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Introduction

Polymer-based composites have been widely used in joint arthroplasty and dentistry as bone cements and dental restorative materials, respectively . Due to their potential for replacing the controversial dental amalgam, polymer-based dental restorative composites are increasingly becoming popular in posterior restorative application . Similarly, acrylic based cemented arthroplasties are drawing significant attention world-wide, because of the excellent primary fixation of the implant and fast recovery times for the patient . However, in-spite of having admirable records of performance, both of these composites suffer from poor mechanical performance which facilitates their failure due to bending induced breakage and fracture related cracking . Commercial bone cements contain barium sulfate (BaSO ₄ ) or zirconium dioxide (ZrO ₂ ) as X-ray contrast agents in particulate form, which are known to adversely affect the mechanical properties. Moreover, as there is no chemical reaction between the inorganic particles and the PMMA bone cements, it can be understood that the filler particles act like pores when a tensile stress is applied to the bone cement . Bone cement failure is responsible for a significant portion of the estimated 36,000 revision hip arthroplasties performed in 2003 in the United States, which is mainly attributed to a lack of sufficient mechanical properties of the commercial acrylic cement . Therefore, there is a sustained interest to improve the mechanical properties of polymer-based dental and orthopedic composites.

Fiber reinforcement is one of the most followed approaches to enhance the mechanical properties of polymer-based composites. Ceramic and metal fibers as well as whiskers were incorporated into orthopedic and dental resin matrices in different studies in order to improve the mechanical properties of the composites . These efforts have been less than ideally successful due to the large size of the fillers, poor filler-resin matrix bonding (and subsequent debonding), nonuniform filler material distribution, and the pores acting as stress concentration sites .

The introduction of nano-scale materials offers new promise for augmenting the mechanical properties of bone cements and dental composites due to their high surface area to volume ratio which enhances their interfacial interaction with the resin matrix . Marrs et al. reported the incorporation of multi-walled carbon nanotubes (MWCNT) as reinforcement into bone cement matrix. However, a slight increase in the mechanical properties was obtained due to the weak bonding between the non-functionalized MWCNT and the resin matrix. In addition, the biocompatibility of carbon nanotube reinforced bone cements seems to be challenging . Similarly, functionalized single-walled carbon nanotubes capable of establishing strong chemical bonds with the matrix polymer have been reported as reinforcing agents in the urethane dimethacrylate dental resin matrix with resulting higher flexural strength (FS) compared to the un-reinforced counterpart . However, addition of the carbon nanotubes may impose negative effects on the aesthetic requirements and biocompatibility of the dental composites. In other study, poor increase in the mechanical properties of bone cements was observed following the incorporation of calcium carbonate nanoparticles due to the agglomeration of nanoparticles and lack of adhesion between the nanoparticles and the resin matrix .

In a previously published work, we examined nanostructured functionalized titania fibers to augment a poly (methyl methacrylate) (PMMA) matrix, which provided significantly enhanced dynamic mechanical properties . Nanotubes, like nanofibers, have a high aspect ratio and a high surface area to volume ratio which may lead to significantly enhanced physical and mechanical properties . Moreover, the hollow structure of the nanotube provides additional interlocking with the matrix through both the interior and exterior surfaces of the tubes. Although titania is an established biocompatible material , nanomaterials are well known to exhibit completely different characteristics at their nanodimensions, often being more toxic. This is attributed to their larger surface area, enhanced chemical reactivity, and easier penetration into cells . Hence, the polymer matrix reinforced with n-TiO ₂ tubes needs to be evaluated in terms of cell adhesion, proliferation and cytotoxicity.

In the present study, we have studied the inclusion of n-TiO ₂ tubes in an acrylic polymer matrix. We have selected a commercial PMMA bone cement (CMW ^® 1) as the control and incorporated the n-TiO ₂ tubes with a loading of 0.5, 1.0, 1.5 and 2.0 wt.% and investigated the following properties of the resulting nanocomposites: complex viscosity-versus-time, maximum polymerization temperature, dough time, setting time, radiopacity, fracture toughness, flexural strength and flexural modulus. In order to obtain maximum enhancement of the mechanical properties, we have functionalized the n-TiO ₂ tubes in an attempt to make the nanotubes more compatible with the matrix according to our previously described technique . The in vitro biocompatibility of the cement reinforced with n-TiO ₂ tube was also investigated in this work using primary osteoblasts obtained from rat calvarias. We should note that PMMA bone cement was used as the model system in this study to investigate the potential of the n-TiO ₂ tubes in strengthening low filled resin composites.

The hypothesis of this study is that functionalized n-TiO ₂ tubes added to commercial resin cement will significantly enhance its mechanical properties without altering its rheology, radiopacity and biocompatibility.