Hydrogel sensors are gaining attention as promising wearable technologies for advanced health monitoring. Their flexible, soft, and stretchable nature enables them to comfortably conform to the human body while maintaining sensitive signal detection. These sensors can monitor multiple physiological parameters, including temperature, pressure, and strain, in real time.1-4
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Importance of Hydrogel Sensors
Hydrogel sensors, using intricately prepared and designed cross-linking structures, offer benefits in the biomedical domain and leverage cross-linking design and preparation approaches. The sensor design depends on the distinct properties of hydrogel materials. These materials comprise a three-dimensional (3D) network structure with high biocompatibility and water retention.1
Specifically, the fundamental design principle focuses on the cross-linking structure of hydrogels, achieved by adjusting the material composition and degree of cross-linking to enable the detection of specific biomolecules or physiological parameters with high sensitivity.1
Thus, hydrogels can be precisely engineered to display intelligent responses in particular biological settings, making them suitable for biosensing applications.1
Hydrogel Sensors: Advantages
Hydrogel sensors demonstrate exceptional biocompatibility, decreasing the risk of immune responses and forming strong bonds with biological tissues. Hence, hydrogel sensors are suitable for in vivo monitoring in the long term and provide opportunities for personalized healthcare.1
They are highly sensitive to minor physiological changes, enabling real-time monitoring of physiological indicators such as heart rate and blood glucose. Reliable data can be obtained with these sensors to enable timely interventions.1
Additionally, reliable operation over extended periods in biological environments is critical for monitoring cases requiring prolonged surveillance or acute conditions, owing to the stability of hydrogels.1
Hydrogel sensors can also be developed as multifunctional monitoring tools to detect several biomolecules and physiological parameters simultaneously, making them suitable for diagnosis and monitoring of various diseases.1
In health monitoring, major applications of hydrogel sensors include biological parameter monitoring, respiratory monitoring, physiological state monitoring, and disease monitoring.1
Decoupled Oxygen and Temperature Sensing
Wearable self-powered sensors that can monitor several physiological signals are desired for advanced healthcare electronics. Yet, realizing decoupled sensing of multiple signals within one compact system is difficult.2
In a recent paper published in Advanced Functional Materials, researchers proposed a multilayered zinc-air battery-design-based self-powered, rechargeable, multimodal soft (SRMS) sensor for decoupled oxygen and temperature monitoring.2
Using a potassium polyacrylate/polyacrylamide hydrogel as the polyelectrolyte, the SRMS sensor employed a zinc-electrodeposited graphite film anode to detect temperature and a cobalt(II,III) oxide/graphene-coated nickel foam cathode to monitor oxygen concentration.2
The sensor can be used for thermal imaging, contactless temperature sensing, and data input via sensor-array integration. In response to temperature and oxygen stimuli, decoupled positive and negative responses in relative current change were observed with the SRMS sensor during respiratory gas monitoring.2
Additionally, the sensor successfully identified sleep-related respiratory disturbance events, monitored variations in skin temperature during sleep, and distinguished various respiratory states.2
Thus, the findings of the work demonstrated the feasibility of this novel approach for developing self-powered, multimodal soft sensors with decoupled sensing capabilities for contactless information interaction and wearable healthcare monitoring.2
Posture Recognition and Respiratory Monitoring
Accurately monitoring the human motion states and respiratory events is crucial for disease prognostication and health surveillance. To achieve this, it is necessary to design an environmentally resilient, portable, multifaceted, and precise sensor for health monitoring.3
A paper published in Biosensors and Bioelectronics proposed a posture recognition and breath monitoring system that uses hydrogel electrolytes based on starch, sodium alginate (SA), and polyvinyl alcohol (PVA) for multimodal wearable sensors and supercapacitors.3
A hydrogel sensor with dual-mode capacitance/resistance capabilities was synthesized by adjusting the electrode assembly and designing a multifunctional gel based on SA, PVA, and starch.3
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The hydrogel network exhibited excellent long-term stability and mechanical strength across different temperature conditions due to the synergistic effects of multiple hydrogen-bonding interactions and chemical cross-linking. These smart multimodal sensors can independently detect thermal and mechanical changes via their resistance and capacitance output signals, respectively.3
Researchers harnessed deep learning algorithms to improve the sensor's recognition accuracy, achieving 99.259% finger-pressing posture recognition in capacitance mode.3
Additionally, the hydrogel sensors were employed successfully in the diagnosis of obstructive sleep apnea syndrome. When applied to real-time sleep respiratory monitoring, the integrated hydrogel sensor demonstrated high accuracy and reliability.3
The findings demonstrated that the flexible electronic device developed in the study has several functionalities. Thus, the study offered a novel perspective on the design and fabrication of next-generation electronic devices for medical applications.3
Multi-signal Sensor based on MXene Hydrogels
Wearable high-performance sensors with multi-sensing capabilities for diverse signals were developed in another study.4
In a paper published in Chemical Engineering Journal, researchers fabricated a novel catechol functionalization of PVA-3′,4′-dihydroxyacetophenone (CA)/poly(N-acryloyl glycinamide)/MXene hydrogels (PcNA-M) by combining the self-powered properties of the catechol functionalization of PVA (Pc)/MXene hydrogels and the thermosensitivity of poly(N-acryloyl glycinamide) (PNAGA) hydrogel.4
By adjusting the mixing ratio of Pc and PNAGA, the resulting PcNA-M exhibited good compression, tensile, and rebound properties, as well as rapid temperature sensing and self-healing properties. The network of multiple hydrogen bonds between PNAGA was either connected or disintegrated at different temperatures, resulting in a significant change in resistance.4
Electrical and ionic conductivities contributed to the hydrogel's overall conductivity. The PcNA-M sensor was used to monitor voltage and resistance signals during body movements, handwriting motion, and temperature changes, producing various electrical signals from diverse sources.
During hydrogel deformation and self-recovery, the PcNA-M sensor exhibited high stability, accuracy, and sensitivity in monitoring human physiological signals. The PcNA-M sensor also displayed excellent stability and sensitivity for temperature monitoring.4
Overall, the PcNA-M sensor exhibited high reliability, identifiability, and sensitivity in wearable self-powered sensor applications. Thus, the findings demonstrated the potential of the proposed multi-signal sensor and created new opportunities for self-powered wearable multi-signal sensors to monitor various human motion-related signals.4
Hydrogel Sensors Multi-signal Monitoring?
In conclusion, hydrogel sensors are emerging as soft, stretchable, and versatile platforms for multi-signal health monitoring. Recent developments demonstrate decoupled sensing, posture recognition, sleep monitoring, self-powered operation, and self-healing capabilities, indicating their significant potential for reliable, multifunctional, and personalized healthcare applications.
References and Further Reading
- Wang, K. et al. (2024). Smart Hydrogel Sensors for Health Monitoring and Early Warning. Advanced Sensor Research, 3(9), 2400003. DOI: 10.1002/adsr.202400003, https://advanced.onlinelibrary.wiley.com/doi/10.1002/adsr.202400003
- Wu, J., Wang, X., Liu, B., Cao, R., & Weng, G. (2026). A Self-Powered and Rechargeable Multimodal Hydrogel Sensor with Decoupled Oxygen and Temperature Sensing. Advanced Functional Materials, e78074. DOI: 10.1002/adfm.78074, https://advanced.onlinelibrary.wiley.com/doi/abs/10.1002/adfm.78074
- Liu, J. et al. (2024). Multimodal and flexible hydrogel-based sensors for respiratory monitoring and posture recognition. Biosensors and Bioelectronics, 243, 115773. DOI: 10.1016/j.bios.2023.115773, https://www.sciencedirect.com/science/article/abs/pii/S0956566323007157
- Chen, F. et al. (2024). High-sensitivity wearable multi-signal sensor based on self-powered MXene hydrogels. Chemical Engineering Journal, 489, 151221. DOI: 10.1016/j.cej.2024.151221, https://www.sciencedirect.com/science/article/abs/pii/S1385894724027086
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