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).