In trying to find natural systems to study for my dissertation project, I’ve noticed what I see as a gap between knowledge and products. This trend is similar to what Bill described a couple of weeks ago with spider silk. We see many cool natural materials, but we don’t know exactly how to make them yet. For instance, earlier this year, a new contender for the stronger natural material was discovered in limpet teeth. The tensile strength of this biomineralized material was measured to be 3.0 to 6.5 GPa.1 To put that in perspective, that is about 100 times stronger than typical polymers and even 5 times stronger than steel.2-3 We know the mineralized goethite structure is high strength, and we know some of the mechanisms of biomineralization. However, having an unbreakable plate based on the limpet tooth may be a ways off.
I think there are a few factors which create this gap from inspiration to products. One is the ways in which we create our products. Melting and quenching is common throughout metals and polymers with casting, injection molding, extruding, etc. If we want to make nanoscale designs or objects we can use lithography, deposition, and other techniques. Nature uses nearly ambient temperature and self-assembly to create hierarchical materials over many length scales. Unless we can find scalable methods to produce hierarchy, we will not be able to create products based on these cool natural materials (or they will have very niche applications).
This leads into what I really think will be the limiting factor for biomimetic materials: complexity. Natural materials are not simple. There are many factors influencing a wide variety of functions. For instance, take the gecko adhesion system. It provides a directional, reversible, non-matting, non-sticky by default, and self-cleaning adhesion system which attaches strongly with minimal preload and detaches quickly and easily.4 The early thought for the gecko adhesion system was to look at the hair structure. The first synthetic versions simply created small pillars to try to achieve the Van der Waals adhesion characteristic of the gecko adhesion system.5-6 However, just copying one aspect of the gecko structure did not lead to all of the properties listed above. As more research was performed on the gecko itself, more aspects of the setae were discovered, which led to shifts in the synthetic versions. Two notable examples are the addition of spatulae for the pillars and oriented synthetics.7-8
I think we will need to be patient with the research of biomimetic materials. The systems which we are trying to mimic are complicated, and it would be hasty to believe that our first shot at a synthetic material without sufficient knowledge of the natural system will produce a successful product. It will take time, but proper understanding of the natural system will help to highlight the structures necessary to produce desired functions within synthetics. In addition, producing synthetics as we go can help illuminate the structures responsible for functions as well. If we realize that we may not get it right on the first try, then we can create realistic expectations for the realm of biomimetic materials. There are biological materials out there with fascinating properties, and with time, I believe we will be able to learn from nature and synthesize our own materials.
Asa H. Barber, Dun Lu, Nicola M. Pugno. J.R. Soc. Interface 2015 12 20141326; DOI: 10.1098/rsif.2014.1326.Published 18 February 2015.
Autumn, K. “Properties, Principles, and Parameters of the Gecko Adhesive System.” Biological Adhesives. Springer: (2006).
Sitti, M. and Fearing R. IEEE-Nano. (2002)
Geim et al. Nature Materials. (2003)
Lee et al. Applied Physics Letters. (2008)
Northen et al. Adv. Mater. (2008)