Bioceramics are versatile materials for hard tissue engineering. Hydroxyapatite (HA) is a widely studied biomaterial for bone grafting and tissue engineering applications. The crystal structure of HA allows for a wide range of substitutions, which allows for tailoring materials properties. Transition metals and lanthanides are of interest since substitution in HA can result in magnetic properties. In this study, experimental results were compared to theoretical calculations of HA substituted with a transition metal. Calculation of a 10 atomic percent substitution of a transition metal ion Mn(2+), Fe(2+), and Co(2+) substituted HA samples lead to magnetic moments of 5, 4, and 3 Bohr magnetons, respectively. Hydroxyapatite substituted by transition metals (MHA) was fabricated through an ion exchange procedure and characterized with X-ray diffraction, Fourier transform infra-red spectroscopy (FTIR), X-ray photoelectron spectroscopy, and vibrating sample magnetometer, and results were compared to theoretical calculations. All the substitutions resulted in phase-pure M(2+)HA with lattice parameters and FTIR spectra in good agreement with calculations. Magnetic measurements revealed that the substitution of Mn(2+) has the greatest effect on the magnetic properties of HA followed by the substitution of Fe(2+) and then Co(2+). The present work underlines the power of synergistic theoretical-experimental work in guiding the rational design of materials.

A biomimetic collagen-apatite (Col-Ap) scaffold resembling the composition and structure of natural bone from the nanoscale to the macroscale has been successfully prepared for bone tissue engineering. We have developed a bottom-up approach to fabricate hierarchically biomimetic Col-Ap scaffolds with both intrafibrillar and extrafibrillar mineralization. To achieve intrafibrillar mineralization, polyacrylic acid (PAA) was used as a sequestrating analog of noncollagenous proteins (NCPs) to form a fluidic amorphous calcium phosphate (ACP) nanoprecursor through attraction of calcium and phosphate ions. Sodium tripolyphosphate was used as a templating analog to regulate orderly deposition of apatite within collagen fibrils. Both X-ray diffraction and Fourier transform infrared spectroscopy suggest that the mineral phase was apatite. Field emission scanning electron microscopy, transmission electron microscopy, and selected area electron diffraction confirmed that hierarchical collagen-Ap scaffolds were produced with both intrafibrillar and extrafibrillar mineralization. Biomimetic Col-Ap scaffolds with both intrafibrillar and extrafibrillar mineralization (IE-Col-Ap) were compared with Col-Ap scaffolds with extrafibrillar mineralization only (E-Col-Ap) as well as pure collagen scaffolds in vitro for cellular proliferation using MC3T3-E1 cells. Pure collagen scaffolds had the highest rate of proliferation, while there was no statistically significant difference between IE-Col-Ap and E-Col-Ap scaffolds. Thus, the bottom-up biomimetic fabrication approach has rendered a group of promising Col-Ap scaffolds, which bear high resemblance to natural bone in terms of composition and structure.

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Many magnetic materials lack intrinsic biocompatibility and require surface functionalization for in vivo applications. In this study, we fabricated an intrinsically biocompatible, biodegradable magnetic material through iron substitution into hydroxyapatite (FeHA) via ion-exchange. Controlling the oxidation state of iron in the ion exchange solution resulted in 'tunable' magnetic properties. Superparamagnetic behavior was observed in Fe 2+ HA and Fe 2+3+ HA and paramagnetism in Fe 3+ HA. FeHA powders were characterized using field emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), Fourier transform infra-red (FTIR) and X-ray photoelectron spectroscopy (XPS) with no detection of iron oxide phases. The as-synthesized HA and Fe 2+3+ HA samples were characterized in vitro using MC3T3-E1 cells. Cellular proliferation of Fe 2+3+ HA was found comparable to HA, while our cytotoxicity results from a lactate dehydrogenase assay showed Fe 2+3+ HA to be less toxic than pure HA, a widely used implantable biomaterial. Thus, these results collectively suggest that we have fabricated a magnetic biomaterial with intrinsic biocompatibility.

Hydroxyapatite (HA) is a widely studied biomaterial for bone grafting and tissue engineering applications. The crystal structure of HA lends itself to a wide variety of substitutions, which allows for tailoring of material properties. Iron is of interest in ion substitution in HA due to its magnetic properties. The synthesis and characterization of iron-substituted hydroxyapatite (FeHA) have been widely studied, but there is a lack of studies on the sintering behaviors of FeHA materials compared to pure HA. Studying the sintering behavior of a substituted apatite provides information regarding how the substitution affects material characteristics such as stability and bulk mechanical properties, thereby providing insight into which applications are appropriate for the substituted material. In this study both pure HA and FeHA were synthesized, pressed into pellets, and then sintered at temperatures ranging from 900-1300°C and 600-1100°C, respectively. The study thoroughly examined the comparative sintering behaviors of the two materials using density measurements, mechanical testing, X-ray diffraction, and electron microscopy. It was found that FeHA is considerably less thermally stable than pure HA, with decomposition beginning around 1200°C for pure HA samples, while at 700°C for the FeHA. The FeHA also had a much lower mechanical strength than that of the pure HA. An in vitro cell culture study was conducted by immersing FeHA powder in cell culture media, with HA powder at equivalent doses as a control, verified that FeHA is a biocompatible material. Although the FeHA would be unsuitable for bulk applications, it is a potential material for a variety of biomedical applications including drug delivery, cancer hyperthermia, and bone tissue engineering composites.