## 3 Results and Discussion
### Mtt  
The cell survival rates at 0 h, 2 h, 24 h, and two months after encapsulation in the hydrogels are presented in Figure 4. Specifically, the cell viability right after the injection process was measured and set as the baseline for the comparison with those measured at different time periods, which is shown as a percentage thereof.  
For the SCL hydrogels, data from 2 h after encapsulation show that cell viability decreases with an increasing alginate concentration. Specifically, the 2% SCL hydrogels had the lowest cell viability and the 0.8% SCL hydrogels had the highest. This suggests that cell viability during the injection process varies with hydrogel viscosity, with more cells surviving in the lower viscosity hydrogels. After 24 h, living cell rates in all of the groups increased, compared to those at 2 h, particularly the 1.5% and 2% SCL hydrogels, which had a higher cell viability than the other two groups. The increased number of living cells indicates that cells proliferated after being encapsulated in the hydrogels, and the low cell viability in lower viscosity hydrogels indicates that the cells survived better in the higher viscosity hydrogels. After two months, cell numbers decreased compared to those at 2 h and 24 h, and 0.8% SCL hydrogels had the lowest living cells rate out of all of the groups. This indicates that living cell rates decreased with the degradation of hydrogels, due to a lack of protection.  
For the Ca\({}^{2+}\) hydrogels, cell viability was not significantly different across the four groups after 2 h, and all viability rates were less than those for the SCL hydrogels. This indicates a lower retention rate of living cells in Ca\({}^{2+}\) hydrogels, which have a higher viscosity than the SCL hydrogels, during the cell encapsulation process. After 24 h, the cell viability sharply increased compared to the baseline, especially for the 1.1% Ca\({}^{2+}\) hydrogel, which had the highest cell viability. As such, cells seem to prefer hydrogels with a higher mechanical strength, in order to survive and proliferate. After two months, cell viability rates in the Ca\({}^{2+}\) hydrogels still demonstrated an increasing trend, when compared to both the baseline and 24 h values. In particular, the 1.1% Ca\({}^{2+}\) hydrogel had the highest living cell rate among the four groups. This suggests that cells were effectively regenerated during extended cultivation in hydrogels and proliferated the most in the high mechanical strength 1.1% Ca\({}^{2+}\) hydrogels. After two months, the living cell rates for all Ca\({}^{2+}\) hydrogels were much higher than those for SCL hydrogels. This reflects the fact that Ca\({}^{2+}\) hydrogels have a slower degradation process than SCL hydrogels and that cells are still protected in the hydrogels after two months. We speculate that, during these two months, the cells proliferated and interconnected to each other, forming a network structure similar to the ECM that provides protection for cells in in vivo conditions.  
The mechanical properties of hydrogels are important factors affecting the cell viability, as discussed above. It should be noted that, in addition to the mechanical properties, many other factors, such as the presence of residual reagents, pore density, and apoptotic potential of gelatin fragments, may also contribute to the cell viability.