Biosensors convert the analyte–receptor reactions into a quantitative electrical, electrochemical (amperometric, potentiometric, conductimetric), piezoelectric, calorimetric, acoustic, mechanical, magnetic, or optical signals (fluorescence absorbance).
Types of Bioresistance/Bioimpedance Sensors
All living cells, tissues, or organs are conductive materials. For example, blood has approximately 0.67 S ⋅ m-1 conductivity (or 1.50 Ω ⋅ m resistivity), urine 3.33 S ⋅ m-1 (or 0.30 Ω ⋅ m resistivity), and fatty tissue 0.04 S ⋅ m-1 (or 25 Ω ⋅ m resistivity). Living organisms are also composed of positive and negative ions in various quantities and concentrations. Bioresistance and bioimpedance sensors extract physiological and pathological information of living organisms through measuring a sample’s conductivity, ion migration, electron increase/decrease, resistance change, electrical potential difference, and impedance variation. Thus, bioresistive sensors can be classified into five categories:
- Sensing based on the migration of ions through a region that causes an electrical potential difference or current flow between two points.
- Sensing based on electrochemical reactions that generate ions or electrons, which alters the overall conductivity of a living sample.
- Sensing based on employing other resistive sensors (e.g., potentiometers, piezoresistive sensors, or chemoresistive sensors) to perform a biosensing task. For example, in an enzyme–substrate reaction (a biometabolic process), instead of measuring how many ions and electrons are produced, the released products, such as O2, pH, CO2 , or NH3, are measured using chemoresistive gas sensors.
- Sensing based on bioelectrodes that convert ionic conduction into electronic conduction (e.g., Ag/AgCl electrodes) so that a signal can be processed in an electronic circuit.
- Sensing based on measuring bioimpedance instead of resistance only.
where f is the frequency (in Hz) of the measuring signal. Indeed, many of the impedance measurements conducted in physiology laboratories are actually AC (alternating current) resistance measurements. This is because a direct conductance measurement often has relatively low sensitivity, while using a sinusoidal current or voltage with a frequency f in the measurement can minimize undesirable effects such as Faradaic processes (electrons transfer at the interface between the electrode and the chemical solution), double-layer charging, and concentration polarization in living samples. Proper selection of the AC frequency f is important since impedance is frequency dependent. For example, the impedance of skin is about 100 Ω with a 40 Hz AC current and about 200 Ω with a 20 Hz AC current. Besides frequency f, the current level is also important since higher current stimulates cells and causes measurement artifacts and discomfort in the subject. For DC (direct current) or lowfrequency AC, the current level can be 1 mA, while for high-frequency AC (e.g., 25 kHz), the RMS value of the current can be 0.1 mA.
Modeling of Bioresistance/Bioimpedance Sensors
Bioimpedance Z is defined as the total opposition to the flow of an AC current. The magnitude of Z can be calculated by
A cell, tissue, or the entire human body is often modeled as a resistor and a capacitor in parallel, since the inductance tends to be very small relative to the capacitance (i.e., XL ≪ XC). Figure 1 shows a cell membrane modeled as a capacitor (Cm) and a resistor (Rm) in parallel, while its intra-cellular and extra-cellular regions function as resistors (Ri and Re, respectively). The capacitance value of a cell membrane is about 10 μF per cm2.