Chitosan-integrated asymmetric-adhesion Janus hydrogels via solid-gel interfacial engineering for continuous ultrasound Doppler monitoring.

Despite significant progress in Janus-type adhesion hydrogels, achieving programmable adhesion asymmetry through simple interfacial engineering remains a formidable challenge. Herein, we introduce a mold-directed solid - gel interface strategy to fabricate Janus polyacrylamide/chitosan hydrogels with special unilateral adhesion contrast based on the abundant reactive groups (-NH2, -OH) of chitosan and through hydrogen bonding or electrostatic interactions., exhibiting an 18-fold difference in adhesion strength between the two sides. The amine groups on the surface of the hydrogels are spatially regulated by engineering interfacial reactions between the precursor solutions and asymmetric mold materials, achieving complete group coordination interaction at the aluminum (Al) interface while preserving cationic groups at the polytetrafluoroethylene (PTFE) interface. This interfacial confinement strategy effectively prevents metal ion migration between surfaces, thereby overcoming the interlayer contamination limitations inherent to conventional coating methods. The optimized hydrogel demonstrates remarkable acoustic performance, with an impedance of 1.59 MRayl that is comparable to commercial ultrasound gels. Moreover, the asymmetric adhesion endows the hydrogel with dual functionality: the adhesion surface ensures stable probe coupling during prolonged monitoring, while the slippery interface allows for tissue-adaptable sliding, enhancing the signal-to-noise ratio compared to conventional gels in carotid artery Doppler measurements. This solid-gel interfacial engineering paradigm establishes a new pathway for designing medical adhesions with spatiotemporal property gradients, offering particular advantages for dynamic bio-interfaces that require simultaneous attachment and sliding functionalities.