This is the first study that looks at the plasma dynamics of ultraviolet lasers in living tissue, says Shane Hutson, assistant professor of physics at Vanderbilt University who conducted the research with post-doctoral student Xiaoyan Ma. 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 such difference involves the elasticity, or stretchiness, of tissue. By stretching and absorbing energy, the biological matrix constrains the growth of the micro-explosions. As a result, the explosions tend to be considerably smaller than they are in water. This 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 surprising difference involves the origination 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 po
|Contact: David F. Salisbury|