Cicada.Over two thousand years ago Aristotle is reputed to have said, “Nature does nothing in vain”. This view of organisms being endowed with an inherent purposefulness is also true when one considers the tiny structures that make up such creatures. Indeed, through the process of evolution, Nature has been able to produce a vast library of intricate structural designs, which in most cases are readily available for scientists and engineers to examine. These natural designs, especially on surfaces, are finely tuned for specific and often multiple functions from self-cleaning to anti-reflection. Animals and plants also need to maintain this functionality in an ever-changing and often hostile environment.
While Nature has been constrained somewhat in its fabrication routes and range of chemistry, it has managed to by-pass this limitation by modifying the surface architecture at very small scales. This refinement of surface structuring is at times deceptively simple in form (see, for example, the cicada wing), however, it can be, and often is, comprised of numerous layers of construction forming more elaborate architectures (such as the termite wing and gecko lizard skin). These tiny features make up an intricate landscape at dimensions only visible with an optical or high powered microscope.
The potential applications of these natural templates cannot be merely based on the simplicity of construction. For example, the small structures shown on the cicada wing represents a surface which is anti-wetting, has low adhesion to dirt and other contaminants, and is anti-reflective. All these features are incorporated into a thin, flexible and transparent membrane. These properties help the organism to shed water from its wings when flying, keep its wings clean, and hide from predators by reflecting very little light. Another interesting feature of this type of surface is that condensation due to dew forming on the outer layer can actively carry away rubbish from the surface by an intriguing droplet self-propulsion mechanism (automatic self-cleaning) whereby two droplets in close proximity spontaneously merge and fly off the surface (analogous to popcorn popping). Even more fascinating, the surface structures have additional properties that do not seem to be a consequence of their intended purpose. For example, these structures also exhibit an antibacterial effect where bacteria are killed upon contact (a property of the surface structure and not the surface chemistry). By contrast, animal cells can be grown on the surface, demonstrating biocompatibility. This relatively simplistic surface exhibits multiple properties that have potential applications beyond their natural purposes in a range of areas including the medical, environmental, and industrial sectors.
Termite (left) and gecko (right).More ornate architectures can be found on a range of other organisms such as termites and small lizards like geckos. These creatures afford us even more sophisticated templates. Tiny hairs on the surface act like springs and are very effective in aiding water droplets to roll off the surface. Small micro grooves along the hair shafts enhance this ability and are found on a range of other insects that also need to resist water. Other star-like features on the termite wing help resist even tinnier water droplets. These examples demonstrate how structuring at different dimensions can cope effectively with environmental liquids and solids over a range of sizes. The hairs on the gecko are spaced much closer than on the termite, which allows them to interact with much smaller objects. For example, as well as being biocompatible with human stem cells, gecko hairs also kill bacteria with similar dimensions to the hair spacing by physical attack.
Scientists, researchers, and engineers can approach the investigation of these natural tiny structures in a variety of ways. The first is knowledge of a particular structure and why the organism utilizes that architecture. In this case, the investigator knows the functions and properties associated with the structure and can try to replicate the natural form. The second, and more difficult, approach is going in without knowledge of either the structure or the most likely properties and functions. This approach does, however, have its merits in minimizing preconceived ideas. Indeed, sometimes this approach is the only option if there is a lack of data on an organism either in terms of structure or behavior.
For our purposes, we believe the next step is to exploit our vast knowledge of specific chemistry and add to or augment the physical effects presented by Nature. In addition, exposure of these interesting structures to environments other than those occurring naturally such as vacuum, aqueous, or marine conditions may yield additional properties and extend the scope of potential applications. We believe that the future of exploration in this area must involve ‘fishing’ for new structures and investigating a gamut of properties, many of which may not seem connected to the functional purpose of the structuring.
References
Wisdom KM, Watson JA, Qu X, Liu F, Watson GS, Chen C-H, Self-cleaning of superhydrophobic surfaces by self-propelled jumping condensate, Proceedings of the National Academy of Science (2013) 110(20), 7992, http://dx.doi.org/10.1073/pnas.1210770110
Green DW, Watson GS, Watson JA, Abraham SJK, New biomimetic directions in regenerative ophthalmology, Advanced Healthcare Materials (2012) 1(2), 140-148, http://dx.doi.org/10.1002/adhm.201100039
Watson GS, Cribb BW, Watson JA, Experimental determination of the efficiency of nano-structuring on non-wetting legs of the water strider, Acta Biomaterialia (2010) 6, 4060-4064, http://dx.doi.org/10.1016/j.actbio.2010.04.016
Watson GS, Green DW, Schwarzkopf L, Li X, Cribb BW, Myhra S, Watson JA, A gecko skin micro/nano structure–A low adhesion, superhydrophobic, anti-wetting, self-cleaning, biocompatible, antibacterial surface, Acta Biomaterialia, (2015), http://dx.doi.org/10.1016/j.actbio.2015.03.007
Watson GS, Cribb BW, Watson JA, How Micro/Nano Architecture Facilitates Anti-Wetting: An Elegant Hierarchical Design on the Termite Wing, ACS Nano, (2010), 4, 129–136, http://dx.doi.org/10.1021/nn900869b