Static friction force also was measured to assess the frictional strength of PVS- and PU-based adhesives. The results are similar to what was found with the data produced on pull-off force. Varenberg says, “The frictional force results demonstrated are very high, though we found that the PVS figures are lower than those from molding and from PU.”

In general, the results shown with PU were somewhat inferior to PVS. Varenberg says, “We will be examining the mechanical properties of PU to determine if we can find an approach for improving its shear-activated adhesive performance.”

Additional information on this research can be found in a recent article³ or by contacting Varenberg at varenberg@gatech.edu.

REFERENCES
1. Singh, R., Yoon, E. and Jackson, R. (2009), “Biomimetics: The science of imitating nature,” TLT, 65 (2), pp. 40-46.
2. Tysoe, W. and Spencer, N. (2015), “Gecko’s feet and sticky tape,” TLT, 71 (6), pp. 108-110.
3. Kim, J. and Varenberg, M. (2020), “Drawing-Based Manufacturing of Shear-Activated Reversible Adhesives,” Applied Materials & Interfaces, 12 (17), pp. 20075-20083.