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Subjects:
Engineering, Biomedical
Biomaterial implants are utilized to fix fractures or replace parts of the body. For the majority of these implant cases, either metal or polymer biomaterials are chosen in order to have a similar functional capacity to the original bone material. Additive manufacturing, also known as 3D printing, is a manufacturing technique that creates 3D objects by adding material layer by layer. The use of biodegradable polymers, additive manufacturing, and advanced 3D printing technologies offers promising solutions for improving biocompatibility, mechanical properties, and customization in the development of implants and tissue engineering constructs.
- orthopedic
- bone
- biodegradable
- corrosion resistance
1. Introduction
Additive manufacturing, also known as 3D printing, is a manufacturing technique that creates 3D objects by adding material layer by layer. It is considered the third pillar of overall manufacturing technology, alongside subtractive manufacturing techniques such as milling or lathing, and formative manufacturing techniques such as casting or forging. Additive manufacturing was formerly known as rapid prototyping (RP) and is now commonly referred to as 3D printing. In 1984, Chuck Hull developed stereolithography, the first additive process for polymers, which was later commercialized. He coined the term “stereolithography” and patented the technology in 1986 [233,234]. There are various additive manufacturing techniques, including fused-deposition modeling, 3D inkjet printing, stereolithography, direct powder extrusion, and selective laser sintering. These techniques involve digitally controlled layer-by-layer deposition of materials to create different geometries of printlets [235].
Additive manufacturing has emerged as a prominent research topic in the past decade due to its low cost, ease of use, and the reliability of 3D printing equipment. It enables the straightforward and customized production of complex 3D structures and components through the layer-by-layer deposition of materials, eliminating the need for specialized tools or molds [234,236].
Additive manufacturing offers a unique opportunity for fabricating personalized dosage forms, which is crucial in addressing the diverse medical needs of patients. Despite having been in existence for four decades, AM has only recently gained wider usage in both surgical and non-surgical fields [48].
Furthermore, 3D printing is becoming a popular method of producing medical devices for orthopedic applications, tissue engineering, and the rehabilitation of patients suffering from disabling neurological diseases such as spinal cord injuries and amyotrophic lateral sclerosis. This is due to 3D printing enabling the creation of patient-specific designs, highly complex structural elements, and affordable on-demand manufacture [237]. Moreover, human organs can be manufactured according to the principles of AM using specialized 3D bioprinters [238]. Techniques of 3D printing have great potential for fabricating porous, complex-formed substances, and forms with highly complex internal structures. As a result, 3D printing technology allows the creation of hierarchical substances with mechanical qualities (strength and elastic Young’s modulus) and porous structures comparable to natural bone, while reducing the stress-shielding impact created by orthopedic implants [239,240,241].
Although 3D printing enables researchers to create parts that meet these requirements, the majority of clinical work in orthopedics focuses on metallic biomaterials, and most commercial representation is centered around metal-related approaches. However, polymers and polymeric composites receive significant attention in bone engineering applications due to the strong similarities between their thermomechanical properties and those of tissues, as well as their biodegradability and biocompatibility [242]. Furthermore, 3D printing technologies offer several advantages, including mass production capability, economic efficiency, and repeatability [243]. Moreover, when combined with computer-aided design (CAD) [244], 3D technology can be used to create completely patient-specific implants [245,246].
Despite the significant advances that have been made in 3D printing technology, there are still notable problems to overcome. These include software design, standardization and integration of a comprehensive bio-fabrication platform, repeatability, limitations of 3D printers’ capabilities, biomaterial characterization, regulatory hurdles, and quality by design. Addressing these challenges is crucial for 3D printing to be recognized as a traditional bio-manufacturing method in medicine and to gain access to the medical market. Among these challenges, the lack of heterogeneous biomaterials that would enable their reliable clinical utilization is the main obstacle [237].
2. Additive Manufacturing of Metallic Implants
Over the years, porous metal biomaterials have been fabricated using conventional manufacturing techniques, primarily based on powder metallurgy, such as metal injection molding and spacer processes. Although these fabrication techniques have made remarkable progress, certain limitations still exist, including the inability to precisely control pore shape and distribution, as well as dimensional inaccuracies [247]. Additionally, implants manufactured using conventional processing methods, known as standard-type implants, cannot match the structure, performance, and required physical and chemical characteristics for addressing specific bone flaws. This limitation restricts the therapeutic efficacy and longevity of implants [52].
On the other hand, additive manufacturing (AM) of metallic biomaterials demonstrates excellent medical potential, encompassing prostheses, implants, drug delivery systems, scaffolds, and stents. The process of AM involves creating and manufacturing 3D designs, which can be achieved using a CAD program. Furthermore, these 3D designs can be manufactured using various techniques such as fused deposition modeling (FDM), selective laser melting (SLM), and selective laser sintering (SLS) [248].
AM technology offers several advantages, including more precise fabrication and greater flexibility in designing both the internal and external macro- and micro-architectures of orthopedic implants [249]. The geometrical and topological porosity qualities of metallic biomaterials can be accurately tuned through controlled AM manufacturing techniques, improving their mechanical properties to mimic bone [250,251]. This leads to improved rates of bone tissue regeneration [252,253,254], altered biodegradation kinetics [255,256], and the formation of a vast, interconnected osteocyte lacuno-canalicular network [257,258].
However, certain characteristics such as wear resistance, hardness, anti-ferromagnetic properties, or antibacterial characteristics cannot be easily modified through geometrical design alone, as these properties require modifications to the underlying base material(s) prior to AM processing [247].
3. Additive Manufacturing of Polymeric Implants
Polymers play a crucial role in 3D printing manufacture due to their versatility, excellent processability, and compatibility with various AM processes [48,259]. They possess notable characteristics such as surface detailing, high precision, temperature resistance, accuracy, and improved strength [260]. In the realm of additive manufacturing, polymers contribute significantly, accounting for 51% of the polymer parts produced, 29% of metal and polymer combinations, and 19.8% of metal products [261]. Reactive monomers, thermoplastic filaments, powder, and resin are commonly utilized forms of polymers in AM techniques [259].
Despite the wide array of available AM techniques, advancements in polymer printing primarily focus on three key strategies: (1) powder bed fusion processes such as selective laser sintering (SLS), (2) deposition-on-demand processes, including extrusion-based technologies such as fused deposition modeling (FDM) and direct-ink-write printing [262], as well as inkjet or drop-wise deposition methods, and (3) photo-polymer-based printing techniques, such as stereolithography (SLA). These printing methods have successfully incorporated various polymers as raw materials [259,263].
The production of polymer composites through 3D printing entails both advantages and drawbacks, with each method having specific requirements regarding the polymer’s structure, state (liquid or solid), and physical characteristics (melting temperature and viscosity) [264]. When it comes to load-bearing applications, the range of materials that can be employed for polymer AM is comparatively limited compared to other applications. For instance, commonly used polymers in SLS processes for load-bearing applications include PEEK, UHMWPE, PMMA, PLA, PCL, polypropylene (PP), polyvinyl alcohol (PVA), and polyamide (PA) [48].
This entry is adapted from the peer-reviewed paper 10.3390/polym15122601
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