Influence of cross-linking in loading/release applications of polyelectrolyte multilayer assemblies. A review

Design of polymeric matrices for loading/release purposes is of great interest in various applications, such as drug delivery systems, antimicrobial surfaces, biosensors, water purification. Compared with other strategies to fabricate materials for such applications, the Layer-by-Layer (LbL) assembly remarked itself by the countless possibilities to tailor the organic architectures at nanoscale owing to the structural diversity of "nano-bricks" suitable for assembly and easiness to control the deposition features. LbL assembled systems have been extensively used as matrices to load/release low molecular compounds such as drugs and dyes, proteins and enzymes, or DNA (RNA). In many studies, cross-linking the layers was investigated as a mean to stabilize and to induce new functionalities in the obtained architectures, as well as to tune their loading/release properties. In this review we discuss recent progress in the use of LbL constructions in loading/release of bioactive species, with a main focus on the role of cross-linking on such features. Overviews of the LbL assembly strategy describing the parameters which influence the build-up process and of the main synthetic routes used to cross-link the obtained architectures are briefly presented. The use of LbL systems (either as thin films deposited on solid surfaces or as hollow capsules) to load/release low molecular compounds and proteins/enzymes, highlighting the role of cross-linking in such processes (construction of porous architectures capable to load high molecular compounds or decreasing the assemblies permeability to delay the release of encapsulated compounds) was thoroughly discussed.