Calcium-responsive hydrogels

Two approaches are discussed for the preparation of enzymatically controlled drug delivery systems a calcium-responsive biodegradable drug delivery system based on a mixture of starch with HPMC (hydroxypropyl methyl cellulose ether) (biodegradable) and the starch hydrolytic enzyme, alpha-amylase, in its non-active form and a glucose responsive insulin delivery system based on the hydrogel poly(2-hydroxyethyl methacrylate-co-N,N-dimethylaminoethyl methacrylate), with entrapped glucose oxidase, catalase and insulin. In both systems, the sensitivity... 

The third mechanism of actuation is based on changes in conformation of natural proteins (Table 2.1, entries 13 and 14). These systems incorporate a natural protein into the hydrogel that undergoes a conformation change and thus alters the characteristics of the material. The use of proteins as actuators is a new development in bio-responsive hydrogels. An example of this system was developed by Mrksich and co-workers [24]. In this study the functional nature of the hydrogel was given by the conformational properties of the protein calmodulin (CaM). Calmodulin has two distinct conformational states. In the presence of calcium ions, CaM has an extended, dumb-bell shaped conformation (extended CaM). 

Calcification is also an undesirable response of the host to an implanted material, because the biomaterial becomes hard and brittle as exemplified in biological heart valves and artificial hearts. The mechanism of such calcification is not clear, thus no effective method to avoid calcification has been proposed. However, if a hydrophobic biomaterial is free of adherent cells, calcification will be greatly reduced, as calcification is thought to be due, in part, to these dead cells. On the other hand, it seems also probable that calcium and phosphate ions sorbed within the material may provide the nucleus for calcification. If this is the case, care should be taken when hydrogel-type polymers are implanted in the body. 

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