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A number of biomaterials have been proposed, including hydrophilic, phase-separated and zwitterionic polymers. It has not been clearly elucidated which mechanisms are responsible for biocompatibility on a molecular level. Water interactions have been recognized as a fundamental part of the biological response to contact with biomaterials. We have proposed the ‘intermediate water’ concept; the water exhibited both clear peak for cold crystallization in differential scanning calorimetry chart and a strong peak at 3400 cm−1 in a time-resolved infrared spectrum. We found the localized hydration structure consisting of the three hydrated water in poly(2-methoxyethyl acrylate) (PMEA).