Transfection of HUVECs was evaluated through the reduction in CD31 expression, quantified by fluorescence microscopy, compared to non-transfected controls. HUVECs were seeded at a density of 50,000 cell/well on glass cover slips pre-treated with collagen in 24 well plates, 24 h prior to the transfection assay. The PDMAEMA-coated nanoparticles used for transfection (30 nm PDMAEMA brush, 330 nm silica core) were prepared according to protocols previously reported . A final siRNA concentration of 50 nM/well was used for all transfection assays. 100 μL Si-PDMAEMA/PECAM1_1 siRNA complexes were prepared at N/P=10 in OPTI-MEM, PBS or RNAse free water. The RNA is firstly dissolved in RNAse free water and transferred to the particle suspension in RNAse free water. After removing the EGM2 medium, cells were washed twice with pre-warmed serum free OPTI-MEM medium and another 400 μL was added. 100 μL siRNA complex was then added dropwise to each well and mixed by shaking gently. Cells were incubated with siRNA complexes for 4 h in the incubator and the medium was then replaced by 500 μL EGM2 medium for a further 24 h of incubation. Lipofectamine® 2000 complexed with PECAM1_1 siRNA (protocol according to the manufacturer's instruction with a final siRNA concentration of 50 nM/well) was used as a positive control.