Multi-doped apatite: Strontium, magnesium, gallium and zinc ions synergistically affect osteogenic stimulation in human mesenchymal cells important for bone tissue engineering

Functional calcium phosphate biomaterials can be designed as carriers of a balanced mixture of biologically relevant ions able to target critical processes in bone regeneration. They hold the potential to use mechanisms very similar to growth factors naturally produced during fracture healing, while circumventing some of their drawbacks. Here we present a novel phase of carbonated-apatite containing Mg²⁺, Sr²⁺, Zn²⁺ and Ga³⁺ ions (HApMgSrZnGa). While all dopants decrease the crystallinity, Ga³⁺ limits crystal growth and enables the formation of a nanosized apatite phase with enhanced specific surface area. Coexistence of the ions enhances degradability and controls solubility of low crystalline, distorted, multi-doped apatite structure, controlled by Ga³⁺ ions accumulated at the surface. Consequently, HApMgSrZnGa supports the viability of human mesenchymal stromal cells (MSCs) and induces their stimulation along the osteogenic lineage. In addition, the co-released ions has a synergistic antimicrobial effect, particularly within the HApMgSrZnGa-Au(arg) composite with Au(arg) as contact-based antimicrobial. The activity is stable up to two months in vitro. Osteogenic nature and antimicrobial activity, combined in a single biomaterial, are suggesting a well-balanced, multi-doped apatite design applicable as future option in bone regeneration and tissue engineering.

Beta-phase Stabilization and Increased Osteogenic Differentiation of Stem Cells by Solid-State Synthesized Magnesium Tricalcium Phosphate

In this study, magnesium and strontium-doped β-tricalcium phosphates were synthesized to understand dopant impact on substrate chemistry and morphology, and proliferation and osteogenic differentiation of mesenchymal stem cells. Under solid-state synthesis, magnesium doping stabilized the β-phase in tricalcium phosphate, with 22% less α-phase content than control. Strontium doping increased α-phase formation by 17%, and also resulted in greater surface porosity, leading to greater crystal precipitation in vitro. Magnesium also significantly enhanced the proliferation of stem cells (P < 0.05) and differentiation into osteoblasts with increased alkaline phosphatase production (P < 0.05) at all time points. These results indicated that magnesium stabilizes β-tricalcium phosphate in vitro and enhanced early and late-time-point osteoconduction and osteoinduction of mesenchymal stem cells.

Substituted Hydroxyapatite, Glass, and Glass-Ceramic Thin Films Deposited by Nanosecond Pulsed Laser Deposition (PLD) for Biomedical Applications: A Systematic Review

The deposition of thin films of bioactive materials is the most common approach to improve the bone bonding ability of an implant surface. With this purpose, several wet and plasma assisted deposition methods were proposed in the scientific literature. In this review, we considered films obtained by nanosecond Pulsed Laser Deposition (PLD). Since hydroxyapatite (HA) has composition and structure similar to that of the mineral component of the bone, the initial studies focused on the selection of experimental conditions that would allow the deposition of films that retain HA stoichiometry and crystallinity. However, biological apatite was found to be a poorly crystalline and multi-substituted mineral; consequently, the attention of researchers was oriented towards the deposition of substituted HA, glass (BG), and glass-ceramic (BGC) bioactive materials to exploit the biological relevance of foreign ions and crystallinity. In this work, after a description of the nanosecond ablation and film growth of ceramic materials, we reported studies on the mechanism of HA ablation and deposition, evidencing the peculiarities of PLD. The literature concerning the PLD of ion substituted HA, BG, and BGC was then reviewed and the performances of the coatings were discussed. We concluded by describing the advantages, limitations, and perspectives of PLD for biomedical applications.

A Review on Ionic Substitutions in Hydroxyapatite Thin Films: Towards Complete Biomimetism

Plasma sprayed coatings composed of stoichiometric hydroxyapatite have been extensively used to improve integration of metallic implants in the host bone, as hydroxyapatite (HA) is normally regarded as similar to the mineralized phase of bone. However, these coatings exhibited several drawbacks that limited their success. On the one hand biological apatite is a carbonated-HA, containing significant amounts of foreign ions, having low crystallinity and a small crystals size. This means that it differs from stoichiometric HA in terms of composition, stoichiometry, crystallinity degree, crystal size/morphology and, as a direct consequence, solubility, and ions release in the peri-implant environment. On the other hand, thick plasma sprayed coatings can undergo cracking and delamination and are scarcely uniform. For these reasons, research is pushing into two directions: (i) Increasing the similarity of apatite coatings to real bone, and (ii) exploring deposition by alternative plasma assisted techniques, allowing to achieve thin films, and having superior adhesion and a better control over the coating composition. In this article, we review the latest advances in the field of plasma-assisted deposition of ion-substituted hydroxyapatite thin films, highlighting the state of the art, the limitations, potentialities, open challenges, and the future scenarios for their application.

Systematic comparison of biologically active foreign ions-codoped calcium phosphate microparticles on osteogenic differentiation in rat osteoporotic and normal mesenchymal stem cells

Osteoporosis is a disease characterized by structural deterioration of bone tissue, leading to skeletal fragility with increased fracture risk. Calcium phosphates (CaPs) are widely used in bone tissue engineering strategies as they have similarities to bone apatite except for the absence of trace elements (TEs) in the CaPs. Bioactive glasses (BGs) have also been used successfully in clinic for craniomaxillofacial and dental applications during the last two decades due to their excellent potential for bonding with bone and inducing osteoblastic differentiation. In this study, we evaluated the osteogenic effects of the ionic dissolution products of the quaternary Si-Sr-Zn-Mg-codoped CaP (TEs-CaP) or 45S5 Bioglass® (45S5 BG), both as mixtures and separately, on rat bone marrow-derived mesenchymal stem cells (rOMSCs & rMSCs) from osteoporotic and normal animals, using an MTT test and Alizarin Red S staining. The materials enhanced cell proliferation and osteogenic differentiation, especially the combination of the BG and TEs-CaP. Analysis by quantitative PCR and ELISA indicated that the expression of osteogenic-specific genes and proteins were elevated. These investigations suggest that the TEs-CaP and 45S5 BG operate synergistically to create an extracellular environment that promotes proliferation and terminal osteogenic differentiation of both osteoporotic and normal rMSCs.

Bioimaging: An Useful Tool to Monitor Differentiation of Human Embryonic Stem Cells into Chondrocytes

To improve the recovery of damaged cartilage tissue, pluripotent stem cell-based therapies are being intensively explored. A number of techniques exist that enable monitoring of stem cell differentiation, including immunofluorescence staining. This simple and fast method enables changes to be observed during the differentiation process. Here, two protocols for the differentiation of human embryonic stem cells into chondrocytes were used (monolayer cell culture and embryoid body formation). Cells were labeled for markers expressed during the differentiation process at different time points (pluripotent: NANOG, SOX2, OCT3/4, E-cadherin; prochondrogenic: SOX6, SOX9, Collagen type II; extracellular matrix components: chondroitin sulfate, heparan sulfate; beta-catenin, CXCR4, and Brachyury). Comparison of the signal intensity of differentiated cells to control cell populations (articular cartilage chondrocytes and human embryonic stem cells) showed decreased signal intensities of pluripotent markers, E-cadherin and beta-catenin. Increased signal intensities of prochondrogenic markers and extracellular matrix components were observed. The changes during chondrogenic differentiation monitored by evaluation of pluripotent and chondrogenic markers signal intensity were described. The changes were similar to several studies over chondrogenesis. These results were confirmed by semi-quantitative analysis of IF signals. In this research we indicate a bioimaging as a useful tool to monitor and semi-quantify the IF pictures during the differentiation of hES into chondrocyte-like.