THERMAL AND DIFFUSION-PROCESSES IN LASER-INDUCED STRESS-RELAXATION AND RESHAPING OF CARTILAGE

The thermodynamic characteristics of the 'bound-to-free' phase transfo rmation of water are studied by means of differential microcalorimetry and FTIR spectroscopy. This phase transition controls the stress rela xation and reshaping of cartilage which we have observed previously un der moderate laser heating. It is shown that the FTIR spectrum of boun d water in cartilage differs from that of free water in cartilage, and that both show differences to the FTIR spectrum of pure water. The pr oportion of bound water in cartilage is found to be of the order 4%. W e have examined water liberation and absorption kinetics and found the m to be controlled by diffusion through the tissue and also by the bou nd-to-free water transformation. The theoretical modelling and experim ental data allowed calculation of diffusion coefficients and the activ ation energy for water transfer. The latter was found to be close to t he heat of phase transformation of water. We have established that the drying and wetting processes in cartilage are reversible (fully or in a part, depending on the temperature of preliminary drying) and can b e described by identical solutions of the diffusion problem, with coef ficients of water diffusion being the same. The mechanism of water dif fusion in cartilaginous tissue is also discussed. (C) 1997 Published b y Elsevier Science Ltd.