Flexible electronics have been drawing significant attention for healthcare applications. Establishing ideal flexible device/tissue interfaces plays a crucial role in achieving high-precision physiological signal monitoring for accurate diagnosis. However, existing methods for the design and fabrication of interfaces with human skin still cannot meet the challenging clinical requirements of superior adhesion during monitoring and avoiding wound damage during peel-off. Taking free flap monitoring as an example, free flap transfer has become the common treatment for soft tissue reconstruction. For postoperative free flaps, blood circulation complications such as venous congestion, arterial spasm, and arterial occlusion always present a critical challenge to their viability. Therefore, timely detection and accurate differentiation of complications are crucial, which enables prompt intervention and improve the survival rate of free flaps. Although various techniques were proposed over the past decades, there is still no effective approach in clinical environment for blood circulation monitoring of postoperative flaps so far due to several drawbacks: 1, high equipment costs, wired connections, incapability of portable and facile use; 2, inconvenient data interpretation, inability to differentiate between arterial and venous complications, and poor reproducibility of measurements. Although developing flexible electronic systems is a potential path to alleviate these issues, flexible electronics still have difficulty in meeting stringent interfacial requirements including high adhesion to skin for high-quality signal recording and low adhesion after monitoring to achieve benign detachment from fragile flap skin to avoid damage.
In order to solve the above challenges, the research team developed a soft biosensor with printable responsive hydrogel interfaces for precise detection and differentiation of blood circulation complications. This study developed a printable hydrogel with tunable adhesion to serve as the universal biosensor interface with human skin. The hydrogel interface layers could maintain superior adhesion and high-fidelity signal acquisition during on-skin measurements while exhibiting low adhesion after monitoring to avoid wound damage. Furthermore, to meet the complex requirements of system integration, it is required to achieve high-efficiency customized patterning of hydrogel interfaces. By regulating the rheological properties of hydrogel inks, this work achieved direct printing with the linewidth precision below 720 µm and patterned hydrogel interface layers of the biosensor within 30 s. Due to the decoration of thermoresponsive monomers and adhesive functional groups, the NZDH exhibited superior initial adhesion (27.8 kPa) and broad adhesion-regulation range (10.8 folds). Furthermore, the prepared flexible hydrogel biosensor was used to monitor the blood circulation of postoperative free flaps. In clinical cases, the hydrogel biosensor can not only achieve intimate coupling with flap skin to collect reflective infrared PPG signals and temperature signals from free flaps, but also enable benign detachment to avoid tissue damage. Based on the morphological analysis of photoplethysmography (PPG) signals, the research team proposed a new indicator ( i.e., the balance index) to quantitatively detect venous congestion. Pilot clinical studies confirm that the hydrogel biosensor can achieve precise detection and differentiation of arterial and venous complications. Compared with the clinical assessment and a commercial microcirculation monitoring system, the hydrogel biosensor shows advantages of low risk of mechanical damage, low cost, and wireless communication.
The printable thermoresponsive hydrogel can be adopted as a universal interface in flexible electronics for healthcare applications, and the biosensor represents a promising platform for blood circulation monitoring.
National Science Review
Experimental study