Learning from Nature – Manipulating Light with the Help of Leafhoppers

A new study led by researchers at Pennsylvania State University has found that the small particles secreted by leafhoppers, which are commonly found in our backyard, can help us build next-generation technology. 

Published recently in the PANAS, Proceedings of the National Academy of Sciences of the United States of America, the researchers, for the first time ever, were successful in the exact replication of the complex geometry of the mysterious particles, which are called brochosomes, and just how they absorb light.

The thing is many natural functional materials consist of hierarchical micro- and nanostructures that are integral parts of biological surfaces. Leafhoppers, in particular, excrete brochosomes and use them as deployable materials on their body surfaces.

Brochosomes are intricately structured, buckyball-shaped spheroids with through-holes produced by the leafhopper. They are hollow and nanoscopic, meaning they are extremely small and are on the nanometer scale, which is one billionth of a meter. These hydrophobic particles are secreted by leafhoppers and found on their body surface. 

While first discovered many decades ago in the 1950s, we still don't have an understanding of brochosomal geometry's functional significance, something this study aims to change.

Tak-Sing Wong, a professor of biomedical engineering and mechanical engineering at Penn State, led the study. It found that the through-holes of brochosomes can help reduce light reflection. This marks the first biological example demonstrating short-wavelength, low-pass antireflection functionality, referring to an optical coating that reduces reflection over a certain range of shorter wavelengths while allowing longer wavelengths to pass through.

The unique geometry of brochosomes, according to the study, could allow the development of bioinspired optical materials (finely tuned and responsive materials inspired by natural structures found in living organisms). 

This development differs from the anti-reflective moth-eye effect, which has inspired highly effective coatings for use in solar panels, smartphones, and tablet computers. Moths' eyes are non-reflective due to the surface's periodic nanoscale structure, causing incident light to scatter in random directions. As a result, rather than being reflected, the light is transmitted into the eye.

So, leafhopper's brochosomes provide a distinct approach for bioinspired optical manipulation, with possible applications in coatings, invisible cloaking devices, and more efficient solar energy harvesting.

Replicating the Complex Structures of Brochosomes

Given the limited understanding of brochosomes' unique geometry, their consistent presence across various leafhopper species, and the uncertainty surrounding their sizes and through-holes, typically ranging in the hundreds of nanometers, the researchers at Penn State undertook this study to uncover these mysteries and elucidate their significance for these species.

The team has been working on this for several years now. Back in 2017, it was actually the first time brochosomes' synthetic version was developed to gain a better understanding of their function.

Making these brochosomes in the lab has been a challenging task despite the fact that scientists have known about them for so long. This has been due to the geometrical complexity of these particles. It has also been unknown as to why these backyard insects even secrete particles that have such complex structures.

Even years ago, while brochosomes' features like dimples and their distribution were successfully mimicked using synthetic materials, researchers weren't able to create an exact replica. Now is the first time the team has been able to replicate the exact geometry of the natural brochosome in a scaled form that is 20,000 nanometers in size.

Now, to create these complex structures in the lab, the research team made use of a high-tech three-dimensional printing method. They utilized two-photon polymerization (2PP) 3D printing to fabricate high-fidelity synthetic versions of brochosomes.

The study noted that while top 3D printers can create objects with a 200 nm to 500 nm resolution, they fall short of replicating the nanoscale geometries of natural brochosomes, which are in the range of ~300 nm to 600 nm. So, they used a scaling model technique and then fabricated microscopic synthetic brochosomes as a model system.

The synthetic brochosomes and their through-hole were designed with diameters of about 20 µm and 5 µm, respectively, to make sure that the printed structures were much larger than the 3D printer's resolution. The resulting structure had 12 pentagonal through-holes and 20 hexagonal through-holes that were connected via a hollow core. While the shell thickness was 7% of the overall diameter, the through-hole wall thickness was 20%, mimicking the natural brochosomes.

The fabricated sample further consisted of an array of 20 by 20 synthetic brochosomes in an HCP lattice that resulted in a packing density of about 91%. A control sample that had no through-hole structures was also fabricated.

The team then examined the optical form-to-function relationship of brochosomes, which showed that brochosomes' hierarchical geometries are engineered within a narrow size range with through-hole architecture to significantly reduce light reflection.

To investigate just how the brochosomes interact with infrared light of different wavelengths, the researchers used a Micro-Fourier transform infrared (FTIR) spectrometer that helped them understand how the light is manipulated by the structures.

The lab-made particles were found to be able to reduce light reflection by as much as 94%, which is a big deal as this is the very first time that a species in nature has been seen controlling light in such a particular manner using hollow particles.

This discovery also suggests the strong possibility of leafhoppers coating themselves with brochosome armor to cloak themselves to avoid predators instead of the previous theories, which speculated this to be a way for them to be free of contaminants and water, as per co-author Wong.

Moreover, the team discovered that the hollow, buckyball-shaped appearance of the brochosome, along with the size of its holes, serves a dual purpose of absorbing ultraviolet (UV) light and scattering visible light.

Interestingly, the size has been found to be consistent across leafhopper species, regardless of the insect's body size. So, brochosomes are around 600 nanometers in diameter, and the pores are about 200 nanometers.

This consistency reduces visibility to predators like birds and reptiles, which have UV vision, by absorbing UV light facilitated by the holes' size. Additionally, the scattering of visible light creates an anti-reflective shield against potential threats.

Click here to read why 3D printing is a potential $500B market.

Marvelous Potential Applications

So, supported by the Office of Naval Research, the study was able to find the answer to the complexity of structures, showing that they were engineered to enhance broadband light scattering. On top of that, the through-holes further reduced light reflection by functioning as short-wavelength, low-pass filters.

These effects allow brochosomes to accomplish up to 80 to 94% reduction in specular reflection across a broadband wavelength range. These findings, according to Lin Wang, the lead author of the study who's a postdoctoral scholar in mechanical engineering, “could be very useful for technological innovation.”

Having a new way to regulate the reflection of light on a surface can enable scientists to hide the thermal signatures of both machines and humans. Wang postulates that one day we may even be able to utilize the tricks of leafhoppers to create a thermal invisibility cloak for ourselves, which holds promise in military and surveillance applications.

“Our work shows how understanding nature can help us develop modern technologies.” 

– Wang

Some potential applications of the findings include advanced sunscreens to protect against sun damage, reducing the risk of skin cancer. Additionally, coatings can be developed to protect pharmaceuticals from light-induced damage, enhancing the efficacy and durability of sensitive medications. Researchers also noted that this knowledge could lead to the development of more efficient solar energy harvesting systems.

While the team has successfully created exact replicas of natural brochosomes, their work is far from over. In the next step, researchers will focus on improving the fabrication of synthetic brochosomes to more closely match the size of their natural counterparts.

The team will also explore additional applications for synthetic brochosomes, such as in information encryption. The complex structures of brochosomes could be integrated into an encryption system where data is only visible under specific light wavelengths, enabling secure communication.

According to Wang, the study shows the value of taking inspiration from nature, which “has been a good teacher for scientists to develop novel advanced materials.”

Leafhoppers are just one of many insect species, marking only the beginning; with so “many more amazing insects out there that are waiting for material scientists to study,” Wang anticipates that such research will “help us solve various engineering problems.”

The researchers have filed a US provisional patent for their work, which also saw contributions from Zhuo Li, a doctoral candidate in mechanical engineering at Carnegie Mellon University, and Sheng Shen, a professor of mechanical engineering at Carnegie Mellon University.

Products Inspired by Successful Nature Reverse-Engineering 

The study shows the great implications of learning from nature. But this is not a new phenomenon; humans have been inspired by nature since the beginning of time. 

Historically, the ancient Greeks proportioned their buildings according to the golden ratio. In the 19th century, Spanish settlers in Colorado utilized the depths of Columbian ground squirrels' burrows to build their homes above ground. More recently, scientists have even reverse-engineered the balance systems of insects like cockroaches to design steadier robots. 

Geckos, in particular, have been a great source of inspiration in the robotics industry. Engineers at the University of Waterloo studied the gripping ability of geckos and inchworms' efficient locomotion to create a tiny robot that is expected to help doctors perform surgery one day. 

The Max Planck group aims to harness the insights obtained from geckos' hard landings to help robotic aerial vehicles accomplish more disciplined perching. Even NASA is taking inspiration from the reptiles, with its gecko adhesive already being tested on the International Space Station. Meanwhile, Boston Dynamics' RiSE robot, with its gecko-inspired feet, shows just how nature's brilliance can be used for amazing inventions and products.

So, let's take a look at a couple of companies that have designed and developed products that are inspired by nature:

#1. Velcro Industries

Back in 1941, the Swiss engineer and entrepreneur George de Mestral invented Velcro, a breakthrough inspired by the burrs he found on himself and his dog. On examining the burr under a microscope, he discovered a simple yet ingenious mechanism: small hooks and fabric loops that allowed the burr to adhere tightly to surfaces. This led him to replicate the burr's structure as a potential fastener. 

Velcro's brand ALFA-LOK fasteners are another example of this, which draws inspiration from the unique structure of mushrooms. The fastener features tiny hooks shaped like mushrooms' caps, which are little umbrella-like tops to cover and protect tiny spores, and at the top of each hook is a cap to help create a secure closure.  

Inspiration from nature, as Velcro puts it, has secured infant diapers, sent us to the moon, and created over 2,000 patents. Today, Velcro has become a generic term and is being used in a wide range of sectors, including computing, healthcare, and the automotive industry.

#2. Sharklet Technologies 

To combat the spread of bacteria, this company develops surfaces that inhibit the growth of bacteria and microbes—a development finding application in healthcare settings. Instead of relying on harsh chemicals or antibiotics, these surfaces derive their antibacterial properties from their very structure.

These surfaces feature a unique coating of microscopic patterns inspired by the texture of shark skin. Measured just about 3 microns tall and 2 microns wide, these patterns are imperceptible to the naked eye or the touch of a finger. The company develops several variations of the Sharklet micropattern, with positive ones protruding from the surface while inverse ones are recessed into the material's surface.

The pattern came about when, in 2002, materials science and engineering professor Dr. Anthony Brennan found the answer to the problem of keeping algae from covering ships and submarines' hulls. Upon examining the impression of shark skin with scanning electron microscopy, he discovered it was arranged in a diamond pattern with tiny riblets that prevent the settlement of microorganisms on it.

#3. BioMASON

In the infrastructure sector, this company is utilizing biomimicry to produce cement, which accounts for 8% of global carbon dioxide emissions. To reduce the climate impact of construction as demand for cement grows, Biomason changed the way it produces cement.

The company, which markets its technology to other companies, drew inspiration from one of nature's most robust and enduring structures: coral reefs. Mimicking the natural formation of calcium carbonate in shells and coral reefs, Biomason adopts a similar process for manufacturing its biocement instead of heating limestone to extract its carbon.

It now aims to use biomimicry to create strong and sustainable materials for buildings. It incorporates aggregate from mine waste and employs microorganisms as the binding agent for this aggregate, relying on the sun's heat for the drying process after it has been cleaned. The company has secured funding from the Department of Defence Advanced Research Project Agency to develop Engineered Living Marine Cement. Furthermore, Project MEDUSA is underway, aiming to build military-grade runways and structures in remote areas using microorganisms.

Conclusion

As Wang said, leafhoppers and other insects “are not bugs; they are inspirations.” For years, scientists have studied nature and utilized them to design products, as we noted above. Through continued research, promising results, and their subsequent applications in areas such as multispectral camouflage, optical encryption, and omnidirectional antireflection coatings, studies like the anti-reflective leafhopper brochosomes can lead to many technological advancements and bring about immense changes across various industries.

Click here to learn how robotics can take a cue from nature.