There are about 450,000 different species of land plants on the earth and the most complex land plants emerged 500 million years ago in the Ordovician or Silurian period. If we include grasses such plants cover at least 50% of the worlds land surface. The vast majority of land plant leaves exhibit superhydrophobicity and we know, from well-known examples, that plant leaves utilise surface structuring in combination with natural waxes to achieve these properties. However, evolution and selection processes over millions of years have refined complex surface topographies to impart a wide array of other properties which should be of great interest to product designers in companies.
Tens of thousands of species have been examined in detail by optical, SEM and AFM microscopy and a great deal is known about the variety of structures and the purpose of the architecture.
For example, plants utilise hierarchical surface structuring to manage both the reflection and absorption of light. Leaf surfaces which harvest light for photosynthesis in low light conditions often have convex shaped epidermal cells. These structures increase internal surface reflection between and within features to minimise loss of light. Certain mosses which occur only in caves have incredibly complex lens-like surfaces for this purpose. These properties are clearly of interest in solar cell developments and sensor technology.
In order to flourish plants must attract pollinators such as bees. Not surprisingly surface topography is used to intensify petal colours by internal scattering of incident light, for example, in Viola tricolor. Examples also exist of flower petal surfaces which use UV optical signals or iridescence to attract pollinators. There are existing examples of flat micropatterned polymer films being used to manage light in the commercial lighting industry but the range of properties achieved without hierarchical structuring found in Nature are small in comparison.
Dark cave mosses.
In Death Valley California the average summer temperatures are around 47C with the highest temperature recorded there being 56.7C. Plant surfaces can only tolerate temperatures below around 45C and utilising only transpiration/water loss to cool is not practical for desert plants. Surface reflection of light by structuring is a tactic employed but it is now believed that surface induced mechanical turbulence in the presence of light winds may improve heat exchange between the plant surface and the cooler wind. Indeed, it is the case that many cacti exhibit extreme forms of hierarchical surface structuring.
Other cactus species in the Atacama desert have surfaces which are superhydrophylic thereby enhancing wettability to harvest moisture from fog. Most water plants are superhydrophylic but retain an ability to introduce hydrophobicity in new shoots above the water line.
Some plants exhibit super hydrophobicity not by dense surface structuring but by virtue of microscopic hairs. An example of this is found in Lady’s Mantle (Alchemilla).
Super Hydrophobic Surface of Lady’s Mantle.
There are many properties which exist in the plant kingdom that are of great interest to product designers, materials scientists and engineers. There is not one company product pipeline which would not benefit from the knowledge and practical application of millions of years of evolution. While there are some examples of biomimetic application found in different industries there remains a wealth of information and potential applications yet to be commercialised.
The reasons for this lie in the complexity of the surface structures found in Nature which are not just micron scale but hierarchical with nano structures superimposed on high aspect ratio micron scale features which vary from cones to hairs, hooks and grooves. To reproduce such features in large scale production on not only flat film but complex 3D articles has proved extremely difficult.
Biomimetica has developed a novel process that allows these complex structures to be produced in volume on soft materials such as common polymers and even elastomers.
The company seek collaboration with academics and industrialists to further explore the remarkable potential of biomimetics in relation to the plant kingdom.
Biomimetica Patterns on Medical Devices, Bearings and Pipes
Lotus Leaf Hierarchical Pattern
Fine structure hierarchical patterns
Regular array of features