Time-domain semi-analytical modeling of photoacoustic wave generation in layered human skin with explicit basal-layer...

The basal layer (BL) of human skin is an ultrathin, melanin-rich absorber that plays a dominant role in near-surface optical absorption and photoacoustic (PA) wave generation. In many PA skin models, the epidermis is treated as a single effective layer to reduce anatomical complexity and computational cost; however, the impact of explicit BL resolution on time-domain PA waveforms remains unclear. This study aims at determining whether the BL must be modeled as an independent ultrathin absorber for quantitatively reliable time-domain PA analysis and reduced-layer representations of human skin. Because skin is a strongly scattering stratified medium, optical transport is diffusion-dominated. To address this question, a coupled optical-thermal-acoustic semi-analytical framework is applied to a physiologically representative nine-layer skin model and to reduced configurations with and without an explicitly resolved BL. Under excitation at a visible wavelength (λe=532 nm), increasing BL absorption raises the peak PA pressure by more than a factor of three, demonstrating the strong sensitivity of the PA amplitude to this localized source. Variations in BL thickness modify the amplitude and temporal-spectral characteristics of the PA waveform. Reduced models that preserve the BL reproduce the full-model response, whereas merging the BL into the epidermis leads to attenuation and temporal broadening due to source delocalization. These results show that preserving the spatial localization of the dominant absorber, rather than the anatomical layer count itself, is the governing physical criterion for model reduction and is essential for physically consistent time-domain PA analysis and inverse reconstruction in layered biological tissues.