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Professor Hanington's Speaking of Science: The science of gripping
Professor Hanington’s Speaking of Science

Professor Hanington's Speaking of Science: The science of gripping

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When you use Velcro you are joining two surfaces together with what is called a probabilistic fastener. The little hooks of one piece grab and intertwine with the wispy fabric of the other, allowing a gripping force that sometimes is not easy to dislodge. Attachments such as these via mechanical interlocking of three-dimensional protruded features is of importance for many items in our daily lives and also for numerous species in nature. In fact, we covered Velcro in my May 2014 article where I told the story how Swiss electrical engineer George de Mestral had gotten the inspiration for this invention while picking burrs from his dog’s fur.

But burdock plants are not the only living thing to use “hook and loop” attachment mechanisms. Consider the climbing ability of the Gallium Aparine plant. Sometimes called a cleaver plant, they are annuals that have creeping straggly stems which branch and grow along the ground and even sometimes over other plants. Because the stalks can reach in excess of three feet in length, they must attach themselves to nearby surfaces or run the risk of buckling and cutting off their internal nutrient supply. By using small hooked hairs that grow out of the leaves they grab every little nook and cranny they can and prop themselves up for safety.

In the insect world, attachment via mechanical interlocking of three-dimensional knobs and alleys is of importance for many species. Taking a look at the Odonata Anisoptera or common dragonfly, they come equipped with a system for locking their head in place that consists of muscles and small hairs on the back that grip structures on the front of the first thoracic segment. This arrester system is unique to the Odonata order and is activated when feeding and during tandem flight to protect its slim neck. Likewise, many other similar fixation systems found in numerous other species such as wasps, bees, and beetles have developed to securely attach their wings to the body while at rest.

Last week, researchers at Wageningen University — located in the picture postcard tiny town of that name in central Netherlands — published an interesting paper in the journal Biointerphases called: “Hooked on Mushrooms: Preparation and Mechanics of a Bioinspired Soft Probabilistic Fastener”. In that report the team, under Professor Joshua Dijksman, describes a surface patterned with soft micrometric features inspired by the mushroom shape that shows a nondestructive mechanical interlocking feature and one non-destructive to fabrics. Looking at the photo you will see that it’s a novel structure that will rival Velcro someday because the gripping hooks have been replaced with tiny mushrooms that somehow fit into the cavities of ordinary fabric. Thus only a one sided fastener is needed.

The group initially studied different types of grabbing surfaces such as the pestle shape as well as the mushroom form and compared them to standard Velcro hooks. It was found that peg-like or pestle shapes created the least amount of damage to the fiber weave and lint of the fabric they were attaching to.

Unfortunately they also displayed the weakest detachment force in a perpendicular direction as compared to the mushroom shaped geometry. It became clear that features with overhangs were required to enhance attachment strength normal to the surface. Having soft patterned surfaces with soft interlocking features would thus be an innovative step toward a new class of safe and residue-free dry adhesives that can actually attach to fabrics of different mesh sizes and potentially even to other surfaces with microscopic roughness. They made a scalable production array of tiny mushrooms by combining simple 3D printing and special lithography to achieve microscopic patterns of the soft polymer.

The molding procedure is quite interesting. First you create a molding pattern as shown in the figure with this article from a 3D printer. Unfortunately, the cured resin from the 3D printer is too stiff to use as a grabber because its Young’s modulus is 2.8 GPa — comparable to polycarbonate — and repeated attachment and subsequent detachment from an opposing surface in fabric can break either the small 3D printed features or damage the fabric itself. Therefore, they replicated the 3D printed structure with a silicone elastomer, using a double molding process. Now the mushroom was rubbery.

Samples of identical size with varying feature densities up to 83 mushrooms/square centimeter were used to test the attachment performance on a nylon based synthetic fabric. Here they found a gripping force of over 200 mN per mushroom – more than enough to act as a button if enough microscopic mushrooms were used. They also found that the samples adhered to all fabrics. They suggested that in future work the mechanical properties of the silicone can be varied to get larger adhesion capabilities.

If this works, your shirts someday will just have a strip of gripping mushrooms along the front. No more buttons to lose.

Gary Hanington is Professor Emeritus of physical science at Great Basin College and Vice President of Engineering at AHV. He can be reached at garyh@ahv.com or gary.hanington@gbcnv.edu.

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