Leveraging 3D-printable PLGA composites with mineral fillers, biomaterial-based bone therapies become clinically relevant. Herein, long-term in vitro experiments were conducted to characterize degradability and evidence cytocompatibility.

Regeneration of large bone defects remains a clinical challenge until today. While existing biomaterials are predominantly addressing bone healing via direct, intramembranous ossification (IO), bone tissue formation via a cartilage phase, so-called endochondral ossification (EO) has been shown to be a promising alternative strategy. However, pure biomaterial approaches for EO induction are sparse and the knowledge how material components can have bioactive contribution to the required cartilage formation is limited. Here, we combined a previously developed purely architecture-driven biomaterial approach with the release of therapeutic metal ions from tailored silicate microparticles. The delivery platform was free of calcium phosphates (CaP) that are known to support IO but not EO and was employed for the release of lithium (Li), magnesium (Mg), strontium (Sr) or zinc (Zn) ions. We identified an ion-specific cellular response in which certain metal ions strongly enhanced cell recruitment into the material and showed superior chondrogenesis and deposition collagen II by human mesenchymal stromal cells (MSCs). At the same time, microparticle incorporation in some cases altered the mechanical properties of the biomaterial with consequences for cell-induced biomaterial contraction and scaffold wall deformation. Collectively, the results suggest that the incorporation of metal-doped silicate microparticles has the potential to further improve the bioactivity of architectured biomaterials for bone defect healing via EO. STATEMENT OF SIGNIFICANCE: Endochondral bone healing, a process that resembles embryonic skeletal development, has gained prominence in regenerative medicine. However, most therapeutic biomaterial strategies are not optimized for endochondral bone healing but instead target direct bone formation through intramembranous ossification. Here, we report on a novel approach to accelerate biomaterial-guided endochondral bone healing by combining cell-guiding collagen scaffolds with therapeutic metal-doped silicate microparticles. While other strategies, such as hypoxia-mimic drugs and iron-chelating biomaterials, have been documented in the literature before to enhance endochondral ossification, our approach uniquely implements enhanced bioactivity into a previously developed biomaterial strategy for bone defect regeneration. Enhanced cell recruitment into the material and more pronounced chondrogenesis were observed for specific hybrid scaffold formulations, suggesting a high relevance of this new biomaterial for improved endochondral bone healing.

A promising therapeutic option for the treatment of critical-size mandibular defects is the implantation of biodegradable, porous structures that are produced patient-specifically by using additive manufacturing techniques. In this work, degradable poly(DL-lactide) polymer (PDLLA) was blended with different mineral phases with the aim of buffering its acidic degradation products, which can cause inflammation and stimulate bone regeneration. Microparticles of CaCO₃, SrCO₃, tricalcium phosphates (α-TCP, β-TCP), or strontium-modified hydroxyapatite (SrHAp) were mixed with the polymer powder following processing the blends into scaffolds with the Arburg Plastic Freeforming 3D-printing method. An in vitro degradation study over 24 weeks revealed a buffer effect for all mineral phases, with the buffering capacity of CaCO₃ and SrCO₃ being the highest. Analysis of conductivity, swelling, microstructure, viscosity, and glass transition temperature evidenced that the mineral phases influence the degradation behavior of the scaffolds. Cytocompatibility of all polymer blends was proven in cell experiments with SaOS-2 cells. Patient-specific implants consisting of PDLLA + CaCO₃, which were tested in a pilot in vivo study in a segmental mandibular defect in minipigs, exhibited strong swelling. Based on these results, an in vitro swelling prediction model was developed that simulates the conditions of anisotropic swelling after implantation.