Photomechanical, Photothermal and Photothermomechanical Mechanisms of Interaction of Nanosecond-Long Laser Pulses With Artificial Tissue Models and PigmentedMelanoma Cells in Medical Applications

Department of Mechanical Engineering
Advisor: Professor Guillermo Aguilar

Nanosecond-long  laser  pulses  are  used  in  both  branches  of  biomedical  optics:  where  photons  affect tissue  (therapy)  and  where  tissue  affects  photons  (diagnostics).  A  current  problem  in  vascular  laser surgery  is  that  small  blood  vessels, with  short  thermal  relaxation  time, remain  after  the  treatment  with laser pulses longer than such relaxation time. This problem is approached irradiating artificial skin models with  nanosecond  laser  pulses.  The  objective  is  to  take advantage  of  its  high  intensity  to  induce  plasma that   produces   cavitation   bubbles,   which   may   serve   as   blood   vessel   photodisruption   mechanism. Permanent  and  transient  bubbles  were identified  as  a  function  of  the laser  dose,  number  of  pulses  and repetition rate.  Additionally,  scattering  effects  were  added  to  the  skin  models,  which  increased  the threshold fluence for plasma formation.  

Energy  from  nanosecond  laser  pulses  couples  to  a material  through  the  combination  of  linear  and nonlinear  absorption  according  to  both,  laser  light  intensity  and  material  properties.  The  first  results  in heat generation and thermoelastic expansion; while the second results in an expanding plasma formation that launches a shock wave and a cavitation/boiling bubble. Such mechanical effects were studied using three   different experimental   techniques:   piezoelectric   sensors,   time-resolved   imaging   (TRI)   and time-resolved  interferometry  (TRIF).  The  relative roll  of  linear  and  nonlinear absorption  upon  bubble formation is discussed. 

A  melanoma  detector  takes  advantage  of  pressure waves  originated  upon  absorption  of  energy  from nanosecond  laser  pulses  within  melanoma  cells.  Excessive  energy  creates  boiling  bubbles  around  melanosomes  that  damage  the  plasma  membrane.  It is  important  to  elucidate  the optimum  laser parameters for this application. Melanoma cells were irradiated with nanosecond laser pulses at λ = 355 and 532 nm to: determine cell survival rate, compare the photoacoustic signal, determine the critical laser fluence  for  melanin  leakage  from  melanoma  cells  and  study the intracellular  interactions  and  their  effect on  the  plasma  membrane  integrity.  Cell  survival decreased  with  increasing  laser  fluence  for  both wavelengths, although the decrease is more pronounced for λ=355 nm. Melanin leaks from cells equally for  both  wavelengths.  No  significant difference  in photoacustic  signal  was  found  between  wavelengths. TRI showed damage to plasma membrane due to bubble formation.