Water and the acute biological response to surfaces

Molecular self association in water through hydrogen bonding is a powerful organizational force leading to a three-dimensional hydrogen-bonded network (water structure) that profoundly influences solvent properties. Localized perturbations in the chemical potential of water as by, for example, contacting with a solid surface, induces compensating changes in water structure that can be sensed tens of nanometers from the point of origin using the surface force apparatus (SFA) and ancillary techniques. These instruments reveal attractive or repulsive forces between opposing surfaces immersed in water, over-and-above that anticipated by continuum theory (DLVO), that are attributed to a variable density (partial molar volume) of a more-or-less ordered water structure, depending on the water wettability (surface energy) of the water-contacting surfaces. Water structure at surfaces is thus found to be a manifestation of hydrophobicity and, while mechanistic/theoretical interpretation of experimental results remains the subject of some debate in the literature, convergence of experimental observations permit a quantitative definition of the heretofore relative terms 'hydrophobic' and 'hydrophilic'. In particular, long-range attractive forces (< 100 nm) are detected only between surfaces exhibiting a water contact angle theta > 65 deg (defined as hydrophobic surfaces with pure water adhesion tension tau(0) = gamma(0)cos theta < 30 dyn cm(-1) where gamma(0) is water interfacial tension = 72.8 dyn cm(-1)). Short range repulsive forces (< 5 nm) are detected between surfaces exhibiting theta < 65 deg (hydrophilic surfaces, tau(0) > 30 dyn cm(-1)). These findings together with other lines of chemical evidence suggest at least two distinct kinds of water structure and reactivity: a relatively less-dense water region against hydrophobic surfaces with an open hydrogen-bonded network and a relatively more-dense water region against hydrophilic surfaces with a collapsed hydrogen-bonded network. Solvent properties of interfacial water profoundly influence the biological response to materials in a surprisingly straightforward manner when key measures of biological activity sensitive to interfacial phenomenon are scaled against water adhesion tension tau(0) of contacting surfaces. Protein adsorption, activation of blood coagulation, and bioadhesion are offered as examples in point, illustrating that the hydrophobic/hydrophilic contrast in the biological response to materials, often disputed in biomaterials science, is very clear when viewed from the perspective of water structure and reactivity at surfaces.