# Microbristle in gels: Toward all-polymer reconfigurable hybrid surfaces

Entry by Helen Wu, AP225 Fall 2010

## Reference

P. Kim, L. D. Zarzar, X. Zhao, A. Sidorenkod and J. Aizenberg, Soft Matter, 6, 750-755 (2010).

Figure 1. fabrication of polymer surface presented in paper. (a) scheme used for polymer post synthesis. (b) hydrogel preparation components. (c) SEM image of honeycomb pattern on confiner. (d) SEM image of pillars.

## Keywords

hydrogel, microactuator, bioactuator

## Overview

Smart and responsive actuation systems are used in nature for various forms of movement, from antifouling mechanisms to Venus fly traps opening and closing. These systems are ususally based on soft structures. Being able to fabricate similar structures could be very useful in many applications.

The group previously combined a rigid silicon nanobristles with a soft hydrogel and demonstrated that it could be used for dynamic switching and actuated miropatterns. However, the system was limited by a large Si aspect ratio requirement for the passive component. This paper improves on the previous system by fabricating the passive component out of a polymer (PDMS) that more closely resembles the properties of bioactuators. They also demonstrate the structures' reversible switching and use a finite element simulation to show how controlled deposition of the hydrogel layer can control actuation direction.

## Description of System

Figure 2. Wet (left) and dry (right) states of the micropost/hydrogel surfaces. (a) flat confining surface. (b) array of microposts in confining surface. (c) honeycomb microwells in confining surface. 20$\mu$m scale bars.

The researchers created arrays of polymer microposts (replicas of the previous silicon structures), shown in Figure 1, by curing pre-polymers in a mold.

The polymer system used was a mixture of:

• photo-curable epoxy resin (UVO114)
• glycidylmethacrylate (GMA), a bifunctional monomer

The hydrogel was a UV-cross linkable acrylamide (AAm). GMA allows polymerization of the hydrogel on the surface of the microstructures. A confiner covered the system while the hydrogel forming nd removed after swelling it with DI water.

The actuator operates by hydration and dehydration of the hydrogel. When the hydrogel dehydrates, the microposts bend due to strain from the hydrogel pulling away. When the hydrogel rehydrates, the stored elastic energy in the microposts is released. The system is responsive to pH, temperature, and electrical stimuli.

## Results and discussion

The polymer microposts had a modulus 2-3 times lower than that of Si, meaning they could be bent by applying much less force than in the previous design. The figure to the right shows the wet and dry states of the fabricated system.

Patterning hydrogel layers around the microposts or sections with local thickness gradients was shown to result in coordinated bending of the microposts. In the Figure 2c, the cross section of the honeycomb-shaped confining surface in the shows that opening microflorets are formed.

Figure 3. Spiral actuation due to a chiral defect. (a) dry state. (b) dry to wet transition. (c) wet to dry transition. (d) wet state.

Experimental observations corresponded with modeling results, which indicated that when the thicker hydrogel part corresponds to contact with the walls of the honeycomb structure, bending goes outward toward the thicker gel at the wall (we see the opening florets). When the thicker regions are inside the microwell, the bending direction is inward (closing florets).

Hydrogel thickness variation comes from large scale imperfections and defects and the researchers postulate that using these systematically can lead to even more elaborate systems. The structure of the hybrid surfaces also change the color and transmission or reflection of light from the surface. The opening and closing structures may also be suitable to use for particle trapping and release.

The use of soft matter in this system created a much more versatile one in terms of sizes and mechanical properties that would be impossible with the stiffer materials.