A generalized model for the interaction of microwave radiation with bound water in biological material.

Calculations have been performed concerning the deposition of microwave energy in bound water surrounding a biological macromolecule immersed in a continuum consisting of free water and dissolved ions. In particular a previous model of a hydrated macromolecule has been generalised to one where the relaxation frequency of the bound water varies across the layer, and the calculations have been carried out for three combinations of values of ionic conductivity of the bound water and the continuum. When these conductivities are equal, but low, the average energy deposition per unit volume in the bound water is greater, sometimes by at least an order of magnitude, than that in the continuum at frequencies in the region of hundreds of MHz to a few GHz. As the ionic conductivity increases this effect decreases and at conductivities equal to that of physiological saline the specific energy deposition in the bound water is not more than around twice that in the surrounding electrolyte continuum over this frequency range. Therefore, for tissues of high bound water content exposed to microwaves of a given power density the biological effect produced is enhanced. For a biological tissue of high ionic conductivity with 20% of its water in the bound state the overall energy absorption would be 25% greater at certain frequencies than if all the water were in the free state. For materials of low ionic conductivity the increase would be more than one order of magnitude.