Have you ever wondered what inspires the creation of new products like Velcro, or paint that remains dirt-free [1]? Peeking into the secrets of nature has inspired countless technological applications such as these. Responsive, adaptive, and functional, these “smart” materials are becoming more and more pervasive in our society. And yet, still, “smart” objects cannot compare to the proficiency found in nature. With billions of years of evolution on its resume, nature has already optimized the efficiency of many processes. Anywhere you look, you will find a simple and elegant answer to sustaining life developed by nature. For example, sense and response are pervasive in organisms: trees are able to add wood to areas that bear a large load, just as our bodies alter bone density to achieve optimal function [1]. Membranes, from those in cells to the skin itself, are very selectively permeable to allow only certain ions or molecules in and out, which is essential for the day-to-day survival of many organisms, including ourselves. With the natural inspiration of dynamic adaptivity all around us and the availability of microscopes and instruments that are able to resolve materials with high precision down to nanometer scales, scientists and innovators are constantly trying to mimic the wonders of natural materials. The results? Increasingly sophisticated materials that can do things that previously seemed impossible.
What makes a material “smart”?
Man-made materials that are functional or respond reversibly in some way to external stimuli or potential, such as pH, temperature, stress, electric, or magnetic fields are considered to be “smart” [2]. Although these materials don’t actually have brains, “smart” materials can be dynamic and adaptive, lending them to diverse applications, including improving energy efficiency, sensing cancer markers, or even functioning as effective therapeutic devices or as responsive armor for soldiers in war. Smart materials are often made up of different components working together hierarchically down from the nanometer scale (interacting molecules) up to visibly macroscopic scales to produce responsive or functional behavior.
For instance, an index card-sized chunk of smooth wall coated with the smart adhesive “Geckskin” can hold 700 pounds, roughly equivalent to the weight of a Ford pick-up truck [5,6]! So how does it work? Geckskin is a material consisting of a compliant polymer pad, made of a silicone or other elastomeric material, incorporated into a stiffer fabric of a different material such as carbon fiber, polyester, or kevlar. The elasticity of the elastomer allows Geckskin to conform to the wall’s shape and achieve more surface contact, while the stiffness of the integrated fabric helps the material to bear large loads [3,4].
What’s more, the adhesive can then be easily removed from the surface without leaving any residue behind, simply by giving it a light tug. Geckskin works by relying on weak, yet omnipresent and attractive forces known as van der Waals forces that occur between molecules at the surface interface to achieve uniformity of contact. Importantly, since van der Waals forces are very weak and reversible forces between molecules, this interaction allows for reversibility in adhesion between the elastomer and the wall [5,6]. At the same time, the stiffer fabric fibers that penetrate the elastomer prevent too much compliance of the material as it accommodates the weight of the load. Additionally, to allow for the adhesive to be used in a variety of situations where it might support weight at different angles, the developers of Geckskin attached another piece of fabric to the center of the adhesive. This extra piece of fabric allows the load to pull on the center of the adhesive rather than only on the edge and thus prevents de-stabilizing the adhesion. Geckskin has promise for a wide variety of applications in today’s world: hanging everyday household appliances, supporting heavy medical equipment in a clinical setting, or even stabilizing the infrastructures of buildings. Where did the scientists who fabricated such an all-star material get their inspiration? The answer lies in nature.
How does nature inspire “smart” materials?
In order to really understand the mechanisms behind the ingenuity of natural materials, scientists often zoom in further and further to examine the components of the natural material at very small scales. By understanding how features at this scale work to create a certain function, scientists can begin to design synthetic versions of the material that will model the chemistry, physics, or biology seen in nature. But there’s a reason nature is always one step ahead: we often find that cost or resources limit our ability to exactly replicate nature’s efficiency. Yet we are able to come close enough to produce some very interesting things.
Figure 1. From interesting observation to smart material.
The scientists who developed Geckskin drew their inspiration from the tokay gecko’s feet, which allow the gecko to climb not only vertical surfaces, but also to suspend themselves upside down [3,4]! They followed the process delineated in Figure 1. Recognizing the gecko’s amazing ability to cling to surfaces, scientists took a closer look at the its feet, depicted in Figure 2, delving into its structures from the micron scale and up, learning how they integrate hierarchically. Scientists uncovered how these different features in the gecko foot interacted with one another to produce its unique adhesive functionalities. This included looking at how the tendons and skin of the gecko feet worked together while a gecko walked over a vertical surface. They investigated how the gecko skin is intertwined with its tendon and found that the tendon of the gecko is actually quite intricate, branching into many individual pads called scansors, the skin of the gecko’s toe. The flexible scansors that are attached to the tendon allowed for high and conformal surface contact. Further, the tendon attaches the scansor to the gecko’s skeleton, helping to bear the weight of the gecko. It was the interplay of these complex structures that inspired the scientists to formulate Geckskin. The fabric of Geckskin mirrors the tendon’s relative stiffness, whereas the elastomer mimics the conformity of the scansor, which exhibits high surface area contact and reversible adhesion. Furthermore, the attached fabric at the center of the adhesive acts as the tendon “joint” to allow the material to bear more weight at different angles (Figure 2) [3,4]. This understanding of how the structure-function properties of gecko feet work allowed scientists to form a basis from which to design a synthetic version. In the end, the scientists were able to develop reversibly sticky adhesives that leverage smart and dynamic capabilities borrowed from the gecko.
Figure 2. From gecko toe to Geckskin.
Geckskin is just one example of the many smart materials that are becoming more and more pervasive in today’s society. Cutting-edge research and newer equipment are pushing the boundaries of how closely we can mimic nature. All we have to do is notice the little things that surround us and ask how nature has achieved these remarkable adaptations. The answer can be complex or remarkably simple, but understanding nature can change not only how we look at things, but also fuel creativity and innovation.
Ankita Shastri is a PhD candidate in the Chemistry and Chemical Biology program at Harvard University, doing research in an interdisciplinary, bio-inspired materials science laboratory.
References:
[1] AskNature http://www.asknature.org
[2] BBC. Smart Materials. GSCE Bitesize. http://www.bbc.co.uk/schools/gcsebitesize/design/electronics/materialsrev5.shtml
[3] Bartlett, M. D., Croll, A. B., King, D. R., Paret, B. M., Irschick, D. J. and Crosby, A. J. (2012), Looking Beyond Fibrillar Features to Scale Gecko-Like Adhesion. Adv. Mater., 24: 1078–1083.
[4] Gecko feet inspire amazing glue that can hold 700 pounds on smooth wall. ScienceDaily. http://www.sciencedaily.com/releases/2012/02/120216165500.htm