A MEMS Four-Electrode Conductivity Sensor With On-Chip Temperature Sensing for Real-Time Monitoring of Yogurt Fermentation

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Abstract
Conductivity sensing plays a crucial role in biochemical and industrial process monitoring, particularly in complex liquid environments such as fermentation systems. This study developed a monolithically integrated sensor based on microelectromechanical systems (MEMS) technology, which innovatively combines a concentric four-electrode conductivity sensor with a four-wire platinum resistance temperature detector (RTD). The sensor is designed to track fermentation progress by performing in situ, real-time monitoring of conductivity and temperature variations. Electric field simulations based on rigorous geometric modeling confirmed the establishment of a uniform sensing field between the current excitation electrodes, effectively confining the measurement region. This concentric ring electrode design successfully generated a uniform current density field between the two voltagesensing electrodes, thereby providing an ideal condition for accurate solution conductivity measurement. The sensor was fabricated on a BF33 glass substrate using standard MEMS processes, including photolithography, ion beam etching (IBE), and plasma-enhanced chemical vapor deposition (PECVD) of a SiO2 insulation layer, achieving high integration and batch manufacturability. Comprehensive performance characterization verified its excellent microstructural characteristics and electrochemical performance. The sensor demonstrated outstanding overall performance: a temperature measurement accuracy of ±0.3%, a conductivity measurement accuracy of ±2%; excellent long-term stability with a monthly drift of less than 0.2% for the temperature sensor and less than 3% for the conductivity sensor. A compensation model, established using the integrated temperature sensor, effectively eliminated the impact of temperature fluctuations on conductivity measurements. Ultimately, in a practical yogurt fermentation application, the conductivitytime profile obtained by this sensor showed close agreement with the trend measured by a pH sensor, successfully revealing the dynamic changes of key fermentation stages. This study not only validates the high reliability of the integrated sensor but also provides a solid foundation for its broad application in monitoring complex biological processes.
Funding Information
  • National Key Research and Development Program of China (2022YFB3205300)
  • National Natural Science Foundation of China (62205308)
  • Postgraduate Research Innovation Project of Shanxi Province (2024SJ276)
  • Shanxi Province Science and Technology Innovation Young Talent Team (202304051001030)

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