Replacing extraocular muscles with anisotropic, strain-stiffening hydrogels

Strabismus surgeries rely on weakening or tightening extraocular muscles. While effective for alignment, these procedures disrupt the length-tension relationships. Recession induces slack, causing delayed force transmission; resection induces stretch, increasing passive drag. Neither technique can augment a muscle that has lost function due to paralysis or atrophy. An implant must replace the passive constraint of the natural muscle. Here, we report a hydrogel implant that replicates the passive mechanics of extraocular muscles. We fabricate the hydrogel through a four-step hierarchy: directional freezing for alignment, salting-out for density, mechanical training for the J-shaped stress-strain curve, and chemical crosslinking for stability. The hydrogel reproduces native tissue's J-shaped stress-strain response and resists fatigue. In a rabbit model of lateral rectus deficiency, the implant restores physiological rotation ranges (∼25°) and saccade velocities, avoiding pathological over-rotation after muscle removal. This work establishes passive muscle replacement as a therapeutic strategy, restoring alignment while preserving physiological dynamics.