Hollow Hydroxyapatite Microspheres Research Articles

Chestnut-Inspired Hollow Hydroxyapatite 3D Printing Scaffolds Accelerate Bone Regeneration by Recruiting Calcium Ions and Regulating Inflammation.

This study aims to overcome the drawbacks associated with hydroxyapatite (HAP) dense structures after sintering, which often result in undesirable features such as large grain size, reduced porosity, high crystallinity, and low specific surface area. These characteristics hinder osseointegration and limit the clinical applicability of the material. To address these issues, a new method involving the preparation of hollow hydroxyapatite (hHAP) microspheres has been proposed. These microspheres exhibit distinctive traits including weak crystallization, high specific surface area, and increased porosity. The weak crystallization aligns more closely with early mineralization products found in the human body and animals. Moreover, the microspheres' high specific surface area and porosity offer advantages for protein loading and facilitating osteoblast attachment. This innovative approach not only mitigates the limitations of conventional HAP structures but also holds the potential for improving the effectiveness of hydroxyapatite in biomedical applications, particularly in enhancing osseointegration. Three-dimensional printed hHAP/chitosan (CS) scaffolds with different hHAP concentration gradients were manufactured, and the physical and biological properties of each group were systematically evaluated. In vitro and in vivo experiments show that the hHAP/CS scaffold has excellent performance in bone remodeling. Furthermore, in-scaffold components, hHAP and CS were cocultured with bone marrow mesenchymal stem cells to explore the regulatory role of hHAP and CS in the process of bone healing and to reveal the cell-level specific regulatory network activated by hHAP. Enrichment analysis showed that hHAP can promote bone regeneration and reconstruction by recruiting calcium ions and regulating inflammatory reactions.

Titanium Carbide MXene Quantum Dots-Modified Hydroxyapatite Hollow Microspheres as pH/Near-Infrared Dual-Response Drug Carriers.

Titanium carbide MXene quantum dots (MQDs) possess intrinsic regulatory properties and selective toxicity to cancer cells. Here, MDQs were selected for the modification of hydroxyapatite (HA) microspheres, and MXene quantum dots-modified hydroxyapatite (MQDs-HA) hollow microspheres with controllable shapes and sizes were prepared as bone drug carriers. The results show that the prepared MQDs-HA hollow microspheres had a large BET surface area (231.2 m2/g), good fluorescence, and low toxicity. In addition, MQDs-HA showed a mild storage-release behavior and good responsiveness of pH and near-infrared (NIR). Thus, the MQDs-HA hollow microspheres have broad application prospects in the field of drug delivery and photothermal therapy.

Experimental study of dexamethasone-loaded hollow hydroxyapatite microspheres applied to direct pulp capping of rat molars.

Dexamethasone (DEX) exerts anti-inflammatory and osteogenic effects. Hydroxyapatite is commonly used in bone repair due to its osteoconductivity, osseointegration, and osteogenesis induction. Hollow hydroxyapatite (HHAM) is often used as a drug carrier. This study aimed to investigate the histological responses of exposed dental pulp when dexamethasone-loaded nanohydroxyapatite microspheres (DHHAM) were used as a direct capping agent. Cavities were created in the left maxillary first molar of Wistar rats and filled with Dycal, HHAM, and DHHAM. No drug was administered to the control group. The rats were sacrificed at 1, 2, and 4 weeks after the procedure. The molars were extracted for fixation, demineralization, dehydration, embedding, and sectioning. H&E staining was performed to detect the formation of reparative dentin. H&E and CD45 immunohistochemical staining were performed to detect pulp inflammation. Immunohistochemical staining was performed to assess the expressions of dentin matrix protein 1 (DMP-1), interleukin (IL)-6, tumor necrosis factor (TNF)-α, and IL-1β. The results of H&E and CD45 immunohistochemical staining showed that the degree of inflammation in the DHHAM group was less than that in the Control and HHAM groups at 1, 2, and 4 weeks after capping of the rat molar teeth (p<0.01). The H&E staining showed that the percentage of reparative dentin formed in the DHHAM group was higher than that in the Control, HHAM (p<0.001), and Dycal groups (p<0.01) at 1 and 2 weeks, and was significantly higher than that in the Control group (p<0.001) and the HHAM group (p<0.01) at 4 weeks. The immunohistochemical staining showed a lower range and intensity of expression of IL-1β, IL-6, and TNF-α and high expression levels of DMP-1 in the DHHAM group at 1, 2, and 4 weeks after pulp capping relative to the Control group. DHHAM significantly inhibited the progression of inflammation and promoted reparative dentin formation.

Hollow Hydroxyapatite Microspheres Loaded with rhCXCL13 to Recruit BMSC for Osteogenesis and Synergetic Angiogenesis to Promote Bone Regeneration in Bone Defects.

Bone tissue engineering is a promising method to treat bone defects. However, the current methods of preparing composite materials that mimic the complex structure and biological activity of natural bone are challenging for recruitment of bone marrow mesenchymal stem cells (BMSCs), which affects the application of these materials in situ bone regeneration. Hollow hydroxyapatite microspheres (HHMs) possess a natural porous bone structure, good adsorption, and slow release of chemokines, but have low ability to recruit BMSCs and induce osteogenesis. In this study, The HHM/chitosan (CS) and recombinant human C-X-C motif chemokine ligand 13 (rhCXCL13)-HHM/CS biomimetic scaffolds that optimize bone regeneration and investigated their mechanism of BMSC recruitment and osteogenesis through cell and animal experiments and transcriptomic sequencing. Evaluate the physical characteristics of the HHM/CS and rhCXCL13-HHM/CS biomimetic scaffolds through Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and the cumulative release curve of rhCXCL13. Transwell migration experiments and co-culture with BMSCs were conducted to study the recruitment ability and osteogenic differentiation of the scaffolds. Transcriptomic sequencing was performed to analyze the osteogenic differentiation mechanism. The osteogenesis and bone healing performance were evaluated using a rabbit radial defect model. SEM demonstrated that the rhCXCL13-HHM/CS scaffold comprised hydroxyapatite microspheres in a porous three-dimensional network. The rhCXCL13 showed excellent sustained release capability. The rhCXCL13-HHM/CS scaffold could recruit BMSCs and induce bone regeneration. Transcriptome sequencing and experimental results showed that the osteogenesis mechanism of rhCXCL13-HHM/CS was through the PI3K-AKT pathway. In vivo, the rhCXCL13-HHM/CS scaffold significantly promoted osteogenesis and angiogenesis at 12 weeks after surgery. The rhCXCL13-HHM/CS scaffold demonstrates excellent potential for BMSC recruitment, osteogenesis, vascularized tissue-engineered bone reconstruction, and drug delivery, providing a theoretical basis for material osteogenesis mechanism study and promising clinical applications for treating large bone defects.

Tamoxifen Delivery to Breast Cancer Cells (MCF-7) Via Hydroxyapatite Microspheres

Drug delivery systems have been used in cancer treatment to increase drug effectiveness. The hydroxyapatite (HAP) based materials used in this area can provide drug transport to the target site without its deterioration. In this study, porous hollow hydroxyapatite microspheres (PHHMs) were produced by using the hydrothermal method. Tamoxifen (TAM) used in the treatment of breast cancer has been covalently attached to the produced microspheres. The obtained microsphere structures (tamoxifen-loaded hydroxyapatite, TAM/H) were successfully characterized by ATR-FTIR, FE-SEM, XRD, and DLS methods. The breast cancer cell line MCF-7 was used to examine the effect of the hybrid structure. The cytotoxic and genotoxic effects of TAM/H was compared with the TAM groups on MCF-7. Our results have showed that, the decrease in cell viability at 24 and 36 hours were still continued at 48 hours only in TAM/H groups. In addition, TAM/H was found to show a genotoxic affect by the increment in genetic damage index (GDI) and damaged cell percentage (DCP%). As a result, use of hydroxyapatite was suitable for the transport of TAM and that covalent binding was suitable for drug particle interaction with hybrid structure and thus controlled drug release occurred.

Fabrication and properties of multi-functional polydopamine coated Cu/F-codoped hydroxyapatite hollow microspheres as drug carriers

Due to its excellent bone conductivity and drug adsorption as well as pH-responsive drug release property, hydroxyapatite (HAp) is widely used as a drug carrier in bone repair field. Here, we report for the first time a novel multi-functional polydopamine (PDA) coated Cu/F-codoped HAp (Cu/F-HAp-PDA) hollow microspheres. Both Cu2+ and F- were successfully doped into the lattice of HAp and uniformly distributed in the shell of hollow microspheres through a one-step hydrothermal synthesis. Then PDA was coated homogeneously on the outer layer of Cu/F-HAp hollow microspheres. Both Cu/F-HAp and Cu/F-HAp-PDA samples displayed high drug loading efficiency and pH responsive drug release behavior. Moreover, the obtained Cu/F-HAp-PDA hollow microspheres exhibited excellent photothermal conversion efficiency and photothermal stability. The molecular dynamics simulations showed that PDA and HAp can form mutual binding mainly through Ca-O bonding, while doxorubicin (DOX) is mainly bound to PDA molecules through hydrogen bonding and π-π stacking interaction.

Facile synthesis, in vitro biocompatibility, and antibacterial activity of porous hollow hydroxyapatite microspheres

AbstractIn this study, porous hollow hydroxyapatite (HAp) microspheres are prepared using chitosan microspheres as novel sacrificial templates and their microstructure, biocompatibility, and drug delivery properties are evaluated. Scanning electron microscope (SEM) observations show that HAp microspheres are spherical in morphology with a diameter of 100–300 μm and have a porous and core–shell structure. X‐ray diffractometer patterns show that HAp microspheres consist of apatite phase. MTT assay indicates that HAp microspheres are biocompatible and have no significant cytotoxicity. SEM observations show that HAp microspheres support attachment and proliferation of osteoblast MC3T3‐E1 cells. After being soaked in the solution of tetracycline hydrochloride (TH, model drug), HAp microspheres adsorb TH with an adsorption capacity of 47% to derive TH‐loaded HAp microspheres. When exposed to two types of representative bacteria: Escherichia coli and Staphylococcus aureus, TH‐loaded HAp microspheres maintain the biological activity of TH to inhibit the growth of bacteria.

Recent progress on biomedical applications of functionalized hollow hydroxyapatite microspheres

Hollow hydroxyapatite microspheres (HHAM), a type of mesoporous nanomaterials with a specific internal cavity and hierarchical architectures, have attracted enormous interest over the past years owing to their remarkable potential for biomedical applications. However, the as-obtained HHAM fail to meet the need of the ever-updating diagnosis, treatment, and theranostics technologies. With the attempt to enhance performances and expand applications of HHAM, scientists have tailored the properties of HHAM by various methods including -doping modification and surface functionalization to give a versatile type of biomaterials, i.e., functionalized hollow hydroxyapatite microspheres (f-HHAM). Actually, significant achievements in the realm of f-HHAM have been obtained and various intractable clinic problems have been resolved. Nevertheless, designing desirable f-HHAM is expected to accelerate the development of the biomedicine field. In this contribution, the state-of-the-art progress f-HHAM is overviewed, which covers from the developments and preparation methods to bioapplications of these biomaterials. In addition, the current trends and key points as well as the future prospects and challenges for the applications of f-HHAM are discussed, aiming at providing a guideline for rational design of more creative f-HHAM according to different therapeutic requirements.

A combined experimental and molecular dynamics simulation study on doxorubicin adsorption on strontium-substituted hydroxyapatite hollow microspheres

Hydroxyapatite (HAp) materials with the functions of bone reconstruction as well as drug release have important applications in treatment of bone diseases. However, it still remains a challenge to synthesize ion-doped HAp with low crystallinity and high surface area to improve its biological responses and drug loading capacity. Herein, strontium-substituted hydroxyapatite (Sr-HAp) hollow microspheres that exhibit high drug loading efficiency and excellent biocompatibility are reported via a simple one-step hydrothermal method. The specific surface area of the as-prepared Sr-HAp microspheres can be as high as 214.69 m2/g. The simulation results show that there is no significant difference in the total binding energy between doxorubicin (DOX) and HAp as well as Sr-HAp. Both HAp and Sr-HAp show high DOX loading efficiency and sustained DOX release behavior. The binding between DOX and Sr-HAp is mainly through the electrostatic interaction between Ca or Sr ions on Sr-HAp surfaces and carbonyl-O or hydroxyl-O in DOX molecule. The findings provide insights into the design and development of drugs used in drug delivery system of HAp or ion-doped HAp materials.

Biomineralization synthesis of hollow microspheres of hydroxyapatite using tea saponin as a template

ABSTRACT Porous hollow hydroxyapatite (HA) microspheres were synthesized by biomineralization using Ca(NO3)2 · 4H2O and (NH4)3PO4 · 3H2O as starting materials and tea saponin as a template. With increasing amounts of tea saponin, the HA morphology exhibited a variation from irregular floccules to rod-like, which formed a spherical prototype, and then grew into porous hollow microspheres. With further increase in the tea saponin concentration, the rods became sheet-like and then formed cross-flakes. A possible mechanism for this morphology change was also discussed.

Surface engineered hollow hydroxyapatite microspheres: Hydrothermal synthesis and growth mechanisms

Well-organized hollow hierarchical hydroxyapatite microspheres were prepared hydrothermally via a template free process. Citrate was used as chelating or morphology regulating agent in this study. In the presence of regulating agent two-dimensional monetite phase evolved into three-dimensional hollow hydroxyapatite microspheres with an average diameter of 4 μm. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies revealed the importance of Citrate/Calcium (Cit/Ca) ratio in regulating the particle morphology. The results indicate that the surface morphology of microspheres can be engineered from nanosheets to nanorods by regulating agent concentration. X-ray diffraction (XRD) and Fourier transform infrared (FTIR) spectroscopy confirmed crystallinity and purity in the synthesized powders. Brunauer-Emmett-Teller (BET) indicated an increase in the specific surface area with the change from hydroxyapatite microspheres with nanosheets (HAMNS) to hydroxyapatite microspheres with nanorods (HAMNR) on the surface. Thermodynamic and kinetic theories are proposed to account for the observed changes in the surface morphology and crystal growth.

Preparation and mechanism of hydroxyapatite hollow microspheres with different surface charge by biomimetic method.

To meet the different application requirements in various fields, hydroxyapatite (HA) hollow microspheres with different surface charge were synthesized successfully by biomimetic method using Ca(NO3)2·4H2O and (NH4)2HPO4 in the presence of polyethylene glycol (PEG). Scanning electron microscopy (SEM), High-resolution TEM (HRTEM), X-ray powder diffraction (XRD), and Zeta PALS were used to characterize the obtained samples. The results indicated that the concentration of PEG and temperature significantly affect the morphology of the obtained samples. After incubation for 5 d, the HA hollow microspheres with positive surface charge, HA spherical nanoparticles with surface charge close to zero and calcium deficiency HA (d-HA) hollow microspheres with negative surface charge were obtained respectively in the presence of 5% PEG, 6% PEG and 7% PEG at 15 °C. Brunauer-Emmett-Teller (BET) revealed that the specific surface area of HA hollow microspheres reached 98.50 m2/g, while that of HA spherical nanoparticles were only 4.12 m2/g, hollow microspheres show a better application prospect. The possible formation mechanism was also discussed. Ca/P molar ratio >1.67, the surface charge of HA hollow microspheres inclines to be positive. Ca/P molar ratio <1.67, the surface charge of d-HA hollow microspheres tends to be negative.

Relaxin enhances bone regeneration with BMP‐2‐loaded hydroxyapatite microspheres

Our aims were to 1) evaluate the capacity of hollow hydroxyapatite (HA) microspheres (212-250 μm) to serve as a delivery system for controlled release of BMP-2 in vitro and 2) examine relaxin as an enhancer of BMP-2 for bone regeneration. Hollow HA microspheres were converted from borate glass microspheres and characterized using X-ray diffraction, Fourier-transform infrared spectroscopy, scanning electron microscopy, and the Brunauer-Emmett-Teller method. The microspheres loaded with BMP-2 and relaxin were implanted for 6 weeks in Sprague Dawley rats with calvarial defects. BMP-2 alone in the range up to 1 μg per defect exhibited dose-dependent bone regeneration while relaxin alone in the range up to 0.25 μg per defect did not promote bone regeneration. When compared with BMP-2 alone (1 μg per defect), a 50% reduction in the BMP-2 dose was achieved with the addition of 0.05, 0.1, or 0.25 μg of relaxin per defect. These results show that loading HA microspheres with a combination of relaxin and BMP-2 can significantly reduce the BMP-2 dose required to regenerate an equivalent amount of bone.

Novel and efficient red phosphorus/hollow hydroxyapatite microsphere photocatalyst for fast removal of antibiotic pollutants

Photocatalysis is a well-established and green technique in environmental pollutant remediation. In the present study, a novel and green red phosphorus (5.0 wt%)/hollow hydroxyapatite microsphere photocatalyst was fabricated and applied forthe degradation of rifampicin, tetracycline, and levofloxacin. The as-prepared material exhibits outstanding photocatalytic activity and stability, and has universal application for degradation of each antibiotic. Notably, tetracycline, rifampicin, andlevofloxacin can be completely degraded within 10 min, 20 min, and 50 min, respectively, under a 300 W Xe lamp with full spectrum illumination, where the optimal conditions were an initial pH of 6.0 and a photocatalyst dosage of 1.0 g/L. In addition, a plausible photocatalytic reaction mechanism was proposed. The low-cost, highly efficient, and green red phosphorus (5.0 wt%)/hollow hydroxyapatite microsphere photocatalyst is a promising candidate for the removal of many hazardous pollutants.

Hollow hydroxyapatite microspheres modified by CdS nanoparticles for efficiently photocatalytic degradation of tetracycline

A novel CdS nanoparticles/hollow hydroxyapatite microsphere (CdS/HAP) photocatalysts, with excellent photocatalytic performance, was firstly prepared by using a facile hydrothermal method under low temperature. The CdS/HAP composite catalysts were explored by various techniques including X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV–vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET) and photoelectrochemical measurements. The photocatalytic ability of the as-prepared composite catalysts was investigated by using tetracycline (TC) as a model contaminant. Compared to pure CdS and HAP, the CdS (10 wt.%)/HAP composite exhibited much higher photocatalytic removal efficiency with 90.2% degradation of TC in 30 min, where the best conditions were an initial pH at 5 and catalyst dosage of 0.50 g/L. Moreover, a probable reaction mechanism of CdS/HAP composite was evaluated based on the analysis of electron spin resonance (ESR) and trapping experiments. It was worth pointing out that the CdS nanoparticles can promote the photo-generated charges separation of HAP microspheres, and thus to improve the photocatalytic degradation performance of the catalysts. It can be inferred that the CdS/HAP composite's photocatalytic superiority could be used for applications in the removal of many hazard pollutants.

Novel design of porous hollow hydroxyapatite microspheres decorated by reduced graphene oxides with superior photocatalytic performance for tetracycline removal

Photocatalysis is a well-established and green technology with cost-effective, high-performance and environment benign for tetracycline (TC) removal in wastewater. Recently, HAp-based materials have been proved to be cheap and green photocatalysts for wastewater pollutant treatment. However, the photocatalytic removal efficiency of these existing HAp-based materials is still not desirable. Herein, a series of new porous hollow hydroxyapatite microspheres decorated with small amounts of reduced graphene oxides (0.5, 1.5 and 3 wt%) were firstly and successfully fabricated by a facile hydrothermal method. The as-prepared RGO/HAp composites were characterized by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, X-ray photoelectron spectroscopy (XPS), Brunauer Emmett-Teller (BET) and photoelectrochemical measurements. Furthermore, their photocatalytic applications were investigated by using TC as a model contaminant. It turned out that the as-prepared RGO (1.5 wt%)/HAp composite exhibits an outstanding photocatalytic activity for TC degradation (60 mg/L) under a 300 W xenon lamp with full spectrum irradiation (92.1%, 30 min). Note that its adsorption efficiency for TC in dark period was only 7.9% after 30 min. Thus, the total removal efficiency for TC was nearly 100%. Furthermore, the as-prepared RGO (1.5 wt%)/HAp exhibited remarkable stability and repeatability, demonstrating its promising potential as an efficient photocatalyst. A plausible photocatalytic reaction mechanism was proposed on the basis of electron spin resonance (ESR) and trapping experiments. This fundamental research will provide a promising strategy for developing highly efficient and compatible photocatalysts with wide applications.

Dexamethasone-loaded hollow hydroxyapatite microsphere promotes odontogenic differentiation of human dental pulp cells in vitro

A sustained-release system was established by synthesis of dexamethasone-loaded hollow hydroxyapatite microspheres (DHHAM). The in vitro effect of DHHAM on odontogenic differentiation of human dental pulp cells (hDPCs) was evaluated. Hollow hydroxyapatite microspheres (HHAM) are successfully manufactured using simple biomimetic one-step strategy in the presence of glycine and sodium dodecyl sulfonate. Dexamethasone (DEX) was loaded to the system after the formation of HHAM. The drug encapsulation capacity of DEX in HHAM is 40.3% and its loading efficiency is 16.7%. The cumulative release of DEX in vitro is 55% up to 35 days. Results of Real-time Polymerase Chain Reaction (Real-time PCR), alkaline phosphatase (ALP) activity and Alizarin Red S staining revealed that DHHAM can obviously promote bio-mineralization of hDPCs in the absence of osteogenic medium and enhance the gene expression of ALP, Runt-related transcription factor 2 (RUNX2), osteocalcin, dentin sialophosphoprotein (DSPP) and dentin matrix protein 1 (DMP1). The data suggest that sustained release of DEX from DHHAM could efficiently enhance odontogenic differentiation of hDPCs.

Hydrothermal synthesis of hierarchical hollow hydroxyapatite microspheres with excellent fluoride adsorption property

Abstract Hierarchical hollow hydroxyapatite microspheres (HHHMs) were fabricated by a facile hydrothermal process and used as an adsorbent for removing fluoride from aqueous solutions. The adsorbent was characterized by microscopic, spectroscopic, diffractometric and thermal analysis techniques. The adsorption kinetics, adsorption isotherms, and the effects of both the initial solution pH and coexisting anions on fluoride adsorption were evaluated. The results showed that the excellent fluoride adsorption capacity of the HHHMs was related to their hierarchical porous hollow structure and large specific surface area. Furthermore, the adsorption kinetics of the HHHMs were accurately described by the pseudo-second-order kinetic model, the adsorption isotherms were perfectly fitted by the Langmuir isotherms model. The adsorption process was pH dependent. The fluoride adsorption capacity decreased the most in the presence of carbonate, followed by phosphate and sulfate, and then nitrate and chloride, which had equal effect on the fluoride adsorption process. Moreover, the adsorption mechanism was studied by FT-IR, XPS, and pHpzc analysis. The results confirmed that the interaction of fluoride with the HHHMs was mainly dominated by ion exchange, electrostatic interactions, and hydrogen bonding. All these results indicated that the low-cost and scalable production of HHHMs has excellent applicability for wastewater treatment.

Protein adsorption characteristics of nanoparticle-assembled hollow microspheres of hydroxyapatite and their composites with PLLA microporous membranes

Nanoparticle-assembled hydroxyapatite (HA) hollow microspheres have a high surface area and are convenient to handle, owing to their characteristic structure. In this study we characterized the protein adsorption of HA hollow microspheres prepared from CaCl2 and K2HPO4 by a water-in-oil-in-water (W/O/W) emulsion method assisted by two surfactants: Span 80 and Tween 20. The HA hollow microspheres adsorbed bovine serum albumin, bovine γ-globulin, equine skeletal muscle myoglobin, and chicken egg white lysozyme in 10 mM sodium phosphate buffer (pH 6.8) in a Langmuir-type adsorption and desorbed the proteins in 800 mM sodium phosphate buffer (pH 6.8). The maximum adsorbed amounts of the HA hollow microspheres were 7.5–9.0 times higher than those of the microrods with a similar size range. The composite membranes of the HA microspheres and the poly(l-lactic acid) (PLLA) microporous membranes exhibited a high adsorption capacity for γ-globulin.

Rapid and effective removal of uranium (VI) from aqueous solution by facile synthesized hierarchical hollow hydroxyapatite microspheres.

Rapidly increasing development of nuclear power stimulates the exploration of low-cost and highly efficient materials to selectively remove uranium (VI) from contaminated wastewater streams. Herein, we successfully developed a novel hydroxyapatite (HAP) adsorbent by using a facile and template-free hydrothermal method. The XRD results demonstrated that the HAP was crystallized in hexagonal structure (space group P63/m(176)), and the images of SEM and TEM indicated that the HAP possessed hollow and hierarchical nanostructure. A large BET specific surface area (182.6 m2/g) and average pore size of 10.5 nm, suggested that the hierarchical hollow HAP microspheres could provide sufficient active sites for highly efficient removal of uranium from aqueous solutions, indicated the HAP might be a prompt emergency material for the remediation of nuclear leakage accident. Freundlich isotherm and pseudo-second-order kinetics model fitted well to sorption experimental data. The study was further advanced by FT-IR and XPS. The sorption mechanism was mainly attributed to surface chemisorption between U(VI) and HAP, forming a new U-containing compound, viz., autunite (Ca(UO2)2(PO4)2·3H2O).