Carbon nanotube–reinforced hyaluronic acid dispersions as a rheological model for synovial fluids

Biohybrid materials that integrate a biopolymer matrix with a nanoscale reinforcing network offer a promising strategy for developing synthetic fluids with tissue-mimetic mechanical functionality. Here, an in vitro model synovial fluid was engineered by incorporating ultralow concentrations of multi-walled carbon nanotubes (MWCNTs) into a high-molecular-weight hyaluronic acid (HA) matrix to reproduce the frequency- and temperature-dependent viscoelasticity of healthy synovial fluid. The optimized HA matrix reproduced key features of native fluid, including pronounced shear thinning and a viscous-to-elastic transition within the frequency range of human locomotion. MWCNT incorporation at concentrations below 0.05 vol% substantially broadened the linear viscoelastic region and introduced reversible thixotropic behavior. At physiological temperature, enhanced CNT network formation combined with thermal softening of the HA matrix increased the elastic contribution and shifted the G ′– G ″ crossover toward the frequency of normal walking. An optimal formulation emerged that combined broad strain tolerance, physiologically relevant viscous–elastic switching, non-monotonic intermediate-shear viscosity, and reversible structural recovery while maintaining low viscosity (η ≈ 20–50 mPa s) at high-shear rate ( γ ˙ ∼ 10 3 s –1 ). These findings establish ultralow CNT reinforcement as a rheological design principle for engineering temperature-responsive HA-based fluids that reproduce, in vitro , the adaptive rheological behavior of native synovial fluid prior to further tribological and biocompatibility evaluations.