Industrial additive manufacturing has emerged from the trough of disillusionment and is now experiencing a renaissance of sorts. This is in good part
due to a proliferation of industrial printer manufacturers with new technologies and more open approaches to materials and parameters.
From my vantage point at 3Diligent, a rapid manufacturing service provider that helps companies with their 3D printing projects, one industry in which additive manufacturing applications are especially prevalent and gaining acceptance is the medical fi eld. Since 3D printing builds objects one
voxel (3D pixel) at a time, it is uniquely suited to construct complex organic shapes that are implausible or impossible with traditional manufacturing techniques. These shapes can, for instance, contain intricate internal geometries and custom-tailored porosity to mimic different bone densities.
Moreover, this complexity is available virtually free of charge. Recognizing the value proposition and economics are compelling for the medical
community, are the currently available materials sufficient to meet market demands? Which materials are currently available? Which are still to come? This update on the state of the 3D printing materials market will show that there are more available now than is generally perceived, and more are rapidly on the way.
Aqueous Solutions/Bio Inks
Aqueous solutions – sometimes referred to as bio inks within the 3D-printing domain include Poly (2-hydroxyethyl methacrylate) (pHEMA) and Polyethylene (glycol) Diacrylate (PEGDA). However, this is an area of vast investment and research with many promising new hydrogel materials
under development. These are commonly printed using pneumatic or hydraulic extrusion. This can be at room temperature to preserve living cells, for instance, or through a heated nozzle. Aqueous solutions can also be “indirectly” 3D printed, in the sense that part patterns can sometimes be fabricated with high-accuracy wax or resin printers, and those are in turn used to create transparent molds. The aqueous solutions can then be poured into the transparent mold and cast, typically with the use of UV light.
Applications for these solutions are wide, as the material range is virtually unlimited, so long as the active agent/material can be carried in water. In the case of reconstructive surgery, bone material can be carried within a hydrogel and printed onto bone. The deposited material can act as a binding agent between bone fragments, facilitating the ingrowth of bone to reconnect previously shattered bone.
Polymers
Thanks to their versatility and relatively low cost, the number of medical applications for polymers is growing rapidly.
Low-Temperature Polymers
Low melting point polymers are often printed using extrusion technologies – otherwise referred to by brand names like Fused Deposition Modeling (FDM) or Fused Filament Fabrication (FFF). The most popular biodegradable polymer is Polylactic Acid (PLA). It is the default material for many desktop FFF printers. Due to that position in the market, it has been affordably printed on a wide variety of machines, not just extrusion printers but also laser sintering printers, for a number of applications like biodegradable stents.
Figure 1. Researchers from Harvard utilize extruded bio inks to print various organic shapes, including this proximal tubule, a critical component of kidney nephrons. (Credit: The Lewis Lab)
Other popular low-melting temperature polymers include Polycaprolactone (PCL), a biodegradable polyester, which is used for things like skin grafts due to its ability to facilitate cell growth, and Polycylcolide (or polyglycolic acid) (PGA). There is still a good deal of research to be done with respect to low- temperature polymers. However, as Roger Narayan, Special ASME Fellow to America Makes notes, what has not yet been offered by manufacturers to this point is biodegradable polymers with several different molecular weights. For example, the ability to print materials with customized molecular weights that will facilitate more controlled degradation of printed devices is an area of relative immaturity and significant potential research.
High-Temperature Polymers
High-temperature polymers are printed using high-end extrusion technologies and powder bed fusion technologies, known by the branded name Selective Laser Sintering (SLS). SLS’s unique combination of heated build chambers and focused laser beams allow for materials with higher melting points. The most common printed material in powder bed systems is nylon. While Nylon 11 has been commercialized to some extent, Nylon 12 is the most popular laser sintered material, and it is sometimes used to create surgical guides. With respect to implants, Polyetherketoneketone (PEKK) parts are most common, although they are utilized for lower-impact implant applications like spinal and maxillofacial. For higher stress applications, such as hip and knee appliances, metals are frequently used to insure the implants can endure the frequent impacts and other dynamic loads they will be exposed to.
Not every high-temperature plastic is printed with ease. Nylon 6 and Nylon 6,6 are under development and used mostly in academic settings. Polyether ether ketone (PEEK), another material being tested in a variety of academic settings, is also on the horizon. The challenge of maintaining tight tempera ture windows across the totality of a build for existing machines has limited PEEK’s commerciality to date.
Thermoset Polymers
Resins are also used in the medical fi eld, but the applications are primarily for instrumentation and surgical guides. Material-jetting technologies, which behave much like traditional inkjet printers, can deposit multiple materials within a single print. This can be especially useful for applications like authentic reproductions of internal organs. In such situations, digital imaging and communications in medicine (DICOM) data can be converted to printable fi les. These can be printed in multiple materials and colors to offer surgeons a practice run before operating on a live patient.
Figure 2. Printed anatomical bodies, like this full-size and miniature liver with cancerous bodies (blue), allow surgeons to analyze and practice surgeries before entering the OR. (Credit: 3Diligent)
Surgeons report increased effi ciency in the operating room as a result of these practice models. Peter Denmark, sales head for Envisiontec, says surgeons report several minutes of savings from the use of surgical guides before entering the OR.
“When you consider that an hour in the OR may cost $15,000, saving just a few minutes can drive some very meaningful efficiency savings,” Denmark says.
Bills are being reviewed in Congress that may facilitate increased insurance coverage for such prints, which would accelerate adoption of the technology in hospital environments, according to Denmark. The frontier seems to be multi-material printing in the thermosets world. Stratasys
unveiled its multi-material Connex J750 earlier this year, capable of printing with up to six materials and more than 360,000 colors to allow for complex anatomical models. Its competition has not yet followed suit with machines providing similar levels of multi-material printing, although
this is something to watch.
Metals
Metal material suppliers started developing their products with an eye to the aerospace industry, but in recent years have been expanding their offerings to meet the demands of the medical market. Powder bed printing is the default printing process for metal medical parts. Using either a laser (e.g., DMLS, SLM, LaserCUSING) or an Electron Beam (EBM) system, powdered metal is selectively melted, layer-by-layer, to build a part. Printed metal implants are growing in popularity, due in part to the ability to build parts to fit particular anatomies. In addition, the natural surface roughness of powder bed printing has been found to accelerate osteointegration. The biggest announcements in this area have come from Europe and Australia, although signifi cant movement has been taking place in the US with the FDA’s recent draft guidance to device manufacturers. A variety of stainless steels with strong anticorrosive properties are available, most notably 316 and 17-4, which are typically processed using laser systems. Titanium alloys, which are printed by both laser and electron beam systems, are also prevalent in the medical space, with applications across instrumentation and implants.
Ti6AlV4 and Ti6AlV4 ELI are two alloys that are popular for things like spinal implants, acetabular hip cups, and knee joints. Recent announcements include commercially pure titanium, which some doctors believe facilitates faster integration with bone structures, although clinical data is still being gathered to see if that hypothesis is correct. Cobalt chrome is another material that sees use, although not with the frequency of titanium.
Looking Ahead
With respect to the future, 3D printer OEMs are generally mum on which materials are on the way, although they are active in encouraging the market to highlight which materials they’d like to see commercialized next. It’s also expected that the growing number of metal machines which provide more open architectures will allow for the development and printing of bespoke alloys for hyper-specifi c applications.
