Physical mechanics of laser energy in living systems
Living systems often have many uncontrolled variables that make research difficult. Xiaoyan Ma studied how short pulse of 355nm light creates a plasma and sebsequent shock wave in the tissue of fruit fly embryos.
When a laser pulse of sufficient strength strikes the tissue , it knocks free some of the electrons from atoms in the tissue. The qussi-free electrons quickly rejion their atoms which heats up the tissue and vaporizes nearby water. The vapor rapidly expands outward as a cavitation bubble,creating a series of shock waves that can be detected. The process is interesting for two reasons. The cavitation bubble causes tissue damage and the shock wave formation can be used to understand the laser energy t,affect plasma generation , and to look further into how NAdh affects plasma formationhreshold needed to create the plasma.
In a series of experiments using a continuum Q switched Nd:YAG laser delivering nanosecond pulses at 532 or 355nm. They determined the difference in the energy threshold for both distilled water and fruit fly embryo tissue. The result in distilled water were similar to other research. However in tissue the thershold for longer wave length light was 38 times higher.
Plasma formation takes place in two stages. At first the laser must be strong enoungh to generate a few quasi free seed electrons in the focal spot. Then as the seed elctrons absorb laser energy they drive a cascade of ionizations that forms the full plasma. In water the seed electrons come from multiphoton ionization. In the fruit fly tissue the seed electrons most likely come from a two photon ionization of nicotinamide adenine dinucleotide (NADH). which requires much less energy. They also determined that the size of the cavitation bubbles that the 355nm laser creates is much smaller in living tissue then in pure water.
Because NADH is ubiquitous molecule central to biological metabolism the research is appicable to a wide range of tissue. In fact NADH concentrations vary greatly between metabolically active tissue and tissue excised and stored for even a few hours.
They intend to continue the investigation in three main areas – to explore laser microsurgery as a way to understand developing embryos, to explore how lasers parameters such as pulsewidth
affect plasma generation, and look further into how levels of NADH affect plasma formation.