The transfection efficiency when using albumin MBs of three different sizes. (a, a′) Fluorescence and bright-field images of living cells in the groups of 0.66 μm albumin MBs combined with plasmid, (b, b′) US combined with 0.66 μm albumin MBs and plasmid, (c, c′) 1.32 μm albumin MBs combined with plasmid, (d, d′) US combined with 1.32 μm albumin MBs and plasmid, (e, e′) 2.83 μm albumin MBs combined with plasmid, and (f, f′) US combined with 2.83 μm albumin MBs and plasmid. (g) Fluorescence intensities for MBs of three sizes quantified and normalized relative to the control group. Scale bar = 200 μm. Results are expressed as mean ± standard deviation with n = 5 for each bar. *indicates P < 0.05.