Designs And Applications Of Bioresistance/Bioimpedance Sensors

Sunday, November 26th, 2017 - Chemical, Resistive Transducers

Designs And Applications Of Bioresistance/Bioimpedance Sensors

Several electrode configurations are available for clinical applications. Figure 1.a shows an Ag–AgCl disk electrode with a flexible lead wire attached to the back. It can be used as a direct-contact skin electrode for electrocardiogram recording.

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Figure 1. Typical clinical electrode designs: (a) disk electrode; (b) pellet electrode; (c) suction-cup electrode; (d) button electrode with gel-filled column; (e) needle electrode.

Figure 1.b is a pellet electrode that has a cylindrical AgCl pellet embedded on a wire. A 0.8–2.0 mm diameter Ag–AgCl matrix is at the tip. Pellet electrodes are suitable for probing tissues. Figure 1.c is a suction cup electrode (chest electrode). Figure 1.d shows a column electrode. In this configuration, an Ag–AgCl metal contact button is placed at the top of a conductive gel-filled column. The assembly is held in place by an adhesive-coated rubber ring. This design reduces movement artifacts since the electrode is lifted off the surface and it is often used for long-term recording and monitoring. Figure 1.e shows a typical needle electrode. A very fine platinum or tungsten wire is slip-fit through a 1.5–2.0 mm glass pipette. The tip is etched and then fire-formed into the shallow-angle taper.

Some applications of bioresistance/bioimpedance sensors are shown as follows.

Body Composition Monitor

Commercial body composition monitoring systems, such as BIA 310e, use measured body resistance to estimate the percentage of body fat, fat weight, lean weight, basal metabolic rate, and hydration status. In BIA 310e, sensor pads are placed on the subject’s right wrist and ankle. To measure the body’s resistance, the system emits a low-level high-frequency electrical current (<1 mA, 50 kHz) that flows through the body. Lean tissue is more conductive than fat tissue due to the lean tissue’s high water content. Thus, the resistance is an indication of how well the body conducts an electrical current. For individuals who have the same height and weight, the lower the resistance, the lower the percentage of body fat. A typical measurement report with automatically generated target recommendations (based on optimal body fat values stored in the analyzer’s memory) is shown in Table 1.

Test Results from a BIA 310e Body Composition Monitor

Table 1. Test Results from a BIA 310e Body Composition Monitor

Resistance Measurement at Acupuncture Points

The electrical characteristics (especially resistance) of the acupuncture meridians in Chinese medicine have been extensively studied since 1950 to establish their existence by scientific methods. It is now generally accepted that both meridian and acupuncture points have lower electrical resistance or impedance than nearby surrounding points. These low resistance points are thought to be the result of sensory and motor nerves emerging from deep tissue to superficial layers of the skin. The estimation of resistance at acupuncture points can be made through skin resistance measurement, which is a function of the cutaneous region, skin humidity, and pressure of the measurement electrode. Measurement methods include using an excitation current or measuring the current (0.01–5 μA DC current) produced by the electromotive force resulting from differences in these tiny points’ potentials using appropriate electrodes.

Ovulation Predictor

The CUE Fertility Monitor, developed by Zetek Inc. (Aurora, Colorado, USA), consists of a hand-held digital monitor with an oral and a vaginal sensor. The system detects and records the changes in electrical resistance and ionic concentration of saliva and vaginal secretions, in response to the cyclical changes in estrogen. Based on the measurement results, the CUE monitor is able to predict and confirm an ovulation. The peak electrical resistance in the saliva occurs 5~7 days before ovulation, and the lowest electrical resistance in cervical secretions occurs about a day before ovulation.

Venous Blood Volume Measurement

Medis Medizinische Messtechnik GmbH in Germany has used an impedance method to measure changes in venous blood volume as well as pulsation of the arteries. As blood volume changes, the electrical impedance also changes proportionally. This measurement requires passing a small magnitude of high-frequency AC current through the body using four electrodes. The two middle electrodes measure the voltage, and their positions define the measurement region. The outer two electrodes are used to emit the small AC current mentioned earlier. The locations of these outer electrodes are not critical. This method allows doctors to detect blood flow disorders, early stage arterioscleroses, functional blood flow disturbances, deep venous thromboses, migraines, and general arterial blood flow disturbances. This bioimpedance sensing method is safe, noninvasive, inexpensive, and easy to operate.

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