Nanostructured titanium dioxide for use in bone implants: a short review

Dióxido de titânio nanoestruturado para uso em implantes ósseos: uma breve revisão

Abstract

Titanium dioxide (TiO₂) based nanostructured materials have shown great potential for use in implants thanks to their excellent physicochemical properties, such as high specific surface area, ability to elicit positive cell response, and stability in body fluids. However, there are few studies in the literature that focus on the use of nanostructured TiO₂ to support cell growth and bone regeneration. The purpose of this survey is to review the state of the art of TiO₂ for use in bone implants, as well as provide insight into its characteristics and synthesis methods. Studies of the biological properties of nanostructured TiO₂ are described, establishing its potential for the biomedical field.

Keywords: nanostructured materials; titanium dioxide; biomaterials

Resumo

Os materiais nanoestruturados à base de dióxido de titânio (TiO₂) têm demonstrado grande potencial para uso em implantes graças às suas excelentes propriedades físico-químicas, como alta área superficial específica, capacidade de provocar resposta celular positiva e estabilidade nos fluidos corporais. No entanto, existem poucos estudos na literatura que se concentram no uso de TiO₂ nanoestruturado para fins de suporte ao crescimento celular e regeneração óssea. O objetivo desta pesquisa é revisar o estado da arte do TiO₂ para uso em implantes ósseos, além de fornecer informações sobre suas características e métodos de síntese. São descritos estudos das propriedades biológicas do TiO₂ nanoestruturado, estabelecendo seu potencial para o campo biomédico.

Palavras-chave: materiais nanoestruturados; dióxido de titânio; biomateriais

INTRODUCTION

Materials for bone repair or regeneration are needed in many situations in orthopedics and orthodontics and are critical for people suffering from osteoporosis, bone cancer, joint and spinal disease, and disorders. As the world population ages and cases of osteoporosis and bone trauma increase sharply among the elderly, the number of individuals that need bone implants to boost bone regeneration has grown dramatically in recent decades ^{1), (2}. The use of autogenous and allogeneic bone grafts may help in the treatment of bone lesions. However, they have numerous disadvantages, such as the need for secondary surgery, limitation in the number of grafts, risk of infections, and immune responses, which may cause other severe health problems ^{3), (4}. In recent years, most orthopedic research has focused on nanostructured biomaterials due to their similarities to the natural physiological environment ^{5)- (11}. Nanoscale morphology has unique properties such as high surface area and a higher degree of biological plasticity than other microstructures ^{12)- (14}. Studies have shown that the surface or chemical properties of these microstructures are similar to those of native bone, affecting how cells adhere to their surface, their biochemical functions, and cell differentiation ^{15)- (17}.

Several materials have been studied for the fabrication of nanostructures for biomedical applications in bone repair ^{18)- (24}, including TiO₂. The mechanical properties, biocompatibility, low cytotoxicity, stability in body fluids ^{25)- (30}, and corrosion resistance of TiO₂ give it a great potential for use in bone implants ^{31), (32}. Researches ^{15), (33)- (35} have shown that nanostructured TiO₂ elicits a favorable molecular response and osseointegration, with better bone formation than non-nanostructured materials. However, despite advances in the development of nanostructured TiO₂ systems for bone repair, review articles addressing this topic are still scanty. Therefore, this paper aims to review the characteristics and fabrication methods of nanostructured titanium dioxide and to focus on studies that evaluated its potential for bone implants.

CHARACTERISTICS OF TITANIUM DIOXIDE

Titanium dioxide is a white solid that is technologically important due to its numerous properties ^{36), (37}, such as low modulus of elasticity, good tensile strength, biocompatibility, and corrosion resistance, as well as its abundance ^{38)- (41}. TiO₂ has been used in a variety of applications, e.g., in biomedicine as catalyst support ^{37), (42}, in water and air purification systems ^{17), (43}, in pigments or opacifiers, in cosmetics ^{36), (37} and solar cells ^{44), (45}, and in various biomedical applications ^{46), (47}.

Under atmospheric pressure, TiO₂ exists in 3 polymorphic forms (Table I), known as rutile, anatase, and brookite ^{38), (39}. These polymorphs can be described by a Ti⁴⁺ cation coordinated by 6 oxygen atoms, forming a distorted octahedron ^{45), (48), (49}, whose polymorphic shapes are dictated by the way these octahedrons combine. The anatase form has a tetragonal structure and a unit cell with 4 titanium dioxide molecules (Fig. 1a). The rutile form also has a tetragonal structure and two titanium dioxide molecules per unit cell (Fig. 1b). Brookite has an orthorhombic structure and a unit cell containing 8 molecules (Fig. 1c). Compared to anatase and rutile, brookite is the least dense form of TiO₂^{42), (44), (50), (51}. The properties of TiO₂ vary according to the polymorphic phase (Table I). Rutile is the most stable phase, while anatase and brookite are metastable and can be irreversibly transformed into rutile at high temperatures. This phase transition and stability can be influenced by impurities, defects, grain size, reaction atmosphere, and synthesis conditions ⁴⁹. In addition, the stability of polymorphic phases depends on grain size. It has been reported that anatase is stable in crystallite sizes smaller than 11-45 nm due to the high surface free energy associated with this particle size ^{50), (52}.

Table I
Properties of anatase, rutile, and brookite.
Tabela I
Propriedades do anatásio, rutilo e brookita.

Property	Anatase	Rutile	Brookite	Ref.
Structure	Tetragonal	Tetragonal	Orthorhombic	45 ), ( 53 )-( 56
Space group	I41/amd	P42/mnm	Pbca	43 ), ( 44 ), ( 57 )-( 59
Unit cell parameters (nm)	a=0.3733-0.3785	a=0.45936-0.4594	a=0.917-0.9184	44 ), ( 45 ), ( 50 ), ( 57 ), ( 58 ), ( 60 )-( 62
	c=0.937-0.9515	c=0.2953-0.29589	b=0.5447-0.546
			c=0.514-0.5154
Cell volume (Å3)	34.02-34.061	31.12-31.216	32.172-32.20	43 ), ( 45 ), ( 57 )-( 59
Ti-O bond length (Å)	1.937-1.94(4)	1.946-1.95(4)	-	43 ), ( 51 ), ( 57 ), ( 58
	1.965-1.97(2)	1.980-1.984(2)
O-Ti-O bond angle	77.7-102°	81.1-98.9°	80.7-105°	45 ), ( 57 ), ( 58 ), ( 63
Specific density (g/cm3)	3.79-3.83	4.13-4.25	3.99-4.170	44 ), ( 50 ), ( 54 ), ( 59 ), ( 62
Band gap energy (eV)	3.20-3.30	3.01-3.02	3.13-3.26	41 ), ( 61 ), ( 64 )-( 70

Figure 1:
Crystalline structure of: a) anatase; b) rutile; and c) brookite. Gray and white spheres represent O and Ti atoms, respectively.
Figura 1:
Estrutura cristalina de: a) anatásio; b) rutilo; e c) brookita. Esferas cinza e branca representam átomos de O e Ti, respectivamente.

TiO₂ is an n-type semiconductor whose electronic and optical properties are influenced by the nature of the conduction band ^{71), (72}. The photocatalytic process is based on the absorption of UV light by titanium dioxide. When the level of photon energy is greater than or equal to that of the band gap, the photoexcited electrons jump from the valence band to the conduction band, leaving electron holes (Eq. A). In aqueous media, reactions may occur between excited TiO₂ and the medium. Electron holes interact with water or with the hydroxyl radical, forming hydrogen and hydroxyl ions (Eqs. B and C). Oxygen can also be adsorbed on titanium dioxide particles, reacting with electrons in the conduction band and generating superoxide (Eq. D) ^{37), (73)- (75}:

${\mathrm{TiO}}_2+\mathrm{hv}\left(\mathrm{UV}\right)\leftrightarrow {\mathrm{h}}^{+}+{\mathrm{e}}^{-}$ (A)

${\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O}\leftrightarrow {\mathrm{H}}^{+}{+}^{\dot{}}\mathrm{OH}$ (B)

${\mathrm{h}}^{+}+{\mathrm{OH}}^{-}{\leftrightarrow }^{\dot{}}\mathrm{OH}$ (C)

${\mathrm{e}}^{-}+{\mathrm{O}}_2{\leftrightarrow }^{\dot{}}{\mathrm{O}}_2^{-}$ (D)

where, h⁺ and e^- represent an unpaired electron, an electron hole in the valence band, and an electron in the conduction band, respectively, and hν is the photon energy ⁷⁴. Rutile and anatase are photoactive, and their electrons are excited in the range of ultraviolet radiation ^{72), (76)- (79}. The structure of anatase has higher photocatalytic activity than that of rutile ^{53), (80}, because of its greater density of localized states and its lower electron-hole recombination rate than rutile ⁷⁴.

Nanostructured TiO₂ has attracted great attention in recent years because it can be used in various fields of science and technology ^{81)- (87}. At least one of the dimensions of nanostructured TiO₂ polymorphs ranges from 1 to 100 nm. Depending on the field of science and technology, materials that have one of its dimensions larger than 100 nm and smaller than 1000 nm are also referred to as nanometric or are considered submicrometric. Nanometric materials, which have a high surface area to volume ratio, comprise thin films, nanotubes, nanowires, nanostrips, nanofibers, and nanoparticles ^{72), (78), (88}. Nanostructured TiO₂ has been described as a promising non-resorbable material for use in bone implants ^{41), (89), (90}. Recent studies ^{16), (21), (91)- (94} indicate that nanoscale modification of the TiO₂ surface topography can positively influence cell behavior, improving implant osseointegration and aiding greater initial biointegration with surrounding tissue ^{95), (96}. Bone is a nanostructured compound of collagen and hydroxyapatite fibers ^{14), (17), (97), (98}. Hence, nanoscale surface topography mimics the topographic characteristics of the extracellular matrix of bone tissue, providing cellular support, and in some cases, adequate porosity for the process of cell adhesion, proliferation, and differentiation during the bone formation process ^{29), (99), (100}.

TiO₂ can also be used as a coating for metal implants in tissue engineering applications. Ti osseointegration can be improved by changing the surface, such as depositing a thicker layer of TiO₂ on the metal surface ^{101)- (103}. It has been reported that TiO₂ coatings manufactured on the surface of a metal alloy improve corrosion resistance, biocompatibility, and promote good cell functionality and bone formation ^{104)- (107}. Composite materials also have satisfactory properties for biomaterials due to the good combination of mechanical properties and excellent biocompatibility. Studies have shown that hydroxyapatite and titanium dioxide (HAp-TiO₂) composites improve mechanical resistance, osseointegration, and cell fixation due to the similarity of hydroxyapatite to various calcified tissues of vertebrates. TiO₂ is able to improve the bond strength of the hydroxyapatite layer and Ti substrate, as well as corrosion resistance ^{108)- (111}. Some authors ^{112)- (116} have found that the presence of hydroxyapatite increases the resistance to corrosion, the integration of implants in bone tissue, and bone growth. In addition, the HAp-TiO₂ composite has excellent chemical and structural uniformity.

FORMS OF SYNTHESIS USED IN THE PRODUCTION OF BIOMATERIALS

Nanostructured TiO₂ can be synthesized by several methods, including anodization ¹¹⁷, sol-gel ^{118), (119}, hydrothermal ^{89), (120}, and electrospinning ^{121)- (123}. Among these techniques, anodization and sol-gel have attracted the attention of research groups for biomedical applications, due to their simplicity, low-cost ^{124), (125}, ability to produce nanostructures with high surface area to volume ratios ^{124), (126}, and their ability to generate highly organized and corrosion resistant structures ^{12), (127}. Table II describes the characteristics of nanostructured TiO₂ produced by anodization and sol-gel. Several properties of nanoscale TiO₂ can significantly improve the performance of biomaterials, which can be affected by the method employed for their fabrication ^{41), (128)- (132}. Therefore, depending on the experimental conditions, synthesized materials have different characteristics ^{31), (133), (134}, including pore structure and interconnectivity, nanotopography, and mechanical properties ^{135)- (139}.

Table II
Processing methods of nanostructured TiO₂.
Tabela II
Métodos de síntese de TiO₂ nanoestruturado.

Method	Calcination temperature	Morphology	Phase	Size	Ref.
Anodization of Ti	530 °C	Nanotube	Anatase	Nanotube diameter: 70-110 nm	105
Anodization of Ti	450 °C	Nanotube	Anatase	Nanotube diameter: 160-200 nm	140
Anodization of Ti	420 °C	Nanotube	Anatase	Nanotube diameter: 45-60 nm	141
Anodization of Ti	450 °C	Nanotube	Anatase	Nanotube diameter: 26-115 nm	142
Anodization of Ti	450 °C	Nanotube	Anatase	Nanotube diameter: 65.7-87 nm	143
Anodization of Ti	750 °C	Nanotube	Anatase, rutile	Nanotube diameter: 50-70 nm	144
Anodization of Ti	-	Nanotube	-	Nanotube diameter: 174.2-235.3 nm	145
Anodization of Ti	450 °C	Nanotube	Anatase	Nanotube diameter: 45 nm	146
Anodization of Ti	500 °C	Nanotube	Anatase	Nanotube diameter: 30-90 nm	147
Anodization of Ti	480 °C	Nanotube	Anatase	Nanotube diameter: 40-80 nm	148
Anodization of Ti	450 °C	Nanotube	Anatase	Nanotube diameter: 20-120 nm	149
Anodization of Ti	500 °C	Nanotube	Anatase	Nanotube diameter: 80-100 nm	150
Anodization of Ti	580 °C	Nanotube	Anatase	Nanotube diameter: 165 nm	151
Anodization of Ti	-	Nanotube	-	Nanotube diameter: 15-120 nm	152
Anodization of Ti	480 °C	Nanotube	Anatase, rutile	Nanotube diameter: 40-110 nm	153
Anodization of Ti	-	Nanotube	-	Nanotube diameter: 50-100 nm	154
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 11-22 nm	155
Sol-gel	500 °C	Nanoparticle	Anatase	Particle size: 51.98 nm	156
Sol-gel	800 °C	Nanoparticle	Anatase, rutile	Particle size: 47 nm	157
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 17.7 nm	158
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 13.8 nm	159
Sol-gel	450 °C	Nanoparticle	Anatase	Crystallite size: 10 nm	160
Sol-gel	500 °C	Nanoparticle	Anatase	Crystallite size: 19.3 nm	161
Sol-gel	550 °C	Nanoparticle	Anatase	Particle size: 0.8 mm	162
Sol-gel	950 °C	Nanoparticle	Rutile	Particle size: 1.0 mm	162
Sol-gel	700 °C	Nanoparticle	Anatase, rutile	Crystallite size: 80 nm	163
Sol-gel	500 °C	Nanoparticle	Anatase	Crystallite size: 20 nm	163
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 3.9 nm	164
Sol-gel	450 °C	Nanoparticle	Anatase	Particle size: 50-100 nm	165
Sol-gel	350 °C	Nanoparticle	Anatase	Particle size: 30-90 nm	166
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 27.4 nm	167
Sol-gel	400 °C	Nanoparticle	Anatase	Crystallite size: 13.7 nm	56
Sol-gel	600 °C	Nanoparticle	Anatase, rutile	Crystallite size: 25.8 nm	56

Anodization is a process that involves the application of an electric field between the metal and an anode, which triggers ionic diffusion, resulting in the formation of ordered nanotubes on the anode surface. The process is performed when a two-electrode system is exposed to alternating voltage in an electrolyte under controlled conditions ^{90), (168}. TiO₂ nanotubes can be grown in aqueous hydrogen fluoride (HF) electrolytes containing acid ^{144), (150), (153} or in neutral mixtures with added fluoride salts ^{143), (146), (151}. The voltage applied in the anodization process affects the TiO₂ nanotube diameter. Several studies ^{142), (147)- (149), (152} reported that the TiO₂ nanotube diameter increases in response to increasing voltage (Fig. 2). In addition, as the applied voltage is increased, the surface morphology changed from particulate to tubular. Recently, other studies ^{105), (140), (141} also demonstrated that different TiO₂ nanotube diameters can be obtained by controlling the voltage applied in the anodization process. TiO₂ nanotube diameter can also be influenced by the type of electrolyte. In an early study ¹⁵⁴, titanium metal sheet in phosphate and fluorine electrolytes was used to form nanotubes; the results indicated that the nanotube diameters were 50 and 100 nm using NH₄F and H₃PO₄ electrolytes, respectively. In another study ¹⁴⁵, TiO₂ nanotubes were produced by anodizing titanium metal sheet in NH₄F and H₃PO₄ electrolyte; the authors also stated that changing the electrolyte from NH₄F to H₃PO₄ caused the nanotube diameter to increase from 174.2 to 235.3 nm.

Figure 2:
Influence of applied voltage on the diameter of TiO₂ nanotubes obtained by anodization.
Figura 2:
Influência da tensão aplicada no diâmetro dos nanotubos de TiO₂ obtidos por anodização.

In the sol-gel method, a solution transitions to a gel state that can solidify into an interconnected network of particles. The solution may be formed by means of hydrolysis and polymerization reactions ^{124), (126}. The firing temperature of gels affects the morphology of nanostructured TiO₂. Several studies ^{56), (155), (156), (158)- (161), (163)- (165), (167} reported that firing at up to 550 ºC resulted in spherical particles, and only anatase phase was found. At temperatures above 600 ºC, larger particles and development of the rutile phase were observed ⁵⁶. Peaks of the rutile phase observed after heating at 700 ºC were attributed to anatase to rutile phase transition ^{157), (162), (163}. The morphology of nanostructures can also be influenced by the ratio between quantities of reactants, notably the ratio of ceramic precursor to water. Nanocrystalline titanium dioxide powders were synthesized by the sol-gel method, varying the rate of hydrolysis ¹⁶³. The nanoparticles were synthesized by hydrolysis using as precursor 20 mL of titanium isopropoxide mixed with 100 mL of ethanol and different amounts of water to control the hydrolysis rate. The results indicated that powder prepared with 2 mL of water contained very fine anatase crystals and quasi-spherical nanoparticles. Upon increasing the amount of water to 5 and 10 mL, spherical and homogeneous structures were obtained in response to the increase in hydrolysis rate, which proves that increasing the amount of water facilitates particle formation ¹⁶³. TiO₂ nanoparticles were also produced by the sol-gel method using 0.5 g of titanium tetrabutoxide, 50 mL of diethylene glycol, and different amounts of water; it was found that, as the amount of water increased from 0.5 to 5 mL, the mean particle diameter decreased from 550 to 250 nm ⁵⁶.

STUDIES OF THE BIOLOGICAL PROPERTIES OF NANOSTRUCTURED TiO₂ USED IN BIOMEDICINE

As mentioned earlier herein, nanostructured TiO₂ has attracted the attention of researchers for the development of biomaterials ^{169)- (173} and implant technology ^{6), (71), (174)- (178}. Studies have shown that the cellular response is affected by the material’s topography ^{179)- (184}. Cells respond positively to nanotopography, with changes in cell morphology, cytoskeleton organization, and proliferation. This means that nanostructuring plays an important role in cellular interaction ^{15), (31}. Table III lists several studies on nanostructured TiO₂, revealing how nanoscale topography and crystal structure affect cell-surface interactions. It has been reported ^{31), (191), (204)- (208} that nanotubular surfaces produced by anodization are able to display cellular responses and simulate a bioactive matrix for cellular accommodation. This is due to increased surface wettability and selective adsorption of proteins in body fluids ^{12), (47}. TiO₂ nanotubes have good biocompatibility due to their low cytotoxicity, good stability, and cytocompatibility, including adhesion, proliferation, and differentiation of osteoblasts with high surface area to volume ratio ²⁰⁹. In this context, the TiO₂ nanotube surface favors osteoblast adhesion and exhibits a strong ability to bind to bone ²¹⁰. Moreover, it facilitates fluid exchange, which is responsible for improving molecular signaling in bone remodeling and functioning characteristics ²¹¹.

Table III
Studies of the biological properties of nanostructured TiO₂.
Tabela III
Estudos das propriedades biológicas do TiO₂ nanoestruturado.

Method	Phase	Type of cell	Type of test	Cell response	Ref.
Anodization	-	MG-63	Contact angle, atomic force microscopy, in vitro assay	Hydrophilicity; cells exhibit rounded morphology with small filopodia on the surface; high adhesion of MG-63 cells to TiO2 nanotubes	185
Anodization	Anatase	MC3T3	Contact angle, alkaline phosphatase (ALP) activity	Hydrophilicity; after 7 days of culture, ALP of MC3T3 grown in different samples exceeded the Ti substrate	17
Anodization	Anatase	G8080	Contact angle, cell proliferation	Hydrophilicity, with cells favoring cell proliferation	186
Anodization	Anatase + rutile	G8080	Contact angle, cell proliferation	TiO2 nanotubes less hydrophilic, with more stimulated proliferation, compared to that containing only anatase phase	186
Anodization	-	MC3T3-E1	Contact angle, atomic force microscopy, in vivo	Hydrophilicity; surface grown osteoblastic cells exhibit a well-distributed cytoskeleton organization; in vivo tests indicate that the anodized surface of the implant promotes osseointegration and greater bone binding strength than pure Ti substrate	187
Anodization	Anatase	hBMSCs	Contact angle, immersion in SBF, in vitro assay	Hydrophilicity; formation of hydroxyapatite after 21 days in simulated body fluid (SBF)	188
Anodization	Anatase	-	Contact angle	Hydrophilicity	189
Anodization	Anatase	MC3T3-E1	Contact angle, atomic force microscopy, immersion in SBF	Hydrophilicity; formation of hydroxyapatite after 12 days in SBF	190
Anodization	Anatase + rutile	SAOS-2	Contact angle, cell proliferation, fluorescence microscopy	Hydrophilicity; cells adhere to all the surfaces; seeded osteoblasts promote adhesion	191
Anodization	Anatase + rutile	-	Immersion in SBF	Formation of hydroxyapatite after 7 and 14 days in SBF	144
Anodization	Anatase	G-292	Scanning electron microscopy, immersion in SBF	Cells attached to the surface of nanotubes became increasingly elongated and formed filopodia; formation of hydroxyapatite after 5 and 10 days in SBF	192
Anodization	Anatase	MC3T3-E1	Immersion in SBF, in vitro assay	Formation of hydroxyapatite after 3 days in SBF, with size increasing as a function of immersion time	193
Anodization	Anatase + rutile	MC3T3-E1	Immersion in SBF, in vitro assay	Formation of hydroxyapatite after 3 days in SBF; a greater amount of hydroxyapatite was formed in the synthesized sample containing the anatase and rutile mixture than in the sample containing only anatase	193
Anodization	Anatase + rutile	MC3T3-E1	Atomic force microscopy, confocal laser scanning microscopy, Alizarin Red staining protocol	Cells grown on nanotubes disperse better and put forth more filopodia than smooth layers; cytoskeleton of osteoblasts grown on anatase display a regular arrangement	149
Anodization	-	HSCs	Scanning electron microscopy, cell proliferation	Cells normally disperse on nanotubes, forming several filopodia	194
Anodization	Anatase	MC3T3-E1	Atomic force microscopy, contact angle, alkaline phosphatase (ALP) activity	Osteoblasts grown on the surface of TiO2 nanotubes showed several filopodia extending along the main edges; hydrophilicity; extremely elongated cell morphology and high levels of alkaline phosphatase	174
Anodization	Anatase + rutile	OPC1	Atomic force microscopy, contact angle, alkaline phosphatase activity	Hydrophilicity; filamentous network-like structure with excellent cell to cell connection; ALP in osteoblasts was high after the 5th day of culture	195
Anodization	Anatase	-	Immersion in SBF	Formation of hydroxyapatite after 2 days in SBF	196
Anodization	-	G8080	Alkaline phosphatase activity, in vitro assays	Approximately 50% increase in ALP levels on nanotube surfaces after 3 weeks of culture; TiO2 nanotubes did not cause chronic inflammation or fibrosis under in vivo conditions	197
Anodization	Anatase	MC3T3-E1	Alkaline phosphatase activity, environmental scanning electron microscopy	Cells grown on TiO2 nanotubes presented filopodia; cell growth accelerated by up to 300-400%	198
Sol-gel	Anatase	-	Atomic force microscopy	Ca-doped TiO2 gels showed good adhesion	199
Sol-gel	Anatase	-	Immersion in SBF	Formation of hydroxyapatite after 28 days in SBF	200
Sol-gel	Anatase	-	Immersion in SBF	Formation of hydroxyapatite after 2 days in SBF	201
Sol-gel	Anatase	-	Immersion in SBF	Amount of hydroxyapatite formed after 14 days in SBF was greater in the gels containing only anatase than in those containing anatase and rutile mixture	202
Sol-gel	Anatase + rutile	-	Immersion in SBF	Formation of hydroxyapatite after 14 days in SBF	202
Sol-gel	-	-	Immersion in SBF	Formation of hydroxyapatite after 3 to 6 days in SBF	203

Several properties can affect cellular responses and protein adsorption in biological systems ^{47), (212}. Hydrophilicity and surface roughness ^{101), (213)- (216}, associated with the high surface area to volume ratio characteristic of nanostructured materials, have been shown to promote high cell adhesion, proliferation, and differentiation ^{217)- (220}. Numerous studies ^{16), (17), (174), (185), (189)- (191), (195} have shown that nanoscale TiO₂ nanotube surfaces present uniform structure, roughness, and hydrophilicity, with contact angles smaller than 55°. Porosity is essential for bone implants, being responsible for the formation of bone tissue, proliferation of osteoblasts, vascularization, and bioactivity. In addition, a porous structure provides good mechanical stability due to the better mechanical interlock between the biomaterial implant and the natural bone ^{203), (221), (222}. Some studies ^{223)- (227} report that a porous structure significantly accelerates cell adhesion and bone growth capacity. The pores must be interconnected with dimensions between 200 to 500 µm to meet the migration and transport requirements of the cell, providing adequate space and permeability for viable bone formation in nanostructured TiO₂.

Nanoscale topography has been shown to be the ideal size for the best osteoblast function ^{194), (228), (229}. According to ¹⁹⁸, osteoblast adhesion and propagation was improved by the topography of TiO₂ nanotubes, forming an interconnected cell structure. The nanoscale structure hastened the growth rate of MC3T3-E1 osteoblastic cells by up to 300% to 400%. Several studies ^{149), (174), (185), (187), (192), (194), (195} also found that cells attached to the surface of nanotubes became increasingly elongated and formed small filopodia, suggesting that they could be used for orthopedic purposes. Over the years, several researchers ^{144), (188), (190), (192), (193), (196} have found that the surfaces of TiO₂ nanotubes immersed in simulated body fluid (SBF) favor the formation of osteoblast-like hydroxyapatite, which is critical for osseointegration, with strong adhesion and binding ^{230), (231}. As noted by several authors ^{16), (35), (198), (232), (233}, the anatase and rutile phases provide adequate atomic arrangements for the formation of hydroxyapatite. Anatase provides the best effects for cell adhesion, proliferation, and differentiation because it is more chemically reactive. Nevertheless, the rutile phase is also interesting due to its higher modulus of elasticity, hardness, and adhesive strength than anatase. Hence, the anatase phase and the mixed anatase-rutile phase are beneficial for cell growth, due to their hydroxyapatite mineralization and increased corrosion resistance. It has been reported that the crystal structure of anatase facilitated osteoblast growth and exhibited structural stability in physiological medium ^{149), (174), (186), (188), (190), (192), (193), (196), (198}. Moreover, it has been suggested that osteoblasts disseminate better in anatase-rutile mixtures ^{186), (193), (195), (234}.

Alkaline phosphatase (ALP) activity serves to analyze the functionality of cells on a surface. TiO₂ nanotubes were produced through the anodization process and evaluated for bone cell interactions using ALP ¹⁹⁵. During each day of analysis, the cells showed a filamentous network-like structure scattered over the entire surface. It was reported that after the 5^th day of cultivation, the ALP level exceeded the titanium substrate ¹⁹⁵. It has been pointed out that ALP in osteoblasts gradually increased on the surface of TiO₂ nanotubes as the number of days of cultivation increased, showing excellent cell-to-cell binding ^{15), (17), (197), (198}. TiO₂ gels produced by the sol-gel technique can potentially be used in biomedical applications. The formation of hydroxyapatite immersed in SBF in amorphous and crystalline TiO₂ gels was investigated ^{202), (203}. TiO₂ gels with an amorphous structure did not induce the formation of hydroxyapatite on their surfaces in simulated body fluid, while gels with anatase or mixed anatase-rutile induced hydroxyapatite formation on their surfaces. Some authors ^{199)- (201} have discussed the influence of doping elements via the sol-gel method. They found that doping with calcium ions stimulates bioactivity, improving the formation of hydroxyapatite on the surface of nanostructured TiO₂.

For in vivo evaluations, it is essential for nanostructured biomaterials to be biocompatible and prevent inflammatory response. Such studies can be performed using animal models that can mimic the growth environment and bone infection. Several animals such as rats and pigs can be used for in vivo experiments, depending on research needs, proper bone size, ease of handling, and low costs ^{16), (235)- (238}. Popat et al. ¹⁹⁷ synthesized TiO₂ nanotubes by anodization and used the G8080 cell line. They assessed in vivo biocompatibility on surfaces of subcutaneous implants in male Lewis rats and made a histological analysis of the tissue around the implants after 4 weeks. The results indicated that TiO₂ nanotubes were uniform and caused no inflammation, and that the tissue was normal and healthy. Li et al. ¹⁸⁷ evaluated the behavior of MC3T3-E1 osteoblasts in experiments with Sprague Dawley rats. After 4 weeks they observed that the anodized implant surface promoted osseointegration. Their study demonstrated the good biological performance of TiO₂ nanotubes, which are able to withstand the forces they undergo during and after insertion into the bone.

CONCLUSIONS

The studies presented in this review of nanostructured TiO₂ demonstrate a promising perspective for tissue engineering, given the possible use of this nanomaterial in bone reconstruction. Biological responses can be controlled by modifying fabrication procedures, e.g., by applying variations in voltage, electrolytes, and using heat-treatments. Studies have shown that nanostructured TiO₂ immersed in simulated body fluid favors the formation of osteoblast-like hydroxyapatite. Equally important, in vivo assays have confirmed the biocompatibility and absence of inflammatory response of TiO₂ nanotubes, which is a material with great potential for application in bone regeneration.

ACKNOWLEDGEMENTS

The authors thank the research funding agencies CAPES (Finance Code 001: scholarship granted to Mrs. Ione Amorim Bezerra Neta) and CNPq (project 308822/2018-8 and 420004/2018-1).

REFERENCES

• ¹ J.I. Dawson, R.O. Oreffo, Arch. Biochem. Biophys. 473 (2008) 124.
• ² X. Lin, S. Yang, K. Lai, H. Yang, T.J. Webster, L. Yang, Nanomedicine 13 (2017) 123.
• ³ A.R. Amini, C.T. Laurencin, S.P. Nukavarapu, Crit. Rev. Biomed. Eng. 40 (2012) 363.
• ⁴ J.R. Venugopal, S. Low, A.T. Choon, A.B. Kumar, S. Ramakrishna, Artif. Organs 32 (2008) 388.
• ⁵ S. Balasubramanian, B. Gurumurthy, A. Balasubramanian, Int. J. Pharm. Sci. Res. 8 (2017) 4950.
• ⁶ E.M. Christenson, K.S. Anseth, J.J. van den Beucken, C.K. Chan, B. Ercan, J.A. Jansen, C.T. Laurencin , W.J. Li, R. Murugan, L.S. Nair, J. Orthop. Res. 25 (2007) 11.
• ⁷ L. Hermansson, J. Korean Ceram. Soc. 55 (2018) 95.
• ⁸ M. Jäger, H. Jennissen, F. Dittrich, A. Fischer, H. Köhling, Materials 10 (2017) 1302.
• ⁹ N. Shadjou, M. Hasanzadeh, Mater. Technol. 31 (2016) 806.
• ¹⁰ M. Vallet-Regí, D. Arcos, Curr. Nanosci. 2 (2006) 179.
• ¹¹ J. Venugopal, M.P. Prabhakaran, Y. Zhang, S. Low , A.T. Choon , S. Ramakrishna , Philos. Trans. Royal Soc. A 368 (2010) 2065.
• ¹² M. Kulkarni, A. Mazare, E. Gongadze, Š. Perutkova, V. Kralj-Iglič, I. Milošev, P. Schmuki, A. Iglič, M. Mozetič, Nanotechnology 26 (2015) 62002.
• ¹³ J. Souza, L. Bins-Ely, M. Sordi, R. Magini, C. Aparicio, T. Shokuhfar, L. Cooper, in “Nanostructured biomaterials for cranio-maxillofacial and oral applications”, J. Souza, D. Hotza, B. Henriques, A. Boccaccini (Eds.), Elsevier (2018) 13.
• ¹⁴ T. Xu, N. Zhang, H.L. Nichols, D. Shi, X. Wen, Mater. Sci. Eng. C 27 (2007) 579.
• ¹⁵ K.S. Brammer, C.J. Frandsen, S. Jin, Trends Biotechnol. 30 (2012) 315.
• ¹⁶ L. Lv, Y. Liu, P. Zhang, X. Zhang, J. Liu, T. Chen, P. Su, H. Li, Y. Zhou, Biomaterials 39 (2015) 193.
• ¹⁷ G. Wang, Y. Wan, B. Ren, Z. Liu, Mater. Sci. Eng . C 95 (2019) 114.
• ¹⁸ P. Gkomoza, M. Vardavoulias, D. Pantelis, C. Sarafoglou, Surf. Coat. Technol. 357 (2019) 748.
• ¹⁹ J.I. Lim, B. Yu, K.M. Woo, Y.-K. Lee, Appl. Surf. Sci. 255 (2008) 2456.
• ²⁰ S. Mondal, G. Hoang, P. Manivasagan, H. Kim, J. Oh, Ceram. Int. 46, 3 (2019) 3443.
• ²¹ L. Salou, A. Hoornaert, G. Louarn, P. Layrolle, Acta Biomater. 11 (2015) 494.
• ²² Y.-T. Sul, Int. J. Nanomedicine 5 (2010) 87.
• ²³ R. Valiev, I. Sabirov, E. Zemtsova, E. Parfenov, L. Dluhoš, T. Lowe, in “Titanium in medical and dental applications”, F. Froes, M. Qian (Eds.), Elsevier (2018) 393.
• ²⁴ X.-M. Zhuang, B. Zhou, K.-F. Yuan, Biomed. Pharmacother. 112 (2019) 108649.
• ²⁵ X. Chen, A. Selloni, Chem. Rev. 114, 19 (2014) 9281.
• ²⁶ G. Fostad, B. Hafell, A. Førde, R. Dittmann, R. Sabetrasekh, J. Will, J. Ellingsen, S. Lyngstadaas, H. Haugen, J. Eur. Ceram. Soc. 29 (2009) 2773.
• ²⁷ H. Haugen , J. Will , A. Köhler, U. Hopfner, J. Aigner, E. Wintermantel, J. Eur. Ceram. Soc . 24 (2004) 661.
• ²⁸ T. Kasuga, H. Kondo, M. Nogami, J. Cryst. Growth 235 (2002) 235.
• ²⁹ K. Lee, A. Mazare , P. Schmuki , Chem. Rev . 114 (2014) 9385.
• ³⁰ H. Nygren, P. Tengvall, I. Lundström, J. Biomed. Mater. Res. 34 (1997) 487.
• ³¹ Y. Fu, A. Mo, Nanoscale Res. Lett. 13 (2018) 187.
• ³² M. Garcia-Lobato, A. Mtz-Enriquez, C. Garcia, M. Velazquez-Manzanares, F. Avalos-Belmontes, R. Ramos-Gonzalez, L. Garcia-Cerda, Appl. Surf. Sci . 484 (2019) 975.
• ³³ K. Das, S. Bose, A. Bandyopadhyay, J. Biomed. Mater. Res. A 90 (2009) 225.
• ³⁴ Y. Peng, Y. Song, Q. Wang, Y. Hu, Y. He, D. Ren, L. Wu, S. Liu, H. Cong, H. Zhou, Int. J. Biol. Macromol. 127 (2019) 48.
• ³⁵ N. Wang, H. Li , W. Lü, J. Li, J. Wang, Z. Zhang, Y. Liu , Biomaterials 32 (2011) 6900.
• ³⁶ D.B. Warheit, Toxicol. Lett. 220 (2013) 193.
• ³⁷ J. Wen, X. Li, W. Liu, Y. Fang, J. Xie, Y. Xu, Chinese J. Catal. 36 (2015) 2049.
• ³⁸ J. Feng, X. Yan, K. Lin, S. Wang, J. Luo, Y. Wu, Mater. Lett. 214 (2018) 178.
• ³⁹ I. Shepa, E. Mudra, M. Vojtko, P. Tatarko, V. Girman, O. Milkovic, T. Sopcak, V. Medvecka, J. Dusza, Ceram. Int . 44 (2018) 17925.
• ⁴⁰ V. Simi, N. Rajendran, Mater. Charact. 129 (2017) 67.
• ⁴¹ A. Tan, B. Pingguan-Murphy, R. Ahmad, S. Akbar, Ceram. Int . 38 (2012) 4421.
• ⁴² K.-R. Zhu, M.-S. Zhang, J.-M. Hong, Z. Yin, Mater. Sci. Eng. A 403 (2005) 87.
• ⁴³ N. Rahimi, R.A. Pax, E.M. Gray, Prog. Solid. State Ch. 44 (2016) 86.
• ⁴⁴ O. Carp, C.L. Huisman, A. Reller, Prog. Solid. State Ch. 32 (2004) 33.
• ⁴⁵ V. Zainullina, V. Zhukov, M. Korotin, J. Photochem. Photobiol. C 22 (2015) 58.
• ⁴⁶ M. Kulkarni , A. Mazare , E. Gongadze , S. Perutkova, V. Kralj-Iglic, I. Milosev, P. Schmuki , A. Iglic, M. Mozetic, Nanotechnology 26 (2015) 62002.
• ⁴⁷ K.M. Kummer, E. Taylor, T.J. Webster , Nanosci. Nanotechnol. Lett. 4 (2012) 483.
• ⁴⁸ V. Binas, D. Venieri, D. Kotzias, G. Kiriakidis, J. Materiomics 3 (2017) 3.
• ⁴⁹ S. Miszczak, B. Pietrzyk, Ceram. Int . 41 (2015) 7461.
• ⁵⁰ D.A. Hanaor, C.C. Sorrell, J. Mater. Sci. 46 (2011) 855.
• ⁵¹ M. Pelaez, N.T. Nolan, S.C. Pillai, M.K. Seery, P. Falaras, A.G. Kontos, P.S. Dunlop, J.W. Hamilton, J.A. Byrne, K. O’Shea, Appl. Catal. B 125 (2012) 331.
• ⁵² I. Ali, M. Suhail, Z.A. Alothman, A. Alwarthan, RSC Adv. 8 (2018) 30125.
• ⁵³ M. Erol, O. Ertugrul, Ceram. Int . 44 (2018) 2991.
• ⁵⁴ H. Lin , L. Li, M. Zhao, X. Huang, X. Chen , G. Li, R. Yu, J. Am. Chem. Soc. 134 (2012) 8328.
• ⁵⁵ N.A. Ramos-Delgado, M.Á. Gracia-Pinilla, R.V. Mangalaraja, K. O’Shea , D.D. Dionysiou, Nanotechnol. Rev. 5 (2016) 467.
• ⁵⁶ L. Wang, Y. Cai, L. Song, W. Nie, Y. Zhou , P. Chen, Colloids Surf. A Physicochem. Eng. Asp. 461 (2014) 195.
• ⁵⁷ I. Ali , M. Suhail , Z.A. Alothman , A. Alwarthan , RSC Adv . 8 (2018) 30125.
• ⁵⁸ S.M. Gupta, M. Tripathi, Chin. Sci. Bull. 56 (2011) 1639.
• ⁵⁹ R. Singh, S. Dutta, Fuel 220 (2018) 607.
• ⁶⁰ Z. Li, Z. Yao, A.A. Haidry, T. Plecenik, L. Xie, L. Sun, Q. Fatima, Int. J. Hydrog. Energy 43, 45 (2018) 21114.
• ⁶¹ R. Verma, J. Gangwar, A.K. Srivastava, RSC Adv . 7 (2017) 44199.
• ⁶² X. Yan , X. Chen , in “Encyclopedia of inorganic and bioinorganic chemistry”, John Wiley Sons (2015) 1.
• ⁶³ S. Livraghi, M. Rolando, S. Maurelli, M. Chiesa, M.C. Paganini, E. Giamello, J. Phys. Chem. C 118 (2014) 22141.
• ⁶⁴ H. Abdullah, M.M.R. Khan, H.R. Ong, Z. Yaakob, J. CO2 Util. 22 (2017) 15.
• ⁶⁵ M.R. Al-Mamun, S. Kader, M.S. Islam, M.Z.H. Khan, J. Environ. Chem. Eng. 7, 5 (2019) 103248.
• ⁶⁶ R. Fatima, M.N. Afridi, V. Kumar, J. Lee, I. Ali , K.-H. Kim , J.-O. Kim, J. Clean. Prod. 231 (2019) 899.
• ⁶⁷ N. Khalid, A. Majid, M.B. Tahir, N. Niaz, S. Khalid, Ceram. Int . 43 (2017) 14552.
• ⁶⁸ A. Mishra, A. Mehta, S. Basu, J. Environ. Chem. Eng. 6 (2018) 6088.
• ⁶⁹ Z. Shayegan, C.-S. Lee, F. Haghighat, Chem. Eng. J. 334 (2018) 2408.
• ⁷⁰ A. Zaleska, Recent Pat. Eng. 2 (2008) 157.
• ⁷¹ T. Li, K. Gulati, N. Wang , Z. Zhang , S. Ivanovski, Mater. Sci. Eng. C 88 (2018) 182.
• ⁷² X. Zhao, L. Huang, Ceram. Int . 43 (2017) 3975.
• ⁷³ R. Ahmad , Z. Ahmad, A.U. Khan, N.R. Mastoi, M. Aslam, J. Kim, J. Environ. Chem. Eng. 4 (2016) 4143.
• ⁷⁴ K. Cowden, M.F. Dias-Netipanyj, K.C. Popat, Nanomedicine (2019).
• ⁷⁵ U.I. Gaya, A.H. Abdullah , J. Photochem. Photobiol. C 9 (2008) 1.
• ⁷⁶ M. Qamar, B. Merzougui, D. Anjum, A.S. Hakeem, Z.H. Yamani, D. Bahnemann, Catal. Today 230 (2014) 158.
• ⁷⁷ X. Qu, Y. Yi, F. Qiao, M. Liu, X. Wang, R. Yang, H. Meng, L. Shi, F. Du, Ceram. Int . 44 (2018) 1348.
• ⁷⁸ L. Rožić, S. Petrović, D. Lončarević, B. Grbić, N. Radić, S. Stojadinović, V. Jović, J. Lamovec, Ceram. Int . 45 (2019) 2361.
• ⁷⁹ Z. Tan, K. Sato, S. Ohara, Adv. Powder Technol. 26 (2015) 296.
• ⁸⁰ T. Peng, D. Zhao, H. Song, C. Yan, J. Mol. Catal. A Chem. 238 (2005) 119.
• ⁸¹ A.J. Haider, R.H. Al-Anbari, G.R. Kadhim, C.T. Salame, Energy Procedia 119 (2017) 332.
• ⁸² Y. Li, J. Luo , X. Hu, X. Wang , J. Liang, K. Yu, J. Alloys Compd. 651 (2015) 685.
• ⁸³ P. Roy, S. Berger, P. Schmuki , Angew. Chem. Int. Ed. 50 (2011) 2904.
• ⁸⁴ B. Sarkar, N. Singhal, R. Goyal, A. Bordoloi, L.S. Konathala, U. Kumar, R. Bal, Catal. Commun. 74 (2016) 43.
• ⁸⁵ S. Shen, J. Chen, M. Wang, X. Sheng, X. Chen , X. Feng, S.S. Mao, Prog. Mater. Sci. 98 (2018) 299.
• ⁸⁶ J. Singh, S.A. Khan, J. Shah, R. Kotnala, S. Mohapatra, Appl. Surf. Sci . 422 (2017) 953.
• ⁸⁷ T. Tao, I.-T. Bae, K.B. Woodruff, K. Sauer, J. Cho, Ceram. Int . 45 (2019) 23216.
• ⁸⁸ C. Qizheng, D. Xiangting, W. Jinxian, L. Mei, J. Rare Earths 26 (2008) 664.
• ⁸⁹ R. Sabetrasekh , H. Tiainen, J. Reseland, J. Will , J. Ellingsen , S. Lyngstadaas , H. Haugen , Biomed. Mater. 5 (2010) 15003.
• ⁹⁰ H. Xu, Q. Zhang, C. Zheng, W. Yan, W. Chu, Appl. Surf. Sci . 257 (2011) 8478.
• ⁹¹ E. Beltrán-Partida, A. Moreno-Ulloa , B. Valdez-Salas, C. Velasquillo, M. Carrillo, A. Escamilla, E. Valdez, F. Villarreal, Materials 8 (2015) 867.
• ⁹² S. Chen, Y. Guo, R. Liu, S. Wu, J. Fang, B. Huang, Z. Li , Z. Chen, Z. Chen , Colloids Surf. B 164 (2018) 58.
• ⁹³ A. Hajjaji, M. Elabidi, K. Trabelsi, A. Assadi, B. Bessais, S. Rtimi, Colloids Surf. B 170 (2018) 92.
• ⁹⁴ M.-H. Kim , K. Park, K.-H. Choi, S.-H. Kim , S. Kim, C.-M. Jeong, J.-B. Huh, Int. J. Mol. Sci. 16 (2015) 10324.
• ⁹⁵ D. Khang, J. Lu, C. Yao, K.M. Haberstroh, T.J. Webster , Biomaterials 29 (2008) 970.
• ⁹⁶ B.K. Mutuma, G.N. Shao, W.D. Kim, H.T. Kim, J. Colloid Interface Sci. 442 (2015) 1.
• ⁹⁷ N. Swami, Z. Cui, L.S. Nair , J. Heat Transfer 133 (2011) 34002.
• ⁹⁸ A. Tampieri, G. Celotti, E. Landi, M. Sandri, N. Roveri, G. Falini, J. Biomed. Mater. Res. A 67 (2003) 618.
• ⁹⁹ G. Wang , H. Feng, W. Jin, A. Gao, X. Peng, W. Li, H. Wu, Z. Li , P.K. Chu, Appl. Surf. Sci . 414 (2017) 230.
• ¹⁰⁰ S. Winardi, R.R. Mukti, K.-N.P. Kumar, J. Wang , W. Wunderlich, T. Okubo, Langmuir 26 (2010) 4567.
• ¹⁰¹ N.K. Awad, S.L. Edwards, Y.S. Morsi, Mater. Sci. Eng. C 76 (2017) 1401.
• ¹⁰² L.M. Bjursten, L. Rasmusson, S. Oh, G.C. Smith, K.S. Brammer , S. Jin , J. Biomed. Mater. Res. A 92 (2010) 1218.
• ¹⁰³ S.M.G. El-Rab, S.A. Fadl-Allah, A. Montser , Appl. Surf. Sci . 261 (2012) 1.
• ¹⁰⁴ A. Bandyopadhyay , A. Shivaram, I. Mitra, S. Bose , Acta Biomater . 96 (2019) 686.
• ¹⁰⁵ K. Cowden , M.F. Dias-Netipanyj , K.C. Popat , Nanomedicine 17 (2019) 380.
• ¹⁰⁶ F. Visentin, A. Galenda, M. Fabrizio, S. Battiston, N. Brianese, R. Gerbasi, V. Zin, N. El Habra, Appl. Surf. Sci . 490 (2019) 568.
• ¹⁰⁷ T. Wang, Z. Weng, X. Liu , K.W. Yeung, H. Pan, S. Wu , Bioact. Mater. 2 (2017) 44.
• ¹⁰⁸ M. Enayati-Jazi, M. Solati-Hashjin, A. Nemati, F. Bakhshi, Superlattices Microstruct. 51 (2012) 877.
• ¹⁰⁹ A.E. Hannora, S. Ataya, J. Alloys Compd. 658 (2016) 222.
• ¹¹⁰ H.-W. Kim, Y.-H. Koh, L.-H. Li , S. Lee, H.-E. Kim, Biomaterials 25 (2004) 2533.
• ¹¹¹ P. Sharma, A. Trivedi, H. Begam, Mater. Today Proc. 16 (2019) 302.
• ¹¹² S. Pushpakanth, B. Srinivasan, B. Sreedhar, T. Sastry, Mater. Chem. Phys. 107 (2008) 492.
• ¹¹³ S.F. Robertson, A. Bandyopadhyay , S. Bose , Surf. Coat. Technol . 372 (2019) 140.
• ¹¹⁴ D. Sidane, D. Chicot, S. Yala, S. Ziani, H. Khireddine, A. Iost, X. Decoopman, Thin Solid Films 593 (2015) 71.
• ¹¹⁵ F. Visentin , N. El Habra, M. Fabrizio , N. Brianese , R. Gerbasi , L. Nodari, V. Zin, A. Galenda , Surf. Coat. Technol . 378 (2019) 125049.
• ¹¹⁶ H. Zhou , J.G. Lawrence, S.B. Bhaduri, Acta Biomater . 8 (2012) 1999.
• ¹¹⁷ N.K. Allam, C.A. Grimes, Langmuir 25 (2009) 7234.
• ¹¹⁸ M. Imran, S. Riaz, S. Naseem, Mater. Today Proc. 2 (2015) 5455.
• ¹¹⁹ S. Mahshid, M. Askari, M.S. Ghamsari, N. Afshar, S. Lahuti, J. Alloys Compd. 478 (2009) 586.
• ¹²⁰ S. Kim , M. Kim, S.-H. Hwang, S.K. Lim, Appl. Catal. B 123 (2012) 391.
• ¹²¹ C. Kuchi, G. Harish, P.S. Reddy, Ceram. Int . 44 (2018) 5266.
• ¹²² M. Someswararao, R. Dubey, P. Subbarao , S. Singh, Results Phys. 11 (2018) 223.
• ¹²³ M. Vahtrus, A. Šutka, S. Vlassov, A. Šutka , B. Polyakov, R. Saar, L. Dorogin, R. Lohmus, Mater. Charact . 100 (2015) 98.
• ¹²⁴ G.J. Owens, R.K. Singh, F. Foroutan, M. Alqaysi, C.-M. Han, C. Mahapatra, H.-W. Kim , J.C. Knowles, Prog. Mater. Sci. 77 (2016) 1.
• ¹²⁵ X. Yan , X. Chen , in “Encyclopedia of inorganic and bioinorganic chemistry”, John Wiley Sons (2015) 1.
• ¹²⁶ W. Xu, W.Y. Hu , M.H. Li , Q.Q. Ma, P.D. Hodgson, C.E. Wen, Trans. Nonferrous Met. Soc. China 16 (2006) s209.
• ¹²⁷ M.F. Kunrath, R. Hubler, R.S. Shinkai, E.R. Teixeira, ChemistrySelect 3 (2018) 11180.
• ¹²⁸ S. Areva, V. Ääritalo, S. Tuusa, M. Jokinen, M. Lindén, T. Peltola, J. Mater. Sci Mater. Med. 18 (2007) 1633.
• ¹²⁹ G. Crawford, N. Chawla, K. Das , S. Bose , A. Bandyopadhyay , Acta Biomater . 3 (2007) 359.
• ¹³⁰ M.A. Khan, U. Farva, Surf. Interfaces 6 (2017) 202.
• ¹³¹ S. Sista, A. Nouri, Y. Li , C. Wen, P.D. Hodgson , G. Pande, J. Biomed. Mater. Res. A 101 (2013) 3416.
• ¹³² W. Xu , W. Hu, M. Li, Q. Ma, P. Hodgson, C. Wen , Trans. Nonferrous Met. Soc. China 16 (2006) s209.
• ¹³³ H.P. Quiroz, C.P. Barrera-Patiño, R.R. Rey-González, A. Dussan, Photonics Nanostruct. 22 (2016) 46.
• ¹³⁴ M.A. Rasheed, K. Ahmad, N. Khaliq, Y. Khan, M. Aftab Rafiq, A. Waheed, A. Shah, A. Mahmood, G. Ali, Curr. Appl. Phys. 18 (2018) 297.
• ¹³⁵ X. Chen , S.S. Mao , Chem. Rev . 107 (2007) 2891.
• ¹³⁶ D. Kowalski, D. Kim, P. Schmuki , Nano Today 8 (2013) 235.
• ¹³⁷ D. Regonini, C.R. Bowen, A. Jaroenworaluck, R. Stevens, Mater. Sci. Eng . R Rep. 74 (2013) 377.
• ¹³⁸ Y. Wang, Y. He, Q. Lai, M. Fan, J. Environ. Sci. 26 (2014) 2139.
• ¹³⁹ C.L. Wong, Y.N. Tan, A.R. Mohamed, J. Environ. Manage. 92 (2011) 1669.
• ¹⁴⁰ T. Tenkyong, J.S.S. Mary, B. Praveen, K. Pugazhendhi, D. Sharmila, J.M. Shyla, Mater. Sci. Semicond. Process. 83 (2018) 150.
• ¹⁴¹ C.T. Teodorescu-Soare, C. Catrinescu, M. Dobromir, G. Stoian, A. Arvinte, D. Luca, J. Electroanal. Chem. 823 (2018) 388.
• ¹⁴² B.B. Çirak, S.M. Karadeniz, T. Kilinç, B. Caglar, A.E. Ekinci, H. Yelgin, M. Kürekçi, Ç. Çirak, Vacuum 144 (2017) 183.
• ¹⁴³ T. Hoseinzadeh, Z. Ghorannevis, M. Ghoranneviss, Appl. Phys. A 123 (2017) 436.
• ¹⁴⁴ F. Nasirpouri, I. Yousefi, E. Moslehifard, J. Khalil-Allafi, Surf. Coat. Technol . 315 (2017) 163.
• ¹⁴⁵ S. Chen, Q. Chen, M. Gao, S. Yan, R. Jin, X. Zhu, Surf. Coat. Technol . 321 (2017) 257.
• ¹⁴⁶ E. Zalnezhad, E. Maleki, S. Banihashemian, J. Park, Y.B. Kim, M. Sarraf, A. Sarhan, S. Ramesh, Mater. Res. Bull. 78 (2016) 179.
• ¹⁴⁷ L. Qin, Q. Chen , R. Lan, R. Jiang, X. Quan, B. Xu, F. Zhang, Y. Jia, J. Mater. Sci. Technol. 31 (2015) 1059.
• ¹⁴⁸ Z. Lockman, S. Sreekantan, S. Ismail, L. Schmidt-Mende, J.L. MacManus-Driscoll, J. Alloys Compd. 503 (2010) 359.
• ¹⁴⁹ W. Yu, Y. Zhang , X. Jiang, F. Zhang , Oral Dis. 16 (2010) 624.
• ¹⁵⁰ Z. Liu , X. Zhang , S. Nishimoto, T. Murakami, A. Fujishima, Environ. Sci. Technol. 42 (2008) 8547.
• ¹⁵¹ H.E. Prakasam, K. Shankar, M. Paulose, O.K. Varghese, C.A. Grimes, J. Phys. Chem. C 111 (2007) 7235.
• ¹⁵² S. Bauer, S. Kleber, P. Schmuki , Electrochem. Commun. 8 (2006) 1321.
• ¹⁵³ Q. Cai, M. Paulose , O.K. Varghese , C.A. Grimes, J. Mater. Res. 20 (2005) 230.
• ¹⁵⁴ A. Ghicov, H. Tsuchiya, J.M. Macak, P. Schmuki , Electrochem. Commun . 7 (2005) 505.
• ¹⁵⁵ M. Andrade-Guel, L. Díaz-Jiménez, D. Cortés-Hernández, C. Cabello-Alvarado, C. Ávila-Orta, P. Bartolo-Pérez, P. Gamero-Melo, Bol. Soc. Esp. Ceram. V. 58 (2019) 171.
• ¹⁵⁶ J. Liang, J. Wang, K. Song, X. Wang, K. Yu, C. Liang, J. Rare Earths 37 (2019) 1.
• ¹⁵⁷ V. Singh, A. Rao, A. Tiwari, P. Yashwanth, M. Lal, U. Dubey, S. Aich, B. Roy, J. Phys. Chem . Solids 134 (2019) 262.
• ¹⁵⁸ E. Blanco, M. Domínguez, J. González-Leal, E. Márquez, J. Outón, M. Ramírez-del-Solar, Appl. Surf. Sci . 439 (2018) 736.
• ¹⁵⁹ R.G. Nair, S. Ojah, P.M. Kumar, S. Nikhil, S. Samdarshi, Mater. Lett. 221 (2018) 313.
• ¹⁶⁰ T.N. Singh, T.G. Devi, S.D. Singh, Nano-Struct. Nano-Obj. 11 (2017) 65.
• ¹⁶¹ T. Balaganapathi, B. Kaniamuthan, S. Vinoth, P. Thilakan, Mater. Chem. Phys. 189 (2017) 50.
• ¹⁶² D. Dastan, Appl. Phys. A 123 (2017) 699.
• ¹⁶³ F. Dejene, M. Onani, P. Tarus, Physica B Condens. Matter 480 (2016) 213.
• ¹⁶⁴ B. Tryba, M. Tygielska, C. Colbeau-Justin, E. Kusiak-Nejman, J. Kapica-Kozar, R. Wróbel, G. Żołnierkiewicz, N. Guskos, Mater. Res. Bull . 84 (2016) 152.
• ¹⁶⁵ B. Liu, J. Xiao, L. Xu, Y. Yao, B.F. Costa, V.F. Domingos, E.S. Ribeiro, F.-N. Shi, K. Zhou, J. Su, Int. J. Hydrog. Energy 40 (2015) 4945.
• ¹⁶⁶ Z. Li, Y. Zhu, J. Wang , Q. Guo, J. Li, Ceram. Int . 41 (2015) 9057.
• ¹⁶⁷ A.K. Tripathi, M.C. Mathpal, P. Kumar, M.K. Singh, M. Soler, A. Agarwal, J. Alloys Compd . 622 (2015) 37.
• ¹⁶⁸ S. Berger, R. Hahn, P. Roy, P. Schmuki , Phys. Status Solidi B 247 (2010) 2424.
• ¹⁶⁹ G. Cao, Nanostructures and nanomaterials: synthesis, properties and applications, World Sci. (2004).
• ¹⁷⁰ L.L. Hench, J.M. Polak, Science 295 (2002) 1014.
• ¹⁷¹ M. Navarro, A. Michiardi, O. Castano, J. Planell, J. R. Soc. Interface 5 (2008) 1137.
• ¹⁷² Y. Xia, Nat. Mater. 7 (2008) 758.
• ¹⁷³ Z.-G. Zhang, Z.-H. Li , X.-Z. Mao, W.-C. Wang, Cytotechnology 63 (2011) 437.
• ¹⁷⁴ K.S. Brammer , S. Oh, C.J. Cobb, L.M. Bjursten, H. van der Heyde, S. Jin, Acta Biomater . 5 (2009) 3215.
• ¹⁷⁵ S.A. Catledge, M.D. Fries, Y.K. Vohra, W.R. Lacefield, J.E. Lemons, S. Woodard, R. Venugopalanc, J. Nanosci. Nanotechnol. 2 (2002) 293.
• ¹⁷⁶ S. Minagar, C.C. Berndt, J. Wang , E. Ivanova, C. Wen, Acta Biomater . 8 (2012) 2875.
• ¹⁷⁷ A.W. Tan, B. Pingguan-Murphy, R. Ahmad, S.A. Akbar, J. Mater. Sci. 48 (2013) 8337.
• ¹⁷⁸ B. Zhang, D. Myers, G. Wallace, M. Brandt, P. Choong, Int. J. Mol. Sci. 15 (2014) 11878.
• ¹⁷⁹ S. Bauer , P. Schmuki , K. Von Der Mark, J. Park , Prog. Mater. Sci. 58 (2013) 261.
• ¹⁸⁰ R. Junker, A. Dimakis, M. Thoneick, J.A. Jansen , Clin. Oral Implants Res. 20 (2009) 185.
• ¹⁸¹ H. Liu, T.J. Webster , Biomaterials 28 (2007) 354.
• ¹⁸² L.E. McNamara, R.J. McMurray, M.J. Biggs, F. Kantawong, R.O. Oreffo , M.J. Dalby, J. Tissue Eng. 1 (2010) 120623.
• ¹⁸³ A. Wennerberg, T. Albrektsson, B. Andersson, J. Krol, Clin. Oral Implants Res. 6 (1995) 24.
• ¹⁸⁴ A. Wennerberg , C. Hallgren, C. Johansson, S. Danelli, Clin. Oral Implants Res. 9 (1998) 11.
• ¹⁸⁵ M. Fathi, B. Akbari, A. Taheriazam, Mater. Sci. Eng. C 103 (2019) 109743.
• ¹⁸⁶ M.F. Dias-Netipanyj, K. Cowden, L.S. Santos, S.C. Cogo, S. Elifio-Esposito, K.C. Popat, P. Soares, Mater. Sci. Eng. C 103 (2019) 109850.
• ¹⁸⁷ Y. Li, Y. You, B. Li, Y. Song, A. Ma, B. Chen, W. Han, C. Li, J. Hard Tissue Biol. 28 (2019) 13.
• ¹⁸⁸ W.-E. Yang, H.-H. Huang, Appl. Surf. Sci . 471 (2019) 1041.
• ¹⁸⁹ P. Agilan, N. Rajendran, Appl. Surf. Sci . 439 (2018) 66.
• ¹⁹⁰ D.P. Bhattarai, S. Shrestha, B.K. Shrestha, C.H. Park, C.S. Kim, Chem. Eng. J. 350 (2018) 57.
• ¹⁹¹ R. Aguirre, M. Echeverry Rendón, D. Quintero, J.G. Castaño, M.C. Harmsen, S. Robledo, F. Echeverría E., J. Biomed. Mater. Res. A 106 (2018) 1341.
• ¹⁹² N. Khoshnood, A. Zamanian, A. Massoudi , Mater. Lett . 185 (2016) 374.
• ¹⁹³ Y. Bai, I.S. Park, H.H. Park, M.H. Lee, T.S. Bae, W. Duncan, M. Swain, Surf. Interface Anal. 43 (2011) 998.
• ¹⁹⁴ J. Park , S. Bauer , K.A. Schlegel, F.W. Neukam, K. Von Der Mark , P. Schmuki , Small 5 (2009) 666.
• ¹⁹⁵ K. Das, S. Bose, A. Bandyopadhyay, J. Biomed. Mater. Res. A 90 (2009) 225.
• ¹⁹⁶ J. Kunze, L. Müller, J.M. Macak , P. Greil, P. Schmuki , F.A. Müller, Electrochim. Acta 53 (2008) 6995.
• ¹⁹⁷ K.C. Popat , L. Leoni, C.A. Grimes , T.A. Desai, Biomaterials 28 (2007) 3188.
• ¹⁹⁸ S. Oh , C. Daraio, L.H. Chen, T.R. Pisanic, R.R. Finones, S. Jin , J. Biomed. Mater. Res. A 78 (2006) 97.
• ¹⁹⁹ B. Burnat, J. Robak, A. Leniart, I. Piwoński, D. Batory, Ceram. Int . 43 (2017) 13735.
• ²⁰⁰ B. Burnat , J. Robak , D. Batory , A. Leniart , I. Piwoński , S. Skrzypek, M. Brycht, Surf. Coat. Technol . 280 (2015) 291.
• ²⁰¹ J. Coreño, A. Martínez , O. Coreño, A. Bolarín, F. Sánchez, J. Biomed. Mater. Res. A 64 (2003) 131.
• ²⁰² M. Uchida, H.M. Kim, T. Kokubo, S. Fujibayashi, T. Nakamura, J. Biomed. Mater. Res. A 64 (2003) 164.
• ²⁰³ T. Peltola, M. Jokinen, H. Rahiala, M. Pätsi, J. Heikkilä, I. Kangasniemi, A. Yli-Urpo, J. Biomed. Mater. Res . 51 (2000) 200.
• ²⁰⁴ E. Beltrán-Partida, B. Valdéz-Salas, A. Moreno-Ulloa, A. Escamilla, M.A. Curiel, R. Rosales-Ibáñez, F. Villarreal, D.M. Bastidas, J.M. Bastidas, J. Nanobiotechnology 15 (2017) 10.
• ²⁰⁵ A.L. Escada, R.Z. Nakazato, A.P.R.A. Claro, Mater. Res. 20 (2017) 1282.
• ²⁰⁶ K. Indira, U.K. Mudali, T. Nishimura, N. Rajendran , J. Bio Tribo-Corros. 1 (2015) 28.
• ²⁰⁷ L. Liu, T.J. Webster , in “Orthopedic biomaterials”, B. Liu , T. Webster (Eds.), Springer (2017) 31.
• ²⁰⁸ A. Rafieerad, E. Zalnezhad , A. Bushroa, A. Hamouda, M. Sarraf , B. Nasiri-Tabrizi, Surf. Coat. Technol . 265 (2015) 24.
• ²⁰⁹ Y. Fu, A. Mo, Nanoscale Res. Lett. 13 (2018) 187.
• ²¹⁰ K.S. Brammer , S. Oh , C.J. Cobb , L.M. Bjursten , H. van der Heyde , S. Jin , Acta Biomater . 5 (2009) 3215.
• ²¹¹ K.S. Brammer , C.J. Frandsen , S. Jin , Trends Biotechnol . 30 (2012) 315.
• ²¹² B.S. Smith, S. Yoriya, T. Johnson, K.C. Popat , Acta Biomater . 7 (2011) 2686.
• ²¹³ C. Eriksson, H. Nygren , K. Ohlson, Biomaterials 25 (2004) 4759.
• ²¹⁴ R.A. Gittens, L. Scheideler, F. Rupp, S.L. Hyzy, J. Geis-Gerstorfer, Z. Schwartz, B.D. Boyan, Acta Biomater . 10 (2014) 2907.
• ²¹⁵ Z. Jia, P. Xiu, M. Li, X. Xu, Y. Shi, Y. Cheng, S. Wei, Y. Zheng, T. Xi, H. Cai, Biomaterials 75 (2016) 203.
• ²¹⁶ S. Rani, S.C. Roy, M. Paulose ,O.K. Varghese, G.K. Mor, S. Kim, S. Yoriya , T.J. LaTempa, C. Grimes, Phys. Chem. Chem. Phys. 12 (2010) 2780.
• ²¹⁷ U.F. Gunputh, H. Le, R.D. Handy, C. Tredwin, Mater. Sci. Eng. C 91 (2018) 638.
• ²¹⁸ H.J. Haugen, M. Monjo, M. Rubert, A. Verket, S.P. Lyngstadaas, J.E. Ellingsen, H.J. Rønold, J.C. Wohlfahrt, Acta Biomater . 9 (2013) 5390.
• ²¹⁹ R. Imani, M. Pazoki, D. Zupančič, M.E. Kreft, V. Kralj-Iglič , P. Veranič, A. Iglič , Protoplasma 253 (2016) 1439.
• ²²⁰ D. Loca, I. Narkevica, J. Ozolins, Mater. Lett . 159 (2015) 309.
• ²²¹ F.A. Akin, H. Zreiqat, S. Jordan, M.B. Wijesundara, L. Hanley, J. Biomed. Mater. Res . 57 (2001) 588.
• ²²² F. Utku, E. Seckin, G. Goller, C. Tamerler, M. Urgen, Appl. Surf. Sci . 350 (2015) 62.
• ²²³ D.W. Hutmacher, J.T. Schantz, C.X.F. Lam, K.C. Tan, T.C. Lim , J. Tissue Eng. Regen. Med . 1 (2007) 245.
• ²²⁴ A.C. Jones, C.H. Arns, D.W. Hutmacher , B.K. Milthorpe, A.P. Sheppard, M.A. Knackstedt, Biomaterials 30 (2009) 1440.
• ²²⁵ V. Karageorgiou, D. Kaplan, Biomaterials 26 (2005) 5474.
• ²²⁶ T.S. Karande, J.L. Ong, C.M. Agrawal, Ann. Biomed. Eng. 32 (2004) 1728.
• ²²⁷ H. Tiainen, D. Wiedmer, H.J. Haugen , J. Eur. Ceram. Soc . 33 (2013) 15.
• ²²⁸ S. Bauer , J. Park , J. Faltenbacher, S. Berger , K. Von Der Mark , P. Schmuki , Integr. Biol. 1 (2009) 525.
• ²²⁹ S. Bauer , J. Park , K. Von Der Mark , P. Schmuki , Acta Biomater . 4 (2008) 1576.
• ²³⁰ A. Kar, K. Raja, M. Misra, Surf. Coat. Technol . 201 (2006) 3723.
• ²³¹ H. Tsuchiya , J.M. Macak , L. Müller , J. Kunze , F. Müller, P. Greil , S. Virtanen, P. Schmuki , J. Biomed. Mater. Res. A 77 (2006) 534.
• ²³² J. He, W. Zhou, X. Zhou, X. Zhong, X. Zhang , P. Wan, B. Zhu, W. Chen, J. Mater. Sci. Mater. Med. 19 (2008) 3465.
• ²³³ H. Shi, R. Magaye, V. Castranova, J. Zhao, Part. Fibre Toxicol. 10 (2013) 15.
• ²³⁴ R. Sawada, Y. Katou, H. Shibata , M. Katayama, T. Nonami, Int. J. Biomater. 2019 (2019) 7826373.
• ²³⁵ B. Malhotra, M.A. Ali, Nanomaterials for biosensors: fundamentals and applications, Elsevier (2018) 183.
• ²³⁶ V.D. Rani, L. Vinoth-Kumar, V. Anitha, K. Manzoor, M. Deepthy, V.N. Shantikumar, Acta Biomater . 8 (2012) 1976.
• ²³⁷ C. von Wilmowsky, S. Bauer , R. Lutz, M. Meisel, F.W. Neukam , T. Toyoshima, P. Schmuki , E. Nkenke, K.A. Schlegel , J. Biomed. Mater. Res. B 89 (2009) 165.
• ²³⁸ H. Zhang, Y. Sun, A. Tian, X.X. Xue , L. Wang, A. Alquhali, X. Bai, Int. J. Nanomedicine 8 (2013) 4379.

Publication Dates

• Publication in this collection
  30 Oct 2020
• Date of issue
  Oct-Dec 2020

History

• Received
  05 Dec 2019
• Reviewed
  20 May 2020
• Reviewed
  03 June 2020
• Accepted
  07 June 2020