Structural and dynamical characterisation of intermediate water interacting polyvinyl pyrrolidone

Polyvinyl pyrrolidone (PVP) is well known as a biocompatible polymer used in the fields of medical devices and pharmaceuticals, and is associated with intermediate water, which is said to play an important role in developing the biocompatibility of medical materials. The states of water molecules interacting with PVP, i.e., the non-freezing water, slow and fast intermediate water are classified using H-2 NMR. The non-freezing water is directly hydrogen-bonded to ca. 1.5 amide groups of PVP. The slow intermediate water interacts with PVP indirectly through non-freezing water. It then shows slow diffusion dynamics and forms cubic-form ice during cold crystallisation, while the fast intermediate water can diffuse as fast as free water, and can freeze to form normal hexagonal-form ice. Statement of significance For many of the medical materials, including artificial organs such as dialysis membranes and artificial blood vessels, polymers are used. Biocompatibility of polymer is an important property for materials used in medical devices and medicine. Although there are lots of studies about surface design of materials in terms of hydrophilicity, charge, or morphology, the unified expression factor of biocompatibility have not been clear. Recently, it is suggested that water molecules, especially intermediate water, on materials surface play important roles in the development of biocompatibility. However, the characteristics of the intermediate water have not been understood. It is important issues to be solved in the development of new and high-performance biocompatible polymer materials based on the concept of intermediate water. In present study, we have investigated the intermediate water on polyvinyl pyrrolidone (PVP), which is biocompatible polymer widely used in medical device and medicine. Differential scanning calorimeter (DSC), X-ray diffraction (XRD), and various nuclear magnetic resonance (NMR) methods for PVP aqueous solutions were performed in order to clarify the physical properties of the intermediate water. We found that the intermediate water has structural order and diffusive mobility just between free water and non-freezing water. These features of intermediate water are induced by structural interaction with polymer. The knowledge from present study expected to provide guidance for the design of biocompatible materials.