|Year : 2020 | Volume
| Issue : 2 | Page : 68-73
Strategies and modification of polyetheretherketone for prosthodontic driven implants
U Krishna Kumar1, Sanjay Murgod2
1 Department of Prosthodontics Including Crown and Bridge and Implantology, Rajarajeshwari Dental College and Hospital, Bengaluru, Karnataka, India
2 Department of Oral Pathology, Rajarajeshwari Dental College and Hospital, Bengaluru, Karnataka, India
|Date of Submission||06-Feb-2020|
|Date of Decision||24-Feb-2020|
|Date of Acceptance||09-Mar-2020|
|Date of Web Publication||16-Feb-2021|
Dr. U Krishna Kumar
# 135, II Block, II Cross, HMT Layout, Vidyaranyapura, Bengaluru - 560 097, Karnataka
Source of Support: None, Conflict of Interest: None
Polyetheretherketone (PEEK) is regarded as one of the most potential materials for replacing current implant applications. To obtain good bone-implant interfaces, many modification methods have been developed to enable PEEK and PEEK-based composites from bioinert to bioactive. Among them, physical and chemical methods have aroused significant attention and been widely used to modify PEEK for dental implants. This review summarizes current modification techniques of PEEK for dental applications, which include composite strategies and surface-coating methods. The positive consequences of those modification methods will encourage continuing investigations and stimulate the wide range of applications of PEEK-based implants in prosthodontics.
Keywords: Bioactivity, coating, composite, polyetheretherketone, surface modification
|How to cite this article:|
Kumar U K, Murgod S. Strategies and modification of polyetheretherketone for prosthodontic driven implants. Int J Oral Health Sci 2020;10:68-73
| Introduction|| |
Only a limited number of polymers such as polytetrafluoroethylene, polymethyl methacrylate, polylactide, polyglycolide, and polyhydroxybutyrate have been used for bone replacement purposes, but they tend to be too flexible and too weak to meet the mechanical demands as dental implant. Besides, they may absorb liquids and swell, leach undesirable products, and may be affected by the sterilization process. Polyetheretherketone (PEEK) is a semi-crystalline linear polycyclic aromatic thermoplastic that was first developed by a group of English scientists. The emergence of carbon fiber reinforced PEEK was exploited for fracture fixation and femoral prosthesis in artificial hip joints. PEEK, a member of the polyaryletherketone family, has an aromatic molecular backbone, with combinations of ketone and ether functional groups between the aryl rings. This special chemical structure makes PEEK exhibit stable chemical and physical properties, and it is wear resistant and stable at high temperatures resistant to attack by all substances apart from concentrated sulfuric acid; it remains stable in sterilization processes. Besides, PEEK exhibits good biocompatibility in vitro and in vivo, causing neither toxic or mutagenic effects nor clinically significant inflammation. More importantly, the mechanical properties of PEEK are close to that of human cortical bone. However, PEEK is biologically inert and poorly hydrophilic surface presents a serious challenge for osseointegration which has limited its potential applications. Therefore, improving the bioactivity of PEEK is a significant challenge that must be solved to fully realize the potential benefits. This review article will focus on different physical and chemical modification ns of PEEK, which have been used to improve the bone-implant interface.
| Surface Modification of Dental Implants|| |
PEEK is always physically and chemically stable; it can be modified by some kind of physical or chemical treatments. The commonly used physical treatments are plasma modifications, such as oxygen plasma, ammonia plasma, nitrogen and oxygen plasma, methane and oxygen plasma, oxygen and argon plasma, ammonia and argon plasma, and hydrogen and argon plasma, and accelerated neutral atom beam (ANAB). The chemical treatments were rare. Only wet chemistry modification or sulfonation treatment can chemically modify the surface of PEEK. Besides, some materials can be coated onto PEEK to impose bioactive effects using various methods, including cold spray technique, radiofrequency (RF) magnetron sputtering, spin coating techniques, aerosol deposition (AD), ionic plasma deposition (IPD), plasma immersion ion implantation and deposition (PIII and D), electron-beam deposition, vacuum plasma spraying (VPS), physical vapor deposition (PVD), and arc ion plating (AIP). Surface treatment alone or in combination with surface coating can greatly improve the bioactivity of PEEK.
| Surface Treatments – Physical Treatment|| |
Plasmas are ionized gases that can be produced in a closed reactor system containing a low-pressure gas mixture by excitation with electromagnetic waves. The reactive particles generated in this way can interact with the surface of the biomaterial placed in the reactor and modify its physical and chemical surface properties without changing the mechanical, electrical, and optical properties of the material that are relevant to its application. The method of plasma modification has been used to modify PEEK material for a long time. PEEK surface was treated with two plasma process (a microwave plasma in NH4/Ar and a downstream microwave plasma in H2/Ar) and found to have good proliferation and differentiation of primary fibroblasts and osteoblasts on plasma-treated PEEK. It was also found that the osteogenic activity of cells on treated PEEK was comparable to that of tissue culture polystyrene, and a reproducible stimulation and suppression of cell proliferation could be achieved by the methods of plasma modification. PEEK was also treated with N2/O2 low-pressure plasma to improve the bioactivity of PEEK. Cell testing with osteoblastic cell lines (MC3T3-E1) showed that plasma-treated PEEK had no disadvantageous effects on cell viability. PEEK was treated with RF plasma with a mixture of CH4/O2 gases to modify the surface of PEEK; it was found that the treatment with CH4/O2 gases resulted in a significantly higher bond strength than untreated samples. Using a PIII and D technique with a CH4/O2 gas mixture, the deposition of oxygen-rich nanofilms on PEEK with a high surface energy was detected, which greatly improved cell adhesion. It was also found that there was a strong correlation between cell adhesion and the water contact angle, the polar component of surface energy, and to a lesser extent, oxygen concentration of the PEEK surfaces. Nanopatterned PEEK rods were etched with O2 plasma to improve their bioactivity as a novelty. O2/Ar or NH4 plasma was applied to treat the PEEK surface and was found to increase adhesion, proliferation, and osteogenic differentiation of adipose tissue-derived mesenchymal stem cells on plasma-treated PEEK. A novel ANAB technique was employed to intense directed beams of neutral gas atoms (comprised van der Waals bonded argon atoms) with average energies that could be controlled resulted in a controllable nanometer-scale texturing of the surface to a depth of no more than 5 nm. ANAB technique was employed to enhance the surface bioactivity of PEEK without modification of surface chemistry and without the addition of bioactive substances. In vitro experiments demonstrated that the ANAB-treated PEEK fostered enhanced growth of human fetal, osteoblast cells compared with untreated PEEK, as evidenced by cell proliferation assays and microscopy.
| Surface Treatments – Chemical Treatment|| |
Wet surface chemistry has been used to chemically modify PEEK to create a series of surface-functionalized PEEK. They are hydroxylated polymer (PEEK–OH) obtained by reduction, carboxylated polymer (PEEK–NCO) prepared by coupling a diisocyanate reagent to PEEK–OH, aminated polymer (PEEK–NH2) acquired by hydrolysis of PEEK–NCO, and aminocarboxylated polymers (PEEK–GABA and PEEK–Lysine) resulting from the coupling of aminoacids to PEEK–NCO. These chemical modifications promoted higher levels of fibronectin covalently fixed and adsorbed on various treated PEEK compared with untreated PEEK. By sulfonation and subsequent water immersion, a three-dimensional porous and nanostructured network with biofunctional groups is produced on PEEK to prepare two kinds of sulfonation-treated PEEK samples, namely SPEEK-W meaning water immersion and rinsing after sulfonation and SPEEK-WA meaning SPEEK-W with further acetone rinsing. The results showed that SPEEK-WA induced preosteoblast functions including initial cell adhesion, proliferation, and osteogenic differentiation in vitro as well as substantially enhanced osseointegration and bone-implant bonding strength in vivo and apatite-forming ability. Although SPEEK-W has a similar surface morphology and chemical composition as SPEEK-WA, its cytocompatibility is inferior due to residual sulfuric acid. Sulfonation can improve hydrophilicity and introduce bioactive sulfonate groups, but PEEK sulfonation has traditionally been applied for fuel cells, employing elevated temperatures and long reaction times to recast PEEK into sulfonated films. Little research has been systematically studied on PEEK surface modification by short reaction time and ambient-temperature sulfonation for biomedical applications. Three ambient-temperature sulfonation treatments under varying reaction time of 5 to 90 s and evaluation of the hydrophilicity and morphology of 15 modified PEEK surfaces were done. An optimal treatment was using 30-s H2SO4 followed by 20 s rinsing, and then, 20-s immersion in sodium hydroxide (NaOH) followed by 20 s rinsing. Thirty-second ambient-temperature sulfonation is found to be more effective than conventional plasma treatments and reduced PEEK water contact angle from 78° to 37° impregnating bioactive materials.
| Surface Coating on Dental Implants|| |
Various materials have been deposited on the surface of PEEK, including hydroxyapatite (HA), titanium (Ti), gold, Ti dioxide (TiO2), diamond-like carbon (DLC), and tert-butoxide. The most commonly used bioactive material as the coating of PEEK is HA. HA is the most widely used calcium phosphate-based bioceramic, which is the closest pure synthetic equivalent to human bone mineral. Numerous studies have consistently shown that HA typically exhibits excellent biocompatibility, bioactivity, and osteoconduction in vivo. Cold spray technique was used to fabricate HA-coated PEEK and evaluated its bioactivity in vitro and in vivo. Nanocrystalline HA-coated PEEK was fabricated with a spin-coating technique and inserted into the rabbit femurs as cylindrical implants, with uncoated cylinders as controls. Highly dense and well-adhered HA coating could be developed on PEEK using AD without the thermal degradation of PEEK. In another study, HA coatings were deposited onto PEEK surfaces using RF magnetron sputtering. Before HA deposition, an yttria-stabilized zirconia (YSZ) coating layer was deposited onto PEEK substrates to prevent degradation of PEEK substrates and the coating-substrate interface, then the HA/YSZ coated PEEK was heat treated using microwave and hydrothermal annealing to form the crystalline HA. Cell tests showed a significant increase in initial cell attachment and growth on the microwave-annealed HA/YSZ-coated PEEK compared with uncoated PEEK and amorphous HA. Jung et al. prepared a PEEK/Mg composite with an Mg content of 30 vol % by compression molding process, then the composite was treated in a specifically prepared aqueous solution for HA coating which led to the formation of an HA coating layer only on Mg particles exposed to the surfaces of the composite. Ti is the most widely used implant material for load-bearing dental and orthopedic applications because of its excellent mechanical and biological properties. Osteoblast adhesion on PEEK coated with either Ti or gold using the IPD process was studied, which created a nanostructured surface (with features <100 nm). Compared with the commonly used Ti and uncoated PEEK, PEEK coated with either Ti or gold significantly increased osteoblast adhesion and spreading. PVD was applied to coat Ti onto PEEK surface and placed coated PEEK and uncoated PEEK cylindrical implants into the femurs of mongrel dogs. The histological evaluation and mechanical evaluation revealed that the Ti-coated specimens had significantly higher percentages of bone contact than the uncoated. Coated Ti onto PEEK using an electron-beam (e-beam) deposition process was done, which produced a dense, uniform film on the substrate at a low temperature. In one study, CF/PEEK was coated with Ti by VPS process and chemically treated in NaOH solution. A carbonate-containing calcium phosphate layer was formed on the NaOH-treated Ti-coated CF/PEEK surface during immersion in simulated body fluid (SBF). In another study, CF/PEEK screws were coated with Ti using two different techniques, VPS and PVD; the results showed that Ti-coated CF/PEEK screws significantly improved bone deposition and removal torque compared with uncoated screws, whereas no statistical difference was detected between VPS and PVD coating types. TiO2 material has been demonstrated with good biocompatibility, bioactivity, hydrophilicity, and corrosion resistance. Anatase phase (A-TiO2) and/or rutile phase (R- TiO2) can be deposited onto PEEK substrate by an AIP technique following three steps (argon ion bombardment, bottom Ti layer deposition, and TiO2-coating deposition) at a low deposition temperature without damaging PEEK substrate while providing satisfactory film adhesion. From the results of cell adhesion, proliferation, and osteodifferentiation abilities, the researchers concluded that the TiO2-coated PEEK exhibited better osteoblast compatibility than bare PEEK and R-TiO2/PEEK exhibited better osteoblast compatibility than A-TiO2/PEEK. Uniform nanoporous TiO2 layer with a pore diameter of 70 nm by anodizing a Ti film was created, then deposited the created TiO2 onto a PEEK substrate via e-beam evaporation technique, and immersed the specimens in a bone morphogenetic protein-2 (BMP-2) solution to immobilize BMP-2. Successful coating of PEEK with DLC by PIII and D technique was done. A cell viability assay, scanning electron microscopy (SEM), and real-time polymerase chain reaction analysis indicated that osteoblast attachment, proliferation, and differentiation were better on DLC-coated PEEK than on bare PEEK. In another study, vapor of zirconium or Ti tetra (tert-butoxides) was deposited on the surface of PEEK at room temperature in a process reminiscent of deposition and partial thermolysis of metal alkoxides on oxide surfaces. Controlled thermolysis of the deposited alkoxide gives the metal a mixed oxide-alkoxide layer, which reacts with solutions of phosphonic acids to attach monolayer films of phosphonates, several of which are shown to significantly enhance osteoblast attachment and spreading compared with the untreated surface. In addition to coating various materials onto PEEK, PEEK material can also be coated onto other materials. Using the electrophoretic deposition method, PEEK and PEEK/bioglass particles were coated onto shape memory alloy (nickel and Ti, NiTi) wires or on two-phase (α + β) Ti–6Al–7Nb Ti alloy substrates with a uniform coating surface and negligible microcracking or porosity. As corrosion protective layers, the PEEK and PEEK/bioglass coatings were able to impede the leakage of ions in contact with body fluids. In particular, the bioglass containing coatings improved the bonding of bone or soft tissue to the implant. After immersion of PEEK/bioglass-coated NiTi in SBF, HA layers formed on the surface of the coated specimens after 1 week.
| Impregnating Bioactive Materials on Dental Implants-Polyetheretherketone Composites|| |
The PEEK composites were classified into two kinds by the size of the impregnating bioactive materials: the conventional PEEK composites and the nano-sized PEEK composite. Conventional PEEK composites: a research group fabricated an HA/PEEK composite with an HA content of up to 40 vol % via a process of melt compounding, granulation, and injection molding. Increasing the HA content resulted in increasing of the tensile modulus and microhardness. These authors also found that PEEK with 30 vol % HA exhibited an elastic modulus within the range of human cortical bone. However, few coworkers found that the spray-dried spherical HA particles in conventional or micro-sized HA/PEEK (μm-HA/PEEK) composites could debond from the PEEK matrix during long-term loading due to the poor interfacial adhesion. HA/PEEK composites were manufactured via the selective laser sintering (SLS) technique and found that HA/PEEK supported osteoblast growth and also found that composites with higher HA contents exhibited enhanced cell proliferation and osteogenic differentiation (increased ALP activity, and produced more osteocalcin), compared to Thermanox™ and polyvinyl chloride. The in situ synthesized composite exhibited good biocompatibility without toxicity, and the composite with 5.6 vol % HA exhibited satisfactory bioactivity without compromising its excellent mechanical performance. HA/PEEK composite was prepared by mixing, compaction, and pressureless sintering process, and when evaluated, it was found that the bioactivity of the HA/PEEK composite increased with increasing HA content in the composite. HA-filled PEEK compound with microscale HA particles called “PEEK-OPTIMA HA enhanced biomaterial provides excellent mechanical properties and performance, proven biocompatibility, and has a modulus of elasticity similar to cortical bone. Using an SLS rapid prototyping system, porous HA/PEEK composite scaffolds starting with 10 wt % HA to 40 wt % HA have been produced. Apart from HA, other bioactive materials were also used to make bioactive PEEK composites, including strontium-containing HA (Sr-HA), calcium silicate (CS), glass fibers, bioglass, and β-tricalcium phosphate (β-TCP). Considering both mechanical properties and bioactivity, these authors selected 20 vol % CS/PEEK as a promising implant material. Glass fiber/PEEK (GPEEK) composites were developed using PEEK and 10% randomly chopped E glass fibers. GPEEK supported proliferation, ALP activity, and osteocalcin production in vitro, suggesting that GPEEK could improve the growth and differentiation of bone cells. β-TCP was also incorporated into PEEK, and β-TCP was not found to improve the bioactivity of PEEK. One research group compared human osteoblast proliferation on pure PEEK, PEEK/1 wt % carbon, and PEEK/1 wt % carbon/10 wt % β-TCP. The results showed that PEEK composites containing 10 wt % β-TCP did not improve the proliferation of osteoblasts in vitro. The rates of proliferation of human osteoblasts growing on PEEK/1 wt % carbon/10 wt % bioglass were significantly higher than those on the other groups. However, some evidence indicated the inhibitory effect of β-TCP/PEEK on osteoblast proliferation.
| Nano-Sized Polyetheretherketone Composites|| |
Conventional HA/PEEK composite may not bear long-term critical loading due to debonding between HA filler and PEEK matrix. Nanotechnology was applied by material scientists to overcome this problem. HA/PEEK nanocomposites were prepared by a compounding and injection molding process. They found that this novel HA/PEEK nanocomposite exhibited satisfactory mechanical properties. More importantly, no debonding occurred between the well-dispersed HA nanoparticles and the PEEK matrix. It was difficult to uniformly disperse the nanoscale powders in a viscous polymer matrix using the conventional methods. To overcome the agglomeration of HA nanoparticles during manufacturing, a technique was adopted an in situ synthetic process to prepare HA/PEEK nanocomposites. The strong bonding between HA and PEEK has been attributed mainly to physical factors such as the mechanical interlock between PEEK molecules and the HA surface. When the feature size of a material is decreased from micrometers to nanometers, the material exhibits several unique characteristics, including a very high surface-area-to-volume ratio, flexible surface functionality, and superior mechanical performance, including stiffness and tensile strength. Few found that nanostructured materials may promote osteoblast adhesion, proliferation, and differentiation, and stimulate new bone growth compared to conventional materials. Few authors did n-TiO2-reinforced PEEK composites (n-TiO2/PEEK) and studied the bioactivity of these nanocomposites in vitro and in vivo in vitro tests which showed that n-TiO2 promoted cell attachment and improved osteoblast spreading. In vivo tests showed that n-TiO2 improved bone regeneration around the implants compared with pure PEEK, as assessed by micro-CT and histological analysis. Thus, n-TiO2 was considered to significantly improve the bioactivity of PEEK, especially for composites with rough surfaces.
| Noncoating Treatment|| |
In a study, hydrophilic surface on PEEK was formed without coating layers using hydroprocessing (aqueous solution processing) and examined the osteoconductivity and anti-inflammatory properties of the surface-treated PEEK compared with the surface-treated Ti implants in vivo. The Water Contact Angle (WCA) (contact angle) value of PEEK reached 20° using a combination of immersion in > 16.2 M H2SO4 and ultraviolet irradiation (172 nm). Although as polished PEEK could not adsorb protein, hydrophilization gave rise to protein adsorption. In vivo, the hydrophilization of PEEK by surface modification without a coating layer improved the osteoconductivity and anti-inflammatory properties.
| Conclusions|| |
PEEK is biocompatible, chemically and physically stable, and radiolucent and exhibits a similar elastic modulus to normal human bone, making it an attractive dental implant material. However, PEEK is biologically inert, preventing good bonding with surrounding bone tissue when it is implanted in vivo. Surface modification and composite preparation are two main strategies to improve the bioactivity of PEEK. For surface modification, including surface chemical treatment, physical treatment, and surface coating, the stability of the modified surface will be the key issue requiring further investigation. For the preparation of bioactive PEEK composites, the main challenge is to keep the excellent mechanical properties of PEEK when impregnating bioactive materials.
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Conflicts of interest
There are no conflicts of interest.
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