The four photo-biological reactions you will encounter as laser therapist

Photo-biological reactions are the interactions between light and biological material (tissue). What happens when the light energy is absorbed by tissue?

The four most common types of photo-biological reactions you will encounter during aesthetic laser treatments are: photothermal reactions, photoablative reactions, photochemical reactions and photoacoustic reactions.

A photothermal reaction is when the light energy is converted into heat. For the aesthetic treatments based on photothermal reactions, it is not the light that is responsible for the results, but the heat generated from the light.

Most common example of this is laser hair removal. The light is absorbed by melanin and then converted to heat. This heat dissipates to the surrounding tissue where it is responsible for the destruction of the dermal papilla. Most vascular and pigmentation treatments also fall in this category.

Photoablative reactions are mostly associated with CO2 or Erbium:YAG skin peels. During this type of treatments the light is absorbed by the water molecules in the top layer of the skin. Because of the high energy contained in the laser beam and short pulse length the water molecule evaporates and in the proses ablate the top layer of the skin.

Due to water being the primary target and the abundance of water in the skin, you have an extremely limited penetration depth.

Photochemical reactions are caused by very low-energy levels. It is the activation or deactivation of biochemical pathways present in the body. The most common reaction you will encounter is the stimulation of adenosine triphosphate (ATP).

Photons of the correct wavelength have a direct working on the electron chain in the mitochondria, which leads to the increased production of ATP. Biomodulation is also well known in would healing and sports injury rehabilitation.

Important to remember with biomodulation is that very little energy is required and a too high level of energy will lead to a decrease in effectiveness.

Photoacoustic reactions are the magic behind laser tattoo removal. This is when an extreme short and energy rich pulse causes an acoustic explosion. The ink molecule is then broken-up into smaller pieces that the lymph system can remove in the weeks following the treatment.

Due to the extreme short pulse duration, there is no time for heat to build up and transfer to surrounding tissue.

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