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DRAFT — For Practitioner Review Only Version 0.1  ·  June 2026  ·  Not for clinical use or distribution beyond this review cohort
Micro-Learning · Module 03 03 of 03 — This Series AptiusIQ

Skin tissue interaction —
lasers & energy devices

The physics and biology of how light, heat, and radiofrequency energy interact with skin tissue — the mechanistic foundation of every energy-based treatment decision.

Reading time35–45 minutes
LevelAll levels — tiered content
CategoryDevice science & physics
Last reviewedJune 2026

Learning objectives

Section 01

Overview — energy and skin

Every energy-based aesthetic device achieves its clinical effect by delivering a controlled form of energy to a targeted tissue component. Whether that energy is coherent monochromatic light (laser), broadband light (IPL), or an oscillating electrical field (radiofrequency), the fundamental goal is the same: to induce a precise, controlled biological response — typically thermal — whilst minimising collateral injury to surrounding structures.

Understanding how energy interacts with skin tissue is not supplementary knowledge — it is the clinical framework within which every treatment parameter, every indication and contraindication, every adverse event and its management, and every device comparison makes sense. Without this foundation, treatment decisions become protocol-dependent rather than principle-driven.

Foundation note — two categories of skin heating

Energy devices deliver heat to the skin through one of two primary mechanisms. Photothermolysis uses light energy (laser or IPL) that is absorbed by a specific chromophore (a light-absorbing molecule), converting light to heat within that target. Dielectric heating (radiofrequency) uses oscillating electrical current to generate heat within tissues based on their electrical impedance — a chromophore-independent mechanism. Both produce thermal injury, but at different depths, with different selectivity, and through different biophysical pathways.

Section 02

Selective photothermolysis

The theory of selective photothermolysis (SP), proposed by Anderson and Parrish in 1983, is the cornerstone of modern laser medicine. It explains how a laser can selectively destroy a specific skin target (e.g. a pigmented lesion or a blood vessel) without damaging the surrounding tissue — provided three conditions are met simultaneously.

Parameter Definition Clinical implication
Wavelength (λ) The laser's wavelength must be strongly absorbed by the target chromophore and relatively poorly absorbed by surrounding structures Determines which chromophore (and therefore which target) is treated. A 532 nm KTP laser targets melanin and oxyhaemoglobin; a 1064 nm Nd:YAG targets deeper vessels with less melanin competition
Pulse duration (τ) The laser pulse must be equal to or shorter than the thermal relaxation time (TRT) of the target — the time for the target to lose 50% of its heat to surrounding tissue Shorter pulses confine heat to the target. Longer pulses allow heat diffusion, risking collateral damage. TRT varies by target size: small targets (melanosomes ~1 µs) require nanosecond to microsecond pulses; large targets (leg veins) tolerate millisecond pulses
Fluence (F) The energy delivered per unit area (J/cm²) must be sufficient to raise the target to its thermal damage threshold Too low: subtherapeutic effect. Too high: heat diffusion beyond the target, blistering, or scarring. Fluence must be titrated against patient skin type, target depth, and chromophore density
Advanced note — thermal relaxation time calculation

The thermal relaxation time (TRT) of a target is proportional to the square of its diameter and inversely proportional to its thermal diffusivity. For clinical reference: epidermal melanosomes (0.5–1 µm) have a TRT of ~1 µs; individual melanocytes (~7 µm) ~100 µs; a port wine stain vessel (~100 µm) ~10 ms; a large leg vein (~1 mm) ~100 ms. This is why picosecond lasers (10⁻¹² seconds, far below melanosome TRT) produce a predominantly photoacoustic rather than photothermal effect on pigmented lesions — generating pressure waves rather than heat — which is a more selective and less damaging mode of fragmentation for recalcitrant pigment and tattoo ink. (Anderson & Parrish, 1983; Nouri, 2011)

Section 03

Primary skin chromophores

A chromophore is any molecule that absorbs light at a specific wavelength. In aesthetic laser medicine, three primary chromophores determine the targets available for treatment — melanin, oxyhaemoglobin (in blood), and water. Each has a distinct absorption spectrum, and the choice of laser wavelength must account for the relative absorption of all three to achieve selective tissue targeting.

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Melanin
Peak absorption: 300–800 nm (broad spectrum)
Located in melanosomes within melanocytes (stratum basale) and keratinocytes. Primary chromophore for treatment of pigmented lesions: solar lentigines, café-au-lait macules, epidermal melasma, naevi, and tattoo ink (which behaves as an exogenous chromophore). Melanin absorbs broadly across the visible and near-UV spectrum, with absorption decreasing at longer wavelengths. Eumelanin (brown/black) and phaeomelanin (red/yellow) have slightly different spectra — relevant in treating red and yellow tattoo pigments.
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Oxyhaemoglobin
Peak absorption: 418 nm (Soret band), 542 nm, 577 nm
Located within red blood cells in cutaneous vasculature. Primary chromophore for vascular lesions: telangiectasia, rosacea, port wine stains, haemangiomas, poikiloderma. Laser wavelengths selected for vascular targets (KTP 532 nm, pulsed dye 585/595 nm, Nd:YAG 1064 nm for deeper vessels) exploit oxyhaemoglobin absorption to heat the vessel wall, producing thermocoagulation and subsequent vessel collapse. Deoxyhaemoglobin has a different spectrum, relevant in venous lesion treatment.
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Water
Peak absorption: 2940 nm (Er:YAG); 10,600 nm (CO₂)
Water constitutes 65–72% of dermal tissue and is the dominant chromophore in the infrared spectrum. Lasers targeting water (ablative resurfacing lasers) heat intracellular and extracellular water to vaporisation, causing controlled tissue ablation. The depth of ablation and residual thermal damage (RTD) determine both efficacy and recovery profile. Water absorption peaks at specific infrared wavelengths — Er:YAG (2940 nm) is 12–18× more absorbed by water than CO₂, resulting in more precise ablation with less RTD. CO₂ (10,600 nm) ablates with greater RTD, producing stronger collagen contraction and longer recovery.
Advanced note — competing chromophore absorption

In clinical practice, competing chromophore absorption is a constant consideration. A wavelength selected to target melanin (e.g. 532 nm KTP) also has significant oxyhaemoglobin absorption — potentially causing purpura in vascular skin or vessel rupture. The same wavelength is more strongly absorbed by epidermal melanin in darker skin types (Fitzpatrick IV–VI), increasing the risk of post-inflammatory hyperpigmentation (PIH) and epidermal damage. Longer wavelengths (1064 nm Nd:YAG) are relatively less absorbed by melanin, penetrate deeper, and are generally safer for darker skin — but less efficient at superficial pigment targets. Chromophore competition directly drives the clinical decision to use longer wavelengths, longer cooling times, lower fluences, or alternate modalities in higher Fitzpatrick phototypes.

Laser systems

Section 04

Laser classification by tissue target

Lasers used in aesthetic medicine are classified by their wavelength (which determines chromophore affinity), their pulse characteristics, and their delivery mode (ablative, non-ablative, fractional). The following covers the major classes in clinical use.

CO₂ laser
Ablative — water chromophore — 10,600 nm
MechanismWater absorption causes rapid vaporisation of tissue cells (ablation). Residual thermal damage (RTD) to adjacent tissue drives collagen contraction and stimulates collagen neosynthesis during healing. RTD of 20–150 µm depending on mode.
Clinical applicationsFull-face resurfacing; deep rhytids; acne scarring; skin laxity; actinic keratoses; seborrhoeic keratoses. The gold-standard ablative resurfacing laser.
Recovery7–14 days for re-epithelialisation (ablative full-beam); 5–7 days (fractional CO₂). Prolonged erythema 2–6 months.
Key risksPost-inflammatory hyperpigmentation (particularly Fitzpatrick III–VI); post-treatment hypopigmentation; scarring; prolonged erythema; herpes simplex reactivation. Requires antiviral prophylaxis.
Er:YAG laser
Ablative — water chromophore — 2940 nm
Mechanism12–18× greater water absorption than CO₂. More precise ablation with minimal RTD (~5–25 µm). Less collagen contraction per pass compared to CO₂; requires more passes for equivalent effect but with faster healing.
Clinical applicationsSuperficial to moderate resurfacing; fine rhytids; pigmentary irregularity; patients requiring faster recovery than CO₂ allows. Dual-mode Er:YAG systems can modulate RTD toward CO₂-like levels.
Recovery4–7 days re-epithelialisation (full-beam); 3–5 days (fractional). Erythema resolves faster than CO₂.
Key risksLess haemostasis than CO₂ (more bleeding intraoperatively); slightly less collagen stimulation per session than CO₂; lower PIH risk but not absent.
Pulsed dye laser (PDL)
Non-ablative vascular — oxyhaemoglobin — 585/595 nm
MechanismWavelength (585 or 595 nm) coincides with oxyhaemoglobin absorption peaks. Pulse duration matched to TRT of target vessels (~0.45–6 ms). Selective heating of vessel wall → thermocoagulation → vessel obliteration → resorption.
Clinical applicationsPort wine stains; telangiectasia; facial erythema; rosacea (vascular component); hypertrophic scars; striae rubra. Considered the gold standard for vascular lesions.
RecoveryMinimal to moderate erythema; purpura may occur (vessel rupture) at therapeutic fluences — typically resolves in 7–14 days. Bruising is a marker of adequate treatment.
Key risksPurpura (expected therapeutic response at certain settings); dyspigmentation in higher Fitzpatrick types; blistering at excessive fluences.
KTP / Nd:YAG (532 nm)
Non-ablative vascular & pigment — 532 nm
MechanismFrequency-doubled Nd:YAG (1064 nm halved to 532 nm). Strong absorption by both oxyhaemoglobin and melanin. Shallow penetration (~0.75 mm).
Clinical applicationsSuperficial telangiectasia; rosacea; epidermal pigmented lesions; poikiloderma; lentigines; facial erythema.
RecoveryUsually minimal; temporary darkening of pigmented lesions (correct response) before shedding over 7–14 days.
Key risksHigh melanin competition — increased PIH risk in Fitzpatrick III+; shallow penetration limits efficacy for deeper targets.
Nd:YAG 1064 nm
Non-ablative — deep vascular & pigment — 1064 nm
MechanismLonger wavelength penetrates to 5–6 mm depth; lower melanin absorption than 532 nm makes it safer in darker skin types. Q-switched/picosecond modes target pigment via photoacoustic fragmentation; long-pulse modes target deeper vessels by thermocoagulation.
Clinical applicationsDeep vascular lesions (leg veins, reticular veins); laser hair removal (darker hair in skin of colour); tattoo removal (black/blue inks in darker skin); dermal pigmented lesions; skin toning ("Hollywood laser peel" at low fluence).
RecoveryVariable. Long-pulse: moderate erythema. QS/picosecond: transient erythema, petechiae possible.
Key risksDeep heating — risk of subepidermal blistering if melanin still sufficient to compete; pain; nerve proximity in certain face zones.
IPL (Intense Pulsed Light)
Non-laser broadband light — 500–1200 nm
MechanismNon-coherent, broadband light filtered to selected wavelength ranges (e.g. 515–1200 nm for pigment; 560–1200 nm for vascular). Not a laser — lacks monochromaticity and collimation. Selectivity achieved via cut-off filters. Multiple chromophores can be targeted simultaneously at the cost of reduced selectivity.
Clinical applicationsPhotorejuvenation; facial erythema; solar lentigines; poikiloderma; rosacea; mild vascular and pigmented lesion treatment. Excellent for combined pigmented-and-vascular presentations.
RecoveryMinimal to moderate erythema; temporary darkening of pigmented lesions. Generally low downtime.
Key risksReduced selectivity vs. laser — higher risk of epidermal collateral damage in darker skin types; burns if epidermal cooling is insufficient or parameters are inappropriate.

Section 05

Fractional laser technology

Fractional photothermolysis, introduced by Manstein et al. in 2004, fundamentally changed the risk–benefit profile of laser resurfacing by replacing confluent tissue treatment with an array of discrete microscopic treatment zones (MTZs). By treating only a fraction of the skin surface in each session, the untreated skin between MTZs acts as a reservoir of viable cells that rapidly repopulate the treated columns — dramatically accelerating healing whilst preserving cumulative collagen stimulation efficacy.

Parameter Ablative fractional (AFR) Non-ablative fractional (NAFR)
Mechanism Microscopic ablative columns extending through epidermis and into dermis. RTD zone surrounding each column. Epidermis is disrupted. Microscopic coagulative columns (microscopic thermal zones, MTZs) within dermis — epidermis intact but heated. No ablation.
Common wavelengths CO₂ 10,600 nm (e.g. Fraxel Re:pair, DEKA SmartXide); Er:YAG 2940 nm fractional modes 1440 nm, 1540 nm, 1550 nm, 1927 nm (e.g. Fraxel Restore, Fraxel Dual, Clear + Brilliant)
Treatment fraction Typically 10–40% surface area per session Typically 15–50% surface area per session (density adjustable)
Depth of effect Epidermis to 1–4 mm reticular dermis (parameter-dependent) 0.4–1.5 mm mid-dermis (parameter-dependent); epidermis relatively spared
Downtime 3–7 days (re-epithelialisation); 1–4 weeks erythema 1–3 days erythema and oedema; micro-exfoliation over 3–7 days
Clinical applications Deep rhytids; acne scarring; significant skin laxity; photodamage reversal; surgical scars Mild to moderate rhytids; skin tone and texture; early photodamage; maintenance; dyspigmentation (1927 nm); periorbital and neck (with parameter adjustment)
Session frequency 1–3 sessions (higher per-session efficacy) 3–6 sessions (lower per-session efficacy; cumulative benefit)
Advanced note — density vs. depth as treatment variables

In fractional laser treatment, two independent variables determine outcome: treatment density (percentage of surface area treated per session) and treatment depth (how deep each MTZ extends into the dermis). Increasing density increases total collagen stimulus but reduces the healing reservoir — raising downtime and adverse event risk. Increasing depth targets deeper dermal collagen for structural improvements such as laxity and deep scarring. A conservative approach for sensitive patients or challenging regions (neck, décolletage, periorbital) is to reduce density (8–15%) and depth, trading per-session efficacy for reduced risk and faster recovery — then accumulate results over multiple sessions. Understanding these variables allows the practitioner to meaningfully explain the protocol to patients and respond intelligently to comparative device claims.

Radiofrequency

Section 06

Radiofrequency — mechanism and device types

Radiofrequency (RF) devices use alternating electrical current at frequencies in the range of 0.3–10 MHz to generate heat within tissue via dielectric heating (also termed resistive heating or Joule heating). Unlike laser, RF is chromophore-independent — it heats tissue based on its electrical impedance, not its light-absorbing properties. This means RF is equally effective across all skin phototypes and is unaffected by melanin content.

When the alternating current of an RF device passes through tissue, it causes oscillation (rotation) of water molecules and charged ions in an attempt to follow the alternating electric field. This molecular friction generates heat. The depth and distribution of this heat is determined by the RF device type, electrode geometry, and the impedance characteristics of the target tissue.

Foundation note — why RF works for skin tightening

Collagen fibres denature (partially "melt") at temperatures of 60–70°C. This denaturation causes immediate collagen fibril contraction — the heat-induced tightening effect visible immediately post-treatment. The subsequent biological response — fibroblast stimulation, new collagen synthesis — produces the longer-term remodelling benefit over 3–6 months following treatment. RF devices are calibrated to achieve these therapeutic temperatures within the target dermal or subdermal layer without damaging the overlying epidermis, which is actively cooled throughout treatment.

Monopolar RF
Single active electrode — ground plate return
MechanismCurrent passes from a single active electrode through the body to a return (ground) plate placed elsewhere. Current path is broad and penetrates deeply — to the subdermal fat and SMAS layer (3–6 mm depth). Suitable for significant skin laxity requiring deep tissue remodelling.
Clinical applicationsFacial and neck laxity; jawline and jowl improvement; brow lifting; body contouring. Examples: Thermage (Solta Medical), Pelleve.
Treatment profileSingle treatment sessions with cumulative benefit developing over 3–6 months. Significant patient discomfort requiring adequate topical or systemic analgesia. Results can be durable at 1–2 years.
Key considerationsDepth of heating and volume treated is large — patient discomfort can be significant. Requires careful energy titration to avoid burns. Contraindicated with implanted metal devices in the treatment field. The return plate must be appropriately positioned to ensure current flows through the target tissue.
Bipolar RF
Two electrodes — current confined between poles
MechanismCurrent flows between two electrodes on the handpiece surface, confining heating to the tissue between them. Depth of penetration is approximately half the distance between the electrodes. More superficial and localised heating than monopolar. Often combined with other modalities (diode laser, optical energy) in multi-platform devices.
Clinical applicationsSkin texture and tightening; early laxity; combination devices (e.g. ELOS technology — combined bipolar RF + optical energy). Examples: eMatrix, Sublime (Syneron-Candela), Exilis Ultra.
Treatment profileGenerally more comfortable than monopolar. Requires multiple sessions (typically 3–6). Depth limited by electrode spacing — less suitable for deep SMAS remodelling than monopolar or HIFU.
Key considerationsConsistent skin contact required — air gaps cause impedance spikes and risk hot spots. Coupling gel must be adequate. Skin type does not affect mechanism — safe across all Fitzpatrick phototypes.
Fractional RF (microneedling RF)
Microneedle electrode arrays — precise dermal delivery
MechanismInsulated microneedles penetrate the epidermis to a programmable depth (typically 0.5–4 mm) and deliver RF energy directly within the dermis from the needle tips. The insulated shaft minimises epidermal heating, while RF delivery at the needle tips produces coagulative columns (similar in concept to fractional laser MTZs) within the dermis. Combines mechanical stimulus (microneedling collagen response) with thermal stimulus (RF collagen denaturation/neogenesis).
Clinical applicationsAcne scarring; skin tightening and laxity; pore refinement; stretch marks; hyperhidrosis (at deeper settings targeting eccrine glands); melasma (due to chromophore independence). Examples: Morpheus8 (InMode), Genius (Lutronic), Sylfirm X, Secret RF, Potenza.
Treatment profileTypically 1–3 sessions at 4–6 week intervals. Downtime 2–5 days (erythema, micro-crusting at needle entry points). Highly customisable by needle depth and RF energy level.
Key considerationsSafe across all skin phototypes (chromophore-independent). Contraindicated in active acne, open wounds, and over implanted metal. Topical anaesthesia required. Risk of grid-pattern PIH in darker skin types if settings are too aggressive — reduce energy and needle depth, test patch recommended.
Multi-polar RF
Multiple electrodes — rotating field delivery
MechanismThree or more electrodes arranged so that current alternates between different electrode pairs in a rotating sequence. Distributes heating more uniformly across a larger tissue volume. Often combined with pulsed electromagnetic fields (PEMF) for synergistic biostimulation.
Clinical applicationsFacial and body skin tightening; cellulite treatment; body contouring. Examples: Venus Freeze/Legacy (Venus Concept), Accent Prime (Alma).
Treatment profileWeekly or fortnightly sessions (typically 6–8 for body; 4–6 for face). Comfortable — often described as a "warm massage." Suitable for maintenance programmes.
Key considerationsLower per-session energy delivery than monopolar — less dramatic single-treatment results but high patient comfort and compliance. Well-suited to patients who require treatment without downtime.

Section 07

High-intensity focused ultrasound (HIFU)

HIFU is distinct from both laser and RF in its delivery mechanism. It uses focused acoustic energy to generate precise focal points of thermal injury deep within tissue — bypassing the epidermis and superficial dermis entirely. The convergence of multiple low-intensity ultrasound waves at a focal point causes a rapid temperature rise (65–70°C) within a discrete coagulation zone of 1–2 mm at a targeted depth.

In aesthetic practice, HIFU devices (e.g. Ultherapy, Ultraformer III, Doublo) deliver energy to three primary depths: 1.5 mm (papillary–reticular dermis junction), 3 mm (reticular dermis), and 4.5 mm (SMAS layer). The ability to target the SMAS non-invasively — with real-time ultrasound imaging in some devices — represents a unique capability among non-surgical modalities.

Advanced note — HIFU vs. RF for tissue depth

HIFU and monopolar RF both target the SMAS layer, but through fundamentally different mechanisms and with different tissue effects. RF produces a broad column of resistive heating that extends from the dermis downward — depth is operator-controlled but not focal. HIFU produces discrete, precise coagulation points at a fixed depth determined by the transducer — the tissue between the surface and the focal point is not heated. This focal precision means HIFU can deposit energy at the SMAS without necessarily heating the overlying fat — a potential advantage when avoiding fat atrophy. However, HIFU's focal energy deposition also means that incorrect positioning produces no effect rather than a partially satisfactory one — the learning curve for optimal transducer placement is steeper than for RF. Both modalities stimulate fibroblast response and collagen I upregulation as their mechanism of long-term benefit.

Biological response

Section 08

The wound healing cascade

Every energy-based treatment that achieves a therapeutic response does so by inducing a controlled wound — and triggering the body's wound healing cascade. Understanding this cascade is not merely academic: it directly informs post-treatment management, the expected timeline of results, and the rationale for recovery-phase skincare and follow-up.

Phase 01
Haemostasis
0–minutes
Immediately following tissue injury, damaged blood vessels vasoconstrict and a platelet plug forms at the wound site. Platelets aggregate via von Willebrand factor and fibrin, activating the coagulation cascade. The fibrin clot provides an initial matrix for cell migration. Platelets also release growth factors — including platelet-derived growth factor (PDGF), transforming growth factor-beta (TGF-β), and vascular endothelial growth factor (VEGF) — that recruit and activate subsequent healing cells. In laser and RF treatment, this phase is reflected clinically as the immediate erythema and oedema following treatment.
Phase 02
Inflammation
Hours–4 days
Neutrophils (arriving within hours) and macrophages (days 2–4) migrate to the wound site. Neutrophils debride damaged tissue and prevent infection via phagocytosis and reactive oxygen species (ROS). Macrophages are the pivotal cells of this phase — they phagocytose cellular debris and pathogens, and importantly secrete growth factors (TGF-β, bFGF, VEGF) that orchestrate subsequent tissue repair. Macrophage polarisation between pro-inflammatory (M1) and anti-inflammatory/pro-regenerative (M2) phenotypes is a key determinant of whether healing proceeds toward scar (M1 dominant) or regeneration (M2 dominant). Clinically: the erythema, oedema, warmth, and discomfort of the first 1–4 days post-treatment represent this phase. Premature suppression of this phase (e.g. aggressive topical steroid) may impair collagen neosynthesis.
Phase 03
Proliferation
Days 4–21
The proliferative phase involves four overlapping processes: re-epithelialisation, angiogenesis, fibroplasia (fibroblast proliferation and collagen synthesis), and wound contraction. Keratinocytes migrate from the wound margins and surviving follicular units to re-epithelialise the surface — the biological reason why follicular density determines healing speed in the neck and décolletage. Fibroblasts, stimulated by TGF-β and bFGF, proliferate and synthesise type III collagen (provisional matrix), which is progressively replaced by type I collagen during remodelling. Angiogenesis restores the vascular supply, producing the characteristic "granulation tissue" appearance with capillary buds visible through the healing epidermis. Clinically: erythema peaks in this phase before gradually resolving. The skin feels tight and new. Effective post-treatment skincare — particularly barrier repair agents, antioxidants, and SPF — supports this phase.
Phase 04
Remodelling
3 weeks–2 years
The longest phase of wound healing — and the one responsible for the durable aesthetic results of energy-based treatments. Type III collagen (disordered, provisional) is progressively cross-linked and replaced by type I collagen (organised, mature) through the coordinated action of matrix metalloproteinases (MMPs) and their inhibitors (TIMPs). Collagen fibres reorganise along lines of mechanical stress, increasing tensile strength from ~20% at week 3 to ~80% at 3 months, stabilising at ~80% of original skin strength — never fully returning to the pre-wound state. Elastin synthesis (reduced in wound healing relative to collagen) is also upregulated, though less dramatically. Clinically: the visible improvement of skin texture, laxity, and pigmentation continues for 3–6 months after a single treatment session, consistent with this remodelling timeline. Patients should be counselled that the full result of a collagen-stimulating treatment is not visible at 1 month — reassessment at 3–6 months is more clinically meaningful.
Clinical application — post-treatment skincare rationale

Understanding the wound healing cascade directly informs post-treatment protocols. During re-epithelialisation (days 1–7): barrier repair is the priority — occlusives (petrolatum, mineral oil) maintain moisture, reduce TEWL, and accelerate keratinocyte migration. Avoid active ingredients (retinoids, AHAs, vitamin C) that could irritate the compromised barrier. During proliferation (days 4–21): introduce antioxidants (vitamin C, niacinamide) to support collagen synthesis and neutralise ROS. Introduce SPF immediately — photodamage during this phase can redirect melanocyte activity toward PIH. During remodelling (weeks 3 onwards): retinoids can be reintroduced to amplify collagen I synthesis, normalise turnover, and support long-term result maintenance. Sunscreen remains non-negotiable throughout.

Reference index

All physics, physiology, and clinical statements in this module are drawn from or consistent with the following peer-reviewed and authoritative sources. References marked * are primary sources for the indicated content area. This index is maintained as a living document and updated on each module review cycle.

Learning Check

Test your understanding before moving on. Select the best answer for each question, then click Check Answers.

1. The principle that allows lasers to target specific tissue structures without damaging surrounding tissue is called:

2. The primary chromophore targeted by a 532 nm laser for vascular lesion treatment is:

3. Thermal relaxation time (TRT) represents:

4. Radiofrequency (RF) energy generates heat in tissue by:

5. Fitzpatrick skin type is a critical variable in laser treatment planning because:

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