Selective Photothermolysis is a laser phenomenon that was discovered in the early 1990s. It allows the laser energy to be delivered to specific chromophores in the skin, which leads to the selective destruction of targeted lesions like pigmentation and hair. This makes it an extremely valuable tool for laser hair removal and other cosmetic procedures. In this blog post, we will discuss how Selective Photothermolysis works and some of its potential applications.
The wavelength is important for Selective Photothermolysis because the wavelength determines which chromophores in the skin are affected by the laser. For hair removal, a wavelength between 700 nm and 1100 nm is ideal because it targets melanin, a pigment found in hair follicles that absorbs the laser energy and destroys them. The laser should have a longer wavelength of over 1500 nm for skin resurfacing and remodelling.
Chromophores are molecules in the skin that absorb certain wavelengths of light. For example, melanin absorbs the 700-1100nm wavelength used for hair removal, and hemoglobin absorbs the 600 to 700 nm wavelength for vein removal. Laser energy is absorbed by these targeted chromophores, which leads to Selective Photothermolysis and allows us to target unwanted lesions such as pigmentation and hair selectively.
The laser type and setting may be modified to target a specific lesion according to the tissue type, scattering pattern, and absorption spectrum. In general, lasers work by causing photothermolysis (the process of light amplification).
The light is absorbed by a specific chromophore in the tissue, converting it to heat energy. Water, melanin, and oxyhemoglobin are the primary endogenous chromophore. Carbon particles from the colloidal charchol masked applied before the carbon peel procedure using the Lutronic spectra q-switch laser can also be used as exogenous chromophores.
The degree of laser absorption and, as a result, the heat produced is determined by the absorption spectrum of each chromophore. Another critical component is the penetration depth.; as the wavelength of the laser beam increases, the depth of penetration increases. The light scattering is inversely proportional to the depth of laser beam penetration. Only before the mid-infrared region of the light spectrum is this true. The longer the wavelength, the more light is absorbed by water, and the more superficial the laser becomes.
A medical aesthetician should know these fundamental concepts to choose the correct settings for the desired outcomes. In other words, if we’re going after a lesion like wrinkles or pigmentation, we need to get as close to the target absorption profile as possible. This will aid in delivering energy to the target tissue while preserving the surrounding tissues.
Thermal Relaxation Time
Thermal relaxation time is another important concept: the time it takes for the target to heat up and then release the heat to the surrounding tissue. Regardless of how precise the laser is, if we deliver energy to a target for a long time, the energy will be passed to the surrounding tissue. The laser pulse width addresses this concept.
The period over which laser energy is delivered is known as the pulse width. The time is measured in milliseconds, nanoseconds, and picoseconds. Because the medical professional can control the pulse width, this concept is important in all treatments, especially hair removal (it is not the case with the nano and pico range). The pulse width must be shorter than the thermal relaxation time to give the target tissue a long enough break or cool down without damaging the surrounding tissue. On the other hand, a very short pulse width indicates that all laser energy will be delivered quickly, which is considered an aggressive treatment.
Ablation and Coagulation
We must use a resurfacing laser to remove wrinkles, and there are two types: ablative and nonablative. Each of these lasers can be fractional or nonfractional. Ablative lasers, such as CO2 and ER: YAG, are the most aggressive laser treatments because they target water and vaporize the entire tissue. The fractional head is one way to team up with the beast.
These heads will direct the laser beam to create specific treatment areas while leaving most of the surrounding tissues undamaged. The epithelium, papillary dermis, and reticular dermis are all destroyed using an ablative nonfractional laser. Collagen synthesis, tissue remodelling, and skin tightening occur as a result. The Er: Yag Fraxel laser emits a wavelength of 2940 nm, readily absorbed by water, resulting in superficial penetration.
It has been proven that Er: YAG and CO2 have the same effect on acne scars, with the ER: YAG group experiencing fewer side effects. The most common side effect of using an ablative laser is post-inflammatory pigmentation (PIH). Research shows that using Hydroquinone 4 percent and tretinoin 1 percent for six weeks before laser treatment significantly reduced the risk. Skin tightening, scar removal, pigmentation removal, wrinkle removal, and acne treatment are the most common indications for fractional ablative and nonablative laser treatment.
The nonablative laser has a lower affinity for water and is linked to better skin tightening, acne scar removal, and wrinkle removal with less downtime and side effects.
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