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Facile Preparation of Controllable-Aspect-Ratio Hydroxyapatite Nanorods with High-Gravity Technology for Bone Tissue Engineering Bo-Yang Lv,† Li-Sheng Zhao,∥ Yuan Pu,*,‡ Yuan Le,† Xiao-Fei Zeng,† Jian-Feng Chen,†,‡,§ Ning Wen,∥ and Jie-Xin Wang*,†,‡,§ †

State Key Laboratory of Organic−Inorganic Composites, ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, and §Beijing Advanced Innovation Center for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ∥ Department of the Prosthodontics, The General Hospital of Chinese PLA, Beijing, 100853, People’s Republic of China ABSTRACT: As the main inorganic component of human hard tissue, hydroxyapatite (HAp) has attracted extensive interest in biomedical and clinical applications, especially for HAp nanoparticles. In this study, HAp nanorods with a controllable aspect ratio were successfully prepared using NH3·H2O and CO(NH2)2 as aspect ratio modifier by a high-gravity reactive precipitation in a rotating packed bed (RPB) combined with hydrothermal treatment. The influences of the molar ratio of NH3·H2O and CO(NH2)2 and the rotating speed on the average size and aspect ratio of HAp nanorods were systematically explored. The as-synthesized HAp nanorods were investigated with TEM, XRD, and FTIR. The results indicated that the average aspect ratio of HAp nanorods could be facilely controlled in the range 2.2−39. As compared to a traditional batch stirred tank reactor, the RPB reactor had HAp nanorods with a smaller particle width and a wider range of aspect ratio, and a much shorter reaction time from 20 min to 1 s. Further, the potential use of HAp nanorods in the pectin/HAp nanocomposite cements was investigated. The pectin/HAp nanocomposite cement with the highest compressive strength of 29.7 MPa was achieved with the addition of HAp nanorods with the aspect ratio of 15. such as chemical precipitation,18,19 hydrothermal treatment,20,21 microemulsion,22 molten salt,23 and soft template.24,25 However, it is still a challenge to facilely obtain uniform HAp nanorods with a wide range of aspect ratio with the above routes.26−28 The rotating packed bed (RPB) is a novel process intensification device which is suitable for a continuous large-scale industrialization preparation.29,30 It can produce high gravity environment (tens to hundreds of g) by centrifugal force. The micromixing process in the RPB reactor is thus significantly intensified, helpful to achieving a more rapid nucleation process and providing more even reaction surroundings compared to the traditional precipitation process in a batch stirred tank reactor (STR).31−33 As a consequence, the precise controllability of particle size and distribution can be obtained, which is efficiently utilized for the preparation of various inorganic and organic nanoparticles, such as CaCO3, Mg(OH)2, ZnS, Ag, HAp, and drug.30,34−39 However, the previous study on HAp was mainly focused on the attainment of uniformly small HAp nanoparticles;38 it is hard to prepare HAp nanorods with a wide range of aspect ratios.

1. INTRODUCTION Since hydroxyapatite (HAp) is the main inorganic component of host hard tissues, HAp has been widely applied in bone tissue engineering and drug delivery.1−8 In addition, HAp has also found other potential biomedical applications including cell targeting,9 fluorescent labeling and imaging,10−13 etc. Nanocrystalline HAp exhibits better sinterability and compactness performances because of high specific surface area from the nanometer size effect. This will be helpful to the improvement of many mechanical properties including fracture toughness. Furthermore, HAp nanoparticles are believed to have a better biological activity than coarser particles.14 Therefore, as tissue implant materials, they exhibit better biocompatibility compared to other implants. Further investigation indicated that the performances of HAp nanoparticles could be efficiently adjusted by regulating shape, size, and distribution, as well as dispersity of HAp nanoparticles.15 Therefore, controlled synthesis of HAp nanoparticles has became an extensive research focus in recent years.16 In a living body, bone is mainly composed of rodlike HAp and collagen substrate. Thus, it is an effective way to utilize synthetic HAp nanorods in biomedical ceramics and composites to improve the related mechanical performances owing to the unique characteristics of HAp nanorods.17 Presently, some methods have been developed to synthesize HAp nanorods, © XXXX American Chemical Society

Received: Revised: Accepted: Published: A

December 19, 2016 February 28, 2017 March 1, 2017 March 1, 2017 DOI: 10.1021/acs.iecr.6b04902 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of experimental setup for high-gravity reactive precipitation process ((1) Ca(NO3)2 solution tank; (2) (NH4)2HPO4 solution tank; (3, 4) pumps; (5, 6) flow meters; (7) RPB reactor; (8) motor; (9) shell; (10) outlet tank).

purification system (Shanghai Hogan Scientific Instrument Co. Ltd., China) was used throughout the study. The experimental equipment for the preparation of HAp nanorods is schematically displayed in Figure 1. The major component of the whole procedure was the RPB, consisting of a wire-mesh packed rotator, a fixed casing, two liquid inlets, and a suspension outlet. The wire meshes have a porosity of 0.85 and a specific surface area of 860 m2/m3. The rotator is installed inside the fixed casing and rotates at an adjustable speed. More descriptions of the RPB can be seen in the previously published papers.30,34 2.2. Preparation of HAp Nanorods. In a typical preparation process, 100 mL of 0.2 mol/L Ca(NO3)2 solution and 60 mL of 0.2 mol/L (NH4)2HPO4 solution were first prepared. A 2 mL volume of the mixed solution was prepared according to some molar ratios (4:0, 3:1, 2:2, 1:3, or 0:4) of NH3·H2O and CO(NH2)2 solutions with the same separate concentration of 10 mol/L, and immediately put into Ca(NO3)2 solution. Subsequently, Ca(NO3)2 solution with a flow rate of 200 mL/min and (NH4)2HPO4 solution with a flow rate of

In this study, our objective is to facilely prepare HAp nanorods with a controllable aspect ratio based on high-gravity technology by introducing CO(NH2)2 and NH3·H2O as aspect ratio modifier. The effects of the molar ratio of CO(NH2)2 and NH3·H2O and the rotating speed were explored. Furthermore, a batch STR was adopted for a comparison study. By comparison, the RPB reactor exhibited the advantages of producing HAp nanorods with a smaller particle width and a wider range of aspect ratios , and having a much shorter reaction time from 20 min to 1 s. Further, pectin/HAp nanocomposite cements were fabricated by adding as-prepared HAp nanorods, and the corresponding mechanical properties were evaluated.

2. EXPERIMENTAL SECTION 2.1. Materials and Setup. Analytical reagent grade ammonium hydrogen phosphate ((NH4)2HPO4), urea (CO(NH2)2), ammonium water (NH3·H2O), sodium citrate (Na3Cit), calcium nitrate tetrahydrate (Ca(NO3)2·4H2O), and ethanol (C2H5OH) were bought from Sinopharm Chemical Reagent Beijing Co. Ltd. Deionized water from a Hitech-K flow water B

DOI: 10.1021/acs.iecr.6b04902 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Industrial & Engineering Chemistry Research 120 mL/min were pumped into the RPB reactor at 80 °C. The rotating speed of the RPB was set at 2500 rpm. The contact and residence time of the reactants in the RPB reactor was about 1 s. The entire HAp precursor collected from the outlet of the RPB was immediately transferred to a 200 mL Teflon autoclave. Afterward, the autoclave was tightly sealed, heated to 200 °C, and maintained for 2 h. After hydrothermal crystallization, the slurry was filtered and washed several times with deionized water. Last, the filter cake was dried at 75 °C overnight to obtain HAp nanorods. As a comparison, HAp nanorods were also similarly prepared with a conventional STR route. A 60 mL volume of (NH4)2HPO4 solution with a concentration of 0.2 mol/L was added dropwise into a flask with 100 mL of Ca(NO3)2 solution with the same concentration of 0.2 mol/L and the addition of the same molar ratio of NH3·H2O and CO(NH2)2 under a vigorous stirring. The whole addition process lasted for 20 min. The following hydrothermal treatment was carried out in the above-mentioned same way. 2.3. Preparation of Pectin/HAp Nanocomposite Cements. The pectin/HAp nanocomposite cements were prepared by mixing the above as-prepared HAp nanorods and the pectin solution. Briefly, 1.0 g of pectin and 2.0 g of Na3Cit were put into 20 mL of deionized water to prepare the pectin solution containing 5 wt % pectin and 10 wt % Na3Cit. Subsequently, HAp nanorods with different aspect ratios were added into the above pectin solution to form an adhesive paste with a liquid−solid ratio of 0.5 mL/g. After uniform stirring, the cement slurry was filled into a cylinder mold with a radius of 4 mm and a height of 15 mm to prepare the sample for the measurement of mechanical properties. Last, the samples were incubated at 37 °C in a oven with a relative humidity of 100%. After storage for 24 h, pectin/HAp nanocomposite cements were obtained, and different samples were denoted as HAC1, HAC2, HAC3, HAC4, and HAC5 for HAp nanorods prepared using NH3·H2O and CO(NH2)2 with different molar ratios of 4:0, 3:1, 2:2, 1:3, and 0:4, respectively. 2.4. Characterization. 2.4.1. X-ray Diffraction (XRD). X-ray diffraction (XRD) measurements were performed by a D8 Advance diffraction meter (Bruker, Germany) equipped with Cu Kα radiation, an accelerating voltage of 40 kV, and a current of 40 mA. The scanning range was 20−60°, and the scanning rate was 5°/min with a step size of 0.01°. 2.4.2. Fourier Transform Infrared Spectroscopy (FTIR). HAp nanorods were mixed with KBr and compressed to form a pellet at 10 MPa. FTIR data were recorded in the range 400− 4000 cm−1 with a Nicolet 6700 spectrometer (Nicolet Instrument Co., USA). 2.4.3. Transmission Electron Microscopy (TEM). A transmission electron microscope (JEOL JSM-2010F, Japan) was operated at an accelerating voltage of 200 kV. The powder was dispersed in C2H5OH by sonication for 30 min. TEM samples were perpared by dropping the sample onto a Cu grid. The particle size and the aspect ratio of HAp nanorods were obtained based on the measurement of over 200 particles of TEM images with image analysis and processing software (Image-Pro Plus 6.5, Media Cybernetics Inc., U.S.). 2.4.4. Scanning Electron Microscopy (SEM). The surface morphologies of pectin/HAp nanocomposite cements were observed with a scanning electron microscope (JSM-6701F, JEOL, Japan). 2.4.5. Compressive Strength. The samples were detected by a testing instrument (Instron-5566, USA) with a load cell of

500 N at a crosshead speed of 2 mm/min. The maximum load for the breakage of every sample was tested, and the compressive strength was calculated.

3. RESULTS AND DISCUSSION 3.1. Effect of Molar Ratio of NH3·H2O and CO(NH2)2. Figure 2 reveals TEM images, the corresponding particle widths,

Figure 2. TEM images of HAp nanorods prepared at different molar ratios of NH3·H2O and CO(NH2)2 (A, 4:0; B, 3:1, C, 2:2; D, 1:3; E, 0:4) and corresponding particle widths and aspect ratios (F).

and the aspect ratios of HAp nanorods prepared with different molar ratios of NH3·H2O and CO(NH2)2 in the RPB reactor. Clearly, the as-prepared HAp nanoparticles had uniform rodlike shape and good dispersity. With the changed molar ratio of NH3·H2O and CO(NH2)2 from 4:0 to 0:4, HAp nanorods became larger and much longer. The average particle width increased from 22 to 79 nm. It was worth noting that the average aspect ratio markedly increased from 2.2 to 39. Figure 3 shows XRD patterns (A) and FTIR spectra (B) of HAp precursor and HAp nanorods prepared at three representative molar ratios of NH3·H2O and CO(NH2)2, as well as typical TEM images of HAp precursor (C) and HAp nanorods (D) prepared with the addition of NH3·H2O:CO(NH2)2 = 2:2 as an example. Before hydrothermal treatment, the HAp precursor appeared to have a similar rod shape, but much smaller and thinner, as compared to the final product of HAp nanorods (Figure 3C,D). Furthermore, HAp precursor was not pure, C

DOI: 10.1021/acs.iecr.6b04902 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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Figure 4. Schematic diagram for possible formation mechanism of HAp nanorods.

to diffuse and adsorb OH− ions onto the surface. The quantity of OH− ions and its hindrance effect vary with different facets owing to the surface energy difference. The OH− ions are preferentially absorbed on some facets of the cluster. Therefore, the growth rate of the OH− absorbed facets will be limited by the shielding effect of OH− ions on the interface.26,43 When the molar ratio of NH3·H2O and CO(NH2)2 was 4:0, the reactant solution had a higher pH value of about 11. In this case, the high pH value could effectively increase the supersaturation degree, which would cause more HAp nuclei. Also, the crystal growth would be inhibited, resulting in small particles. Simultaneously, owing to the formed higher concentration of OH− ions, their high diffusion flux and strong adsorption probability led to isotropic or weak-anisotropic growth, thereby benefiting the generation of very short HAp nanorods. However, when the molar ratio of NH3·H2O and CO(NH2)2 was changed from 4:0 to 0:4, the pH value of the reactant solution declined from 11 to 7. At a lower OH− concentration, there was a limited probability of the adsorption of OH− ions on the surface of HAp nuclei, preferentially leading to anisotropic growth along the c-axis direction ({001} facet).43 HAp nanorods with a higher aspect ratio were thus formed. Moreover, hydrothermal treatment is an effective way to lead to one-dimensional anisotropic growth of HAp, and has an apparent effect on the crystallization, the size, and the aspect ratio of HAp crystals.44 During the hydrothermal treatment of 200 °C for 2 h, the further large amount of the decomposition of CO(NH2)2 created more OH− ions, which met the requirements of the complete formation of HAp crystals, and produced highly crystalline HAp nanorods with larger size and higher aspect ratio. 3.2. Effect of RPB Rotating Speed. The rotating speed of the RPB reactor, which is the most important equipment structure parameter, can directly affect the intensified degree of micromixing,29−31 thereby influencing the initial nucleation particle size. Figure 5 exhibits the particle widths and aspect ratios of HAp nanorods prepared with three representative molar ratios of NH3·H2O and CO(NH2)2 at different RPB rotating speeds, and typical TEM images of HAp nanorods prepared with NH3·H2O:CO(NH2)2 = 2:2 at different RPB rotating speeds. As shown in Figure 5A−C, the average particle width of the HAp nanorods greatly decreased with the increased rotating speed in the range 500−2500 rpm, which can be clearly observed in the TEM images of Figure 5D−F with NH3·H2O:CO(NH2)2 = 2:2 as a representative. Obviously, the enhancement of the rotating speed could greatly increase the

Figure 3. XRD patterns (A) and FTIR spectra (B) of HAp precursor (d, e, f) and HAp nanorods (a, b, c) prepared at different molar ratios of NH3·H2O and CO(NH2)2 (4:0 (a, d); 2:2 (b, e); 0:4 (c, f)), as well as TEM images of HAp precursor (C) and HAp nanorods (D) prepared with the addition of NH3·H2O:CO(NH2)2 = 2:2.

but had a composition of Ca10(PO4)6(OH)2 and CaHPO4 (Figure 3A). With the decreased NH3·H2O and increased CO(NH2)2 (NH3·H2O:CO(NH2)2 = 4:0, 2:2, and 0:4), the diffractions located at 2θ = 26.7 and 30.2° indexed to CaHPO4 became stronger beside weak peaks belonging to Ca10(PO4)6(OH)2 (curves d−f). This could be ascribed to an obvious decrease of the concentration of key OH− ions owing to the weak supply of OH− ions from CO(NH2)2 in the precipation process. However, after hydrothermal treatment, all three corresponding HAp samples (curves a−c) had five typical peaks located at 2θ = 25.8, 31.7, 32.2, 32.9, and 46.7°, belonging to (002), (211), (112), (300), and (222) crystallographic facets of HAp, respectively, which well accorded with the standard card (JCPDS NO. 74-0565). No impurity peaks were found, proving that the as-prepared products were pure HAp. The composition of the product was also measured with FTIR. As shown in curves a−c of Figure 3B, the positions of characteristic bands at 1089, 1035, 597, and 560 cm−1 and the bands at 3570 and 633 cm−1 could be attributed to phosphate stretching and bending and hydroxyl stretching of HAp, respectively,26,40 indicating pure HAp. Before hydrothermal treatment (curves d−f), the band around 870 cm−1 belonged to the symmetrical stretching vibration of the HPO42− group. Also, the band at 1450 cm−1 was ascribed to the CO32− group because of the possible introduction of CO2 that resulted from the decomposition of CO(NH2)2.41 The formation process of HAp nanorods with controllable aspect ratio by altering the molar ratio of NH3·H2O and CO(NH2)2 is complicated, but it is plausible to present the following possible mechanism. Figure 4 gives the schematic diagram for the possible formation mechanism of HAp nanorods. Generally, the size of crystals intimately depends on the nucleation rate, with higher supersaturation tending to generate a mass of smaller crystals. However, the aspect ratio is mainly determined by the relative surface energies of the growth planes.42 For HAp, it is necessary for crystallization and growth D

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Figure 5. Particle widths and aspect ratios of HAp nanorods prepared with three representative molar ratios of NH3·H2O and CO(NH2)2 at different RPB rotating speeds (A, 4:0; B, 2:2; C, 0:4), and TEM images of HAp nanorods prepared with NH3·H2O:CO(NH2)2 = 2:2 at different RPB rotating speeds (D, 500 rpm; E, 1500 rpm; F, 2500 rpm).

shearing force in the RPB reactor. Therefore, the fluids passing through the porous packing were violently split into tinier droplets, thinner films, and threads, thereby intensifying the micromixing of fluid elements.30,45 Thus, an uniform reaction surrounding was achieved, helpful to the generation of HAp particles with a smaller size or thinner particle width. At the same time, the corresponding average aspect ratios rose with different increasing extents. For NH3·H2O:CO(NH2)2 of 4:0, there was a very slight increase of the average aspect ratio owing to the generation of very short nanorods (Figure 5A). However, with the increased content of CO(NH2)2, the rising trend of the average aspect ratio became more and more apparent. For NH3·H2O:CO(NH2)2 of 2:2, the average aspect ratio increased from 7.8 to 15 by about 1.92 times with the increased rotating speed from 500 to 2500 rpm (Figure 5B,D−F). Correspondingly, for NH3·H2O:CO(NH2)2 of 0:4, the average aspect ratio increased by about 5.3 times, reaching 39 (Figure 5C). 3.3. Comparison with a Stirred Tank Reactor (STR). Figure 6 shows TEM images of HAp nanorods prepared at three representative molar ratios of NH3·H2O and CO(NH2)2 in the STR and the RPB reactor, as well as the corresponding particle widths and aspect ratios. For NH3·H2O:CO(NH2)2 of 4:0, the product prepared in the RPB reactor (Figure 6B)

Figure 6. TEM images of HAp nanorods prepared at three representative molar ratios of NH3·H2O and CO(NH2)2 in STR (A, C, E) and RPB (B, D, F), as well as corresponding particle widths and aspect ratios (G, H, I) (4:0 (A, B, G); 2:2 (C, D, H); 0:4 (E, F, I)).

tended to be more nanoparticles and much shorter nanorods owing to the greatly intensified micromixing, while the product prepared in the STR (Figure 6A) appeared as more apparently E

DOI: 10.1021/acs.iecr.6b04902 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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of HAp nanorods with different aspect ratios. It could be seen that the as-prepared pectin/HAp nanocomposite cements had smooth and dense surfaces. SEM images indicated that the HAp nanorods had the original appearance and had no fractures even at higher aspect ratio in the hardened cements. The mechanical properties of the pectin/HAp nanocomposite cements were further studied. Figure 8 depicts the compressive

rodlike nanoparticles with larger particle width and a little higher aspect ratio. With the increased content of CO(NH2)2, despite the STR or the RPB, the particle width and the aspect ratio of HAp nanorods had a rapid increase. In particular, there was more obvious difference of the particle width and the aspect ratio between the STR and the RPB. HAp nanorods obtained in the RPB exhibited smaller particle width and higher aspect ratio. Furthermore, HAp nanorods with a larger aspect ratio ranging from 2.2 to 39 could be prepared in the RPB, while the counterpart had only an aspect ratio range of 3.4 to 22. Such a result could be mainly ascribed to the truth that the decrease of the particle width directly led to the increase of the aspect ratio (except NH3·H2O:CO(NH2)2 of 4:0). From the above results, it could also be deduced that high gravity environment contributed to the preferentially anisotropic growth of HAp nuclei along the c-axis direction at low OH− ions from the action of high-content CO(NH2)2. Certainly, the more detailed formation mechanism will be studied in the future. More importantly, the RPB reactor could achieve a continuous precipitation process (about 1 s) while the STR had a batch operation and longer reaction time (20 min), which is very benefical to the mass production of HAp nanorods.45 3.4. Application of HAp Nanorods for Pectin/HAp Nanocomposite Cements. A calcium phosphate cement scaffold is commonly adopted to repair bone defects. However, its low compressive strength and poor osteogenesis greatly limit its clinical application. As a natural, biocompatible, biodegradable, water-soluble polysaccharide, pectin can improve the osteogenic activity of biomaterials.46 Figure 7 presents the digital photograph and SEM images of the surface morphologies of pectin/HAp nanocomposite cements with the addition

Figure 8. Compressive strengths (A) and stress−strain curves (B) of pectin/HAp nanocomposite cements.

strengths and the corresponding stress−strain curves of pectin/ HAp nanocomposite cements. Clearly, the compressive strength of pectin/HAp nanocomposite cement first increased and then declined with the increased average aspect ratio in the range 2.2−39. At an average aspect ratio of 15, there was the highest compressive strength of 29.4 MPa (Figure 8A), which was much higher than that (2−12 MPa) of cancellous bone.47 Furthermore, as shown in the stress−strain curves of Figure 8B, HAC1 exhibited a mechanical response belonging to brittle materials. Also, the increase of the stress had an almost linear relationship with the increase of strain, and a following failure. At failure, the cements fractured into several pieces. Other samples showed a plastic response owing to the addition of higher aspect ratio, had no fractures, and retained integrity after the testing. The results indicated that the rodlike morphology was beneficial to the improvement of the mechanical properties, and could effectively increase the elastic modulus and tensile strength.47−49 However, the compressive strength reached a maximum and dropped with the increasing aspect ratio since HAp nanorods with excessive aspect ratio were easily broken. Therefore, too short or too long HAp nanorods are not the most suitable for the preparation of high-performance pectin/ HAp nanocomposite cement.

Figure 7. Phototgraph of pectin/HAp nanocomposite cements (A) (a, HAC1; b, HAC3; c, HAC5), and SEM images of surface morphologies of pectin/HAp nanocomposite cements (B, HAC1; C, HAC2; D, HAC3; E, HAC4; F, HAC5). F

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(8) Zhang, C.; Li, C.; Huang, S.; Hou, Z.; Cheng, Z.; Yang, P.; Peng, C.; Lin, J. Self-activated luminescent and mesoporous strontium hydroxyapatite nanorods for drug delivery. Biomaterials 2010, 31, 3374. (9) Kozlova, D.; Chernousova, S.; Knuschke, T.; Buer, J.; Westendorf, A. M.; Epple, M. Cell targeting by antibody-functionalized calcium phosphate nanoparticles. J. Mater. Chem. 2012, 22, 396. (10) Wagner, D. E.; Eisenmann, K. M.; Nestor-Kalinoski, A. L.; Bhaduri, S. B. A microwaveassisted solution combustion synthesis to produce europium-doped calcium phosphate nanowhiskers for bioimaging applications. Acta Biomater. 2013, 9, 8422. (11) Chen, F.; Huang, P.; Zhu, Y. J.; Wu, J.; Cui, D. X. Multifunctional Eu3+/Gd3+ dual-doped calcium phosphate vesicle-like nanospheres for sustained drug release and imaging. Biomaterials 2012, 33, 6447. (12) Morgan, T. T.; Goff, T. M.; Adair, J. H. The colloidal stability of fluorescent calcium phosphosilicate nanoparticles: the effects of evaporation and redispersion on particle size distribution. Nanoscale 2011, 3, 2044. (13) Ashokan, A.; Gowd, G. S.; Somasundaram, V. H.; Bhupathi, A.; Peethambaran, R.; Unni, A. K.; Palaniswamy, S.; Nair, S. V.; Koyakutty, M. Multifunctional calcium phosphate nano-contrast agent for combined nuclear, magnetic and near-infrared in vivo imaging. Biomaterials 2013, 34, 7143. (14) Dorozhkin, S. V. Nanosized and nanocrystalline calcium orthophosphates. Acta Biomater. 2010, 6, 715. (15) Fratzl, P.; Gupta, H. S.; Paschalis, E. P.; Roschger, P. Structure and mechanical quality of the collagen-mineral nano-composite in bone. J. Mater. Chem. 2004, 14, 2115. (16) Murphy, W. L.; Mooney, D. J. Bioinspired growth of crystalline carbonate apatite on biodegradable polymer substrata. J. Am. Chem. Soc. 2002, 124, 1910. (17) Byrappa, K.; Yoshimura, M. Handbook of Hydrothermal Technology: A Technology for Crystal Growth and Materials Processing; N. J. Noyes Publications: 2001. (18) Gao, S.; Sun, K.; Li, A.; Wang, H. Synthesis and characterization of hydroxyapatite nanofiber by chemical precipitation method using surfactants. Mater. Res. Bull. 2013, 48, 1003. (19) SL Shanthi, S.; Ashok, M.; Balasubramanian, R.; Riyasdeen, A.; Akbarsha, M. A. Synthesis and characterization of nano-hydroxyapatite at ambient temperature using cationic surfactant. Mater. Lett. 2009, 63, 2123. (20) Manafi, S.; Rahimipour, M. R. Synthesis of nanocrystalline hydroxyapatite nanorods via hydrothermal conditions. Chem. Eng. Technol. 2011, 34, 972. (21) Taheri, M. M.; Kadir, M. R. A.; Shokuhfar, T.; Hamlekhan, A.; Assadian, M.; Shirdar, M. R.; Mirjalili, A. Surfactant-assisted hydrothermal synthesis of fluoridated hydroxyapatite nanorods. Ceram. Int. 2015, 41, 9867. (22) Sun, Y.; Guo, G.; Wang, Z.; Guo, H. Synthesis of single-crystal HAP nanorods. Ceram. Int. 2006, 32, 951. (23) Galea, L.; Bohner, M.; Thuering, J.; Doebelin, N.; Ring, T. A.; Aneziris, C. G.; Graule, T. Growth kinetics of hexagonal submicrometric beta-tricalcium phosphate particles in ethylene glycol. Acta Biomater. 2014, 10, 3922. (24) Nga, N. K.; Giang, L. T.; Huy, T. Q.; Viet, P. H.; Migliaresi, C. Surfactant-assisted size control of hydroxyapatite nanorods for bone tissue engineering. Colloids Surf., B 2014, 116, 666. (25) Nguyen, N. K.; Leoni, M.; Maniglio, D.; Migliaresi, C. Hydroxyapatite nanorods: soft-template synthesis, characterization and preliminary in vitro tests. J. Biomater. Appl. 2013, 28, 49. (26) Zhang, H.; Darvell, B. W. Morphology and structural characteristics of hydroxyapatite whiskers: effect of the initial Ca concentration, Ca/P ratio and Ph. Acta Biomater. 2011, 7, 2960. (27) Zhang, H.; Darvell, B. W. Synthesis and characterization of hydroxyapatite whiskers by hydrothermal homogeneous precipitation using acetamide. Acta Biomater. 2010, 6, 3216.

4. CONCLUSIONS In this study, HAp nanorods with a controlled aspect ratio were successfully synthesized with NH3·H2O and CO(NH2)2 as aspect ratio regulator by a combination of a high-gravity precipitation method in the RPB reactor and hydrothermal treatment. The average aspact ratio of HAp nanorods could be adjusted from 2.2 to 39 by altering the molar ratio of NH3· H2O and CO(NH2)2 from 4:0 to 0:4. By comparing with the STR, it was found that the RPB reactor had HAp nanorods with a smaller particle width and a wider range of aspect ratio, as well as a shorter reaction time from 20 min to 1 s. Afterward, pectin/HAp nanocomposite cements were prepared by using HAp nanorods with various aspect ratios. With the increase of the average aspect ratio, the compressive strength had a first increase and subsequent decrease, reaching the highest value of 29.4 MPa at an average aspect ratio of 15. This study will provide a new route to the preparation of high-quality HAp nanorods for bone tissue engeering.

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-10-64447274. Fax: +86-10-64423474. E-mail: [email protected] (J.-X.W.). *Tel.: +86-10-64421905. Fax: +86-10-64434784. E-mail: [email protected] (Y.P.). ORCID

Jie-Xin Wang: 0000-0003-0459-1621 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Key Research and Development Program of China (2016YFA0201701/ 2016YFA0201700), National Natural Science Foundation of China (21622601 and 21576022), National 973 Program of China (2015CB932100), and Guangdong Provincial Applied Science and Technology Research and Development Project (2015B090927001).

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REFERENCES

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DOI: 10.1021/acs.iecr.6b04902 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX