Mechanical and in vitro investigation of a porous PEEK foam for medical device implants

Immobilization of Collagen on the Surface of a PEEK Implant with Monolayer Nanopores

Polyetheretherketone (PEEK) is the only polymer material that can replace titanium implants in the field of orthopedics. This is because the mechanical properties of PEEK are similar to those of bone, and PEEK has natural radiolucency, chemical stability, and sterilization resistance. Despite these advantages, PEEK has a disadvantage—that it is bio-inert. Therefore, many studies have attempted to change the bio-inertness of PEEK into bioactivity. Among them, a method of forming pores by acid treatment is attracting attention. In this study, an attempt was made to form pores on the surface of PEEK implant using a mixed acid of sulfuric acid and nitric acid. As a result, it was found that the condition when the PEEK surface is in contact with the acid is very important. That is, it was possible to form single-layered nanopores on the surface by contacting PEEK with a mixed acid under ultrasound. Additionally, by immobilizing type I collagen on the porous PEEK surface through dopamine coating, it was possible to obtain collagen-immobilized porous PEEK (P-PEEK-Col) with high compatibility with osteoblasts. This P-PEEK-Col has high potential for use as a bone substitute that promotes bone formation.

Fabrication and in vitro study of 3D novel porous hydroxyapatite/polyether ether ketone surface nanocomposite

The unique characteristics of polyether ether ketone (PEEK) including low elastic modulus, high mechanical strength, and biocompatibility have made it an attractive alternative for the metallic biomaterials. However, its bioinert property is always the main concern, which could lead to poor osseointegration and subsequent clinical failure of the implant. Changing the surface structure to porous structure and mixing it with bioactive hydroxyapatite (HA) are the common methods, which could be used to enhance the properties of the PEEK-based implants. In this study, friction stir processing was utilized for the fabrication of porous HA/PEEK surface nanocomposite. Scanning electron microscopic image of the nanocomposite surface showed nano-scale roughness of the porous structure. Water contact angle test confirmed the increase in the wettability of the treated specimens. In vitro bioactivity test via simulated body fluid solution, initial cell adhesion, cell proliferation, and cell differentiation assay also confirmed the enhancement in bioactivity of the treated surface in comparison to the bare PEEK. This surface modification method requires no special equipment and would not damage the heat-sensitive PEEK substrate due to the low temperature used during the fabrication process.

A Multi-Criteria Assessment Strategy for 3D Printed Porous Polyetheretherketone (PEEK) Patient-Specific Implants for Orbital Wall Reconstruction

Pure orbital blowout fractures occur within the confines of the internal orbital wall. Restoration of orbital form and volume is paramount to prevent functional and esthetic impairment. The anatomical peculiarity of the orbit has encouraged surgeons to develop implants with customized features to restore its architecture. This has resulted in worldwide clinical demand for patient-specific implants (PSIs) designed to fit precisely in the patient's unique anatomy. Material extrusion or Fused filament fabrication (FFF) three-dimensional (3D) printing technology has enabled the fabrication of implant-grade polymers such as Polyetheretherketone (PEEK), paving the way for a more sophisticated generation of biomaterials. This study evaluates the FFF 3D printed PEEK orbital mesh customized implants with a metric considering the relevant design, biomechanical, and morphological parameters. The performance of the implants is studied as a function of varying thicknesses and porous design constructs through a finite element (FE) based computational model and a decision matrix based statistical approach. The maximum stress values achieved in our results predict the high durability of the implants, and the maximum deformation values were under one-tenth of a millimeter (mm) domain in all the implant profile configurations. The circular patterned implant (0.9 mm) had the best performance score. The study demonstrates that compounding multi-design computational analysis with 3D printing can be beneficial for the optimal restoration of the orbital floor.

Superior Osteo-Inductive and Osteo-Conductive Properties of Trabecular Titanium vs. PEEK Scaffolds on Human Mesenchymal Stem Cells: A Proof of Concept for the Use of Fusion Cages

Fusion cages composed of titanium and its alloys are emerging as valuable alternative to standard polyetheretherketone (PEEK) ones routinely used in cervical and lumbar spine surgery. Aim of this study was to evaluate osteo-inductive and osteo-conductive ability of an innovative trabecular titanium (T-Ti) scaffold on human mesenchymal stem cells (hMSCs), in both absence and presence of biochemical osteogenic stimuli. Same abilities were assessed on PEEK and standard 2D plastic surface, the latter meant as gold-standard for in vitro differentiation studies. hMSCs adhered and colonized both T-Ti and PEEK scaffolds. In absence of osteogenic factors, T-Ti triggered osteogenic induction of MSCs, as demonstrated by alkaline phosphatase activity and calcium deposition increments, while PEEK and standard 2D did not. Addition of osteogenic stimuli reinforced osteogenic differentiation of hMSCs cultured on T-Ti in a significantly higher manner with respect to standard 2D plastic culture surfaces, whereas PEEK almost completely abolished the process. T-Ti driven differentiation towards osteoblasts was confirmed by gene and marker expression analyses, even in absence of osteogenic stimuli. These results clearly indicate superior in vitro osteo-inductive and osteo-conductive capacity of T-Ti compared to PEEK, and make ground for further studies supporting the use of T-Ti cages to improve bone fusion.

Gaseous sulfur trioxide induced controllable sulfonation promoting biomineralization and osseointegration of polyetheretherketone implants

Fabricating a desired porous structure on the surface of biomedical polyetheretherketone (PEEK) implants for enhancing biological functions is crucial and difficult due to its inherent chemical inertness. In this study, a porous surface of PEEK implants was fabricated by controllable sulfonation using gaseous sulfur trioxide (SO₃) for different time (5, 15, 30, 60 and 90 min). Micro-topological structure was generated on the surface of sulfonated PEEK implants preserving original mechanical properties. The protein absorption capacity and apatite forming ability was thus improved by the morphological and elemental change with higher degree of sulfonation. In combination of the appropriate micromorphology and bioactive sulfonate components, the cell adhesion, migration, proliferation and extracellular matrix secretion were obviously enhanced by the SPEEK-15 samples which were sulfonated for 15 min. Finding from this study revealed that controllable sulfonation by gaseous SO₃ would be an extraordinarily strategy for improving osseointegration of PEEK implants by adjusting the microstructure and chemical composition while maintaining excellent mechanical properties.

Formulation of a covalently bonded hydroxyapatite and poly(ether ether ketone) composite

Spinal fusion devices can be fabricated from composites based on combining hydroxyapatite and poly(ether ether ketone) phases. These implants serve as load-bearing scaffolds for the formation of new bone tissue between adjacent vertebrae. In this work, we report a novel approach to covalently bond hydroxyapatite and poly(ether ether ketone) to produce a novel composite formulation with enhanced interfacial adhesion between phases. Compared to non-linked composites (HA_PEEK), covalently linked composites (HA_L_PEEK), loaded with 1.25 vol% hydroxyapatite, possessed a greater mean flexural strength (170 ± 5.4 vs 171.7 ± 14.8 MPa (mean ± SD)) and modulus (4.8 ± 0.2 vs 5.0 ± 0.3 GPa (mean ± SD)). Although the mechanical properties were not found to be significantly different (p > 0.05), PEEK_L_HA contained substantially larger hydroxyapatite inclusions (100-1000 µm) compared to HA_PEEK (50-200 µm), due to the inherently agglomerative nature of the covalently bonded hydroxyapatite and poly(ether ether ketone) additive. Larger inclusions would expectedly weaken the HA_L_PEEK composite; however, there is no significant difference between the flexural modulus of poly(ether ether ketone) with respect to HA_L_PEEK (p = 0.13). In addition, the flexural modulus of HA_PEEK is significantly lower compared to poly(ether ether ketone) (p = 0.03). Ultimately, covalent linking reduces hydroxyapatite particulate de-bonding from the polymeric matrix and inhibits micro-crack development, culminating in enhanced transfer of stiffness between hydroxyapatite and poly(ether ether ketone) under loading.

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.

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.

Improving PEEK bioactivity for craniofacial reconstruction using a 3D printed scaffold embedded with mesenchymal stem cells

Objective

Polyetheretherketone (PEEK) is a bioinert thermoplastic that has been investigated for its potential use in craniofacial reconstruction; however, its use in clinical practice is limited by a poor integration with adjacent bone upon implantation. To improve the bone–implant interface, two strategies have been employed: to modify its surface or to impregnate PEEK with bioactive materials. This study attempts to combine and improve upon the two approaches by modifying the internal structure into a trabecular network and to impregnate PEEK with mesenchymal stem cells. Furthermore, we compare the newly designed PEEK scaffolds' interactions with both bone-derived (BMSC) and adipose (ADSC) stem cells.

Design

Customized PEEK scaffolds were designed to incorporate a trabecular microstructure using a computer-aided design program and then printed via selective laser sintering (SLS), a 3D-printing process with exceptional accuracy. The scaffold structure was evaluated using microCT. Scanning electron microscopy (SEM) was used to evaluate scaffold morphology with and without mesenchymal stem cells (MSCs). Adipose and bone marrow mesenchymal cells were isolated from rats and cultured on scaffolds. Cell proliferation and differentiation were assessed using alamarBlue and alkaline phosphatase assays, respectively. Cell morphology after one week of co-culturing cells with PEEK scaffolds was evaluated using SEM.

Results

SLS 3D printing fabricated scaffolds with a porosity of 36.38% ± 6.66 and density of 1.309 g/cm2. Cell morphology resembled viable fibroblasts attaching to the surface and micropores of the scaffold. PEEK scaffolds maintained the viability of both ADSCs and BMSCs; however, ADSCs demonstrated higher osteodifferentiation than BMSCs ( p < 0.05).

Conclusions

This study demonstrates for the first time that SLS 3D printing can be used to fabricate customized porous PEEK scaffolds that maintain the viability of adipose and bone marrow-derived MSCs and induce the osteodifferentiation of the adipose-derived MSCs. The combination of 3D printed PEEK scaffolds with MSCs could overcome some of the limitations using PEEK biopolymers for load-bearing bone regeneration in craniofacial reconstruction.

Implant materials generate different peri-implant inflammatory factors: poly-ether-ether-ketone promotes fibrosis and microtextured titanium promotes osteogenic factors

STUDY DESIGN

An in vitro study examining factors produced by human mesenchymal stem cells on spine implant materials.

OBJECTIVE

The aim of this study was to examine whether the inflammatory microenvironment generated by cells on titanium-aluminum-vanadium (Ti-alloy, TiAlV) surfaces is affected by surface microtexture and whether it differs from that generated on poly-ether-ether-ketone (PEEK).

SUMMARY OF BACKGROUND DATA

Histologically, implants fabricated from PEEK have a fibrous connective tissue surface interface whereas Ti-alloy implants demonstrate close approximation with surrounding bone. Ti-alloy surfaces with complex micron/submicron scale roughness promote osteoblastic differentiation and foster a specific cellular environment that favors bone formation whereas PEEK favors fibrous tissue formation.

METHODS

Human mesenchymal stem cells were cultured on tissue culture polystyrene, PEEK, smooth TiAlV, or macro-/micro-/nano-textured rough TiAlV (mmnTiAlV) disks. Osteoblastic differentiation and secreted inflammatory interleukins were assessed after 7 days. Fold changes in mRNAs for inflammation, necrosis, DNA damage, or apoptosis with respect to tissue culture polystyrene were measured by low-density polymerase chain reaction array. Data were analyzed by analysis of variance, followed by Bonferroni's correction of Student's t-test.

RESULTS

Cells on PEEK upregulated mRNAs for chemokine ligand-2, interleukin (IL) 1β, IL6, IL8, and tumor necrosis factor. Cells grown on the mmnTiAlV had an 8-fold reduction in mRNAs for toll-like receptor-4. Cells grown on mmnTiAlV had reduced levels of proinflammatory interleukins. Cells on PEEK had higher mRNAs for factors strongly associated with cell death/apoptosis, whereas cells on mmnTiAlV exhibited reduced cytokine factor levels. All results were significant (P < 0.05).

CONCLUSION

These results suggest that fibrous tissue around PEEK implants may be due to several factors: reduced osteoblastic differentiation of progenitor cells and production of an inflammatory environment that favors cell death via apoptosis and necrosis. Ti alloy surfaces with complex macro/micro/nanoscale roughness promote osteoblastic differentiation and foster a specific cellular environment that favors bone formation.

LEVEL OF EVIDENCE

N/A.

High-strength, surface-porous polyether-ether-ketone for load-bearing orthopedic implants

Despite its widespread clinical use in load-bearing orthopedic implants, polyether-ether-ketone (PEEK) is often associated with poor osseointegration. In this study, a surface-porous PEEK material (PEEK-SP) was created using a melt extrusion technique. The porous layer was 399.6±63.3 μm thick and possessed a mean pore size of 279.9±31.6 μm, strut spacing of 186.8±55.5 μm, porosity of 67.3±3.1% and interconnectivity of 99.9±0.1%. Monotonic tensile tests showed that PEEK-SP preserved 73.9% of the strength (71.06±2.17 MPa) and 73.4% of the elastic modulus (2.45±0.31 GPa) of as-received, injection-molded PEEK. PEEK-SP further demonstrated a fatigue strength of 60.0 MPa at one million cycles, preserving 73.4% of the fatigue resistance of injection-molded PEEK. Interfacial shear testing showed the pore layer shear strength to be 23.96±2.26 MPa. An osseointegration model in the rat revealed substantial bone formation within the pore layer at 6 and 12 weeks via microcomputed tomography and histological evaluation. Ingrown bone was more closely apposed to the pore wall and fibrous tissue growth was reduced in PEEK-SP when compared to non-porous PEEK controls. These results indicate that PEEK-SP could provide improved osseointegration while maintaining the structural integrity necessary for load-bearing orthopedic applications.