Live/dead staining
A two-colour fluorescence assay (LIVE/DEAD Assay; Molecular Probes, consisting of calcein as a marker of viable cells and ethidium homodimer as a marker of dead cells; Invitrogen) was employed to determine the cell viability in the constructs. Each sample was washed in PBS 3 times before staining. The samples were stained for 15 min in the dark and washed 3 times in PBS after staining. A confocal microscope (Xcellence; Olympus) was used for image acquisition. Three different fields were counted for each sample. Cell viability was calculated as (number of green stained cells/number of total cells) ×100%.