Soldering of Blood Vessels using a System based on a CO2 Laser and AgClBr Fibers

Prof. Amos Hardy, Faculty of Engineering Tel-Aviv University
Prof. Abraham Katzir, Faculty of Exact Sciences Tel-Aviv University


During the last few years researchers in the Applied Physics Group, under the supervision of Prof. Abraham Katzir, have developed a fiber-optic laser system for bonding incisions in tissues. The system is based on three components:

  1. A CO2 laser which heats a spot on an incision;
  2. A fiber-optic radiometer which measures the temperature of the heated spot on the incision and
  3. A computer which controls the heating and stabilizes the temperature.M/li.
These researchers have actually used a method called "laser soldering" – heating an incision covered by a layer of biological solder (e.g. albumin). Laser soldering has been successfully used for bonding of cuts in skin, cornea, urinary bladder, dura and other tissues.

The main goal of the present research was to change the method and adapt it for the soldering of blood vessels. In the past, Dr. Eyal Gur and Dr. Dudi Leshem of the Plastic Surgery Department of Ichilov hospital in Tel Aviv carried out experiments on blood vessels of rats. The research was carried out in the Animal Facilty in Tel Aviv University with the help of the veterinary Dr. Noam Kariv. In these experiments it has been discovered that the transition from skin to blood vessels was not trivial and that it would be a real challenge.

This research project involved several stages: (a) Theory - Preliminary work has already been done by Avi Ravid, who studied the temperature distribution in skin which was irradiated by a CO2 laser. The theoretical work had to be expanded to blood vessels. This theoretical work was a project given to Yaron Rabi, a graduate student, who worked under the supervision of Prof. Amos Hardy. Mr. Rabi also tested the idea of replacing the CO2 laser by a GaAs laser which is much easier to use (b) Experimental work on the fiber laser system - The system had to be adapted for work with tiny, cylindrical, blood vessels. (c) Experimental work on albumin - the process of soldering with albumin had also to be adapted for cylindrical blood vessels. Also, it has been planned to start experiments using GaAs lasers. (d) Animal experiments -Experiments on animals were planned. These experiments were to be carried out by Dr. Eyal Gur, and some younger physicians.

Theoretical Results
The absorption and scattering of GaAs laser light in tissues was investigated theoretically by means of Monte-Carlo simulation. This study together with extension of Avi Ravid’s study on temperature distribution in tissues enabled us to predict the optimal parameters for laser soldering. The temperature control module influence is currently under investigation. Temperature control system is essential for a successful laser soldering procedure. The theoretical study reduced significantly the number of animal experiments needed in order to reach satisfactory results. We believe that system which utilizes GaAs laser, in addition to mixture of ICG and albumin as solder, and temperature control system will result in a superior laser tissue soldering system.

Experimental Results - Improvements in the Fiber-optic Laser System
The original fiber-laser system was based on unclad AgClBr fibers of typical diameter 1mm. The CO2 laser beam divergence from these fibers was relatively large, and as a result a spot of diameter 3-4mm was heated. The system controlled the temperature on a spot whose diameter was much larger than the typical diameter of a small blood vessel. Therefore such a system would not be useful for bonding of cuts in such blood vessels. The spot diameter should be smaller than 1mm.

During the research year, the Applied Physics Group has developed AgClBr fibers which consisted of core and cladding. These fibers offered two advantages: (i) The diameter of the core was relatively small (ii) The divergence from the core was relatively small. Preliminary results that had been obtained with core/clad fibers indicate that it would be possible to heat and to control the temperature in a spot of diameter smaller than 1mm.

Also, preliminary results were carried out in vitro using a solder made of albumin mixed with ICG. The laser used was a GaAs laser. The results were very encouraging.

Experimental Results - Albumin Stent
In the past we tried to make a longitudinal incision in a coronary artery in a rat and to try to bond this incision with the aid of controlled heating.

This approach presented several problems:

  1. It was necessary to stop the flow of blood from both sides of the incision with the aid of clamps. This is the usual procedure when blood vessels are sutured clinically. When the clamps are used, the walls of the blood vessels "collapse" in the absence of blood pressure. In the case of suturing this is not much of a problem. In the case of soldering it is much more difficult to keep the sides of the incision approximated.
  2. Immediately after the soldering, the clamps stopping the flow of blood are released and the blood pressure in the blood vessel rises rapidly. Therefore the immediate strength of the bond is very important, because it has to hold the blood vessel together, and must not permit blood leakage.

During the research year we gradually developed a partial solution to these problems. We managed to fabricate tubes made of dried albumin. These tubes serve as stents. We try to insert such a stent into a cut end of a blood vessel and then attach to it another cut end, and then attach the two ends together. We then try to use albumin solder for bounding the two ends together, i.e. to perform anastomoses. This albumin stent will hopefully solve the two problems mentioned above. Some preliminary experiments have already been carried out on bonding of two plastic tubes, and they showed the great potential of the new method for bonding blood vessels.

Results of the animal experiments.
In all of our preliminary experiments we failed to generate reliable and strong bonding of cuts in blood vessels, and the mortality rate of rats in these experiments was high. We therefore stopped the animal experiments for a few months. Now that we seem to have solved the problems, we plan to resume the laser bonding of longitudinal cuts in blood vessels in rats. We will also plan to carry out anastomoses experiments in these animals.

We dedicated most of our efforts, during the year of research, to the development of a better system for laser soldering of blood vessels. We made significant progress in the theoretical work, which was adapted for use with blood vessels. We improved the CO2 laser soldering system and made it possible to heat small spots. We also established a totally new idea for soldering of blood vessels, based on albumin tubes. At the same time we also tried to use the more practical GaAs laser for bonding of blood vessels, using albumin + ICG solder.

We plan to use all these development for further studies of bonding of blood vessels, first in vitro and then in rats, in vivo. With the success of such experiments we will be able to move to larger animals, such as pigs. This should pave the way for clinical trials, in the future.