Electrical responses of polymer gels

Cross-linked polymer gels that switch on and off the supply of drug or biomolecules in response to changes in temperature and/or pH have been developed [36-41], An electrically controlled DDS has also been developed. For example, insulin was entrapped in a poly(dimethylaminopropyl acrylamide) (PDMAPAA) gel and the delivery was controlled by electrical stimulation. As seen in Fig. 15, periodic on and off release of the protein molecule was obtained with numerous alternating applications of an electric field [42]. The response of the gel is quite sharp, and no leakage of the insulin during the off time was observed. Similarly, endrophonium chloride was released in an of-off pattern from poly(AMPS) copolymers with electric stimuli [43]. 

Another type of gel expands and contracts as its structure changes in response to electrical signals and is being investigated for use in artificial limbs that would respond and feel like real ones. One material being studied for use in artificial muscle contains a mixture of polymers, silicone oil (a polymer with a (O—Si—O—Si—) — backbone and hydrocarbon side chains), and salts. When exposed to an electric field, the molecules of the soft gel rearrange themselves so that the material contracts and stiffens. If struck, the stiffened material can break but, on softening, the gel is reformed. The transition between gel and solid state is therefore reversible. 

The model of electric field-controlled artificial muscles has been described in 1972 [5], Fragala et al. fabricated an electrically activated artificial muscle system which uses a weakly acidic contractile polymer gel sensitive to pH changes. The pH changes are produced through electrodialysis of a solution. The response of the muscle as a function of pH, solution concentration, compartment size, certain cations, and gel fabrication has been studied. The relative change in length was about 10%, and the tensile force was 1 g/0.0025 cm2 under an applied electric field of 1.8 V and 10 mA/cm2. It took 10 min for the gel to shrink. 

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