4.2. Rheological properties of carrageenan composite hydrogel
The rheological study was performed for composite hydrogel with alginate concentrations of 0.5%, 1.0%, and 1.5%, and with MCNT concentrations of 0.01%, 0.03%, 0.05%, and 1.5%, respectively. A rheological study was performed to evaluate the effects on the viscoelastic properties and the printability of carrageenan composite hydrogel. As shown in Figure 3, all the samples demonstrated shear-thinning properties, which indicated that bioinks with low viscosity avoid blocking nozzle during printing and cell vitality during cell-laden printing. Shear thinning of composite hydrogel was caused by the destruction of ion cross-linking and rearrangement of the molecular chain by shearing. Moreover, high initial viscosity makes the hydrogel undergo high shear forces during the printing process, which affects the ultimate printability and vitality of the cells. Therefore, we introduced alginate and MCNTs; the initial viscosity of the carrageenan composite hydrogel was decreased with increasing concentrations of alginate and MCNTs, while the shear-thinning properties remained unchanged. The result demonstrated that the introduction of alginate and MCNTs can improve printability and vitality of cells as well as the cytocompatibility of kc-based bioink.
As shown in Figure 4, the initial modulus of kc and composite hydrogel with sa increased slightly. However, the modulus of the composite hydrogel with MCNT and sa increased obviously with the shear frequency. Lower initial modulus endows better survival of cells in the microenvironment. High modulus in a high frequency of hydrogel could avoid the structure disruption and supports the cell well integrity.
The storage modulus (G’) of kc was higher than the loss modulus (G”) ,gel point(G’= G”) was not observed in a range of the test. On the other hand, the gel point was observed in kc-sa and kc-sa-c in the range of measured frequency, which was within the range of shear frequency of the printing nozzle. The results cumulatively implied that kc composite hydrogel was liquid during the process of printing, which ensures that the cells loaded underwent one lower shear force.