The first of its kind study was conducted by Shane Hutson, assistant professor of physics at Vanderbilt University along with post-doctoral student Xiaoyan Ma who found that despite the increasing popularity of laser surgeries, the knowledge regarding the ways in which laser light interacts with living tissue were not clear among lot of scientists.
The study states that the effect of powerful lasers on actual flesh differs both with the wavelength, or colour, 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, in different medical procedures different types of lasers work best.
For lasers with pulse lengths of a millionth of a second or less, there are two basic cutting regimes. First are the mid-infrared lasers with long wavelengths cut by burning.
In this regime, tissue is heated up to the point where the chemical bonds holding it together break down. It is done so because the chemical bonds automatically cauterise the cuts that they make, infrared lasers are used frequently for surgery in areas where there is a lot of bleeding.
Second are the shorter wavelength lasers in the near infrared, visible and ultraviolet range cut by an entirely different mechanism. In this a series of micro-explosions that break the molecules apart are created. During each laser pulse, high-intensity light at the laser focus creates an electrically charged gas known as plasma. At the end of each laser pulse, the plasma collapses and the energy released produce the micro-explosions.
Due to which these lasers, particularly the ultraviolet ones, are able to 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. "This is the first study that looks at the plasma dynamics of ultraviolet lasers in living tissue," Hutson said.
"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," he said. One difference involved the elasticity, or stretchiness, of tissue. The biological matrix constrains the growth of the micro-explosions by stretching and absorbing energy. As a result, the explosions tend to be considerably smaller than they are in water, which reduces the damage that the laser beam causes while cutting flesh.
Another difference involved the origination of the individual plasma 'bubbles'. A few free electrons are all that are needed to seed such a bubble. These electrons pick up energy from the laser beam and start cascade process that produces a bubble that grows till it contains millions of quadrillions of free electrons. The collapse of this plasma bubble causes a micro-explosion. It is very difficult to get those first few electrons in pure water. Water molecules have to absorb several light photons at once before they will release any electrons and 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," Hutson said.
This meant that tissues containing significant amounts of NADH, ultraviolet lasers don't need as much power to cut effectively as people had thought. "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," Hutson said.
The study is published online in Physical Review Letters.