Impact of Cell Density and Collagen Concentration on the Electromagnetic Properties of Dermal Equivalents in the Terahertz Band

In this paper, the measurement of collagen gels and dermal equivalents (DEs) with three kinds of fibroblast cell densities and collagen concentrations is performed, respectively, using terahertz time-domain spectroscopy (THz-TDS) in transmission mode. With the objective to reduce the thickness uncertainty, an algorithm to extract material parameters and sample thickness of thin samples simultaneously is employed. The statistical mean and standard deviation of the refractive indexes and absorption coefficients in a THz band from 0.2 to 1.5 THz are presented. It is shown that the refractive indexes of DEs drop by around 10%, whereas the absorption coefficients decrease by 20% when cell density increases by one order from 0.1 to 1 
$ \text{M/mL}$
. In addition, the complex refractive indexes of DEs with 3-mg/mL collagen are higher than those with 2 
$\text{mg/mL}$
, whereas the THz optical parameters of DEs with 2.5 mg/mL are the lowest. The obtained results indicate that the difference in the intrinsic biological features, initial cell density, and collagen concentration changes the tissue hydration, which in turn brings changes to the electromagnetic properties.