The use of three-dimensional (3D) printing techniques to address challenging fabrication problems has become mainstream over the past decade. While this rich resource has extended fabrication of personalized medical devices to the limit of our imagination, the myriad materials and morphologies available present a unique concern from a toxicological perspective. A range of standalone 3D printers are commercially available with compatible materials ranging from plastics to oxides and metals. Raw materials used in the fabrication process often have highly customized properties, achieved through the use of proprietary additives and specific microscale morphologies which can affect the overall biocompatibility of the finished device. Therefore, 3D printed medical devices require versatile approaches to the assessment of their biocompatibility that consider several factors which will be addressed in turn over this four-part blog series:
- Possible additives to raw materials which enhance workability
- Details of the material curing process
- Post-printing finishing and rinsing processes
- Time allowed for aeration between device manufacture and use
Possible additives to raw materials which enhance workability
3D printed plastic materials can be grouped by the printing technology used; generally either photolithography or direct writing of thermoplastic materials. In both of these cases, one or more materials are printed in tandem with a sacrificial filler material that provides structural support during the printing process. Photolithographic methods use a mixture of polymer precursors called photoresist which polymerize into a durable solid on exposure to light. If the photoresist requires light with intensity above a certain threshold, extremely fine resolution on the order of hundreds of nanometers is possible by scanning tightly focused laser light through the photoresist. Direct writing involves the partial melting of raw materials through a heated nozzle into fine layers. The structure and support material are deposited layer by layer, gradually building from the ground up. A compromise between photolithography and direct writing is also possible. In a process similar to inkjet printing, which produces thousands of colors by mixing three or four primary colors, different combinations of photoresists can be mixed and printed followed by exposure and polymerization with UV light.
Each technology for 3-D printing of plastic involves materials with highly customized properties, enabled by their unique chemistries. Photolithography involves polymer precursors, photosensitizers, other additives, and solvents. Following exposure, precursors and reaction byproducts remain embedded in the structure raising concerns regarding their potential to leach out during clinical use. Thermoplastics used in direct writing processes include plasticizers and other additives essential for their workability but which may cause concern as some of these additives are not biocompatible. Following melting and drawing through the writing nozzle, the additives are redistributed through the material and the surface area is increased exponentially. These processes increase the availability of potential toxicants to their surrounding matrix in the body and potentially a clinical exposure risk if not understood.
Evaluation of the biocompatibility of 3D printed devices should consider chemicals which are novel additives to otherwise well-known materials, as well as byproducts of the polymerization process. The availability of these chemicals for extraction into the matrix surrounding the device must be evaluated along with an assessment of their potential toxicological impact on a case-by-case basis.
Tune in next week to learn more about the details of the material curing process and the role it plays in the biocompatibility of 3D printed medical devices.