For antibacterial photodynamic therapy (A-PDT), only continuous-wave light excitation has been used so far; the effect of PDT using pulsed light excitation on bacteria is not clear. In this study, pulsed photodynamic effect on Pseudomonas aeruginosa was investigated in vitro with the use of methylene blue (MB) as a photosensitizer. Bacterial suspensions were exposed to pulsed light from an optical parametric oscillator (wavelength, 665 nm; pulse width, 6 ns; repetition frequency, 30 Hz) under the various drug and light dose conditions. It was shown that bacterial survival fraction decreased with increasing drug and light doses, and a survival fraction of∼10^-4 was obtained. The results demonstrated that the MB-mediated PDT using pulsed light excitation is effective in killing Ps. aeruginosa.

View full abstract

In order to demonstrate basically the usefulness of 6.05-μm-light as the light source for non-invasive soft tissue cutting, we have preliminarily conducted gelatin ablation experiments using a tunable mid-infrared Free Electron Laser (MIR-FEL). It is essential for non-invasive cutting to enable us to precisely predict irradiation effects (such as ablation and denaturation depths) before laser treatment. 6.05-μm-light can resonantly excite both the OH bending of water and the amide-I of proteins. Wet gelatins (including 80-wt% water concentration) were used as the model substance of soft tissue, which consists of 80-wt% water and 20-wt% proteins. The MIR-FEL wavelength was varied within the wavelength range of 5.6-6.7 μm. The incident laser energy density was fixed at 3.6±0.3 J/cm². The exposure time was 10 s or 100 s. The structural changes of irradiated gelatins were observed with an optical microscope coupled to a CCD camera. The followings can be derived from these observations. (1) Efficient gelatin ablation occurred around 6.05 μm but not 6.45 μm (corresponding to the amide-II of proteins). (2) In the case of 6.05 μm, the wet gelatins were removed due to the pure vaporization of water according to the absorption properties of the wet gelatins before laser treatment, resulting to a predictable laser treatment. In addition, the denaturated area of the gelatins remarkably decreased due to a spectral shift from the OH bending to the amide-I in the absorber as the exposure time is increased (the water in the irradiated gelatins is depleted), leading to the prevention of the formation of thermal coagulum. Thus, 6.05-μm-light can remove predictably soft tissue without unexpected effects and also prevent the formation of undesirable thermal coagulum even if the water concentration remarkably declines. We have proved that 6.05-μm-light is one of the candidates for a predictable soft tissue cutting and/or surgery.

View full abstract