Development of electrical transduction based wearable tactile sensors for human vital signs monitor: Fundamentals, methodologies and applications

Abstract

With the advent of advanced materials processing techniques, and physical sensors’ output data acquisition, transmission and processing by embedded systems through internet of things (IoT), artificial intelligence (AI), machine learning, etc., tools are opened up many possibilities in distributed healthcare monitor, robotic process automation, smart home, etc., domains. To further augment the efficiency of these applications wearable sensors, electronic skin, smart sensors, etc., were fabricated with a variety of materials combination. Among many sensors, temperature, pressure and volume (plethysmo) sensors are widely used in healthcare to monitor vital signs: body temperature(BT), heart rate (HR), blood pressure(BP), respiration rate (RR) and oxygen saturation (Osat). These readings are called vital signs because they all must be present for life to continue. Vital signs measurement by wearable tactile/ pressure sensors for healthcare monitor are most widely researched in the recent years due to their promising application in a distributed healthcare system. Hence, it is important to understand both the basics of vital signs and their measurement by various transduction based tactile sensors in details to devise a suitable sensor for vital signs monitor. In the literature, there are many research articles on vital signs monitor by pressure sensors with qualitative data and, some times, exaggerated claims of measurements and suitability of a sensor for specific vital sign measurement. However, for real applications one need quantitative parameters of a sensor for a particular application. Thus it may worth to separate the wheat from the chaff by studying the fundamentals of vital signs, measurement requirements and limitations of measuring techniques etc., in details. This information is missing in the literature. Hence we have systematically presented the various electrical transduction based tactile or pressure sensor technologies, sensor fabrication methods, sensor characteristics modification, basics of vital signs, and measurement techniques. The past three years literature on respective vital sign measurement were also presented here. It is inferred that the tactile sensor should be highly sensitive in linear working range of 30–190 mmHg (4–25 kPa) to measure blood pressure, heart rate and respiration rate. However, the required sensitivity value is not yet determined for each of the above measurements, in the literature. Both resistance and capacitance tactile sensors’ sensitivity and linear working range can be enhanced by employing biomimetic surface microstructures, 3D porous microstructures, intrafillable microstructures, metallic fillers, ionic liquids, etc., The details of each method is described in the article under suitable section. It is still a great challenge to simultaneously achieve high sensitivity, wide linear working range, excellent stability in a sensor for meeting the requirements of full-scale healthcare monitoring from subtle deformations (due to blood pulse, and cheek occlusion) to large deformations (due to finger bending and pressing).