Lasers have been widely used in surgery for making cuts or for the removal of tissues, but they may also be used for the closure of wounds and incisions. The bonding of the edges of human tissues is an important step in most surgical procedures. The edges have to be approximated, and this allows the beginning of the natural wound-healing processes.
Bone needles from the Paleolithic period (around 30,000 years ago) may indicate that the edges of wounds were already stitched at that prehistoric time. Suturing was used 4000 years ago in ancient Egypt, Greece, and India, and the tradition continues in modern times.
Today, the standard procedures for tissue closure rely on sutures or on metallic and polymeric staples or clips to approximate the incision edges. These technologies are easily adapted to most types of tissues. However, these procedures injure the tissue and leave a foreign body in it. They do not provide watertight closure, and the closure is highly dependent on the physician’s skill. Moreover, these bonding techniques cannot be easily adapted for endoscopic procedures.
An alternative method makes use of various chemical adhesives (e.g. cyanoacrylates). Many of these adhesives are toxic, however, and they cannot be used for internal or endoscopic tissue bonding.
A new way of healing
In the 1970s, physicians discovered that local heating might speed closure and healing, in a process akin to the welding of solids. Many groups tried to use this laser welding technique as a novel method for bonding of cuts in tissues, using different lasers.
It was then suggested that a biological substance such as albumin or fibrin could be added over the cut and heated with the underlying tissues, with the heated substance acting as biological glue.
This laser soldering procedure produced a stronger bond.
Laser bonding of tissues is, potentially, a very promising technique. It introduces no foreign material, it is less traumatic, it provides a watertight closure, and it may leave minimal scars. It is also faster and easier to apply, useful for various types of tissues, and much less skill dependent.
Yet, neither laser welding nor laser soldering has been widely used in the clinical setting. This is probably because of the unreliable and irreproducible results reported by many groups, and because the initial strength of the repair was not sufficient for practical applications.
The underlying reason for these problems may have to do with the temperature.
On the one hand, there is a need to heat the tissue edges in order to obtain strong and reliable bonding. On the other hand, heating causes biological damage in tissues. The damaging effects depend exponentially on temperature and linearly on time. There is only a narrow temperature range in which tissue welding or tissue soldering is expected to be efficient (e.g. 60º–65º C). A lower temperature may lead to no bonding and a higher temperature to significant thermal damage.
In both cases the immediate tensile strength is expected to be insufficient. Most of the groups who worked on laser bonding did not address these issues, and this may have led to the inconsistent results.
Controlling laser temperature
The Applied Physics Group at Tel Aviv University assumed that accurate temperature control of the bonded tissue site is essential in order to reduce thermal damage, increase the immediate bond strength, and yield reproducible results. They developed a smart fiber-optic laser system that accurately monitored and controlled the temperature of the soldered spot, as shown below.
A cross section of incised tissue with albumin layer spread over it. CO2 laser radiation transmitted through a silver halide fiber heats a spot on the albumin. IR radiation emitted from the surface is transmitted to a second silver halide fiber, onto an IR detector. A computer uses the signal obtained from the detector to control the temperature of the spot.
The basic idea is to approximate the edges of a cut, apply some biological solder, and heat a spot on the solder using a CO2 laser. The heated spot emits infrared radiation whose intensity is proportional to the temperature T of the spot. The intensity is measured by an IR detector, which generates a voltage V that is read by a personal computer (PC). A dedicated computer program uses the value of V to control the power emitted by the CO2 laser, so that the temperature T is kept constant for a time t.
After this time, the laser beam is moved to a neighboring spot which is again heated to temperature T for a time t.
Strong bonding occurred for such spot soldering if each spot was heated to T = 60º – 65º C for ≈ 10 seconds.
Flexible optical fibers
An important development for this technique was the incorporation of optical fibers in the system. The standard optical fibers, used in medical applications, are totally opaque in the IR, and special optical fibers, made of AgClBr, were developed by the Applied Physics Group.
These fibers made it possible to transmit the CO2 laser radiation onto the heated spot and to transmit the IR radiation emitted from the spot onto the IR detector. These fibers are flexible, insoluble in water, and bio compatible. The two distal tips of the fibers were held in a handpiece, which made it much easier for the surgeon to carry out the procedure.
This system made it possible to heat a spot on tissue to a desired temperature, with accuracy of roughly 3º C. Such a system enables laser welding (without any solder) or laser soldering of tissues under accurate temperature control.
Testing on animals
During the last few years, the Applied Physics Group has been working on laser welding and soldering of different types of tissues. We collaborated with many physicians in several medical centers in Israel and bonded incisions in animal models (in vivo) in numerous types of tissues, including cornea, dura, trachea, small bowel, urinary bladder and blood vessels.
The smart system has generated higher immediate and long-term tensile strength, faster wound healing, and reduced scarring. The system is based on IR fibers, which made it possible to carry out endoscopic bonding of incisions in the kidneys of farm pigs in vivo.
In experiments for bonding of incisions on the skins of large farm pigs, it was established that the wound healing was faster than with standard suturing, and there were practically no scars after laser soldering. As a result, we obtained permission to carry out clinical trials.
In the clinical trials, 10 patients underwent laparoscopic cholecystectomy, an endoscopic procedure for removal of the gallbladder. In each patient, four incisions were left in the abdomen area at the end of the procedure. Two of the incisions were sutured, using the standard surgical procedure, and two of the incisions were laser soldered, using the laser-soldering system and a human albumin solder.
A sutured cut (left) versus laser-soldered cut (right) on the skin of a human patient’s abdominal area, 30 days postoperatively.
Laboratory tests showed that our laser soldering system was able to control surface temperature very accurately. Our system succeeded to bond 20 cuts in 10 patients. The scar in the laser-treated cuts seemed much smaller one month after the operation. We are waiting now for photographs that were taken one year after the procedure.
Other laser applications
Laser bonding of tissues will have a wide range of applications. The technique could be used in plastic surgery, bonding cuts with practically no scarring. It could be used in complex surgeries on blood vessels (especially microsurgery of small blood vessels).
Laser bonding might be used in ophthalmic surgery, such as cornea transplants, perhaps reducing the discomfort and inflammation caused by standard sutures that must remain in the cornea for a long time. It may be used in endoscopic procedures, or in robotic surgery, where the use of sutures requires great skill.
We have been working in all these fields. Laser bonding will not replace standard bonding techniques in surgery, but it will add an important tool for many surgeons.
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TAU’s Applied Physics Group
The Applied Physics Group at Tel Aviv University is involved with the research and development of devices that operate in the mid-IR (3-30 microns). Consisting of some 20 graduate students, technicians, post-docs, and other researchers, the group has developed semiconductor lasers, electro-optical systems, and optical fibers for this spectral range.
The group was among the first to develop crystalline fibers made of silver halides (AgClBr), and most of its research projects make use of the unique properties of these fibers.
Other research topics include material science, solid state physics, lasers and electro-optics, biomedical engineering, and environmental studies.
SPIE Fellow Abraham Katzir founded and directs the Applied Physics Group at Tel Aviv University (Israel). He organized SPIE’s first conference on biomedical optics in 1984 and coined the term BiOS. The BiOS symposium, part of SPIE Photonics West, is now the largest such meeting in the world.
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