By Crystal Morrison, Ph.D.
The additive manufacturing community is abuzz with discussions of what the future will look like—for aerospace, for medicine, and for countless other fields. Those who approach additive manufacturing (AM) from a material science perspective are also considering what these advances mean at the material level—how will a polymer or plastic that is injection molded for a medical device differ from one that is 3D printed? What happens to raw materials when subjected to new processing and manufacturing methods?
In the case of 3D-printed polymeric medical devices, these questions only begin to scratch the surface. The field is quickly moving from using AM to develop medical models and prototypes to using AM to produce highly customizable devices including implants. Given the strict regulatory environment, it is critical that materials considerations and selection are thoroughly examined when producing medical devices with AM. A systematic materials assessment that focuses on requirements, material screening, manufacturability, and ranking will help the manufacturer meet regulations and avoid potential liability while delivering innovative, safe, and effective medical devices to patients.
The first step in the systematic assessment is to create a comprehensive list of all requirements for the plastic medical device. Consider regulatory and operational as well as functional, mechanical, and testing requirements. Sometimes establishing this list is straightforward, but in some cases, the form, fit, and function of the device are vague at best. Complicated or not, going through this exercise gives an interdisciplinary team a better understanding of the big picture and what obstacles may come later in the process.
Once the team outlines device requirements, the next step is materials screening. Perhaps the most critical screening requirement for medical polymer selection is biocompatibility. The two most common biocompatibility standards are USP class VI and ISO 10993. The ISO 10993 standard is more stringent and suited for medical devices. ISO 10993 covers the nature and duration of physical contact vs. biological risks like cytotoxicity, sensitization, and irritation.
As of now, the list of biocompatible materials available for additive manufacturing is specific to each process and piece of equipment. Several of the materials are acrylonitrile butadiene styrene-like (ABS-like) or “polycarbonate-like” but not necessarily just ABS or polycarbonate. The first FDA approval for an additively manufactured polymer implant was Oxford Performance Material’s OsteoFab cranial device made from poly(etherketoneketone) or PEKK, with more materials sure to be approved in the future.
In terms of materials screening, other factors such as sterilization and chemical resistance must also be considered. The common approaches to sterilization are radiation, chemicals, and steam; common cleaners for medical devices include isopropyl alcohol (IPA), bleach, and peroxides. Although some materials, such as ABS, are considered stable to radiation, it is critical to remember that the materials used in AM are often modified in a number of ways to make them amenable to AM methods and equipment. Not only is the “ABS-like” material not the exact same formulation, but the process of conventional vs. additive manufacturing is inherently different. This aspect opens the door for many questions. For “ABS-like” materials, does the specific AM process create defects or changes in the bulk material that actually improve (or reduce) its stability toward radiation? This question, and many others, should be addressed with testing.
Mechanical properties are also an important part of material screening. There are fundamental differences in conventional vs. additive manufacturing techniques that create different mechanical behavior for the same material. A number of groups within the additive manufacturing community are exploring these differences, including researchers at the Direct Manufacturing Research Center (DMRC) in Germany. To understand the comparative performance of AM parts to traditional injection molded specimens, the researchers at DMRC generated standard stress-strain curves for Ultem 9085 mode from injection molded material vs. material made with the AM method of fused deposition modeling (FDM). The researchers observed that the AM manufactured parts can exhibit similar strength to the injection molded parts, but exhibit much lower strain break. The lower strain can most likely be attributed to the internal structure of the parts. Test results showed that compared to AM processes, injection molding typically has lower porosity and greater homogeneity, leaving fewer imperfections in the structure of the material to encourage crack propagation.
Not only are there mechanical differences when comparing conventional vs. additive manufacturing, but mechanical properties will also depend on the type of AM process used and the build direction or print orientation. The differences in properties can be a challenge, especially if mechanical isotropy is required, but can also be used for design advantages. For example, a device may require higher tensile strength in one direction rather than uniform strength in all directions. Perhaps the build direction can be changed to yield directional orientation and mechanical strength that meet the requirements?
Since it is well known that physical mechanical properties can differ for AM materials, it is also possible that the wear resistance and surface properties can differ for devices made with traditional vs. AM methods. It is not safe to assume that years of data collected from testing implants made with traditional techniques will have the same surface properties and wear debris results when created with additive. Filler, orientation, and degree of crystallinity can modify the thermal properties of a material. Inherent differences between conventional and additive mean that the thermal properties can also be different for the “same” material. In injection molding, orientation can be induced by the injection flow and crystallinity gradients develop from slowly crystallizing polymers. Filler, orientation, and crystallinity are all possible with additive, but the properties will vary across the component. Understanding how the properties vary and how they can be controlled is important and may require testing to avoid assumptions and minimize potential issues.
Legal issues are becoming more of a focus area within additive manufacturing. Currently, major concerns revolve around intellectual property issues. In the future, there could be claims related to device failures and questionable quality. Those looking to produce medical devices with additive manufacturing should be aware of the Biomaterials Access Assurance Act (BAAA) of 1998. The BAAA was created to provide some protection for materials suppliers for the medical device industry. This responsibility requires manufacturers to guarantee high quality materials are received from suppliers and used across the lifecycle of their devices. Including quality measurers and testing during the initial planning phases can help to avoid liability and protect the end user.
With the constantly evolving landscape of additive manufacturing, new materials, new processes, and new equipment are emerging at a dizzying pace. It can certainly be overwhelming, especially for medical device manufacturers who are more experienced at developing the therapeutic or engineering aspects of their products. The key is not to leave materials as an afterthought. Moving forward it will be very important to approach materials selection using a systematic assessment while emphasizing the considerations unique to AM. To be effective, this assessment must include interdisciplinary discussion. What will the impact be if these steps are followed? Expect an overall reduction in cost and liability, and the opportunity to harness incredible innovation at the intersection of materials, medicine, and manufacturing.