Material science for 3D printing in medicine

Materials science and 3D printing

As discussed in Chapter 1 , materials for 3D printing include polymers, ceramics, metals, and composites. The materials used to fabricate a 3D part are dependent on the performance, or the “requirements” of the final part for its application. Material selection is also dependent on the 3D-printing technology used, as not all materials can be manufactured using all available 3D-printing methods. Thus, the intersection of material science with 3D printing has grown into a vast and popular field since the 2000s. Continued work in this field will produce a better understanding of materials, manufacturing, and their use in medical applications.

Processing-structure-property-performance relationships

In material science, processing-structure-property-performance relationships define the correlation between the way a part is made and the resulting behavior and application of the part. Processing refers to the steps a raw material undergoes during the manufacture of a final part. This can include both primary and secondary processing steps. For a 3D-printed part, additive manufacturing is typically the primary processing step in which a raw material in the form of a filament, resin, powder, or other type, is converted to a 3D part. Secondary processing of the part may include heat treatment, surface treatment, or other additional steps to achieve the desired structure and therefore properties. The performance, or function, of the part is dependent on its various properties. In orthopedic applications, performance is typically driven by a tradeoff between the mechanical and surface properties of a given application. For example, a hemiarthroplasty implant must be high strength and fatigue resistant, as well as have a bearing surface with a low coefficient of friction to minimize wear. These relationships help material scientists and engineers to select materials for a given application, as well as to discover new materials.

Materials selection

As described earlier, processing of a material defines its structure, and thus the resulting material properties. In load-bearing orthopedic applications, mechanical (structural) performance is typically a priority when designing an implant. In additive manufacturing, due to the wide selection of materials and processing technologies, a top-down strategy can be employed to first define the desired properties of a part for a given application. For example, for a tibial tray of a total ankle prosthesis, engineering requirements may prescribe a minimum strength before failure, as well as a coefficient of friction of the surface interfacing with the polyethylene insert, amongst other requirements. These properties and their interrelationships must be considered in order to define the appropriate processing steps to achieve the end goal.

Most importantly, these properties help to define the materials that can be used for the part. Material selection charts, also often called Ashby plots, are often used in these cases to identify materials that can meet defined the specifications for a given application ( Fig. 3.1 ). The following sections give an overview of the current state of the art for materials produced by 3D printing for medical applications and biomedical research.

Aside from selecting the type of material (metals, polymers, ceramics, or composites thereof), there are other important material and final implant characteristics that need to be addressed for the successful fabrication of a 3D-printed orthopedic device ( Fig. 3.2 ).

Titanium and titanium alloys

Titanium, both commercially pure (CP) and the most common alloy, Ti6Al4V, have long been used in the human body for various load-bearing and nonloaded applications. In fact, the first 3D-printed implant cleared for use by the FDA was a titanium alloy acetabular hip cup manufactured by Exactech, Inc. Due to its high strength and fatigue resistance, Ti6Al4V is the standard choice for load-bearing orthopedic implants. The manufacture of porous implants with complex, interconnected lattices has been a primary use case for additive manufacturing in implants. Of the devices produced by additive manufacturing (AM) with a porous lattice, 72% are Ti6Al4V. These include devices such as spinal fusion cages, osteotomy wedges, and arthroplasty components, where osseointegration of the implant is critical to a successful clinical outcome. Thus, research efforts at the intersection of AM and the biomedical field are focused on optimization of porous lattices to improve the efficacy of such implants. This includes investigation of the tradeoff between design and manufacturing, the mechanical properties of porous scaffolds, and the associated in vivo performance. ^,

Nickel titanium (NiTi) is another alloy used in medical device applications. To date, no additively manufactured implants fabricated from NiTi have been cleared by the FDA. However, there many research efforts have been dedicated to overcoming the challenges related to additive manufacturing of the alloy. AM has also led to the development of new alloys that are designed with compositions that are well suited for the additive process. One such example is titanium niobium (TiNb) alloys, which have received increasing research attention due to their ability to reduce the bulk material modulus while maintaining high strength similar to Ti6Al4V. ^, Challenges in the 3D printing of titanium and its alloys include the sensitivity to oxidation, which is discussed further here. This is a particular challenge for NiTi, where even small changes in atomic composition drastically effect the material’s properties, and the ability to achieve super elasticity.

Cobalt alloys

Cobalt chromium (Co ₂₈ Cr ₆ Mo) is a high-strength alloy with a low coefficient of friction, which facilitates its use in articulating applications where reduction of wear is critical ( Fig. 3.3 ). While its adoption into the additive manufacture of implants has lagged behind titanium alloys, there are an increasing number of implants that use this alloy, including the talar implant for a total ankle arthroplasty system (Wright Medical Infinity TAR). Similar to titanium, CoCrMo alloy forms a spontaneous oxide layer. Thus, similar solutions to allow high-temperature processing with oxidation of the surface are employed. Additionally, the resulting surface of as-printed parts requires significant postprocessing to reduce the roughness to within the specifications defined for articulating arthroplasty components (<0.05 μm per ASTM 2033).

Other metals

Stainless steels, including 17-4 and 316L, are commonly used in surgical applications for both implants and instruments. The use of powder bed fusion (PBF) for producing stainless steel implants has been less widely adopted than for titanium alloys. However, opportunity exists to use 3D printing for the production of stainless steel plates or other orthopedic applications.

3D printing of precious metals has also been demonstrated in the scientific literature; for example, pure silver bone scaffolds which exhibit antibacterial properties. Other work has investigated the addition of copper (Cu) or gold (Au) to traditional alloy systems to impart antimicrobial properties to the implant. These additions to the alloy system also have an effect on the mechanical properties of the materials, which needs ongoing research to be better understood.

A number of bulk metallic glass systems have also been under investigation for the AM of implants, including Ti ₄₇ Cu ₃₈ Zr _7.5 Fe _2.5 Sn ₂ Si ₁ Ag ₂ and Zr _52.5 Cu _17.9 Ni _14.6 Al ₁₀ Ti ₅ . The advantage of bulk metallic glass is its ability to reduce stiffness while maintaining a high strength. Additional degradable metals such as magnesium, zinc, and iron-based materials have also been discussed for their use in bone-regeneration applications in the “Degradable Materials” section.

3D-printing technologies for metals

Introduction

A polymer is a macromolecule composed of smaller repeating units known as monomers. A polymer is formed when monomers are linked together by a primary bond that is typically covalent in nature. Colloquially, a subset of synthetic polymers is better known as plastics. Polymers are of significant interest in medical technology owing to their unique characteristics and large range of properties. For example, the mechanical properties of polymers can range from being akin to gel, rubber, or hard plastic. Many common polymers also possess good processability, making them amenable to 3D printing. These qualities have led to a vast number of commercial products and scientific research that utilize 3D-printed polymers for medical applications. In orthopedics, commercial applications of 3D-printed polymers have ranged from simple surgical guides and instruments to a bioresorbable bone graft cage implant. The types of 3D-printed polymers available for orthopedic implants are limited due to stringent biocompatibility requirements but a much larger range of polymers is available for nonimplant applications (e.g., surgical guides, surgery planning aids, orthoses).

The properties of a polymer are largely determined by its chemistry, structure, and morphology. By varying these aspects, polymers can be tuned to achieve specific objectives and improve the performance of a device. The chemistry of a polymer is the combination of atoms contained in a polymer chain and the manner in which they are bonded to one another. The structure or configuration of a polymer refers to the way the atoms in a polymer are arranged, which could vary in a polymer with the same chemistry, such as in linear versus branched polymers. The morphology of a polymer is the arrangement of polymer chains in relation to all other polymer chains and any ordering associated with it, and the morphology is strongly related to its processing history. In addition to these three factors, the property of a polymer can also be altered by using additives to achieve specific goals. For example, plasticizers are commonly added as an additive to promote plasticity and reduce the brittleness of a polymer.

Nondegradable polymers used in orthopedics 3D printing

From the perspective of orthopedic applications, there are some 3D-printable polymers that are frequently in use in both scientific research and commercial products. These polymers are described and their applications in the context of orthopedics are presented in the following section. All the polymers shown here are discussed in the context of their use as a nondegradable material, which is defined as a material that does not degrade appreciably within its service life.

Most polymers commonly used for 3D printing are not suitable for implant applications. The requirements for implant materials are much more stringent than nonimplants and include specifications spanning from biocompatibility and physical properties, to mechanical properties. Generally, nonimplant applications (e.g., surgical planning aids) only require two properties: safety for short-term contact and suitable mechanical properties ( Fig. 3.4 ).

In this chapter, synthetic polymers are categorized into thermoplastics, thermosets, and photopolymers. This classification reflects the nature of their processing, which in turn strongly determines the 3D-printing methods available to them.

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.

Nylon

Nylon is a colloquial name for a group of polymers based on aliphatic polyamides. An aliphatic polyamide is a polymer with aliphatic repeating units connected by amide bonds. There is a large variety of nylon available, the most common being PA6, PA66, PA11, and PA12. Nylon is a semicrystalline thermoplastic polymer with high toughness and strength, and is steam sterilizable. It also has a wide processing window, allowing it to be processed in a large variety of ways. Owing to this processing window, current PBF printing of polymers mainly uses nylon. In medical devices, nylon is often used in sutures and in short-term devices such as balloon catheters; nylon is rarely used long term. In orthopedics, 3D-printed nylon sees use in nonimplant applications such as surgical guides and tools due to its excellent mechanical properties and its amenability to PBF 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.

Other thermoplastic polymers

Polymers that find some use in 3D printing for orthopedic research and commercial devices are discussed here:

• 1.

  Acrylonitrile butadiene styrene (ABS): ABS is one of the most common polymers for FFF 3D printing, especially in a home 3D printer setting. Outside of medical applications it is often used in products such as toys and enclosures for consumer electronics. In medical devices, ABS finds the most use outside of implants. In orthopedics, FFF 3D-printed ABS is used in the fabrication of surgical planning aids as it is sterilizable using ethylene oxide.

• 2.

  Polylactic acid (PLA): PLA is another common polymer in FFF 3D printing along with ABS. It is an aliphatic polyester composed of lactic acid. It has favorable properties for processing via extrusion and molding. In medical devices, PLA is most often used in implant applications requiring bioabsorbability. In orthopedic applications without need for bioabsorbability, FFF 3D-printed PLA has been used in surgical planning aids.

• 3.

  Ultra-high-molecular-weight polyethylene (UHMWPE): UHMWPE is a common engineering polymer with excellent mechanical properties and good wear resistance, self-lubricating properties, and biocompatibility. In orthopedics, it is commonly used in hip, knee, and spine implants for articulating surfaces. The 3D-printing method of choice for UHMWPE is PBF. However, its high melt viscosity hinders optimal printing via PBF, resulting in inferior properties. At its current state, 3D printing of UHMWPE appears to be unsuitable for use in common UHMWPE orthopedic implants.

• 4.

  Polymethyl methacrylate (PMMA): PMMA, also known as bone cement, is a ubiquitous polymer in orthopedics. PMMA is also used in other implant applications, such as in craniofacial implants (e.g., PMMA Customized Implant, Stryker). Its thermoplastic nature allows it to be printed using extrusion and PBF methods. PMMA can also be printed via BJ. However, the printed parts possess significantly inferior mechanical properties and/or lower density compared to injection-molded samples.

• 5.

  Polypropylene (PP): PP is a semicrystalline polymer composed of propylene monomers and the second most produced thermoplastic in the world behind polyethylene. PP is a widely used material in FFF 3D printing. It is also one of the most used polymers in medical devices, frequently seen in sutures, packaging, noncontact devices, and implants. In orthopedics, an example of PP application is in ligament augmentation devices. SLS-printed PP shows properties close to injection-molded PP.

• 6.

  Thermoplastic polyurethane (PU) : PU belongs to a large family of polymers that are used in a large variety of industries. PU’s main characteristic is its urethane links that joins its monomers together. Polycarbonate urethane (PCU) is a member of the PU family that has garnered much attention for its potential use in orthopedics, particularly as articulating surfaces for knee implants, ^, and has been shown to be very amenable to FFF 3D printing, having similar properties to injection-molded samples.

• 7.

  Polysulfones (PSUs): PSUs are a group of high-performance polymeric materials. Similar to PAEKs, PSUs possess high hydrolytic resistance, high temperature resistance, and good mechanical properties. They are often used in reusable devices requiring sterilization such as surgical trays and sizers for implants. Commercial PSU 3D-printing filaments are available for extrusion printing, such as Ultem 9085 (Stratasys). Extrusion-printed PSUs have shown potentially similar properties to injection-molded PSUs in past studies.

Thermosets

Thermosets are chemically crosslinked polymers with a highly restricted chain motion. Because of the covalent nature of the crosslinks, thermosets are not reprocessable, unlike thermoplastics and their physical crosslinks.