The novel autonomous, responsive polymer shells undergo morphological changes when triggered by an initial deformation. Image: McCormick School of Engineering.
The novel autonomous, responsive polymer shells undergo morphological changes when triggered by an initial deformation. Image: McCormick School of Engineering.

Engineers at Northwestern University have developed a theoretical model for designing soft materials able to demonstrate autonomous oscillating properties that mimic biological functions. This work, reported in a paper in the Proceedings of the National Academy of Sciences, could advance the design of responsive materials used to deliver therapeutics, as well as robot-like soft materials that operate autonomously.

The design and synthesis of materials with biological functions requires a delicate balance between structural form and physiological function. During embryonic development, for instance, flat sheets of embryonic cells morph through a series of folds into intricate three-dimensional structures such as branches, tubes and furrows. These, in turn, become dynamic, three-dimensional building blocks for organs performing vital functions like heartbeat, nutrient absorption and information processing by the nervous system.

Such shape-forming processes are controlled by chemical and mechanical signaling events, which are not fully understood at the microscopic level. To bridge this gap, researchers led by Monica Olvera de la Cruz designed computational and experimental systems that mimic these biological interactions.

Hydrogels, a class of hydrophilic polymer materials, have emerged as candidates for reproducing the shape changes upon chemical and mechanical stimulation observed in nature. The researchers developed a theoretical model for a hydrogel-based shell that underwent autonomous morphological changes when induced by chemical reactions.

"We found that the chemicals modified the local gel microenvironment, allowing swelling and deswelling of materials via chemo-mechanical stresses in an autonomous manner," said de la Cruz, professor of materials science and engineering at Northwestern's McCormick School of Engineering. "This generated dynamic morphological change, including periodic oscillations reminiscent of heartbeats found in living systems."

In the study, the researchers designed a chemical-responsive polymeric shell meant to mimic living matter. They applied the water-based mechanical properties of the hydrogel shell to a chemical species, by incorporating a chemical substance that produces specific patterned behavior – in this case, wave-like oscillations –within the shell. After conducting a series of reduction-oxidation reactions, which transfer electrons between two chemical species, the shell generated microcompartments capable of expanding or contracting, or inducing buckling-unbuckling behavior when mechanical instability was introduced.

"We coupled the mechanical response of the hydrogel to changes in the concentration of the chemical species within the gel as a feedback loop," explained Daniel Matoz-Fernandez, a postdoctoral fellow in de la Cruz's lab. "If the level of chemicals goes past a certain threshold, water gets absorbed, swelling the gel. When the gel swells, the chemical species gets diluted, triggering chemical processes that expel the gel's water, therefore contracting the gel."

The researchers' model could be used as the basis for developing other soft materials that demonstrate diverse, dynamic morphological changes. This could lead to new drug delivery strategies, with materials that enhance the rate of diffusion of compartmentalized chemicals or release cargos at specific rates.

"One could, in principle, design catalytic microcompartments that expand and contract to absorb or release components at a specific frequency. This could lead to more targeted, time-based therapeutics to treat disease," said Siyu Li, another postdoctoral fellow in de la Cruz's lab.

This work could also inform the future development of soft materials with robot-like functionality that operate autonomously. These 'soft robotics' have emerged as candidates to support chemical production, tools for environmental technologies and smart biomaterials for medicine. Yet existing materials rely on external stimuli, such as light, to function.

"Our material operates autonomously, so there's no external control involved," Li said. "By 'poking' the shell with a chemical reaction, you trigger the movement."

The researchers plan to build on their findings and further bridge the gap between what's possible in nature and the science lab. "The long-term goal is to create autonomous hydrogels that can perform complex functions triggered by clues as simple as a local mechanical deformation," said de la Cruz.

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