Sonication
This is a simple method to produce silk gels. Since there is a lag time between the sonication and the onset of gelation, this method is also useful when cells are to be encapsulated in the gel. Once again, to ensure a proper time window for cell seeding, sonication conditions should be optimized first without cells. The silk concentration can be varied from 1 to 20 w/v% and sonication time can be varied from 5 to 30 s at 10-20% amplitude. Cells can be added after the sonication step and prior to the gelation, to provide homogeneous and effective encapsulation in the gel. As an example, we have encapsulated cells into sonicated gels by autoclaving 4 w/v% silk and sonicating 5 ml of it using the methods below. Separately, human mesenchymal stem cells were harvested and brought up to a concentration of 50 × 106 cells/ml. Fifty microliters of the cell suspension was added to the sonicated silk and then incubated at 37 °C and 5% CO2 for 2 hours prior to adding cell culture medium.