Remote healthcare monitoring systems are crucial for reducing infection risks and improving patient care. Blood pressure (BP) monitoring is particularly important, as hypertension is linked to various chronic diseases. Conventional cuff-based sphygmomanometers are unsuitable for continuous monitoring, necessitating cuffless systems combining electrocardiogram (ECG) and photoplethysmography (PPG) sensing. Pulse arrival time (PAT), calculated from ECG R-wave and PPG systolic peak, is used to estimate BP. However, ECG sensing requires reliable skin contact, often achieved with wet electrodes that can cause irritation. Dry electrodes are desirable but suffer from poor contact. This research aims to address these limitations by developing a novel dry electrode system using origami principles and the suction mechanism of leeches, enabling reliable, continuous BP monitoring with a sensing robot.

Existing research highlights the need for improved dry electrodes for ECG sensors. Conducting polymers like PEDOT:PSS with Ag/AgCl have been explored, along with self-adhesive electrodes using soft or micro-patterned polymers. Printing on stretchable polymers improves adhesion, but often requires additional support systems like belts or tapes. Origami offers advantages due to its tunable mechanical rigidity, force direction-shifting properties, and ability to create suction for improved grasp. The leech's suction mechanism, a combination of auxetic and non-auxetic behaviors, inspires the design of the LIO sensor.

The LIO sensor was designed based on the leech's suction mechanism, mimicking the simultaneous expansion of the sucker and contraction of the body. A 3D printed origami structure, combining non-auxetic and auxetic parts, was created using fused filament fabrication (FFF). A parametric study optimized the origami design (α and β angles) by analyzing Poisson's ratio, rotation angle, and elastic modulus. Serpentine-patterned electrodes were 3D printed onto the non-auxetic part for robustness and conductivity. The LIO sensors were integrated into a humanoid robot, enabling ECG and PPG sensing. ECG signals were compared to those from conventional wet electrodes. BP monitoring was performed using a paired ECG-PPG system, with PAT calculated from the time difference between the R-peak and PPG peak. A calibration process, using a sphygmomanometer, established a relationship between PAT and BP. Ethical approvals were obtained from the Simon Fraser University research ethics committee.

The optimized LIO sensor (α = 30°, β = 40°) demonstrated a superior suction capability, as evidenced by its ability to maintain stable contact with the skin even during movement. The 3D printed serpentine electrodes with three layers (n=3) showed reliable conductivity up to a strain of 1.8. The LIO sensor integrated into the humanoid robot exhibited ECG signal quality comparable to conventional wet electrodes, with a signal-to-noise ratio of 21.7 ± 0.56 dB. Paired ECG and PPG sensing allowed for accurate BP monitoring. The average difference in systolic BP between the sensing robot and sphygmomanometer was only 0.03 mmHg. The system accurately reflected BP changes under different exercise conditions (climbing stairs, push-ups, squats, fast walking, and sprinting). The repeatability and stability of the LIO sensor over 60 sensing cycles and after 30 days of storage were also demonstrated.

The results demonstrate the feasibility of using a bio-inspired origami sensor for reliable, cuffless BP monitoring in a robotic system. The LIO sensor effectively addresses the challenges of dry electrode contact and enables continuous monitoring even with subject movement. The high signal quality of the ECG measurements combined with PPG data allowed accurate BP estimation, validated against a sphygmomanometer. The close agreement between estimated and measured BP under various exercise conditions highlights the robustness and potential clinical applicability of the system. The integration of this sensor into a humanoid robot demonstrates a promising platform for future remote healthcare applications.

This study successfully demonstrated a novel leech-inspired origami (LIO) sensor integrated into a robotic system for accurate and continuous blood pressure monitoring. The use of 3D-printed serpentine electrodes and an optimized origami design enabled reliable ECG sensing with dry electrodes. Future research should focus on long-term studies, involving a larger and more diverse population, to further validate the clinical utility and improve the robustness of the system. Exploration of additional bio-inspired designs and advancements in sensor technology holds promise for more sophisticated and versatile remote healthcare monitoring.

The study was conducted with a single male participant, limiting the generalizability of the findings. Further research with a larger and more diverse cohort is necessary to validate the robustness and accuracy of the BP estimation across different demographics and physiological conditions. Long-term stability and durability of the sensor in real-world settings should also be further investigated. The current calibration method is subject-specific, requiring individual calibration for each user.