Powder bed fusion

PBF has been the primary technology adopted for 3D printing of medical devices. PBF of metallic materials can be further divided into subcategories of technologies based on the high energy source used to achieve fusion of the powder material. The two most common are electron beam melting and laser PBF. While both technologies have been used to produce metallic medical implants and there are advantages and disadvantages to each depending on the end part and its application, laser PBF has seen wider adoption. This wider adoption can be attributed to its ability to achieve smaller feature sizes and control complex geometry, which are advantageous for the fabrication of complex porous lattices to allow for osseointegration.

While PBF has unlocked an unprecedented opportunity to produce medical devices, there are some processing-related challenges that must be considered. First, the sensitivity of reactive metals to oxidation at high temperatures must be managed. In PBF, this is overcome by use of inert gases and/or a vacuum in the build chamber to reduce the risk of oxidation. Typically, oxygen levels are well below 100 ppm throughout the duration of the build. Further, the metallic material produced from PBF usually has a microstructure that differs from that of the wrought metal. Due to the layer-wise process, anisotropy in the grain structure is often observed, as well as a non-equilibrium phase. For example, titanium alloy is a biphasic material and the as-printed microstructure is a non-equilibrium alpha resulting from rapid heating and cooling during the PBF process. To homogenize the material microstructure, reduce residual thermal stresses, and improve the material properties, PBF parts typically require postprocessing thermal treatments. These include standard stress relieving, high-temperature annealing, as well as hot isostatic pressing (HIP) processes. Lastly, the as-printed surface of parts resulting from PBF can have roughness (R _a ) ranging from 5 to 50 μm due to the adherence of partially fused powder particles. While in some osseointegration applications this topography may be favorable, in those at risk of high wear, the surface of implants must be treated through chemical or physical methods.

Others

While PBF technologies have dominated the AM of implants to date, other technologies are poised to offer advantages for the production of metallic implants. Directed energy deposition (DED) has been investigated for printing of multimaterial components due to its ability to switch between feedstock more easily, which allows the local deposit of different materials or the introduction of material gradients. ^, One such example would be a hip stem with a porous titanium alloy stem and a CoCrMo head for articulation.

Similarly, material extrusion and binder jetting (BJ) of metallic parts have been demonstrated in both research and commercial settings. However, the material selection and geometric complexity are currently limited in these processes. Further, both technologies require a postprinting workflow that includes debinding and sintering steps. One major hurdle for these technologies in implantable medical devices will be complete removal of the binder material to ensure no contamination occurs in the body.

Thermoplastics

Thermoplastics are polymers that can be melted when exposed to temperatures above their melting temperature. In thermoplastics, the polymer chains are held together by secondary bonds which form physical crosslinks such as hydrogen bonds, which are reversible. The physical crosslinks dissipate when heated past a critical temperature but reform when cooled, granting thermoplastics their reprocessable nature. Reprocessable refers to the capability of an already shaped polymer to be melted and reshaped. Because of these properties, thermoplastics lends themselves well to fused filament fabrication (FFF) and PBF 3D printing.

Poly(aryl ether ketone) polymers

Poly(aryl ether ketone) (PAEK) is a relatively new family of thermoplastic that is composed of a backbone of paraphenylene groups interspersed with ketone and ether groups. Prominent polymers in the PAEK family include poly(ether ether ketone) (PEEK), poly(ether ketone) (PEK), and poly(ether ketone ketone) (PEKK). Of the members of the PAEK family, PEEK is the most commonly used in commercial applications. PEEK is a polymer with a high melting temperature (allowing for steam sterilization), radiolucency, and excellent mechanical and chemical resistance properties. PEEK also has a stiffness that is closer to bone than metal, minimizing stress shielding. In medical devices, PEEK has vast applications, particularly in orthopedics. It is also one of the few polymers commonly used for load-bearing orthopedic implants. PEEK has a long and established history of clinical use, exhibiting osteoconduction and biocompatibility. Some examples of PEEK implants include internal fixations for bones, spinal cages, and craniofacial implants. PEEK also has nonimplant applications as surgical instruments and its stability allows for autoclaving, enabling sterilization, and repeated use.

The most common processing method for PEEK is injection molding, which is commonly used as a benchmark for 3D-printed samples. PEEK is amenable to both extrusion and PBF 3D printing. 3D printing of PEEK is a frequent research subject because of its perceived potential for commercial applications. In past studies of PEEK FFF 3D printing, a wide variety of resulting properties have arisen because of the wide-ranging testing parameters and set up used by the authors. In general, printed PEEK is highly anisotropic and has vastly inferior mechanical properties to injection-molded PEEK. ^, Many approaches have been proposed to mitigate the reduction in mechanical properties, such as annealing/heat treatment, adjustments to the printing environment, and other print parameter optimizations ; however, none have yielded significant improvement on the observed anisotropy. Currently, no commercial PEEK implant is 3D printed via FFF. Studies on PBF 3D printing of PEEK have generally demonstrated mechanical properties that are superior to FFF printing. ^, However, although these studies did not test for anisotropy, printed PEEK is expected to be anisotropic, as has been shown in other PBF-printed materials. ^, Currently, to the best of the authors’ knowledge, there is no commercial PBF-printed PEEK implant.

PEKK is another member of the PAEK family that is useful in orthopedic implants. Chemically it is similar to PEEK, but with one ether group and two ketone groups interspersing the paraphenylene group backbone. In orthopedics, PEKK is used in similar applications to PEEK because its properties are in many ways comparable. One of the earliest 3D-printed polymer implants cleared by the FDA is a craniofacial device (OsteoFab, Oxford Performance Materials) fabricated out of PEKK via selective laser sintering (SLS). Currently, there are also a number of FDA-cleared PEKK 3D-printed load-bearing spinal implants (SpineFab, Oxford Performance Materials; Tetrafuse, RTI Surgical) fabricated via SLS ( Fig. 3.5 ). A key advantage for PEKK 3D printing compared to PEEK 3D printing is printed PEKK’s superior properties, a direct result of its thermal behavior which is suited for PBF printing.

Commercial 3D-printed orthopedic implants. (A) OsteoFab Patient Specific Cranial Device. (B) SpineFab VBR System.

( https://oxfordpm.com/osteofab-medical-devices )

Silicone

Silicone, or polysiloxanes, are polymers containing a siloxane (Si-O-Si) linkage. Silicones are used in many applications owing to their very versatile properties. Medical device applications for silicones are particularly vast because of their biocompatibility and chemical stability. An example of clinical applications of silicones include contact lenses and catheters. In orthopedics, polysiloxanes are used in hand and foot joint implants. 3D printing of polysiloxanes is commonly performed via direct extrusion and a base silicone material is typically mixed with substances to achieve a consistency that is suitable to be used as a printable ink.

Photopolymers

Photopolymers are polymers that are crosslinked under exposure to light of a particular wavelength and are printed using vat photopolymerization and material jetting (MJ). Most photopolymers are not reprocessable, hence they are often a subset of thermoset polymers. A photopolymerization system is typically composed of three components: a mixture of monomers and oligomers, photoinitiators, and other additives such as stabilizers and photosensitizers. Most commercial mixtures are proprietary, and their list of components are not readily searchable. The most common photopolymer systems are described here. It is important to note that, in practice, these different systems are often combined into a complex mixture to improve properties.

Currently, commercial photopolymers find most of their applications in nonimplant devices such as surgical planning aids and guides. Commercial photopolymers have a wide range of mechanical properties and are suitable for most nonimplant applications. However, to the best of the author’s knowledge and at the time of writing, no commercial photopolymer has been present in an FDA-cleared human implant.

Extrusion

The main advantages of extrusion-based polymer printing are its simplicity and a large library of material that is available for use, some of which possess an established clinical use history. However, structures with overhanging sections are difficult or impossible to print via extrusion and should be minimized during device design.

Printing parameters that are important to polymer extrusion are temperatures, and print pathing and orientation. Typically, the overarching objective of adjusting these parameters are to (1) achieve good mechanical properties, and (2) print parts with good fidelity. In achieving good mechanical properties, layer bonding is the primary consideration. For good part fidelity, the extruded polymer needs to maintain its shape in a predictable way when extruded out to form layers.

Nozzle and bed temperature are considered one of the most important print parameters for melt-based extrusion. Excessively high temperature leads to polymer degradation and undesired spreading of the molten polymer, while inadequate temperature leads to low polymer flow and weak layer bonding. In addition, temperature also has a more nuanced effect on printed polymers, such as by affecting crystallinity. Print pathing and orientation are also major factors in improving print quality. Print pathing includes aspects such as layer thickness, path width, raster pattern, and many others, whereas print orientation refers to the orientation at which the object is printed. Broadly speaking, parameters that maximize part density (such as smaller layer thickness) and direct mechanical loads away from layer bonding surfaces (such as via raster pattern adjustment) will improve quality.

There are a vast number of parameters that can be adjusted for extrusion printing. All these parameters are interrelated; therefore, it is important to evaluate each printed part using the exact same parameter as production parts, including the same environment, same device, and same material. In orthopedics, extrusion printing has found the most use in surgical planning models. The simplicity of extrusion systems also means it can be used on site in clinical settings. At the time of writing, to the best of the author’s knowledge, no commercial orthopedic device is fabricated via extrusion printing.

Vat photopolymerization

VP printing is a process unique to polymers. The main advantage of VP is its ability to print complex structures. VP-printed parts are also accurate, with a smooth surface finish. This has led VP parts to find use in applications that require accurate form and fitting. However, photopolymers tend to be nonbiocompatible for implant use. Scarce clinical usage history is a major obstacle for use as an implant by device developers. In comparison to injection-molded parts, VP parts have lower durability and strength, in addition to the anisotropy common with 3D-printing techniques.

From a processing perspective, an ideal resin mixture for VP would have low viscosity, minimize undesired light leakage, polymerize rapidly, and have no shrinkage when cured. The formulation of the resin strongly affects this. Important parameters in VP include cure depth and scan pathing. Cure depth is a commonly used VP measure that includes the effect of laser power, laser penetration, laser speed, resin cure-threshold, and laser spot size. Along with cure depth, scan pathing is an important parameter that is able to minimize residual stress and shrinkage when adjusted well. It is also worth noting that VP systems can use different types of radiations (e.g., ultraviolet [UV], visible light, or infrared) to initiate photopolymerization.

Research directions in VP include novel VP approaches such as two-photon microprinting. In the context of medical devices and orthopedics specifically, there is much interest in the development of novel photopolymers that are biocompatible for implant application. Some past research on this subject includes the development of biocompatible hydrogels, ^, biodegradable scaffolds, filler-added resins, and so on. In orthopedics, VP finds most often used in nonimplant applications.

Powder bed fusion

PBF of polymers has the distinct advantages of both being able to fabricate complex structures and having a large library of biocompatible thermoplastics. However, a limiting factor of PBF is that it requires that the polymers have a narrow set of thermal properties to print viably. The most common variety of PBF printing for polymers is SLS. For nondegradable polymers, the material that is by far the most commonly used for PBF is polyamides, making up more than 95% of the current PBF market, with PA12 and PA11 being the most common varieties.

In SLS printing, the major build parameters are laser power, bed temperature, layer thickness, scan spacing, scan speed, and part raster pattern. Laser power, scan spacing, and scan speed are often bundled and expressed as the amount of energy delivered by the laser to the polymer powder. Appropriate energy delivery by laser and bed heating is important for printing serviceable parts. Powder properties are also well documented to be crucial to print quality, which are primarily affected by powder morphology. In addition, environmental effects such as humidity and electrostatic charging should also be taken into account. ^, Another important consideration in polymer PBF is powder aging and reuse. The ability to reuse polymer powders from prior print runs improves the economy of PBF printing significantly. Powders that are used in a print run are subject to thermal aging and often result in parts with inferior print quality, but a mitigation approach that is frequently used is to mix virgin powders with aged powders to minimize the decrease in part quality. Postprinting powder removal is also important to consider.

In the context of orthopedics, polymer PBF is used in both implant and nonimplant applications, and currently it is the only printing approach that has been used for commercial orthopedic implants. For implant applications, PAEK polymers are a common choice because of their vast clinical usage history and good printed properties. Commercial PAEK devices include a suture anchor, craniofacial implants, and spinal implants. For nonimplant materials, a common choice is any nylon variety, with common applications in surgical guides. Although there is obviously much potential in the use of PBF for printing orthopedic devices, currently there are relatively few commercially available powders that are optimized for PBF, with the majority of them being nonimplant grade.

Inkjet

Inkjet (IJ) printing is considered to be the most complex yet most accurate printing method, resulting in multicolor parts with a good finish. However, the printed parts are mechanically weaker than standard injection-molded parts. Material availability is also very limited compared to other printing methods because of stringent property requirements, and commercial resins are typically proprietary and machine specific. IJ can be classified into MJ and BJ. Both use photopolymers, although BJ processes can also use other substances for binding, such as solvents. Some processing considerations of IJ are similar to VP, wherein the photopolymer needs to cure properly during the build process. A crucial difference to VP is the need to consider the jetting process. Additionally, for BJ there is also a need to take powder properties into account. Postprint powder removal is also important in medical applications. In medical devices and orthopedics specifically, IJ is most often used for planning and visualization aids.