Noninvasive blood pressure monitor using strain gauges, a fastening band, and a wrist elasticity model

This paper proposes a novel continuous noninvasive blood pressure estimation method that is based on the variation of wrist skin strain occurring with changes in diastolic and systolic blood pressure. The wrist elasticity model used to estimate blood pressure was constructed according to an ultrasonic probe indentation method used for measuring and calculating the thicknesses and material properties of skin layers. The strain value of the wrist skin was measured using a dual strain sensor to avoid temperature drift. The dual strain sensor was fabricated using a screen-printing process on a polyimide film. To cause the dual strain sensor to indent the skin for ascertaining the depth to measure skin strain, the wristband is driven by a compact ultrasonic linear motor consisting of a piezoelectric slab and a flap clip. The compact ultrasonic linear motor provides an adequate driving distance, which was estimated using the energy method, and a constant holding force used for strain measurement without input power. In blood pressure measurement experiments performed with 30 subjects, the estimation mean error against a validated cuff-type blood pressure monitor was 1.6 mmHg at rest for diastolic pressure. The error increased when the subjects exercised, but returned to near the rest value when the subjects rested after exercise. These results showed that the noninvasive blood pressure monitor can estimate blood pressure with a small fastening force. This device can be used for bedridden patients who require continuous blood pressure monitoring.