Would you like to know about a more compact, higher performance, and lower power consumption bioimpedance analyzer?

Bioelectrical impedance analysis is a low-cost, non-invasive technique used to measure body composition and assess clinical conditions. Bioimpedance is a complex number consisting of a resistance value R (real part) and a reactance value Xc (imaginary part), the former mainly due to the total amount of water in the body and the latter mainly due to the capacitance generated by the cell membrane. The impedance can also be represented by a vector with magnitude |Z| and phase angle φ. The phase angle plays a major role in determining the body composition.

The resistance R of a conductor of cross-sectional area S and length l, and the capacitance C of a parallel plate capacitor of surface area S and distance d are given by the following formulas:

As can be seen from Equation 4 and Equation 5, resistance and capacitance depend on geometric parameters (length, distance, and surface area), which means that they are related to the measurement system and physical parameters adopted; that is, the resistivity ρ and the dielectric constant ε are closely related to the type of material to be measured (in this case, biological tissue). Figure 1 shows a simplified model of the electrical characteristics of a bioimpedance and its measurement instrument. RE considers the resistance of the extracellular fluid, RI represents the resistance of the intracellular fluid, and Cm is the capacitance of the cell membrane. The connection between the instrument and the human body is achieved through electrodes placed on the skin. The instrument provides an excitation voltage to the electrodes and measures the resulting current. The excitation signal is generated by a digital-to-analog converter (DAC) connected to a back-end driver. The DAC is programmed by a microcontroller, which allows the signal amplitude and frequency to be set. For current measurement, a transimpedance amplifier (TIA) is used, which is connected to a high-resolution analog-to-digital converter (ADC) for accurate measurement. The acquired data is processed using a system microcontroller to extract the information required for analysis.

In bioimpedance measurement, the human body is divided into five sections: two upper limbs, two lower limbs, and the trunk. This distinction is important to understand the measurement method used. The most common are hand to foot, foot to foot, and hand to hand.

During the Bioelectrical Impedance Analysis (BIA) test, several factors are considered, including anthropometric parameters; namely height, weight, skin thickness and body shape. Other factors include gender, age, ethnicity and, especially, the patient's health status; namely, any malnutrition or dehydration. If these factors are not taken into account, the test results may be distorted. The interpretation of the measurement results is based on statistical data and formulas that take into account the various factors mentioned above.

When studying the composition of the human body, we refer to the three-compartment model, which includes :

• Fat body mass

• Cell quality

• Extracellular mass

Figure 2 explains these concepts, starting with the well-known two-compartment model of lean body mass (fat-free body mass) and fat body mass. Fat body mass has two components, essential fat and storage fat. Lean body mass is divided into body cellular mass (composed of protein mass and intracellular water) and extracellular mass (composed of extracellular water and bone mass). The last parameter is the basis for determining water content and is the total body water obtained by the sum of intracellular and extracellular water.

From an electrical properties perspective, the intracellular and extracellular electrolyte solutions behave like good conductors, while fat and bone tissue are poor conductors.

The various bioimpedance measurement techniques in widespread use differ in the frequency of the excitation signal used. The simplest instruments are based on fixed-frequency measurements (single-frequency bioimpedance analysis or SF-BIA), some use multi-frequency systems (multi-frequency bioimpedance analysis or MF-BIA), while the most sophisticated instruments perform actual spectrum measurements over a range of frequencies (bioimpedance spectroscopy or BIS). There are also many techniques for evaluating the results, the most important of which are bioimpedance vector analysis and real-time analysis.

In the SF-BIA instrument, the frequency of the current injected into the body is 50kHz; this is based on the inverse relationship between the measured impedance and the total body water (TBW) (the conductive part of the impedance), which is composed of intracellular water (ICW) and extracellular water (ECW). This technology provides good results for test subjects with normal hydration, but loses its effectiveness for test subjects with drastic changes in hydration due to the limited ability to evaluate changes in ICW.

The MF-BIA technique overcomes the limitations of SF-BIA by performing measurements at both low and high frequencies. Low-frequency measurements allow for a more accurate estimate of ECW, while high frequencies provide an estimate of TBW. The difference between the two estimates yields the ICW. However, the technique is not perfect and has shown limitations in estimating body fluid in elderly populations with medical conditions.

Finally, BIS is based on the measurement of impedance, which, according to the model of Figure 1, is the resistance RE generated by the extracellular fluid at zero frequency and the parallel connection of RE and RI at infinite frequency. At these two frequency extremes, the capacitance generated by the cell membrane behaves as an open circuit or a short circuit. Intermediate frequency measurements provide information about the capacitance value. BIS provides more detailed information than other techniques, but in this case the measurement takes longer.

Bioimpedance vector analysis (bioelectrical impedance vector analysis or BIVA) is a human health assessment technique based on the measurement of the absolute value of bioimpedance. It uses a graph to display a vector representation of impedance, where the horizontal axis shows the resistance value and the vertical axis shows the capacitive impedance value, both of which are benchmarked to the patient's height. The method is based on the formula of 3 tolerance ellipses: 50%, 75% and 95%. The 50% tolerance ellipse defines a population with an average body composition. Moving along the horizontal axis of the ellipse, individuals with a low percentage of lean body mass are identified on the right and vice versa; that is, individuals with a high percentage of lean body mass are identified on the left. Moving along the vertical axis determines the water content, with tendencies towards the upper half of the ellipse representing levels below the norm and towards the lower half of the ellipse above the norm.

Observing fluctuations in body composition (e.g., deviations from normal values ​​for lean body mass, fat body mass, and total body water) is a key factor in determining a patient's health status. Significant loss of lean body mass and fluid imbalances are both primary parameters used in diagnosing disease. Currently, bioelectrical impedance analysis is used to diagnose diseases of the following human systems:

• Pulmonary system
  

• Lung cancer

• Pulmonary Edema

• Cardiovascular System
  

• Postoperative effusion

• Circulatory system
  

• Intravascular volume

• Hyponatremia

• Hydration

• Renal system

• Hemodialysis

• Estimated dry weight

• nervous system

• Alzheimer's disease

• Anorexia nervosa

• Muscular system

• Changes in body composition during training

• Immune system

• Assessment of HIV-infected patients

• Cancer patient assessment

• dengue

ADI has a broad portfolio of impedance analysis products, including devices such as ADuCM35x, which is a highly integrated system-on-chip (SoC) designed for impedance spectroscopy. The recently launched AD5940 is a high-precision, low-power analog front end that is ideal for portable applications. The AD5940 is designed for measuring bioimpedance and skin conductivity and consists of two excitation loops and a universal measurement channel.

The first excitation loop is capable of generating a signal with a maximum frequency of 200Hz and can be configured as a potentiostat for measuring different types of electrochemical cells. Its basic components include a dual-output DAC, a precision amplifier to provide the excitation signal, and a transimpedance amplifier to measure the current. This loop operates at a low frequency and consumes low power, so it is also called a low-power loop.

The second excitation loop has a similar configuration, but is capable of processing signals up to 200kHz, hence the name high-speed loop. The device is equipped with an acquisition channel that includes a 16-bit, 800kSPS SAR-type ADC and the converter analog signal processing chain front end, which includes a buffer, a programmable gain amplifier (PGA), and a programmable anti-aliasing filter. To complete the architecture, a switch matrix multiplexer is used, which allows multiple signals from multiple internal or external signal sources to be connected to the ADC. In this way, in addition to the main impedance measurement function, accurate system diagnostics can be performed to verify the full functionality of the instrument.

Figure 4 shows the connections for the AD5940 for absolute human body impedance measurement in a four-wire configuration. For this type of measurement, a high frequency loop is used; a programmable ac voltage generator provides the excitation signal. A second generator provides the common-mode voltage for proper measurement. The current resulting from the body impedance is measured by a transimpedance amplifier and converted using a 16-bit ADC. The system is capable of measuring frequencies up to 200kHz and provides a 100dB signal-to-noise ratio (SNR) at 50kHz. The digital data is sent to a hardware accelerator to extract the desired values; namely, the real and imaginary parts of the impedance.

As a medical device, the bioimpedance analyzer must comply with the IEC60601 standard. This standard sets limits on the voltage and current that can be applied to the human body. To this end, this device provides a resistor Rlimit to limit the maximum current and four coupling capacitors CisoX to prevent the DC component from being applied to the human body.

Bioimpedance measurement is a low-cost, versatile method for quickly and noninvasively assessing body composition and diagnosing certain types of diseases. Thanks to devices such as the AD5940, current technology allows compact, high-performance, low-power bioimpedance analyzers that can be battery powered. The high level of integration, small size, and low power consumption of the AD5940 also make it particularly suitable for wearable applications.