Dynamic molecular arrangements shaped by light

From its early ceremonial roots in ancient Japan, origami is now serving as inspiration for modern-day applications: collapsible medical implants for supporting blood vessels, compact satellite technology and foldable solar panels all showcase origami’s timeless ingenuity.

Now, scientists are taking the art of origami to the molecular level to create dynamic structures. A new wave of smart materials made from light-reactive polymer hydrogels can revolutionise applications where precise control over material behaviour is critical.

Thioindigo belongs to the class of ‘photoswitches’ that upon exposure to green light, elegantly rotates its molecular structure around a carbon-carbon double bond axis, inducing a major change in the attached molecular payload and its surrounding environment. Conversely, under blue light, the compacted structure unfolds, seamlessly reverting to its initial stiff structure.

“Light-responsive switchable molecules hold significant potential in applications ranging from soft robotics to wearable electronics and photonic devices to personalised medicine,” commented Vinh Truong, Principal Scientist at A*STAR’s Institute of Sustainability for Chemicals, Energy and Environment (ISCE2). Truong also described how optically controllable molecules are advancing optogenetics, offering the unprecedented control of neurons function to treat conditions like Parkinson's disease.

Nevertheless, incorporating light-sensitive thioindigos into large-scale soft materials has so far been unsuccessful due to their insolubility in water, which is essential for building soft water-swollen polymer networks, such as hydrogels.

Truong explained that thioindigos’ planar configurations encourage aggregation and limit solubility. “This behaviour is similar to how flat sheets of paper can be packed together more easily compared to crumpled up papers,” said Truong.

To address this, Truong and co-corresponding author, Xian Jun Loh, Executive Director at A*STAR’s Institute of Materials Research and Engineering (IMRE), collaborated with researchers across various disciplines from Queensland University of Technology, Australia; National University of Singapore; and Singapore University of Technology and Design; to devise a novel method to tweak the molecule's architecture. By chemically attaching a hydrophilic (water-attracting) segment to the thioindigo, the modified molecule dissolved readily, making it perfect for large-scale polymer creation.

These modified thioindigos, now woven into the polymer matrix, enabled control over the material’s physical properties. The team successfully demonstrated how the molecules can repeatedly fold and unfold, switching between states without losing integrity. Truong also noted that while attempts have been made to enhance thioindigo solubility, their work pioneers the integration of thioindigos into crosslinked polymer networks like hydrogels.

Moreover, the team found the polymer to be non-toxic, which broadens its application scope in medicine. Preliminary studies showed promising results, such as the growth of human kidney cells on the polymer substrate.

Their research is progressing rapidly, said Truong, highlighting upcoming projects focused on molecular-scale light-guiding soft robotics and collaborations with Monash University, Australia, to explore regenerative therapies using this innovative material platform.

The A*STAR-affiliated researchers contributing to this research are from the Institute of Materials Research and Engineering (IMRE) and the Institute of Sustainability for Chemicals, Energy and Environment (ISCE2).

Source research.a-star.edu.sg