By Amanda Campanaro

Above, a typical optical microscopic image of the moisture responsive wrinkling surface.

Songshan Zeng, Ph.D. student in Dr. Luyi Sun’s lab of IMS, recalls watching a YouTube video showing the camouflage capabilities of the octopus underwater. The marine animal can switch its skin color and texture instantly to mimic the appearance of the surrounding environment. He was inspired to study how he can use this natural occurrence to further his research.

“This phenomenon sparked my curiosity because I found out the octopus can contract or release its skin muscle to tune the exposed area of the pigment and thus change its skin color,” Songshan says. “I think this mechanism can be incorporated into my research project.

Understanding the color or transparency change mechanism of the squid, jellyfish and the wrinkling phenomenon in human skin, has been at the center of his research.

Recently, Songshan’s research has focused on two projects involving these natural phenomena, which he outlines below.

Bio-inspired sensitive and reversible mechanochromism based on strain-dependent cracks and folds

“These bio-inspired mechanochromism devices can create transparency and color change in response to mechanical force. They were primarily inspired by marine life – specifically animals similar to jellyfish and squids. Jellyfish are commonly seen as transparent. Directly in response to touch, their muscles contract, resulting in a wrinkled or folded morphologies of their skin. Because of that folding form, light will be scattered instead of travelling directly through the skin, resulting in an opaque appearance.  For squids, instead of changing from transparent to opaque, they can make color appear and disappear. When they contract their muscles, they expose pigment sacs that allow color to be shown. Once they release their muscles, the pigment sacs are shrunk into a tiny area, causing the color to be invisible.”

Inspired by these display tactics, Songshan and the group have developed analogous deformation-controlled surface-engineering approaches via strain-dependent cracks and folds to realize four mechanochromic devices: transparency change mechanochromism (TCM), luminescent mechanochromism (LM), color alteration mechanochromism (CAM) and encryption mechanochromism (EM). These devices are based on a simple bilayer system that exhibits a broad range of mechanochromic behaviours with high sensitivity and reversibility.

“The TCM device can reversibly switch between transparent and opaque states. The LM can emit intensive fluorescence as stretched with very high strain sensitivity. The CAM can turn fluorescence from green to yellow to orange as stretched within 20% strain. The EM device can reversibly reveal and conceal any desirable patterns.”

These breakthrough devices exhibit artificial mechanisms like those of the octopus, squid, and jellyfish in nature. This research work was published on a high impact journal – Nature Communications.  (Zeng, S. et al. Bio-inspired sensitive and reversible mechanochromisms via strain-dependent cracks and folds. Nat. Commun. 7:11802 doi: 10.1038/ncomms11802 (2016).)

Moisture responsive wrinkling surface with tunable dynamics

“After lengthy exposure to moisture, one’s fingers and toes may become wrinkly, while a flat skin surface returns as the skin dries. Moreover, the aged face skin becomes thinner and less elastic due to the dehydration, leading to the formation of wrinkles, creases, and lines. Cosmetic surgery can remove these unwanted aging signs by restoring the skin’s thickness and elasticity,” he explains.

According to Songshan, the skin wrinkle dynamics (such as reversibility and stability) can be tuned by external stimuli, as well as the skin’s structure and properties. Inspired by these tunable responses, they have achieved for the first time three types of moisture-responsive wrinkle dynamics, through a single film–substrate system. These dynamics include: completely reversible wrinkles formation; irreversible wrinkles formation by which the initially formed wrinkles can be permanently erased and never reappear; and irreversible wrinkles formation by which once the wrinkles form, they can no longer be erased.

“The key to success is to control the stiffness and thickness ratios of the film and the substrate, and tailor the crosslink degree/gradient of the film to allow for moisture-dependent changes of modulus and swelling degree. These unique responsive dynamics motivate the invention of a series of optical devices triggered by moisture, including anti-counterfeit tabs, encryption devices, water indicators, light diffusors, and anti-glare films.” This research was also published on another high impact journal – Advanced Materials. (Zeng, S. et al. Moisture‐Responsive Wrinkling Surfaces with Tunable Dynamics. Adv. Mater. 2017, 1700828.)

A Future of Possible Uses

For Songshan’s first project above, all the designs for different types of

mechanochromisms are based on a bilayer structure containing rigid film, made of polyvinyl alcohol (PVA) or laponite composite, with or without fluorescent dye, tightly bonded to a soft polydimethylsiloxane (PDMS). The second project uses the designs for different types of wrinkling device consisting of a stiff hydrophilic polyvinyl alcohol (PVA) film tightly adhered onto a hydrophobic soft PDMS substrate. The thickness of the PVA film, PVA crosslinking degree/gradient, PDMS modulus, and PVA-PDMS interface were carefully optimized.

“The mechanochromic devices can be attractive for a wide range of applications in smart windows, strain sensors, encryption, tunable wetting systems and more,” says Songshan.

“For the second project, three types of bilayer PVA-PDMS wrinkling devices are firstly prepared which demonstrate reversible, temporary or permanent wrinkles as moisturized. It can be promising for various applications, such as anti-counterfeit tabs, encryption devices, water indicators, light diffusor, and anti-glare films.” Songshan explains.

Recently, the national ACS meeting recognized Songshan and Dr. Luyi Sun’s work during a press conference. “The participation of the press conference allows a large number of media, peers, and companies to know our work, which in turn leads to opportunities for collaboration,” Songshan says. “For example, we got an interview from the Smithsonian Magazine after the press conference.” Many online media, including WIRED and ScienceDaily, reported their work presented at the ACS press conference.

Along with Dr. Dianyun Zhang, IMS associated Assistant Professor of Mechanical Engineering, Songshan worked closely with a group of outstanding undergraduate and high school student assistants. “The list of undergraduate assistants who made the main contribution are Stephan Freire, Andrew Smith (now a UConn Ph.D. student majoring in Chemical Engineering); and Vivian M. M. Garbellotto. The high school student who made the main contribution is Emily Huang. I also worked closely with Rui Li, a graduate student supervised by Dr. Dianyun Zhang,” he says.

“Songshan is a truly innovative student,” says Professor Luyi Sun, Songshan’s advisor.  “He has also done a great job in leading a team of undergraduate students to work with him efficiently and effectively. It is great to work with such a coworker and I expect more significant research with him in the coming days.”