Contributions of Re-entrant Honeycomb Lattice Geometry and Shear-Thickening Fluid Infill to Peak Deflection of an Artificial Joint Under Pneumatic Impact

Protective systems for joints, whether human or robotic, must balance impact shielding against natural range of motion. Materials stiff enough to resist high-impact forces usually also restrict normal movement. Shear-thickening fluids (STFs), which stiffen under rapid loading, and auxetic re-entrant honeycomb lattices, which redistribute compressive loads, both show promise for impact protection, but few studies have separated their individual contributions to joint deflection. This study tested a bare 3D-printed knee-like joint, empty 3D-printed TPU re-entrant honeycomb lattices (1.0 mm and 1.5 mm wall thicknesses), and the same lattices filled with a 54 wt% cornstarch–water STF at three pneumatic pressure settings (30, 45, and 60 PSI). We measured peak pitch deflection of the upper and lower limbs with inertial sensors over 12 impacts per condition. Empty lattices reduced peak deflection relative to the bare joint in every condition (by 70–84% in the upper limb and 49–73% in the lower limb). Filling the lattices with STF reduced deflection up to 20% further. This added reduction was statistically significant in all four combinations at 60 PSI but mostly not at lower pressures, and it did not consistently favor either wall thickness. STF-filled lattices thus merit further study for joint protection in athletic, robotic, and aerospace applications; confirming the mechanism will require multiple specimens, a mass-matched non-thickening fluid control, and direct force measurement.