  Traveling-wave optical coherence elastography (OCE) is a promising technique
to measure the stiffness of biological tissues. While OCE has been applied to
relatively homogeneous samples, tissues with significantly varying elasticity
through depth pose a challenge, requiring depth-resolved measurement with
sufficient resolution and accuracy. Here, we develop a broadband Rayleigh-wave
OCE technique capable of measuring the elastic moduli of the 3 major skin
layers (epidermis, dermis, and hypodermis) reliably by analyzing the dispersion
of leaky Rayleigh surface waves over a wide frequency range of 0.1-10 kHz. We
show that a previously unexplored, high frequency range of 4-10 kHz is critical
to resolve the thin epidermis, while a low frequency range of 0.2-1 kHz is
adequate to probe the dermis and deeper hypodermis. We develop a dual
bilayer-based inverse model to determine the elastic moduli in all 3 layers and
verify its high accuracy with finite element analysis and skin-mimicking
phantoms. Finally, the technique is applied to measure the forearm skin of
healthy volunteers. The Young's modulus of the epidermis (including the stratum
corneum) is measured to be ~ 4 MPa at 4-10 kHz, whereas Young's moduli of the
dermis and hypodermis are about 40 and 15 kPa, respectively, at 0.2-1 kHz.
Besides dermatologic applications, this method may be useful for the mechanical
analysis of various other layered tissues with sub-mm depth resolution.
