Medical implants range from a wide array of materials including metals, metallic alloys, plastic polymers, ceramics, hydrogels, and composites depending on the use-case. They often fixed into tissues with glues and therefore must be compatible with the host body. They must be clinically and commercially viable materials and must meet regulatory standards for biocompatibility and prove to be nontoxic. Improvements in biomanufacturing have led to different manufacturing strategies being adopted and deployed. AM is uniquely suited to manufacture medical implants than traditional machining processes due to the flexibility it offers in tooling requirements and lower manufacturing constraints. For example, it is possible to manufacture customised lattice structures designed for bone growth, suited to individual patient needs, which may be quite difficult with traditional machining processes. With increasing interest in the use of additive manufacturing to develop medical implants, it is a necessary trajectory to explore the environmental impact of AM over machining processes to produce medical implants.

For effective analysis of environmental impacts, the whole manufacturing lifecycle must be considered from an energy and emissions standpoint. This starts in the lifecycle from mining raw material to primary metal production and refinement, feedstock manufacturing, printing, post-processing, and material recycling throughout the lifecycle.

Discussion

• Raw Material Extraction (Mining) and Primary Metal Production (Refinement): Both AM and Machining processes have the same process in place for raw material extraction and refinement. The ore is mined and processed, and then the metal is alloyed and cast into an ingot. The energy consumed in these first steps must be understood to calculate the impact of downstream material waste. One of the main materials used in medical implants is Titanium, which requires a significant amount of energy for the processing.
• Feedstock Manufacturing: Almost all metal AM processes consume powder or wire as the feedstock, while machining usually uses billet as the feedstock. The powder is created through gas atomization, a slow and expensive process. The wire is created by drawing the metal through progressive dies. Metal plates are formed by hot rolling the ingot and then cut into various billet sizes. Metal AM feedstocks require 3-4x more energy to manufacture than the billet used in machining.
• AM Printing Process: Printing process energy includes both the primary printing energy and secondary energy to operate the machine. Energy losses and secondary processes of metal 3D printers are usually the largest contributors to total energy consumption. The total system energy use can be 5-10x the energy of the primary printing energy.
• Machining Process: Roughing is the first step to mill material away from the billet to achieve a near-net-shape part. Titanium and tool steel cannot be machined rapidly and which would wear the cutting tool. The calculation of material waste is more complex since material waste is highly dependent on material, geometry and application.
• Post-Processing: There is a lot of variability in post-processing energy demands because of dependencies on geometry, support options and finish machining requirements, process control, and application-specific requirements like heat-treatment.
• Material and Energy Recycling: A large percentage of material wasted throughout the feedstock manufacturing, printing and post-processing can be re-processed and re-used. On average, about 60% of the material waste can be recycled and about 80% of the energy already spent processing it can be recaptured.

Conclusion

AM could be a more environmentally viable and sustainable manufacturing process for medical implants. Highly complex geometric parts lead to poor material utilisation in Machining processes and a large amount of wastage. Integrated geometric complexity and techno-economic models need to be developed to provide simultaneous economic and environmental sustainability analyses in more detail.

References:

1. Barchowsky, Systemic and Immune Toxicity of Implanted Materials, in: Biomater. Sci., Elsevier, 2020: pp. 791–799. https://doi.org/10.1016/B978-0-12-816137-1.00051-9
2. R. Lyons, A. Newell, P. Ghadimi, N. Papakostas, Environmental impacts of conventional and additive manufacturing for the production of Ti-6Al-4V knee implant: a life cycle approach, Int. J. Adv. Manuf. Technol. 112 (2021) 787–801. https://doi.org/10.1007/s00170-020-06367-7.
3. Huckstepp, Environmental Impact of Metal AM, (2019). https://3dprinting.com/news/environmental-impact-of-metal-am/ (accessed March 5, 2021).