Temperature Controlled GaAs Laser System for the Soldering of Blood Vessels

Dr. Eyal Gur, Department of Plastic Surgery, Ichilov Hospital,Tel Aviv

We proposed to develop a system based on three novel features:

  1. A hollow stent made of dried albumin.
  2. Albumin solder which would include ICG coloring agent, and this would make it possible to use small and portable GaAs laser for soldering.
  3. A fiber-optic system which can controllably heat spots of diameter 1mm - as needed for the anastomoses of small blood vessels. It was also proposed to try the system for bonding of blood vessels in animals

Experimental and Theoretical Studies

  1. Albumin Stents: We developed sheets of dried (semi-solid) albumin and studied their interaction with both CO2 and GaAs laser radiation. Strips of semi-solid albumin were cut from the sheets and then rolled to form small tubes. Some of these tubes were further dried and used as intralumenal stents. We also developed a special device, which could form tubular stents of outer diameter smaller than 1 mm.
  2. Albumin + ICG: Pure albumin does not absorb well the near - IR radiation emitted by a GaAs laser and it cannot be heated directly by this radiation. In order to heat albumin it is necessary to add to it some coloring agent. We have used for this purpose indocyanine green (ICG), which is approved for clinical use and has a strong absorption at the GaAs laser wavelength.
  3. Temperature Control: We constructed a temperature controlled Ga:As laser system based on the previous experience of our group with the CO2 laser soldering system. Our group has developed silver halide flexible optical fiber capable of delivering the emitted IR radiation from the solder surface to the IR detector. The laser output power is controlled by the IR detector
  4. Soldering Experiments with Colored Albumin Stents: We have tested two different schemes for laser heating of the albumin:
    1. Colored Stent: The stent was prepared from albumin, which was mixed with ICG, so that the stent itself was green. A blood vessel was cut in two, the albumin tube served as a stent over which the two halves of the blood vessels were approximated, in vitro. An albumin solution (of concentration 45%) was mixed with ICG and used as a "wet" solder that was spread outside the cut. The laser radiation heated both the outer albumin layer and the inner stent.
    2. Colorless Stent: In this case the stent was prepared from pure albumin (without any coloring agent). The albumin solution was again mixed with ICG. The experiment was repeated and the colored albumin was spread on the cut, on the outside. The colored albumin was then heated by a GaAs laser, under temperature control.
  5. Burst pressure measurements: Measurements of soldered vessels were carried out in both cases (A) and (B). We did not see any difference between the cases, and therefore we concluded that the coloring of the albumin stent itself did not contribute to the bonding strength.
  6. Studies of Laser Heating of Albumin ''enriched'' by ICG: As mentioned above, using a diode laser as the heating source for the albumin solder, requires the addition of a chromophore (i.e. ICG) which will absorb the laser radiation in the solder. The heating depends on the concentration of the absorbing dye (i.e. ICG) and we found the minimum concentration needed to obtain good heating. Most of the laser energy was absorbed in the upper layer of the albumin solder, closer to the laser source. Irradiation of the solder produced a temperature gradient throughout the depth of the solder. The gradient was a function of the irradiance, the absorption coefficient of the solder/dye combination and the thermal conduction coefficient of the solder. It was difficult to determine the temperature at the solder-tissue interface. In the actual experiments we controlled the temperature of the upper layer of the colored albumin, at values close to 65oC. Based on a set of preliminary measurements, we estimated that after 2 seconds the temperature at the solder tissue interface was 5oC lower than that of the upper surface of the solder during laser irradiation.
  7. The Full Soldering System: We developed a full soldering system based on a GaAs laser, which was small, portable and could be battery operated. An infrared fiberoptic system provided temperature control on a spot of diameter 1 mm. A GaAs semiconductor laser diode, operating at a central wavelength of 828 nm (Sharplan 6040, Lumenis, Yokneam, Israel) was used to heat a spot on the vessel covered with albumin doped with ICG. The temperature control system changed the laser power according to the surface temperature in order to maintain a preset temperature. The laser was operated in the CW mode, with an output power range 0 - 2.2 W. The laser emission was delivered to the vessel using a standard silica fiber, located 3 mm from the tissue to achieve minimal spot diameter. The preliminary results obtained with this system were very good.
  8. Study of the Interaction between GaAs Laser Radiation and Albumin and Tissue: students of Prof. Hardy, using Monte-Carlo simulations, investigated theoretically the absorption and scattering of GaAs laser light in tissues. This study together with extension of previous study on thermal distribution in tissues enabled us to predict optimal parameters for laser soldering. The theoretical study reduced significantly the number of animal experiments needed in order to reach satisfactory results.

The preliminary experiments with albumin solder + ICG and with the controlled temperature laser soldering system were very successful. We believe that the system, which utilizes GaAs laser, ''colored'' albumin as a solder and a reliable temperature control system, will be extremely useful for the anastomoses of small blood vessels. It is planned at this stage to continue this research and preform laser bonding experiments on animal models.