A number of materials have been proposed for use as biomaterials, including hydrophilic, phase-separated and zwitterionic polymers. The mechanisms responsible for the bio/blood compatibility (bioinertness) of these polymers at the molecular level have not been clearly demonstrated, although many theoretical and experimental efforts have been made to understand these mechanisms. Water interactions have been recognized as fundamental for the biological response to contact with biomaterials. We have proposed the 'intermediate water' concept, in which water clearly exhibits defined peaks for cold crystallization in the differential scanning calorimetry chart and presents a strong peak at 3400 cm-1 in a time-resolved infrared spectrum. We found a localized hydration structure consisting of three hydrated waters in poly(2-methoxyethyl acrylate). We hypothesized that intermediate water, which prevents the proteins and blood cells from directly contacting the polymer surface, or non-freezing water on the polymer surface has an important role in the bio/blood compatibility of polymers. We will provide an overview of the recent experimental progress and a theoretical description of the bio/blood compatibility mechanisms as determined by thermal, spectroscopic and surface force measurements.
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