Ørjan G. Martinsen

Sources of error in tetrapolar impedance measurements on biomaterials and other ionic conductors

Tetrapolar electrode systems are commonly used for impedance measurements on biomaterials and other ionic conductors. They are generally believed to be immune to the influence from electrode polarization impedance and little can be found in the literature about possible pitfalls or sources of error when using tetrapolar electrode systems. In this paper we show that electrode polarization impedance can indeed influence the measurements and that also other phenomena such as negative sensitivity regions, separate current paths and common-mode signals may seriously spoil the measured data.

Cole electrical impedance Model-a critique and an alternative

The Cole single-dispersion impedance model is based upon a constant phase element (CPE), a conductance parameter as a dependent parameter and a characteristic time constant as an independent parameter. Usually however, the time constant of tissue or cell suspensions is conductance dependent, and so the Cole model is incompatible with general relaxation theory and not a model of first choice. An alternative model with conductance as a free parameter influencing the characteristic time constant of the biomaterial has been analyzed. With this free-conductance model it is possible to separately follow CPE and conductive processes, and the nominal time constant no longer corresponds to the apex of the circular arc in the complex plane.

The development of a software program for analyzing spontaneous and externally elicited skin conductance changes in infants and adults

OBJECTIVES Changes in palmar and plantar skin conductance (SC) are due to outgoing bursts in the postganglionic efferent sympathetic cholinergic fibres, which responds to changes in central arousal state. The purpose of this study was designed to develop a software program for analyzing spontaneous and external elicited SC changes for infants and adults. METHODS The program was designed to calculate the number and mean amplitude of the waves as well as the mean basal level in a given period. Different pre-set values for the minimum amplitude, maximum slope and minimum width of the spontaneous waves were used in the analysis program, and the results were compared with manually counted waves. The program was also used to perform coherent averaging of repeated elicited SC changes. For the mean elicited skin conductance responses, the latency time, response amplitude and recovery time were calculated. The habituation pattern could be calculated semi-automatically by analyzing each response. RESULTS For SC waves, the minimum amplitude and the maximum slope should be, respectively, 0.02 microsiemens (microS) and 2 microS/s for infants and adults, and the width of the waves should be at least 1 s for adults, and unlimited for infants. The coherent average method was found to be a satisfactory method for revealing whether a subject responded to stimuli, and is recommended, especially for infants. CONCLUSIONS Spontaneous and stimulated skin conductance are easily analyzed by this software program.

Electrical methods for skin moisture assessment

Skin sites on 8 test subjects were treated with moisturizers, and different electrical measuring methods were compared regarding their quality in the assessment of the induced changes in the stratum corneum hydration level. Low frequency susceptance measurements were found preferable to high frequency admittance measurements, and the advantages of monopolar measurements with the three-electrode system are described.

Impedance-based tissue discrimination for needle guidance

Measurement of electrical impedance can discriminate between tissues of different electrical properties. A measurement system with adequate spatial resolution focused on a volume around the tip of a needle or other invasive clinical equipment can be used to determine in which type of tissue the tip is positioned. We have measured the sensitivity zone of a needle electrode with an active electrode area of 0.3 mm(2), and measured impedance spectra in porcine tissue in vivo. Small electrode impedance data will be influenced by electrode polarization impedance (EPI) at low frequencies. To refine existing methods for needle guidance with higher spatial resolution, we have used multivariate analysis and new interpretations of EPI, and tissue data gathered with selected needle electrodes. The focus of this study is on discrimination between muscle and fat/subdermis for drug administration, but our results also indicate that these refinements will facilitate new clinical applications for impedance-based needle guidance in general.

A study on electrode gels for skin conductance measurements

Low-frequency skin conductance is used within several clinical applications and is mainly sensitive to sweating and the moisture content of the stratum corneum, but also how electrodes introduce changes in the electrical properties. Four electrode gels were investigated with regard to sorption characteristics and electrical properties. Skin conductance time series were collected from 18 test subjects during relaxation, exercise and recovery, wearing different pairs of electrodes contralaterally on the hypothenar and the T9 dermatome. Pressure test was applied on the T9 electrodes. Impedance frequency sweeps were taken on the T9 electrodes the same day and the next, parameterized to the Cole model. ANOVA on the initial skin conductance level change, exercise response amplitude, recovery offset and pressure-induced changes revealed significant differences among gel types. The wetter gels caused a higher positive level change, a greater response amplitude, larger recovery offset and greater pressure-induced artifacts compared to the solid gels. Sweating on the T9 site led to negative skin conductance responses for the wetter gels. Correlations were found between the desorption measurements and the initial skin conductance level change (hypothenar: R = 0.988 T9: R = 0.901) RM-ANOVA on the Cole parameters revealed a significant decrease in R(s) of the most resistive gel. Clinical implications are discussed.

Facts and Myths about Electrical Measurement of Stratum corneum Hydration State

Some of the views presented in the chapter on ‘Examination of stratum corneum hydration state by electrical methods’ in Skin Bioengineering – Techniques and Applications in Dermatology and Cosmetology (Karger, 1998) are in strong disagreement with the results from basic research that has been conducted on skin impedance measurement over the last decades. This research has e.g. non-ambiguously shown that the frequency response of the stratum corneum does not obey the Cole equation and that measurement depth is strongly dependent on measurement frequency. One consequence of these findings is that multifrequency electrical measurements on stratum corneum are impossible to achieve in vivo with any electrode system known today. Hence, electrical measurements of stratum corneum hydration must be conducted at one single, low frequency.

Electrical measurement of sweat activity

A multichannel logger for long-term measurements of sweat activity is presented. The logger uses skin surface electrodes for unipolar admittance measurements in the stratum corneum. The logger is developed with emphasis on clinical use. The portability of the logger enables recording of sweat activity under circumstances such as daily errands, exercise and sleep. Measurements have been done on 24 healthy volunteers during relaxation and exercise with heart rate monitoring. Recordings of sweat activity during sleep have been done on two healthy subjects. Early results show good agreement with the literature on sweating physiology and electrodermal activity. Results are presented showing measurements related to physical exercise, dermatomes, distribution of sweat glands and sympathetic activity. This study examines the normal sweating patterns for the healthy population, and we present results with the first 24 healthy volunteers. Comparing these results with similar measurements on hyperhidrosis patients will make it possible to find the most useful parameters for diagnosis and treatment evaluation.

Non-invasive measurements of post-mortem changes in dielectric properties of haddock muscle - a pilot study

Significant changes can be measured in the electrical properties of organs and muscle during ischemia. This will undoubtedly form the basis of new diagnostic tools and tools for assessing food quality in the future. In this paper, we present measurements of the electrical properties of haddock muscle from 1 Hz to 100 kHz as a function of time after the fish was sacrificed. Clear alpha and beta dispersions were found. Most of the alpha dispersion disappeared after a few hours. The low frequency resistance of the beta dispersion increased during the first 5 h as the fish went into rigor, and then decreased as cell destruction developed.

Skin conductance changes during the first year of life in full-term infants

Skin conductance changes (SCC) reflect the activity in the sympathetic postganglionic cholinergic fibers, which innervate the sweat glands located in the palm of the hand and the sole of the foot. The purpose of this study was to measure the changes in the number of waves per second, the wave amplitude, and the mean skin conductance level during the 1st year of life. During SCC elicited by an auditory stimulus we measured the percentage of infants that responded, the amplitude, latency and recovery times, and any habituation pattern. Thirty-nine full-born, healthy infants were investigated during their 1st and 3rd days of life, at 3 and 10 wk of life, and at 6 and 12 mo of life. The mean skin conductance level (p < 0.001), the number of waves with an amplitude threshold higher than 0.5 and 1.0 μsiemens (p < 0.001), and the amplitude of the waves (p < 0.001) all increased during the first 10 wk of life. The percentage that responded to stimuli increased from 8% to 50%, and the amplitude of the response increased during the first 10 wk of life (p < 0.001). The level of arousal influenced the mean skin conductance level, the number of waves per second, and the amplitude of the waves during the 1st year of life. In conclusion, these results indicate that the part of the sympathetic nervous system associated with arousal develops during the first 10 wk of life.