intracellular and extracellular water. By the 1990s, the market included several multi-frequency analyzers and a couple of BIS devices. The use of BIA as a bedside method has increased because the equipment is portable and safe, the procedure is simple and noninvasive, and the results are reproducible and rapidly obtained. More recently, segmental BIA has been developed to overcome inconsistencies between resistance (R) and the body mass of the trunk. In 1996, an eight-polar stand-on BIA device, InBody, that did not utilize empirical equations was created and was found to "offer accurate estimates of TBW and ECW in women without the need of population-specific formulas." In 2018, AURA Devices brought the fitness tracker AURA Band with built-in BIA. In 2020 BIA became available for Apple Watch users with the accessory AURA Strap with built-in sensors. By the early 2020’s smartwatches like the Samsung Galaxy Watch 4 contained built in BIA’s. Measurement configuration The impedance of cellular tissue can be modeled as a resistor (representing the extracellular path) in parallel with a resistor and capacitor in series (representing the intracellular path, the resistance that of intracellular fluid and the capacitor the cell membrane). This results in a change in impedance versus the frequency used in the measurement. Whole body impedance measurement is generally measured from the wrist to the ipsilateral ankle and uses either two (rarely) or four (overwhelmingly) electrodes. In the 2-elctrode (bipolar) configuration a small current on the order of 1-10 μA is passed between two electrodes, and the voltage is measured between the same whereas in the tetrapolar arrangement resistance is measured between as separate pair of proximally located electrodes. The tetrapolar arrangement is preferred since measurement is not confounded by the impedance of the skin-electrode interface Phase angle In bioelectrical impedance analysis in humans, an estimate of the phase angle can be obtained and is based on changes in resistance and reactance as alternating current passes through tissues, which causes a phase shift. A phase angle therefore exists for all frequencies of measurement although conventionally in BIA it is phase angle at a measurement frequency of 50 kHz that is considered. The measured phase angle therefore depends on several biological factors. Phase angle is greater in men than women, and decreases with increasing age. See also Body fat percentage Impedance spectroscopy Electrical impedance tomography References Further reading External links The International Society for Electrical Bioimpedance Bioimpedance Analysis