On the Role of Water Molecules in the Interface between Biological Systems and Polymers

The excellent biocompatibility of PMEA was ascribed by Tanaka to the predominant population of intermediate water in the hydrated polymer matrix. The intermediate water concept was examined using the 'nano-plate model' on the basis of new results (by Morita) of a time-resolved IR study on the water sorption process into PMEA. The examination showed that the image picture proposed by Tanaka concerning the role of intermediate water was in consistent with experimental results so far obtained. Morita showed that the intermediate water exhibited a strong peak at 3400 cm−1 in its IR spectrum. Water sorption profiles of MMA, PEG and PMVE were found to be similar to that of PMEA. It was shown that the biocompatibility of these polymers could be explained by the intermediate water concept. It was also pointed out that PVP and PDMAA have a considerable amount of intermediate water under appropriate circumstances. The PHEMA–water system showed neither clear peak for cold crystallization in DSC chart, nor the strong peak at 3400 cm−1 in its IR spectrum, because the PHEMA system did not contain enough intermediate water to show similar behavior to PMEA. The hydrated PHEMA matrix contains a stable network structure of water molecules including the nodes of OH groups of its side-chains. In the stable network system, most water molecules should be hydrogen bonded strongly to form non-freezing water, but not intermediate water. The inferior biocompatibility of PHEMA was ascribed to the stable network structure of water molecules. Some of the PHEMA co-polymers, on the other hand, were found to have highly improved biocompatibility. Mechanism for the improvement was discussed in terms of loosening the network structure, which could be brought about by introducing ionic groups or branching to the polymer chains through co-polymerization with appropriate 'key monomers'. The mobility of polymer chains, as well as the population of three kinds of water in polysaccharide molecules in aqueous medium could change in response to their chemical structure such as nature of ionic groups, the degree of branching, etc. Polysaccharides located at the utmost-outer layer of blood cells probably possess a loosened network structure to form soft biological surface where intermediate water predominates. Cellulose, on the other hand, has a common feature with PHEMA in the sense of predominance of the non-freezing water in its hydrated system. Note: The word 'biocompatibility' is used in general as the term evaluating properties of materials which do not cause adverse effect when the materials come into contact with living organisms, such as proteins, biological cells and tissues. This review paper primarily deals with 'biocompatibility' of polymer materials against various biological elements in blood flow system.