Measurement of blood pressure is one of the most important parameters for personal healthcare. Arteries are responsible for carrying blood from the heart to all parts of the body and the pressure of blood in arteries is measured in many abnormalities. The blood pressure is explained by two parameters. The highest blood pressure during heart pumping is measured by systolic pressure whereas the diastolic pressure measures the lowest blood pressure when the heart is not pumping the blood. For current work, the principle of light absorption is used to measure blood pressure in arteries. Light is more strongly absorbed by blood compared to skin or tissues and therefore, blood flow change can be detected by the change in intensity of light. This blood flow change is proportional to blood pressure and hence the blood pressure can be measured by measurement of change in light intensity.

In this work, we have developed the InGaP photodiode module with higher signal to noise ratio for blood pressure measurement. The InGaP photodiode External Quantum Efficiency (EQE) is 50% higher than Silicon (Si) photodiode. The EQE is ever higher than Si photodiode equipped with the filter as shown in Fig. 1(a). The newly developed InGaP photodiode has the advantage of selective EQE at different wavelength. The photodiode has almost zero EQE outside the desired region of wavelength (300 nm- 700 nm) where the Si photodiode EQE is spread in the longer wavelength region (300 nm - 1200 m). This higher region of EQE can lower the signal to noise ratio. The InGaP photodiode has an increased signal to noise ratio (24.8) compared to other photodiodes (24.0) due to the smaller wavelength region of EQE. Both systolic blood pressure and diastolic blood pressure measurement was compared for actual value and predicted value. The R² correlation value for systolic and diastolic blood pressure is measured to be 0.86 and 0.91 respectively as shown in Fig. 1(b) and Fig. 1(c).

This works explore new doorway for LED and photodiode application in the field of biomedical sensors. The application of our new photodiode with enhanced EQE and the wide range of LED has numerous possibilities. The higher possibility of multichip design with Flip chip packaging technique can provide a single module for measurement of blood pressure, blood glucose, blood lipid and other parameters using LED of different wavelength.