Caterpillar spins a silky sunscreen
The cocoons spun by silkworm caterpillars are providing more than just a luxurious fibre. Researchers have found compounds in the silk cocoons have UV-protecting properties, offering a potential source for future UV-shielding products such as sunscreens and protective clothing.
Researchers at Deakin University’s Institute for Frontier Materials, with colleagues at ANU and CSIRO, have investigated protection given by the silky cocoons of semi-domestic and wild silk worms.
“Silkworms tend to spin their cocoons in low light conditions but there is always the risk of exposure to harmful ultraviolet rays,” explains Jasjeet Kaur, a PhD student who carried out the research.
“The cocoon is less than 1mm thick but it shields the silkworm from visible light. We wanted to know what factors in the cocoon’s structure might preclude the entry of UV rays.”
Jasjeet and her colleagues found that the fibre coating, made of a gummy mixture of proteins called sericin, was mainly responsible for absorbing incident UV-A light, acting as an effective sunscreen for the silkworm pupae and also protecting the silk fibres from the sun.
An insight into these UV blocking properties is particularly important from the perspective of designing lightweight structures with the ability to shield UV radiation for many applications, such as UV protective garments, umbrellas and silk tents.
It is well known to those in the silk trade that removing the gum from silk, a necessary process carried out before spinning for cloth production, makes silk susceptible to light damage. Artificial sunscreen must be added to protect clothing from damage. Sericin is already used in cosmetics because of its known moisturising properties and now this research has shown that it also acts as an antioxidant on exposure to UV-A light.
The researchers looked at four different silk worm species – cocoons produced in very cold regions of northern China where the temperature plummets to -40°C and others harvested from tropical regions of North East India, where the maximum temperature can reach 40°C.
In all the cocoon varieties, sericin absorbed UV-A radiation and the chemical changes triggered by UV-A radiation were smallest for the silk species with the highest amount of sericin. These UV-induced chemical changes (release of free radicals) increased when sericin was removed in all silk cocoon varieties.
The findings in this study, published in the journal Biomacromolecules, suggest that silk cocoons and their gummy matrix have components that are responsible for significant UV blocking. Further studies are needed to understand the mechanism of absorbance to develop organic UV shielding materials such as sun creams and UV protective clothing.