The core mechanism of the 755 nm wavelength is selective photothermolysis. This specific wavelength is engineered to be highly absorbed by the melanin found in the hair shaft and the follicle matrix. By converting light energy into localized heat, it precisely targets and destroys the hair follicle stem cells housed in the bulge and dermal papilla. This process achieves permanent hair reduction while sparing the surrounding skin tissues, which typically contain lower concentrations of melanin.
By leveraging the high absorption rate of the 755 nm wavelength by melanin, the technology ensures that thermal energy is concentrated solely on the hair's regenerative structures, preserving the integrity of the surrounding dermal tissue.
The Physics of Follicle Destruction
Targeting Melanin as a Chromophore
The effectiveness of the 755 nm Alexandrite laser relies on melanin acting as the primary target, or chromophore. The wavelength is "tuned" to seek out dark pigment.
Because the hair shaft contains a dense concentration of melanin, it absorbs the laser energy much more rapidly than the surrounding skin. This differential absorption creates a stark contrast in how energy is distributed.
Ablation of Regenerative Cells
Once absorbed, the optical energy transforms into thermal energy. This heat travels down the hair shaft to the root.
The ultimate goal is to thermally damage the hair follicle stem cells located in the follicle bulge and the dermal papilla. Destroying these specific structures eliminates the follicle's regenerative capacity, leading to permanent reduction.
The Role of Pulse Width and Safety
Applying Thermal Relaxation Theory
While wavelength determines absorption, the pulse width (the duration of the laser shot) determines confinement.
According to the theory of thermal relaxation, the laser pulse must be long enough to heat the follicle to its destruction threshold. However, it must also be controlled to prevent that heat from diffusing outward into the surrounding dermis.
Optimizing Energy Release
Professional devices automatically adjust the pulse width based on the selected energy level (fluence).
This optimization ensures that the power density—the rate of energy release—remains within a safe range. By managing the pulse width, the system provides enough energy to disable the follicle without creating an instantaneous power spike that could damage the skin or collagen.
Understanding the Trade-offs
The Necessity of Pigment Contrast
The mechanism of the 755 nm wavelength is strictly dependent on the presence of melanin.
Consequently, this technology is most effective when there is a significant contrast between the hair color and the skin tone. If the surrounding skin has a high melanin concentration, the "selective" nature of the photothermolysis is compromised, increasing the risk of surface heating.
Pulse Duration Limits
While adjusting pulse width adds a layer of safety, it is not a cure-all for incorrect settings.
If the pulse width is too short for the specific follicle size, the heat may not penetrate deeply enough to reach the stem cells. Conversely, if the pulse width is essentially mismatched to the skin's thermal relaxation time, the risk of collateral tissue damage increases despite the correct wavelength.
Making the Right Choice for Your Goal
To maximize the efficacy of long-pulse 755 nm laser technology, you must align the physics with your specific clinical objectives.
- If your primary focus is Efficacy: Prioritize this wavelength for patients with high contrast between hair and skin color, as the melanin absorption is maximized for rapid stem cell destruction.
- If your primary focus is Safety: Ensure your device allows for precise pulse width adjustments to respect the thermal relaxation time of the skin, preventing heat diffusion into the dermis.
Mastering the balance between high melanin absorption and controlled thermal relaxation is the key to achieving permanent results without compromising skin integrity.
Summary Table:
| Feature | Description |
|---|---|
| Core Mechanism | Selective Photothermolysis targeting melanin |
| Target Structures | Follicle bulge and dermal papilla stem cells |
| Primary Chromophore | Melanin (highest absorption at 755nm) |
| Safety Control | Pulse width adjustment based on thermal relaxation time |
| Ideal Patient | High contrast between hair color and skin tone |
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References
- Elisabeth Russe, Katharina Russe‐Wilflingseder. Evaluation of Safety and Efficacy of Laser Hair Removal With the Long‐Pulsed 755 nm Wavelength Laser: A Two‐Center Study With 948 Patients. DOI: 10.1002/lsm.23160
This article is also based on technical information from Belislaser Knowledge Base .
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