Wearable System Integrating\nDual Piezoresistive and\nPhotoplethysmography Sensors for Simultaneous Pulse Wave Monitoring

Wearable, flexible piezoresistive pressure sensors have\ngarnered\nsubstantial interest due to their diverse applications in fields such\nas electronic skin, robotic limbs, and cardiovascular monitoring.\nAmong these applications, arterial full pulse waveform monitoring\nstands out as a critical area of research. The emergence of piezoresistive\npressure sensors as a prominent tool for capturing pulse waveforms\nhas led to extensive investigations. However, the lack of a linear\nresponse while achieving high sensitivity, limited compactness of\nsignal collection systems, and scaling issues are a few potential\ncauses of the gap between research advances and technology market\nreadiness. Here, we present a scalable dual piezoresistive sensor\nthat uses two complementary resistance-changing mechanisms to balance\nthe trade-off between linear response and high sensitivity. This synergic\ndesign enables excellent sensitivity of 8.4 kPa<sup>–1</sup> and near linear pressure response. It boasts a fast response time\nof 95/145 ms for loading and unloading at a pressure of 10.1 kPa.\nThe durability tests, encompassing nearly 5000 two-stage compression\ncycles, validate the reliable performance of the sensor, even after\nprolonged use. Leveraging these performance metrics, we developed\na high-resolution and compact signal collection device capable of\naccurately detecting the three distinct peaks associated with the\nfull pulse waveform, including the systolic and reflected diastolic\npeaks. Moreover, the signal acquisition system incorporates a photoplethysmography\nsensor, which, when paired with the dual piezoresistive sensor, offers\nnew insights into localized vascular health monitoring. The unique\nfeature of our approach is its ability to perform localized vascular\nmonitoring using multiple sensors placed on different veins. The simultaneous\ndetection of forward- and backward-going pulse waves at the body extremities\nallows for a comprehensive evaluation of localized vascular health,\nwhich is not achievable with a single sensor. Finally, the comparison\nof wrist pulse waveforms between the compact signal collection device\nand a standard sourcemeter (such as the Keithley 2460) confirmed that\nthe wearable sensing system is on par with conventional sourcemeters\nin terms of capturing the typical details of the full pulse waveform\n(i.e., systolic peak, dicrotic notch, and diastolic peak). This validation\nunderscores the reliability and effectiveness of the developed wearable\nsensing system for practical and continuous pulse waveform monitoring.