Titanium–SiO2 nanocomposites and their scaffolds for dental applications
Highlights
► Ti–10 wt.% SiO2 nanocomposites are more corrosion resistant than the microcrystalline titanium. ► Porous Ti–10 wt.% SiO2 nanocomposites displays good in vitro cytocompatibility. ► An innovative product for dental implant applications.
Introduction
Nanomaterials can include metals, ceramics, polymers, and composite materials, and they demonstrate novel properties compared to conventional (microcrystalline) materials because of their nanoscale features. Furthermore, researchers have exhibited an increased interest in exploring the numerous biomedical applications of nanomaterials and nanocomposites [1], [2], [3]. It has been demonstrated that the use of metallic, carbon, or oxide bionanomaterials to produce implants considerably improved the prosthesis strength and biocompatibility of the implants.
Metal/alloy nanocrystalline materials can be produced using non-equilibrium processing techniques, such as mechanical alloying (MA) [4], [5] and severe plastic deformation (SPD) [6]. Recent studies have clearly demonstrated that the nanostructuring of titanium can considerably improve not only its mechanical properties but also its biocompatibility [6]. This approach also has the benefit of enhancing the biological response of the commercially pure titanium — (CP Ti) surface [7].
Mechanical alloying is a powder processing technique that enables the production of nanomaterials starting from mixed elemental powders in the right proportion followed by loading the powder mixture into the mill along with the milling balls. During the mechanical alloying process, the powder particles are repeatedly flattened, cold welded, fractured and rewelded. The milled nanocrystalline powders are finally compacted and heat-treated to obtain the desired microstructure and properties [8].
A number of SPD methods for producing bulk, ultra-fine grained metals/alloys have been developed [6]. Valiev and co-workers used a process known as equal channel angular pressing (ECAP), which is a viable processing route for grain refinement and property improvement. Their study reports on nanostructured titanium that was produced as long-sized rods with superior mechanical and biomedical properties and that demonstrates applicability for dental implants.
Ti and Ti-based alloys are preferred materials for the production of implants in medical applications. These biomaterials have relatively poor tribological properties because of their low hardness. One of the methods that allow the biological properties of Ti alloys to be altered is the modification of its chemical composition [8], [9]. The other method is to produce a composite that will exhibit the favorable mechanical properties of titanium and the excellent biocompatibility and bioactivity of a ceramic [10]. The most commonly used ceramics employed in medicine are hydroxyapatite (HA), silica, and bioglass [11]. Silica (SiO2) is a bioactive material that has high corrosion resistance. Silica bioceramics are used as prosthetic bone and dental implants because they promote the formation of apatite at their surfaces when immersed in simulated body fluid (SBF) [11], [12]. A biological basis for the role of silica in bone formation was established by Carlilse in a study of the role of silicon in bone calcification [13]. SiO2 improves materials' bioactivity by leading to the formation of Si–OH groups on the material surface.
However, SiO2 cannot be used for load-bearing applications because of its poor mechanical properties compared to natural bone. The ceramic coatings often flake off as a result of poor ceramic/metal interfacial bonding [14]. The above mentioned problems may be overcome by fabricating metal/bioceramic composites. Some studies have been reported on the preparation of Ti-bioceramic (HA, 45S5 Bioglass) composite materials [10], [15], [16].
Current studies are focused on fabricating Ti-based porous scaffolds to promote bone or tissue ingrowth into pores and to provide biological anchorage. Several factors have been demonstrated to have an influence on bone ingrowth into porous implants, such as the porous structure (pore size, pore shape, porosity and interconnecting pore size) of the implant, duration of implantation, biocompatibility, implant stiffness, micromotion between the implant and adjacent bone, etc. The architecture of a porous implant has been suggested to have a significant effect on the implant integration with newly grown bone [17], [18], [19].
Porous metallic scaffolds are generally fabricated using a variety of processes to provide a high degree of interconnected porosity to allow bone ingrowth. These fabrication techniques include chemical vapor infiltration for depositing tantalum onto vitreous carbon foams, solid freeform fabrication, self-propagating high-temperature synthesis, and powder metallurgy [20], [21], [22], [23], [24], [25]. Although these porous metals have been successful for encouraging bone ingrowth in both in vivo and in clinical trials, the range of materials and microstructures available is still rather limited. It is important to use appropriate surface modifications to increase the anti-corrosive and biocompatible properties of Ti implants for long-term clinical applications.
Recently, the mechanical alloying method and the powder metallurgy process for the fabrication of bulk Ti-HA, Ti-45S5 Bioglass and Ni-free austenitic stainless steel-HA nanocomposites with a unique microstructure have been developed [10], [26], [27]. These studies provided the first evidence for an enhancement of the properties due to the nanoscale structures in consolidated Ti-HA, Ti-45S5 Bioglass and Ni-free austenitic stainless steel-HA nanocomposites. The structure, corrosion and biological properties of porous titanium-45S5 Bioglass nanocomposite scaffolds have also been studied [23]. The produced nanocomposites have a low density and a structure composed of an interconnected network of pores.
As a continuation of the previous work, the present study examines the structure and surface roughness on the in vitro cytocompatibility of Ti–10 wt.% SiO2 porous scaffolds under static conditions. Our previous results related to bulk titanium–SiO2 nanocomposites were also incorporated [28], [29]. The properties of titanium–10 wt.% SiO2 nanocomposite scaffolds have not been previously investigated.
Section snippets
Sample Preparation
Mechanical alloying was performed under an argon atmosphere using a SPEX 8000 Mixer Mill. The starting materials included titanium (< 45 μm, 99%, Alfa Aesar) and silicon dioxide (44 μm, 99.6%, Sigma Aldrich). The vial was loaded and unloaded in a Labmaster 130 glove box under a high-purity argon atmosphere. The mixed powders were mechanically alloyed for 20 h. The blended Ti–10 wt.% SiO2 was mixed with ammonium hydrogen carbonate (NH4HCO3), which was used as the space-holder material. The size of
Results
The titanium–10 wt.% SiO2 scaffold nanocomposites were prepared by mechanical alloying and with a space-holder sintering process. X-ray diffraction was employed to examine the effects of mechanical alloying on the Ti–10 wt.% SiO2 composite. Fig. 1a, b present the XRD patterns of the initial titanium (ICDD: 5-682) and amorphous SiO2 powders. During the MA process, the intensity of the titanium Bragg reflections decrease with increasing milling time, and after 20 h of milling, the sample transformed
Discussion
Mechanical alloying is a typical top down method for preparing nanoparticles. When a mixture of elemental powders is milled, the formation of the amorphous phase is primarily due to an ultimate interdiffusion of atoms that occurs at fresh surfaces and interfaces created by mechanical milling. This interdiffusion is promoted by defects and chemical disorder in the crystalline structure [4], [38]. The considerable deformation during the MA process causes a large dislocation density and the
Conclusions
In this work, an enhancement of the properties due to the nanoscale structure in a consolidated bulk Ti–10 wt.% SiO2 nanocomposite and their scaffolds was observed. The following conclusions can be drawn:
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nanostructured Ti–10 wt.% SiO2 composites possess a greater Vicker's hardness,
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Ti–10 wt.% SiO2 nanocomposite scaffold has a porous architecture, with macropores of 400–800 μm and micropores of some tens of micrometers,
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nanostructured Ti–10 wt.% SiO2 composite is characterized by a smaller Young's
Acknowledgments
The partial financial support of the Polish Ministry of Education and Science under the contract no. N N507 295039 is gratefully acknowledged.
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