Veroblack

All the uniaxial tensile/compressive tests were conducted on an MTS uniaxial testing machine (Criterion model 43, MN, USA). For the comparison of the stretch-stress behavior between the AP hydrogels initiated by TPO and APS-TEMED (Fig. 3C), we prepared the hydrogel samples consisting of 80 wt % water and 20% AP mixture with the PEGDA (700)/acrylamide mixing ratio of 1.25 wt %. For the bonding demonstrations and tests (Fig. 3, D to Q), we prepared the hydrogel consisting of 80 wt % water and 20% AP mixture with the PEGDA (700)/acrylamide mixing ratio of 0.625 wt %. For the uniaxial tensile test of the horseshoe structure–reinforced hydrogel composite (Fig. 4B), we printed the matrix with the hydrogel consisting of 80 wt % water and 20% AP mixture with the PEGDA (700)/acrylamide mixing ratio of 0.625 wt % and printed the horseshoe structure with VeroBlack from Stratasys Ltd. (Eden Prairie, MN, USA). For the uniaxial compressive tests for the lattice-reinforced hydrogel cubes (Fig. 4, D to F), we printed the matrix with the hydrogel with the same recipe as the one used above and printed the lattice structures with VeroClear from Stratasys Ltd. (Eden Prairie, MN, USA) added with 0.05% Sudan I (Sigma-Aldrich) to improve the printing resolution. The same solutions were used to print lattice-reinforced meniscus (Fig. 4, G to K).