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Thermoresponsive Hydrogel Mobility

Motion at any scale attracts attention and interest. Several mechanisms for the generation of motion exist in nature on a wide range of length scales: from a macroscopic scale on which a snake crawls over a rough surface to a micro-scale where molecular motor proteins walk on microtubules. It is desirable to mimic these natural systems to make efficient devices for useful applications. While a biological approach is limited by the working environment of the device, a synthetic approach gives freedom to tune parameters as per the requirement. We take a synthetic approach to solve this problem and work with thermoresponsive hydrogels.

phases

Hydrogels are a class of crosslinked polymers that can hold large volumes of water. Some of these polymer gels are highly sensitive to their environment and on small changes in the environment makes them release their water. This is known as volume phase transition (VPT) and is similar to gas-liquid phase transition.

We use a special kind of hydrogels, hybrid hydrogels, in which nanocomposite clay particles are used as crosslinker and N-Isopropylacrylamide is used as the monomer to form polymer chains between the crosslink points. The clay particles give the hydrogel robustness and the monomer makes the gel thermoresponsive. This hydrogel shows a Volume Phase Transition (VPT) at around 32°C which means when heated above 32°C the gel releases all its water. And since the process is reversible, on cooling the gel retains its size.

schematic

These gels move in a direction when VPT is asymmetrically inducing along the length of the cylindrical gel, as shown at the left.

For this we make long cylindrical gels inside a glass capillary and then raise temperature locally using our Peltier Element device, shown below. This device has 24 Peltier Elements which are individually controlled with a switch. Each element heats/cools a small section of the capillary from the bottom.

We can move these gels several times in a millimeter size capillary, see figure 3, with velocities ~ 1 µm/sec. In comparison, one of the fastest crawling eukaryotes, Amoebae of Acrasis (with cell surface area of 759 m m2), moves with an average speed of 71.6 m m/min.

peltier elements

We have also shown that this device is capable of doing work by attaching a glass bead at the end of the gel and showing that the gel carries the bead with it ( VIDEO 1 ).

We anticipate that such devices can be widely utilized in a variety of areas in biotechnology including microfluidics, small-scale robotics and drug delivery.

References:

L. Yeghiazarian, H. Arora, V. Nistor, C. Montemagno, U. Wiesner. "Teaching hydrogels how to move like an earthworm" Soft Matter 3 (2007) 939-944

L. Yeghiazarian, S. Mahajan, C. Montemagno, C. Cohen, U. Wiesner. "Directed Motion and Cargo Transport Through Propagation of Polymer-Gel Volume Phase Transitions", Advanced Materials, 17(15), 2005, 1869-1873

K. Haraguchi, H. Li, K. Matsuda, T. Takehisa, E. Elliot. "Mechanism of Forming Organic/Inorganic Network Structures during In-situ Free-Radical Polymerization in PNIPA-Clay Nanocomposite Hydrogels" Macromolecules 38, 2005, 3482-3490

All images and text Copyright 2008

Wiesner Research Group - Cornell University