Microstructural control of modular peptide release from microporous biphasic calcium phosphate

Strategies to Control or Mimic Growth Factor Activity for Bone, Cartilage, and Osteochondral Tissue Engineering

Growth factors play a critical role in tissue repair and regeneration. However, their clinical success is limited by their low stability, short half-life, and rapid diffusion from the delivery site. Supraphysiological growth factor concentrations are often required to demonstrate efficacy but can lead to adverse reactions, such as inflammatory complications and increased cancer risk. These issues have motivated the development of delivery systems that enable sustained release and controlled presentation of growth factors. This review specifically focuses on bioconjugation strategies to enhance growth factor activity for bone, cartilage, and osteochondral applications. We describe approaches to localize growth factors using noncovalent and covalent methods, bind growth factors via peptides, and mimic growth factor function with mimetic peptide sequences. We also discuss emerging and future directions to control spatiotemporal growth factor delivery to improve functional tissue repair and regeneration.

The role of calcium phosphate surface structure in osteogenesis and the mechanisms involved

Calcium phosphate (CaP) ceramics have been widely used for bone regeneration because of their ability to induce osteogenesis. Surface properties, including chemical composition and surface structure, are known to play a crucial role in osteoconduction and osteoinduction. This review systematically analyzes the effects of surface properties, in particular the surface structure, of CaP scaffolds on cell behavior and new bone formation. We also summarize the possible signaling pathways involved in the osteogenic differentiation of bone-related cells when cultured on surfaces with various structures in vitro. The significant immune response initiated by surface structure involved in osteogenic differentiation of cells is also discussed in this review. Taken together, the new biological principle for advanced biomaterials is not only to directly stimulate osteogenic differentiation of bone-related cells but also to modulate the immune response in vivo. Although the reaction mechanism responsible for bone formation induced by CaP surface structure is not clear yet, the insights on surface structure-mediated osteogenic differentiation and osteoimmunomodulation could aid the optimization of CaP-based biomaterials for bone regeneration. STATEMENT OF SIGNIFICANCE: CaP ceramics have similar inorganic composition with natural bone, which have been widely used for bone tissue scaffolds. CaP themselves are not osteoinductive; however, osteoinductive properties could be introduced to CaP materials by surface engineering. This paper systematically summarizes the effects of surface properties, especially surface structure, of CaP scaffolds on bone formation. Additionally, increasing evidence has proved that the bone healing process is not only affected by the osteogenic differentiation of bone-related cells, but also relevant to the the cooperation of immune system. Thus, we further review the possible signaling pathways involved in the osteogenic differentiation and immune response of cells cultured on scaffold surface. These insights into surface structure-mediated osteogenic differentiation and osteoimmunomodulated-based strategy could aid the optimization of CaP-based biomaterials.

Mesoporous Hydroxyapatite Nanoparticles Mediate the Release and Bioactivity of BMP-2 for Enhanced Bone Regeneration

Efficient delivery of bone morphogenetic protein-2 (BMP-2) with desirable bioactivity is still a great challenge in the field of bone regeneration. In this study, a silk fibroin/chitosan scaffold incorporated with BMP-2-loaded mesoporous hydroxyapatite nanoparticles (mHANPs) was prepared (SCH-L). BMP-2 was preloaded onto mHANPs with a high surface area before mixing with a silk fibroin/chitosan composite. Bare (without BMP-2) silk fibroin/chitosan/mHANP (SCH) scaffolds and SCH scaffolds with directly absorbed BMP-2 (SCH-D) were investigated in parallel for comparison. In vitro release kinetics indicated that BMP-2 released from the SCH-L scaffold showed a significantly lower initial burst release, followed by a more sustained release over time than the SCH-D scaffold. In vitro cell viability, osteogenic differentiation of bone marrow mesenchymal stem cells (BMSCs), and the in vivo osteogenic effect of scaffolds in a rat calvarial defect were evaluated. The results showed that compared with bare SCH and SCH-D scaffolds, the SCH-L scaffold significantly promoted the osteogenic differentiation of BMSCs in vitro and induced more pronounced bone formation in vivo. Further studies demonstrated that the mHANP-mediated satisfactory conformational change and sustained release benefited the protection of the released BMP-2 bioactivity, as confirmed by alkaline phosphatase (ALP) activity and a mineralization deposition assay. More importantly, the interaction of BMP-2/mHANPs enhanced the binding ability of BMP-2 to cellular receptors, thereby maintaining its biological activity in osteogenic differentiation and osteoinductivity well, which contributed to the markedly promoted in vitro and in vivo osteogenic efficacy of the SCH-L scaffold. Taken together, these results provide strong evidence that mHANPs represent an attractive carrier for binding BMP-2 to scaffolds. The SCH-L scaffold shows promising potential for bone tissue regeneration applications.

Mineralization in micropores of calcium phosphate scaffolds

With the increasing demand for novel bone repair solutions that overcome the drawbacks of current grafting techniques, the design of artificial bone scaffolds is a central focus in bone regeneration research. Calcium phosphate scaffolds are interesting given their compositional similarity with bone mineral. The majority of studies focus on bone growth in the macropores (>100 µm) of implanted calcium phosphate scaffolds where bone structures such as osteons and trabeculae can form. However, a growing body of research shows that micropores (<50 µm) play an important role not only in improving bone growth in the macropores, but also in providing additional space for bone growth. Bone growth in the micropores of calcium phosphate scaffolds offers major mechanical advantages as it improves the mechanical properties of the otherwise brittle materials, further stabilizes the implant, improves load transfer, and generally enhances osteointegration. In this paper, we review evidence in the literature of bone growth into micropores, emphasizing on identification techniques and conditions under which bone components are observed in the micropores. We also review theories on mineralization and propose mechanisms, mediated by cells or not, by which mineralization may occur in the confined micropore space of calcium phosphate scaffolds. Understanding and validating these mechanisms will allow to better control and enhance mineralization in micropores to improve the design and efficiency of bone implants. STATEMENT OF SIGNIFICANCE: The design of synthetic bone scaffolds remains a major focus for engineering solutions to repair damaged and diseased bone. Most studies focus on the design of and growth in macropores (>100 µm), however research increasingly shows the importance of microporosity (<50 µm). Micropores provide an additional space for bone growth, which provides multiple mechanical advantages to the scaffold/bone composite. Here, we review evidence of bone growth into micropores in calcium phosphate scaffolds and conditions under which growth occurs in micropores, and we propose mechanisms that enable or facilitate growth in these pores. Understanding these mechanisms will allow researchers to exploit them and improve the design and efficiency of bone implants.