title: Laser microbial inactivation and detection
Creators: name: Yeo, Allen Chiew Beng
Publisher: name: University of Glasgow
Year: 1999
Subjects: subject: Electrical engineering.

description: The experimental work in this thesis was aimed at optimising both the Nd: YAG and CO2 laser-bacteria sterilisation systems and to test synergistic bactericidal effects with other sources of electromagnetic waves, namely ultra-violet and microwaves. Such novel applications could be used in decontamination and hygiene systems in the food and medical industries. A novel study of laser-bacteria detection method, speckle, was used to detect the level of contamination before a decontamination process could be optimised. First, the absorption properties of water were investigated. This was necessary because water is the main constituent in any living bacterial cells. Water is often the main interactor with laser light in processes such as photolysis, photocoagulation and photo-thermal medical and dental treatments. The efficacy of laser irradiation is governed also by the penetration and absorption abilities of the cells. The knowledge of such optical properties will enhance the efficiency of laser-cell interaction processes. The temperature dependency of the extinction coefficient of water was found between 25 to 70° C at 1.064 mum. The a-r decreased with temperature and there was an apparent sudden transition from 0.060 cm-1 to 0.038cm-1 between about 40-45° C. The scattering coefficient did not decrease at this temperature, but did with increasing temperature. The laser parameters, namely pulse repetition frequency (PRF), pulse energy, exposure time and beam diameter of 400 W Nd:YAG laser were investigated to quantify the bactericidal effects of S. aureus on agar plates. The effects of varying the applied energy density from 800 Jcm-2 to 2700 Jem-2 were investigated by changing the pulse repetition frequency and pulse energy. Zones of clearing observed on lawned agar plates after incubation at 37° C for 24 hr were measured and the energy density at which 50 % (IA50) of the beam area was inactivated determined. An increase in PRF and/or pulse energy increased the area of clearing. The IA50 values were dependent on the pulse energy, frequency and exposure time. Nd:YAG irradiation produced rapid sterilisation without damaging the substrate. A method of modelling the three dimensional beam output characteristic of the Nd:YAG high power laser with a two dimensional pyrolectric detector array scanning slit was described. The three dimensional beam profiles of both sets of laser parameter were superimposed from the detector array to the lawned agar plates' positions. A two dimensional energy density profiles across the burn print diameters along the highest peak plane obtained at the lawned agar plate position were mapped onto the IA50 values beam diameters of the lawned clearance for both sets of laser parameter used. With the CO2 laser, the biocidal activity on different contaminated biomaterials namely, polypropylene, polyethylene, polyvinyl chloride and stainless steel was investigated. Complete inactivation of S. aureus was achieved before damage was induced on the substrate surfaces with the exception of polyvinyl chloride. Two different types of high power CO2 scanning mechanism were used to study the bactericidal effects on nutrient agar, collagen film and stainless steel; both systems produced sterilisation. Complete killing was achieved at 1.3 and 0.9 cm s-1 scanning speed with E. coli and S. aureus on stainless steel respectively. With 15 W low power CO2 system, similar work was done to compare with the high power CO2 and to quantify the inactivation effect over various powers (2 to 13.4 W) and speed from 3 to 100 mm s-1. The investigation showed the feasibility of scaling down the sterilisation capability at this wavelength of laser light to minimise laser cost and increase portability. The novel and efficacious use of lasers as sterilisation tools can be further exploited. By combining emerging processes, effective minimal processing and decontamination may be achieved more effectively than conventional techniques. (Abstract shortened by ProQuest.).