Facile Preparation of Controllable-Aspect-Ratio Hydroxyapatite Nanorods with High-Gravity Technology for Bone Tissue Engineering

3D printing of biological tooth with multiple ordered hierarchical structures

Natural teeth fulfill functional demands by their heterogeneity. The composition and hydroxyapatite (HAp) nanostructured orientation of enamel differ from those of dentin. However, mimicking analogous materials still exhibit a significant challenge. Herein, a bottom-up, sequential approach was formulated by combining shear-induced and magnetic-assisted 3D printing technology, enabling the fabrication of the intricate microstructure of a multi-material dental crown, where the HAp nanostructure is highly ordered and almost perpendicular to each other at the dentinoenamel junction (DEJ). The HAp nanorods were first induced to achieve high orientation in each printed line, then formed a plane with a vertical structure of DEJ under the shear force and magnetic field at dentin and enamel, respectively, and finally 3D-printed into a dental crown with bilayered parts exhibiting site-specific composition, texture, and outstanding biocompatibility. This novel approach can be applied to design and fabricate natural tooth crowns, indicating the potential for multi-level and multi-dimensional texture control.

3D-Printed Strong Dental Crown with Multi-Scale Ordered Architecture, High-Precision, and Bioactivity

Mimicking the multi-scale highly ordered hydroxyapatite (HAp) nanocrystal structure of the natural tooth enamel remains a great challenge. Herein, a bottom-up step-by-step strategy is developed using extrusion-based 3D printing technology to achieve a high-precision dental crown with multi-scale highly ordered HAp structure. In this study, hybrid resin-based composites (RBCs) with "supergravity +" HAp nanorods can be printed smoothly via direct ink writing (DIW) 3D printing, induced by shear force through a custom-built nozzle with a gradually shrinking channel. The theoretical simulation results of finite element method are consistent with the experimental results. The HAp nanorods are first highly oriented along a programmable printing direction in a single printed fiber, then arranged in a layer by adjusting the printing path, and finally 3D printed into a highly ordered and complex crown structure. The printed samples with criss-crossed layers by interrupting crack propagation exhibit a flexural strength of 134.1 ± 3.9 MPa and a compressive strength of 361.6 ± 8.9 MPa, which are superior to the corresponding values of traditional molding counterparts. The HAp-monodispersed RBCs are successfully used to print strong and bioactive dental crowns with a printing accuracy of 95%. This new approach can help provide customized components for the clinical restoration of teeth.

Functionalized Porous Hydroxyapatite Scaffolds for Tissue Engineering Applications: A Focused Review

Biomaterials have been widely used in tissue engineering applications at an increasing rate in recent years. The increased clinical demand for safe scaffolds, as well as the diversity and availability of biomaterials, has sparked rapid interest in fabricating diverse scaffolds to make significant progress in tissue engineering. Hydroxyapatite (HAP) has drawn substantial attention in recent years owing to its excellent physical, chemical, and biological properties and facile adaptable surface functionalization with other innumerable essential materials. This focused review spotlights a brief introduction on HAP, scope, a historical outline, basic structural features/properties, various synthetic strategies, and their scientific applications concentrating on functionalized HAP in the diverse area of tissue engineering fields such as bone, skin, periodontal, bone tissue fixation, cartilage, blood vessel, liver, tendon/ligament, and corneal are emphasized. Besides clinical translation aspects, the future challenges and prospects of HAP based biomaterials involved in tissue engineering are also discussed. Furthermore, it is expected that researchers may find this review expedient in gaining an overall understanding of the latest advancement of HAP based biomaterials.

Controllable Synthesis of Biomimetic Hydroxyapatite Nanorods with High Osteogenic Bioactivity

The development of biodegradable materials with high osteogenic bioactivity is important for achieving rapid bone regeneration. Although hydroxyapatite (HAp) has been applied as a biomaterial for bone engineering due to its good osteoconductivity, conventional synthetic HAp nanomaterials still lack sufficient osteogenesis, likely due to their high crystallinity and uncontrollable architecture. A design of HAp nanoparticles mimicking bone features may create good microenvironments that promote osteogenesis for rapid bone regeneration. In this study, HAp nanoparticles with a comparatively less crystalline structure and nanorod shapes mimicking biological HAp nanocrystals of natural bone were fabricated using a simple chemical precipitation approach with mild temperature control in the absence of any organic solvents. Transmission electron microscopy (TEM) indicated that HAp nanorods with aspect ratios from 2.0 to 4.4 were synthesized by adjusting the reaction time as well as the reaction temperature. Fourier transform infrared spectroscopy and X-ray diffraction experiments displayed that HAp nanorods prepared at 30 °C (HAp-30 with an aspect ratio of 2.9) had a low crystalline structure and B-type CO₃^2- substitution similar to those of natural HAp originating from bone tissue. The energy-dispersive spectroscopy (EDS) results showed that the Ca/P ratio of HAp-30 was 1.66 ± 0.13. An in vitro biological evaluation against rat bone marrow-derived mesenchymal stem cells indicated that the resulting HAp nanorods had excellent biocompatibility (with an ∼80-fold increase in IC50 compared to that of conventional HAp nanoparticles). Interestingly, the alkaline phosphatase (ALP), alizarin red S, and immunofluorescence staining results all showed that stem cells display an obvious osteogenesis dependence on the HAp nanostructure. Specifically, HAp nanorods with a moderate aspect ratio had the optimal osteogenic capacity (e.g., HAp-30 offered a 2.8-fold increase in ALP expression and a 4-fold increase in OCN expression relative to that provided by irregular HAp at day 14). It is expected that HAp nanorods with controllable architectures and size have potential as a kind of new bioactive bone filler for bone defect repair.

Design and structural investigations of Yb3+ substituted β-Ca3(PO4)2 contrast agents for bimodal NIR luminescence and X-ray CT imaging

The quest for the development of bone substitutes with contrast agents for diagnostic imaging subsists to distinguish synthetic bone from native human tissue. To this aim, ytterbium (Yb³⁺) substitutions in β-tricalcium phosphate (β-Ca₃(PO₄)₂, β-TCP) as contrast agents has been developed to differentiate implant materials thereby, facilitating as host for bimodal imaging application by means of NIR luminescence and X-ray computed tomography techniques. A facile aqueous chemical precipitation route with the aid of surfactant is used for the synthesis of Yb³⁺ substitutions in β-Ca₃(PO₄)₂. The characterization results affirms the ability of β-Ca₃(PO₄)₂ to host 4.36 mol% of Yb³⁺ while the excess Yb³⁺ crystallizes as YbPO₄. The structure refinement results favour the occupancy of Yb³⁺ at the Ca²⁺(5) site of β-Ca₃(PO₄)₂. The absorption and photoluminescence spectra in the near infrared region with emission intensity ~1024 nm in the second biological window correspond to ²F_5/2 → ²F_7/2 transitions of Yb³⁺. The designed Yb³⁺ substituted β-Ca₃(PO₄)₂ does not exhibit any toxicity in human osteosarcoma cell lines and delivers an excellent in vitro CT contrast ability allied by the enhanced signal intensity and high X-ray absorption coefficient.