Evaluation of the Mechanical Properties of a Hydrogel Fiber in the Development of a Polymeric Actuator

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Abstract
The ability of polymer gels to undergo reversible conformational variations when some particular environmental parameter is changed, such as temperature, pH, salt concentration, etc., can be used in the development of microactuators which are characterized by high power-to- weight ratio and built-in compliance. Such actuators could fit the requirements of some advanced bioengineering and robotics applications. Hydrogel fibers, derived from poly(acrylonitrile), show remarkable shrinking and swelling capabilities when the surrounding solution becomes acid or basic, respectively, as well as good resistance to mechanical stresses (Umemoto et al., 1991). An experimental protocol for evaluation of the electro-mechano-chemical properties of this material has been formulated, with the aim of extensively evaluating its performance for possible use as a microactuator. Tests have been grouped into two classes: mechano-chemical tests, in order to evaluate the concentration of the charged sites on the polymeric chain which are responsible for the shrinking and swelling of the gel fiber and to estimate the response of the fiber in terms of produced force density, speed of contraction, resistance to fatigue, maximum borne stress, etc., (when work ing in solutions of different pH values); and electro-mechano-chemical tests, in order to estimate the response of the fiber when the shrinking and swelling are caused by local changes of the pH induced, applying an electric field through the solution in which the hydrogel is immersed. The results show that the hydrogel fiber can generate a force density of 15 kg/cm2 during the shrinking phase, and 60 kg/cm2 in resistance to mechanical stresses. From the rest length, the maximum force exerted can be reached in about 5 seconds. The power-to-weight ratio is found to be about 5 10 4J/kg. A major shortcoming is chemical hysteresis, which could theoretically be eliminated by modifying the initial treatment used to ionize the polymer. The methods used for the characterization of the fiber and the results are presented and discussed.

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