Friday 6 March 2015

Infrared and tissue penetration

A recent article in New Scientist caught my eye. My drug-filled nanospheres heal at the speed of light reports work by a team led by Professor Adah Almutairi at the University of California, San Diego.

Her work explains that by making use of near infrared's ability to penetrate skin and tissue, it is possible to use a laser of the appropriate wavelength to trigger a polymer nanotube to break down and, if it's carrying a drug, to release it. Since the light can be tightly targeted it would be possible to therefore tightly target a release site for the drug. There are other mechanisms for triggering release, such as the temperature of inflammation or even sunlight on the skin. This latter has the neat prospect of a sunscreen that activates when you get into the sun.

While the NS article is very recent, I found a paper from 2011 by Almutairi's team that explains the concept:

Low Power, Biologically Benign NIR Light Triggers Polymer Disassembly Fomina et al
Macromolecules, 2011, 44 (21), pp 8590–8597

You can see the abstract or buy the paper from ACS Publications.

One interesting thing, for me, is the statement at the top of the abstract that "Near infrared (NIR) irradiation can penetrate up to 10cm deep into tissues". Admittedly, from a photographic point of view you need to remember that the IR has to penetrate the tissue and then get back out again, but I believe the figure of 'a few millimetres' has been a good rule of thumb for years. A figure of between one and two cm has been cited from a paper by Gao et al, In vivo cancer targeting and imaging with semiconductor quantum dots from 2004.

So I decided to see what Lou Gibson had to say on the matter in his third edition of Clark 'Photography by Infrared' in 1978. He quotes Balderry and Ewald, in a 1924 paper called 'Life Energy in Theraputics' as saying that sunlight can penetrate up to 25 cm into the body. So that 10cm seems quite reasonable.

Photographically, however, a near infrared photograph will often show veins under the skin, and will almost always give people a 5-o'clock shadow (even some women). This is also, as I've pointed out before, the cause of the alabaster look you can see in infrared portraits. I'll leave you with this image of Jude (a lady with whom I used to work) demonstrating the infrared look, with the added 'bonus' of 35mm infrared film grain.