Cuffless blood pressure (BP) measurement is an all-inclusive term for a method that aims to measure BP without using a cuff. Recent cuffless technology has made it possible to estimate BP with reasonable accuracy. However, mainstream methods require an electrocardiogram and photoplethysmogram measurements, and frequent calibration procedures using a cuff sphygmomanometer. We therefore developed a far simpler cuffless method, using only heart rate (HR) and modified normalized pulse volume (mNPV) that can be measured using a smartphone, based on the knowledge that ln BP = ln cardiac output (CO) + ln total peripheral resistance (TPR), where CO and TPR are correlated with HR and mNPV, respectively. Here, we show that mean arterial pressure (MAP), systolic BP (SBP), and diastolic BP (DBP) could be estimated using the exponential transformation of linear polynomial equation, (a × ln HR) + (b × ln mNPV) + constant, using only a smartphone, with an accuracy of R > 0.70. This implies that our cuffless method could convert a large number of smartphones or smart watches into simplified sphygmomanometers.

Photoplethysmography (PPG) is a simple method to measure various physiological indices, including heart rate (HR). To prevent motion artifacts, the optimal light wavelength for PPG measurements should be selected. However, this countermeasure has not been examined thoroughly. This study addressed PPG robustness against motion artifacts for different light wavelengths and measuring modes to accurately determine HR. Twelve healthy volunteers underwent motion artifact experiments during PPG measurements, in which they were asked to either remain still or wave their hands horizontally or vertically as fast and rhythmically as possible. Reflectance mode blue (RB), green (RG), red (RR), and near-infrared (RNIR) lights and transmittance mode red (TR) and near-infrared (TNIR) lights were evaluated for PPG signals acquired along with electrocardiogram (for reference HR) and hand acceleration measurements. The analysis revealed that the RB and RG PPG modes increased the signal-to-noise ratio by approximately 8 dB compared to TR PPG, and the HR obtained from both did not exhibit fixed or proportional bias, with a Pearson's correlation coefficient above 0.986. Furthermore, RNIR PPG was superior to TR PPG by approximately 4 dB, and its calculated HR did not show fixed or proportional bias, with a Pearson's correlation coefficient of 0.967. The RR, TNIR, and TR PPG modes showed comparable and inferior performance. Therefore, blue and green lights followed by near-infrared light in reflectance mode are the recommended settings to measure HR using PPG. These findings may serve as guidelines for researchers and engineers to improve PPG measurements and devices.

For transillumination imaging of animal tissues, we have attempted to suppress the scattering effect in a turbid medium using the time-reversal principle of phase-conjugate light. We constructed a digital phase-conjugate system to enable intensity modulation and phase modulation. Using this system, we clarified the effectiveness of the intensity information for restoration of the original light distribution through a turbid medium. By varying the scattering coefficient of the medium, we clarified the limit of time-reversal ability with intensity information of the phase-conjugate light. Experiment results demonstrated the applicability of the proposed technique to animal tissue.

Routine blood pressure measurement is important for the early detection of various diseases. Recently, cuffless blood pressure estimation methods that do not require cuff pressurization have attracted attention. In this study, we investigated the effect of the light source wavelength on the accuracy of blood pressure estimation using only two physiological indices that can be calculated with photoplethysmography alone, namely, heart rate and modified normalized pulse volume. Using a newly developed photoplethysmography sensor that can simultaneously measure photoplethysmograms at four wavelengths, we evaluated its estimation accuracy for systolic blood pressure, diastolic blood pressure, and mean arterial pressure against a standard cuff sphygmomanometer. Mental stress tasks were used to alter the blood pressure of 14 participants, and multiple linear regression analysis showed the best light sources to be near-infrared for systolic blood pressure and blue for both diastolic blood pressure and mean arterial pressure. The importance of the light source wavelength for the photoplethysmogram in cuffless blood pressure estimation was clarified.