Light and Tissue
The light used to control the state of the photo-responsive drug should not interfere with other biological processes. UV/Vis light is often absorbed by light-absorbing molecules in the body.[a] The more energy-rich the light delivered is, the shorter the penetration depth in living tissue and the higher the chance for photodamage.[b] This thus poses a problem for the use of energy-rich UV light, that has been shown to have carcinogenic effects and damages tissue.[c–e] Thus, photopharmacology, similarly to photodynamic therapy (PDT), should make use of the "near-infrared phototherapeutic window"[f] that is set by the absorption of hemoglobin (λ < 650 nm) and water (λ > 900 nm).[g,h]
There has been a plethora of technological advances (often due to developments in photodynamic therapy, PDT)[c] that allows facile, but targeted light-delivery. It is based on improved light-sources (LEDs and lasers), computer-aided delivery systems, endoscopes, fiberoptic devices, and light diffusers.
The depth of tissue penetration depends strongly on the wavelength of the light. Irradiation at around 800 nm penetrates up to 2 cm into tissue.[i] Oftentimes problems with tissue penetration can be circumvented by the use of endoscopy or by inserting a fiber optical cable through a small incision.[j]
Instrumentation of light-delivery:
© 2017 Michael M. Lerch |