In Vitro Studies

For tooth repair and regeneration, biomedical scaffolds must meet the following requirements in vitro: good mechanical properties, biocompatibility, biodegradability and non-toxic; they can promote cell adhesion, proliferation and growth. These scaffolds should allow various of cells including ameloblasts, odontoblasts, cement- oblasts, to carry out its function and further promote regeneration for single or multiple tooth tissue. Therefore, the performance of the scaffolds should be as close as possible to the above requirements. In fact, fracture toughness, which represents material absorbed energy before the fraction for crack propagation, is one of the most significant mechanical properties for the dental materials. In order to acquire excellent fatigue resistance and fracture toughness, many novel fabrication techniques have been used in the proceeding of synthesis of fiber or tube reinforced scaffolds. With regard to fiber reinforced composites, they can be prepared from thermosets reinforced by electronic-grade glass (E-glass) fibers are promising to be used as metallic implants. Bioactive glass granules can be combined with a polymer matrix to improve the osteointegration of fiber-reinforced composites implants.

However, it is difficult to use the bioactive glass granules to create a viable surface layer. Kulkova et al. investigated the potential application of excimer laser ablation to achieve selectively remove the matrix to expose the surface of bioactive glass granules [62]. To study the differences of light in the thermoset matrix and bioactive glass during the penetration process, they studied ultra-violet-visible spectroscopy. In addition, they established the optimal excimer laser ablation parameters. The research verified that a calcium phosphate layer formed on the surface of the laser-ablated specimens in simulated body fluid. Moreover, they investigated the proliferation of human osteosarcoma cells on the surfaces of the laser-ablated specimens. Their results showed that excimer laser ablation had the potential to create a bioactive surface on fiber-reinforced composites implants.

Air jet spinning technique has been successfully developed to fabricate 3D porous and micro-nano plain organic or organic-inorganic hybrid/composite fibers [63]. Although the initial and operational costs of air jet spinning are very low, the production rate is relative high. Air jet spinning can cause a high-speed compressed gas onto the suitable solution to get a polymer jet. Abdalla et al. used hybrid nanofibers to improve the biocompatibility and biodegradation rate of AM50 magnesium

An optical morphology observation ofMC3T3-E1 pre-osteoblast cell culture 2 days at

Fig. 7.2 An optical morphology observation ofMC3T3-E1 pre-osteoblast cell culture 2 days at (a) negative control; (b) magnesium alloys; (c) magnesium alloys treated with plain PLA using air jet spinning; (d) magnesium alloys treated withPLA and 3% nHA using air jet spinning, showing a higher cytocompatibilityof air jet spinning-coatings compared to neat Mg alloys (Adapted with permission from Ref. [64]. Copyright 2016 Elsevier Ltd)

alloy [64]. They prepared biodegradable hybrid membrane fiber layers by nanohydroxyapatite particles and poly(lactide) nanofibers and coated layer-by-layer on AM50 coupons by air jet spinning approach. The corrosion performance of coated and uncoated coupon samples was investigated by means of electrochemical measurements. The results showed that compared to the neat AM50 coupon samples, the air jet spinning 3D membrane fiber layers effectively decreased the initial degradation rate and induced a higher biocorrosion resistance. Due to the presence of nano-hydroxyapatite particles in the air jet spinning layer, the adhesion strength improved highly. In addition, the air jet spinning-coatings significantly controlled the long biodegradation rate of AM50 alloy in Hank’s balanced salt solution. Moreover, the cytocompatibility of air jet spinning-coatings was higher than that for neat Mg alloys (Fig. 7.2). Therefore, the nanostructured hydroxyapatite embedded hybrid poly(lactide) nanofibers coating could be a suitable coating material for Mg alloy as a potential material for biodegradable metallic orthopedic implants.

Another great advantage of fiber reinforced composites in implant prostheses is that they can solve several problems associated with metal alloy frameworks such as high cost, the complexity of fabrication and corrosion. Erkan et al. built a 3D finite element analysis model to evaluate the stress distribution in implant-abutment complex, bone and prosthetic structures [65]. Two distinctly different models, composed of fiber-reinforced composite and particulate composite (FRC-FPD model) or Cr-Co and porcelain(M-FPD model) were studied. The results showed that all the stress values in the M-FPD model were higher than in the FRC-FPD model, except for the stress values in the implant-abutment complex. They concluded that the implant supported FRC-FPD could maintain normal physiological loading of the surrounding bone and eliminate the excessive stresses in the bone-implant interface, thus reducing the risk of peri-implant bone loss. Similarly, Boudeau et al. injected model of polyetheretherketone reinforced with carbon fibers (CF/PEEK) to obtain composite stems used for hip replacement [66]. They combined process simulations and structural analysis to explore different compositions of CF/PEEK and injection conditions. The obtained implants were compared to the bone-implant system based on conventional metallic materials and the bone alone under walking load. Comparisons were done from four objective criteria: stress deficiency, stress shielding, global deformation and debonding. CF/PEEK injected implant might reduce three of the criteria. The carbon fibers presented in the interface bone-implant could exhibit health problem for the patient with bone fragments due to wear and friction between implant and bone.

To investigate the effects of short fibers such as silk microfibers (SMFs), Ceria nanofibers (CNFs) and alumina microfibers (AMFs) as reinforced materials in 2,2-bis[4-(20-hydroxy-30-ethacryloyloxypropoxy)phenyl]propane(Bis-GMA) and triethyleneglycol dimethacrylate (TEGDMA) resin on the development of composite dental filler. Rameshbabu et al. studied the morphology, phase, degree-of- conversion, viscosity, flowability and physical properties of AMFs, SMFs, CNFs and their representative fracture surfaces of the reinforced dental resins/composites [67]. Their results showed that addition of short fibers into base resin during formulation of composites reduced the degree of conversion and depth of cure. The resultant composites had significantly better mechanical properties compared to the base resin for both alumina and silk as additives. SMFs, CNFs and AMFs reinforced composite resins significantly improved the physical and mechanical properties. Therefore, they may be used as additives in composite dental filler for the plausible application.

Cellulosic fibers with low cost, low density, specific strength and high elasticity modulus have been extensively used for reinforcing the physical structure of some composites. Silva et al. applied cellulosic fibers to modify a conventional restorative glass ionomer cement (GIC), and used scanning electron microscopy, light microscopy, and energy dispersive X-ray spectroscopy to characterize the composite and its precursors [68]. The addition of cellulosic fibers reinforced the solubility in water, compressive strength, resistance to abrasion and bond strength of the fiber- reinforced glass ionomer cement. Similarly, Anderson et al. studied the physical, chemical and degradation properties of three different kinds of epoxide-based materials used for restorative dental applications by Raman, Fourier transform infrared, X-ray fluorescence spectroscopy, Brunauer-Emmett-TellerAnalysis, and degradation experiments [69]. Physicochemical characterization showed that the materials had similar chemical composition, the differences between them mainly depended on the different phase distribution, not chemical composition. Maryam et al. further used polyurethane combined with hydroxyapatite and fibers to prepare composites for dental applications by solution casting technique [70]. The concentrations of hydroxyapatite and polyurethane hard segment accompanied with the duration of immersion in artificial saliva were two main factors of the modification of solid- state properties of hydroxyapatite. The use of an optimum amount of hydroxyapatite and polyurethane hard segment in composites and fibers could respectively improve the properties of the dental filling material.

To study the combined effect of dental luting cement and posts on fracture resistance and failure mode of the endodontically-treated tooth, Mohamed et al. randomly restored 60 endodontically-treated upper central incisors with fiber, titanium, or stainless steel posts and luted by zinc phosphate or composite resin cement [71]. They also analyzed a 3D tooth model by the finite element method to contrast the stress distributions caused by different post-core systems. Their results indicated that zinc phosphate cement showed reversely higher fracture resistances, however, luting with composite resin led to failures that are more restorable. What is more, compared to the tooth restored by titanium posts or stainless steel, those treated by fiber posts showed satisfactory fracture resistances. In addition, to enhance the mechanical performance of fiber-reinforced composite for dental applications, structural optimization was used to obtain an alternative design for a 3D inlay- retained fiber-reinforced composite dental bridge [72]. Chen et al. tried to validate in vitro the improved adhesive fixed partial denture design obtained by the stress- induced material transformation technique by simulating occlusal loading [73]. The 3D inlay-retained fiber-reinforced composite dental bridge was made with glass fibers as the substructure and a veneering composite. The samples were loaded to 400 N on a universal test machine and recorded the force and displacement. At the same time, they used a two-channel acoustic emission system to monitor the development of cracks during loading. The results showed that the optimized fiber- reinforced composite bridge design had a higher fracture resistance. Similarly, Baaran et al. compared the load bearing capacity of fiber-reinforced or unreinforced composites for fixed dental prostheses by simulated manufacturing fabrication [74]. The results showed that fixed partial dental prostheses with fiber-reinforced composite resin blocks existed higher load-bearing capacities than that unreinforced composite resin blocks. The shock-absorbing mechanism in commercial dental implants may be the factor caused bone loss and possible implant failure. Therefore, Sheikhhassani et al. tried to generate a shock-absorbing dental implant that is similar to the periodontal ligament by using polyethylene fibers to reinforce polycarbonate-urethane [75]. To further study the implant type, tests based on mechanical testing and finite element analysis was proposed. The reinforced composite dental implant showed good shock-absorbing properties.

To enhance osseointegration and decrease healing time after implantation, Lee et al. studied the physiology interaction of drug (N-acetyl cysteine) loaded nanotube titanium (NLN-Ti) implants in vitro. MC-3T3 E1 osteoblast-like cells were cultured on the surfaces of drug loading Ti nanotubes. They demonstrated that the cellular viability was evidently enhanced. Additionally, osteogenesis molecule tumor necro?sis factor-alpha and interleukin were observed in MC-3T3-E1 cells by means of reducing the inflammatory response [76]. The biological effects of the drug on Ti surfaces have been reported to improve osteoconduction and osseointegrate capacity [77, 78].

To clarify fracture toughness of tubular materials, Dafar et al. investigated two different kinds of composites consisted of titania (TiO2) nanotubes. The research found that functionalized n-TiO2 modified composites showed greater values of fracture toughness than control composite, and n-TiO2-reinforced composites exhibited outstanding biocompatibility in vitro [79].

Chen and his group prepared a novel dental resin containing silanized halloysite nanotubes (HNTs). The HNTs were mixed with 2.2'-bis-[4-(methacryloxypropoxy) -phenyl]-propane/tri(ethyleneglycol) dimethacrylate (Bis-GMA/TEGDMA) resin by three different techniques to synthesize uniform scaffold containing different content of HNTs. They found that mechanical properties of the composites improved substantially, when the weight content of the HNTs is low (e.g., 1 and 2.5%). However, the mechanical properties never further enhance even if adding their amount (e.g., 5%). This result showed that the different content of HNTs into dental composites could lead to opposite enhancement effect [80].

As for tooth materials, it is well known that CNTs could serve as collagen fibers during the tooth formation. They always control crystal nucleation and inorganic component growth [81]. Many studies have shown that CNTs could boost the growth of osteoblasts and osteogenesis. Cheng et al. studied a composite scaffold consisted of CNT and PLGA. The research indicated that the addition of CNT into PLGA facilitated the proliferation and differentiation of osteoblast cell. In addition, CNT modified composite scaffolds exhibited excellent mechanical strength. The results could be ascribed to higher surface roughness and qualified mechanical properties of CNTs, which further promotes the adsorbent of growth factors on the surface of CNT. Moreover, the incorporation of CNT is conducive to control phenotypic expression and lineage specification of stem cells [82].

In recent studies, CNT-reinforced scaffolds have exhibited great potential in the repair of dental defects. Mohamed et al. investigated the biocompatibility of carbon nanotube reinforced tetragonal zirconia polycrystalline (TZP) composite in vitro. In their study, osteoblast-like cell MG63 was cultured on the scaffold. The result showed that CNTs enhanced differentiation and proliferation of MG63 cell, which could ascribe to their large specific surface area that provides more opportunities to absorb osteogenic-related proteins. In addition, the addition of CNTs enhanced fracture toughness for the CNTs reinforced materials compare to monolithic zirconia [83].

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