Bio-inspired Design of Submerged Hydrogel-Actuated Polymer Microstructures Operating in Response to pH

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Title: Bio-inspired Design of Submerged Hydrogel-Actuated Polymer Microstructures Operating in Response to pH Authors: Lauren D. Zarzar, Philseok Kim, and Joanna Aizenberg @Harvard:


This paper discussed the possibility of creating chemically-actuated structures by using a modified kind of hydrogel. Applications of such structures could include microfluidic devices (i.e. lab-on-a-chip devices or micro-mixing systems) and biomimetics (i.e. cilia or pedicellaria). Actuation of structures was achieved by modifying the pH of the system in such a way that osmotic pressure on the submerged hydrogel caused either contraction or swelling such that the hydrogel acts like a muscle analog while actuating an array of microposts or microfins.

Methods and Results

Some effort was made to allow for the plates to bend in a predictable direction over a large area. The first attempt (using microposts) contained many small domains of bent plates with no preferential direction observed from domain to domain. The method by which the hydrogel actuates these structures is shown in the figure below. So, anisotropy was introduced in the system by switching from microposts to microfins with a large aspect ratio such that the bending occurs preferentially in only two directions. To further allow for all of the microfins to bend in the same direction, several methods were implemented to increase the anisotropy: notably tilted plates and tilted application of the hydrogel to the structure.


Conclusions and Link to Soft Matter

The addition of anisotropy to the system and the ability to use chemical gradients to reliably test the repeatability of the actuation shows that these structures have fairly good actuation. However, the requirement that the structures be covered mostly with hydrogel and submerged in water whose pH must be controlled limits the potential applications of such a design. Taller plates that bend to the same angle would be needed for some of the micromixing, antifouling, or other microfluidic applications.

This paper relates to the soft matter course in that it utilizes osmotic pressure within a gel-structure to actuate a polymer structure. All of these processes (the osmotic pressure, the shrinking/expansion of the hydrogel, and the bending of the posts/fins) can be modeled using tools and techniques from the field of soft matter.

A movie showing the actuation of the plates: