Measurement of Pulsation Strain at the Fingertip Using a Plastic FBG Sensor

There is a need to develop sensors that can measure vital signs such as pulse rate and blood pressure easily; such vital signs can be calculated by measuring arterial strain. To this end, a new method that uses a plastic fiber Bragg grating (FBG) sensor to measure pulsation strain at human fingertips is proposed herein. Plastic fiber has a smaller Young’s modulus than that of the glass fiber. Under the same applied pressure, the plastic FBG sensor deforms more than the silica glass FBG sensor, and the Bragg wavelength shift length becomes longer, which results in higher detection sensitivity. The pulsation strain of the fingertip was measured by the proposed method based on this characteristic. The most sensitive signal detection point in the fingertip was the measurement point 10 or 15 mm from the distal interphalangeal joint in the fingertip direction. Strain signals with periodic peaks were measured from five participants. From the strain signal measured with the cuff attached to the arm, the peak disappeared when the cuff moved, and it reappeared when the cuff was opened. Therefore, the measured signal waveform changes based on the blood flow condition. The pulse rate calculated from the peak interval of the measured signal showed high correlation with the reference pulse rate, and the measurement accuracy was ±5.5 bpm. It may be possible to calculate the blood pressure and pulse rate from the measurement signal of the pulsation strain of the fingertip obtained using the plastic FBG sensor.