Evaluation of penetration speed of liquids into skin using a terahertz time-of-flight method

Analyzing the penetration of cosmetic/medical lotions and creams into the skin is important for evaluating their effectiveness. Hence, the penetration speed of liquids into chicken and pig skins was evaluated using a terahertz (THz) time-of-flight method. An arsenide-free disposable THz emitter comprising a silicon film on a sapphire substrate was developed for practical measurements using skin samples. The skin sample was mounted on the THz emitter surface directly so that the generated THz pulses in the silicon film were easily focused on the skin sample. To evaluate the time evolution of the penetration depth, the arrival times of the reflected THz pulses at the edge of the liquid-penetrated region on the skin were observed. As a preliminary experiment, measurements using a stack of conventional glass slides were conducted. In addition, the penetration speed into the chicken and pig skin was compared to prove the practical applicability of our method.