While hydrogels enable a variety of applications in wearable sensors and electronic skins, they are susceptible to fatigue fracture during cyclic deformations owing to their inefficient fatigue resistance. Herein, acrylated β-cyclodextrin with bile acid is self-assembled into a polymerizable pseudorotaxane via precise host-guest recognition, which is photopolymerized with acrylamide to obtain conductive polymerizable rotaxane hydrogels (PR-Gel). The topological networks of PR-Gel enable all desirable properties in this system due to the large conformational freedom of the mobile junctions, including the excellent stretchability along with superior fatigue resistance. PR-Gel based strain sensor can sensitively detect and distinguish large body motions and subtle muscle movements. The three-dimensional printing fabricated sensors of PR-Gel exhibit high resolution and altitude complexity, and real-time human electrocardiogram signals are detected with high repeating stability. PR-Gel can self-heal in air, and has highly repeatable adhesion to human skin, demonstrating its great potential in wearable sensors.
a Illustration of the topological networks constructed by the polyrotaxane of PEG with α-CD in SR hydrogels; b illustration of the topological networks of PR-Gel constructed by the polymerizable pseudorotaxane crosslinkers of BA with β-CD derivatives; c design and assembly of polymerizable pseudorotaxane crosslinker and further applications of superior fatigue-resistant hydrogels and 3D printing fabrication of wearable flexible sensors. DLP 3D printer and PAm refer to the digital light processing (DLP) based 3D printer and polyacrylamide (PAm), respectively. It should be noted that CD-AC is a mixture of acrylate modified β-CD and the acrylate units on each CD-AC were averaged to be ca. 1.9.
A conductive PR-Gel system was designed and developed through the photopolymerization of a polymerizable pseudorotaxane crosslinker with acrylamide in the presence of choline chloride and in a binary solvent system. The mobile junctions between β-cyclodextrin and bile acid units on the topological networks endowed the PR-Gel with extraordinary fatigue resistance, high strain sensitivity, fast self-adhesion and self-healing capability, and low temperature tolerance. PR-Gel based wearable sensors were successfully exploited to detect human motions in different modes, such as bending of fingers and wrist, as well as subtle muscle movements of swallowing, and exhibited excellent sensitivity, wide strain range and outstanding cyclic stability. The marriage of DLP 3D printing technology with PR-Gel realized personalized and rapid preparation of flexible strain sensors with complex geometry and high resolution. The 3D printed PR-Gel based electrode could detect human electrocardiogram signals in real-time with high repeating stability. This work not only represents one example of PR topological networks in the applications of hydrogel based wearable sensors, but also provides a new paradigm for the design of 3D printable flexible devices. Considering the precise and simple preparation, and excellent comprehensive performance, it is believed that this study offers new opportunities for the development of 3D printed smart and flexible electronic devices with PR topological networks in the future.
Xiong, X., Chen, Y., Wang, Z. et al. Polymerizable rotaxane hydrogels for three-dimensional printing fabrication of wearable sensors. Nat Commun 14, 1331 (2023). https://doi.org/10.1038/s41467-023-36920-3