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, conta cting with a solid surface, induces compensating changes in water structure that can be sensed tens of nanometers from the point of origin using the s urface force apparatus (SFA) and ancillary techniques. These instruments re veal attractive or repulsive forces between opposing surfaces immersed in w ater, over-and-above that anticipated by continuum theory (DLVO), that are attributed to a variable density (partial molar volume) of a more-or-less o rdered water structure, depending on the water wettability (surface energy) of the water-contacting surfaces. Water structure at surfaces is thus foun d to be a manifestation of hydrophobicity and, while mechanistic/theoretica l interpretation of experimental results remains the subject of some debate in the literature, convergence of experimental observations permit a quant itative definition of the heretofore relative terms 'hydrophobic' and 'hydr ophilic'. In particular, long-range attractive forces (< 100 nm) are detect ed only between surfaces exhibiting a water contact angle theta > 65 deg (d efined as hydrophobic surfaces with pure water adhesion tension tau(0) = ga mma(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 bet ween 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 re latively less-dense water region against hydrophobic surfaces with an open hydrogen-bonded network and a relatively more-dense water region against hy drophilic 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 me asures of biological activity sensitive to interfacial phenomenon are scale d against water adhesion tension tau(0) of contacting surfaces. Protein ads orption, activation of blood coagulation, and bioadhesion are offered as ex amples in point, illustrating that the hydrophobic/hydrophilic contrast in the biological response to materials, often disputed in biomaterials scienc e, is very clear when viewed from the perspective of water structure and re activity at surfaces.