The fundamental mechanism of action is selective photothermolysis targeting the hair follicle. Long-pulse Ruby and long-pulse Alexandrite lasers emit high-energy beams at specific wavelengths designed to be absorbed by melanin. This light energy is converted into heat, causing precise thermal ablation of the follicular structure to inhibit growth while sparing the surrounding epidermis.
The success of these lasers relies on the "long-pulse" duration (milliseconds), which allows sufficient time for thermal energy to travel from the hair shaft deep into the follicle, permanently damaging the growth center without burning the skin surface.
The Physiology of Thermal Ablation
To understand how these lasers achieve long-term hair reduction, one must look at how light interacts with biological tissue.
Targeting the Chromophore
The primary target (chromophore) for both Ruby and Alexandrite lasers is melanin.
Because these lasers utilize specific wavelengths—such as the 755 nm wavelength used in Alexandrite systems—the energy is preferentially absorbed by the pigment within the hair shaft rather than the surrounding tissue.
The Conversion of Energy
Once the melanin absorbs the laser light, that energy is instantly converted into heat.
This thermal reaction is not merely surface-level; it is designed to induce thermal ablation, effectively cauterizing the hair structure.
Deep Follicular Penetration
For permanent reduction, the heat must reach the root of the problem.
The thermal energy generated in the hair shaft conducts outward to the surrounding follicular cells. This transfer impairs the follicle's ability to regenerate, leading to long-term inhibition of hair growth.
The "Long-Pulse" Advantage
The distinction of using a "long-pulse" is critical to both efficacy and safety.
Optimizing Pulse Width
These systems operate with a pulse width in the millisecond range.
This duration is optimized to match the thermal relaxation time of the hair follicle. It ensures the laser fires long enough to heat the follicle to a destructive temperature.
Protecting the Epidermis
The pulse width also serves a protective function.
By controlling the energy release, the system ensures the heat penetrates the depths of the hair follicle but does not linger excessively on the epidermis. This prevents burns and protects the normal skin structure surrounding the target.
Understanding the Trade-offs
While effective, the mechanism of action for Ruby and Alexandrite lasers presents specific constraints based on the physics of light absorption.
The Melanin Dependency
Because the mechanism relies entirely on targeting melanin, the efficacy is directly tied to the presence of pigment in the hair.
Hair with little to no melanin (white, grey, or very blonde hair) absorbs less energy, making thermal ablation difficult or impossible to achieve.
Epidermal Competition
The high absorption rate of these wavelengths by melanin creates a challenge for darker skin tones.
Since the epidermis in darker skin also contains significant melanin, there is a risk that the skin will absorb the energy intended for the follicle. This requires precise calibration of the pulse width to maintain the safety profile described in the references.
Making the Right Choice for Your Goal
When evaluating laser technologies for hair reduction, understanding the interaction between wavelength and pulse duration is key to predicting results.
- If your primary focus is targeting dark, coarse hair: The high melanin absorption of these specific wavelengths ensures rapid heating and effective thermal ablation of the follicle.
- If your primary focus is safety during treatment: Rely on the millisecond pulse width, which is engineered to transfer heat deep into the tissue while simultaneously sparing the skin surface.
Ultimately, the efficacy of long-pulse Ruby and Alexandrite lasers comes down to their ability to deliver heat precisely where it is needed—the follicle depth—while leaving the surface unharmed.
Summary Table:
| Feature | Long-Pulse Ruby Laser | Long-Pulse Alexandrite Laser |
|---|---|---|
| Primary Wavelength | 694 nm | 755 nm |
| Target Chromophore | Melanin (High absorption) | Melanin (High absorption) |
| Pulse Duration | Millisecond range | Millisecond range |
| Core Mechanism | Selective Photothermolysis | Selective Photothermolysis |
| Ideal Hair Type | Dark hair on light skin | Dark to medium hair |
| Primary Benefit | Precise thermal ablation | Deep follicular penetration |
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References
- Amy McMichael. Hair disorders update: Alopecia to hirsutism. DOI: 10.1016/s1040-0486(00)90029-2
This article is also based on technical information from Belislaser Knowledge Base .
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