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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 (PSDd-sEIS). PSDd-sEIS is composed of three stages: (1) dermis voltage measurement, (2) PSDd 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 PSDd-sEIS 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 R2 = 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.

Gastric Function has been successfully estimated by gastric electrical impedance tomography (gEIT) Suit with dual-step fuzzy clustering. The gEIT Suit which are made of elastic cloth with dual-planar electrodes and compact data acquisition (DAQ) system measures gastric impedance Z to visualize the gastric conductivity distribution σ. The dual-step fuzzy clustering extracts the clustered gastric conductivity distribution kσ, which accurately estimates the gastric function. The gEIT Suit with dual-step fuzzy clustering are applied to eight healthy persons during liquid meal consumption to estimate the gastric function under gastric accommodation phase of 200, 400 and 600 mL based on the gastric emptying phase. As the results, the gEIT Suit successfully estimate the gastric function. By the measured impedance Z, the subjects have a mean temporal impedance $$\overline{\Delta \mathbf{Z} }$$ Δ Z ¯ = − 9.27 [Ohm] and p-value of that $$\overline{\mathbf{Z} }$$ Z ¯ p(Z) = 0.0013[–]as the t-test result. In the case of gastric conductivity distribution σ, the subjects have a value of spatial mean conductivity distribution ⟨σ⟩ = 0.23[–] and p-value of that ⟨σ⟩ p(σ) = 0.0140[–]. Lastly, in the case gastric volume V, subjects have a gastric volume V = 12.44 [%] and p-value p(V) = 0.0664[–].

In this study, sodium concentration in the dermis layer is imaged by the square wave open electrical impedance tomography (SW-oEIT) with spatial voltage thresholding (SVT). The SW-oEIT with SVT consists of three steps which are 1) voltage measurement, 2) spatial voltage thresholding, and 3) sodium concentration imaging. In the 1st step, the root mean square voltage v ̃ is calculated based on the measured voltage v under the square wave current I through the planar electrodes on the skin domain Ω. In the 2nd step, the m-th measured voltage v is converted to a compensated voltage v^* based on the voltage electrodes distance d^v and threshold distance d^Γ in order to highlight the region of interest of the dermis layer Ω^d. In the 3rd step, sodium concentration is imaged by the Gauss-Newton reconstruction method. The SW-oEIT with SVT was applied to multi-layer skin simulation and ex-vivo experiments under various dermis sodium concentrations c in the range of 5-50 mM. As an image evaluation result, the spatial mean conductivity distribution 〈σ^* 〉 in Ω^d is successfully determined as increasing c on both simulations and experiments. The relationship between 〈σ^* 〉 and c was evaluated by the determination coefficient R2 and the normalized sensitivity 〈S〉. The optimized d^Γ with the highest evaluation values of R^2= 0.84 and 〈S〉= 0.83 is under the condition of d^Γ = 2 mm. Based on the signal evaluation, the SW-oEIT with SVT has a 15.32 % higher correlation coefficient CC compared to the conventional oEIT based on sinewave injection.

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