Microcontroller-Based Implementation of the DRT for Bioimpedance Spectroscopy

Bio-Impedance Spectroscopy (BIS) is a non-invasive method for characterizing biological systems. Converting impedance spectra into the distribution of relaxation times (DRT) allows the separation of overlapping effects without relying on equivalent circuits as impedance models. This transformation significantly reduces signal processing time from several seconds to milliseconds. Therefore, there is a significant need for embedded solutions that implement DRT in microcontroller-based systems. Following a detailed evaluation of various methodologies, NNLS combined with Tikhonov regularization was selected for its ability to achieve an 83.33% reduction in execution time. This paper presents a novel approach for implementing this optimized DRT extraction in STM32-based embedded systems. The novel implementation achieves processing times of 74 ms for a 101-frequency data set on the STM32 while utilizing only 2.69 kB of RAM. The system successfully extracts DRT peaks with different magnitudes and their relative relaxation times for 51-frequency and 101-frequency datasets. We demonstrate equivalent circuit parameter extraction with relative errors below 3.5%. This work establishes a foundation for portable BIS analysis, with potential applications in areas such as medical diagnostics, food state characterization, and plant monitoring.