Anisotropic contraction

Fig. 5.13 Photomechanical effects. Schematic illustration of UV-light-induced bending of a cross-linked liquid-crystalline polymer film containing azobenzene groups. Light is absorbed at the upper surface layer of the film and causes anisotropic contraction. Adapted from Ikeda et al. [51] with permission from Wiley-VCH.

It is well known that human muscles are made of many bundles of muscle fibers and their anisotropic contractions are induced by electric stimulus. To construct artificial muscles, crosslinked PLCP fibers were fabricated due to their high mechanical flexibility [40]. As shown in Figure 7.18, crosslinked PLCP fibers wifh high orientational order of the mesogens along the fiber axis were fabricated. 

When a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of electrodes and a DC voltage is applied, it undergoes anisotropic contraction and concomitant fluid (water) exudation [13,20]. Figure 4 shows a schematic view of the shape change in an anionic gel with time in an electric field [21]. It is found that... 

If a water-swollen cross-linked polyelectrolyte gel is inserted between a pair of planar electrodes and a voltage difference is applied, the material can undergo anisotropic contractions and concomitant fluid exudations [197,198], Electrically induced contractions of the gel are caused by transport of hydrated ions and water in the network (electrokinetic phenomena). In fact, when an outer electric field is applied across a gel, both macro- and micro-ions are subjected to electrical forces in opposite directions. However, macro-ions are typically in a stationary phase, being chemically fixed to the polymer network, while counter ions are mobile and are capable of migrating along the electric field, dragging water molecules with them. 

Based on the theory of bending mechanism, surfactants bound on the gel surface generate stress on the surface, because of anisotropic contraction. Stress cause strain on the surface of gel. 

Okuzaki H, Hattori T (2003) Electrically induced anisotropic contraction of polypyrrole films. 

Finlay, J.B. and Hardie, W.R. (1994) Anisotropic contraction of cortical bone caused by dehydration of samples of the bovine femur in vitro. Proc. Inst. 

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