Tailored Polyelectrolyte Multilayer Systems by Variation of Polyelectrolyte Composition and EDC/NHS Cross-Linking: Physicochemical Characterization and In Vitro Evaluation

Abstract

The layer-by-layer (LbL) self-assembly technique is an effective method to immobilize components of the extracellular matrix (ECM) such as collagen and heparin onto, e.g., implant surfaces/medical devices with the aim of forming polyelectrolyte multilayers (PEMs). Increasing evidence even suggests that cross-linking influences the physicochemical character of PEM films since mechanical cues inherent to the substrate may be as important as its chemical nature to influence the cellular behavior. In this study, for the first-time different collagen/heparin films have been prepared and cross-linked with EDC/NHS chemistry. Quartz crystal microbalance, zeta potential analyzer, diffuse reflectance Fourier transform infrared spectroscopy, atomic force microscopy and ellipsometry were used to characterize film growth, stiffness, and topography of different film systems. The analysis of all data proves a nearly linear film growth for all PEM systems, the efficacy of cross-linking and the corresponding changes in the film rigidity after cross-linking and an appropriate surface topography. Furthermore, preliminary cell culture experiments illustrated those cellular processes correlate roughly with the quantity of newly created covalent amide bonds. This allows a precise adjustment of the physicochemical properties of the selected film architecture regarding the desired application and target cells. It could be shown that collagen improves the biocompatibility of heparin containing PEMs and due to their ECM-analogue nature both molecules are ideal candidates intended to be used for any biomedical application with a certain preference to improve the performance of bone implants or bone augmentation strategies.

Tailored Polyelectrolyte Multilayer Systems by Variation of Polyelectrolyte Composition and EDC/NHS Cross-Linking: Controlled Drug Release vs. Drug Reservoir Capabilities and Cellular Response for Improved Osseointegration

“…As shown in Figure 4 A, the coating of PLL–HEP initially inhibits the cell viability at early time points. This effect can be explained by the reduced initial adhesion and proliferation of the cells on the relatively soft multilayers, as we have shown recently [ 36 ].…”

“…Probably, this effect could be explained by the higher layer thickness of the coating. As we have shown elsewhere, the substitution of PLL by collagen results in a sudden increase in the coating thickness, which is a result of an incomplete charge compensation by the weak poly-cation collagen [ 36 ].…”

“…Obviously, drug delivery can also be targeted by a defined variation of the degree of cross-linking. A higher cross-linking of the multilayer films initially leads to a higher stiffness of the coatings and causes a significant inhibition of the release of BMP-2 [ 36 ]. This observation is particularly true for the PEM3 multilayer, as could be shown by the near zero order release kinetic for higher degrees of cross-linking ( Figure 3 C).…”

“…In both cases, with or without BMP-2 loading, an obvious correlation between the cross-linking and the ARS assay is evident. This effect might have mechanical reasons, as the cross-linked coatings show a drastic increased layer stiffness [ 36 ]. Nevertheless, it suggests that the immobilized rhBMP-2 cytokine plays a major key role concerning the observed differentiation behavior.…”

Self-Cross-Linkable Chitosan–Alginate Complexes Inspired by Mussel Glue Chemistry

“…The stability of Chi–Alg complexes in response to pH and salinity of the environment is of importance in many applications. − In the complexed state, these oppositely charged weak biopolyelectrolytes are held together mainly by electrostatic interactions; hence, variations in pH and salinity perturb the charge balance and produce swelling/dissociation . This inherent feature has been used to develop pH-responsive materials, e.g., vesicles releasing the encapsulated agent. − In many other systems though, one requires the complex to be stable against variations in pH and salt. − Herein, various chemical modifications are suggested to covalently cross-link the biopolymers by postmodification of the complex through EDC/NHS coupling or using a cross-linker such as glutaraldehyde . Such methods may not always be favorable considering the need for an extra postmodification step as well as safety concerns over using volatile toxic reagents.…”