Wireless Wearable Photoplethysmography Sensors for Continuous Blood Pressure Monitoring Yiyuan Zhang, Melissa Berthelot, Benny Lo Abstract— Blood Pressure (BP) is a crucial vital sign taken into consideration for the general assessment of patient’s con- dition: patients with hypertension or hypotension are advised to record their BP routinely. Particularly, hypertension is emphasized by stress, diabetic neuropathy and coronary heart diseases and could lead to stroke. Therefore, routine and long-term monitoring can enable early detection of symptoms and prevent life-threatening events. The gold standard method for measuring BP is the use of a stethoscope and sphygmo- manometer to detect systolic and diastolic pressures. However, only discrete measurements are taken. To enable pervasive and continuous monitoring of BP, recent methods have been proposed: pulse arrival time (PAT) or PAT difference (PATD) between different body parts are based on the combination of electrocardiogram (ECG) and photoplethysmography (PPG) sensors. Nevertheless, this technique could be quite obtrusive as in addition to at least two contacts/electrodes to measure the differential voltage across the left arm/leg/chest and the right arm/leg/chest, ECG measurements are easily corrupted by motion artefacts. Although such devices are small, wearable and relatively convenient to use, most devices are not designed for continuous BP measurements. This paper introduces a novel PPG-based pervasive sensing platform for continuous measurements of BP. Based on the principle of using PAT to estimate BP, two PPG sensors are used to measure the PATD between the earlobe and the wrist to measure BP. The device is compared with a gold standard PPG sensor and validation of the concept is conducted with a preliminary study involving 9 healthy subjects. Results show that the mean BP and PATD are correlated with a 0.3 factor. This preliminary study shows the feasibility of continuous monitoring of BP using a pair of PPG placed on the ear lobe and wrist with PATD measurements is possible. I. INTRODUCTION Despite the fact that hypertension is one of the most important preventable contributors to diseases, it has no symptoms. Many people are unaware of their hyperten- sion condition until an adverse event occurs. Long-term abnormal blood pressure (BP), such as hypertension, can lead to heart disease, stroke, and damage to major organs. It was estimated that over 14 millions of patients in the US (39.4%) were not aware of their hypertension, while 66.9 million (53.5%) estimated patients did not have their condition controlled [1]. The World Health Organization reported in 2016 that one-third of worldwide adults have high BP. To assess this condition and prevent complications, patients with hypertension have to be monitored routinely by healthcare providers. BP measurement is usually performed during routine health examinations, but the collected data can be affected by environmental factors or patient’s mood, The Hamlyn Centre, Imperial College London, UK {meb14, benny.lo}@imperial.ac.uk nervousness or emotion - also referred as ”white coat” effect, which can lead to an elevated BP. To better monitor the condition, patients are generally advised to record their BP themselves on a daily basis to provide more accurate data for their healthcare providers to decide on appropriate therapeutic intervention. Taddei et al. demonstrated that a large population of patients cannot maintain their BP in the normal range; anti-hypertension drugs are proven to be effective, but are often not administrated at the correct dosage [2]. It is suggested that pharmacological treatments for hypertension should consider the degree of BP reduction, and the effective duration of the drug. As patient’s condition may deteriorate over age, treatments have to be reviewed regularly, and long term BP measurement is essential to enable accurate treatment design. There are two major types of non-invasive BP measurement methods. One method widely applied clinically is the use of a sphygmomanometer and stethoscope. The point of systolic and diastolic BP can be detected by auscultation, capturing the sound of pulses when blood flows through the upper arm arteries under pressure, or oscillometry, using the empirical method to detect the oscillation [3]. However, this approach is prone to motion artefacts [4], and cannot be used for 24- hour monitoring. Non-invasive long term and continuous BP measurement methods have been proposed and are mostly based on pulse arrival time (PAT). With wearable devices placed at strategic body parts, this technique is more com- fortable compared to volume-clamp and tonometry-based methods [4], [5]. A combination of electrocardiogram (ECG) and photoplethysmogram (PPG) sensors are usually used to measure PAT and estimate BP. As ECG sensors are designed to capture electrical signals of the cardiac cycle with no latency, this signal is used as the reference for measuring PAT. PPG sensor can measure the light intensity variation of the oxygenated (HbO 2 ) haemoglobin in the blood. The measured pulses are the result of the fluctuation of blood volume during systolic and diastolic heart phases [6]. PPG sensors have been used to detect pulses on different body parts such as fingertip, wrist, brachial, forehead, chest, earlobe, and ankle [6]. Different approaches have been suggested for measuring PAT by capturing the latency of PPG signals on different positions of the body using the ECG signal as reference. Difference of PAT (PATD) have also been stated to measure PAT difference between the toe and finger, or the brachial and ankle [7], [8]. Y. Zheng et al. [9], [10] have developed a wearable cuff-less system for BP monitoring, using an armband for measuring ECG and PPG signals. H. Lin et al. [5] similarly introduced