# Light- and Solvent-Responsive Bilayer Hydrogel Actuators with Reversible Bending Behaviors

## TL;DR

This paper introduces a light- and solvent-responsive hydrogel system that can bend and fold, which could be useful in soft robotics.

## Contribution

A simple and efficient bilayer hydrogel design with rapid and reversible bending behavior is introduced.

## Key findings

- SP-PAAm hydrogels showed a swelling ratio of ∼370% under blue LED in DMF.
- Four-armed bilayer hydrogels folded to ∼75° in ∼15 minutes, faster than one-armed designs.
- The system demonstrates potential for use in soft robotic actuators.

## Abstract

Light-responsive hydrogel systems have gained significant
attention
due to their unique ability to undergo controlled and reversible swelling
behavior in response to light stimuli. Combining light-responsive
hydrogels with nonresponsive polymers offers a unique self-folding
feature that can be used in soft robotic actuator designs. However,
simple formulation of such systems with rapid response time is still
a challenging task. Herein, we demonstrate a simple but versatile
bilayer polymeric design combining light-responsive spiropyran–polyacrylamide
(SP-PAAm) with polyacrylamide (PAAm) hydrogels. The photochromic spiropyran
in our polymer design is a closed-ring, hydrophobic compound and turns
into an open-ring, hydrophilic merocyanine isomer under light irradiation.
The swelling degree of SP-PAAm and PAAm hydrogels was evaluated using
LED lights with different wavelengths and solvent media (e.g., water,
ethanol, DMF, and DMSO). We observed that SP-PAAm hydrogels reached
a swelling ratio of ∼370% with the illumination of the blue
LED in the DMF medium. By combining light-responsive SP-PAAm hydrogels
with nonresponsive PAAm, a proof-of-concept demonstration was performed
to demonstrate the applicability of our fabricated platforms. Although
fabricated one-armed bilayer hydrogels possessed self-folding ability
with a folding angle of ∼40° in 30 min, the four-armed
bilayer platforms demonstrated more efficient and rapid folding behavior
and reached a folding angle of ∼75° in ∼15 min.
Given their simplicity and efficiency, we believe that such polymeric
designs may offer new avenues for the fields of polymeric actuators
and soft robotic systems.