A research team from the University of Connecticut led by Songshan Zeng, Rui Li, Dianyun Zhang and Luyi Sun has developed a unique series of materials based on controlling the dynamics of moisture responsive wrinkles. The research was inspired by the everyday experience of one’s finger and toes becoming wrinkled after lengthy exposure to water and then returning to a flat skin surface once they are dried, as well as by changes to facial skin through ageing – with the skin becoming thinner and less elastic due to dehydration, leading to the formation of wrinkles, creases, and lines.

As cosmetic surgery has shown, both the skin’s thickness and elasticity can be somewhat restored, making it apparent that features of skin wrinkle dynamics, such as reversibility and stability, can be altered. The team were inspired by these tunable responses to develop, for the first time, three types of moisture-responsive wrinkle dynamics with distinctively different reversibility and stability.

“These novel responsive dynamics [could] initiate the invention of a series of optical devices triggered by moisture”Luyi Sun

As reported in the journal Advanced Materials [Zeng et al Adv. Mater. (2017) DOI: 10.1002/adma.201700828], based on a single PVA–PDMS film substrate system, these three types were: completely reversible wrinkle formation; irreversible wrinkles formation, where the wrinkles that are initially formed can be erased permanently to the extent that they never reappear; and a second irreversible wrinkle formation where, once the wrinkles are formed, they can no longer be erased.

Unlike their earlier work that focused just on the formed wrinkles, this study aims at controlling the dynamics of wrinkle formation. To reveal the various wrinkling dynamics, it was crucial to control the moisture-dependent film properties of PVA through tuning the film’s cross-linking degree, as well as the gradient and macro-/microstructures of the film–substrate bilayer system. To study the underlying mechanism, the team developed a finite element model to capture the wrinkling phenomena and which could also predict the amplitude of the wrinkles.

As well as improving our knowledge of the possibilities of the dynamics and manipulation of human skin wrinkling, the approach could lead to applications in optical devices, such as anti-counterfeiting tabs, encryption devices, water indicators, light diffusors, and anti-glare films. As lead researcher Luyi Sun told Materials Today, “These novel responsive dynamics [could] initiate the invention of a series of optical devices triggered by moisture”. The team are now looking to develop similar wrinkling devices that can respond to other stimuli, such as light, heat, and electricity.