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.




Tissue Penetration



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]


[a]      J. Am. Acad. Dermatol. 2000, 42, 389 – 413;

[b]      Proc. Natl. Acad. Sci. USA 1991, 88, 10124 – 10128;

[c]      Mol. Cell. Biol. 1986, 6, 3349−3356;

[d]      Cancer Lett. 2005, 223, 275−284;

[e]      Biosci. Rep. 2000, 20, 99−108;

[f]       Clin. Endosc. 2013, 46, 7 – 23;

[g]      Nat. Med. 2003, 9, 123 – 128;

[h]      J. Controlled Release 2015, 219, 18−30;

[i]       Int. J. Dermatol. 1996, 35, 312– 316;

[j]       CA: Cancer J. Clin. 2011, 61, 250– 281


Instrumentation of light-delivery:

- J. Biomed. Opt. 2010, 15, 010506.

- Front. Optoelectron. 2015, 8, 141 – 151.

- Indian Dermatol. Online J. 2015, 6, 145 – 157.





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© 2017 Michael M. Lerch

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Background: subtle patterns