# Toward Light-Responsive Hydrogel-Based Valves for Flow Regulation

## TL;DR

This paper introduces light-responsive hydrogel valves that can open and close quickly, offering potential for biomedical and microfluidic applications.

## Contribution

The novel use of light-triggered hydrogel deformation with gold nanorods enables tunable and rapid valve operation.

## Key findings

- Valves can open or close based on their aspect ratio and confinement during heating.
- Light enables reversible valve operation with response times in seconds.
- Valves withstand pressures up to 18 kPa, suitable for biomedical use.

## Abstract

Smart hydrogels are
promising materials for soft actuators in biomedical
applications thanks to their varied responses to external stimuli.
Light is a particularly attractive trigger for contactless stimulation
of hydrogels that can induce reversible morphological changes without
damaging the fragile gels. To meet the varied needs of applications
in microfluidics, soft robotics, biomedicine, and other fields, there
is significant demand for novel valve designs that are highly tunable,
miniaturizable, and respond quickly to stimuli while maintaining their
function over many activation cycles. Additionally, it is crucial
to develop a more quantitative understanding of the mechanics of valve
operation in response to different stimuli, especially when active
hydrogels are combined with other materials in multicomponent devices.
Here, stimulus-responsive valves are fabricated using active hydrogels
deformed upon temperature changes and exposure to near-infrared radiation.
Gold nanorods (AuNRs) acting as photothermal transducers are embedded
inside cross-linked poly N-isopropylacrylamide (PNIPAM),
allowing local morphological changes in response to light with high
spatiotemporal control. These changes are described precisely as a
function of the valve’s confinement, aspect ratio, and the
parameters of the stimulus using quantitative image analysis, providing
novel mechanistic insights. Changing the aspect ratio of the valves
and the degree of confinement of the hydrogel causes valves to either
open or close during heating and can be used to control the magnitude
of their response to different stimuli. These varied morphological
changes are due to local, inhomogeneous deformations of the gel. The
use of light as a trigger enables local confinement of the valve,
reversible opening and closing, and fast response times on the order
of seconds. The valves are shown to withstand hydrostatic pressures
of up to 18 kPa, providing high potential for biomedical applications
where precise pressure control and quick switching between open and
closed states is critical.