Stimulated emission light source technology for leather unhairing: Proof of concept using diode laser systems and process parameter optimisation

Selective photothermolysis of melanin-rich chromophores is well established in clinical laser hair removal; however, its application to dense keratinised substrates of industrial thickness—such as fresh bovine hide—remains insufficiently characterised in a systematic manner. This study presents an experimental investigation of near-infrared diode laser irradiation for photothermal follicular disruption in bovine hide, a substrate that differs fundamentally from human skin in dermal thickness (4–8 mm vs. 1–2 mm), follicular implantation depth (3–6 mm), melanin distribution heterogeneity, and the overriding engineering constraint of collagen structural preservation. Two laser platforms were evaluated: a triple-wavelength diode system (755/808/1064 nm, RS P-13) and an industrial Nd:YAG scanning laser (e-QUBE, ON-LASER). Nineteen parameter-screening trials systematically varied fluence (5–45 J/cm 2 ), pulse repetition rate (1–9 Hz), working distance (contact to 10 cm), substrate hydration state, hair pigmentation, and hair length to map the operative laser–tissue interaction space. A well-defined photothermal window—30–45 J/cm 2 , 5 Hz, 5–7 cm working distance on hydrated substrate—produced consistent thermally-mediated follicular weakening sufficient for mechanical extraction of intact hair units including follicle bulbs. Hair pigmentation governed the energy threshold: eumelanin-rich (black/brown) hair responded at 30–35 J/cm 2 , whereas low-melanin (blond) hair required 40–45 J/cm 2 and up to 20 irradiation passes. Heat-affected zone analysis confirms that the estimated radial thermal diffusion radius (≈0.3 mm) remains smaller than the inter-follicular spacing under optimised parameters, preserving the collagen fibre network of the corium. Fluences exceeding 45 J/cm 2 or contact-distance irradiation caused macroscopic collagen contraction consistent with triple-helix denaturation above the shrinkage temperature. The operative parameter space, chromatic selectivity constraints, and heat-affected zone boundaries identified here define the laser engineering specifications required for scalable industrial implementation in zero-effluent hide processing.