Porous poly-ether ether ketone (PEEK) manufactured by a novel powder route using near-spherical salt bead porogens: Characterisation and mechanical properties

Bone ingrowth into a porous structure is achieved by preceding fibrogenesis and vascularization

Porous structures are frequently used in surgical implants to strengthen the interlocking power produced by bone ingrowth. Therefore, we aimed to elucidate the mechanism underlying bone ingrowth into a porous structure accompanied by vascularization. A nonbioactive polyetheretherketone implant with a 3D-printed porous structure was prepared and implanted in a bone hole created in the tibias of rabbits. We observed bone ingrowth in the same individual specimens immediately and at 2, 4, 8, and 12 weeks post-implantation using in-vivo computed tomography (CT). Furthermore, a detailed evaluation with blood vessels of each specimen at 2, 4, and 12 weeks was performed with ex-vivo CT and histological specimen. Additional histological evaluation was performed using thin sections of an implant made with thermoplastic polyurethane having the same structure. As a result, the bone invasion began after four weeks, when the construction of fibrous tissue and the spread of new blood vessels within the voids matured. As the bone matured in the load-bearing area, new blood vessels outside the bone matrix regressed. This longitudinal evaluation study suggests that preceding fibrogenesis and vascularization may be key in developing bone ingrowth. STATEMENT OF SIGNIFICANCE: A porous structure is an essential structure for dental and orthopedic implants because it provides strong fixation through bone invasion. Although it was known that vascularization was involved in this, the details were not known. This in vivo study revealed that in order for bone ingrowth to begin, a preparatory period of approximately 4 weeks was required to establish blood flow inside and outside the implant. Furthermore, it was confirmed that by spreading the fibrous structure in advance, it has an advantageous effect on the migration of cells involved in the formation of bones and blood vessels. We pointed out that it is necessary to consider fibrogenesis and vascularization when creating future implants.

Multiscale Porous Poly (Ether-Ether-Ketone) Structures Manufactured by Powder Bed Fusion Process

The aim of the study is to create a multiscale highly porous poly (ether-ether-ketone) (PEEK) structure while maintaining mechanical performance; the distribution of pores being generated by the manufacturing process combined with a porogen leaching operation. Salt at 70 wt% concentration was used as a porogen in a dry blend with PEEK powder sintered in the powder bed fusion process. The printed porous PEEK structures were examined and evaluated by scanning electron microscopy, microcomputed tomography, and mechanical testing. The PEEK structures incorporating 70 wt% salt achieved 79-86% porosity, a compressive yield strength of 4.1 MPa, and a yield strain of ∼60%. Due to the salt leaching process, the PEEK porous frameworks were fabricated without the need to drastically reduce the process parameters (defined by the energy density [ED]), hence maintaining the structural integrity and good mechanical performance. The compression results highlighted that the performance is influenced by the printing orientation, level of the PEEK particle coalescence (controlled here by the ED), pore/cell wall thickness, and subsequently, the overall porosity framework. The porous printed PEEK structures could find potential uses in a wide range of applications from tissue engineering, filtration and separation to catalysts, drug release, and gas storage.

Fabrication and properties of interweaved poly(ether ether ketone) composite scaffolds

This paper interweaved scaffolds with poly(ether ether ketone) (PEEK) and poly(lactic acid)/Walnut shell/hydroxypatite (PLA/WS/HA) composites by using fused filament fabrication technology, although there was a huge difference in thermal property term between PLA and PEEK. In order to keep mechanical properties of PEEK scaffold and remedy the stress loss produced by pores, PLA/WS/HA composites were used to fill the pores with gradient form outside-in (0.4-0.8 mm, 0.6-1.0 mm, 0.8-1.2 mm and 1.6-2.0 mm). The thermal stability, tensile and compression properties, tensile fracture surface morphology, cytotoxicity and in vivo experiment were investigated. The results showed: the scaffolds were intact without any flashes and surface destruction, and kept a well thermal stability. Compared with the PEEK porous scaffolds, the tensile fracture stress and strain, compression yield stress and strain of interweaved scaffolds were dramatically enhanced by 24.1%, 438%, 359.1% and 921.2%, respectively, and they climbed to the climax at 8 wt% of WS. In vivo experiment showed that the degradation of PLA/WS/HA composites synchronized with the adhesion, proliferation and ingrowth of bone cells, keeping the stable biomechanical properties of interweaved scaffolds. Those experiments showed that interweaved PEEK-PLA/WS/HA scaffolds had the potential to be used as bone implant in tissue engineering.

Use of Piranha Solution as An Alternative Route to Promote Bioactivation of PEEK Surface with Low Functionalization Times

This study aimed to achieve bioactivity on the PEEK surface using piranha solution through a lower functionalization time. For this purpose, the functionalization occurred with piranha solution and 98% sulfuric acid in the proportions of 1:2, 1:1, and 2:1 at periods of 30, 60, and 90 s. The samples treated for longer times at higher concentrations registered the characteristic spectroscopy band associated with sulfonation. Additionally, both chemical treatments allowed the opening of the aromatic ring, increasing the number of functional groups available and making the surface more hydrophilic. The piranha solution treatments with higher concentrations and longer times promoted greater heterogeneity in the surface pores, which affected the roughness of untreated PEEK. Furthermore, the treatments induced calcium deposition on the surface during immersion in SBF fluid. In conclusion, the proposed chemical modifications using sulfuric acid SPEEK 90 and, especially, the piranha solution PEEK-PS 2:1-90, were demonstrated to be promising in promoting the rapid bioactivation of PEEK-based implants.

Bio-Activated PEEK: Promising Platforms for Improving Osteogenesis through Modulating Macrophage Polarization

The attention on orthopedic biomaterials has shifted from their direct osteogenic properties to their osteoimmunomodulation, especially the modulation of macrophage polarization. Presently, advanced technologies endow polyetheretherketone (PEEK) with good osteoimmunomodulation by modifying PEEK surface characteristics or incorporating bioactive substances with regulating macrophage polarization. Recent studies have demonstrated that the fabrication of a hydrophilic surface and the incorporation of bioactive substances into PEEK (e.g., zinc, calcium, and phosphate) are good strategies to promote osteogenesis by enhancing the polarization of M2 macrophages. Furthermore, the modification by other osteoimmunomodulatory composites (e.g., lncRNA-MM2P, IL-4, IL-10, and chitosan) and their controlled and desired release may make PEEK an optimal bio-activated implant for regulating and balancing the osteogenic system and immune system. The purpose of this review is to comprehensively evaluate the potential of bio-activated PEEK in polarizing macrophages into M2 phenotype to improve osteogenesis. For this objective, we retrieved and discussed different kinds of bio-activated PEEK regarding improving osteogenesis through modulating macrophage polarization. Meanwhile, the relevant challenges and outlook were presented. We hope that this review can shed light on the development of bio-activated PEEK with more favorable osteoimmunomodulation.

Development of an architecture-property model for triply periodic minimal surface structures and validation using material extrusion additive manufacturing with polyetheretherketone (PEEK)

Additively manufactured structures designed from triply periodic minimal surfaces (TPMSs) have been receiving attention for their potential uses in the medical, aerospace, and automobile industries. Understanding how these complex geometries can be designed to achieve particular architectural and mechanical properties is essential for tuning their function to certain applications. In this study, we created design tools for visualizing the interplay between TPMS design parameters and resulting architecture and aimed to validate a model of the relationship between structure architecture and Young's modulus. A custom MATLAB script was written to analyze structural properties for families of Schoen gyroid and Schwarz diamond structures, and a numerical homogenization scheme was performed to predict the effective Young's moduli of the structures based on their architecture. Our modeling methods were validated experimentally with polyetheretherketone (PEEK) structures created using material extrusion additive manufacturing. The architectural characteristics of the structures were determined using micro-computed tomography, and compression testing was performed to determine yield strength and Young's modulus. Two different initial build orientations were tested to determine the behavior both perpendicular and parallel to the layer deposition direction (referred to as z-direction and xy-direction, respectively). The z-direction Young's modulus ranged from 289.7 to 557.5 MPa and yield strength ranged from 10.12 to 20.3 MPa. For the xy-direction, Young's modulus ranged from 133.8 to 416.4 MPa and yield strength ranged from 3.8 to 12.2 MPa. For each initial build orientation, the mechanical properties were found to decrease with increasing porosity, and failure occurred due to both strut bending and interlayer debonding. The mechanical properties predicted by the modeling agreed with the values found for z-direction samples (difference 2-11%) but less so for xy-direction samples (difference 27-62%) due to weak interlayer bonding and print path irregularities. Ultimately, the findings presented here provide better understanding of the range of properties achievable for additive manufacturing of PEEK and encouraging results for a TPMS architecture-property model.

Modification of polyetheretherketone (PEEK) physical features to improve osteointegration

Polyetheretherketone (PEEK) has been widely applied in orthopedics because of its excellent mechanical properties, radiolucency, and biocompatibility. However, the bioinertness and poor osteointegration of PEEK have greatly limited its further application. Growing evidence proves that physical factors of implants, including their architecture, surface morphology, stiffness, and mechanical stimulation, matter as much as the composition of their surface chemistry. This review focuses on the multiple strategies for the physical modification of PEEK implants through adjusting their architecture, surface morphology, and stiffness. Many research findings show that transforming the architecture and incorporating reinforcing fillers into PEEK can affect both its mechanical strength and cellular responses. Modified PEEK surfaces at the macro scale and micro/nano scale have positive effects on cell-substrate interactions. More investigations are necessary to reach consensus on the optimal design of PEEK implants and to explore the efficiency of various functional implant surfaces. Soft-tissue integration has been ignored, though evidence shows that physical modifications also improve the adhesion of soft tissue. In the future, ideal PEEK implants should have a desirable topological structure with better surface hydrophilicity and optimum surface chemistry.

Morphology and properties of foamed high crystallinity PEEK prepared by high temperature thermally induced phase separation

Polyetheretherketone (PEEK) is a high‐performance semi‐crystalline thermoplastic polymer with outstanding mechanical properties, high thermal stability, resistance to most common solvents, and good biocompatibility. A high temperature thermally induced phase separation technique was used to produce PEEK foams with controlled foam density from PEEK in 4‐phenylphenol (4PPH) solutions. Physical and mechanical properties, foam and bulk density, surface area, and pore morphology of foamed PEEK were characterized and the role of PEEK concentration and cooling rate was investigated. Porous PEEK with densities ranging from 110 to 360 kg/m³ with elastic moduli and crush strength ranging from 13 to 125 MPa and 0.8 to 7 MPa, respectively, was produced.

3D porous PCL-PEG-PCL / strontium, magnesium and boron multi-doped hydroxyapatite composite scaffolds for bone tissue engineering

Bioceramic/polymer composite systems have gained importance in treating hard tissue damages using bone tissue engineering (BTE). In this context, it was aimed to develop 3D porous composite PCL-PEG-PCL scaffolds containing different amounts of B, Sr and Mg multi-doped HA that can provide bone regeneration in the bone defect area and to investigate the effect of both the amount of inorganic phase and the porosity on the mechanical and the biological properties. B-Sr-Mg multi-doped HA and PCL-PEG-PCL copolymer were successfully synthesized. PCL-PEG-PCL composite scaffolds containing different amounts of hydroxyapatite (HA) (10% and 20 wt%) were produced with the desired porosity (50% and 60%) by compression-molding and particulate leaching method. The porosity of the scaffolds was determined between 47% and 59%. HA/PCL-PEG-PCL composite scaffolds were subjected to a 3-week degradation test and showed negligible (0.2-0.5%) degradation. The water uptake percentage of the composite scaffolds with 60% porosity was the highest among all groups. Presence of HA in the scaffolds improved the water adsorption and the mechanical properties. Compressive strength of the scaffolds was between 9.32 and 24.27 MPa and 20% 2Sr0.5BHA scaffolds were found to have the maximum compressive strength. Compressive strength of 50% porous samples was higher than that of 60% porous samples. In the relative cell viability (%) test, the highest viability was observed on the scaffolds with HA and 2Sr0.5BHA. The specific ALP activity level of the cells on the scaffolds containing 2Sr0.5BHA was significantly higher (2.6 times) than that of the control group. The amount of porosity did not make a significant difference in cellular response. It was concluded that PCL-PEG-PCL composite scaffolds with 2Sr0.5BHA have the potential to be used in BTE.

Surface porous poly-ether-ether-ketone based on three-dimensional printing for load-bearing orthopedic implant

Poly-ether-ether-ketone (PEEK) possesses excellent biocompatibility and similar elastic modulus as bones but yet suffers from poor osseointegration. In order to balance PEEK's mechanical and osseointegration properties, a novel surface porous PEEK (SP-PEEK) is successfully fabricated by fused deposition modelling three-dimensional printing (FDM 3DP) and characterized by mechanical and osteogenesis in vitro tests. Moreover, the effects of pore diameter and pore layer number on the mechanical behaviors of SP-PEEK are investigated by theoretical model and numerical simulation. Comparison among experimental, theoretical and simulation results show good agreement. As pore diameter decreases, the equivalent strength and modulus become more sensitive to the decrease of pore layer number. In addition, the SP-PEEK exhibits the mechanical properties within the range of human trabecular bone and cortical bone, and thus can be tailored to mimic human bone by adjusting the pore diameter and pore layer number, which is benefit to mitigate stress shielding. The effects of pore diameter on the cell proliferation and osteogenic differentiation of SP-PEEK are tested by the co-culture of osteoblast precursor cells (MC3T3-E1) and SP-PEEK round discs. Results showcase that porous surface improves the osteogenesis in vitro, and the SP-PEEK group that the pore diameter is 0.6 mm exhibits optimal-performance osteogenesis in vitro.

Fused Filament Fabrication of PEEK: A Review of Process-Structure-Property Relationships

Poly (ether ether ketone) (PEEK) is a high-performance engineering thermoplastic polymer with potential for use in a variety of metal replacement applications due to its high strength to weight ratio. This combination of properties makes it an ideal material for use in the production of bespoke replacement parts for out-of-earth manufacturing purposes, in particular on the International Space Station (ISS). Additive manufacturing (AM) may be employed for the production of these parts, as it has enabled new fabrication pathways for articles with complex design considerations. However, AM of PEEK via fused filament fabrication (FFF) encounters significant challenges, mostly stemming from the semi crystalline nature of PEEK and its associated high melting temperature. This makes PEEK highly susceptible to changes in processing conditions which leads to a large reported variation in the literature on the final performance of PEEK. This has limited the adaption of FFF printing of PEEK in space applications where quality assurance and reproducibility are paramount. In recent years, several research studies have examined the effect of printing parameters on the performance of the 3D-printed PEEK parts. The aim of the current review is to provide comprehensive information in relation to the process-structure-property relationships in FFF 3D-printing of PEEK to provide a clear baseline to the research community and assesses its potential for space applications, including out-of-earth manufacturing.

Compression moulding and injection over moulding of porous PEEK components

A simple and adaptable process for the production of porous PEEK has been demonstrated herein, which uses compression moulding to infiltrate molten PEEK into of a packed bed of salt beads. The process has the capacity to vary the pore size and porosity within the range suitable for materials to replace bone, but compressive testing showed the stiffness to be well below the target to match trabecular bone. This issue was addressed by creating a hybrid structure, integrating "pillars" of solid PEEK into the porous structure, by the injection over-moulding of compression moulded PEEK-salt inserts that contained drilled holes. Good bonding between the moulding and the insert was demonstrated and it was found that as little as 35 mm² of support, in the form of PEEK "pillars" was required to achieve the target performance.

High-Performance Polymer Foams by Thermally Induced Phase Separation

Macroporous, low-density polyetheretherketone, polyetherketoneketone, and polyetherimide foams are produced using a high-temperature, thermally induced phase separation method. A high-boiling-point solvent, which is suitable to dissolve at least 20 wt% of these high-performance polymers at temperatures above 250 °C, is identified. The foam morphology is controlled by the cooling procedure. The resulting polymer foams have porosities close to 80% with surface areas up to 140 m² g^-1 and elastic moduli up to 97 MPa.

Foaming Behavior and Microcellular Morphologies of Incompatible SAN/CPE Blends with Supercritical Carbon Dioxide as a Physical Blowing Agent

The foaming process and cellular morphologies of poly(styrene-co-acrylonitrile) (SAN)/chlorinated polyethylene (CPE) blends with supercritical carbon dioxide (scCO₂) as a blowing agent were investigated in this study. As compared to pure SAN foam in the same batch, the foamed blends with various CPE elastomer content had smaller average pore size and larger cell density. This is probably related to the inhibition of bubble growth by elastomer, resulting in poor melt flowability and strong viscoelasticity, and the efficient bubble heterogeneous nucleation caused by numerous phase interfaces inside the incompletely compatible blend system. In addition, many tiny interconnected holes through the pore walls were formed to connect adjacent micropores in foamed blend samples. The formation mechanism of such interconnected pores is probably due to the fracture of stretched melt around the bubble from phase interfaces with weak interactions. These facts suggest an effective path to control pore size, cell density and even interconnected pores of blend foams depends on the compatibility of the blend system and difference in foamability of individual components in supercritical CO₂.

Getting PEEK to Stick to Bone: The Development of Porous PEEK for Interbody Fusion Devices

Interbody fusion cages are routinely implanted during spinal fusion procedures to facilitate arthrodesis of a degenerated or unstable vertebral segment. Current cages are most commonly made from polyether-ether-ketone (PEEK) due to its favorable mechanical properties and imaging characteristics. However, the smooth surface of current PEEK cages may limit implant osseointegration and may inhibit successful fusion. We present the development and clinical application of the first commercially available porous PEEK fusion cage (COHERE^®, Vertera, Inc., Atlanta, GA) that aims to enhance PEEK osseointegration and spinal fusion outcomes. The porous PEEK structure is extruded directly from the underlying solid and mimics the structural and mechanical properties of trabecular bone to support bone ingrowth and implant fixation. Biomechanical testing of the COHERE^® device has demonstrated greater expulsion resistance versus smooth PEEK cages with ridges and greater adhesion strength of porous PEEK versus plasma-sprayed titanium coated PEEK surfaces. In vitro experiments have shown favorable cell attachment to porous PEEK and greater proliferation and mineralization of cell cultures grown on porous PEEK versus smooth PEEK and smooth titanium surfaces, suggesting that the porous structure enhances bone formation at the cellular level. At the implant level, preclinical animal studies have found comparable bone ingrowth into porous PEEK as those previously reported for porous titanium, leading to twice the fixation strength of smooth PEEK implants. Finally, two clinical case studies are presented demonstrating the effectiveness of the COHERE^® device in cervical spinal fusion.

Two-year performance study of porous, thermoset, shape memory polyurethanes intended for vascular medical devices

The long-term shape-recovery behavior of shape memory polymers has often been shown to be dependent on the length of time the material has been stored in the secondary shape. Typically, recovery performance and shape fixity will decrease with increased time in the secondary shape. In medical materials, a shelf-life is crucial to establish as it sets the upper threshold for device performance in a clinical setting, and a reduction in shape recovery would limit the development of SMP medical devices. Here, we present a two-year study of strain recovery, strain fixity, and shape recovery kinetics for passively and actively actuated SMPs intended for vascular devices. While kinetic experiments using immersion DMA indicate slight material relaxation and a decrease in the time to recovery, these changes are not found for bulk recovery experiments. The results indicate that a two-year shelf-life for these SMPs is very reasonable, as there is no change in the recovery kinetics, strain recovery, or strain fixity associated with this aging time. Further, a thermal accelerated aging test is presented for more rapid testing of the shape memory behavior of these SMPs and is compared with the real time aging results, indicating that this test is a reasonable indicator of the two-year behavior.

Macro-mesoporous composites containing PEEK and mesoporous diopside as bone implants: characterization, in vitro mineralization, cytocompatibility, and vascularization potential and osteogenesis in vivo

Superior in vitro bioactivity, cytocompatibility, and in vivo osteogenesis and vascularization potential.

A new light triggered approach to develop a micro porous tough hydrogel

A porous tough hydrogel (alginate–polyacrylamide) is successfully synthesized using a photo-gelling polymer as a templating agent.

Mechanical properties and in vivo study of modified-hydroxyapatite/polyetheretherketone biocomposites

Polyether ether ketone (PEEK) has received much attention for its excellent mechanical properties and biocompatibility. Here, the silane coupling agent KH560 [γ-(2,3-epoxypropoxy)propyltrimethoxysilane] is used for graft modification of bioactive HA (hydroxyapatite) particles and for preparing HA/PEEK composites via a hot-press molding method. The prepared HA/PEEK composites were tested for their mechanical properties with SEM (scanning electron microscopy), infrared spectroscopy, and thermo-analysis. The results show that silane coupling KH-560 modifies HA successfully and that the tensile strengths of HA/PEEK and m-HA/PEEK composites indicate an increasing and then a decreasing tendency with increasing HA contents. The non-modified HA/PEEK composites display the same trend as the modified specimens with lower tensile strength and consist of sharp points. When the HA content is 5wt.%, the tensile strength of m-HA/PEEK composite reaches its maximum, which is 23% higher than that of pure PEEK specimens. The in vivo experiments of m-HA/PEEK used a biomechanical push-out test, SEM, optical microscopy, and an Image-Pro Express C image analysis system. The growth of the bone tissues around the m-HA/PEEK composites with an HA content of 5wt.% is better than that of specimens with different HA contents. This finding shows the nano-scale effect of the bioactive filler HA in PEEK substrates, which obviously contributes to the growth of the surrounding bone issues in vivo. This study could provide theoretical support for the further promotion and application of high-performance engineering plastics such as PEEK in biomedical fields.

Do Surface Porosity and Pore Size Influence Mechanical Properties and Cellular Response to PEEK?

BACKGROUND

Despite its widespread use in orthopaedic implants such as soft tissue fasteners and spinal intervertebral implants, polyetheretherketone (PEEK) often suffers from poor osseointegration. Introducing porosity can overcome this limitation by encouraging bone ingrowth; however, the corresponding decrease in implant strength can potentially reduce the implant's ability to bear physiologic loads. We have previously shown, using a single pore size, that limiting porosity to the surface of PEEK implants preserves strength while supporting in vivo osseointegration. However, additional work is needed to investigate the effect of pore size on both the mechanical properties and cellular response to PEEK.

QUESTIONS/PURPOSES

(1) Can surface porous PEEK (PEEK-SP) microstructure be reliably controlled? (2) What is the effect of pore size on the mechanical properties of PEEK-SP? (3) Do surface porosity and pore size influence the cellular response to PEEK?

METHODS

PEEK-SP was created by extruding PEEK through NaCl crystals of three controlled ranges: 200 to 312, 312 to 425, and 425 to 508 µm. Micro-CT was used to characterize the microstructure of PEEK-SP. Tensile, fatigue, and interfacial shear tests were performed to compare the mechanical properties of PEEK-SP with injection-molded PEEK (PEEK-IM). The cellular response to PEEK-SP, assessed by proliferation, alkaline phosphatase activity, vascular endothelial growth factor production, and calcium content of osteoblast, mesenchymal stem cell, and preosteoblast (MC3T3-E1) cultures, was compared with that of machined smooth PEEK and Ti6Al4V.

RESULTS

Micro-CT analysis showed that PEEK-SP layers possessed pores that were 284 ± 35 µm, 341 ± 49 µm, and 416 ± 54 µm for each pore size group. Porosity and pore layer depth ranged from 61% to 69% and 303 to 391 µm, respectively. Mechanical testing revealed tensile strengths > 67 MPa and interfacial shear strengths > 20 MPa for all three pore size groups. All PEEK-SP groups exhibited > 50% decrease in ductility compared with PEEK-IM and demonstrated fatigue strength > 38 MPa at one million cycles. All PEEK-SP groups also supported greater proliferation and cell-mediated mineralization compared with smooth PEEK and Ti6Al4V.

CONCLUSIONS

The PEEK-SP formulations evaluated in this study maintained favorable mechanical properties that merit further investigation into their use in load-bearing orthopaedic applications and supported greater in vitro osteogenic differentiation compared with smooth PEEK and Ti6Al4V. These results are independent of pore sizes ranging 200 µm to 508 µm.

CLINICAL RELEVANCE

PEEK-SP may provide enhanced osseointegration compared with current implants while maintaining the structural integrity to be considered for several load-bearing orthopaedic applications such as spinal fusion or soft tissue repair.