Photobiomodulation Therapy: Survey and Principal Study Leading to Design Rules for Implants

Photobiomodulation (PBM) therapy using red- and near-infrared (NIR) light has shown beneficial regenerative effects on cell functionalities and consequently on health applications. Light parameter values, particularly power density, significantly affect the treatment outcomes. The limited use of light in transcutaneous applications is due to the power attenuation challenge, which restricts the transmission of light energy to deeper tissues. However, the potential of light therapy to restore cellular function presents a promising strategy for regenerating cells of internal human organs. We have 1) studied the mechanism and impact of red light at the cellular level, 2) reviewed the literature and classified the research done using light therapy, indicating the positive and negative effects associated with the light power density and exposure time levels, and 3) proposed a design rule for designing PBM implants targeting interior organs. Therefore, this work leads to 1) designing safe and efficient implantable devices by extracting the necessary light parameters, and 2) expanding the applications of light therapy. The range of the red-light power density used in different studies starts from 10 mW/cm² to as big as 5000 mW/cm² in a few cases, and from 10 s to 3000 s for different exposure times. According to the light parameters, we have tested different types of off-the-shelf LEDs to experimentally find the correlation between the generated light power densities at different distances from light sources and the LEDs' input electrical power. We have found that for an average light power density of 100 mW/cm² over an area of 1 cm², the input power of 18 mW must be delivered to the LED, which is achievable safely via wireless inductive links for delivering sufficient power deep in the body with under 1.6 W/kg of specific absorption rate (SAR). A design rule and approach are provided in this work as a starting point for designing PBM implants.