Rotary extrusion printing of strong and tough multi-material hydrogel fibers with double-helix architectures

Hydrogel fibers, characterized by the high-water content and remarkable flexibility, demonstrate significant potential for biomedical and flexible electronic applications, yet the inherent challenge of simultaneously enhancing both mechanical strength and toughness remains a critical issue. To address the issue, this study introduces a novel multi-material fiber featuring a double-helix core architecture, designed to achieve synergistic enhancement of mechanical strength and fracture toughness. By developing a pneumatic rotary extrusion system, we achieved precise control over fiber morphology by optimizing rotational speed (ω = 5 r/min), extrusion pressure ratio ( P ratio = 1.4), and gelatin concentration (8 g, 28.5 wt%). The resultant fibers exhibited an exceptional fracture energy of 70.22 ± 3.85 kJ/m 2 (6.8 times higher than pure hydrogel) and a toughness of 10.37 ± 0.69 MJ/m 3 , along with a tissue-matched (cartilage and ligaments) elastic modulus (16.6 MPa). NaCl/glycerol infiltration endows the fiber with ionic conductivity (strain sensitivity GF = 0.12, response time = 608 ms), enabling real-time monitoring of human joint kinematics through strain-responsive electrical signal transduction. Furthermore, doping Fe₃O₄ into PDMS allows the fiber to be programmable magnetically navigated with the superparamagnetic responsiveness of 15.59 emu/g. This work establishes a scalable strategy for designing high-performance multi-material hydrogel fibers, bridging the gap between mechanical robustness, smart responsiveness, and biomedical applicability, with transformative potential in surgical robotics and wearable electronics.