The formation and architecture of surface-initiated polymer brush gene delivery complexes

Understanding the architecture and mechanism of assembly of polyelectrolyte-nucleic acid complexes is critical to the rational design of their performance for gene delivery. Surface-initiated polymer brushes were recently found to be particularly effective at delivering oligonucleotides and maintaining high knock down efficiencies for prolonged periods of time, in highly proliferative cells. However, what distinguishes their binding capacity for oligonucleotides from that of larger therapeutic macromolecules remains unknown. In this report, we characterise the binding capacity and adsorption kinetics of different types of nucleic acid materials for gene delivery (single and double stranded oligo RNA and DNA, mRNA and plasmid DNA) to PDMAEMA and PMETAC brushes, using surface plasmon resonance. The type and size of these nucleic acid macromolecules are found to have an important impact on their maximum surface density, and the association and adsorption constants of the resulting complexes. To gain further insight into the mechanisms that restrict the adsorption of higher molecular weight materials, and promote particularly effective RNA capture, the architecture of PDMAEMA brushes prior and after complexation is investigated by in situ ellipsometry and neutron reflectometry. Deep infiltration of oligonucleotides was found, irrespective of their binding capacity, suggesting that their infiltration is not a limiting factor in their dense capture on polymer brushes. In contrast, mRNA and pDNA were found to partially infiltrate within PDMAEMA brushes, although some of the nucleic acid materials could be found deep into the brush layer. This indicates that the size of these macromolecules and their partial infiltration may restrict further adsorption and high binding capacities, but also suggests that oligonucleotides will experience enhanced protection within polymer brushes, with fewer residues accessible for enzymatic degradation.