More from the FDA on Material Characterization for Additive Manufacturing

In March, I addressed here recent actions by the FDA to increase the specificity of material characterizations. That article focused on a February FDA Draft Guidance on UHMWPE used in orthopedic devices, and also mentioned a Guidance on blood access devices. Now, the FDA has again weighed in on material characterization, this time in the context of a Draft Guidance Document (DGD) on “Technical Considerations for Additive Manufactured Devices,” which includes 3D printing. The general issue of material characterization is how much needs to be specified to know exactly what material is being used in a medical device, and to address whether one material is identical or similar to another material. If similar rather than identical, how similar is it? Is that degree of similarity sufficient to apply experience with one material to expected performance of the other? These issues apply to both the raw materials and those materials as manufactured into a device.

In the DGD, the FDA “recommends,” or is thinking about recommending (since this is a draft) that material specifications begin with the identity of the starting material used in terms of its common name, chemical name, trade name, CAS number, and material supplier. Note that this goes well beyond merely giving a generic polymer name, and reflects that knowing only the chemical name is not knowing very much. In addition, the DGD asks for the identification of processing aids, additives, and cross-linkers used, also by material or chemical name and incoming specifications, certificates of analysis, and test methods. Specifications may include particle size and distribution for powder; filament diameter and diametric tolerances for filaments; viscosity or viscoelasticity, pH, ionic strength, and pot life for fluids; composition, purity, water content, molecular formula, chemical structure, molecular weight, molecular weight distribution, glass transition temperatures, and melting and crystallization point temperatures for polymers or polymer mixtures; and chemical composition and purity for metals. If any material is recycled from manufacturing back into the starting process, this must also be addressed with respect to potential changes in properties and content.

Post-processing, the FDA has identified a number of additional concerns. These include cleaning excess starting material from the device, and annealing and final machining, as potentially affecting the material and the device. Therefore, all post-processing steps should be documented, including a discussion of their effects. Any potentially detrimental effects of post-processing should be identified along with mitigations. In some cases, material performance requirements may be application specific. For example, devices that are intended for applications where fatigue is a factor may require additional specifications.

The DGD also identifies appropriate testing methods for material characterization, which may include both chemical and physical properties. It is noted that because of the iterative nature of 3D printing, the starting material can be exposed to see multiple re-melting and solidification processes, which may result in “unexpected or undesired material chemistries for some polymer systems.” Similarly, the process can raise some new issues, such as interior bonding.

Biocompatibility is addressed only briefly, with the usual requirement of evaluating the final finished device as described in the FDA Guidance “Use of International Standard ISO-10993, Biological Evaluation of Medical Devices Part 1: Evaluation and Testing,” although the FDA notes that under some circumstances “additional information may be necessary.”

In total, these concerns reflect that there is a great deal of detail in knowing what you started with, and even this information isn’t determinative of what you end up with. These differences can occur on scales ranging from chemical to macroscopic. While the focus in this DGD is on 3D printing and other additive processes, many of these questions are really common to all material selections, manufacturing, and products. The era of giving just a generic name for a material may be ending.