This is a schematic representation of optical self-trapping within SP-functionalized hydrogels with two remote beams; each beam is switched on and off to control the interaction. Image: Aizenberg/Saravanamuttu Lab. Proceedings of the National Academy of Sciences Feb 2020, 201902872; DOI: 10.1073/pnas.1902872117.
This is a schematic representation of optical self-trapping within SP-functionalized hydrogels with two remote beams; each beam is switched on and off to control the interaction. Image: Aizenberg/Saravanamuttu Lab. Proceedings of the National Academy of Sciences Feb 2020, 201902872; DOI: 10.1073/pnas.1902872117.

Advances in biomimicry – creating biological responses in non-biological substances – will allow synthetic materials to behave in ways that are typically only found in nature. While light provides an especially effective tool for triggering life-like, dynamic responses within a range of materials, it is typically dispersed throughout the sample, making it difficult to localize the bio-inspired behavior in specific sections of the sample.

Now, however, a convergence of optical, chemical and materials sciences has yielded a new way to utilize light to control the local dynamic behavior within a novel hydrogel. When illuminated, this hydrogel mimics a vital biological behavior: the ability of the iris and pupil in the eye to dynamically respond to incoming light. Furthermore, once the light enters the sample, the material itself modifies the behavior of the light, trapping it within specific sections.

The hydrogel was developed by a team of researchers from the University of Pittsburgh's Swanson School of Engineering, Harvard University and McMaster University in Canada, who report their findings in a paper in the Proceedings of the National Academy of Sciences. The researchers from the University of Pittsburgh include Anna Balazs, professor of chemical and petroleum engineering, and Victor Yashin, visiting research assistant professor.

"Until only a decade or so ago, the preferred state for materials was static. If you built something, the preference was that a material be predictable and unchanging," Balazs said. "However, as technology evolves, we are thinking about materials in new ways and how we can exploit their dynamic properties to make them responsive to external stimuli.

"For example, rather than programming a computer to make a device perform a function, how can we combine chemistry, optics and materials to mimic biological processes without the need for hard-wired processors and complex algorithms?"

This study continues Balazs' work with spiropyran (SP)-functionalized hydrogels and their photo-sensitive chromophores. Although the SP gel resembles gelatin, it is distinctive in its ability to contain beams of light and not disperse them, similar to the way fiber optics passively control light for communication. Unlike a simple polymer, the water-filled hydrogel reacts to light and can ‘trap’ the photons within its molecular structure.

"The chromophore in the hydrogel plays an important role," Balazs explained. "In the absence of light, the gel is swollen and relaxed. But when exposed to light from a laser beam about the width of a human hair, it changes its structure, shrinks and becomes hydrophobic. This increases the polymer density and changes the hydrogel's index of refraction, and traps the light within regions that are denser than others. When the laser is removed from the source, the gel returns to its normal state. The ability of the light to affect the gel and the gel in turn to affect the propagating light creates a beautiful feedback loop that is unique in synthetic materials."

Most surprisingly, the group found that introducing a second, parallel beam of light creates a type of communication within the hydrogel. One of the self-trapped beams can not only control a second beam, but this control can happen even with a significant distance between the two, thanks to the response of the hydrogel medium.

Yashin notes that this type of control is now possible because of the evolution of materials, not because of advances in laser technology. "The first observation of self-trapping of light occurred in 1964, but with very large, powerful lasers in controlled conditions," he said. "We can now more easily achieve these behaviors in ambient environments with far less energy, and thus greatly expand the potential use for non-linear optics in applications."

The researchers believe that opto-chemo-mechanical responses present a potential sandbox for exploring soft robotics, optical computing and adaptive optics. "There are few materials designed with a built-in feedback loop," Balazs said. "The simplicity of the responses provides an exciting way to mimic biological processes such as movement and communication, and open new pathways toward creating devices that aren't reliant on human control."

This story is adapted from material from the University of Pittsburgh, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.