Quantification of dermis sodium concentration in skin layers by power spectral density drop of square-wave electrical impedance spectroscopy (PSDd-sEIS)

Assessing dermis sodium concentration in skin is essential since an excessive amount of sodium is associated with vascular function disorders. In this research, dermis sodium concentration c has been quantified by the power spectral density drop Δ P of square-wave electrical impedance spectroscopy (PSD d-s EIS). PSD d-s EIS is composed of three stages: (1) dermis voltage measurement, (2) PSD d calculation, and (3) dermis sodium quantification. In the 1st stage, three electrodes are arranged to measure voltage v under the constant current I of square-wave. In the 2nd stage, Δ P is evaluated by the first and third harmonic frequency from v . In the 3rd stage, c is quantified by the developed linear regression. To obtain the highest sensitivity between c and Δ P , four measurement factors (square-wave frequency f sw , duty-cycle γ , electrode distance d, and electrode diameter ø ) were optimized using numerical simulations of a skin model. Experiments on multi-layered skin under various c in the range of 5 nM–50 mM were conducted by PSD d-s EIS hardware with optimized measurement factors of f sw ∗ = 100 kHz , γ ∗ = 50 % , d ∗ = 2 mm , and ϕ ∗ = 0.8 mm . As the results, the quantification of c has a good accuracy with the normalized sensitivity ⟨ S ⟩ = 0.87 and determination coefficient R 2 = 0.92. The measurement factors of f sw and γ affect the power of square-wave excitation to pass through the high resistivity barrier of the stratum corneum, while the measurement factors of d and ø affect the stability of current flow in the dermis.