Akron Ascent Innovations is the first company to use electrospinning to produce dry adhesives, which offers the potential for solvent-free, environmentally friendly adhesives that have a remarkable combination of strength and clean, damage-free removability not possible with conventional adhesive technologies.
Dr. Fei Wang, AAI lead chemist, demonstrates lab-scale electrospinning at the Bounce Innovation Hub. A high voltage is applied at the tip of a syringe containing a polymer liquid. As the voltage is increased, the liquid deforms toward the nearest grounded surface and ultimately ejects as a thread with rapidly increasing surface area to offset the electrical charge. Formulations are carefully tuned to balance between the effects of charge build-up, which extend the jet and drive the reduction in diameter, with surface tension, which pulls back and may result in bead or droplet formation. The small collector on the right is used for rapid prototyping of new polymer blends and basic research, particularly the relationship between fiber structure and performance.
Electrospinning is a versatile technique to fabricate nanofibers with finely controlled nanostructures that are useful for a range of applications. Most importantly, electrospinning offers a scalable route to control these structures with a resolution that can typically only be achieved with much more time consuming, batch processes such as lithography.
Electrospun fibers are produced when a high enough voltage is applied to the surface of a polymer liquid to overcome its surface tension. Above this critical voltage, a polymer jet forms and rapidly extends to a grounded collector, decreasing in diameter as it stretches between surfaces. The diameter can decrease by over one million times during this process and result in nanofibers that are only several molecules wide*.
Electron micrograph showing structure of AAI dry adhesive. The dry adhesive is made up on millions of nano-fibers. The individual nanofibers are flexible and can weakly grip a range of surfaces through reversible, non-chemical interactions. The weak forces are multiplied across the fiber network to provide an extremely strong attachment in the direction of the fibers, but low removal force. The result is a non-tacky, high strength adhesive that can easily adjusted, repositioned, and reused without damage or residue.
One advantage of electrospinning is that the rapid decrease in diameter results in a massive increase in surface area, which is very useful for drying the fibers. In contrast to conventional adhesive materials, which often require several drying steps and may still contain a small fraction of solvent after processing, electrospun dry adhesives do not require drying or subsequent processing steps to remove solvent or residual volatile materials. The extreme surface area is the key feature for dry adhesive performance, because it means that the solid fibers can make millions of contacts with surfaces on different length scales without needing to actually flow. As a result, the solid fibers are more resistant to creep over time than pressure-sensitive adhesives, which must be carefully formulated to balance their solid-like and liquid-like character for specific applications.
The small diameter nanofibers produced by electrospinning can be used to strongly engage with different substrates, and are also useful when applied face-to-face. The extreme surface area promotes strong inter-locking between fibers from adjacent surfaces, which is being explored for next-generation hook-and-loop productions, including for apparel, packaging, and other types of closure applications.
* For a remarkable example of the limits of electrospinning, look no further than the work from Professor Darrell Reneker’s lab published in Nanoscale (Nanoscale 2016, volume 8, p. 120-128 - public version available here). They were able to electrospin fibers that were less than 10 molecules wide! This fantastic work allowed for some very impressive experimental testing of single-molecule behavior that are not really feasible with any other methods.
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Microscope image of ultra-thin polymer nanofiber (polyvinylidene difluoride, PVDF) produced by electrospinning. The extremely small fiber diameter allowed for a remarkably detailed study of the motion and organization of individual polymer chains. Previous investigations on similar few-chain dynamics was largely limited to theory and computer modeling. Figure from Lolla et al., Nanoscale 2016 8 120 (figure 3).