The medical segment - as a whole - is one of the first and most important adopters of additive manufacturing (AM)/3D printing technologies (we use these terms interchangeably). In a recent report SmarTech Analysis (VA, USA) forecasted AM adoption in the medical field, including device manufacturing, surgical modeling and orthopedic implants to grow into an almost US$7 billion market within the next 10 years. Orthopedic implants are expected to be the largest segment, accounting for US$3.7 billion. But can all 3D printers really make ‘replacement parts’ that go into our bodies? The answer is yes, it all depends on the materials. The following are the biocompatible materials that are making it possible for each of the primary 3D printing technologies.
Titanium changed the world of 3D printed implants
According to SmarTech Analysis data, the medical sector is leading the way for demand for titanium alloy, largely due to the shift away from other metals that have proven to cause long-term complications with a high percentage of patients. Titanium has very good bio-inert properties and bonds very well to bone with the proper surface finish applied via AM, reducing complications. It also is capable of outperforming heavier alloys with its strength-to-weight ratio.
Current production of titanium orthopedic implants via AM is growing rapidly in traditional areas such as hip cups and meshes and explosively in relatively new implant areas like spinal, knee0 and shoulder implants. The biomedical sector will continue to be dominated by use of Ti6Al4V titanium alloys, including the possibility of more specialized alloys, for the production of orthopedic and similar implants.
Just about every major metal 3D printer manufacturer and atomized metal powder supplier offer Ti6Al4V and other titanium alloys for use in medical implants. Most major orthopedic implant suppliers and medical 3D printing service are able to provide titanium 3D printed implants. Trabecular titanium is particularly optimized geometry inspired by the trabecular bone, that ensures strength and facilitates bone regrowth. Originally developed by Lima Corporate for medical applications, the trabecular approach is now also used in 3D printed industrial components.
Cobalt chrome is also likely to continue to play a role for implants in certain geographical markets where cobalt alloys are still accepted for certain use cases, as well as in the fabrication of custom surgical tools. Stainless steels, though are not expected to generate major revenues through use in AM, will be valued in medical-grade stainless alloys for producing low volume and custom medical devices and surgical instruments. In the future, refractory metals are also expected to find increasing use in medical additive manufacturing to produce both specialized implants in alloys such as tantalum, which provides the potential for greater bio-inert properties, as well as in the production of various components for nuclear imaging used in medical diagnostics technology.
Along with mechanical strength, elasticity and durability, one of the key aspects that can determine if a material is fit for biomedical applications is temperature resistance. For this reason, some of the best fit materials for biomedical 3D printing applications in thermoplastic filament extrusion technologies belong to the PAEK family of polymers. Polyaryletherketone is a family of semi-crystalline thermoplastics with high-temperature stability and high mechanical strength. They include PEEK, PEKK and PEI (ULTEM) polymers. Until only a few years ago only Stratasys (MN, US), the global leader in filament extrusion 3D printing, made machines that could print these materials (ULTEM). Today PEEK, PEKK and PEI filaments are supported by a large number of 3D printer manufacturers.
While the primary focus for PEI (ULTEM) and PEKK filaments remains on industrial sectors, PEEK filaments are often associated with medical applications. In fact, implant-grade PEEK filaments were introduced by Evonik (Essen, Germany), a major thermoplastic company. The 3D printable materials are based on its VESTAKEEP i4 G, highly viscous implant-grade material. The product, which exhibits impressive biocompatibility, biostability, and X-Ray transparency, is easy to process and has been established for years as a high-performance material in medical technology applications such as spinal implants, sports medicine, and maxillofacial surgery.
Several filaments used in healthcare applications, however, not for implants. In 2018 Solvay (Brussels, Belgium), another leading global supplier of specialty polymers, introduced three medical grade products for use in the healthcare industry. These include the KetaSpire polyetheretherketone (PEEK) AM filament (NT1 HC) and a 10% carbon fibre reinforced KetaSpire PEEK AM filament (CF10 HC), together with a neat Radel polyphenylsulfone (PPSU) AM filament (NT1 HC). They are approved for limited contact applications, which means that they can be used for bodily fluid/tissue contact lasting less than 24 hours and are not fit for implants. The filaments, however, can be used for a range of healthcare applications such as patient-specific cutting guides for surgery and for complex components in single-use and reusable medical devices.
American filament manufacturer 3D Printlife (CA, US) introduced the FibreTuff PAPC Bone Replacement filament. This biocompatible, non-toxic material made from polyamide polyolefin and cellulose can be used to 3D print bone-like anatomical orthopedic models based on patient-specific anatomies. The printed models can be used by doctors for a range of medical uses, including pre-surgical planning, assessment and simulation as well as training.
The bone replacement filament is also characterized by adhesive properties that enable the support of advanced printing technology for electronic circuits. In other words, it is possible to integrate or embed 3D printed circuits into PAPC parts to enable temperature, pressure and motion monitoring, as well as other factors that might be useful to medical professionals.
Copper 3D (Santiago, Chile), a Chilean-American startup introduced the PLACTIVE range of filament materials, which are based on a PLA polymer with nanoparticle copper additive concentrations of 1%, 2% and 3% for elimination of microorganisms. PLACTIVE thus eliminates the bacterial burden housed in 3D printed materials and medical devices such as prosthetics and orthotics.
A PEEK at laser sintered implants
The technological leader for production of polymer medical implants is US-based Oxford Performance Materials or OPM (CT, US). The company produces medical devices that utilize the company's OsteoFab process, combining laser sintering additive manufacturing technology and OPM's proprietary OXPEKK material formulation to 3D print orthopedic and neurological implants.
The adoption of OsteoFab implants over metallic and other polymeric options is driven by a combination of 3D printing technology attributes: desirable economics, biocompatibility, radiolucency, and bone-like mechanics and behavior. OXPEKK 3D printed formulations exhibit osteoconductive characteristics – meaning bone grows easily onto the micro-textured surface.
OPM uses modified SLS 3D printers from EOS (Munich, Germany). The German 3D printing leader also supports its own EOS PEEK HP3 3D printing materials, developed specifically for use on the high-temperature system EOSINT P 800. Parts made with this PEEK powder can withstand continuous use temperature ranges within 180 °C (mechanical dynamic), 240 °C (mechanical static) and 260 °C (electrical), meaning the can also easily be sterilized. EOS’s PEEK was developed in collaboration with Victrex (Lancashire, UK), the third major PEEK manufacturer (with Solvay and Evonik).
Ceramics in the future of 3D printed implants
In the medical field advanced ceramics are used for implants such as hip implantable components, joint reconstruction implants and orthopedic implant components. Specific ceramic materials such as hydroxyapatite (HA or HAP) and tricalcium phosphate (TCP) are used in the biomedical segment as a synthetic bone substitute.
HAP is a naturally occurring mineral form of calcium apatite with the formula Ca5(PO4)3(OH). Up to 50% by volume and 70% by weight of human bone is a modified form of hydroxyapatite, known as bone mineral. Carbonated calcium-deficient hydroxyapatite is the main mineral of which dental enamel and dentin are composed. TCP is a calcium salt of phosphoric acid with the chemical formula Ca3(PO4)2. It can be used as a tissue replacement for repairing bony defects when autogenous bone graft is not feasible or possible.
The implementation of ceramics AM thus blurs the line between medical and biomedical applications since HAP and TCP closely mimic the actual composition of
human bone tissue. The primary benefits of ceramics in 3D printed biomedical applications include the fact that ceramics are biocompatible and medium-term resorbable osteoconductors, with good mechanical properties such as strength and hardness.
The most common process for 3D printing both hydroxyapatite and tricalcium phosphate is stereolithography, with several 3D printer manufacturers offering either or both materials for use with their systems. These include 3DMIX HAP from 3DCeram (Limoges, France), PRINT3D Hydroxyapatite and PRINT3D Tricalcium Phosphate from Prodways (Les Mureaux, France), Lithabone 200 (TCP) and Lithabone 300 (TCP) from Lithoz (Vienna, Austria) and Hydroxyapatite from Admatec (Hamburg, Germany).
Additive manufacturing makes it possible to control the location and geometry of the pores of ceramic substitutes, unlike implants that are made porous by adding organic foams. Porosity structured in three dimensions and constant diameter of the fully interconnected pores promote osseointegration and mechanical strength of substitutes. Compressive mechanical strength is between three and five times higher than that of conventional porous structures. The risk of post-operation inflammation caused by micro debris that breaks when handling and positioning the implant is also greatly reduced.