A hybrid 755-nm/1064-nm dual-wavelength laser compared with a single 755-nm alexandrite laser for hair reduction: a prospective, single-blind, split-body controlled trial with alternating allocation

Abstract

Hybrid 755-nm/1064-nm laser systems may improve hair reduction by combining strong melanin absorption with deeper dermal penetration. This study compared a synchronized hybrid dual-wavelength mode with a single 755-nm alexandrite mode. In this prospective, single-blind, split-body controlled trial with alternating allocation, 41 adults underwent three laser sessions at 4-week intervals using one integrated platform. One bilateral subsite was treated with single 755-nm mode and the contralateral subsite with hybrid 755-nm/1064-nm mode. The primary outcome was change in trichoscopic hair thickness from baseline to 20 weeks. Secondary outcomes included hair density, pain, satisfaction, and adverse events. Thirty-six participants completed the per-protocol analysis. Hair thickness decreased significantly with both modes, with a greater reduction after hybrid treatment (between-mode difference, 0.00461 ± 0.00745 mm; 95% CI, 0.00209–0.00713; p = 0.0007), corresponding to an approximately 28% larger relative reduction. Hair density reduction did not differ significantly (p = 0.238). Pain was slightly higher with hybrid treatment at the second session only, but satisfaction was comparable. No device-related adverse events occurred at hybrid-mode sites. The synchronized hybrid 755-nm/1064-nm mode produced superior hair-thickness reduction compared with single 755-nm treatment, with comparable satisfaction and favourable tolerability.

Introduction

Laser hair removal (LHR) is among the most commonly performed aesthetic dermatologic procedures, and global demand continues to grow [1]. The principle underlying LHR is selective photothermolysis, originally described by Anderson and Parrish in 1983: laser energy at a wavelength preferentially absorbed by melanin in the hair follicle is delivered with a pulse duration shorter than the thermal relaxation time of the target, producing irreversible follicular injury while sparing surrounding tissue [2].

Among available laser platforms, the 755-nm alexandrite laser has long been considered a gold standard for LHR in patients with Fitzpatrick skin types I–IV [3]. Its wavelength corresponds to a high absorption peak of eumelanin, allowing efficient photothermal conversion within the hair matrix and shaft. However, dermal penetration of the 755-nm beam is shallower than that of longer-wavelength systems, and efficacy may be suboptimal in patients with coarse terminal hair, deeply seated follicular bulbs, or darker phototypes (Fitzpatrick IV–V), in whom competing epidermal melanin increases the risk of non-specific thermal injury [4, 5].

The 1064-nm Nd: YAG laser overcomes some of these limitations through its substantially lower melanin absorption and correspondingly greater dermal penetration—often beyond 5 mm—allowing thermal energy to reach the outer root sheath, the bulge stem-cell niche, the dermal papilla, and the perifollicular vascular plexus [6, 7]. Although higher fluences are typically needed to achieve follicular damage equivalent to that of the 755-nm system, the 1064-nm wavelength provides complementary photothermal injury at depths that the shorter wavelength may incompletely reach.

Building on this biophysical rationale, integrated platforms have been developed that deliver 755-nm and 1064-nm wavelengths simultaneously in a synchronized “hybrid” mode. The 755-nm component preferentially targets the melanin-rich superficial hair matrix and shaft, while the 1064-nm component extends thermal damage into the deeper follicular and perifollicular structures. This dual-wavelength strategy is intended to span the entire vertical extent of the follicular unit—from the pigmented shaft at the level of the isthmus to the deeply seated bulge and dermal papilla—and may therefore produce broader follicular destruction than either wavelength alone [8, 9].

Despite the biological plausibility of hybrid LHR, prospective clinical evidence directly comparing a synchronized dual-wavelength mode with single 755-nm treatment remains limited. Earlier comparative work has typically used separate devices or different patient cohorts, introducing confounding from inter-individual variation in skin phototype, hair characteristics, and treatment parameters [10, 11]. A split-body design, in which each participant serves as their own control by receiving the two modalities at contralateral subsites, is the most rigorous framework for direct efficacy comparison and removes between-subject variability.

We therefore conducted a prospective, single-blind, split-body controlled trial with alternating allocation to test the hypothesis that the hybrid 755-nm/1064-nm dual-wavelength mode is superior to the single 755-nm mode for the reduction of hair thickness over three treatment sessions. Secondary objectives were to compare the two modes with respect to hair density, intraoperative pain, patient satisfaction, and adverse events.

Materials and methods

Study design and ethics

This was a prospective, single-blind, split-body controlled trial with alternating allocation conducted at a single academic dermatology centre ([department and institution blinded for peer review]). The study was carried out in accordance with the Declaration of Helsinki and was approved by the institutional review board of the participating centre [name and approval number blinded for peer review]. All participants provided written informed consent before enrollment.

Participants

Eligible participants were adults aged 18–70 years who wished to undergo LHR at a bilateral or mirror-image body site and had measurable terminal hair (dark or dark-brown) at the target site. Participants with Fitzpatrick skin phototypes I–V were eligible. Key exclusion criteria were: prior LHR at the target site within 6 months of enrollment; use of photosensitizing medications or presence of a photosensitive dermatosis; active cutaneous infection or inflammatory dermatosis at the treatment site; pregnancy or lactation; history of hypertrophic scarring or keloid formation; pre-existing decorative tattoo or scar within the treatment area (other than the study position-marking dots described in Sect. 2.4); tanning or substantial sun exposure within 30 days before enrollment; and use of hair-affecting medications (e.g., minoxidil, spironolactone) within 3 months of enrollment.

Allocation and blinding

For each participant, the treatment area was divided into two bilaterally symmetric or mirror-image subsites by a study coordinator who was not involved in outcome assessment. Treatment-mode allocation was determined by a pre-specified alternating scheme based on the consecutive enrollment number: participants with odd enrollment numbers received the single 755-nm mode on the left subsite and the hybrid 755-nm/1064-nm mode on the right subsite, while assignment was reversed for participants with even enrollment numbers. Enrollment order was assigned consecutively at clinic entry and was independent of any clinical characteristic, skin phototype, hair morphology, or anatomical site. The alternating allocation scheme was designed to ensure balanced distribution of treatment modes across body sides, although it did not constitute computer-generated randomisation. Participants were blinded to which modality was applied to each subsite (single blinding), and outcome assessors performing trichoscopy were similarly blinded to allocation. To minimise baseline heterogeneity between the two compared fields, only participants with dark or dark-brown terminal hair at the target region were enrolled, and the two adjacent subsites were selected so that their baseline hair thickness and density, measured by trichoscopy before treatment, differed by no more than 5%.

Treatment protocol

All participants received three laser sessions at 4-week intervals (Visit 2 [V2], Visit 3 [V3], and Visit 4 [V4]). Both modalities were delivered using the same integrated alexandrite/Nd: YAG device platform (SandroDual; manufacturer blinded for peer review). The single mode used the 755-nm alexandrite wavelength only. The hybrid mode delivered the 755-nm alexandrite and 1064-nm Nd: YAG components synchronously within a single pulse train, with the 755-nm component preceding the 1064-nm component by a fixed inter-pulse interval determined by the device algorithm. Treatment parameters—fluence (J/cm²), pulse duration (ms), spot size (mm), and repetition rate (Hz)—were standardised in accordance with the manufacturer’s clinical guidelines for the SandroDual platform and applied uniformly across participants and sessions (Table 7). Rather than representing an individually optimised “optimal” setting, these parameters were chosen to lie within clinically effective and safe ranges reported in prior laser hair-removal studies, using deliberately conservative fluences appropriate to this exploratory comparison. The single-mode setting (3-ms pulse, 12-mm spot, 14 J/cm²; protocol-defined range 14–16 J/cm²) is consistent with the range applied for alexandrite hair removal in Asian skin [12]. For the hybrid mode, combining a reduced 755-nm fluence (10 J/cm²) with a lower 1064-nm fluence (8 J/cm²) reflects previously reported low-fluence dual-wavelength strategies [10, 13] and was intended to explore whether a conservative energy combination could achieve effective hair reduction while limiting cumulative thermal load. For the single mode, fluence was 14 J/cm² with a 12-mm spot, 3-ms pulse, 2-Hz repetition rate, and Skin Cooling Spray (SCS) at 4.4 ms applied pre- and post-pulse. For the hybrid mode, the 755-nm component was delivered at 10 J/cm² and the 1064-nm component at 8 J/cm², using the same 12-mm spot, 3-ms pulse, 2-Hz repetition rate, and 4.4 ms pre- and post-pulse Skin Cooling Spray (SCS), with a 20-ms inter-pulse interval between the two wavelengths. Skin Cooling Spray (SCS) was applied continuously throughout each session. Treatment areas were pre-shaved 24 h before each session to standardize baseline hair length. To ensure that the same area was treated and assessed at every visit, a site-marking protocol was applied. At body sites, four dot tattoos were placed at the margin of each treatment field, within the zone overlapped by the handpiece tip, to serve as permanent position markers; the midpoint defined by joining the opposing dots was used as the reference point for repeat treatment and for trichoscopic measurement. At facial sites, where dot tattoos are cosmetically undesirable, reproducible anatomical landmarks (e.g., the philtral midline, cupid’s bow, oral commissure, mental protuberance, and otobasion inferius) together with fixed quantitative distances (e.g., 20 mm from a landmark, or a 12-mm-diameter field) were used to relocate the same site at each visit.

Table 7.

Standardised laser treatment parameters by mode (SandroDual; manufacturer blinded for peer review)

Mode	Wavelength (nm)	Spot size (mm)	Pulse duration (ms)	Fluence (J/cm²)	Frequency (Hz)	Interval (ms)	Cooling pre/post (ms)
Hybrid	755	12	3	10	2	20	4.4/4.4
	1064	12	3	8	2	20	4.4/4.4
Single	755	12	3	14	2	—	4.4/4.4

Interval = inter-pulse delay between the 755-nm and 1064-nm components (hybrid mode only). Hz hertz, J/cm² joules per square centimeter, mm millimetres, ms milliseconds, nm nanometres, — not applicable

Outcome measures

The primary outcome was the change in hair thickness (mm) from V2 (baseline, first treatment session) to V6 (20-week follow-up). Hair thickness was measured by a calibrated trichoscopy device at the predefined reference point of each subsite—the dot-tattoo midpoint at body sites, or an anatomical-landmark-defined point at facial sites (Sect. 2.4)—so that the same location was assessed across visits.

Secondary outcomes were: (i) change in hair density (hairs/cm²) from V2 to V6, also measured by trichoscopy at the same reference point; (ii) intraoperative pain assessed by each participant immediately after each treatment session (V2–V4) on a numerical rating scale (NRS; 0 = no pain, 10 = worst imaginable pain); (iii) patient satisfaction at follow-up visits V5 (week 12) and V6 (week 20) on a 5-point Likert scale (1 = very dissatisfied, 2 = dissatisfied, 3 = neutral, 4 = satisfied, 5 = very satisfied); and (iv) the incidence, nature, and severity of adverse events throughout the study period.

Statistical analysis

The pre-specified superiority hypothesis was that the hybrid mode would produce a greater reduction in hair thickness than the single mode at V6. Sample size was calculated to detect a clinically meaningful between-mode difference in hair-thickness change with 80% power at a two-sided α of 0.05, assuming a within-participant standard deviation of the difference of 0.015 mm based on published trichoscopy data; this yielded a minimum required sample of 36 evaluable participants, and 41 were enrolled to accommodate anticipated attrition.

The primary efficacy analysis was performed in the per-protocol population, defined as participants who completed all three treatment sessions and the V6 follow-up. Safety was evaluated in all treated participants (full analysis set; n = 41). Within-mode change from V2 to V6 was assessed using the paired t-test or Wilcoxon signed-rank test depending on the distributional assumptions verified by the Shapiro–Wilk test. The between-mode comparison of change scores was performed using the paired t-test applied to within-participant differences. Categorical variables were summarized as frequencies and percentages. Results are presented as mean ± standard deviation (SD) unless stated otherwise. A two-sided p value < 0.05 was considered statistically significant. Analyses were performed using SAS version 9.4 (SAS Institute Inc., Cary, NC, USA).

Results

Study population and participant flow

Forty-one participants were enrolled and received at least one treatment session; all 41 were included in the safety analysis. Five participants did not complete the per-protocol follow-up: four were lost to follow-up, and one discontinued after an adverse event. Thirty-six participants were therefore included in the per-protocol efficacy analysis. Baseline characteristics of the per-protocol population are summarized in Table 1.

Table 1.

Baseline characteristics of the per-protocol study population

Characteristic	Value
Participants in efficacy analysis, n	36
Age, years, mean ± SD	38.0 ± 10.5
Male sex, n (%)	29 (80.6)
Fitzpatrick skin phototype, n (%)
III	20 (55.6)
IV	16 (44.4)
Treatment site, n (%)
Leg	22 (61.1)
Face	7 (19.4)
Axilla	4 (11.1)
Arm	2 (5.6)
Back	1 (2.8)
Baseline hair density (V2), hairs/cm²
Single mode, mean ± SD	15.94 ± 11.74
Hybrid mode, mean ± SD	16.06 ± 11.95
Between-mode p value	0.761
Baseline hair thickness (V2), mm
Single mode, mean ± SD	0.0649 ± 0.0217
Hybrid mode, mean ± SD	0.0656 ± 0.0220
Between-mode p value	0.436

Values are mean ± SD or n (%). SD, standard deviation; V2, Visit 2 (baseline, first treatment session)

Mean age was 38.0 ± 10.5 years, and most participants were male (29/36, 80.6%). The most common treatment site was the leg (22/36, 61.1%), followed by the face (7/36, 19.4%), axilla (4/36, 11.1%), arm (2/36, 5.6%), and back (1/36, 2.8%). Twenty participants (55.6%) had Fitzpatrick skin type III and 16 (44.4%) had skin type IV. Baseline hair density and hair thickness were similar between the single-mode and hybrid-mode subsites (density: 15.94 ± 11.74 vs. 16.06 ± 11.95 hairs/cm², p = 0.761; thickness: 0.0649 ± 0.0217 vs. 0.0656 ± 0.0220 mm, p = 0.436), confirming adequate baseline balance.

Primary outcome: change in hair thickness

Hair thickness decreased significantly from baseline to 20 weeks in both modes (Table 2; both p < 0.001). With the single mode, mean hair thickness fell from 0.0649 ± 0.0217 mm at V2 to 0.0494 ± 0.0197 mm at V6 (mean reduction 0.0155 ± 0.0118 mm; 95% CI 0.0115 to 0.0195; 23.9% reduction). With the hybrid mode, mean hair thickness fell from 0.0656 ± 0.0220 mm at V2 to 0.0455 ± 0.0192 mm at V6 (mean reduction 0.0201 ± 0.0135 mm; 95% CI 0.0155 to 0.0247; 30.6% reduction).

Table 2.

Change in hair thickness from Visit 2 (baseline) to Visit 6 (20 weeks) by treatment mode

Variable	Single mode	Hybrid mode	Between-mode comparison
V2 (baseline), mm	0.0649 ± 0.0217	0.0656 ± 0.0220	—
V6 (20 weeks), mm	0.0494 ± 0.0197	0.0455 ± 0.0192	—
Change (V2 − V6), mm	0.0155 ± 0.0118	0.0201 ± 0.0135	0.00461 ± 0.00745
Reduction from baseline, %	23.9	30.6	—
95% CI for change	0.0115 to 0.0195	0.0155 to 0.0247	0.00209 to 0.00713
Within-mode p value†	< 0.001	< 0.001	—
Between-mode p value‡	—	—	0.0007

Values are mean ± SD. Change = V2 − V6; a larger positive value indicates greater reduction in hair thickness. †Paired t-test for within-mode comparison (V2 vs. V6). ‡Paired t-test on within-participant differences in change between hybrid and single modes. CI, confidence interval; SD, standard deviation; V2, Visit 2; V6, Visit 6

The between-mode comparison showed a significantly greater reduction with the hybrid mode (mean between-mode difference 0.00461 ± 0.00745 mm; 95% CI 0.00209 to 0.00713; p = 0.0007). This corresponded to an approximately 28% greater relative reduction in hair thickness with the hybrid mode compared with the single mode. The pre-specified superiority criterion was satisfied. Representative trichoscopy images are shown in Fig. 1, and clinical photographs in Fig. 2.

Fig. 1.

Representative trichoscopy images, Subject 26 (treatment site: leg). Panel A, hybrid-mode subsite at baseline (V2, pre-treatment); Panel B, single-mode subsite at baseline (V2, pre-treatment); Panel C, hybrid-mode subsite at the 20-week follow-up (V6); Panel D, single-mode subsite at the 20-week follow-up (V6). Images were acquired with the FotoFinder trichoscopy system at ×20 magnification. Reduction in shaft diameter and density is visible at both subsites at V6 compared with V2; greater follicular miniaturization is apparent at the hybrid-mode subsite (Panel C) than at the single-mode subsite (Panel D), consistent with the quantitative primary efficacy outcome

Fig. 2.

Representative clinical photographs, Subject 39 (treatment site: leg). Panel A, hybrid-mode (left leg) at baseline (V2, pre-treatment); Panel B, single-mode (right leg) at baseline (V2, pre-treatment); Panel C, hybrid-mode (left leg) at the 20-week follow-up (V6); Panel D, single-mode (right leg) at the 20-week follow-up (V6)

Secondary outcome: change in hair density

Hair density also decreased significantly from baseline to 20 weeks in both modes (Table 3; both p < 0.001). With the single mode, mean density fell from 15.94 ± 11.74 to 9.97 ± 10.34 hairs/cm² (mean change 5.97 ± 7.43; median 5.0 [IQR 2.75–8.25]). With the hybrid mode, mean density fell from 16.06 ± 11.95 to 9.39 ± 10.15 hairs/cm² (mean change 6.67 ± 7.69; median 6.0 [IQR 3.0–8.25]). Although the numerical reduction tended to be greater with the hybrid mode, the between-mode difference (0.69 ± 3.47 hairs/cm²; 95% CI − 0.48 to 1.87; p = 0.238) was not statistically significant.

Table 3.

Change in hair density from Visit 2 (baseline) to Visit 6 (20 weeks) by treatment mode

Variable	Single mode	Hybrid mode	Between-mode comparison
V2 (baseline), hairs/cm²	15.94 ± 11.74	16.06 ± 11.95	—
V6 (20 weeks), hairs/cm²	9.97 ± 10.34	9.39 ± 10.15	—
Change (V2 − V6), mean ± SD	5.97 ± 7.43	6.67 ± 7.69	0.69 ± 3.47
Median change [IQR]	5.0 [2.75, 8.25]	6.0 [3.0, 8.25]	—
95% CI for between-mode difference	—	—	−0.48 to 1.87
Within-mode p value†	< 0.001	< 0.001	—
Between-mode p value‡	—	—	0.238

Values are mean ± SD unless stated otherwise. Change = V2 − V6; a larger positive value indicates greater reduction in density. †Wilcoxon signed-rank test (within-mode). ‡Paired t-test (between-mode comparison of change). CI, confidence interval; IQR, interquartile range; SD, standard deviation; V2, Visit 2; V6, Visit 6

Pain and patient satisfaction

Pain scores at each treatment visit are shown in Table 4. At V2 (first session), pain scores were similar between the single and hybrid modes (3.92 ± 2.03 vs. 4.00 ± 1.82; p = 0.613). At V3 (second session), pain was significantly higher with the hybrid mode than with the single mode (3.61 ± 1.76 vs. 3.06 ± 1.80; p = 0.024). At V4, a borderline non-significant trend favoured the single mode (3.28 ± 1.99 vs. 2.78 ± 1.91; p = 0.053). The mean pain score across all three sessions was modestly higher with the hybrid mode (3.63 ± 1.44 vs. 3.25 ± 1.56; p = 0.076).

Table 4.

Pain scores (NRS 0–10) by treatment mode at each treatment visit

Visit (Treatment session)	Single mode (mean ± SD)	Hybrid mode (mean ± SD)	p value†
V2 (Session 1)	3.92 ± 2.03	4.00 ± 1.82	0.613
V3 (Session 2)	3.06 ± 1.80	3.61 ± 1.76	0.024
V4 (Session 3)	2.78 ± 1.91	3.28 ± 1.99	0.053
Mean of V2–V4‡	3.25 ± 1.56	3.63 ± 1.44	0.076

Values are mean ± SD. Pain was assessed using the numerical rating scale (NRS; 0 = no pain, 10 = worst imaginable pain) immediately after each treatment session. †Wilcoxon signed-rank test (between-mode). ‡Paired t-test (between-mode comparison of mean across V2–V4). NRS, numerical rating scale; SD, standard deviation; V2–V4, treatment visits 2 to 4

Patient satisfaction scores are shown in Table 5. Satisfaction did not differ between modes at V5 (week 12; 3.69 ± 0.71 vs. 3.64 ± 0.72; p = 0.480), at V6 (week 20; 3.17 ± 0.94 vs. 3.14 ± 0.93; p = 0.782), or in the V5–V6 mean (3.43 ± 0.70 vs. 3.39 ± 0.64; p = 0.580).

Table 5.

Patient satisfaction scores by treatment mode at each follow-up visit

Visit	Statistic	Single mode	Hybrid mode	p value†
V5 (Week 12)	Mean ± SD	3.69 ± 0.71	3.64 ± 0.72	0.480
	Median [IQR]	4 [4, 4]	4 [3.75, 4]
V6 (Week 20)	Mean ± SD	3.17 ± 0.94	3.14 ± 0.93	0.782
	Median [IQR]	3 [3, 4]	3 [3, 4]
Mean of V5–V6	Mean ± SD	3.43 ± 0.70	3.39 ± 0.64	0.580
	Median [IQR]	3.5 [3.0, 4.0]	3.5 [3.0, 4.0]

Values are mean ± SD or median [IQR]. Satisfaction was assessed on a 5-point Likert scale (1 = very dissatisfied, 5 = very satisfied). †Wilcoxon signed-rank test. IQR, interquartile range; SD, standard deviation; V5, Visit 5 (week 12); V6, Visit 6 (week 20)

Adverse events and safety

The overall safety profile of both modes was favourable (Table 6). Two device-related adverse events occurred during the study, both at single-mode treatment sites (4.9%); no events were observed at hybrid-mode sites (0.0%). One participant developed crust formation that resolved spontaneously without further treatment. A second participant developed crust formation requiring escharectomy and subsequently withdrew from the study—the single dropout attributable to an adverse event. No serious adverse events were reported in either group (Table 6 and 7).

Table 6.

Adverse events by treatment mode (safety population, n = 41 per mode)

Category	Single mode (n = 41), n (%)	Hybrid mode (n = 41), n (%)
Participants with any AE	2 (4.9)	0 (0.0)
Device-related AEs	2 (4.9)	0 (0.0)
Crust formation (resolved spontaneously)	1 (2.4)	0 (0.0)
Crust formation (requiring escharectomy)	1 (2.4)	0 (0.0)
Serious AEs	0 (0.0)	0 (0.0)
AEs leading to discontinuation	1 (2.4)	0 (0.0)
AEs requiring intervention	1 (2.4)	0 (0.0)

Values are n (%). AE, adverse event

Discussion

In this prospective, single-blind, split-body controlled trial with alternating allocation, the hybrid 755-nm/1064-nm dual-wavelength laser mode was superior to the single 755-nm mode for the reduction of hair thickness, with a statistically significant between-mode difference (p = 0.0007). Both modes produced highly significant within-mode reductions, but the magnitude was approximately 28% greater with the hybrid mode (mean reduction 0.0201 vs. 0.0155 mm). The absolute between-mode difference (0.00461 mm) was reproducibly detected by calibrated trichoscopy and met the pre-specified superiority criterion. Whether this magnitude translates into a perceptible benefit at the individual patient level requires formal validation against an anchor-based minimal clinically important difference (MCID) for trichoscopic hair thickness, which has not yet been established and represents an important direction for future work. Taken together, the findings identify the hybrid mode as achieving statistically greater follicular miniaturization than the single mode under the conditions studied, without compromising safety or satisfaction.

The greater reduction in hair thickness achieved by the hybrid mode is mechanistically consistent with the complementary photothermal properties of its two component wavelengths. The 755-nm alexandrite laser exhibits high melanin absorption and efficiently heats the melanin-rich hair matrix and shaft at the level of the isthmus and the suprabulbar region. The 1064-nm Nd: YAG laser, in contrast, has a melanin absorption coefficient roughly an order of magnitude lower, but its longer wavelength penetrates more deeply into the dermis and reaches the outer root sheath, the bulge stem-cell niche, the dermal papilla, and the perifollicular vascular plexus—structures that are central to follicular cycling and regrowth. When delivered synchronously, the resulting photothermal injury spans the full axial depth of the follicular unit, from the pigmented superficial structures to the deeply seated germinative compartment, producing broader and more comprehensive damage than either wavelength alone [6, 7, 9]. This extended thermal footprint is likely to be particularly advantageous for coarse terminal hair with large, deeply situated bulbs, which are inherently resistant to superficial-wavelength treatment.

A useful additional consideration is that simultaneous delivery of two wavelengths permits each component to be used at a lower fluence than would be required for an equivalent effect with either wavelength alone. In the present study, the hybrid mode used 10 J/cm² at 755 nm (versus 14 J/cm² in the single mode) and 8 J/cm² at 1064 nm. The reduced 755-nm fluence lowers peak epidermal heating, while the added 1064-nm component compensates for any loss of follicular damage at the shorter wavelength by extending the heated volume into deeper layers. This redistribution of thermal load across two penetration depths is a plausible biophysical explanation both for the greater overall hair thickness reduction and for the absence of crust formation observed at hybrid-mode sites [9, 14].

The lack of a statistically significant between-mode difference in hair-density reduction (p = 0.238), despite a numerical trend favouring the hybrid mode (6.67 vs. 5.97 hairs/cm²), warrants comment. Hair density and hair thickness measure different biological endpoints: density reflects the number of follicles still producing terminal hair, whereas thickness reflects shaft diameter from individual follicles—a sensitive measure of subclinical follicular damage that impairs metabolic activity without ablating it. It is therefore biologically plausible that the broader photothermal coverage of the hybrid mode produces preferential follicular miniaturization (manifested as thickness reduction) before achieving complete follicular ablation (manifested as density reduction), particularly after only three sessions. This sequence is consistent with the established biology of laser-induced follicular injury, in which sub-lethal damage to the germinative compartment first leads to progressive miniaturization and only later to terminal loss [15]. Studies with more sessions and longer follow-up will be needed to determine whether the density advantage of the hybrid mode reaches statistical significance with cumulative therapy.

The modest increase in pain scores with the hybrid mode at V3 (mean NRS difference 0.55 points) was statistically significant but well below the commonly accepted minimal clinically important difference of 1.0–2.0 NRS points [16]. This marginal increase in intraoperative discomfort did not translate into any difference in patient satisfaction at any follow-up visit, nor did it lead any participant to discontinue the hybrid arm because of pain. The transient, session-limited nature of the pain elevation (significant only at V3, with a non-significant trend at V4) and its lack of impact on the overall treatment experience suggest that any extra discomfort from the hybrid mode is clinically acceptable in light of the greater efficacy. The slightly higher pain at the second session may reflect a combination of mildly increased dermal heating from the 1064-nm component—because water in the dermis has a slightly higher absorption coefficient at 1064 nm than at 755 nm—and a transient increase in nociceptor sensitization in early treatment cycles, which typically attenuates with subsequent sessions as follicle density decreases.

The safety profile of both modes was favourable across the study period. No device-related adverse events occurred at hybrid-mode sites (0/41), compared with two events of crust formation at single-mode sites (2/41; 4.9%). The present sample is not adequately powered for a definitive between-mode safety comparison: the study was powered for efficacy, not safety, and the observed difference (two events versus none) does not reach statistical significance by Fisher’s exact test (p = 0.49). These observations should therefore be regarded as hypothesis-generating. Even so, the absence of adverse events with the hybrid mode is notable, because it counters the intuitive assumption that combining two wavelengths into one pulse train must increase the risk of cutaneous injury. The most likely explanation is the redistribution of energy described above: by using a lower 755-nm fluence (10 vs. 14 J/cm²) supplemented by 1064-nm energy that bypasses epidermal melanin, peak thermal accumulation at any single tissue layer is reduced, lowering the risk of superficial thermal injury such as crust formation [4, 9]. Adequately powered prospective trials with safety as a co-primary endpoint will be needed to formally characterize the between-mode adverse event profile.

The split-body controlled design is a methodological strength of this study. By using each participant as their own control, we reduced inter-individual variability in skin phototype, hair characteristics, anatomical site, and systemic factors. Single blinding of participants minimised response bias in patient-reported outcomes. To our knowledge, this is among the few split-body clinical studies evaluating synchronized, single-device hybrid 755-nm/1064-nm laser delivery against single-wavelength 755-nm treatment, with trichoscopic hair thickness as the primary endpoint.

Our findings extend earlier literature on combined 755-nm and 1064-nm therapy for hair removal. Previous comparative work has examined Nd: YAG and alexandrite lasers either as separate devices or in sequential combination and has suggested that dual-wavelength approaches may benefit patients with coarse hair or deeply situated follicles, particularly at sites such as the legs and the bikini area [9, 17, 18]. Most of those studies, however, used separate device platforms in separate sessions, which introduces inter-device and inter-session variability and limits direct attribution of efficacy to wavelength combination per se. Using a single integrated platform that delivers both wavelengths simultaneously, the present trial reduces the inter-device and inter-session variability inherent in earlier designs. It should be emphasised, however, that the two modes differed not only in wavelength composition but also in per-wavelength fluence and in total radiant exposure (single mode, 755 nm at 14 J/cm²; hybrid mode, 755 nm at 10 J/cm² plus 1064 nm at 8 J/cm²). The observed between-mode difference therefore reflects the combined effect of the hybrid mode as configured—the addition of the 1064-nm wavelength together with the accompanying redistribution of fluence—rather than the isolated contribution of the 1064-nm wavelength per se.

Several limitations should be acknowledged. First, treatment-mode allocation was performed using a pre-specified alternating scheme based on enrollment sequence rather than computer-generated random allocation; therefore, allocation concealment was not possible. Although enrollment order was independent of clinical characteristics and the scheme produced balanced distribution of treatment modes across body sides, future trials should adopt concealed computer-generated randomisation to fully meet contemporary standards. Second, the primary efficacy analysis was conducted in the per-protocol population (n = 36); an intention-to-treat sensitivity analysis was not performed, and the influence of the five excluded participants on the primary estimate cannot be formally excluded. Third, secondary outcome comparisons across multiple visits were not adjusted for multiplicity, so the significant pain difference at V3 (p = 0.024) should be interpreted with appropriate caution. Fourth, although treatment parameters were standardised and are reported in Table 1, individual variation in real-time parameter selection (e.g., adjustments for patient comfort) cannot be entirely excluded; future studies should record and report any intra-session adjustments to ensure complete reproducibility. Fifth, an MCID for trichoscopic hair thickness has not been formally established, which limits the patient-centred interpretation of the absolute between-mode difference (0.00461 mm). Sixth, blinding of the treating physician was not feasible because the operational characteristics of the two modes are externally distinguishable—a recognised constraint of procedural split-body trials. Seventh, the 20-week follow-up may not capture late hair regrowth; longer follow-up will be needed to establish the durability of the observed superiority. Eighth, the predominance of male participants (80.6%) and the concentration of treatment at the leg (61.1%) may limit generalizability to female patients and to other anatomical sites. In addition, the two treatment modes differed in per-wavelength fluence and in total radiant exposure; consequently, the greater efficacy of the hybrid mode cannot be attributed solely to the addition of the 1064-nm wavelength, and the design does not isolate an independent wavelength effect. Relatedly, treatment parameters were standardised at conservative, protocol-defined values informed by the manufacturer’s guidance and prior literature rather than individually optimised for each participant’s hair or skin characteristics; although the split-body design ensured that both modes were applied under identical conditions within each participant, the findings may not generalise to fully optimised, individualised treatment protocols. Finally, blinded photographic grading was not incorporated as a formal quantitative outcome, and the sample size did not permit pre-specified subgroup analyses by skin phototype, hair colour, or anatomical site—each an important direction for future research.

In this prospective, single-blind, split-body controlled trial with alternating allocation, the hybrid 755-nm/1064-nm dual-wavelength laser mode was superior to the single 755-nm mode in reducing hair thickness after three treatment sessions, with an approximately 28% greater relative reduction that met the pre-specified superiority criterion. Both modalities were well tolerated; the hybrid mode showed a numerically more favourable safety profile, with no device-related adverse events at any hybrid-mode site, although the present study was not powered for a definitive safety comparison. Patient satisfaction was comparable between modes despite a small, clinically non-meaningful increase in intraoperative pain in the hybrid arm.

These results indicate that, under the standardized conditions studied, the synchronized hybrid 755-nm/1064-nm mode was an effective and well-tolerated option for reducing hair thickness, achieving greater follicular miniaturization than the single 755-nm mode. Because the two modes differed in fluence as well as in wavelength composition, this benefit should be interpreted as an effect of the hybrid mode as configured rather than of the 1064-nm wavelength in isolation. The hybrid mode may be a useful option—particularly for coarse or deeply situated terminal hair—although its relative advantage across the full range of Fitzpatrick skin phototypes and anatomical sites remains to be confirmed. Larger, longer-term, multi-centre randomised trials covering a wider range of Fitzpatrick skin types and anatomical sites will be needed to define optimal treatment algorithms.

Author contributions

Won-Serk Kim: Conceptualization, Methodology, Funding acquisition, Supervision, Writing—review and editing. Young-Jun Choi: Conceptualization, Investigation, Data curation, Writing—review and editing. Na Yeon Baek: Investigation, Data curation, Formal analysis, Writing—original draft. Ji Soo Park: Formal analysis. All authors critically reviewed the manuscript and approved the final version for submission.

Funding

This study was supported by an unrestricted research grant and equipment provided by Wontech Co., Ltd., Republic of Korea. The funder had no role in the study design, data collection, analysis, interpretation, or manuscript preparation.

Data availability

The de-identified individual participant data underlying this study are available from the corresponding author on reasonable request.

Declarations

Consent to publish

Written informed consent for publication of the clinical and trichoscopic images shown in Figs. 1 and 2 was obtained from the participants concerned.

Clinical trial number

not applicable.

Competing interests

The authors received equipment and an unrestricted research grant from Wontech Co., Ltd., Republic of Korea. The sponsor had no role in the design, conduct, analysis, interpretation, or reporting of the study, or in the decision to submit the manuscript for publication. The authors have no other competing interests to declare.