Hydrogel-Actuated Integrated Responsive Systems (HAIRS): Moving towards Adaptive Materials
Written by: Grant England AP225 Fall, 2011
Title: Hydrogel-actuated integrated responsive systems (HAIRS): Moving towards adaptive materials
Authors: Philseok Kim, Lauren D. Zarzar, Ximin He, Alison Grinthal, Joanna Aizenberg
This paper shows a new method for creating adaptive structures by combining soft lithography to replicate silicon structures in materials with different elastic properties, and combining these replicas with hydrogels to allow for actuation. The types of hydrogels used can vary in their responsiveness from temperature-sensitive to humidity-sensitive to pH-sensitive to light-sensitive to glucose-sensitive with threshold values which can be tuned by choosing the hydrogel used carefully. Since the structures being actuated are of such a small scale that light can interact with them, such systems could be envisioned to create self-reporting systems; that is, systems using this combined methodology could undergo a colorimetric change upon changes in the system properties. One example of a useful device from such materials would be a bridge material which changes color when it is under too much stress.
Methods and Results
Soft lithography was used to allow silicon structures to be replicated in other materials. By making a mold of the substrate out of PDMS, macroscopic changes can be made to the symmetry and shape of the sample by applying stretching, bending, or twisting to the mold while it is being replicated. Such processes allow for one Silicon master (which is difficult and expensive to create) to be used to generate multiple different structures. However, these methods are not a panacea; in order to generate completely new structures or gradient samples for testing, new masters must be created.
Actuation is achieved by patterning hydrogels onto the surface of the replica and exposing the hydrogel to a stimulus which causes either swelling or deswelling. By controlling the thickness of the hydrogel profile, preference in the direction of actuation can be chosen such that patterned actuation can occur.
The combination of the use of hydrogels and microstructures created via soft lithography were explored in detail in this paper. The possibilities for such designs are as varied as the number of different hydrogels and structures which can be envisioned. Surfaces which are hydrophobic in high-humidity and hydrophobic in low-humidity or transparent in low-light and opaque in high-light or red in low-glucose and blue in high-glucose are just a few examples of the structures which could be designed by using HAIRS.