Xiaoguo Liu

Effects of strontium in modified biomaterials

Strontium (Sr) plays a special role in bone remodelling, being associated with both the stimulation of bone formation and a reduction in bone resorption. Thus, the modification of biomaterials by partial or full substitution by Sr is expected to increase both bioactivity and biocompatibility. However, such effects have to be studied individually. Although no phase transition was found in Sr-substituted hydroxyapatite (Sr-HA), Sr-containing calcium silicate (Sr-CS) or Sr-containing borosilicate (Sr-BS), their biological performance was substantially affected by changes in the physico-chemical properties and Sr content of the materials. Three distinct outcomes were found for the presence of Sr: (1) increased HA solubility; (2) no significant effect on the degradation rate of CS; (3) apparent inhibition of the otherwise rapid degradation of BS. In each case the released Sr affected osteoblast proliferation and alkaline phosphatase activity, with clear evidence that an optimum Sr dose exists. Such chemical and biological variations must be disentangled for the behaviour to be properly understood and materials design to be advanced.

Mussel-inspired bioceramics with self-assembled Ca-P/polydopamine composite nanolayer: preparation, formation mechanism, improved cellular bioactivity and osteogenic differentiation of bone marrow stromal cells.

The nanostructured surface of biomaterials plays an important role in improving their in vitro cellular bioactivity as well as stimulating in vivo tissue regeneration. Inspired by the mussels adhesive versatility, which is thought to be due to the plaque-substrate interface being rich in 3,4-dihydroxy-l-phenylalamine (DOPA) and lysine amino acids, in this study we developed a self-assembly method to prepare a uniform calcium phosphate (Ca-P)/polydopamine composite nanolayer on the surface of β-tricalcium phosphate (β-TCP) bioceramics by soaking β-TCP bioceramics in Tris-dopamine solution. It was found that the addition of dopamine, reaction temperature and reaction time are three key factors inducing the formation of a uniform Ca-P/polydopamine composite nanolayer. The formation mechanism of a Ca-P/polydopamine composite nanolayer involved two important steps: (i) the addition of dopamine to Tris-HCl solution decreases the pH value and accelerates Ca and P ionic dissolution from the crystal boundaries of β-TCP ceramics; (ii) dopamine is polymerized to form self-assembled polydopamine film and, at the same time, nanosized Ca-P particles are mineralized with the assistance of polydopamine, in which the formation of polydopamine occurs simultaneously with Ca-P mineralization (formation of nanosized microparticles composed of calcium phosphate-based materials), and finally a self-assembled Ca-P/polydopamine composite nanolayer forms on the surface of the β-TCP ceramics. Furthermore, the formed self-assembled Ca-P/polydopamine composite nanolayer significantly enhances the surface roughness and hydrophilicity of β-TCP ceramics, and stimulates the attachment, proliferation, alkaline phosphate (ALP) activity and bone-related gene expression (ALP, OCN, COL1 and Runx2) of human bone marrow stromal cells. Our results suggest that the preparation of self-assembled Ca-P/polydopamine composite nanolayers is a viable method to modify the surface of biomaterials by significantly improving their surface physicochemical properties and cellular bioactivity for bone regeneration application.

Synthesis of element-substituted hydroxyapatite with controllable morphology and chemical composition using calcium silicate as precursor

In the absence of any surfactants and template-directing reagents, element-substituted hydroxyapatite [Ca10(PO4)6(OH)2, HAp] powders with controllable morphologies and chemical compositions were synthesized via hydrothermal treatment of a calcium silicate precursor in phosphate solutions. The morphologies (such as nanoparticles, nanowires and nanosheets) of the products could be well tailored through regulating the crystalline nature of the calcium silicate precursors or the type of phosphate solution. While through regulating the chemical compositions of the precursors and the reaction ratio of the precursor/solution, the HAp powders substituted by different kinds and amount of elements (such as Si, Na, Mg, and Sr, etc.) could be facilely obtained. A possible cluster growth mechanism accompanied with ion-releasing and incorporating model was preliminarily proposed for the formation of the controllable structures and compositions. The present work indicated that using calcium silicate as a precursor is an effective strategy to synchronously and delicately control the morphology and chemical composition of HAp crystals.

Dental enamel-like hydroxyapatite transformed directly from monetite

Up to now, the strategy to fabricate biomimetic dental enamel-like hydroxyapatite (HA) has attracted great interest due to the extremely high degree of architectural organization of HA crystals. In this study, in the absence of surfactants, organic solvent or template-directing agents, a facile method via direct transformation from monetite (CaHPO4) single crystals was developed to synthesize HA with enamel-like hierarchical structures on the nano- to microscale. The suitably shaped CaHPO4 precursor was first synthesized under simultaneous irradiation of microwave and ultrasound. The highly oriented HA bundles, constructed of tiny crystals in a parallel structure, were then fabricated through microwave treatment of the precursor in alkaline aqueous solution. The present study indicated that the formation of the highly oriented hierarchical structure of the obtained HA was due to the intrinsically ordered structure of CaHPO4, the structural similarity between CaHPO4 and HA as well as the ultrafast transformation rate.

Growth of Highly Oriented Hydroxyapatite Arrays Tuned by Quercetin

Crystal calls: the remarkable crystal modulation ability of quercetin (QUE) in highly oriented hydroxyapatite (HAp) array crystallization is reported. Organized HAp crystals were obtained by hydrothermal exchange of α-tricalcium phosphate (α-TCP) precursor in solution with a progressive increase in QUE concentration. Experimental results revealed that QUE would be a potentially effective crystal modulation assistant.

Multilevel Hierarchically Ordered Artificial Biomineral

Living organisms are known for creating complex organic-inorganic hybrid materials such as bone, teeth, and shells, which possess outstanding functions as compared to their simple mineral forms. This has inspired many attempts to mimic such structures, but has yielded few practical advances. In this study, a multilevel hierarchically ordered artificial biomineral (a composite of hydroxyapatite and gelatine) with favorable nanomechanical properties is reported. A typical optimized HAp/gelatin hybrid material in the perpendicular direction of the HAp c-axis has a modulus of 25.91 + 1.78 GPa and hardness of 0.90 + 0.10 GPa, which well matches that of human cortical bone (modulus 24.3 + 1.4 GPa, hardness 0.69 + 0.05 GPa). The bottom-up crystal constructions (from nano- to micro- to macroscale) of this material are achieved through a hard template approach by the phase transformation from DCP to HAp. The structural biomimetic material shows another way to mimic the complex hierarchical designs of sclerous tissues which have potential value for application in hard tissue engineering.

Modulation of hydroxyapatite crystals formed from α-tricalcium phosphate by surfactant-free hydrothermal exchange

In the present work, an unusual precursor, α-tricalcium phosphate [Ca3(PO4)2, α-TCP] was used for the modulation of hydroxyapatite [Ca10(PO4)6(OH)2, HA] crystals. Micro-structure and morphology controllable HA crystals were successfully synthesized via direct hydrothermal treatment of α-TCP particles in the absence of any surfactants or additives, and were investigated by field emission scanning electron microscopy, field emission transmission electron microscopy, X-ray diffraction, X-ray fluorescence spectroscopy and Fourier transform infrared spectroscopy. Well developed HA crystals with different structures and morphologies (chrysanthemum-like HA microflowers, enamel-like HA microparticles, rectangle shaped HA microplates and HA microrods) could be obtained by adjusting the reaction temperature and the concentration of Ca2+ ions. The experimental results showed that different aggregation routes of HA nanorods which grow along the c-axis were the reason for the formation of various micro-structures and morphologies. The possible mechanism of the formation of HA crystal was based on the Cluster Growth Model. This study suggested that the hydrothermal phase transformation from α-TCP to HA could be a promising way for the delicate morphology control of HA crystals.

Preparation of hierarchical enamel-like structures from nano- to macro-scale, regulated by inorganic templates derived from enamel

The artificial construction of enamel-like structure has been focusing on the utilization of an organic matrix, since proteins are believed to be essential in directing apatite orientation during enamel formation. Here we report that the orientation of apatite can be regulated by inorganic substrates and we successfully constructed enamel-like cross-arranged structure using natural enamel as template. The templating effect of inorganic substrates on apatite orientation may lead to the design of biomimetic materials.