Osaka, Japan – Natural materials, ranging from butterfly wings to mother of pearl, can exhibit an amazing array of optical effects. In trying to reproduce these effects, scientists have found it challenging to use biodegradable, sustainable raw ingredients while also achieving the high levels of reflectivity required for certain applications. Now, in a study published in Advanced Materials, a team led by the University of Osaka researchers has developed a new process that can make highly reflective surfaces from sustainable, plant-based raw materials.
Earlier work in this field used minerals or petroleum-based compounds to successfully synthesize useful products such as pigments. Later work turned to natural, organic components, including liquid crystals and cellulose, but these struggled to achieve the high reflectivity needed for optical systems, hampered by both the low reflectivity of natural materials and a lack of advanced processing methods. To address this, the research team considered new approaches to processing polysaccharides, which are naturally-occurring compounds made of chains of sugar molecules.
"This work was based on the use of xerogels," points out lead author Takaaki Kasuga. "These are highly porous solids made by drying gels without allowing any shrinkage."
The study used a technique known as electrodeposition to form 100 alternating layers of pectin, a natural material found in plants and often used in jams, along with nanofibers of cellulose, the main component of plant cells. "The specific process that was used, together with the unique properties of the pectin and cellulose, generated layers that were either dense or highly porous," says senior author Masaya Nogi. "These alternating layers had very different optical characteristics, and the contrast between the two led to extreme reflectivity in the finished product."
An exciting aspect of this research is that the final product showed greater than 95% reflectance at a variety of different wavelengths of light, including non-visible light. Although there are some examples of reflective materials in nature, such as the brilliant armor of the jewel beetle, the jewel beetle only reflects 30% of light. The unique products generated in this work have potential applications in optical sensors, decorative displays, and anti-counterfeiting technologies.
The researchers also found that following some tweaking of the material's chemical composition, these naturally-sourced layers could be made to degrade by up to 80% in just two months, meeting the standard benchmark for biodegradability.
By reflecting near infrared light while allowing the transmittance of visible light, these new materials could also help to keep houses cool in the summer when used as windows. This is likely just the beginning: because the color, pattern, and performance of these films can all be finely tuned, this approach points toward a future of sustainable coatings, labels, and building materials that could one day replace their synthetic, non-biodegradable counterparts.
