The Surface Characterisation of Polyetheretherketone (PEEK) Modified via the Direct Sputter Deposition of Calcium Phosphate Thin Films

Polyetheretherketone development in bone tissue engineering and orthopedic surgery

Polyetheretherketone (PEEK) has been widely used in the medical field as an implant material, especially in bone tissue engineering and orthopedic surgery, in recent years. This material exhibits superior stability at high temperatures and is biosecured without harmful reactions. However, the chemical and biological inertness of PEEK still limits its applications. Recently, many approaches have been applied to improve its performance, including the modulation of physical morphology, chemical composition and antimicrobial agents, which advanced the osteointegration as well as antibacterial properties of PEEK materials. Based on the evolution of PEEK biomedical devices, many studies on the use of PEEK implants in spine surgery, joint surgery and trauma repair have been performed in the past few years, in most of which PEEK implants show better outcomes than traditional metal implants. This paper summarizes recent studies on the modification and application of biomedical PEEK materials, which provides further research directions for PEEK implants.

Heteroatom Introduction to Reconstruct Interfacial Chemical Structures for High-Reliability CFRTP/A6061-T6 Hybrid Structures

The application of carbon-fiber-reinforced thermoplastic (CFRTP)/metal hybrid structures is a vital step for realizing the lightweight design concepts in aerospace. However, the CFRTP/metal hybrid structures are usually not reliable enough in practical applications due to the high differences in chemical and physical properties between these two materials. The current work provides a bottom-up strategy of introducing heteroatoms into CFRTP/metal interfaces to reconstruct the interfacial chemical structures and thus manufacture high-reliability hybrid structures. Based on the principle of utmost using reaction sites at metal surfaces, the heteroatoms of oxygen and hydrogen are specially designed and introduced to the CFRTP/A6061-T6 (6061) interfaces by simple and green plasma polymerization. The introduced oxygen and hydrogen heteroatoms react with the aluminum and oxygen of the oxidation film at 6061 surfaces to produce great interfacial Al-O covalencies and hydrogen bonds. The reconstructing interfacial chemical structures strengthen the joint strength of CFRTP/6061 hybrid structures from 8.82 to 23.97 MPa. Our heteroatom introduction strategy is expected to get a fresh insight into the interfacial design concept and has several important implications for the future application of high-reliability CFRTP/metal hybrid structures.

Strategies for the Covalent Anchoring of a BMP-2-Mimetic Peptide to PEEK Surface for Bone Tissue Engineering

Researchers in the field of tissue engineering are always searching for new scaffolds for bone repair. Polyetheretherketone (PEEK) is a chemically inert polymer that is insoluble in conventional solvents. PEEK's great potential in tissue engineering applications arises from its ability to not induce adverse reactions when in contact with biological tissues and its mechanical properties, which are similar to those of human bone. These exceptional features are limited by the bio-inertness of PEEK, which causes poor osteogenesis on the implant surface. Here, we demonstrated that the covalent grafting of the sequence (48-69) mapped on the BMP-2 growth factor (GBMP1α) significantly enhances the mineralization and gene expression of human osteoblasts. Different chemical methods were employed for covalently grafting the peptide onto 3D-printed PEEK disks: (a) the reaction between PEEK carbonyls and amino-oxy groups inserted in the peptides' N-terminal sites (oxime chemistry) and (b) the photoactivation of azido groups present in the peptides' N-terminal sites, which produces nitrene radicals able to react with PEEK surface. The peptide-induced PEEK surface modification was assessed using X-ray photoelectron measurements, while the superficial properties of the functionalized material were analyzed by means of atomic force microscopy and force spectroscopy. Live and dead assays and SEM measurements showed greater cell cover on functionalized samples than the control, without any cytotoxicity induction. Moreover, functionalization improved the rate of cell proliferation and the amount of calcium deposits, as demonstrated by the AlamarBlue™ and alizarin red results, respectively. The effects of GBMP1α on h-osteoblast gene expression were assayed using quantitative real-time polymerase chain reaction.

Polyether Ether Ketone Coated with Ultra-Thin Films of Titanium Oxide and Zirconium Oxide Fabricated by DC Magnetron Sputtering for Biomedical Application

Recently, polyether ether ketone has raised increasing interest in research and industry as an alternative material for bone implants. This polymer also has some shortcomings, as it is bioinert and its surface is relatively hydrophobic, causing poor cell adhesion and therefore slow integration with bone tissue. In order to improve biocompatibility, the surface of polyether ether ketone-based implants should be modified. Therefore, polished disc-shaped polyether ether ketone samples were surface-modified by direct current magnetron sputtering with ultrathin titanium and zirconium coatings (thickness < 100 nm). The investigation results show a uniform distribution of both types of coatings on the sample surfaces, where the coatings mostly consist of titanium dioxide and zirconium dioxide. Differential scanning calorimetry revealed that the crystalline structure of the polyether ether ketone substrates was not changed by the coating deposition. Both coatings are amorphous, as shown by X-ray diffraction investigations. The roughness of both coating types increases with increasing coating thickness, which is beneficial for cell colonization. The coatings presented and investigated in this study improve wettability, increasing surface energies, in particular the polar component of the surface energies, which, in turn, are important for cell adhesion.

Femtosecond laser optimization of PEEK: efficient bioactivity achieved by synergistic surface chemistry and structures

Poly-ether-ether-ketone (PEEK) is considered a potential orthopedic material due to the excellent mechanical properties and chemical resistance, but its biological inertness hampers its further clinical application. In this study, advanced femtosecond laser microfabrication technology was utilized to induce the change of the surface characteristics of PEEK to improve its bioactivity. Meanwhile, the mechanism of surface reaction and improved bioactivity was interpreted in detail from the perspective of material science. The surface physical-chemical characterization results showed that femtosecond laser etching could increase the surface energy, and the contents of active sites including amorphous carbon and carbon-hydroxyl on PEEK surfaces. In vitro validation experiments demonstrated that the samples etched with a femtosecond laser had a better ability to induce apatite deposition and cell proliferation than those treated with popular sulfonation modification, which would lead to better bioactivity and osteointegration. The current work fully presents the mechanism of the femtosecond laser low-temperature plasma effect on PEEK and the resulting surface characteristics, which could broaden the application of PEEK in the orthopedic field. Moreover, it has great potential in the surface design and modification of other biomaterials with enhanced bioactivity.

Mechano-Chemical Properties of Electron Beam Irradiated Polyetheretherketone

In this study, the mechano-chemical properties of aromatic polymer polyetheretherketone (PEEK) samples, irradiated by high energy electrons at 200 and 400 kGy doses, were investigated by Nanoindentation, Brillouin light scattering spectroscopy and Fourier-transform infrared spectroscopy (FTIR). Irradiating electrons penetrated down to a 5 mm depth inside the polymer, as shown numerically by the monte CArlo SImulation of electroN trajectory in sOlids (CASINO) method. The irradiation of PEEK samples at 200 kGy caused the enhancement of surface roughness by almost threefold. However, an increase in the irradiation dose to 400 kGy led to a decrease in the surface roughness of the sample. Most likely, this was due to the processes of erosion and melting of the sample surface induced by high dosage irradiation. It was found that electron irradiation led to a decrease of the elastic constant C₁₁, as well as a slight decrease in the sample’s hardness, while the Young’s elastic modulus decrease was more noticeable. An intrinsic bulk property of PEEK is less radiation resistance than at its surface. The proportionality constant of Young’s modulus to indentation hardness for the pristine and irradiated samples were 0.039 and 0.038, respectively. In addition, a quasi-linear relationship between hardness and Young’s modulus was observed. The degradation of the polymer’s mechanical properties was attributed to electron irradiation-induced processes involving scission of macromolecular chains.

The Surface Characterisation of Fused Filament Fabricated (FFF) 3D Printed PEEK/Hydroxyapatite Composites

Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer which has found increasing application in orthopaedics and has shown a lot of promise for 'made-to-measure' implants via additive manufacturing approaches. However, PEEK is bioinert and needs to undergo surface modification to make it at least osteoconductive to ensure a more rapid, improved, and stable fixation that will last longer in vivo. One approach to solving this issue is to modify PEEK with bioactive agents such as hydroxyapatite (HA). The work reported in this study demonstrates the direct 3D printing of PEEK/HA composites of up to 30 weight percent (wt%) HA using a Fused Filament Fabrication (FFF) approach. The surface characteristics and in vitro properties of the composite materials were investigated. X-ray diffraction revealed the samples to be semi-crystalline in nature, with X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry revealing HA materials were available in the uppermost surface of all the 3D printed samples. In vitro testing of the samples at 7 days demonstrated that the PEEK/HA composite surfaces supported the adherence and growth of viable U-2 OS osteoblast like cells. These results demonstrate that FFF can deliver bioactive HA on the surface of PEEK bio-composites in a one-step 3D printing process.

Effects of Raster Angle and Material Components on Mechanical Properties of Polyether-Ether-Ketone/Calcium Silicate Scaffolds

Polyetheretherketone (PEEK) was widely used in the fabrication of bone substitutes for its excellent chemical resistance, thermal stability and mechanical properties that were similar to those of natural bone tissue. However, the biological inertness restricted the osseointegration with surrounding bone tissue. In this study, calcium silicate (CS) was introduced to improve the bioactivity of PEEK. The PEEK/CS composites scaffolds with CS contents in gradient were fabricated with different raster angles via fused filament fabrication (FFF). With the CS content ranging from 0 to 40% wt, the crystallinity degree (from 16% to 30%) and surface roughness (from 0.13 ± 0.04 to 0.48 ± 0.062 μm) of PEEK/CS scaffolds was enhanced. Mechanical testing showed that the compressive modulus of the PEEK/CS scaffolds could be tuned in the range of 23.3–541.5 MPa. Under the same printing raster angle, the compressive strength reached the maximum with CS content of 20% wt. The deformation process and failure modes could be adjusted by changing the raster angle. Furthermore, the mapping relationships among the modulus, strength, raster angle and CS content were derived, providing guidance for the selection of printing parameters and the control of mechanical properties.

Drug-Releasing Antibacterial Coating Made from Nano-Hydroxyapatite Using the Sonocoating Method

Medical implant use is associated with a risk of infection caused by bacteria on their surface. Implants with a surface that has both bone growth-promoting properties and antibacterial properties are of interest in orthopedics. In the current study, we fabricated a bioactive coating of hydroxyapatite nanoparticles on polyether ether ketone (PEEK) using the sonocoating method. The sonocoating method creates a layer by immersing the object in a suspension of nanoparticles in water and applying a high-power ultrasound. We show that the simple layer fabrication method results in a well-adhering layer with a thickness of 219 nm to 764 nm. Dropping cefuroxime sodium salt (Cef) antibiotic on the coated substrate creates a layer with a drug release effect and antibacterial activity against Staphylococcus aureus. We achieved a concentration of up to 1 mg of drug per cm² of the coated substrate. In drug release tests, an initial burst was observed within 24 h, accompanied by a linear stable release effect. The drug-loaded implants exhibited sufficient activity against S. aureus for 24 and 168 h. Thus, the simple method we present here produces a biocompatible coating that can be soaked with antibiotics for antibacterial properties and can be used for a range of medical implants.