Water molecules play a crucial role in bio-interfacial interactions, including protein adsorption/desorption and cell adhesion behavior. To understand the role of water in the bio-interfaces, it is necessary to compare the states of hydration water with various physicochemical properties of hydrated materials. The states of water were analyzed by differential scanning calorimetry, solid state NMR, in situ attenuated total reflection infrared spectroscopy, soft X-ray emission spectroscopy, surface force measurements, and wide variety of analytical techniques. We found that intermediate water, which is loosely bound to a polymer, is a useful indicator of the biocompatibility of material surfaces. In addition, atomic force microscopy (AFM) revealed the presence of nanostructures on material/water interface. These structures are generated by the phase separation of material and water at the interfacial region. The material-poor (water-rich) regions contribute to the biocompatibility of materials. Our research revealed that the material density on the interface is an important factor. Frequency modulation AFM demonstrated that the hydrated material layer causes the repulsion force in the material-poor region on the material/water interface. This finding on intermediate water provides novel insights for materials design and helps develop novel experimental tools for understanding protein adsorption/cell adhesion on materials/medical devices.

The tribological properties between single hair fibers in wet conditions were investigated to evaluate the tangling/detangling performance of shampoo formulations during washing. A new setup for friction measurements between taut single hair fibers under large load was established. Using the setup, friction of hair fibers treated with different cosmetics solutions were evaluated; particular attention was focused on the static (stick-slip) friction behavior which determined the tangling/detangling performance. We found that a water-soluble silicone-type lubricant, bis-isobutyl polyethylene glycol (PEG)-14/amodimethicone copolymer (BIPA), in dilute aqueous solution eliminated stick-slip behavior; the shampoo formulation including BIPA exhibited excellent detangling performance. The detailed mechanism of why BIPA exhibited such excellent lubricity was further investigated by the precise nanotribological measurement using the surface forces apparatus. BIPA aqueous solution was confined between molecularly smooth mica surfaces, compressed under load, and sheared between each other. The obtained friction coefficient was extremely small, of the order of 10^–5, indicative of the “hydration lubrication” mechanism. BIPA has positive charges in aqueous solution which forms adsorbed film on negatively charged mica surface (hair surface is also negatively charged). The BIPA film has hydrated water layer on the top surface which has high fluidity and contributes to excellent lubricity.