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Review

Current Strategies to Improve the Bioactivity of PEEK

by
Rui Ma
and
Tingting Tang
*
Shanghai Key Laboratory of Orthopedic Implants, Department of Orthopedic Surgery, Shanghai Ninth People's Hospital, Shanghai Jiao Tong University School of Medicine, Shanghai 200011, China
*
Author to whom correspondence should be addressed.
Int. J. Mol. Sci. 2014, 15(4), 5426-5445; https://doi.org/10.3390/ijms15045426
Submission received: 3 January 2014 / Revised: 14 March 2014 / Accepted: 24 March 2014 / Published: 28 March 2014
(This article belongs to the Special Issue Biologic Coatings for Orthopaedic Implant)
Browse Figure
Versions Notes

Abstract

:
The synthetic thermoplastic polymer polyetheretherketone (PEEK) is becoming a popular component of clinical orthopedic and spinal applications, but its practical use suffers from several limitations. Although PEEK is biocompatible, chemically stable, radiolucent and has an elastic modulus similar to that of normal human bone, it is biologically inert, preventing good integration with adjacent bone tissues upon implantation. Recent efforts have focused on increasing the bioactivity of PEEK to improve the bone-implant interface. Two main strategies have been used to overcome the inert character of PEEK. One approach is surface modification to activate PEEK through surface treatment alone or in combination with a surface coating. Another strategy is to prepare bioactive PEEK composites by impregnating bioactive materials into PEEK substrate. Researchers believe that modified bioactive PEEK will have a wide range of orthopedic applications.

1. Introduction

Aging related aggravation and increases in accidental injuries have resulted in a sharp increase in the incidence of many diseases related to the bone and joint system, including fracture, vertebral degeneration, arthritis, bone tumors and tuberculosis. Orthopedic surgery using implants is now the main method to restore the structure and function of damaged bones and joints. Orthopedic implant materials commonly used in the clinic mainly include metals, ceramics, polymers and composites.
Metallic implants (e.g., gold, tantalum (Ta), stainless steel, shape memory alloy (NiTi), titanium (Ti) alloy, cobalt chromium (Co-Cr) alloy), have been widely used in the clinic either as permanent prostheses (such as the hip prosthesis, dental implants, etc.), or as temporary implants (such as plates, pins, screws and rods for the fixation of bone fractures). Metals can provide favorable mechanical strength, excellent friction-resistance and non-toxic properties [1–3]; however, some notable disadvantages have hindered their more widely medical applications [4–8]. Their high strength and elastic modulus that do not match those of normal human bone tissues can cause a stress shielding effect on the peri-implant bones, which will led to adsorption of adjacent bone tissues and cause prosthetic loosening. The radiopacity of metals causes artifacts in computed tomography (CT) images and limits the ability to examine the patient with magnetic resonance imaging (MRI). The long-term presence of metals in vivo can trigger allergic tissue reactions and initiate osteolysis.
Ceramics, including metallic oxides (e.g., Al2O3, MgO), calcium phosphate (e.g., hydroxyapatite (HA), tricalcium phosphate (TCP), octacalcium phosphate (OCP)) and glass ceramics (e.g., bioglass, ceravital), have received a great deal of attention from material scientists. Among these ceramics, metallic oxides are inert ceramics, and calcium phosphate and glass ceramics are bioactive ceramics that are commonly used at present. Bioactive ceramics exhibit favourable non-toxicity and corrosion-resistance, good biocompatibility and bioactivity. However, the mechanical properties of these materials, including their low fracture toughness and ductility, high elastic modulus and brittleness, cannot meet the demands of the load-bearing applications [9].
Polymers, such as ultra-high molecular weight polyethylene (UHMWPE), polytetrafluoroethylene (PTFE), polymethyl methacrylate (PMMA), polylactide (PLA), polyglycolide (PGA) and polyhydroxybutyrate (PHB), are widely used in various biomedical applications. However, only a limited number of polymers have been used for bone replacement purposes because they tend to be too flexible and too weak to meet the mechanical demands as orthopedic implants [10–12]. Besides, they may absorb liquids and swell, leach undesirable products and may be affected by sterilization process [10].
Polyetheretherketone (PEEK) is a semi-crystalline linear polycyclic aromatic thermoplastic that was first developed by a group of English scientists in 1978 [13]. In the 1980s, PEEK was commercialized for industrial applications, such as aircraft and turbine blades [14]. By the late 1990s, PEEK became an important high-performance thermoplastic candidate for replacing metal implant components, especially in orthopedic and traumatic applications [15]. PEEK was commonly used in vertebral surgery as a material of the interbody fusion cage [16–18]. With the emergence of carbon fiber reinforced PEEK (CF/PEEK), this new composite material was exploited for fracture fixation and femoral prosthesis in artificial hip joints [19,20]. Over the past few years, PEEK and its composites have attracted a great deal of interest from material scientists and orthopedists.
PEEK, a member of the polyaryletherketone family, has an aromatic molecular backbone, with combinations of ketone and ether functional groups between the aryl rings [20]. This special chemical structure makes PEEK exhibit stable chemical and physical properties [13,15,20–26]: it is wear-resistant and stable at high temperatures [13]; it is resistant to attack by all substances apart from concentrated sulfuric acid [15,20]; it remains stable in sterilization processes [21]. Besides, PEEK exhibits good biocompatibility in vitro and in vivo, causing neither toxic or mutagenic effects nor clinically significant inflammation [22–25]. More importantly, the mechanical properties of PEEK are close to that of human cortical bone [26]. For example, the elastic modulus of PEEK is approximately 8.3 GPa, which is close to that of human cortical bone (17.7 GPa) and much lower than that of Ti alloy (116 GPa) and Co–Cr alloy (210 GPa) [12]. However, PEEK is biologically inert [26,27], 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. Currently, two major strategies have been used to improve the bioactivity of PEEK, including surface modification and composite preparation, which will be reviewed in our present article.

2. Surface Modification

Although 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 (O2) plasma, ammonia (NH4) plasma, nitrogen and oxygen (N2/O2) plasma, methane and oxygen (CH4/O2) plasma, oxygen and argon (O2/Ar) plasma, ammonia/argon (NH4/Ar) plasma, and hydrogen/argon (H2/Ar) plasma), and accelerated neutral atom beam (ANAB) (Figure 1A). The chemical treatments were rare. Only wet chemistry modification or sulfonation treatment can chemically modify the surface of PEEK (Figure 1B). Besides, some materials can be coated onto PEEK to impose bioactive effects using various methods, including cold spray technique, radio-frequency (RF) magnetron sputtering, spin coating techniques, aerosol deposition (AD), ionic plasma deposition (IPD), plasma immersion ion implantation and deposition (PIII&D), electron beam deposition, vacuum plasma spraying (VPS), physical vapor deposition (PVD), and arc ion plating (AIP) (Figure 1C). Surface treatment alone or in combination with surface coating can greatly improve the bioactivity of PEEK.

2.1. Surface Treatment

2.1.1. Physical Treatment

Plasmas are ionized gases that can be produced in a closed reactor system containing a low pressure gas mixture by excitation with electro-magnetic waves [27]. 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 [27,28]. The method of plasma modification has been used to modify PEEK material for a long time. Briem et al. [27] treated PEEK surface with two plasma process (a microwave plasma in NH4/Ar and a downstream microwave plasma in H2/Ar), and investigated the proliferation and differentiation of primary fibroblasts and osteoblasts on plasma-treated PEEK. They found that the osteogenic activity of cells on treated PEEK was comparable to that of tissue culture polystyrene (TCPS), and a reproducible stimulation and suppression of cell proliferation could be achieved by the methods of plasma modification. Ha et al. [29] treated PEEK 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. After 24 days of immersion in a calcium and phosphate-saturated solution, a carbonate-containing calcium phosphate layer with a thickness of up to 50 μm was formed on the surface of plasma-treated PEEK. Compared with the untreated PEEK, the cell viability on the plasma-treated PEEK coated with calcium phosphate was significantly increased. Awaja et al. [30,31] treated PEEK with RF plasma with a mixture of CH4/O2 gases to modify the surface of PEEK. They found that the treatment with CH4/O2 gases resulted in a significantly higher bond strength than untreated samples [30]. Using a plasma immersion ion implantation and deposition (PIII&D) technique with a CH4/O2 gas mixture, they detected the deposition of oxygen-rich nanofilms on PEEK with a high surface energy, which greatly improved cell adhesion [31]. They also found there are strong correlations 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 [31]. Brydone et al. [32] fabricated novel nanopatterned PEEK rods, etched these PEEK rods with O2 plasma to improve their bioactivity, and then implanted them into a femoral defect rabbit model. Animal testing results proved that this nanopatterned PEEK etched by oxygen plasma exhibited potential osteoinductivity in vivo. Waser-Althaus [33] applied the O2/Ar or NH4 plasma to treat the PEEK surface. They demonstrated an increased adhesion, proliferation, and osteogenic differentiation of adipose tissue-derived mesenchymal stem cells (adMSC) on plasma-treated PEEK, and a doubled mineralization degree on 50 W plasma-treated PEEK relative to the 10 W was observed, indicating the osteogenic differentiation was dependent on the plasma power.
A novel ANAB technique employing intense directed beams of neutral gas atoms (comprised of 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 [34,35]. Khoury et al. [34] employed the ANAB technique 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 (hFOB) compared with untreated PEEK as evidenced by cell proliferation assays and microscopy. Using a rat calvarial defect model, they revealed that ANAB-treated PEEK enhanced osteointegration, with bone tissue formation only evident on the ANAB-treated PEEK.

2.1.2. Chemical Treatment

Wet surface chemistry has been used to chemically modify PEEK to create a series of surface-functionalized PEEKs. 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 [36,37]. These chemical modifications promoted higher levels of fibronectin covalently fixed and/or adsorbed on various treated PEEK compared with untreated PEEK. The carboxylated polymer and aminated polymer promoted the adhesion and growth of CaCo2 cells (cell line derived from a human colon adenocarcinoma) in the presence of serum.
By sulfonation and subsequent water immersion, a 3D porous and nano-structured network with bio-functional groups is produced on PEEK to prepare two kinds of sulfonation-treated PEEK (SPEEK) samples (SPEEK-W (water immersion and rinsing after sulfonation) and SPEEK-WA (SPEEK-W with further acetone rinsing)), and the in vitro cellular behavior, in vivo osseointegration, and apatite-forming ability of the sulfonation-treated PEEK were systematically investigated [38]. The results showed that SPEEK-WA induced pre-osteoblast 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.

2.2. Surface Coating

Various materials have been deposited on the surface of PEEK, including hydroxyapatite (HA), titanium (Ti), gold, titanium dioxide (TiO2), diamond-like carbon (DLC), and tert-butoxides. The bioactivity of PEEK can be greatly enhanced by these surface coatings.
The most commonly-used bioactive material as coating of PEEK is HA. HA (chemical formula Ca10(PO4)6(OH)2) is the most widely used calcium phosphate-based bioceramic, which is the closest pure synthetic equivalent to human bone mineral [39]. Numerous studies have consistently shown that HA typically exhibits excellent biocompatibility, bioactivity, and osteoconduction in vivo [40–42]. Lee et al. [43] used a cold spray technique to fabricate HA-coated PEEK and evaluated its bioactivity in vitro and in vivo. In vitro tests indicated that the adhesion, viability and osteoblast differentiation of human bone marrow mesenchymal stem cells (hBMSCs) were improved on HA-coated PEEK compared with the uncoated one. For in vivo tests, these authors implanted HA-coated PEEK cylinders into a rabbit ilium model with uncoated PEEK as control and demonstrated that HA-coated PEEK promoted implant osteointegration with the surrounding bone using micro-computed tomography (micro-CT) and histomorphometric analysis. Barkarmo et al. [44] fabricated nanocrystalline HA-coated PEEK with a spin coating technique and inserted the cylinder implants into the femurs of rabbits with uncoated cylinders as controls. The nano-HA coated PEEK cylinders exhibited a higher mean bone-implant contact than uncoated cylinders, indicating that nano-HA coated PEEK promoted osteointegration. Highly dense and well-adhered HA coating could be developed on PEEK using aerosol deposition (AD) without thermal degradation of PEEK [45]. In vitro and in vivo bioactivity of PEEK, in terms of cell adhesion, morphology, proliferation, differentiation, and bone-to-implant contact ratio, were remarkably enhanced by the HA coating. In another study [46], HA coatings were deposited onto PEEK surfaces using RF magnetron sputtering. Before HA deposition, a 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. [47] prepared a PEEK/Mg composite with a Mg content of 30 vol % by compression molding process, then they treated the composite 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. The HA-coated PEEK/Mg composite was proved to exhibit enhanced in vitro bio-corrosion resistance and bioactivity (with more attached MC3T3-E1 cells exhibiting active cytoskeletal extension and more significant proliferation) compared with pure PEEK and the uncoated PEEK/Mg composite.
Titanium (Ti) is the most widely used implant material for load-bearing dental and orthopedic applications because of its excellent mechanical and biological properties [48]. Therefore, Ti is an appropriate candidate as the coating material for PEEK. Chang Yao et al. [49] studied osteoblast adhesion on PEEK coated with either Ti or gold using the ionic plasma deposition (IPD) process, which created a nanostructured surface (with features below 100 nm). Compared with the commonly used Ti and uncoated PEEK, PEEK coated with either Ti or gold significantly increased osteoblast adhesion and spreading. They attributed the increased cell adhesion to the nanometer surface roughness and the changed surface wettability. Cook et al. [50] applied plasma vapor deposition (PVD) 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 specimens at both 4 and 8 weeks, and the uncoated implants had significantly higher shear strength values than the coated implants at 4 weeks. Han et al. [51] coated Ti onto PEEK using electron beam (e-beam) deposition process, which produced a dense, uniform film on the substrate at a low temperature. The in vitro cellular responses of the samples were assessed in terms of cell attachment, proliferation, and osteoblastic differentiation, and the in vivo bone conductivity was examined by measuring the bone-to-implant contact (BIC) ratio using a rabbit tibial defect model. The level of proliferation and differentiation of the MC3T3-E1 cells was more than doubled after Ti was coated onto the PEEK surface, and the in vivo animal tests showed that the Ti-coating PEEK implants had a much higher BIC ratio than the pure PEEK implants. In one study [52], CF/PEEK was coated with Ti by vacuum plasma spraying (VPS) process and chemically treated in sodium hydroxide (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), whereas no calcium phosphate precipitation occurred on the untreated PEEK surfaces. In another study [53], CF/PEEK screws were coated with Ti using two different techniques, VPS and PVD. The coated CF/PEEK was implanted into the tibia of sheep with uncoated CF/PEEK as the control. 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.
Titanium dioxide (TiO2) material has been demonstrated with good biocompatibility, bioactivity, hydrophilicity, and corrosion resistance [54,55]. Anatase phase (A-TiO2) and/or rutile phase (R-TiO2) can be deposited onto PEEK substrate by an arc ion plating (AIP) technique following three steps (argon ion bombardment, bottom titanium layer deposition and TiO2 coating deposition) at a low deposition temperature without damaging PEEK substrate, while providing satisfactory film adhesion [56–58]. From results of cell adhesion, proliferation and osteo-differentiation abilities, the authors 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 [56,57]. Surface roughness and hydrophilicity of the AIP-TiO2 films were found to be responsible for significant osteoblast cell growth and the presence of negatively charged hydroxyl groups on R-TiO2 contributed to its better cytocompatibility than A-TiO2. In SBF immersion test, TiO2-coated PEEK presented enhanced HA growth with the crystallinity and film thickness of the grown HA layer proportional to immersion time, and R-TiO2/PEEK exhibited superior ability to induce HA formation due to the pre-absorbed negatively charged groups on R-TiO2 coating surface [58]. Han et al. [59] created a uniform nanoporous TiO2 layer with a pore diameter of ~70 nm by anodizing a Ti film, 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. The in vitro cell tests and in vivo animal tests demonstrated that the nanoporous TiO2 surface immobilized with BMP-2 could significantly enhance the cell attachment, proliferation, differentiation of MC3T3-E1 cells, and the osseoconductivity of PEEK implants. The BMP-immobilized PEEK coated with nanoporous TiO2 showed much higher BIC ratio (60%) than the bare PEEK (30%), PEEK coated with nanoporous TiO2 (50%) and even BMP-immobilized PEEK without coating (32%).
Except for the commonly-used coating materials (HA, Ti and TiO2), some other infrequent materials were used as coating materials on PEEK. Chu et al. [26] successfully coated PEEK with diamond-like carbon (DLC) by plasma immersion ion implantation and deposition technique. A cell viability assay, scanning electron microscopy (SEM) and real-time polymerase chain reaction (PCR) analysis indicated that osteoblast attachment, proliferation and differentiation were better on DLC-coated PEEK than on bare PEEK. In another study [60], vapor of zirconium or titanium 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 (EPD) method, PEEK and PEEK/bioglass particles were coated onto shape memory alloy (nickel and titanium, NiTi) wires [61] or on two phase (α + β) Ti–6Al–7Nb titanium alloy substrates [62] 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, hydroxyapatite layers formed on the surface of the coated specimens after one week.

3. PEEK Composites

Some ceramics, such as HA, TCP, calcium silicate (CS), bioglass, glass-ceramic A-W, are referred to as bioactive materials due to their ability to spontaneously bond to living bone, and these materials are already used as bone substitutes with important clinical applications [9]. Unfortunately, these bioactive materials exhibit a lower fracture toughness and higher elastic modulus compared with human cortical bone [9,40]. Although PEEK can provide favourable mechanical properties, its native inertness prevents good bonding with surrounding bone tissues. Thus, impregnating bioactive materials into PEEK has become one attractive strategy to improve the bioactivity of PEEK while maintaining its mechanical properties. The PEEK composites were classified into two kinds by the size of the impregnating bioactive materials: the conventional PEEK composites and the nano-sized (<100 nm) PEEK composites (Figure 1D). The reported PEEK composites that are related to the bioactivity of the composites are shown in Table 1.

3.1. Conventional PEEK Composites

With good biocompatibility, bioactivity and osteoconduction, HA is not only used as common coating material for PEEK, but also as common filler material to prepare PEEK composite. Several studies has investigated the mechanical properies of the HA incorporated PEEK composite (HA/PEEK). Khor’s research group [63,64] fabricated a 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, but decreasing the tensile strength and strain to fracture. These authors also found that PEEK with 30 vol % HA exhibited an elastic modulus within the range of human cortical bone. All of the specimens (5, 10, 20, 30, and 40 vol % HA) survived cyclic loading at 50% ultimate tensile strength and exhibited a high estimated fatigue strength at 1 million cycles. Similar results can be found in the report by Converse and co-workers [65]. However, Khor and co-workers [63,64,66,67] 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. Fatigue damage of μm-HA/PEEK composites began with filler matrix interface failure, followed by initiation and propagation of matrix cracks from the filler-matrix debonding site, and subsequent development of longer matrix cracks from shorter cracks caused final failure [64]. The biocompatibility and bioactivity of HA/PEEK composites has been studied by several researchers. Zhang et al. [68] manufactured HA/PEEK composites via the selective laser sintering (SLS) technique and evaluated cell attachment, morphology, proliferation and differentiation using primary human osteoblast cells. They found that the SLS-treated HA/PEEK supported osteoblast growth and that composites with higher HA contents exhibited enhanced cell proliferation and osteogenic differentiation (increased ALP activity, and produced more osteocalcin), compared to thermanox (TMX) and polyvinyl chloride (PVC). Ma et al. [69,70] successfully prepared an HA/PEEK composite via an in situ synthetic process. The composite exhibited an excellent improvement in mechanical properties and bonding between HA and PEEK. Subsequently, to examine the possible adverse effects of the residual organic chemicals from the in situ synthesis process, the biocompatibility of the material was investigated. 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. Bioactive materials can form a bone-like apatite layer on their surfaces in vivo and bond to bone through this apatite layer [9]. Thus, the bone-bonding ability of a material is often evaluated by examining the ability of apatite to form on its surface in a simulated body fluid (SBF) with ion concentrations nearly equal to those of human blood plasma [71]. Yu et al. [72] prepared HA/PEEK composite by mixing, compaction, and pressureless sintering process, and evaluated the bioactivities of HA/PEEK composites with 10, 20, 30 and 40 vol % HA by immersing the composite disks in SBF for 4 weeks. Pure PEEK exhibited no significant changes on its surface after 28 days of immersion, and the surface of composite with 40 vol % HA was covered by a layer of bone-like apatite just after 3 days of immersion, while 10 vol % HA was covered after 28 days. The growth rate increased with HA volume fraction, suggesting that the bioactivity of the HA/PEEK composite increased with increasing HA content in the composite. Invibio has released an HA-filled PEEK compound with microscale HA particles called “PEEK-OPTIMA HA enhanced polymer” for use in implants [73]. PEEK-OPTIMA HA enhanced biomaterial provides excellent mechanical properties and performance, proven biocompatibility, a modulus similar to cortical bone, reducing stress shielding and a high degree of radiolucency that allows for clear fusion assessment. Within four weeks of implantation Invibio “PEEK-OPTIMA HA enhanced polymer” demonstrated enhanced bone apposition compared to PEEK-OPTIMA® Natural, in a pre-clinical in vivo study using a sheep model. Within 12 weeks of implantation the bone apposition levels are maintained with the new grade.
To favour bone-in-growth to the composites and make strongly bonded implant/bone interface, some measures were adopted to prepare porous HA/PEEK composite. Abu Bakar et al. [66] prepared 20 vol % HA/PEEK with porosity of 60% and pore size ranging from 300 to 600 mm by leaching of particulate technique employing a suitable pore-forming agent, and implanted these materials into the distal metaphyseal femur in pigs to evaluate the biological responses and tissue in-growth of the material. Histological studies revealed the presence of fibro-vascular tissue within the pores at 6 weeks and mature bone formation at 16 weeks after implantation. Using an SLS rapid prototyping system, porous HA/PEEK composite scaffolds starting with 10 wt % HA to 40 wt % HA have been produced by Tan et al. [74,75]. Both the microporosity and macroporosity of the scaffolds showed that highly porous HA/PEEK scaffolds could be obtained. The immersion of HA/PEEK scaffolds in SBF demonstrated the bioactivity of the specimens by the precipitation of apatite-layers. Cell culture of fibroblast cell lines on HA/PEEK scaffolds demonstrated positive cell adhesion and growth. However, the attainable level of porosity was limited to 70%–74% which was dependent on both the reinforcement level and laser power. To overcome this disadvantage, Converse et al. [76] fabricated HA whisker-reinforced PEEK with high levels of porosity (75%–90%) and HA whisker reinforcement (0–40 vol %) using a powder processing, followed by compression molding and particle leaching, but neither in vitro nor in vivo tests related to this scaffold have been reported. As the mechanical properties may be compromised with increasing the HA content, determining the appropriate HA content to attain both satisfying mechanical properties and bioactivity is crucial in the fabrication of HA/PEEK composites. More studies should be focused on this point.
Apart from HA, other bioactive materials were also used to make bioactive PEEK composites, including strontium-containing hydroxyapatite (Sr-HA), calcium silicate, glass fibers, bioglass, and β-tricalcium- phosphate (β-TCP). Wong et al. [77] developed Sr-HA/PEEK composites with Sr-HA content ranging from 15–30 vol % by a compression molding technique. The addition of Sr-HA outperformed HA in increasing the bioactivity of the composite based on a qualitative comparison of apatite formation in SBF and the quantitative measurement of MG-63 cell-mediated mineralization via alizarin red staining in vitro. However, no difference was observed in the cell proliferation and ALP activity between Sr-HA/PEEK and HA/PEEK composites at each time point. Kim et al. [78] fabricated CS-reinforced PEEK composite (CS/PEEK) with 0–50 vol % CS and soaked specimens in SBF with pure PEEK as the control. Except for pure PEEK, all of the CS-containing composites promoted apatite formation on their surfaces, exhibiting the potential to bond to living bone. The time required for the induction of apatite formation on the composite surfaces decreased with increasing CS content. The mechanical properties of the samples after soaking in SBF did not significantly decrease compared with samples that were not exposed to SBF. 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, and the cell proliferation, ALP activity and osteocalcin production on GPEEK using MG-63 cells were analysed [79,80]. 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. Wilmowsky’s research group [81] 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. They also compared cell growth among pure PEEK, PEEK/1 wt % carbon, PEEK/1 wt % carbon/10 wt % β-TCP and PEEK/1 wt % carbon/ 10 wt % bioglass fabricated by laser sintering [82]. Cell proliferation and cell viability tests using hFOB cells showed that none of these composites induced cytotoxicity. 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. Petrovic et al. [83] studied the effect of PEEK containing 5, 10, 20 and 40 wt % β-TCP processed by injection molding on normal human osteoblast (NHOst) cells. The results showed that the proliferation rates of NHOst cells growing on β-TCP/PEEK were lower than those on pure PEEK, but β-TCP/PEEK showed no concentration-dependent decrease in cell proliferation compared with the pure PEEK. Von Wilmowsky also observed a lower cell viability and proliferation on β-TCP/PEEK compared with pure PEEK [82]. These authors suggested that a shift in the pH of the cell culture medium resulting from the degradation of the β-TCP compound may contribute to the inhibitory effect of β-TCP/PEEK on cell proliferation. Therefore, more detailed investigations are required to understand the effects of β-TCP/PEEK on osteoblasts.

3.2. Nano-Sized PEEK Composites

Conventional HA/PEEK composite may not bear long-term critical loading due to debonding between HA filler and PEEK matrix; which has been illustrated in detail in part 3.1. Nanotechnology was applied by material scientists to overcome this problem. Wang et al. [84,85] prepared HA/PEEK nanocomposites by a compounding and injection molding process. They found that this novel HA/PEEK nanocomposite exhibited satisfactory mechanical properties and a high surface HA content. More importantly; no debonding occurred between the well-dispersed HA nanoparticles and the PEEK matrix. However; the agglomeration of HA nanoparticles became severe as the HA content increased over 10 vol %. This process may be related to the high viscosity of the PEEK matrix at high temperatures during the manufacturing process [84]. Other studies also found a general tendency for nanoparticles to aggregate during the fabrication of nanoparticle-reinforced thermoplastics [84–87]. It was difficult to uniformly disperse the nanoscale powders in a viscous polymer matrix using the conventional methods [40]. To overcome the agglomeration of HA nanoparticles during manufacturing; these authors adopted an in situ synthetic process to prepare HA/PEEK nanocomposites [88]. In this in situ synthetic process, HA particles were first mixed into PEEK oligomers with short chains and a low viscosity and good wetting and contact were achieved between HA and PEEK. Then, continuing polymerization increased the molecular weight of the PEEK oligomers on the HA surface and the oligomers were firmly wrapped on the HA surface. 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.
In native bone tissues, bone cells are exposed to substrates and structures with nanoscale features, such as extracellular matrix (ECM) proteins, minerals and pores in membranes and tissues [89]. By mimicking this nanotopography through the fabrication of nano-sized materials, researchers hope to enhance bone cell growth and tissue integration [90]. 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 [91,92]. Webster and co-workers [93–97] have conducted a great deal of research on the bioactivity and biocompatibility of nanomaterials. They found that nanostructured materials may promote osteoblast adhesion, proliferation, differentiation, and stimulate new bone growth compared to conventional materials.
Therefore, developing PEEK composites reinforced with nano-sized bioactive materials is a promising strategy for obtaining both mechanical and biological benefits. Studies on the bioactivity of nano-sized HA and nano-titanium dioxide (n-TiO2) reinforced PEEK have been reported. Li et al. [98] fabricated HA/PEEK nanocomposites containing 15.1, 21.6, 29.2 and 38.2 vol % nano-sized HA (nHA) by powder processing and sintering. The tensile strength and fracture strain of PEEK nanocomposites filled with 21.6 and 29.2 vol % nHA match closely with those of human cortical bone. The results of SBF immersion, cell adhesion and proliferation in vitro suggested that 29.2 vol % nHA/PEEK nanocomposite possessed better bioactivity and biocompatibility than the other specimens. Wu et al. [99] fabricated n-TiO2 reinforced PEEK composites (n-TiO2/PEEK) and studied the bioactivity of these nanocomposites in vitro and in vivo. The effect of surface morphology or roughness was also considered. In vitro tests 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.

4. Conclusions and Outlooks

PEEK is biocompatible, chemically and physically stable, radiolucent and exhibits a similar elastic modulus to normal human bone, making it an attractive orthopedic 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. The development of PEEK composites containing nano-sized bioactive materials may provide an effective way to obtain both mechanical and biological benefits.

Acknowledgments

This work was supported by a grant from the National Natural Science Foundation of China (No. 31271015, 81271705, and 51173041) and the grants from the Science and Technology Commission of Shanghai Municipality (13JC1403900, 13DZ2294000).

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Adell, R.; Lekholm, U.; Rockler, B.; Branemark, P.I. A 15-year study of osseointegrated implants in the treatment of the edentulous jaw. Int. J. Oral Surg 1981, 10, 387–416. [Google Scholar]
  2. Aziz-Kerrzo, M.; Conroy, K.G.; Fenelon, A.M.; Farrell, S.T.; Breslin, C.B. Electrochemical studies on the stability and corrosion resistance of titanium-based implant materials. Biomaterials 2001, 22, 1531–1539. [Google Scholar]
  3. Ribeiro, D.A.; Matsumoto, M.A.; Padovan, L.E.; Marques, M.E.; Salvadori, D.M. Genotoxicity of corrosion eluates obtained from endosseous implants. Implant. Dent 2007, 16, 101–109. [Google Scholar]
  4. Huiskes, R. Stress shieding and bone resorption in THA: Clinical vs. computer-simulation studies. Acta Orthop. Belg 1993, 59, 118–129. [Google Scholar]
  5. Huiskes, R.; Weinans, H.; Rietberge, V.B. The relationship between stress shielding and bone resorption around total hip stems and the effects of flexible materials. Clin. Orthop. Relat. Res 1992, 274, 124–134. [Google Scholar]
  6. Kitamura, E.; Stegaroiu, R.; Nomura, S. Biomechanical aspects of marginal bone resorption around osseointegrated implants: Considerations based on a three-dimensional finite element analysis. Clin. Oral Implant. Res 2004, 15, 401–412. [Google Scholar]
  7. Isidor, F. Influence of forces on peri-implant bone. Clin. Oral Implant. Res 2006, 17, 8–17. [Google Scholar]
  8. Thomas, P.; Maier, S.; Summer, B. Allergie reactions to metal implants. Materialwissenschaft Werkstofftechnik 2004, 35, 997–1000. [Google Scholar]
  9. Kokubo, T.; Kim, H.-M.; Kawashita, M. Novel bioactive materials with different mechanical properties. Biomaterials 2003, 24, 2161–2175. [Google Scholar]
  10. Ramakrishna, S.; Mayer, J.; Wintermantel, E.; Leong, K.W. Biomedical applications of polyer-composite materials: A review. Compos. Sci. Technol 2001, 61, 1189–1224. [Google Scholar]
  11. Boccaccini, A.R.; Blaker, J.J. Bioactive composite materials for tissue engineering scaffolds. Expert Rev. Med. Devices 2005, 2, 303–317. [Google Scholar]
  12. Black, J.; Hastings, G.W. Handbook of Biomaterials Properties; Chapman and Hall: London, UK, 1998; pp. 270–283. [Google Scholar]
  13. Eschbach, L. Nonresorbable polymers in bone surgery. Injury 2000, 31, 22–27. [Google Scholar]
  14. Rigby, R.B. Engineering Thermoplastics Properties and Applications; Marcel Dekker: New York, NY, USA, 1985; p. 15. [Google Scholar]
  15. Kurtz, S.M.; Devine, J.N. PEEK biomaterials in trauma, orthopedic, and spinal implants. Biomaterials 2007, 28, 4845–4869. [Google Scholar]
  16. Jiya, T.; Smit, T.; Deddens, J. Posterior lumbar interbody fusion using nonresorbable polyetheretherketone vs. resorbable Poly-l-Lactide-Co-d,l-lactide fusion devices. Spine 2009, 34, 233–237. [Google Scholar]
  17. Park, H.W.; Lee, J.K.; Moon, S.J.; Seo, S.K.; Lee, J.H.; Kim, S.H. The efficacy of the synthetic interbody cage and grafton for anterior cervical fusion. Spine 2009, 34, E591–E595. [Google Scholar]
  18. Toth, J.M.; Wang, M.; Estes, B.T.; Scifert, J.L.; Seim, H.B., 3rd; Turner, A.S. Polyetheretherketone as a biomaterial for spinal applications. Biomaterials 2006, 27, 324–334. [Google Scholar]
  19. Jarman-Smith, M. Evolving uses for implantable PEEK and PEEK based compounds. Med. Device Technol 2008, 19, 12–15. [Google Scholar]
  20. Williams, D. Polyetheretherketone for long-term implantable devices. Med. Device Technol 2008, 19, 10–11. [Google Scholar]
  21. Godara, A.; Raabe, D.; Green, S. The influence of sterilization processes on the micromechanical properties of carbon fiber-reinforced PEEK composites for bone implant applications. Acta Biomater 2007, 3, 209–220. [Google Scholar]
  22. Wenz, L.M.; Merritt, K.; Brown, S.A.; Moet, A.; Steffee, A.D. In vitro biocompatibility of polyetheretherketone and polysulfone composites. J. Biomed. Mater. Res 1990, 24, 207–215. [Google Scholar]
  23. Rivard, C.-H.; Rhalmi, S.; Coillard, C. In vivo biocompatibility testing of peek polymer for a spinal implant system: A study in rabbits. J. Biomed. Mater. Res 2002, 62, 488–498. [Google Scholar]
  24. Nieminen, T.; Kallela, I.; Wuolijoki, E.; Kainulainen, H.; Hiidenheimo, I.; Rantala, I. Amorphous and crystalline polyetheretherketone: Mechanical properties and tissue reactions during a 3-year follow-up. J. Biomed. Mater. Res. A 2008, 84, 377–383. [Google Scholar]
  25. Katzer, A.; Marquardt, H.; Westendorf, J.; Wening, J.V.; Foerster, G.V. Polyetheretherketonecytotoxicity and mutagenicityin vitro. Biomaterials 2002, 23, 1749–1759. [Google Scholar]
  26. Wang, H.; Xu, M.; Zhang, W.; Kwok, D.T.; Jiang, J.; Wu, Z.; Chu, P.K. Mechanical and biological characteristics of diamond-like carbon coated poly aryl-ether-ether-ketone. Biomaterials 2010, 31, 8181–8187. [Google Scholar]
  27. Briem, D.; Strametz, S.; Schröder, K.; Meenen, N.M.; Lehmann, W.; Linhart, W.; Ohl, A.; Rueger, J.M. Response of primary fibroblasts and osteoblasts to plasma treated polyetheretherketone (PEEK) surfaces. J. Mater. Sci. Mater. Med 2005, 16, 671–677. [Google Scholar]
  28. Liston, E.M. Plasma treatment for improved bonding: A review. J. Adhes 1989, 30, 199–218. [Google Scholar]
  29. Ha, S.-W.; Kirch, M.; Birchler, F.; Eckert, K.-L.; Mayer, J.; Wintermantel, E.; Sittig, C.; Pfund-Klingenfuss, I.; Textor, M.; Spencer, N.D.; et al. Surface activation of polyetheretherketone (PEEK) and formation of calcium phosphate coatings by precipitation. Mater. Sci. Mater. Med 1997, 8, 683–690. [Google Scholar]
  30. Awaja, F.; Zhang, S.; James, N.; McKenzie, D.R. Enhanced autohesive bonding of polyetheretherketone (PEEK) for biomedical applications using a methane/oxygen plasma treatment. Plasma Process. Polym 2010, 7, 1010–1021. [Google Scholar]
  31. Awaja, F.; Bax, D.V.; Zhang, S.; James, N.; McKenzie, D.R. Cell adhesion to PEEK treated by plasma immersion ion implantation and deposition for active medical implants. Plasma Proc. Polym 2012, 9, 355–362. [Google Scholar]
  32. Brydone, A.S.; Morrison, D.S.S.; Stormonth-Darling, J.; Meek, R.D.M.; Tanner, K.E.; Gadegaard, N. Design and fabrication of a 3D nanopatterned PEEK implant for cortical bone regeneration in a rabbit model. Eur. Cells Mater 2012, 24, 39. [Google Scholar]
  33. Waser-Althaus, J.; Salamon, A.; Waser, M.; Padeste, C.; Kreutzer, M.; Pieles, U.; Müller, B.; Peters, K. Differentiation of human mesenchymal stem cells on plasma-treated polyetheretherketone. J. Mater. Sci. Mater. Med 2014, 25, 515–525. [Google Scholar]
  34. Khoury, J.; Kirkpatrick, S.R.; Maxwell, M.; Cherian, R.E.; Kirkpatrick, A.; Svrluga, R.C. Neutral atom beam technique enhances bioactivity of PEEK. Nucl. Instrum. Meth. B 2013, 307, 630–634. [Google Scholar]
  35. Kirkpatrick, A.; Kirkpatrick, S.; Walsh, M.; Chau, S.; Mack, M.; Harrison, S.; Svrluga, R.; Khoury, J. Investigation of accelerated neutral atom beams created from gas cluster ion beams. Nucl. Instrum. Meth. B 2013, 307, 281–289. [Google Scholar]
  36. Noiset, O.; Schneider, Y.J.; Marchand-Brynaert, J. Fibronectin adsorption or/and covalent grafting on chemically modified PEEK film surfaces. J. Biomater. Sci. Polym. Ed 1999, 10, 657–677. [Google Scholar]
  37. Noiset, O.; Schneider, Y.J.; Marchand-Brynaert, J. Adhesion and growth of CaCo2 cells on surface-modified PEEK substrata. J. Biomater. Sci. Polym. Ed 2000, 11, 767–786. [Google Scholar]
  38. Zhao, Y.; Wong, H.M.; Wang, W.; Li, P.; Xu, Z.; Chong, E.Y.; Yan, C.H.; Yeung, K.W.; Chu, P.K. Cytocompatibility, osseointegration, and bioactivity of three-dimensional porous and nanostructured network on polyetheretherketone. Biomaterials 2013, 34, 9264–9277. [Google Scholar]
  39. Hench, L.L.; Wilson, J. An Introduction to Bioceramics; World Scientific Publishing Co: Singapore, Singapore, 1993; pp. 139–171. [Google Scholar]
  40. Roeder, R.K.; Converse, G.L.; Kane, R.J.; Yue, W. Hydroxyapatite-reinforced polymer biocomposites for synthetic bone substitutes. JOM 2008, 60, 38–45. [Google Scholar]
  41. Thamaraiselvi, T.V.; Rajeswari, S. Biological evaluation of bioceramic materials—A review. Trends Biomater. Artif. Organs 2004, 18, 9–17. [Google Scholar]
  42. Jarcho, M. Calcium phosphate ceramics as hard tissue prosthetics. Clin. Orthop. Relat. Res 1981, 157, 259–278. [Google Scholar]
  43. Lee, J.H.; Jang, H.L.; Lee, K.M.; Baek, H.R.; Jin, K.; Hong, K.S.; Noh, J.H.; Lee, H.K. In vitro and in vivo evaluation of the bioactivity of hydroxyapatite-coated polyetheretherketone biocomposites created by cold spray technology. Acta Biomater 2013, 9, 6177–6187. [Google Scholar]
  44. Barkarmo, S.; Wennerberg, A.; Hoffman, M.; Kjellin, P.; Breding, K.; Handa, P.; Stenport, V. Nano-hydroxyapatite-coated PEEK implants: A pilot study in rabbit bone. J. Biomed. Mater. Res. A 2012, 101A, 456–471. [Google Scholar]
  45. Hahn, B.-D.; Park, D.-S.; Choi, J.-J.; Ryu, J.; Yoon, W.-H.; Choi, J.-H.; Kim, J.-W.; Ahn, C.-W.; Kim, H.-E.; Yoon, B.-H.; et al. Osteoconductive hydroxyapatite coated PEEK for spinal fusion surgery. Appl. Surf. Sci 2013, 283, 6–11. [Google Scholar]
  46. Rabiei, A.; Sandukas, S. Processing and evaluation of bioactive coatings on polymeric implants. J. Biomed. Mater. Res. A 2013, 101A, 2621–2629. [Google Scholar]
  47. Jung, H.-D.; Sun Park, H.; Kang, M.-H.; Lee, S.-M.; Kim, H.-E.; Estrin, Y.; Koh, Y.-H. Polyetheretherketone/magnesium composite selectively coated with hydroxyapatite for enhanced In vitro bio-corrosion resistance and biocompatibility. Mater. Lett 2014, 116, 20–22. [Google Scholar]
  48. Noort, R.V. Titanium: The implant material of today. J. Mater. Sci 1987, 22, 3801–3811. [Google Scholar]
  49. Yao, C.; Storey, D.; Webster, T.J. Nanostructured metal coatings on polymers increase osteoblast attachment. Int. J. Nanomed 2007, 2, 487–492. [Google Scholar]
  50. Cook, S.D.; Rust-Dawicki, A.M. Preliminary evaluation of titanium-coated PEEK dental implants. J. Oral Implantol 1995, 21, 176–181. [Google Scholar]
  51. Han, C.M.; Lee, E.J.; Kim, H.E.; Koh, Y.H.; Kim, K.N.; Ha, Y.; Kuh, S.U. The electron beam deposition of titanium on polyetheretherketone (PEEK) and the resulting enhanced biological properties. Biomaterials 2010, 31, 3465–3470. [Google Scholar]
  52. Ha, S.-W.; Eckert, K.-L.; Wintermantel, E.; Gruner, H.; Guecheva, M.; Vonmont, H. NaOH treatment of vacuum-plasma-sprayed titanium on carbon fibre-reinforced poly(etheretherketone). J. Mater. Sci. Mater. Med 1997, 8, 881–886. [Google Scholar]
  53. Devine, D.M.; Hahn, J.; Richards, R.G.; Gruner, H.; Wieling, R.; Pearce, S.G. Coating of carbon fiber-reinforced polyetheretherketone implants with titanium to improve bone apposition. J. Biomed. Mater. Res. B 2013, 101, 591–598. [Google Scholar]
  54. Shan, C.X.; Hou, X.; Choy, K.-L. Corrosion resistance of TiO2 films grown on stainless steel by atomic layer deposition. Surf. Coat. Technol 2008, 202, 2399–2402. [Google Scholar]
  55. Harle, J.; Kim, H.W.; Mordan, N.; Knowles, J.C.; Salih, V. Initial responses of human osteoblasts to sol-gel modified titanium with hydroxyapatite and titania composition. Acta Biomater 2006, 2, 547–556. [Google Scholar]
  56. Tsou, H.-K.; Hsieh, P.-Y.; Chung, C.-J.; Tang, C.-H.; Shyr, T.-W.; He, J.-L. Low-temperature deposition of anatase TiO2 on medical grade polyetheretherketone to assist osseous integration. Surf. Coat. Technol 2009, 204, 1121–1125. [Google Scholar]
  57. Tsou, H.K.; Hsieh, P.Y.; Chi, M.H.; Chung, C.J.; He, J.L. Improved osteoblast compatibility of medical-grade polyetheretherketone using arc ionplated rutile/anatase titanium dioxide films for spinal implants. J. Biomed. Mater. Res. A 2012, 100, 2787–2792. [Google Scholar]
  58. Chi, M.-H.; Tsou, H.-K.; Chung, C.-J.; He, J.-L. Biomimetic hydroxyapatite grown on biomedical polymer coated with titanium dioxide interlayer to assist osteocompatible performance. Thin Solid Films 2013, 549, 98–102. [Google Scholar]
  59. Han, C.M.; Jang, T.S.; Kim, H.E.; Koh, Y.H. Creation of nanoporous TiO2 surface onto polyetheretherketone for effective immobilization and delivery of bone morphogenetic protein. J. Biomed. Mater. Res. A 2014, 102, 793–800. [Google Scholar]
  60. Dennes, T.J.; Schwartz, J. A nanoscale adhesion layer to promote cell attachment on PEEK. J. Am. Chem. Soc 2009, 131, 3456–3457. [Google Scholar]
  61. Boccaccini, A.R.; Peters, C.; Roether, J.A.; Eifler, D.; Misra, S.K.; Minay, E.J. Electrophoretic deposition of polyetheretherketone (PEEK) and PEEK/bioglass® coatings on NiTi shape memory alloy wires. J. Mater. Sci 2006, 41, 8152–8159. [Google Scholar]
  62. Moskalewicz, T.; Seuss, S.; Boccaccini, A.R. Microstructure and properties of composite polyetheretherketone/bioglass® coatings deposited on Ti-6Al-7Nb alloy for medical applications. Appl. Sur. Sci 2013, 273, 62–67. [Google Scholar]
  63. Bakar, M.S.A.; Cheang, P.; Khor, K.A. Tensile properties and microstructural analysis of spheroidized hydroxyapatite/poly(etheretherketone) biocomposites. Mater. Sci. Eng. A 2003, 345, 55–63. [Google Scholar]
  64. Tang, S.; Cheang, P.; AbuBakar, M.S.; Khor, K.A.; Liao, K. Tension–tension fatigue behavior of hydroxyapatite reinforced polyetheretherketone composites. Int. J. Fatigue 2004, 26, 49–57. [Google Scholar]
  65. Converse, G.L.; Yue, W.; Roeder, R.K. Processing and tensile properties of hydroxyapatite-whisker-reinforced polyetheretherketone. Biomaterials 2007, 28, 927–935. [Google Scholar]
  66. Abu Bakar, M.S.; Cheng, M.H.W.; Tang, S.M.; Yu, S.C.; Liao, K.; Tan, C.T.; Khor, K.A.; Cheang, P. Tensile properties, tension–tension fatigue and biological response of polyetheretherketone-hydroxyapatite composites for load-bearing orthopedic implants. Biomaterials 2003, 24, 2245–2250. [Google Scholar]
  67. Bakar, M.S.A.; Cheang, P.; Khor, K.A. Mechanical properties of injection molded hydroxyapatite–polyetheretherketone. Compos. Sci. Technol 2003, 63, 421–425. [Google Scholar]
  68. Zhang, Y.; Hao, L.; Savalani, M.M.; Harris, R.A.; di Silvio, L.; Tanner, K.E. In vitro biocompatibility of hydroxyapatite-reinforced polymeric composites manufactured by selective laser sintering. J. Biomed. Mater. Res. A 2009, 91, 1018–1027. [Google Scholar]
  69. Ma, R.; Weng, L.; Bao, X.; Ni, Z.; Song, S.; Cai, W. Characterization of in situ synthesized hydroxyapatite/polyetheretherketone composite materials. Mater. Lett 2012, 71, 117–119. [Google Scholar]
  70. Ma, R.; Weng, L.; Bao, X.; Song, S.; Zhang, Y. In vivo biocompatibility and bioactivity of in situ synthesized hydroxyapatite/polyetheretherketone composite materials. J. Appl. Polym. Sci 2013, 127, 2581–2587. [Google Scholar]
  71. Kokubo, T.; Takadama, H. How useful is SBF in predicting in vivo bone bioactivity? Biomaterials 2006, 27, 2907–2915. [Google Scholar]
  72. Yu, S.; Hariram, K.P.; Kumar, R.; Cheang, P.; Aik, K.K. In vitro apatite formation and its growth kinetics on hydroxyapatite/polyetheretherketone biocomposites. Biomaterials 2005, 26, 2343–2352. [Google Scholar]
  73. Invibio Biomaterial Solutions Announces Global Launch of “PEEK-OPTIMA HA Enhanced Polymer”—A New PEEK-Based Biomaterial Designed for Superior Bone Apposition. Available online: http://www.prweb.com/releases/2013/9/prweb11099082.htm (accessed on 14 Match 2014).
  74. Tan, K.H.; Chua, C.K.; Leong, K.F.; Cheah, C.M.; Cheang, P.; Abu Bakar, M.S.; Cha, S.W. Scaffold development using selective laser sintering of polyetheretherketone-hydroxyapatite biocomposite blends. Biomaterials 2003, 24, 3115–3123. [Google Scholar]
  75. Tan, K.H.; Chua, C.K.; Leong, K.F.; Naing, M.W.; Cheah, C.M. Fabrication and characterization of three-dimensional poly(ether-ether-ketone)/hydroxyapatite biocomposite scaffolds using laser sintering. Proc. Inst. Mech. Eng. H 2005, 219, 183–194. [Google Scholar]
  76. Converse, G.L.; Conrad, T.L.; Merrill, C.H.; Roeder, R.K. Hydroxyapatite whisker-reinforced polyetherketoneketone bone ingrowth scaffolds. Acta Biomater 2010, 6, 856–863. [Google Scholar]
  77. Wong, K.L.; Wong, C.T.; Liu, W.C.; Pan, H.B.; Fong, M.K.; Lam, W.M.; Cheung, W.L.; Tang, W.M.; Chiu, K.Y.; Luk, K.D.; et al. Mechanical properties and in vitro response of strontium-containing hydroxyapatite/polyetheretherketone composites. Biomaterials 2009, 30, 3810–3817. [Google Scholar]
  78. Kim, I.Y.; Sugino, A.; Kikuta, K.; Ohtsuki, C.; Cho, S.B. Bioactive composites consisting of PEEK and calcium silicate powders. J. Biomater. Appl 2009, 24, 105–118. [Google Scholar]
  79. Lin, T.W.; Corvelli, A.A.; Frondoza, C.G.; Roberts, J.C.; Hungerford, D.S. Glass peek composite promotes proliferation and osteocalcin production of human osteoblastic cells. J. Biomed. Mater. Res 1997, 36, 137–144. [Google Scholar]
  80. Corvelli, A.A.; Roberts, J.C.; Biermann, P.J.; Cranmer, J.H. Characterization of a PEEK composite segmental bone replacement implant. J. Mater. Sci 1999, 34, 2421–2431. [Google Scholar]
  81. Pohle, D.; Ponader, S.; Rechtenwald, T.; Schmidt, M.; Schlegel, K.A.; Münstedt, H.; Neukam, F.W.; Nkenke, E.; von Wilmowsky, C. Processing of three-dimensional laser sintered polyetheretherketone composites and testing of osteoblast proliferation in vitro. Macromol. Symp. 2007, 253, 65–70. [Google Scholar]
  82. Von Wilmowsky, C.; Vairaktaris, E.; Pohle, D.; Rechtenwald, T.; Lutz, R.; Munstedt, H.; Koller, G.; Schmidt, M.; Neukam, F.W.; Schlegel, K.A.; et al. Effects of bioactive glass and beta-TCP containing three-dimensional laser sintered polyetheretherketone composites on osteoblasts in vitro. J. Biomed. Mater. Res. A 2008, 87, 896–902. [Google Scholar]
  83. Petrovic, L.; Pohle, D.; Munstedt, H.; Rechtenwald, T.; Schlegel, K.A.; Rupprecht, S. Effect of βTCP filled polyetheretherketone on osteoblast cell proliferation in vitro. J. Biomed. Sci. 2006, 13, 41–46. [Google Scholar]
  84. Wang, L.; Weng, L.; Song, S.; Sun, Q. Mechanical properties and microstructure of polyetheretherketone—Hydroxyapatite nanocomposite materials. Mater. Lett 2010, 64, 2201–2204. [Google Scholar]
  85. Wang, L.; Weng, L.; Song, S.; Zhang, Z.; Tian, S.; Ma, R. Characterization of polyetheretherketone-hydroxyapatite nanocomposite materials. Mater. Sci. Eng. A 2011, 528, 3689–3696. [Google Scholar]
  86. Horch, R.A.; Shahid, N.; Mistry, A.S.; Timmer, M.D.; Mikos, A.G.; Barron, A.R. Nanoreinforcement of poly(propylene fumarate)-based networks with surface modified alumoxane nanoparticles for bone tissue engineering. Biomacromolecules 2004, 5, 1990–1998. [Google Scholar]
  87. Shi, X.; Hudson, J.L.; Spicer, P.P.; Tour, J.M.; Krishnamoorti, R.; Mikos, A.G. Rheological behaviour and mechanical characterization of injectable poly(propylene fumarate)/single-walled carbon nanotube composites for bone tissue engineering. Nanotechnology 2005, 16, S531–S538. [Google Scholar]
  88. Ma, R.; Weng, L.; Fang, L.; Luo, Z.; Song, S. Structure and mechanical performance of in situ synthesized hydroxyapatite/polyetheretherketone nanocomposite materials. J. Sol-Gel Sci. Technol 2012, 62, 52–56. [Google Scholar]
  89. Kriparamanan, R.; Aswath, P.; Zhou, A.; Tang, L.; Nguyen, K.T. Nanotopography: Cellular responses to nanostructured materials. J. Nanosci. Nanotechnol 2006, 6, 1905–919. [Google Scholar]
  90. Stevens, B.; Yang, Y.; Mohandas, A.; Stucker, B.; Nquyen, K.T. A review of materials, fabrication methods, and strategies used to enhance bone regeneration in engineered bone tissues. J. Biomed. Mater. Res. B 2008, 8, 573–582. [Google Scholar]
  91. Njuguna, J.; Pielichowski, K.; Desai, S. Nanofiller-reinforced polymer nanocomposites. Polym. Adv. Technol 2008, 19, 947–959. [Google Scholar] [Green Version]
  92. Díez-Pascual, A.M.; Naffakh, M.; Marco, C.; Ellis, G.; Gómez-Fatou, M.A. High-performance nanocomposites based on polyetherketones. Prog. Mater. Sci 2012, 57, 1106–1190. [Google Scholar]
  93. Webster, T.J.; Siegel, R.W.; Biios, R. Design and evaluation of nanophase alumina for orthopaedic dental applications. Nanostruct. Mater 1999, 12, 983–986. [Google Scholar]
  94. Webster, T.J.; Siegel, R.W.; Bizios, R. Osteoblast adhesion on nanophase ceramics. Biomaterials 1999, 20, 1221–1227. [Google Scholar]
  95. Webster, T.J.; Ergun, C.; Doremus, R.H.; Siegel, R.W.; Bizios, R. Enhanced functions of osteoblasts on nanophase ceramics. Biomaterials 2000, 21, 1803–1810. [Google Scholar]
  96. Gutwein, L.G.; Webster, T.J. Osteoblast and chrondrocyte proliferation in the presence of alumina and titania nanoparticles. J. Nanopart. Res 2002, 4, 231–238. [Google Scholar]
  97. Webster, T.J.; Ergun, C.; Doremus, R.H.; Siegel, R.W.; Bizios, R. Enhanced osteoclast-like cell functions on nanophase ceramics. Biomaterials 2001, 22, 1327–1333. [Google Scholar]
  98. Li, K.; Yeung, C.Y.; Yeung, K.W.K.; Tjong, S.C. Sintered hydroxyapatite/polyetheretherketone nanocomposites: Mechanical behavior and biocompatibility. Adv. Eng. Mater 2012, 14, B155–B165. [Google Scholar]
  99. Wu, X.; Liu, X.; Wei, J.; Ma, J.; Deng, F.; Wei, S. Nano-TiO2/PEEK bioactive composite as a bone substitute material: In vitro and in vivo studies. Int. J. Nanomed 2012, 7, 1215–1225. [Google Scholar]
Ijms 15 05426f1 1024
Figure 1. Scheme of current strategies to improve the bioactivity of PEEK.
Figure 1. Scheme of current strategies to improve the bioactivity of PEEK.
Ijms 15 05426f1
Table
Table 1. The reported PEEK composites that are concerned with the bioactivity of the composites.
Table 1. The reported PEEK composites that are concerned with the bioactivity of the composites.
PEEK compositesFillers (name, size, form)Processing techniquesResearch results related the bioactivity of the compositesReference
HA/PEEKConventional HA particlesMelt compounding, granulation and injection moldingN/R[63,64,66,67]
HA/PEEKConventional HA whiskersPowder processing and compression moldingN/R[65]
HA/PEEKConventional HA particlesSelective laser sinteringCell tests: with improved osteoblast growth compared to TMX and PVC; higher HA contents with enhanced cell proliferation and osteogenic differentiation.[68]
HA/PEEKConventional HA powdersIn situ synthetic processIn vivo: the new bone tissues surrounding the composite grow faster with a higher HA content.[69,70]
HA/PEEKConventional HA powdersMixing, compaction and pressureless sinteringSBF immersion test: the 40 vol %-HA composite was covered by apatite-layer after 3 days; the growth rate increased with HA volume fraction.[72]
Porous HA/PEEKConventional HA particlesLeaching of particulate techniqueIn vivo: formation of fibro-vascular tissue within the pores at 6 weeks and mature bone at 16 weeks.[66]
Porous HA/PEEKConventional HA powersSelective laser sinteringSBF immersion test and cell tests: with precipitation of apatite-layers; with positive cell adhesion and growth compared to control (no specimens).[74,75]
Sr-HA/ PEEKConventional Sr-HA powersMixing, compression and moldingSBF immersion test and cell tests: with improved apatite-formation ability and mineralization compared to HA/PEEK or pure PEEK.[77]
CS/PEEKConventional CS powersMixing and compactionSBF immersion test: except for pure PEEK, all of the CS-containing composites promoted apatite-formation.[78]
Glass/PEEKConventional Chopped E-glass fibersN/ACell tests: with improved cell proliferation, ALP activity and OC production compared to polystyrene.[79]
β-TCP/PEEKConventional β-TCP powersInjection and moldingCell tests: with inhibited cell proliferation, but with no concentration-dependent decrease.[83]
Carbon black/ β-TCP/PEEKNano-sized carbon black powers, Conventional β-TCP powersLaser sinteringCell tests: with no improvement of cell proliferation compared to pure PEEK and carbon black/PEEK.[81,82]
Carbon black/ bioglass/PEEKNano-sized carbon black powers, Conventional bioglass powersLaser sinteringCell tests: with improvement of cell proliferation compared to PEEK, carbon black/PEEK, and carbon black/β-TCP/PEEK.[82]
HA/PEEKNano-sized HA particlesCompounding and injection moldingN/R[84,85]
HA/PEEKNano-sized HA particlesIn situ synthetic processN/R[88]
HA/PEEKNano-sized HA rodsPowder processing and sinteringCell tests: with improved apatite-formation ability, cell adhesion and proliferation compared to pure PEEK.[98]
TiO2/PEEKNano-sized TiO2 particlesMixing compression and moldingCell tests: with improved cell attachment and spreading compared with pure PEEK;
In vivo: with improved bone regeneration around the implants compared to pure PEEK.		[99]
N/A, not applicable; N/R, not report.

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Ma, R.; Tang, T. Current Strategies to Improve the Bioactivity of PEEK. Int. J. Mol. Sci. 2014, 15, 5426-5445. https://doi.org/10.3390/ijms15045426

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Ma R, Tang T. Current Strategies to Improve the Bioactivity of PEEK. International Journal of Molecular Sciences. 2014; 15(4):5426-5445. https://doi.org/10.3390/ijms15045426

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Ma, Rui, and Tingting Tang. 2014. "Current Strategies to Improve the Bioactivity of PEEK" International Journal of Molecular Sciences 15, no. 4: 5426-5445. https://doi.org/10.3390/ijms15045426

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