Solar panels, phones, tablets, computers all have a common problem of surface reflection. However nature had anticipated the problem of surface reflection thereby developed the outer surface of corneal lenses of nocturnal insects, such as moths, in such a manner so that it could suppress surface reflection effectively. With the help of electronic microscopes scientists have observed that the outer surface of the corneal lens of moths is covered with a regular array of conical projections or protrusions. Height and spacing of the projections were about 200nm. The scientists have proposed that the array of protrusions present on the outer surface of corneal lenses provides a graded transition of refractive index between the air and the cornea which substantially reduces surface reflection.
The moth-eye antireflection surface can suppress reflection effectively thereby having the potential to form the best antireflection coating. Inspired by nature researchers from Tokyo University of Science and Geomatec Co., Ltd., Japan, have developed a novel anti-reflective coating based on the unique nanostructures present in the eyes of moths. Nanoscale biostructures being highly effective have become a suitable candidate for antireflective structures.
Moths like other nocturnal animals have to evade predators. They have developed non reflective eyes so that they do not attract attention. The periodic nanometric structure present in moth’s eyes makes the surface of the eye graded and not polished. This ensures that most of the incident light bends at the surface thereby gets transmitted through the eye instead of being reflected off the surface. Therefore researchers have fabricated materials to imitate the anti-reflective properties of a moth’s eyes in order to develop advanced antireflective coatings.
The novel antireflective nanostructures based on moth’s eye developed by the team can effectively suppress reflection of a wide range of wavelength of light and incident angles. They have overcome the previous hurdles of scalability and manufacturing cost using an ingenious technique to develop antireflective coatings and films based on nanostructures at large scale. Their work has been published in the journal Micro and Nanoengineering.
In their previous approach the team had successfully developed moth-eye mimicking nanostructured molds by irradiating a glassy carbon (GC) substrate with an oxygen ion beam using an electron-cyclotron resonance-type ion source system. However this method was not scalable. Since fabrication of glassy carbon (GC) substrate is done using powder metallurgy technique which is not suitable for production of large molds. They have overcome this difficulty by using a thin layer of glassy carbon deposited on a large regular glass substrate as the antireflective structure mold material. Also it was not feasible to fabricate large area of moth eye based nanostructures using electron-cyclotron resonance-type ion source system since it has a narrow oxygen ion beam irradiation range. Thus, in their present work, the team switched to inductively coupled plasma (ICP) system which has a wider oxygen ion beam radiation range as compared to electron-cyclotron resonance-type ion source systems. Thus making ICP apt for irradiating or etching large glassy carbon (GC) substrate to fabricate moth-eye structures on a larger area.
The team had successfully formed a uniform 2-μm-thick GC (glassy carbon) layer on a glass substrate as the antireflective structure master mold. After trying different ICP (inductively coupled plasma) parameters the researchers concluded that using a two-step ICP etching process was optimal for creating a good quality moth-eye mimicking nanostructure on the GC master mold. With this fabricated moth-eye structured mold the team further went on to develop a transparent moth eye nanostructured film using a UV-curable resin.
As claimed by the research team the moth-eye based nanostuctured mold and transparent film had remarkable optical properties. According to the team the moth-eye mimicking nanostructred mold had an approximate reflectance of 0.1% in the visible range of light. Also the transparent moth eye nanostructured film had a reflectance of only 0.4% towards visible light which is ten times lower compared to the reflectance rate of a similar film not fabricated with moth-eye nanostructure. This gives the novel moth-eye nanostructure based transparent film a huge advantage over ordinary film. Furthermore, the researchers have claimed, that after using the novel film the transmittance of light through the material was increased thereby usage of the novel film to reduce surface reflection of light did not affect the optical property of the material.
The technology used by the team can be used for large scale production of moth-eye nanostructured films. If produced in meter scale the moth eye mimicking nanostructured film will have a huge number of potential applications as an anti-reflective film according to the researchers. The ingenious antireflective film could be used to improve visibility of digital signs, flat panel displays and transparent acrylic plates. Also the performance and efficiency of solar panels could be improved by using this novel antireflective coating. This novel anti-reflective coating based on the unique nanostructures present in the eyes of moths could be used to solve the problem of surface reflection.
The technology developed by the researchers could be used to boost the development and use of biologically inspired structures. Their scientific breakthrough would help make creation and fabrication of nature inspired structures more scalable.
Moreover this antireflective coating is not the first nature inspired technology used by scientists and engineers to solve human problems. Previously scientists have taken inspiration from various mechanisms found in life forms to develop technologies which are of great use to mankind.
One such example is Velcro. Velcro was invented by chance when Swiss engineer George de Mestral after hiking in the woods with his dog found that burr from the Budrock plant had clung to his clothes and the dog’s fur. Under the microscope he observed that thousands of tiny hooks present on the burr were attached to the small loops present in clothes. Inspired by this mechanism he went on to invent the two sided fastener. After extensive research and trials he created the Velcro which consists of two strips of fabric which can grip together firmly and can be released easily at the same time. Similar to burr attaching to clothes one strip is covered with thousands of tiny hooks while the other with thousands of tiny loops.
Another example of nature inspired technology is an advanced biomaterial, mimicking the unique nanostructural property of spider silk protein, developed by researchers from University of Bayreuth. The novel biomaterial based on the antimicrobial properties of spider silk protein can not only prevent the attachment of microbes on its surface but also allow the attachment of mammalian cells. The research team has observed that the microbe resistant property is due to the spider silk protein’s structure at the nanometre level and not because it is poisonous for cells. By resisting adhesion of pathogens this nature inspired biomaterial when applied to biomedical devices can eliminate the risk of biofilm formation and its associated potential threats. Thus the researchers have developed an innovation solution for biofilms, a major healthcare industry problem.
The technologies inspired by nature should give us all the more reasons to save the various life forms so that scientist can develop more such applications based on the mechanisms found in living beings.
Tomoya Yano, Hiroyuki Sugawara, Jun Taniguchi. Moth-eye structured mold using sputtered glassy carbon layer for large-scale applications. Micro and Nano Engineering, 2020; 9: 100077 DOI: 10.1016/j.mne.2020.100077