The manufacture of polyetheretherketone/hydroxyapatite (PEEK/HA) composites is seen as a viable approach to help enhance direct bone apposition in orthopaedic implants. A range of methods have been used to produce composites, including Selective Laser Sintering and injection moulding. Such techniques have drawbacks and lack flexibility to manufacture complex, custom-designed implants. 3D printing gets around many of the restraints and provides new opportunities for innovative solutions that are structurally suited to meet the needs of the patient. This work reports the direct 3D printing of extruded PEEK/HA composite filaments via a Fused Filament Fabrication (FFF) approach. In this work samples are 3D printed by a custom modified commercial printer Ultimaker 2+ (UM2+). SEM-EDX and µCT analyses show that HA particles are evenly distributed throughout the bulk and across the surface of the native 3D printed samples, with XRD highlighting up to 50% crystallinity and crystalline domains clearly observed in SEM and HR-TEM analyses. This highlights the favourable temperature conditions during 3D printing. The yield stress and ultimate tensile strength obtained for all the samples are comparable to human femoral cortical bone. The results show how FFF 3D printing of PEEK/HA composites up to 30 wt% HA can be achieved.

Polyetheretherketone (PEEK) is a high-performance thermoplastic polymer which has found increasing application in orthopaedics and has shown a lot of promise for ‘made-to-measure’ implants via additive manufacturing approaches. However, PEEK is bioinert and needs to undergo surface modification to make it at least osteoconductive to ensure a more rapid, improved, and stable fixation that will last longer in vivo. One approach to solving this issue is to modify PEEK with bioactive agents such as hydroxyapatite (HA). The work reported in this study demonstrates the direct 3D printing of PEEK/HA composites of up to 30 weight percent (wt%) HA using a Fused Filament Fabrication (FFF) approach. The surface characteristics and in vitro properties of the composite materials were investigated. X-ray diffraction revealed the samples to be semi-crystalline in nature, with X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry revealing HA materials were available in the uppermost surface of all the 3D printed samples. In vitro testing of the samples at 7 days demonstrated that the PEEK/HA composite surfaces supported the adherence and growth of viable U-2 OS osteoblast like cells. These results demonstrate that FFF can deliver bioactive HA on the surface of PEEK bio-composites in a one-step 3D printing process.

Neck and lower back pain, often caused by spinal disorders such as scoliosis and degenerative disc disease, affects over 80% of the global population, with an estimated from 250,000 to 500,000 spinal cord injuries occurring annually according to the WHO. As the demand for spinal procedures continues to rise, advancements in implant materials have become essential. Orthopedic implants play a vital role in restoring mobility and improving the quality of life of patients with musculoskeletal disorders. Metallic implants, such as stainless steel, titanium, and its alloys, are commonly used to make fixation devices for spinal fusion surgery due to their excellent mechanical properties. However, complications such as stress shielding have been recorded. Polymeric materials offer new prospects as an alternative to metal-based materials such as those based on Polyaryletherketone (PEAK). Among the advanced materials used in these implants, PAEK has emerged as the preferred choice due to its exceptional mechanical strength, thermal stability, and chemical resistance. Polyetheretherketone (PEEK) and Polyetherketoneketone (PEKK) offer notable advantages, such as radiolucency and mechanical properties resembling those of natural bone, reducing stress shielding and facilitating postoperative imaging. Although PEEK and PEKK are considered as bioinert, it has been demonstrated that adding bioactive agents such as hydroxyapatite (HA) into the matrix to make composites solves this problem and can help with aiding direct bone apposition. Furthermore, PAEK’s compatibility with 3DP enables the creation of patient-specific implants with intricate geometries, enhancing the surgical outcomes. In addition, the lattice structures of orthopedic implants can alleviate stress shielding, provide an enhanced surface area for the release of bioactive agents (or antimicrobial materials), and eliminate more imaging artifacts compared to that of simple, solid metal implants. PAEK/HA composite implants represent a transformative solution, addressing the psychological, social, and economic burdens of spinal disorders, while enhancing the surgical outcomes. With continuous technological evolution, PAEK/HA composites are poised to play a pivotal role in modern spinal care.

Additively manufactured implants, surgical guides, and medical devices that would have direct contact with the human body require predictable behaviour when stress is applied during their standard operation. Products built with Fused Filament Fabrication (FFF) possess orthotropic characteristics, thus, it is necessary to determine the properties that can be achieved in the XY- and Z-directions of printing. A concentration of 10 wt% of hydroxyapatite (HA) in polyetherketoneketone (PEKK) matrix was selected as the most promising biomaterial supporting cell attachment for medical applications and was characterized with an Ultimate Tensile Strength (UTS) of 78.3 MPa and 43.9 MPa in the XY- and Z-directions of 3D printing, respectively. The effect of the filler on the crystallization kinetics, which is a key parameter for the selection of semicrystalline materials suitable for 3D printing, was explained. This work clearly shows that only in situ crystallization provides the ability to build parts with a more thermodynamically stable primary form of crystallites.

Printing with high-performance polymers such as polyether ether ketone (PEEK) and polyetherimide (PEI) presents issues regarding shrinkage and warpage due to elevated temperatures. One method highlighted to mitigate against this is through polymer blending. This study explores the development and characterization of PEEK and PEI blends as filament for fused filament fabrication (FFF) in additive manufacturing. Filaments were produced via melt extrusion using PEEK/PEI weight ratios 100/0, 80/20, 70/30, 60/40, 50/50, 40/60, 20/80, and 0/100 (wt.%). The aim is to identify an optimum blend which enhances printability and maintains mechanical and thermal integrity. The extruded filaments were first characterized through differential scanning calorimetry (DSC) to determine miscibility with all ratios presenting a single glass transition temperature. Samples were then 3D-printed and assessed through mechanical testing, DSC, X-ray diffraction (XRD), and scanning electron microscopy (SEM). The PEEK/PEI 80/20 (wt.%) blend was recognized as the optimum blend for maintaining crystallinity (35%) as well as good mechanical properties, averaging ultimate tensile strengths (UTSs) of 75.6 MPa and a Young’s modulus of 1338 MPa. Thermal properties also improved while warpage reduced and printability improved.

This review investigates the challenges of Additive Manufacturing using commodity, engineering, and advanced materials, both amorphous and semicrystalline. It explains the reasons behind the weaker mechanical performance of semicrystalline materials compared to amorphous polymers used in the 3D printing process. The performance of 3D printing is discussed to demonstrate the current position of additive manufacturing as one of the promising techniques meeting the requirements of the 5.0 Industrial Revolution, particularly in terms of delivering personalized products. The differences between amorphous and semicrystalline materials on a macromolecular level, including the strength of the single bond in the polymer backbone chain and the effect of electron donation to the backbone, are discussed. Interlayer imperfections are classified into several groups: moisture in the feedstock filament, inconsistent filament diameter, shrinkage of the deposited materials, and, most importantly, crystallization kinetics of semicrystalline materials. Finally, insights on how to achieve properties closer to an isotropic body when advanced semicrystalline materials are printed, in order to overcome layer-layer defects, are provided.