Photothermal effects of laser tissue soldering

Low-strength anastomoses and thermal damage of tissue are major concerns in laser tissue welding techniques where laser energy is used to induce therm al changes in the molecular structure of the tissues being joined, hence al lowing them to bond together. Laser tissue soldering, on the other hand, is a bonding technique in which a protein solder is applied to the tissue sur faces to be joined, and laser energy is used to bond the solder to the tiss ue surfaces. The addition of protein solders to augment tissue repair proce dures significantly reduces the problems of low strength and thermal damage associated with laser tissue welding techniques. Investigations were conducted to determine optimal solder and laser paramet ers for tissue repair in terms of tensile strength, temperature rise and da mage and the microscopic nature of the bonds formed. An in vitro study was performed using an 808 nm diode laser in conjunction with indocyanine green (ICG)-doped albumin protein solders to repair bovine aorta specimens. Liqu id and solid protein solders prepared from 25% and 60% bovine serum albumin (BSA), respectively, were compared. The efficacy of temperature feedback c ontrol in enhancing the soldering process was also investigated. Increasing the BSA concentration from 25% to 60% greatly increased the tens ile strength of the repairs. A reduction in dye concentration from 2.5 mg m l(-1) to 0.25 mg ml(-1) was also found to result in an increase in tensile strength. Increasing the laser irradiance and thus surface temperature resu lted in an increased severity of histological injury. Thermal denaturation of tissue collagen and necrosis of the intimal layer smooth muscle cells in creased laterally and in depth with higher temperatures. The strongest repa irs were produced with an irradiance of 6.4 W cm(-2) using a solid protein solder composed of 60% BSA and 0.25 mg ml(-1) ICG. Using this combination o f laser and solder parameters, surface temperatures were observed to reach 85 +/- 5 degrees C with a maximum temperature difference through the 150 mu m thick solder strips of about 15 degrees C. Histological examination of t he repairs formed using these parameters showed negligible evidence of coll ateral thermal damage to the underlying tissue. Scanning electron microscop y suggested albumin intertwining within the tissue collagen matrix and subs equent fusion with the collagen as the mechanism for laser tissue soldering . The laser tissue soldering technique is shown to be an effective method for producing repairs with improved tensile strength and minimal collateral th ermal damage over conventional laser tissue welding techniques.