<title>Study of possible pulsed-laser plasma use in medical treatment</title>

Abstract

The new method of combined surgical and therapeutic treatment by means of laser produced surface plasma is discussed. The first data on crater formation in tissue modelling samples as well as applications of such plasma in dermatology is presented. Pulsed TEA CO2 lasers were used in the experiments and medical tests. 1.INTRODUCTIONLaser radiation with different wavelengths and temporal structure are widely used in medicine. In the present paper we consider the possibility of action on biological tissues not directly by laser beam but by means of low temperature surface plasma, forming in highly focused beam. Such a plasma is optical breakdown of air and is initiated by evaporation of irradiated target. The plasma cloud spreads in the direction of the incoming beam and can absorb essential part (as high as 90%) of tie laser pulse energy.As it follows from numerous investigations on its ignition near solid surfaces the plasma of the surface air breakdown should act on the irradiated tissue in various ways.First, part of the laser energy, accumulated in plasma, can be reradiated in different spectral regions (X-Ray, ultra-violet, visible, infrared etc.) in all directions including onto the area inside and around the laser spot on the tissue surface.Second, pulsed air breakdown plasma behaves like a point source explosion in gas. Third, because of charge separation on the expanding plasma boundaries pulsed currents, electric and magnetic fields are produced.As a result, the surface laser processing in the presence of plasma cloud gives way, in principle, to the realization of a number of therapeutical and surgical treatment factors.Really, when plasma is formed it becomes possible to ablate tissues with high efficiency both with the laser beam, partially penetrating through plasma, and with light emittance pulse from plasma. The latter can heat and even coagulate tissue at the depth different from the laser light absorption depth.It is not very clear now what can be a positive role of the explosion action of plasma on tissues except for explosion type removal of tissue material from the zone with maximum overpressure. The negative role of this factor should be associated with mechanical damage of the surrounding tissue layers.In parallel with the thermomechanical action the laser produced surface plasma can also cause: -biostimulation by its light emittance and by electrical and magnetic pulses; -tissue treatment by ionizing deep UV and X-Ray reradiation, including UV sterilization. 2.MODEL EXPERIMENTSThe degree of plasma influence on the irradiated biological sample depends on laser beam wavelength ¼' duration t, shape and repetition rate f of laser pulses; radiation energy density E5 on the sample surface, position of the lens focal plane and focused beam diameter d0. By varying these parameters it is possible to amplify one or another plasma action factor. As an example, with the increase of I the energetic threshold of the breakdown E = Et falls as you change from the green to midinfrared while plasma temperature...