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Open Access Publications from the University of California

Laser-Generated Shockwaves for the Disruption of Bacterial Biofilms

  • Author(s): Navarro, Artemio
  • Advisor(s): Gupta, Vijay
  • et al.
Abstract

The goal of this thesis was to explore the use of laser-generated shockwaves as a potential methodology to treat bacterial infected surfaces as they are a major burden on the healthcare industry. A modified version of the Laser Spallation Technique (LST) was built to measure the intrinsic adhesion strength of S. epidermidis biofilms (RP62A) grown on polystyrene surfaces. Previously, the LST was used to successfully measure the adhesion strengths of mammalian cells. The system is based on a Q-switched, Nd:YAG pulsed laser with an output wavelength of 1,064 nm and a pulse width between 2-6 ns that ablates titanium-coated soda-lime glass slides (under confinement), thereby causing unipolar compressive waves. The modified LST successfully measured the adhesion strength of S. epidermidis biofilms grown on polystyrene, under static growth conditions, to be 22.75+/-0.16%. High strain rates of 1.544x10^5 s^-1 and 1.41x10^6 s^-1 in glass and in biofilm, respectively, were calculated, and total strains of 0.3% in glass and 3% in the S. epidermidis biofilm are reported and are below published failure strains. Current techniques that measure biofilm material properties vary significantly due to viscoelastic effects of biofilms under lower strain rates. Due to the high strain rates and only purely compressive/tensile loading, the adhesive failure in the biofilm is an intrinsic strength measurement.

With motivation to use these glass modified shockwaves on infected wounds, studies were implemented to evaluate the effect of the shockwaves on porcine samples ex vivo. Due to the impedance mismatch of the glass slides with the coupling medium (water), a compressive train of shockwaves are created. Peak stresses varying from 21-266.5 MPa of the shockwave, with subsequent stress pulses with 18% less peak stress, reached the pigskin surface and caused no qualitative structural damage. This is a very important result as other studies have used similar stress profiles, as a single shockwave, to deliver drugs through the skin and biofilm structures.

Finally, a low-cost, high-speed imaging system was developed and characterized to capture shockwave-induced phenomenon, i.e. cavitation. Unlike other techniques including extracorporeal shockwave (ESW) therapy, which has a tensile component in the stress profile and thus causing cavitation-induced damage in biological structures, laser-generated shockwaves are purely compressive and will only have tensile components at interfaces, depending on the impedance of each layer. Cavitation bubbles were successfully imaged at the glass/water interface which propagated toward the biofilm and caused localized delaminations.

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