Pressure effects on soft tissues monitored by changes in tissue optical properties

For pulsed laser tissue welding, an appropriate pressure needs to be applied to the tissues to achieve successful welds. In this study, we investigated the influences of pressure on in vitro optical properties of elastin biomaterial. The optical properties were measured as a function of pressure with a double integrating-sphere system. A He-Ne laser (633 nm) was used for all measurements. Each sample was sandwiched between microscope slides and then compressed with a spring-loaded apparatus. Transmittance and diffuse reflectance of each sample were measured under a pressure (0-1.5 kg/cm² and then released to 0). Absorption and reduced scattering coefficients were calculated using the inverse doubling method from the measured transmittance and reflectance values. Results from this study demonstrated: 1) The overall transmittance increased while the reflectance decreased as the tissue thicknesses were reduced up to 72% and the tissue weights were decreased about 40%, 2) The absorption and scattering coefficients increased with increasing the pressure, and 3) The pressure effects on the tissue optical properties were irreversible. Possible mechanisms responsible for the changes in the tissue optical properties were also investigated by changing tissue thicknesses or weights (through dehydration). This study implies that changes in tissue thickness and water content are important factors that affect tissue optical properties in different ways.