Development of a Holographic Sensor for Daylight Photodynamic Therapy Dosimetry

Daylight photodynamic therapy, or D-PDT, uses a daylight-activated topical photosensitizing agent to target and kill cells. For successful drug activation, it is crucial to ensure that the treatment site receives sufficient daylight exposure, or daylight dose. Here, the development and optimization of a holographic transmission mode grating-based sensor for individualized D-PDT dosimetry is presented. The concept involves the use of a holographic grating, recorded in a photosensitive polymer-based film, which undergoes a measurable decrease in diffraction efficiency on exposure to daylight due to bleaching of the film and thus grating erasure. This low-cost, wearable sensor can be positioned on the patient’s skin adjacent to the D-PDT treatment site to measure the cumulative daylight dose at regular intervals during their treatment. Here, the formulation of the photosensitive film has been optimized to respond to daylight on relevant D-PDT time scales. Optimisation of the holographic grating parameters, such as grating thickness and spatial frequency, has led to improvements in initial diffraction efficiency and grating stability. Controlled grating erasure studies were carried out under various light exposure conditions: single wavelength, artificial white light, and daylight. A final sensor configuration which enables grating erasure on relevant D-PDT treatment time scales is described. The ability to modify the grating parameters and photosensitive material provides potential for adapting the technology to different photosensitizing agent types and weather conditions for a variety of visible light therapies.