Swelling studies of freeze-dried hydrogels were tested by a general gravimetric method. Samples were incubated at 37 °C in pH 7.4 PBS until weight is balanced. The swollen hydrogels were removed from the excess of water absorbed with a filter paper, and then weighed. The samples were then dried until constant weight. The swelling ratio was calculated using following Eq: Percentage of swelling (%) = (Wwet-Wdry)/Wdry × 100, where Wwet and Wdry refer to the weight of wet and dry hydrogels respectively.