Synthesis, Characterization, Functionalization and Bio-Applications of Hydroxyapatite Nanomaterials: An Overview

Figures & data

Figure 1 Hierarchical bone configuration at varying size scales. The cortical bone microstructure is made up of osteons with lamellae Haversian channels, while the structural units at the nanoscale are collagen threads having rolls of mineralized collagen fibrils.

Table 1 Different Salts of Calcium Phosphate

Name	Symbols	Chemical Formula	Ca/P Ratio	Ref.
Tetracalcium phosphate	TTCP	Ca4(PO4)2O	2.0	[^Citation141]
Hydroxyapatite	HA or HAp	Ca10(PO4)6(OH)2	1.67	[^Citation9,^Citation10]
Fluoroapatite	FA or FAp	Ca10(PO4)6F2	1.67	[^Citation23]
Oxyapatite	OA or OAp	Ca10(PO4)6O	1.67	[^Citation14]
Calcium deficient hydroxyapatite	CDHA	Ca10-x (HPO4)x (PO4)6-x(OH)2-x (0 < x < 1	1.5–1.67	[^Citation5]
β-Tricalcium phosphate	β-TCP	Ca3(PO4)2	1.5	[^Citation36]
α-Tricalcium phosphate	α-TCP	Ca3(PO4)2	1.5	[^Citation7]
Octacalcium phosphate	OCP	Ca8(HPO4)2(PO4)4.5H2O	1.33	[^Citation106]
Amorphous calcium phosphate	ACP	Cax(PO4)y.nH2O	1.2–2.2	[^Citation28]
Dicalcium phosphate anhydrous (Monetite)	DCPA	CaHPO4	1.0	[^Citation44]
Dicalcium phosphate dehydrate (Brushite)	DCPD	CaHPO4.2H2O	1.0	[^Citation32]
Monocalcium phosphate anhydrous	MCPA	Ca(H2PO4)2	0.5	[^Citation93]
Monocalcium phosphate monohydrate	MCPM	Ca(H2PO4).2H2O	0.5	[^Citation2]

Figure 2 Hydroxyapatite crystal structure, (A) top view and (B) side view. Dashed lines represent one line.

Table 2 Techniques for the Synthesis of Hydroxyapatite Nanomaterials and Their Features

Method	Symbol	Synthesis Time	Temp (^oC)	Size (µm)	Distribution	Shape	Crystallinity	Phase Purity	Cost	Ref.
Core-shell	CS	>24	60–120	> 0.01	Narrow	Diverse	Variable	Variable	Variable	[^Citation10,^Citation88]
Electrospinning	ES	> 24	-	10 x 30 nm	Variable	Fibers	High	Variable	Variable	[^Citation21]
Self-propagating combustion	SPC	< 24	170–500	> 0.45	Wide	Diverse	Variable	High	Low	[^Citation26]
Microemulsion	MEm	> 24	RT	> 1	Narrow	Needle like	Low	Variable	High	[^Citation27,^Citation31]
Microwave irradiation	MI	< 24	-	100 x 25 nm	Narrow	Diverse	High	High	Variable	[^Citation29,^Citation30]
Flux cooling	FC	< 24	200	18 x 2.1 nm	Wide	Hexagonal cylinder	High	Variable	Variable	[^Citation32]
Chemical precipitation	CP	> 24	RT	> 0.1	Variable	Diverse	Low	Variable	Low	[^Citation34,^Citation35]
Hydrothermal	HT	< 24	150–400	> 0.05	Wide	Needle-like	High	High	High	[^Citation40]
Electro spraying	ESp	>	-	75 x 40 nm	Wide	Diverse	Variable	Variable	Low	[^Citation42]
Solid-state	SS	>	1150 ± 100	> 2	Wide	Diverse	High	Low	Low	[^Citation43]
Sol-gel	SG	>	37–85	> 0.001	Narrow	Diverse	Variable	Variable	Variable	[^Citation44]

Figure 3 Hydroxyapatite characterization by (A) FTIR spectrum of HA-NMs calcined at 400°C (a and b), while (c) signifies IR spectra of HA at different temperature. Figure A reprinted with permission from Elsevier, from: Kalita SJ, Verma S. Nanocrystalline hydroxyapatite bioceramic using microwave radiation: Synthesis and characterization. Mater Sci Eng C. 2010;30:295–303.⁶⁵ Copyright © 2010 Elsevier. (B) XPS spectrum showing the presence of Ca, O, P with small amounts of contamination (such as C), (C) TOF-SIMS spectra (a) Positive and (b) negative. Figures B and C reprinted with permission from American Chemical Society, from: Lu HB, Campbell CT, Graham DJ, Ratner BD. Surface characterization of hydroxyapatite and related calcium phosphates by XPS and TOF-SIMS. Anal Chem. 2000;72:2886–2894.⁶⁶ Copyright © 2000 American Chemical Society. (D) Nitrogen physisorption isotherms and BJH pore size distribution (E) adsorption (F) desorption. Figures D-F reprinted with permission from American Chemical Society, from: Munir MU, Ihsan A, Javed I et al. Controllably biodegradable hydroxyapatite nanostructures for cefazolin delivery against antibacterial resistance. ACS Omega. 2019;4(4):7524–7532.⁶⁷ Copyright © 2019 American Chemical Society (https://pubs.acs.org/doi/10.1021/acsomega.9b00541; further permissions related to the material excerpted should be directed to the ACS).

Figure 4 Hydroxyapatite nanomaterials characterization (A) (a) An illustration indicating the synthetic process of hollow mesoporous hydroxyapatite nanoparticles (hmHANP). SEM images of (b) CaCO₃ nanoparticles, (c) CaCO₃/HA core/shell nanocomposites, (d) hmHANP; TEM images of (e) CaCO₃ nanoparticles, (f) CaCO₃/HA core/shell nanocomposites, (g) hmHANP (inset: dark-field STEM image). Figure A republished with permission of Royal Society of Chemistry, from: Yang YH, Liu CH, Liang YH, Lin FH, Wu KCW. Hollow mesoporous hydroxyapatite nanoparticles (hmHANPs) with enhanced drug loading and pH-responsive release properties for intracellular drug delivery. J Mater Chem B. 2013;1:2447–2450.,⁶⁸ copyright © 2013 Royal Society of Chemistry; permission conveyed through Copyright Clearance Center, Inc. (B) XRD pattern of HA-NMs synthesized by hydrothermal treatment at 180 °C for (a) 12, (b) 24, and (c) 48 h. (C) Raman spectra of nanodisks (a) and nanorings (b). Inset shows the deviation of the specific peak. Figures B and C reprinted with permission from the American Chemical Society, from: Nathanael AJ, Hong SI, Mangalaraj D, Ponpandian N, Chen PC. Template-free growth of novel hydroxyapatite nanorings: formation mechanism and their enhanced functional properties. Cryst Growth Des. 2012;12:3565–3574.⁶⁴ Copyright © 2012 American Chemical Society.

Table 3 Various Methods Applied for the Modification of Hydroxyapatite

Modifying Substance	Method	Ref.	Modifying Substance	Method	Ref.
Insulin	Grafting	[^Citation10]	L-lactide (LA)	Polymerization	[^Citation67]
BMP-2	Grafting	[^Citation49]	Poly L-lactide (PLLA)	Grafting	[^Citation68]
Hydro stearic acid	Direct reaction	[^Citation54]	Vinylphosphonic acid	Grafting	[^Citation69]
Decanoic and hexanoic acids	Direct reaction	[^Citation57]	Ethylene glycol methacrylate phosphate	Polymerization	[^Citation121]
Pyrophosphoric acid (PP)	Direct reaction	[^Citation59]	Glycidoxy propyltrimethoxy silane	Coating	[^Citation67]
Oleic acid	Direct reaction	[^Citation60]	Polyacrylic acid	Coating	[^Citation37]
Sodium dodecyl sulphate (SDS)	Direct reaction	[^Citation62]	ϒ-Benzyl-L-glutamate N-carboxy anhydride	Polymerization	[^Citation72]
Polyvinyl pyrrolidone (PVP) and serum albumin	Grafting	[^Citation63]	ϒ-Methacroyloxy trimethyl silane	Coating	[^Citation75]
Silanes	Coating	[^Citation64]	L-Phenylalanine N-carboxy anhydride	Polymerization	[^Citation79]

Figure 5 Schematic diagram to express the role of hydroxyapatite nanomaterials in drug delivery.

Figure 6 (A) Oriented Immobilization of β-Lactamase and enhanced green fluorescent protein (EGFP) on HA through hydrazine bisphosphonates (HBPs). (B) Immobilization of EGFP and (C) β-lactamase on HA surfaces determined by fluorescence and BCA protein assay, respectively. EGFP and β-lactamase were immobilized on HA surfaces via seven different HBPs (1−7) and by simple adsorption like HA-EGFP and HA-BL, respectively. The corresponding EGFP and β-lactamase are denoted as HA-1-EGFP through HA-7-EGFP and HA-1-BL through HA-7-BL, respectively. HA-EGFP and HA-BL are referred to EGFP and β-lactamase physically adsorbed on HA in the absence of HBP. (* indicates the values are significantly different from others p < 0.05, while ** indicates the values are significantly different from HA-1-BL and HA-5-BL p < 0.05). Figures A-C reprinted with permission from American Chemical Society from: Yewle JN, Wei Y, Puleo DA, Daunert S, Bachas LG. Oriented immobilization of proteins on hydroxyapatite surface using bifunctional bisphosphonates as linkers. Biomacromolecules. 2012;13:1742–1749.¹³² Copyright © 2012 American Chemical Society.

Figure 7 Schematic diagram representing phases of colonization mechanisms anticipated for HA-NMs coating. (A) Adhesion phenomenon starts in the cells by confining anchoring/adhesion spots in micropore surfaces. (B) These cells give response to adjacent hierarchical environment. (C) The growth of filopodia is guided by coating material penetration and resulting formation of nano sized lengthy morphology. Figures A-C reprinted with permission from American Chemical Society from: Nasiri N, Mukherjee S, Panneerselvan A, Nisbet DR, Tricoli A. Optimally hierarchical nanostructured hydroxyapatite coatings for superior prosthesis biointegration. ACS Appl Mater Interfaces. 2018;10(29):24840–24849.¹⁴² Copyright © 2018 American Chemical Society. (D) Mixed state including (i) PLGA rich state (ii) HA rich state (E) Grafting state (i) short chain/large number (ii) long chain/small number (F) Tensile strength measurements of PLGA-HA and interaction evaluation of tensile strength with and without PLGA-HA composite. Figures D-F reprinted with permission from Elsevier from: Park JW, Hwang JU, Back JH et al. High strength PLGA/Hydroxyapatite composites with tunable surface structure using PLGA direct grafting method for orthopedic implants. Compos Part B Eng. 2019;178:107449.¹⁶⁰ Copyright © 2019 Elsevier.

Figure 8 Porous HA ceramics (A) macroscopic image (B) SEM image representing spherical pores of diameter 100–200 μm are unified by pores with diameter 10–80 μm. Republished with permission from The Royal Society from: Yoshikawa H, Tamai N, Murase T, Myoui A. Interconnected porous hydroxyapatite ceramics for bone tissue engineering. J R Soc Interface. 2009;6(suppl_3):S341-S348.¹⁹² Copyright © 2009; permission conveyed through Copyright Clearance Center, Inc.

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 Web of Science ®Google Scholar

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 PubMed Web of Science ®Google Scholar