How Lasers Melt Flesh

The first study of ultraviolet lasers ability to cut living tissues reveals some interesting details about the physics of flesh melting.

Lasers, an acronym for Light Amplification by Stimulated Emission of Radiation, are becoming more and more used in healthcare. Medical applications include dental drills, unwanted hair removal, eye surgery, treating wrinkles and stretch marks, pain relief, angioplasty, endoscopy procedures, and photodynamic therapy for cancer. For a host of procedures, intense beams of coherent light are gradually replacing the knife. However, there is still much that scientists dont know about the ways in which laser light interacts with living human tissue

The research, published online in Physical Review Letters October 10, 2007, was headed up by Shane Hutson, assistant professor of physics at Vanderbilt University.

“This is the first study that looks at the plasma dynamics of ultraviolet lasers in living tissue,” says Hutson. “The subject has been extensively studied in water and, because biological systems are overwhelmingly water by weight, you would expect it to behave in the same fashion. However, we found a surprising number of differences.”

One difference comprises the elasticity of tissue. By stretching and absorbing energy, the biological tissue matrix limits the growth of the micro-explosions. These explosions, as a result, tend to be much smaller than they are in water, which in turn reduces the damage that the laser beam causes while cutting flesh. This effect had been predicted, but the researchers found that it is considerably larger than expected.
Another unexpected difference involves the origin of the individual plasma “bubbles.”

All it takes to seed such a bubble is a few free electrons. These electrons pick up energy from the laser beam and start a cascade process that produces a bubble that grows until it contains millions of quadrillions of free electrons.

Subsequent collapse of this plasma bubble causes a micro-explosion. In pure water, it is very difficult to get those first few electrons. Water molecules have to absorb several light photons at once before they will release any electrons. So a high-powered beam is required.

“But in a biological system there is a ubiquitous molecule, called NADH, that cells use to donate and absorb electrons. It turns out that this molecule absorbs photons at near ultraviolet wavelengths. So it produces seed electrons when exposed to ultraviolet laser light at very low intensities”‚  says Hutson. This means that in tissue containing significant amounts of NADH, ultraviolet lasers dont need as much power to cut effectively as people have thought.

NADH is the reduced form of NAD, Nicotinamide adenine dinucleotide, a key coenzyme found in cells. The cornea of the eye is one example of tissue that has very little NADH. As a result, it responds to an ultraviolet laser beam more like water than skin or other kinds of tissue, according to the researchers.

Now that we have a better sense of how tissue properties affect the laser ablation process, we can do a better job of predicting how the laser will work with new types of tissue,” says Hutson.

Effect Variables are Many

The effect that powerful lasers have on actual flesh varies both with the wavelength, or color, of the light and the duration of the pulses that they produce. The specific wavelengths of light that are absorbed by, reflected from or pass through different types of tissue can vary substantially. Therefore, different types of lasers work best in different medical procedures.

For lasers with pulse lengths of a millionth of a second or less, there are two basic cutting regimes:

-Mid-infrared lasers with long wavelengths cut by burning. That is, they heat up the tissue to the point where the chemical bonds holding it together break down. Because they automatically cauterize the cuts that they make, infrared lasers are used frequently for surgery in areas where there is a lot of bleeding.

-Shorter wavelength lasers in the near-infrared, visible and ultraviolet range cut by an entirely different mechanism. They create a series of micro-explosions that break the molecules apart. During each laser pulse, high-intensity light at the laser focus creates an electrically-charged gas known as a plasma.

At the end of each laser pulse, the plasma collapses and the energy released produces the micro-explosions. As a result, these lasers particularly the ultraviolet ones can cut more precisely and produce less collateral damage than mid-infrared lasers. That is why they are being used for eye surgery, delicate brain surgery and microsurgery.

Laser Ablation

In laser ablation, material is removed from a surface by irradiating it with a laser beam. At low laser flux, the material is heated by the absorbed laser energy and evaporates or sublimes. At high laser flux, the material is usually converted to a plasma. Typically, laser ablation involves removing material with a pulsed laser, but it is possible to ablate material with a continuous wave laser beam if the laser intensity is high enough.

One recent application is endovenous laser therapy for varicose vein treatment, in which laser energy is passed to the targeted area inside the vein via a laser fiber the size of a piece of spaghettini. When the laser is triggered, it delivers heat energy to the blood and vein tissues. This causes small areas of localized vein tissue damage.

The laser fiber is slowly drawn along the course of the vein until the entire vessel is closed off and becomes inert, permanently getting rid of the varicose vein. The procedure is done with only local anesthetizia on an outpatient basis, and eliminates general anesthesia, infection risk, pain and swelling, and the lengthy recovery associated with the traditional vein stripping treatment.