Energy transfer occurs through a process of selective photothermolysis, where light energy is specifically absorbed by target tissues and converted into heat. When using a long-pulse 755 nm Alexandrite laser for axillary hair removal, this involves delivering light at specific settings—such as a 20 ms pulse width and energy densities between 16 and 34 J/cm²—to thermally destroy hair follicles without causing explosive damage to the surface skin.
Core Takeaway The long-pulse 755 nm Alexandrite laser works by converting light into thermal energy to destroy the hair follicle's regenerative capacity. While effective for hair removal, the heat generated by large spot sizes can diffuse deeply into the dermis, potentially affecting sweat glands and altering post-operative perspiration.
The Mechanics of Energy Delivery
To understand how this laser removes hair effectively, we must look at how the parameters interact with biological tissue.
Selective Conversion to Thermal Energy
The fundamental mechanism is the selective conversion of light energy into thermal energy.
The laser emits light at a 755 nm wavelength, which travels through the skin and is absorbed by the melanin in the hair.
Once absorbed, this light transforms into heat, raising the temperature of the follicle to a point where its structure is compromised.
The Importance of Pulse Width
The "long-pulse" aspect is critical for safety. The system typically uses a pulse width of around 20 ms.
By matching or exceeding the thermal relaxation time of the hair follicle, the laser allows thermal energy to act slowly and continuously on the germinal cells.
This controlled release avoids the "explosive" damage to the skin surface that shorter pulses might cause, ensuring the energy destroys the follicle's regenerative capacity rather than the surrounding skin.
Energy Density and Spot Size
The total energy delivered is determined by the energy density (fluence), which is generally set between 16 and 34 J/cm².
Simultaneously, a large spot size, such as 12.5 mm, is utilized to ensure the energy is distributed uniformly across the treatment area.
Higher energy densities lead to more thorough thermal destruction of the follicle, but they must be balanced carefully to prevent surface damage.
Understanding the Trade-offs
While the primary goal is hair removal, the physics of energy transfer introduces side effects and risks that must be managed.
Heat Diffusion and Sweat Glands
A specific consequence of using a large spot size (12.5 mm) is the depth of energy penetration.
While this ensures uniform coverage, it increases the likelihood of heat diffusing toward the sweat glands located in the deep dermis.
This thermal diffusion can inadvertently affect the function of these glands, leading to changes in local post-operative perspiration.
Balancing Efficacy with Skin Safety
There is a fine line between effective follicle destruction and tissue damage.
If the energy density is too high for a specific skin type, or if the laser is inappropriate for the patient, there is a risk of adverse effects.
These risks include burns, hyperpigmentation (darkening of the skin), or hypopigmentation (lightening of the skin) caused by excessive or misdirected thermal energy.
Making the Right Choice for Your Goal
When evaluating the use of a long-pulse Alexandrite laser, consider your specific clinical priorities.
- If your primary focus is thorough hair reduction: Prioritize a protocol that optimizes energy density (within the 16–34 J/cm² range) to ensure sufficient thermal destruction of the follicle's regenerative cells.
- If your primary focus is minimizing side effects: Be aware that the large spot size required for uniform heating may cause deep heat diffusion, potentially impacting sweat gland function and perspiration levels.
Successful treatment relies on precisely matching the pulse duration and fluence to the patient's physiology to destroy the follicle while preserving the surrounding tissue.
Summary Table:
| Parameter | Specification | Biological Impact |
|---|---|---|
| Wavelength | 755 nm | High melanin absorption for targeted follicle destruction |
| Pulse Width | 20 ms | Matches thermal relaxation time to prevent skin surface damage |
| Energy Density | 16 - 34 J/cm² | Determines the level of thermal destruction to hair germinal cells |
| Spot Size | 12.5 mm | Ensures uniform coverage; facilitates deep thermal diffusion |
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References
- Josiane Hélou, Roland Tomb. Reversibility of hyperhidrosis post axillary depilatory laser. DOI: 10.1007/s10103-013-1404-4
This article is also based on technical information from Belislaser Knowledge Base .
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