1. Mechanism of selective photothermolysis

The scientific basis for destroying the hair follicle without damaging the epidermis and adjacent dermis structures is based on the principle of "Selective Photothermolysis." In this process, photon energy is emitted at a wavelength that the target substance (chromophore) has the highest absorption in that light range.

In laser hair removal treatment, the target chromophore is melanin in the hair shaft and matrix. After the energy is absorbed by the melanin pigments, the light energy is converted into thermal energy in a fraction of a millisecond, raising the temperature of the follicle to above 65 to 70 degrees Celsius. This level of heat causes protein coagulation of the germinal stem cells in the bulge and dermal papilla area, neutralizing the power of hair regrowth.

2. Follicle growth cycle and the anagen phase

The failure to eradicate 100% of hair in a single session is a direct result of the nature of the hair's biological cycle. Each hair strand goes through three distinct stages during its life:

• Anagen (active growth) phase: The phase in which the hair is rich in melanin pigment and has a strong physical connection to the bulb and blood vessels. This phase is considered the only effective window for definitive destruction.
• Catagen (transitional) phase: The hair shaft is separated from the root and melanin synthesis is reduced to a minimum; therefore, heat transfer to the root is disrupted.
• Telogen (resting) phase: The follicle is active, without an attached stalk, and is waiting to emerge and begin a new cycle.

Since only a certain percentage of hairs (between 20 and 30 percent depending on the body location) are in the anagen phase at any given time, regular intervals between sessions (4 to 8 weeks) are necessary to allow the remaining hairs to enter the active growth phase.

3. Comparison of wavelengths and technologies

Different wavelengths have different penetration depths and absorption tendencies towards melanin. Choosing the right system for your hair thickness and skin tone is the most important factor in efficiency:

4. Skin types and parameter settings

The fluence (Joules per square centimeter), pulse width (milliseconds) and spot size are adjusted based on the Fitzpatrick scale. In darker skin, a longer pulse width and a milder fluence are prescribed along with powerful cooling systems (gas cooling or titanium contact) to prevent epidermal burns.

5. Pre-treatment critical care

The effectiveness of the treatment depends as much on the individual's adherence to skin preparation protocols as on the laser parameters:

• Avoid root hair removal methods: Waxing, epilation, and threading should be stopped at least 4 weeks before the session, as the hair shaft within the follicle must be present for energy transfer.
• Gillette spot correction: The desired area should be completely shaved 12 to 24 hours before the session to prevent heat loss on the skin surface and superficial burns.
• Avoid exposure to ultraviolet rays: Absolute avoidance of solariums or sunbathing is required for at least one month before treatment; inflamed and tanned skin has a high risk of forming hyperpigmentation.
• Avoid using photosensitizing ingredients: Discontinue use of topical retinoids, exfoliating acids, and photosensitive medications such as Roaccutane with the approval of your doctor.

6. Clinical measures after laser

Hot showers, saunas, hot tubs, and sports activities that produce heavy sweating are prohibited for the first 48 hours. Also, in areas exposed to sunlight, continuous use of broad-spectrum sunscreen (SPF 50+) every two hours is mandatory to prevent post-inflammatory hyperpigmentation (PIH).

7. Contraindications and possible side effects

Laser treatment is absolutely or partially prohibited in the following conditions and requires a detailed specialist evaluation:

• Pregnancy and lactation due to severe hormonal fluctuations and lack of definitive clinical data.
• Active skin infections such as herpes simplex, active purulent acne, or open wounds in the treatment area.
• Autoimmune diseases exacerbated by light, such as lupus erythematosus or porphyria.
• History of keloid scars or excess flesh after previous skin injuries.

The most common transient side effects include erythema, edema, and a slight itching sensation; however, failure to match the skin type to the wavelength can result in blisters, second-degree burns, and permanent white (hypopigmentation) or dark (hyperpigmentation) spots. Continuous monitoring of equipment and periodic calibration of systems are the main guarantees that these events will not occur.