Dry Adhesion Primer

AAI’s dry adhesive technology is based on the principle of contact splitting, which is a robust and reversible (elastic) mechanism of adhesion commonly exploited by animals such as flies, beetles, spiders, and the most prolific climber of all, the gecko.  All surfaces have some attractive force between them arising from weak, inter-molecular forces known as van der Waals (vdW) forces.  This force decreases linearly with the size of the contact, but the resulting stress (force divided by area), increases.  As a result, a surface made up on a large number of small contact sites will have an immensely larger interaction with a target surface than a smooth one.

It is interesting to note that this principle does not hold for “vacuum adhesion.”  The adhesive force of a suction cup increases with contact area, so the adhesive stress (force divided by contact area) is actually independent of cup size.  There are a number of “micro”-suction cup technologies emerging on the market, but the micro-technology actually has no inherent advantage regarding adhesive strength.  The advantage is in failure behavior – any defect in the cup or surface will allow air to enter the suction cup and cause failure.  If there are a large number of small cups, then random flaws in the surface will have less of an effect, but this technology will remain ineffective on porous or high roughness surfaces.  This contact mechanics-based claim is well supported by internal competitive analysis of commercially available products carried out by AAI and presented in a later section.

Van der Waals Forces

All materials have an inherent attractive force originating from random imbalances in their atomic-scale structure.  These forces can arise from a few different sources, but are collectively referred to as van der Waals (vdW) forces.  Despite being extremely weak compared to “stronger” forces such as covalent bonds or electrostatic interactions, these “secondary” forces play an immense role in chemistry, biology, polymer science, and surface science.  The most important feature is that they are ubiquitous – vdW forces exist between everything, but are usually overshadowed by other forces.  It is only when things get extremely small that vdW forces become important.

Smaller = Stronger

Geckos are nature's greatest climbers - they can run up or down just about any surface, regardless if it is wet, rough, or dirty.  These heroic abilities originate from the structure of their feet, which consist of millions of setae that unite to form a self-cleaning dry adhesive.  The setae themselves contain hundreds of spatulae.  Image from Biomimetic Millsystems Lab at Berkley - https://robotics.eecs.berkeley.edu/~ronf/Gecko/

Geckos are nature's greatest climbers - they can run up or down just about any surface, regardless if it is wet, rough, or dirty.  These heroic abilities originate from the structure of their feet, which consist of millions of setae that unite to form a self-cleaning dry adhesive.  The setae themselves contain hundreds of spatulae.  Image from Biomimetic Millsystems Lab at Berkley - https://robotics.eecs.berkeley.edu/~ronf/Gecko/

The inverse relationship between the strength of vdW forces and the size of objects can be clearly observed by comparing the feet (attachment pads) of different climbing animals.  The density of “hairs” extended from the feet of different animals increases strongly with body weight, and geckos have the highest hair density of all.  To accommodate the weight of a larger animal with such weak forces, the structure of the gecko foot pad has evolved many different length scales of complexity.  At the finest scale, there are individual spatulae with diameter of only tens of nanometers (less than 1,000 smaller than the diameter of a human hair) that establish an intimate contact with a surface.  It is here that the extremely weak vdW forces are generated and multiplied across the millions of contact points to generate the remarkable bulk strength that allows the gecko to run up walls and on inverted surfaces.  It is this extremely fine structure that also imparts the self-cleaning characteristics to geckos – dust and other contaminants cannot remain in the dense “forest” of nano-hairs and are expelled.  On a more coarse scale, the individual setae with their spatula tips are organized into pads on the micrometer scale, which are themselves organized into discrete regions on the millimeter scale.  It is this hierarchical structure that allows for the gecko to run up and down surfaces in apparent defiance of gravity.

Bio-Mimicry

Processing steps to prepare PDMS micropillars for dry adhesive applications.

Processing steps to prepare PDMS micropillars for dry adhesive applications.

In order to truly replicate this vertical “forest” structure of a gecko, and to achieve similar performance characteristics, then this same fine attention to detail is necessary.  There needs to be long setae made of a semi-rigid polymer to allow for compliance while maintaining stability, and the setae need to be curved at the end to increase surface area.  The synthetic setae need to be sufficiently separated so that they do not clump, which requires careful consideration of the balance in mechanical properties, and should be consolidated into separate “pad” regions to promote non-directional and reversible adhesive characteristics.  There have been remarkable successes in mimicking these fibrillar structures across length scales by a number of research groups at universities around the world, but the precision required almost certainly precludes this avenue from commercial application.  It is truly remarkable that such a fine structure can be achieved on a micrometer scale (and even more rarely on millimeter or centimeter length scales), but the extreme conditions and fine molding requirements necessary will make any scale-up of design prohibitively expensive and extremely flaw-sensitive.

Even on these small scales, the achievable shear strengths of “fibrillar adhesives” are relatively modest, typically on the order of 10 pounds per square inch (psi) or smaller, and with very limited reusability.  One notable exception is for carbon nanotubes that are growth with a dual-layer catalyst bed to promote an initial horizontal growth phase followed by a protracted vertical growth during a low-pressure vapor deposition process.  This strategy results in dense forests of vertically aligned carbon nanotubes with flattened tips that mimic the spatulae ends of gecko attachment pads.  When the forest is removed from the catalyst and carefully inverted, the flat regions on the nanotube ends provide enhanced contact area with smooth surfaces.  Qu and co-workers reported shear strengths on the order of 150 psi on glass for ultra-long CNTs prepared with this approach [Qu et al., Science 322 238 (2008)], which is the highest value reported so far for a dry adhesive.  On rough surfaces, the shear strength was considerably lower, which appears to be a persistent problem for fibrillar dry adhesives.  Crosby and co-workers recently evaluated this limitation and proposed a composite approach using a combination of a soft elastomer to conform to a target substrate and a rigid fiber phase to promote an elastic mechanism for reinforcement and distribute contact loads over a large area [King et al., Advanced Materials 26 4345 (2014)].  AAI’s A’Qat™ dry adhesive has a similar combination of soft and rigid phases that are thought to be responsible for the high performance on rough substrates with easy peel and high reusability.

AAI's Dry Adhesive Strategy

Schematic showing concept of applying electrospinning as a polymer blending technique to formulate adhesive nanofibers with high adhesion strength and conformability to surface asperities.

Schematic showing concept of applying electrospinning as a polymer blending technique to formulate adhesive nanofibers with high adhesion strength and conformability to surface asperities.

At AAI, we have adopted a different approach based on the mechanics exploited by geckos (large number of extremely long, finely structured contact points), rather than the actual fibrillar structure, which appears to suffer from a number of limitations in form and function.  This strategy first emerged with the study based on a simple dead-weight experiment of aligned electrospun nanofibers of nylon 6 (average Tg of 47°C) funded by Dr. Wong’s NSF CAREER award under the Materials Processes and Manufacturing Program.  The fibers, which were deposited by the single-step, scalable electrospinning process, are in the solid state at room temperature, in contrast to elastomers used for PSAs that transition from a glassy to rubbery state (glass transition temperature, Tg) at sub-ambient temperatures and are viscous liquids at room temperature.  The highly aligned fiber arrays showed remarkable shear strength of nearly 40 psi, but were easily peeled and able to be reused.  This was the first demonstration that dry adhesion produced by nanoscale cylindrical solids (nanofibers) is as strong as, and more reversible, than wet adhesion at room temperature and formed the impetus for the formation of AAI.

The research and product development led to a series of preliminary studies evaluating adhesive capability of electrospun polymer species by AAI’s CTO, Prof. Josh Wong, and coworkers.  These studies were greatly benefited by leveraging the collective expertise of electrospinning afforded by UA.  Electrospun polymer nanofibers are vastly different from previous research on dry adhesives - the electrospinning process and resulting laid-flat nanoscale cylindrical solids are unique technologies in their own right.

The extremely small diameter of the nanofibers allows for the contact area between the nanofiber mat and the target substrate to be maximized, whereas the horizontal deposition allows for easy scalability through a coating line process similar to what is used today for PSAs.

We found that this approach was sufficient to provide excellent performance on smooth surfaces, but to tailor performance to real-world surfaces, such as painted drywall, a polymer blend strategy can be adopted.  High Tg and highly electro-spinnable polymers can be blended with controlled weight fractions of acrylics, silicones, elastomers, and tackifiers to enhance their conformability to surface asperities, viscoelasticity and reusability, with an engineered balance of wet and dry adhesive mechanisms.  The flow of the dispersed components also augments the strength of physical interactions such as overcoming surface tension and surface roughness effects.

Without blending, a polymer such as nylon 6 can be envisioned as a homogeneous material.  With discontinuous and co-continuous blends, different morphologies can be promoted to achieve synergistic effects.  Prior to this work, most art in polymer blends was performed using extrusion compounding or other shear mixing operations.  The nanoscale A’Qat™ products are being developed as polymer blend nanofibers for the first time.

This unique combination of a rigid, fiber-forming polymer and a soft polymer in a proprietary blend formulation has formed the basis of AAI’s approach to the next generation of adhesive materials.  These nanofibers are able to conform to surface asperities and mechanically interlock between mating faces.  Shear strengths as high as 128 pounds per square inch (psi) have been achieved on steel substrates using this approach.  The AAI A’Qat materials show good shear adhesion with easy peel to a wide range of substrates including metal, glass, painted walls, white boards, cardboard, cured rubber, and paper.  The nanoscale mechanical interlocking shear adhesion between mating faces is also strong (> 65 psi).