Laser and IPL treatments rely on selective thermal damage of chromophores in pigmented cells, blood vessels, or hair follicles by absorption of high-energy pulses of radiation. Such therapies are used for cosmetic or medical purposes and include hair removal, tattoo removal, treatment of vascular and pigmented lesions, and skin photorejuvenation.
The term ‘selective thermal damage’ refers specifically to photothermolysis.
The physical mechanism of photothermolysis is as follows:
- Chromophores within pigmented tissue absorb photons corresponding with their absorption spectrum.
- The radiative energy is transferred to heat.
- With sufficient heat, the target structure is destroyed.
The method is said to be ‘selective’ since only the pigmented structures, cells, and organelles are affected due to their inherent optical and thermal properties. The pulse duration in all light-based treatments should match or be less than the thermal relaxation time of the pigment to prevent damage to the tissue surrounding it.
The occurrence of photothermolysis depends on the wavelength, pulse duration, and pulse energy. Therefore, both coherent or incoherent heating sources can be employed. IPL devices are broadband and incoherent sources that target multiple chromophores with relatively low intensity. They emit visible to IR wavelengths that can be selectively blocked with a bandpass or edge filter at the exit port. Conversely, lasers target a specific chromophore with high intensity, providing an increased absorption rate.
The major chromophores present in a human body are haemoglobin, water, melanin, and exogenous dyes (i.e. tattoo pigment). The chromophores present in vascular lesions, in addition to other skin conditions, include haemoglobin and/or water. Light-based hair-removal treatments specifically target melanin. Pigmented lesions in the skin also contain excess melanin.
The term ‘photoepilation’ refers to the process of hair removal using IPL and laser devices, though ‘laser hair removal’ is more commonly used for the latter. In either case, light with the prescribed parameters is made incident on the surface of the skin. Radiation propagates through the skin until it strikes the melanin-rich bulb. The heat produced from photothermolysis is transmitted to the bottom of the hair follicle and subsequently destroys the hair-producing tissue known as the dermal papilla.
Photoepilation requires the hair to be in the anagen stage of growth for the follicle to be permanently disconnected from the blood supply. Anagen is the active hair-growth phase. During this phase, nourishment of the hair follicle via a blood supply enables hair growth. There is also a transition phase and a resting phase known as catagen and telogen, respectively.
Tattoo removal requires very short pulse durations due to the small size of tattoo ink pigments. IPL devices can only emit pulse durations in the millisecond range, resulting in prolonged heating of the pigments and subsequent heating of the surrounding tissue. Q-switched lasers have therefore become the preferred modality of tattoo removal because nanosecond laser exposure can specifically target small structures with less collateral damage.
Vascular and pigmented lesions
The terms vascular and pigmented lesions encompass a variety of conditions such as port wine stains (PWS), rosacea, and telangiectasias (otherwise known as spider veins). Both IPL and laser treatments have been used to improve the appearance of such conditions by targeting the dominant chromophores in the lesion.
Photorejuvenation is implemented to improve the appearance of sun-induced photoaging or intrinsic aging. Skin aging is characterised by rhytides (wrinkles), pigmentary alterations, and skin thinning.
There are two objectives in the process of photorejuvenation. The first is to improve the appearance of pigmented lesions like telangiectasias or diffuse redness.
The second objective is to reorganise collagen fibres and increase dermal volume. IR light is used for this purpose because IR absorption in the skin predominantly occurs in water, resulting in dermal heating that increases fibroblast activity in connective tissue. Subsequently, organised and neatly arranged collagen fibres are produced, theoretically leading to a smoother skin texture, increased elasticity, and shallower rhytides.
Laser eye surgery is a renowned laser treatment that employs an excimer laser to reshape the cornea and correct refractive errors. This procedure relies on a mechanism known as photoablation rather than photothermolysis, which essentially breaks down the molecular bonds within all types of tissue, pigmented or otherwise.
Photodynamic therapy (PDT) involves the use of light-sensitive medication, referred to as a photosensitizer. A specific wavelength of light is used to activate the drug, consequently releasing a toxic substance that kills nearby cells. This allows abnormal tissues to be destroyed without the need for surgery.
PDT is particularly useful for the treatment of skin cancers such as ‘actinic keratoses’ and ‘basel cell carcinoma’. It can also alleviate dermatologic diseases such as acne.