Kinetic models of laser-tissue fusion processes

Laser tissue fusion processes depend primarily on thermal denaturization of tissue collagen: the fibrils of apposed collagen strands apparently unravel under sufficient heat and re-entwine during the cooling phase. Excessive heating desiccates the fibers to a brittle state unsuitable for fusion while inadequate heating results in weak bonds. In all cases local heat transfer processes significantly affect, and may dominate, the thermal damage realized. Consequently, in addition to spot size power and beam activation time, the choice of laser wavelength is critically dependent on the particular vessel or tissue geometry (chiefly the thickness). We have conducted parametric studies on tissue welding laser activation protocols in transient finite difference numerical models which include tissue water vaporization processes in parallel with kinetic models of collagen and smooth muscle thermal damage. The results show the complex inter-relationship between laser parameters and tissue geometry which determines whether successful fusion may be obtained. The advantage of the numerical modeling approach is that individual physical processes may be studied singly to determine their relative importance.