Development of a biomimetic hydrogel cervical spinal cord surrogate for transverse compression across loading rates

Abstract Study Design: Experimental laboratory study; fully factorial hydrogel formulation design with mechanical benchmarking against published spinal cord tissue data. Objectives To develop and characterise polyacrylamide-alginate (PAAm-alginate) double-network hydrogels as surrogates for the transverse compressive response of cervical spinal cord tissue, and to identify the formulation variables governing surrogate stiffness. Setting: University biomechanics laboratory, Adelaide, Australia. Methods Twenty-seven formulations combined three alginate concentrations (4.7, 7.8, 10.3% w/v), three ionic crosslinkers (CaCl₂, FeCl₃, AlCl₃) and three immersion durations (1, 2, 3 h). 313 elliptical specimens were tested in transverse compression at four displacement rates (~ 0.01 to 140 s⁻¹). Characteristic curves were generated by arc-length re-parameterisation, compared by statistical parametric mapping, and benchmarked against published human and porcine cervical cord data. Results All formulations showed J-shaped, rate-dependent viscoelastic responses. Crosslinking ion valency had the greatest influence, trivalent species being approximately five times stiffer than divalent CaCl₂; alginate concentration had a moderate, diminishing effect and immersion duration had little effect beyond 2 h. CaCl₂ formulations matched the benchmarks most closely at every rate; at ~ 50 s⁻¹, CaCl₂ / 10.3% / pooled 2 h + 3 h matched porcine cervical cord in tangent modulus (~ 1180 kPa) and stress at benchmark failure strain (~ 490 kPa). Conclusions PAAm-alginate double-network hydrogels reproduce the nonlinear, rate-dependent transverse compressive behaviour of cervical spinal cord tissue and can be tuned to published benchmarks, most closely under dynamic loading. They provide a basis for instrumentable cord surrogates for ex vivo injury biomechanics and surgical simulation. Sponsorship: University of Adelaide Medical School Development Grant.