Terahertz time-domain spectroscopy (THz-TDS) methods have been utilized in previous studies in order to characterize
the optical properties of skin and its primary constituents (i.e., water, collagen, and keratin). However, similar
experiments have not yet been performed to investigate whether melanocytes and the melanin pigment that they
synthesize contribute to skin’s optical properties. In this study, we used THz-TDS methods operating in transmission
geometry to measure the optical properties of in vitro human skin equivalents with or without normal human
melanocytes. Skin equivalents were cultured for three weeks to promote gradual melanogenesis, and THz time domain
data were collected at various time intervals. Frequency-domain analysis techniques were performed to determine the
index of refraction (n) and absorption coefficient (μa) for each skin sample over the frequency range of 0.1-2.0 THz. We
found that for all samples as frequency increased, n decreased exponentially and the μa increased linearly. Additionally,
we observed that skin samples with higher levels of melanin exhibited greater n and μa values than the non-pigmented
samples. Our results indicate that melanocytes and the degree of melanin pigmentation contribute in an appreciable
manner to the skin’s optical properties. Future studies will be performed to examine whether these contributions are
observed in human skin in vivo.
Terahertz time-domain spectroscopy (THz-TDS) is an effective technique to probe the intermolecular and collective
vibrational modes of biological macromolecules at THz frequencies. To date, the vast majority of spectroscopic studies
have been performed on dehydrated biomolecular samples. Given the fact that all biochemical processes occur in
aqueous environments and water is required for proper protein folding and function, we hypothesize that valuable
information can be gained from spectroscopic studies performed on hydrated biomolecules in their native conformation.
In this study, we used a THz-TDS system that exploits photoconductive techniques for THz pulse generation and freespace
electro-optical sampling approaches for detection. We used the THz spectrometer to measure the time-dependent
electric field of THz waves upon interaction with water, phosphate buffered saline (PBS), and collagen gels. By
comparing these waveforms with references, we simultaneously determined each sample's index of refraction (n) and
absorption coefficients (μa) as a function of frequency. Our data show that the properties we measure for the water, PBS
and collagen are comparable to those reported in the literature. In the future, we plan to examine the effect that both
temperature and pH have on the optical properties of other biological macromolecules. Studies will also be performed to
compare our results to those generated using molecular dynamics simulations.
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