From antireflective plastics inspired by moth eyes to the development of super strong materials based upon a desert cactus, researchers from Singapore and Malaysia to Hong Kong and Japan are taking material science to new levels.

Nature has long been a source of inspiration for both scientists and artists alike. Some of the most ingeniously designed products and gadgets familiar to millions of people worldwide owe their origin to seemingly simple forms and patterns found in plants and wildlife. The ability to adapt these natural forms to develop ever more innovative products and processes has given rise to the field of biomimetics — literally meaning ‘imitation of life’.

One of the most notable examples of biomimetic design to date is Velcro, famously inspired by the tiny, adhesive hairs found on the underside of geckos’ feet. Insects, too, have provided intriguing clues for the development of many new technologies ranging from self-cooling systems inspired by termite mounds to cicada wing-inspired nanosensors. Now, a team of researchers based at the A*STAR Institute of Materials Research and Engineering (IMRE) have succeeded in developing a new type of high-quality, anti-reflective plastic inspired by another unlikely source: the eyes of a moth.

 

 

credit: graftedno1

Moths are renowned for their ability to see well in the dark. Moth eyes are coated with a special anti-reflective layer that lends them the unusual distinction of having one of nature’s least reflective surfaces. Composed of a hexagonal array of conical nanostructures, the anti-reflective layer enables moths to maximize light capture and

minimize reflection, thereby reducing the chances of being spotted by predators, even in settings that appear pitch-dark to the human eye.

“Our group has been working on bio-inspired surfaces for a number of years,” explains Low Hong Yee, senior scientist and team leader of the project at the IMRE. “The anti-reflection properties found on some insect eyes are rather well-known and it was natural that we looked into the moth-eye structures. Mimicking moth-eye nanostructures have been attempted by others — however, our unique approach is in the combination of micro and nanostructures in a hierarchical arrangement. These structures are even closer to mimicking the insect eye.” Using a high-precision method known as nanoimprinting, the IMRE team were able to ‘reconstruct’ the moth-eye nanostructures and utilize direct patterning techniques to reduce surface glare.

One of the main advantages of nanoimprinting is that materials can be manipulated in terms of their physical as opposed to their chemical properties. New plastics can therefore be developed without the need to use harmful chemicals. Indeed, this type of nanoimprinting is viewed as a way of moving towards cost-effective, environmentally sustainable manufacturing practices.

Anti-reflective plastics currently on the market typically exhibit a reflectivity of around 1% of visible light. In contrast, the new plastic developed at IMRE reflects less than 0.2% of visible light — attaining a five-fold increase in anti-reflective power. Combined with the reduced amount of glare, the new plastic may find a host of applications in the development of new and improved TV displays, windows and organic solar cells.

Inspired by the lotus leaf 

Dr Linda Wu Yongling and co-workers at the A*STAR Singapore Institute of Manufacturing Technology (SIMTech) have been inspired by the lotus leaf. The leaf’s hierarchical structured surface has self-cleaning properties.

Based on the leaf’s design, the team from Singapore have developed a fast and cost-efficient way to fabricate large-scale superhydrophopic surfaces on a hard material — silica.

The researchers used a laser to carve out a microstructured template that they then used to pattern a sol-gel coating. Nanoparticles were subsequently bound to the surface of the cured sol–gel surface to create a second level of hierarchy. The fabrication methodology can be adjusted to achieve different degrees of micro- and nanostructures.

credit: William Thielicke

In addition to the new fabrication methodology, Wu and co-workers considered various ways to optimise the water repellency of the textured surface. They found that increasing the surface roughness increases the true area of contact between the liquid and the solid, enhancing its intrinsic wetting properties. However, if the surface features are small enough, water can bridge protrusions leading to the formation of air pockets; the wettability of such a nanostructured material is then calculated as a weighted average of the wettability of the pure material and that of air.

These two effects are known respectively as the Wenzel and Cassie-Baxter states. The researchers derived an equation for calculating the surface contact angle between a water droplet and a silica surface with a certain degree of roughness. They found that there was a transition between the Wenzel to the Cassie-Baxter state, as surface structuring enters the nano dimension. The researchers found that for an optimum superhydrophobic effect, the Cassie–Baxter state must dominate the surface structure to allow a massive 83% of the surface state to be involved in air trapping with only 17% of the liquid drop surface actually in contact with the silica itself.

The static water contact angle on such surface is higher than 161° with sliding angle below 1°, which are the same as the natural lotus leaf. Rubbing test by 3M automotive sponge for 1000 cycles indicated the durability of the surface condition suitable for automotive applications. Such functionality is useful if applied to textiles or windows for self-cleaning effect, and may also be used in analytical techniques for controlling fluid flow. The researchers are further developing the technology for real applications such as easy-clean coating for solar films and structured surfaces for personal care products.

The shape of a cactus

Plants are also extensively studied in the field of biomimetics and a team from Universiti Teknologi MARA has been inspired by the cacti specie Cardon, Saguaro and Cholla found in the desserts of the United States of America and Mexico.

The researchers conducted a study to evaluate the strength of perforated hollow sections with the shape and arrangement of perforations inspired by the cactus skeleton.

The Cardon and Saguaro (tallest cactus in the world and America respectively) can achieve 12 to 20m of height with their main stem supporting approximately 6 tonnes of body weight. Their ability to achieve extreme height is thought to be the result of a circular skeleton of inter-connected vascular bundles inside a succulent thick, columnar stem.

The research team has set out to investigate how small perforations found in the hollow sections of the cacti stems may be influencing the cacti’s ability to stay upright under such high loads. A total of 13 circular hollow sections were modeled using computer software. The models included one without perforation, and twelve models with varying degree of perforations, perforation shapes (circular and elliptical) and arrangements.

Elliptical shaped perforations were found to show most desirable responses for longitudinal stress. For the case of flexural loading (a material’s ability to resist deformation under load) models with perforations arranged in an array pattern were found to show the most desirable structural response for longitudinal stress for both circular and elliptical shape perforations.

credit: Saguaro Pictures

For further information contact:

Dr Low Hong Yee

Institute of Materials Research and Engineering (IMRE)

Agency for Science, Technology and Research (A*STAR)

Email: hy-lowimre.a-star.edu.sg

 

Dr Linda Wu Yongling

Singapore Institute of Manufacturing Technology (SIMTech)

Agency for Science, Technology and Research (A*STAR)

Email: ylwu@SIMTech.a-star.edu.sg

 

Woo Yian Peen and Syahrul Fithry Bin Senin

Universiti Teknologi MARA

Email: wy.peen@ppinang.uitm.edu.my and syahrul573@ppinang.uitm.edu.my