Blind Localization of Heating in Neural Tissues Induced by a Train of the Infrared Pulse Laser.

Introduction: Recently, infrared lasers (wavelengths larger than 1100 nm) have been applied to stimulate neural tissues. Infrared neural stimulation (INS) has some advantages over conventional electric stimulation, including contact-free delivery, spatial precision, and lack of stimulation artifacts. In this study and based on a photothermal mechanism, we applied the heat diffusion equation to study temperature variation of a biological phantom during INS. In addition, the impact of laser parameters on spatially localized heating induced by 2 different infrared wavelengths were studied. Methods: We studied the localization of INS inside a phantom similar to cortical neural tissue. First, we analytically solved the heat diffusion equation to study the distribution of temperature inside this phantom. Then, the accuracy of analytical results was verified by heating the phantom using amplitude-modulated infrared lasers (lambda= 1450 and 1500 nm, the energy between 2 and 5 mJ and pulse duration up to 20 ms). The laser light was directed to sample by a multimode optical fiber (NA=0.22, core size= 200 microns). Finally, the impacts of laser properties on the spatial resolution of infrared heating were discerned. Results: In order to verify analytical results, we measured the maximum temperatures of the phantom during illumination of lasers and compared them with analytical results. The analytical results were in agreement with the experimental results. The effects of laser beam properties such as pulse duration, energy and repetition rate frequency on the spatial resolution were investigated. The results indicated that the spatial resolution of INS can be smaller than one millimeter. Conclusion: Here, the influences of laser properties on the localization of INS inside a biological phantom were studied. These results can be applied to improve the spatial selectivity of the peripheral nerve interface.

Influence of radiant exposure and repetition rate in infrared neural stimulation with near-infrared lasers.

In this study, we combine heat diffusion equation and modified Hodgkin-Huxley axonal model to investigate how an action potential is generated during infrared neural stimulation. The effects of temporal and spatial distribution of heat induced by infrared pulsed lasers on variation of electrical membrane capacitance are investigated. These variations can lead to depolarize the membrane and generate an action potential. We estimate the threshold values of laser light parameters such as energy density, pulse duration, and repetition rate are needed to trigger an action potential. In order to do it, we present an analytic solution to heat diffusion equation. Then, the analytic results are verified by experimental results. Furthermore, the modified Hodgkin-Huxley axonal model is applied to simulate the generation of action potential during infrared neural stimulation by taking into account the temperature dependence of electrical membrane capacitance. Results show that the threshold temperature increase induced by a train infrared pulse laser can be smaller if repetition rate is higher. These results also indicate that temperature rise time and axon diameter influence on threshold temperature increase. To verify threshold values estimated by the presented method, we use a train infrared pulsed laser (λ = 1450 nm with repetition rate of 3.8 Hz, pulse duration of 18 ms and energy density of 5 J/cm2) to optically pace an adult rat heart, and we are able to successfully pace the rat heart during an open-heart surgery. The presented method can be used to estimate threshold values of laser parameters required for generating an action potential, and it can provide an insight to how the temperature changes lead to neural stimulation during INS.

Synergic phototoxic effect of visible light or Gallium-Arsenide laser in the presence of different photo-sensitizers on Porphyromonas gingivalis and Fusobacterium nucleatum.

BACKGROUND: According to the development of resistant strains of pathogenic bacteria following treatment with antimicrobial chemotherapeutic agents, alternative approaches such as lethal photosensitization are being used. The aim of this study was to evaluate the effect of visible light and laser beam radiation in conjugation with three different photosensitizers on the survival of two main periodontopathogenic bacteria including Porphyromonas gingivalis and Fusobacterium nucleatum in different exposure periods. MATERIALS AND METHODS: In this in vitro prospective study, strains of P. gingivalis and F. nucleatum. were exposed to visible light at wavelengths of 440 nm and diode laser light, Gallium-Arsenide, at wavelength of 830 nm in the presence of a photosensitizer (erythrosine, curcuma, or hydrogen peroxide). They were exposed 1-5 min to each light. Each experiment was repeated 3 times for each strain of bacteria. Data were analyzed by two-ways ANOVA and least significant difference post-hoc tests. P < 0.05 was considered as significant. After 4 days the colonies were counted. RESULTS: Viability of P. gingivalis was reduced 10% and 20% subsequent to exposure to visible light and diode laser, respectively. The values were 65% and 75% for F. nucleatum in a period of 5-min, respectively. Exposure to visible light or laser beam in conjugation with the photosensitizers suspension caused significant reduction in the number of P. gingivalis in duration of 5-min, suggesting a synergic phototoxic effect. However, the survival rate of F. nucleatum following the exposure to laser with hydrogen peroxide, erythrosine and rhizome of Curcuma longa (curcumin) after 5-min was 10%, 20% and 90% respectively. CONCLUSION: Within the limitations of this study, the synergic phototoxic effect of visible light in combination with each of the photosensitizers on P. gingivalis and F. nucleatum. However, the synergic phototoxic effect of laser exposure and hydrogen peroxide and curcumin as photosensitizers on F. nucleatum was not shown.