Polycaprolactone (PCL) fiber mats with different surface modifications were functionalized with a chitosan nanogel coating to attach the growth factor human bone morphogenetic protein 2 (BMP‐2). Three different hydrophilic surface modifications were compared with regard to the binding and in vitro release of BMP‐2. The type of surface modification and the specific surface area derived from the fiber thickness had an important influence on the degree of protein loading. Coating the PCL fibers with polydopamine resulted in the binding of the largest BMP‐2 quantity per surface area. However, most of the binding was irreversible over the investigated period of time, causing a low release in vitro. PCL fiber mats with a chitosan‐graft‐PCL coating and an additional alginate layer, as well as PCL fiber mats with an air plasma surface modification boundless BMP‐2, but the immobilized protein could almost completely be released. With polydopamine and plasma modifications as well as with unmodified PCL, high amounts of BMP‐2 could also be attached directly to the surface. Integration of BMP‐2 into the chitosan nanogel functionalization considerably increased binding on all hydrophilized surfaces and resulted in a sustained release with an initial burst release of BMP‐2 without detectable loss of bioactivity in vitro.

Bone morphogenetic protein 2 (BMP-2) has a high tendency to aggregate at physiological pH and physiological ionic strength, which can complicate the development of growth factor delivery systems. The aggregation behavior in differently concentrated BMP-2 solutions was investigated using dynamic and static light scattering. It was found that at higher concentrations larger aggregates are formed, whose size decreases again with increasing dilution. A solubilizing effect and therefore less aggregation was observed upon the addition of albumin. Imaged capillary isoelectric focusing and the simulation of the surface charges of BMP-2 were used to find a possible explanation for the unusually low solubility of BMP-2 at physiological pH. In addition to hydrophobic interactions, attractive electrostatic interactions might be decisive in the aggregation of BMP-2 due to the particular distribution of surface charges. These results help to better understand the solubility behavior of BMP-2 and thus support future pharmaceutical research and the development of new strategies for the augmentation of bone healing.

Chitosan nanogel-coated polycaprolactone (PCL) fiber mat-based implant prototypes with tailored release of bone morphogenic protein 2 (BMP-2) are a promising approach to achieve implant-mediated bone regeneration. In order to ensure reliable in vitro release results, the robustness of a commercially available ELISA for E. coli-derived BMP-2 and the parallel determination of BMP-2 recovery using a quantitative biological activity assay were investigated within a common release setup, with special reference to solubility and matrix effects. Without bovine serum albumin and Tween 20 as solubilizing additives to release media buffed at physiological pH, BMP-2 recoveries after release were notably reduced. In contrast, the addition of chitosan to release samples caused an excessive recovery. A possible explanation for these effects is the reversible aggregation tendency of BMP-2, which might be influenced by an interaction with chitosan. The interfering effects highlighted in this study are of great importance for bio-assay-based BMP-2 quantification, especially in the context of pharmaceutical release experiments.

Temporally and spatially controlled growth factor release from a polycaprolactone fiber mat, which also provides a matrix for directional cell colonization and infiltration, could be a promising regenerative approach for degenerated tendon−bone junctions. For this purpose, polycaprolactone fiber mats were coated with tailored chitosan-based nanogels to bind and release the growth factors bone morphogenetic protein 2 (BMP-2) and transforming growth factor-β 3 (TGF-β 3 ), respectively. In this work we provide meaningful in vitro data for the understanding of the drug delivery performance and sterilizability of novel implant prototypes in order to lay the foundation for in vivo testing. ELISA-based in vitro release studies were used to investigate the spatial and temporal control of release, as well as the influence of radiation sterilization on protein activity and release behavior. Layer-by-layer coatings based on BMP-2-containing chitosan tripolyphosphate nanogel particles and negatively charged alginate showed a good sustainment of BMP-2 release from chemically modified polycaprolactone fiber mats. Release control improved with increasing layer numbers. The approach of controlling the release via a barrier of cross-linked chitosan azide proved less promising. By using a simple, partial immersion-based dip-coating process, it was possible to apply opposing gradients of the growth factors BMP-2 and TGF-β 3 . Final radiation sterilization of the growth factor-loaded implant prototypes resulted in a radiation dose-correlated degradation of the growth factors, which could be prevented by lyophilization into protective matrices. For the manufacture of sterile implants, the growth factor loading step must probably be carried out under aseptic conditions. The layer-by-layer coated implant prototypes provided sustained release from opposing gradients of the growth factors BMP-2 and TGF-β 3 and thus represent a promising approach for the restoration of tendon−bone defects. KEYWORDS: bone morphogenetic protein 2 (BMP-2), transforming growth factor-β 3 (TGF-β 3 ), controlled release, polyelectrolyte coating, opposing growth factor gradients Special Issue: Bioconjugates