Post written by: Sisi Leung
IntroductionPolyaryletherketone (PAEK) polymers encompass a versatile group of materials, with polyetheretherketone (PEEK) standing out as a favoured polymer in various medical fields such as orthopaedics, trauma care, and spinal implants (Kurtz, 2012). This article explores PEEK's thermal properties, biocompatibility, resilience to steam and gamma sterilization, and applications in medicine.
Fig. 1: image of PEEK filament.
Thermal Properties
With a molecular structure of (C19H12O3)n and chemical structure as seen in Fig. 2a, PEEK hosts a range of desirable traits, notably its exceptional heat resistance. Despite its stiffness stemming from the aromatic benzene backbone, PEEK's molecular chains exhibit flexibility and rotation around ether and ketone-carbon bonds. Upon gradual cooling from a molten state, PEEK forms disordered amorphous microstructures alongside ordered crystalline formations crucial for its material integrity, as seen in Fig. 2b (Kurtz, 2012).
Fig. 2a (left): chemical structure of PEEK (Carbon, 2024).
Fig. 2b (right): Schematic representation with amorphous and crystalline structures of PEEK (Kurtz, 2012).
Kurtz (2012) highlights four pivotal thermal transitions in polymer heating processes: the glass transition temperature (143 °C), melt temperature (343 °C), flow temperature (400 °C), and recrystallization temperature (280 °C – 320 °C) (Tardif et al., 2014) (Victrex, 2016). These transitions, occurring well above water's boiling point and practical medical temperatures, underscore PEEK's robust heat resistance. The polymer's aromatic backbone, a core component of its monomer unit, ensures its stability during various treatments. However, rapid cooling may induce continuation unwanted crystallization or recrystallization, leading to volume loss, increased density, and component shrinkage, rendering the material unsuitable for use. Therefore, precise temperature control during PEEK processing is imperative for optimal product performance.
Biocompatibility
Given PEEK's clinical applications, its biocompatibility emerges as a critical consideration. Biocompatibility assesses a biomaterial's non-toxic, non-mutagenic, non-carcinogenic, and non-immunogenic properties (Toth, 2019). Extensive studies administering PEEK in animal models have been done to demonstrate its biocompatibility.
Toxicity assessments involving submuscular and subcutaneous implantations in rats and rabbits revealed minimal reactions, with no cell lysis or adverse effects compared to controls without PEEK (Williams et al., 1987) (Wenz et al., 1990). Other investigations on fibroblast proliferation and attachment demonstrated PEEK's ability to enhance treatment processes without inducing cytotoxicity or harm to the individual (Hunter et al., 1995).
Studies on mutagenesis potential in salmonella typhimirium mutants indicated that PEEK did not elicit mutagenic responses. Since PEEK administration failed to cause genotype variants of salmonella typhimirium with a mutation in their histidine-operon to grow in histidine-free environments (Katzer et al., 2002).
Additionally, immunogenicity studies underscored PEEK's non-inflammatory nature, with rare reported cases of immune responses to PEEK implants attributed to allergic reactions, rather than inherent immunogenicity (Maldonado-Naranjo et al., 2015) (Kofler et al., 2016). Coupled with the evidence of the absence of necrosis or swelling after PEEK injections at the spinal cord and nerve roots in rabbits, along with limited local inflammatory cell activity at the injection site, and normal appearance of nerve cells (Rivard et al., 2002), as seen in Fig. 4 and 5. Such findings suggest immunocompetent individuals should not experience PEEK allergy, nor should PEEK cause serious immunogenic responses. Once again highlighting the non-immunogenic properties of PEEK.
Fig. 3 (left): high magnification of test rabbit nerve root, showing accumulation of some inflammatory cells around the PEEK particles within connective tissue four weeks post-surgery.
Fig. 4 (right): high magnification of control rabbit nerve root, showing normal appearance four-week post-surgery (spinal cord tissue, ST).
3D printing utilizing PEEK
In the study exploring the impact of extrusion speed and printing speed on 3D printing using PEEK material for product stability, it was discovered that the correlation between extrusion speed and filament diameter significantly influences product stability. As detailed in Wu et al. (2015) 's research, elevated extrusion speed triggers fluctuations in the extruded filament's diameter, subsequently compromising dimensional accuracy and surface quality. Conversely, increased melt pressure during printing reduces surface defects in the extruded filament by ensuring consistency in filament dimensions, thereby elevating overall print quality (Geng et al., 2019). While fluctuating extrusion force can disrupt extrusion stability, the integration of optimized control algorithms can reduce such a phenomenon via enhancing the stability, dimensional accuracy, and surface quality of printed PEEK components. By precisely adjusting extrusion speed based on filament diameter and regulating melt pressure, these algorithms effectively mitigate extrusion process fluctuations, resulting in heightened precision, smoother surfaces, and improved overall stability of printed parts (Geng et al., 2019). This comprehensive approach underscores the significance of finely tuning extrusion parameters and their intricate control to achieve optimal printing outcomes with PEEK material.
Resilience to Steam and Gamma Radiation
Sterilization is a critical step preceding medical implantation, commonly achieved through steam and gamma radiation methods. Steam sterilization can introduce moisture into polymers via capillary action, altering their physical and mechanical properties through dilatational expansion and inducing stress (Ray, 2006). Conversely, gamma radiation induces chain scission, enhancing polymer crystallinity through cross-linking. If the irradiated polymer is melted and undergoes recrystallization, the presence of irradiation-induced cross-links inhibits the normal crystallization process. This interference disrupts the formation of distinct crystalline regions, leading to alterations in the polymer's mechanical properties (Premnath et al., 1999).
Despite these effects, studies have shown that sterilized PEEK maintains consistent elastic modulus and hardness values (Godara et al., 2007), indicating its resilience to sterilization processes and suitability for medical applications.
Applications in Medicine
Polyaryletherketone (PEEK) stands out in medical applications due to its unique properties of being easily designed and constructed with high precision and complexity as opposed to titanium mesh and bone grafts. Enabling precise and complex designs for reconstruction in maxillofacial, cranial, and frontal-orbital-temporal defects (Hanasonoet al., 2009).
Kim et al. (2009) successfully reconstructed defects in four patients using customized PEEK implants, yielding excellent aesthetic and functional results without adverse post-operative symptoms such as infection, extrusion, or malposition. Similarly, Jalbert et al. (2009) demonstrated the efficacy of PEEK in optimal primary reconstruction during large fronto-orbital lesion resections in 2014, achieving superior outcomes while reducing operating time and donor site morbidity.
Fig. 5: PEEK implant used in reconstruction of orbito-zygomatic and maxillary reconstruction after maxilla facial trauma; A) PEEK LT1 orbito-zygomatic maxillary implant; B) PEEK implant after implantation.
In the realm of dentistry, PEEK is gaining traction for its resistance to mechanical stress in the oral cavity. Lee et al. (2012) investigated the stress shielding and fatigue limits of PEEK dental implants, revealing impressive fatigue limits that surpass ISO standards for posterior tooth restorations. This underscores the promising future of PEEK as a material for dental implants.
In orthopaedics, studies by Steinbergn et al. (2013) highlighted the comparable biomechanical properties of CF-PEEK Optima implants paired with zirconia, alumina femoral, or Co/Cr/Mo heads to titanium devices, showcasing the potential for PEEK to reduce implant replacement frequency. Additionally, Li et al. (2015) lauded CFR-PEEK as an orthopaedic implant material due to its biocompatibility, mechanical durability, and safety profile, supporting its use in orthopaedic applications.
Furthermore, PEEK's utilization in cervical disc disease surgery has revolutionized treatments by mitigating issues like subsidence and bone re-absorption associated with autologous bone grafts. PEEK cages offer corrosion resistance, elasticity akin to natural bone (Hee & Kundnani, 2010), and storage of tissues like banked bone and allografts, aiding in stabilizing the anterior column of the lumbar or cervical spine (Klimo & Peelle, 2009). Studies comparing PEEK with titanium and carbon fibre cages, such as the work by Cabraja et al. (2012) between 2002 and 2007, have demonstrated comparable outcomes between PEEK cages and titanium cages in treating anterior cervical discectomy and fusion. Furthermore, a study indicated that anterior discectomy and fusion with PEEK cage significantly improved a patient with spinal curvature (Zhou et al., 2010) as seen in Fig. 7, reinforcing PEEK's viability as a material for medical devices.
Fig. 6: PEEK cage successfully restored and maintained spinal curvature throughout recovery process, as there is significant difference in degree of spinal curvature between before surgery (a) (3.86 ± 0.87 mm) and immediately after surgery (b) (8.91 ± 0.93 mm). Whereas difference in degree of spinal curvature between immediately after surgery (b), 6 months after surgery (c) (8.78 ± 1.01 mm), and final follow-up examination (d) (8.82 ± 0.97 mm) were not significant.
Recent advancements have extended PEEK's applications to cardiac disease treatment, as seen in Leat & Fisher’s (1994) investigation of a novel geometry for polyurethane leaflet heart valves. By utilizing a PEEK frame with specialized leaflet design, it showcased enhanced hydrodynamic performance, less pressure drops required to open the leaflets, and smoother leaflet opening characteristics, suggesting potential improvements in cardiac valve technologies.
Conclusion
In conclusion, PEEK has solidified its reputation as an invaluable asset in the medical field. Evidenced by its exceptional thermal properties, proven biocompatibility, and robust resilience to sterilization processes, PEEK stands out as a versatile material with a plethora of desired attributes sought in raw materials used to manufacture medical products. Its adaptability is emphasized when utilised in orthopaedics, trauma care, spinal implants, and even advancements in cardiac applications, all of which underscore its significance in improving patient outcomes and propelling medical innovation. As research and development in the medical field continue, PEEK will undoubtedly remain a crucial contender in production of medical devices and treatments, offering an excellent combination of strength, biocompatibility, and reliability that make it a preferred choice for medical professionals.
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