Full Length Articlel-Arginine modified multi-walled carbon nanotube/sulfonated poly(ether ether ketone) nanocomposite films for biomedical applications
Graphical abstract
Introduction
Poly(ether ether ketone) (PEEK) is a semi-crystalline polymer with a rigid aromatic backbone constituted of a hydroquinone and a benzophenone segment. PEEK has advantageous properties over metallic implants such as a low density of 1.3 g/cm3, high wear and corrosion resistance [1], [2]. In addition, it has an elastic modulus of approximately 3.6 GPa, which is close to the elastic modulus of bone and prevention of this enables stress shielding like metallic implants [3], [4], [5]. PEEK’s radiolucent properties also enable X-ray imaging of PEEK implants [6], [7]. In addition to these, the lower costs associated with the preparation and implementation of polymer-based implants is an advantage in comparison with those of metallic competitors. However, unfortunately, PEEK is bioinert and hydrophobic, and therefore osseointegration failure occurs when untreated PEEK is employed. Therefore, enhancing the physical performance of PEEK has been studied by several research groups [7], [8], [9], [10] and different methods have been proposed, including surface coating, chemical modification and addition of bioactive fillers such as apatites [8], carbon nanotubes (CNTs) [9], [10] and carbon fibers [7].
It should be noticed that functionalization or blending of PEEK with other materials is usually difficult as it is insoluble in most of the organic solvents [11]. PEEK is usually functionalized either via its aromatic ring through electrophilic substitution reactions or the reactive carbonyl group in its benzophenone segment. One of the chemical modification methods used to improve the solubility and hydrophilicity of PEEK is the sulfonation of its backbone [12], [13]. Montero et al. [14] suggested that sulfonated PEEK (SPEEK) may reduce biofilm formation on implant surfaces. At sulfonation levels above 30%, the solubility of PEEK in dimethylformamide (DMF), dimethylsulfoxide (DMSO) and N-methylpyrrolidone (NMP) increases. At higher sulfonation levels, above 70%, PEEK can also be dissolved in methanol [15]. It is worth mentioning that the sulfonation changes the chain and packing conformation of PEEK, and therefore reduces crystallinity of its domains. It was found that SO3H group can also modify the intermolecular forces in PEEK hindering the segmental mobility which increases the glass transition temperature of PEEK. These hydrophilic groups also increase moisture affinity and act as a plasticizer [12], [13].
In the literature, SPEEK has also blended with CNT to produce solvent cast nanocomposite films. Discovered in 1991 by Sumio Iijima, CNTs are typical 2D carbon structures which can be categorized into single and multi-walled nanotubes. CNTs possess a combination of interesting properties including low density, high aspect ratio and intrinsically unique mechanical, thermal and electrical properties. An elastic modulus of 1.2 TPa and a tensile strength of 50–200 GPa have been reported for CNTs [9]. This may enable production of nanocomposites with high mechanical performance. The recent studies suggest that the incorporation of CNTs into polymers may improve their biological properties [16].
In this regard the optimization of interfacial adhesion between CNTs and the polymer matrix used has been considered to be an essential factor in extracting positive benefits from CNTs additives. Often CNTs are functionalized with carboxyl groups to achieve appropriate dispersion properties between CNTs and polymers. In general, optimizing CNTs chemistry and processing parameters have been employed as the practical tools towards maximizing their dispersibility and the interfacial bonding with the polymer matrix used, resulting in the enhancement of the polymer’s strength, toughness, and modulus as well as electrical conductivity [17]. However, with the change of the degree of carboxylation of CNTs, the dispersibility of CNTs in a polymeric matrix may vary [9], [18]. There are chemical interactions between carboxylated MWCNTs and SPEEK including hydrogen bonding, van der Waals interactions as well as pi stacking forces. The latter occurs between the aromatic groups of PEEK and the sidewalls of the carboxylated MWCNT [19].
l-Arginine which is the most hydrophilic of the 20 most common natural amino acids is produced by Astragalus membranaceus Bunge [20]. The studies so far indicate that l-arginine improves bone mineralization and supports the healthy fracture healing process [21], [22], [23], [24]. l-Arginine was also previously utilized to modify prosthesis [25]. It has been found that l-arginine activates cell signaling, which induces osteogenic differentiation in human mesenchymal stem cells [20]. Calcium phosphate cement modified with l-arginine enhanced the mesenchymal stem cell adhesion and proliferation. Moreover, l-arginine containing Poly(ε-caprolactone) nanofibers enhance the proliferation of murine endothelial cells [26]. The anti-bacterial properties of l-arginine have also been demonstrated in previous works [27], [28].
Liu et al. [29], successfully conjugated poly(ethylene terephthalate) (PET) films with l-arginine in three steps. The authors, first, aminolyzed PET films, followed by treating with glutaraldehyde. Finally, the films were immersed in l-arginine solution. The same approach was pursued in the current study in order to fabricate l-arginine-conjugated carboxylated MWCNT/SPEEK nanocomposite films. The surface properties of the films have then been analyzed. The results obtained can shed light on the efficient utilization of PEEK in biomedical applications.
Section snippets
Materials
PEEK (Victrex) was provided by Uniplast Company. Commercially available carboxylated MWCNTs (with 0.7% carboxyl groups, outer diameter = 30–50 nm, inside diameter = 5–10 nm length = 10–20 µm, >95% purity) was purchased from Cheaptubes Inc., USA. The other chemical reagents and equipment, including l-arginine mono hydrochloride, H2SO4 (95–98% purity), HNO3 (65%, purity), N,N-Dimethylformamide (DMF), hydrazine monohydrate (%95 purity), glutaraldehyde solution (25% in H2O), dimethyl sulfoxide
Results and discussion
PEEK is one of the most promising alternatives of titanium alloys for orthopedic applications. However, PEEK is bioinert which leads to deficient osteogenesis of PEEK implants. In this study, the surface of PEEK was chemically modified to improve its biological performance. For this, PEEK was first sulfonated and then incorporated with carboxylated MWCNTs. Finally, l-arginine was covalently conjugated on the nanocomposite films. The physical and chemical properties of the films were assessed
Conclusions
In this study, l-arginine conjugated carboxylated MWCNT/SPEEK nanocomposite films were fabricated. The sulfonation of PEEK caused a dramatic improvement in the solubility and hydrophilicity of PEEK. XPS studies provided evidence that nanocomposites were successfully conjugated with l-arginine biomolecules with the potential of further improving the biological performance of carboxylated MWCNT/PEEK. l-arginine conjugated MWCNT/SPEEK nanocomposites had improved mechanical properties than pure
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The support of TUBITAK 3001 Research Fund (ref: research grant 215M727) for this project paper is kindly acknowledged. Uniplast Company is appreciated for providing with PEEK (Victrex). We would like to thank Dr. Bora Garipcan for providing human fetal osteoblasts (hFOB) for cell culture studies.
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