Where \(\tau\) is the shear stress, \(\tau_{\text{HB}}\) is the shear yield stress, \(\eta_{\text{HB}}\) is the consistency, \(\gamma\) is the shear rate and \(p\) is the flow index (n<1 for shear thinning fluids). Theoretically, Herschel-Bulkley fluid has the ability to hold the shape owing to the shear yield stress, which is suitable for extrusion-based 3D printing method. However, 3D printing test with pure GO hydrogel failed with severe collapse owing to the low mechanical strength (data not shown).
Considering the ion crosslinking theory mentioned previously, we chose ferric ion as the crosslinker, owing to its low toxicity and high charge number. In order to find the optimal concentration, we tested the rheology and printability of GO with Fe3+ over the concentration range of 0–8 mmol/L, since the solubility of Fe3+ is 8 mmol/L. According to the result in fig. 1d, the GO hydrogel possessed higher elastic modulus and high shear yield stress at increased concentrations. Compared to that of pure GO hydrogel, both storage modulus and loss modulus of Fe3+-modified hydrogel increased by 1.7 orders of magnitude (fig. 1e). Printability was evaluated according to equation (3), developed by Ouyang et al. (Ouyang, Yao, Zhao, & Sun, 2016).