Design of PPG-ECG detection system using sensor electronic skin

Flexible wearable electronic devices have many unique advantages, such as high conformability to the skin, high data extraction accuracy, and low motion artifacts, which help meet the needs of personalized health monitoring applications. As humans enter a new era of remote and decentralized patient care, wearable technology, especially electronics that fit on the skin, is expected to have great opportunities to move from everyday gadgets to clinical practice. Among various physiological signals, vital signs such as heart rate (HR), respiratory rate (RR), and blood pressure (BP) are important indicators of cardiovascular health status. In addition, blood oxygen saturation (SpO2, defined as the ratio of oxygen-saturated hemoglobin to total hemoglobin in the blood) is an important indicator for detecting hypoxia (abnormally low oxygen levels in the human body).

Flexible photoplethysmography (PPG) sensors and pulse oximeters are built by integrating flexible photodetectors and light-emitting diodes, which can extract pulse information from volume changes of blood vessels in real time and non-invasively. Light with near-infrared (NIR) wavelengths can penetrate deep into the skin and even detect changes in subcutaneous blood volume. The stable penetration depth of the red near-infrared diagnostic window improves the measurement accuracy of SpO2 without damaging tissue. Currently, most of the PPGs reported in the literature are based on silicon-based photodetectors, which have certain limitations in their compliance with soft, moist and dynamic skin. Motion artifacts caused by skin activity during measurement further complicate the algorithm and limit the measurement accuracy.

There has been a great deal of research into developing narrow-bandgap organic semiconductors and facile solution processing to develop near-infrared organic photodetectors (OPDs) with tunable mechanical compliance and high performance. The development of narrow-bandgap n-type molecular semiconductors has demonstrated an effective strategy to be a potential replacement for silicon in photodetectors. Currently, there are few reports of OPDs with ultraflexible configurations and comparable photoresponsivity to their silicon counterparts in the near-infrared band, and their great potential in electronic skin systems remains to be explored.

The pulse signal provided by PPG is based on photoelectric volume changes, while the electrocardiogram (ECG) signal, as the gold standard for measuring heart rhythm and heart rate, provides pulse-related information based on bioelectricity. Traditional ECG systems are difficult to become daily wear detection devices due to their large size, inconvenience of long-term wear, and limited functions. The combination of wearable PPG and portable single-lead ECG sensing components can improve the accuracy and reliability of pulse information and enable home care in a user-friendly way. By post-processing these two sets of signals, more health indicators, including blood pressure, can be obtained.

Although flexible ECG electrodes or PPG sensors have been developed separately, it is still challenging to build a reliable skin-fitted PPG-ECG integrated system. First, commercial ECG electrodes, such as Ag/AgCl gel electrodes or other metal-based dry electrodes, are difficult to stably couple with flexible photodetectors due to their bulky size and mechanical mismatch. Second, individual sensing elements should have high performance and long-term working stability. Third, maintaining close and comfortable contact with the skin and long-term fit without skin irritation are still common challenges. Most skin-fitted electronic devices require additional adhesive layers to enhance interface toughness, which increases the risk that the skin cannot "breathe" freely. In addition, loose adhesion and device contamination due to skin secretions are usually unavoidable, which can lead to deterioration of transmittance and electrical performance. In short, skin patches that can accurately monitor photoelectric and bioelectric signals require a combination of high-performance photodetectors and conductive electrodes, all of which need to be mounted in a soft substrate in direct contact with the skin and do not restrict the movement or breathing of the skin.

According to MEMS Consulting, a joint research team from Tsinghua University Shenzhen International Graduate School and Tsinghua-Berkeley Shenzhen Institute and the Shenzhen Institute of Advanced Technology of the Chinese Academy of Sciences published a paper titled "Near-Infrared Organic Photodetectors toward Skin-Integrated Photoplethysmography-Electrocardiography Multimodal Sensing System" in the journal Advanced Science. This study demonstrated a soft and compact electronic skin patch that can monitor multiple vital signs such as HR, RR, SpO2 and BP, while showing unprecedented mechanical compliance and skin compatibility. The resulting flexible PPG sensor exhibits a high signal-to-noise ratio (SNR) and stable peak-to-peak amplitude under conditions of hypoxia and hypoperfusion, which is superior to commercial finger-clip pulse oximeters, and can ensure accurate extraction of SpO2 under dynamic working conditions.

Design of skin-fitting PPG-ECG detection system

The red bar graph PPG signal comes from the ultra-flexible PPG sensor developed by the research, and the blue bar graph comes from a commercial finger-clip pulse oximeter

Skin-fitting PPG-ECG multi-mode sensing system

Experiments show that the skin-fitted PPG-ECG integrated system developed in this study has great application potential in the field of personalized healthcare. The researchers developed an OPD with ultra-flexibility and long-term stability, showing high photoresponsivity under near-infrared light illumination. This OPD showed a photoresponsivity of 0.53 A/W at a wavelength of 940 nm (−1 V reverse bias) and a cutoff response frequency of more than 1 MHz at −3 dB. After long-term storage for 1272 hours under ambient conditions, the photoresponsivity can still maintain more than 90% of the initial value. Their shot noise-limited specific detectivity is >10¹³ Jones, which is comparable to the corresponding rigid silicon devices. The ultra-flexible PPG based on OPD has a higher signal-to-noise ratio and peak-to-peak amplitude, making it very suitable for accurate measurement of SpO2, especially under dynamic and hypoxic conditions.

In addition, the researchers demonstrated a hydrogel-based soft electrode with a conductivity of >660 S/cm and a low skin contact impedance of 15 kΩ cm² at 100 Hz, which can detect electrophysiological signals with high quality. Its flexible PPG sensing module can be immediately integrated with bioelectrodes and human skin through interfacial coupling of ultrathin hydrogel films. This skin-compatible sensing platform is less than 20 µm thick and can be repeatedly applied or removed without causing any significant degradation to the electronic components. For the first time, the researchers demonstrated a skin-fitting multimodal sensing system that can accurately and stably measure various vital signs, including heart rate, respiratory rate, cuffless blood pressure, and arterial oxygen saturation, even under dynamic working conditions.