Use of catechol modified polymers in Layer by Layer assembly to enhance its stability and sustained release of biomolecules: A bio inspired approach
Although Layer-by-Layer (LbL) assembly technique has been successfully used in various areas of nanotechnology and biotechnology, some LbL-assembled nanostructures have suffered from lack of stability when they are exposed to any changes in aqueous environments (e.g., pH and ionic strength). In addition, this technique has been limited because of the diffusion of polyelectrolytes throughout the film during the deposition process, resulting in poorly organized architectures, thus limiting the control of release characteristics. To improve the stability of LbL-assembled nanostructures, and to control interlayer diffusion, we have developed a strategy to conjugate catechol groups, which are largely present in mussel adhesive proteins, to conventional polyelectrolytes such as branched poly(ethyleneimine) (BPEI) and poly(acrylic acid) (PAA). Only a fraction of amine or acid groups were modified with catechol using this synthetic approach, thereby preserving the original cationic or anionic nature of the polymers for use in LbL assembly while integrating beneficial adhesive features of catechol groups into LbL films. The structure, physico-chemical properties, and stability of LbL films composing BPEI and PAA without and with catechol modifications were compared. The incorporation of catechol groups led to a significant change of the LbL growth curve from exponential to a clearly linear increase of thickness with number of layers, reflecting the nondiffusing characteristic of the catechol-modified polyelectrolytes. When LbL films were exposed to PBS pH 7.4, we observed that the catechol containing LbL films underwent far fewer changes in the degree of ionization and film thickness and exhibited stronger mechanical properties, indicative of their enhanced film stability. When LbL films with catechol modifications were used as physical barrier layers to prevent the undesired initial burst release of radiolabeled 14C-dextran sulfate sodium salt (14C-DS), a clear sustained release was achieved over a period of 40 days. When the bottom layer containing 14C-DS is separated from radiolabeled 3H-heparin sodium salt (3H-HS) by catechol modified polyelectrolytes, we observed two different release rates composing an abrupt release from the surface 3H-HS, together with a sustained release from the underlying 14C-DS. Overall, we conclude that when catechol groups are present in LbL films, linear film growth, enhanced film stability, and extended release times are attributed to the introduction of non-ionic types of interactions such as hydrogen bonding and Π-Π stacking, as well as the formation of covalent diphenyl ether bonds, thus enable these films to provide a general platform for the systematic incorporation and assembly of biological therapeutics into controlled release films at mild conditions for drug delivery.