14 DAY TRIAL // The steel scalpel is rapidly disappearing from the surgeons' arsenal of instruments. Laser now performs from cosmetic to brain surgery, but the technique remains mysterious - we know what it does, but we don't know why it does it.
This is exactly the question to which a new research - published online in Physical Review Letters on October 10: how UV lasers (like those employed in LASIK eye surgery) slice tissues - is attempting to provide an answer.
How lasers chop living tissues was found to be linked to the wavelength (color) of the light and pulse duration, explaining why various types of lasers are proper in specific surgery procedures.
In lasers pulsing up to a millionth of a second, two mechanisms were detected. Mid-infrared lasers slash by burning and because they cauterize the tissue at the same time, they are preferred for areas prone to abundant bleeding. Shorter wavelength lasers (near-infrared, visible and UV range) work totally differently: they induce micro-explosions tearing apart the molecules. Each laser pulse creates plasma (an electrically-charged gas) which breaks off the energy required for creating the micro-explosions.
This last mechanism, especially in the case of UV light, delivers an extremely precise cut, with minimal collateral damage, required in eye and brain surgery or microsurgery. "This is the first study that looks at the plasma dynamics of ultraviolet lasers in living tissue. 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.", said co-author Shane Hutson, assistant professor of physics at Vanderbilt University.
The biological matrix is elastic and because it also absorbs energy, the growth of the micro-explosions is hampered, being much smaller than they are in water, and this is why the laser beam causes limited damage in the tissue. The effect was found to be much larger than researchers forecast. The plasma "bubbles", triggered by a few free electrons, fuel with energy coming from the laser beam and grow up to millions of quadrillions of free electrons. Bubble's collapse translates into a micro-explosion.
In pure water, those first few electrons are difficult to activate, requiring an extremely powerful beam. "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," said Hutson.
This makes UV lasers work with much lower energy levels than believed. The eye's cornea is a tissue poor in NADH, that's why it reacts to an ultraviolet laser beam more like water than skin or other kinds of tissue, as the team concluded. "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," said Hutson.