The primary function of the Long-pulse 755nm Alexandrite laser is to induce selective photothermolysis, a process where light energy is absorbed by melanin in the hair shaft and converted into heat. This thermal energy is precisely conducted to the hair follicle and its germinative center, destroying the structure's regenerative capacity while preserving the surrounding skin tissue.
The core strength of this laser lies in its "long-pulse" capability, which delivers energy in a timeframe (10–15 ms) that sits perfectly between the cooling time of the skin and the hair follicle. This ensures the follicle is destroyed by heat accumulation before the skin has time to suffer thermal damage.
The Mechanism of Selective Photothermolysis
Targeting the Chromophore
The 755nm wavelength is specifically engineered to target melanin, the pigment that gives hair its color. In this context, melanin acts as a "chromophore," or a light-absorbing target.
Conversion to Thermal Energy
When the laser light hits the hair shaft, the melanin absorbs the photon energy and instantly converts it into heat. This heat does not stay in the shaft; it conducts outward into the hair follicle and the "bulge" area.
Destruction of Regenerative Cells
The ultimate goal is not just to burn the hair, but to disable the germinative center. By raising the temperature of these specific cells, the laser inhibits the follicle's ability to produce new hair during its growth phase.
The Critical Role of Pulse Duration
Matching Thermal Relaxation Time (TRT)
The "long-pulse" aspect is critical for safety. The pulse width is tuned to be longer than the TRT of the epidermis (skin surface) but shorter than the TRT of the hair follicle.
Preventing "Explosive" Damage
If the pulse were too short, the energy delivery would be too rapid, potentially causing explosive damage to the skin surface. A longer pulse (10 to 15 ms) allows the heat to build up slowly and continuously within the follicle.
Sparing the Epidermis
Because the pulse duration exceeds the skin's cooling time, the epidermis can dissipate the heat effectively. This creates a safety window where the hair follicle is destroyed, but the surrounding tissue remains unharmed.
Optimizing for Depth and Efficiency
Reducing Energy Scattering
To ensure the laser reaches follicles located deep in the dermis, a large spot size (1 cm or larger) is often utilized.
Enhancing Penetration
A larger spot size minimizes the scattering of laser light as it travels through the dermal layers. This ensures a higher concentration of energy reaches the deeper hair bulbs, improving the overall efficiency of the treatment on large surface areas.
Understanding the Trade-offs
Melanin Competition
Because the 755nm wavelength is highly absorbed by melanin, there is a risk of the laser targeting pigment in the skin rather than the hair. This makes the specific pulse width calibration essential to prevent burns on patients with higher epidermal melanin (darker skin).
Growth Phase Dependency
The mechanism relies on the presence of a hair shaft to conduct heat to the bulb. Consequently, the treatment is most effective when the hair is in its active growth phase, as this provides the physical pathway for thermal destruction.
Making the Right Choice for Your Goal
To maximize the effectiveness of the Long-pulse 755nm Alexandrite laser, consider the following technical parameters:
- If your primary focus is safety: Ensure the pulse duration is calibrated strictly between the thermal relaxation times of the epidermis and the follicle to prevent non-specific thermal damage.
- If your primary focus is deep follicle destruction: Utilize a large spot size (1 cm+) to reduce dermal scattering and deliver maximum energy to the hair bulb.
- If your primary focus is treatment longevity: Target the germinative center with sufficient fluence to permanently disable the reproductive capacity, rather than simply vaporizing the visible hair shaft.
Success with this technology requires balancing high melanin absorption with precise timing to destroy the target while protecting the tissue.
Summary Table:
| Feature | Mechanism/Function | Clinical Benefit |
|---|---|---|
| 755nm Wavelength | High melanin absorption (selective photothermolysis) | Maximum efficiency for light to medium skin types |
| Long-Pulse (10-15ms) | Heat delivery timed between skin and follicle cooling | Protects the epidermis while destroying the follicle |
| Thermal Energy | Converts light to heat in the hair shaft | Disables the germinative center and follicle growth |
| Large Spot Size | Reduces energy scattering in dermal layers | Enhances penetration depth for deep-rooted hair bulbs |
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References
- Zekayі Kutlubay. Alexandrite laser hair removal results in 2359 patients: A Turkish experience. DOI: 10.1080/14764170902984903
This article is also based on technical information from Belislaser Knowledge Base .
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