**5. Optimization of physico-chemical properties of hydrogels**

The hydrogels had been prepared using different ratios of precursors and crosslinkers. The prepared hydrogels had been optimized on the basis of biocompatibility, biodegradability, swelling ratio, mechanical strength, and pore size. The pore-size of hydrogels could be improved by using different inorganic or organic pore-forming agents. The chain length and the molecular weight of organic pore-forming agents such as polyethylene glycol 2000, carbamide, and polyvinyl pyrrolidone had an important influence on the pore size, the compressive strength, and the swelling ratio. However, among inorganic pore-forming agents such as NaCl, CaCO3, and NaHCO3, hydrogels with NaHCO3 displayed a great performance in terms of the pore size of the hydrogels, mechanical properties, and drug release. Moreover, the pore-forming agents had little

influence on the thermal stability of the hydrogels. The strength of hydrogels could also be enhanced by the formation of coordination bonds between Zn2+ and anionic groups such as –COO− when hydrogel is immersed in the solution of ZnCl2 [20].
