Influence of water and temperature on the electrical conductivity of the human nail
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The human toenail was measured in vitro in the alpha-dispersion region of the electric field and the temperature from 22 to 150 °C. The values of dielectric properties are much higher in the wet nails than in those without water of the same temperature and frequency. The peak temperature of dielectric parameters near 100 °C for wet nails is attributed to the water removal process. The dielectric spectra of the nail revealed high-frequency relaxation at 25 kHz irrespective of the water content in the tissue. Our dielectric studies of the nail plate enable deeper analysis of the matrix–keratin–water system, which can facilitate the assessment of the electrical conductivity of this tissue in the state of health and diseased.
KeywordsNail plate Keratin Water Alpha-dispersion Dielectric properties Proton jump
Nail is an interface between living organisms and the external environment and is therefore useful as a marker of physiological changes in the structure of this tissue [1, 2, 3]. The influence of such external factors as relative humidity, temperature and electromagnetic radiation on the nail causes a physical–chemical release of the matrix–keratin–water composite forming this tissue. The results of our previous dielectric studies of the nail [4, 5, 6, 7] indicate the key role of water in protecting the ordered crystalline structure of the keratin molecule before the onset of the melting process. In recent years, nails research also deals with the influence of water on the biophysical and chemical structure of this material [8, 9, 10, 11, 12].
The paper compares the thermal decomposition of the lower water content in the nail, with the decomposition of higher water content obtained in our previous studies of this material. However, our previous work on dielectric properties of nails was related to the frequency and temperature range of 100 Hz–100 kHz and 22–200 °C, respectively, which are wider than those used in this study. Earlier nail data obtained at 100 Hz were necessary for the analysis of low-frequency dispersion (LFD) and to prove that the electrode polarization does not affect the dielectric behavior of the nail below 2 kHz . In this study, the lower limit of the frequency range used was set at 500 Hz, because the main purpose of this article is to show a significant difference between the size of the relaxation time of protons and active polar spaces inside the keratin above 2 kHz. Thus, the current data expand the previous analysis of the mechanism of polarization and nail conduction, taking into account the complex conductivity as an additional dielectric parameter. In addition, compared to our previous articles, the temperature range is limited to 150 °C to release loosely bound water from wet samples, which thus become dry samples for the analysis of dielectric spectra at a physiological temperature range.
The nails of the middle fingers of the upper limbs, 3 mm wide, were obtained from the group of 15 healthy people (21–30 years). The procedure used to obtain the samples was as follows: The nails were immersed in a 0.9% NaCl solution to remove the fat, washed with distilled water, dried at room temperature (22–25 °C) with a relative humidity of about 60% and then cut into a rectangular sample about surface of 8 mm2 and a thickness of 0.3 mm.
Dielectric measurements of relative permittivity, dielectric loss and conductivity of the nail placed between the two silver paste electrodes were taken using the HIOKI 3522-50 LCR impedance analyzer in the frequency, f, range from 500 Hz to 100 kHz at temperatures, T, from 22 to 150 °C. The accuracy of the device provided by the manufacturer depends on the measuring range of the electrical impedance of the sample and the applied measuring frequency and signal level. The impedance measurement range is from 10.00 mΩ to 200.00 MΩ, and the signal level range is from 10 mV (rms) to 5 V (rms). In our experiments for 1 V (rms) nail samples in the 500 Hz–100 kHz range, the impedance values are below 200.00 MΩ, indicating that the instrument used in this test is sufficient. The method of estimating the impedance accuracy and the phase angle between the applied voltage and the current flowing through the nail on the basis of the two factors is given in the instruction table for HIOKI 3522-50 LCR HiTESTER. The accuracy of the impedance and phase angle is needed to calculate the accuracy of electrical resistance and capacitance in the parallel configuration used to determine the dielectric properties of the nail in our measurements. To obtain a high accuracy in the measurement of the electrical stability of the nail sample, the calibration procedure included in the instruction for HIOKI 3522-50 was used. Calibration of dielectric properties was obtained by measuring the properties of a Teflon sample with a known relative permittivity of 2.1, and the accuracy was estimated to be within 0.5%.
These experiments were carried out in the wet and dry states of the sample in the air atmosphere using the measuring chamber described in the previous article . The wet and dry conditions concerned the same sample. Therefore, dielectric measurements include a procedure for heating a wet sample in the range of 22–150 °C and a constant temperature of 150 °C (~ 1 h) and then after rapidly cooling to room temperature, the heating procedure of the dry sample without water as a function of temperature up to 150 °C. The temperature of the nail sample was measured using a constantan-copper thermocouple with an accuracy of ΔT ~ 0.2 K. In the same measuring chamber, we also measured the water content in wet samples without electrodes. After the water removal procedure, the average water content in 5 samples (n = 5) taken from various 15 people (n = 15) participating in the study was ~ 9%. The measurements of the dielectric parameters of nail were taken for each from the 15 persons. The results for all samples are presented in the figures as average values (n = 15) and standard deviations within 5%.
Results and discussion
The presence of water in wet nail samples compared to dry samples is represented by the increase in measured dielectric parameters for each temperature and frequency of the applied electric field for these materials. During heating, partial changes in the secondary structure of keratin macromolecules are preceded by the breakage of intra- and intramolecular hydrogen bonds. In the case of wet nail samples, dielectric behavior is associated with surface polar groups such as OH, CO and NH in the side chains of amino acids as dominant interactions between the keratin–water system. As a result of the temperature increase of these samples, loosely bound water is released from the nail plate. In contrast, dry nail results are attributed to the polar regions of the intermolecular structure of this material. On the basis of dielectric measurements of the nail plate in healthy people, we can identify the proton conduction processes that affect the physiological integrity of the nail with human tissues in the clinical state.
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