A designable twist-densification route to bioactive collagen hydrogel yarns approaching tendon-like mechanics

Reconstituted collagen hydrogels offer the bioactivity that load-bearing tissue engineering requires, but their Pascal to low-kPa moduli have confined them to non-structural roles, and the synthetic and hybrid strategies that close the mechanical gap typically forfeit that bioactivity. We show that a twist-induced densification process resolves this tradeoff in a controllable way, converting soft collagen hydrogel fibers into superhelical hydrogel yarns whose mechanics can be prescribed from fabrication parameters. A parameter-free model drawn from fiber-network mechanics predicts the modulus enhancement from densification, fibril alignment, and helical fiber architecture, and the same surface helix angle independently predicts the nonlinear strain-stiffening response. The resulting yarns exhibit modulus, strength, and toughness approaching the lower range reported for native tendons, representing enhancements of two to three orders of magnitude over the as-fabricated collagen hydrogels. Importantly, they remain amenable to braiding, knitting, and weaving into two- and three-dimensional constructs, including tubular architectures that recover elastically under repeated compression. Short-term cytocompatibility is preserved despite the severe compaction: encapsulated fibroblasts retain over 90% viability, exhibit pronounced alignment within the yarns, and transduce externally applied strain. Twist densification thus provides a designable route to living, load-bearing protein textiles.