Human beings have evolved into an extraordinary species with great command of science, engineering and medicine capable of outstanding feats of engineering and even landing human beings on the moon. However, there are countless plants, insects, reptiles and even single cell organisms which have existed for billions of years that are routinely carrying out chemical processes and creating structures that human technology struggles to mimic.
Nature has utilised a number of strategies to compete and thrive in difficult environments and one of the most interesting is the use of surface topography. The development of advanced microscopy techniques over the years has revealed a myriad of complex micro and nanometre surface patterns and structures on the surface of plant leaves, insect wings shark skin, snake skin, sea coral and a multitude of other creatures. Painstaking model surface studies carried out using advanced lithographic and etching techniques to replicate similar structures on metals indicate these surfaces can impart valuable properties such as hydrophobicity, frictional characteristics, adhesion, light scattering and diffusion and anti-bacterial effects.
The lotus leaf effect is extremely well known. The surface of the leaf consists of a multitude of nano-features superimposed on an array of 10-15-micron wide cone structures. This hierarchical structure is extremely complex and in the presence of natural waxes results in super-hydrophobicity and self- cleaning properties with contact angles in excess of 150 degrees. Probably less well known is that there are many plant species which use different surface topographies for different purposes. There are ferns, (S. molesta), which grow beneath the water level that have developed surface whisk-like hairs resulting in extreme water repellence while submerged. Certain flower petals have a different denser structure which have high contact angles but trap the water in droplets on the plant. Changing the surface wettability of common materials to impart new properties is of great interest to industry.
a) Lotus leaf hierarchical structure
Snakes have a unique ability to travel across rough terrain at speed which suggests a very low friction contact to prevent excessive wear on the epidermis. Indeed, studies indicate that scales of snake bellies have evolved to exhibit a succession of hierarchically micro/nano patterned grooves which run along the direction of travel reducing wear in the presence of a natural exudate. Studies on the Black Viper confirm the theory that frictional effects at the nanoscale are quite different from the macroscale such that very significant reductions in friction coefficient are observed. Reducing friction between industrial components is of great importance in many aspects of engineering.
The Cicada insect wing has been extensively studied and microscopy reveals the surface is covered by exceptionally small nano-pillars. These structures are reported to be biocidal in that bacteria cell membranes are ruptured on landing. (Ivanova et al). Other plant and insect structures are considered non-biocidal but anti-fouling and less prone to bacterial colonisation by virtue of the hierarchical patterns.
Black viper on rough terrain.
Numerous studies on model surfaces have confirmed that certain structures and patterns particularly hierarchical patterns reduce the bioburden at a surface. Such anti-fouling properties may be of value in the hulls of ships, industrial pipes or indeed in medical devices such as catheters and medical tubing.
Production techniques such as Roll to Roll manufacturing where lithographic patterned plates are used to stamp micro-patterns or indeed nano-patterns onto, for example, a flat uv curable coated polymer film for packaging/light management applications are available. Micro injection moulding and advanced 3D printing are also being developed for complex shapes. However, these are normally not hierarchical patterns and the full range of properties seen in nature are not exhibited.
Although small areas of model surfaces have been constructed to mimic these natural hierarchical features by a variety of lithographic and etching techniques mass production of these complex structures particularly on three-dimensional “soft” surfaces, (plastics/elastomers), has remained exceptionally difficult.
Biomimetica now offer a new process technology which allows rapid production of hierarchically patterned three-dimensional plastic/elastomer products. This includes a range of tubular, spherical and complex objects as well as flat film. The company is working with clients to develop applications of the technology including biocidal, anti-fouling, low friction, light management and anti-counterfeit applications.
Examples of Biomimetica Multiscale Printing Technology
a) Hierarchical Lotus Leaf structures on PVC
b) Hair-like structures on elastomer surface