Historical scientific foundation
The concept of selective photothermolysis was formally established by Anderson and Parrish in 1983 in their foundational publication "Selective photothermolysis" (Science 1983;220
:524-527). They demonstrated mathematically and experimentally that selective thermal destruction of a targeted chromophore was possible by meeting three fundamental criteria:
selection of a wavelength absorbed preferentially by the target chromophore compared to surrounding tissues,
limitation of the light pulse duration to a time shorter than the thermal relaxation time of the target, and
delivery of sufficient energy fluence to generate a destructive temperature elevation. This theory revolutionized laser dermatology by rationalizing treatment parameters and predicting chemical and thermal selectivity.
The three fundamental conditions
| Condition | Name | Description | Clinical example |
|---|---|---|---|
| 1 | Wavelength selection (Chemical selectivity) | Select a wavelength λ absorbed preferentially by the target chromophore (hair melanin) rather than adjacent structures (epidermis, superficial dermis). In hair removal, melanin absorbs particularly strongly at 600-900nm. Melanin absorption coefficient at 808nm = 136 cm⁻¹ versus hemoglobin 40-60 cm⁻¹, yielding a selectivity ratio of 2-3:1. The higher the target/surrounding tissue absorption ratio, the lower the risk of collateral damage. | 755nm Alexandrite: melanin absorption coefficient ~180 cm⁻¹, excellent for light hair but increased burn risk in dark skin types. 1064nm Nd:YAG: melanin absorption coefficient 70 cm⁻¹, reduced but tolerated in Fitzpatrick type VI skin. |
| 2 | Pulse duration limitation (Thermal selectivity) | Maintain light pulse duration τ shorter than the thermal relaxation time (TRT) of the target. TRT represents the duration required for heat to diffuse out of the structure and reduce temperature by 50%. According to Altshuler et al. (2001), hair follicle TRT is approximately 10-40ms depending on diameter. If τ < TRT, heat concentrates within the target causing selective destruction. If τ > TRT, heat diffuses radially out of the follicle toward the epidermis causing collateral damage. | 808nm diode laser with 10ms pulse < follicular TRT concentrates heat within the follicle. A 100ms pulse > TRT causes epidermal heat diffusion with risk of excessive erythema and burn. |
| 3 | Sufficient energy fluence | Deliver sufficient light energy (fluence in J/cm²) to elevate intra-target temperature above the irreversible denaturation threshold (65-75°C depending on protein type). Insufficient fluence results in sub-threshold temperatures causing treatment failure. Required fluence depends on: chromophore absorption coefficient, target diameter, TRT, and tissue thermal properties. | 808nm laser: 10 J/cm² fluence is insufficient (hair survives), 25 J/cm² fluence is effective (papilla destruction), 50 J/cm² fluence risks epidermal burn. Optimal 'therapeutic window' fluence is typically 15-35 J/cm². |
Step-by-step process: laser follicle destruction
- The laser device emits photons at 808nm wavelength. Photons traverse the upper epidermal layers (stratum corneum, granular layer) with moderate absorption. Penetration depth at 808nm = 1.5-3.0mm, positioning energy at the level of the deep hair follicle. Photons reach the hair shaft containing concentrated melanin.
- 808nm photons are absorbed by melanin molecules (eumelanin and pheomelanin polymer) located in the cytoplasm and nucleus of follicular cells. Photon absorption excites melanin electrons to higher energy states. Return to the ground state via non-radiative relaxation converts excitation energy into thermal energy (heat). The process is described quantum mechanically: E(photon)=hν must be ≥ the energy gap of the melanin transition.
- Heat generated by absorption accumulates in the hair shaft and adjacent follicular structures (bulb, papilla, germinal matrix). According to Altshuler et al. (2001), with a 10-30ms pulse (< follicular TRT ~20-40ms), temperature rises rapidly to 60-80°C within the follicle while the surrounding epidermis remains at approximately 45-50°C. The thermal differential between target and periphery creates selectivity. Intra-follicular temperature reaches the protein denaturation threshold of approximately 65°C.
- Temperature elevation above 65-70°C denatures structural and enzymatic proteins of the dermal papilla and follicular germinal region: collagen retracts, membrane proteins lose integrity, metabolic enzymes become inactivated, DNA fragments. Germinal cells of the dermal papilla (the source of follicle renewal) undergo irreversible thermal necrosis. Cell death mechanisms include: apoptosis (moderate heat ~65-75°C) and necrosis (elevated heat > 75°C).
- With excessive fluence or very short pulse duration, follicular temperature can reach > 95°C locally causing vaporization of intracellular water. Vapor expansion creates cavitation (bubble formation), generating mechanical forces that rupture follicle walls. The acoustic phenomenon releases additional energy. While useful for complete destruction, excessive vaporization risks follicle rupture causing scarring or deep burns.
- Days to weeks post-treatment: the dermis responds to thermal destruction through inflammation followed by repair. Dermal collagen retracts (contraction of 30-50% from heat). Fibroblasts produce type III collagen (wound healing response). Blood vessels of the destroyed papilla regenerate incompletely. Result: follicle fibrosis, loss of papillary vascularization, and inability of the follicle germinal region to regenerate hair. Absence of regrowth or minimal thin/white hair regrowth if germinal stem cells partially survive.
Thermal dynamics: thermal relaxation time versus thermal damage
The central concept of selective photothermolysis according to Altshuler et al. (2001): comparison between thermal relaxation time (TRT) and thermal relaxation damage time (TRDT).
THERMAL RELAXATION TIME (TRT): The duration required for temperature to drop 50% after pulse termination. Estimated by TRT = d²/(4α) where d=target diameter, α=thermal diffusivity (~0.1-0.3 mm²/s for soft tissue). For a 1.5mm diameter follicle: TRT ≈ 0.56/(4×0.2) ≈ 0.7s = 700ms. For a 0.5mm diameter follicle: TRT ≈ 0.06/(4×0.2) ≈ 7.5ms. Deduction: fine hair has short TRT (5-10ms), thick hair has long TRT (50-100ms).
THERMAL RELAXATION DAMAGE TIME (TRDT): The duration at a given temperature necessary to cause irreversible tissue damage. Denaturation threshold (Arrhenius activation energy): tissue heated to 65°C must remain at that temperature ≥10-30s for damage to occur. At 70°C, damage occurs within seconds. At 100°C, damage is instantaneous. TRDT is inversely proportional to temperature.
THERMAL SELECTIVITY: If pulse duration τ is chosen such that τ < follicular TRT but τ is of sufficient duration for temperature to remain above the denaturation threshold for several seconds post-pulse, selective destruction occurs. Example: 20ms pulse < thick hair TRT (~50ms) = temperature remains elevated 20-40ms post-pulse, causing damage. Adjacent epidermis with shorter TRT (5-10ms for superficial melanocytes) cools rapidly below the damage threshold.
Factors influencing thermal selectivity
Synthetic table of factors increasing or decreasing selectivity:
| factor | increases_selectivity | decreases_selectivity | mechanism |
|---|---|---|---|
| Wavelength absorption | 1 | Higher target/adjacent tissue absorption coefficient increases selectivity. 755nm > 808nm > 1064nm in terms of melanin absorption. | |
| Fluence adaptation to skin phototype | 1 | Insufficient fluence = treatment failure. Excessive fluence = burn. Narrow therapeutic window. Phototype-adjusted fluence = maximum selectivity. | |
| Pulse duration reduction | 1 | Short pulse < TRT concentrates heat at target. Long pulse > TRT allows epidermal diffusion. Ultra-short pulse (< 1ms) maximizes selectivity but risks vaporization/cavitation. | |
| Dynamic epidermal cooling | 1 | Pre/peri/post-pulse cooling reduces epidermal temperature. Deep follicle selectivity increases. Efficacy may decrease slightly if cooling is too aggressive. | |
| Increased power/irradiance | 1 | Higher irradiance (W/cm²) causes rapid temperature rise. Reaches denaturation threshold quickly, cooling diffusion is slow. Maximum selectivity with short pulse + high irradiance. | |
| Very dark skin phototype (VI) | 1 | Epidermal absorption increases with phototype. Deep follicle selectivity is reduced. Longer wavelengths (1064nm) are necessary but intrinsic selectivity decreases. | |
| Fine or light hair | 1 | Melanin absorption is reduced in pale hair. Target/epidermis absorption contrast decreases. Risk of collateral epidermal damage increases at therapeutic fluence. | |
| Recent suntan or thickened epidermis | 1 | Thickened or tanned epidermis absorbs more light. Less energy reaches the deep follicle. Selectivity is degraded, treatment failure or superficial burn possible. |
Technologies exploiting selective photothermolysis
Several types of laser/light hair removal devices exploit the principles of selective photothermolysis:
MONOCHROMATIC WAVELENGTH LASERS
- Ruby Laser 694nm: very high melanin absorption, but reduced selectivity in dark skin types
- Alexandrite Laser 755nm: excellent absorption/penetration balance, moderate selectivity across all skin types
- Diode Laser 808nm: optimal penetration, very good selectivity, versatility
- Nd:YAG Laser 1064nm: extreme penetration, reduced selectivity but excellent for Fitzpatrick type VI skin
COMBINED MULTI-WAVELENGTH SYSTEMS
- Platform 755nm + 808nm + 1064nm: maximum spectral coverage, selectivity adapted to each hair type and skin phototype
- Diode 808nm + 940nm: complementary penetration and safety for dark skin phototypes
IPL (Intense Pulsed Light) SYSTEMS
- Broad spectrum 500-1200nm: reduced selectivity compared to monochromatic devices because absorption is widely distributed
- Compensation through dichroic filters selecting sub-spectrum (e.g., 600-1000nm for hair removal)
- Inferior efficacy to monochromatic lasers according to comparative studies
PARAMETRIC VARIANTS OPTIMIZING SELECTIVITY
- Ultra-short pulse (picosecond, nanosecond): extreme selective photothermolysis, minimizes collateral damage but reduced efficacy for hair removal (thermal pulses too brief)
- Long-pulse 50-100ms: thermal accumulation, optimal efficacy but reduced selectivity, increased epidermal damage
- Variable/dynamic pulse: adapts pulse duration to intra-target temperature measured in real-time
- Dynamic pre/peri/post cooling: increases selectivity by reducing epidermal temperature
Frequently asked questions
Within the 650-1100nm window, melanin absorption is high with good contrast against hemoglobin and water absorption. Tissue penetration of ~1-5mm reaches deep follicles. Wavelengths < 650nm (UV/visible) are absorbed superficially by the epidermis and penetrate poorly. Wavelengths > 1100nm are primarily absorbed by water and lack melanin specificity. The 650-1100nm window represents the optimal compromise.
Laser thermal burn: excessive epidermal temperature elevation > 70°C denatures collagen and proteins, causing immediate visible damage (erythema/blistering). Chemical burn: results from basic or acidic chemical reaction. Laser causes exclusively thermal effects. Prevention: appropriate fluence, pulse duration < TRT, epidermal cooling.
White hair lacks melanin (aged hair follicle stem cells no longer produce melanin). Without the melanin chromophore, 808nm absorption is minimal. White hair absorbs few photons, generates minimal heat, and the papilla survives. Chemical selectivity fails. Mechanism: follicular aging leads to loss of melanin synthesis capacity, not a laser technology limitation.
Yes, partially. Excessive cooling reduces intra-follicular temperature if too aggressive, reducing efficacy. Balance is necessary: moderate cooling (pre/post-pulse) protects the epidermis without compromising deep follicle efficacy. Contact cooling systems plus cryogenic spray provide optimum. Continuous contact cooling can reduce efficacy by 10-20%.
Before 1983, laser dermatology use was empirical: parameter adjustment by trial-and-error with high adverse event risk. Anderson and Parrish mathematically formalized the 3 conditions (wavelength selectivity, pulse < TRT for thermal selectivity, fluence threshold) allowing prediction of outcomes, parameter optimization, and complication reduction. This theory guided the development of all dermatologic lasers over the past 40 years.
Sources scientifiques
- Anderson RR, Parrish JA. Selective photothermolysis. Science (1983) ;220 (4596) :524-527 . PMID: 6836297
- Altshuler GB, Anderson RR, Manstein D, Zenzie HH, Smirnov MZ. Extended theory of selective photothermolysis. Lasers in Surgery and Medicine (2001) ;29 (5) :416-432 . PMID: 12030874
- Nanni CA, Alster TS. Laser-assisted hair removal: side effects of Q-switched Nd:YAG, long-pulsed ruby, and alexandrite lasers. Journal of the American Academy of Dermatology (1999) ;41 (2) :165-171 . PMID: 10426883
- Haedersdal M, Wulf HC. Evidence-based review of hair removal using lasers and light sources. Journal of the European Academy of Dermatology and Venereology (2006) ;20 (1) :9-20 . PMID: 16405602
- Lepselter J, Elman M. Biological and clinical aspects in laser hair removal. Journal of Dermatological Treatment (2004) ;15 (2) :72-83 . PMID: 15204154
- Ibrahimi OA, Avram MR, Hanke CW. Laser hair removal. Dermatologic Therapy (2011) ;24 (1) :94-107 . PMID: 21276162
Vous souhaitez en savoir plus ?
Contactez nos experts pour une démonstration personnalisée des appareils NeoCure.
Demander une démonstration