Evaluation Methods for 3D Printed Implants
Medical additive manufacturing has transformed the industry by introducing new genres of implants and instrumentation that employ complex or porous structures and improved anatomic geometries. The ability to create new designs, that cannot be traditionally manufactured, allows medical device manufacturers to simulate support structures and modify surfaces to promote bone ingrowth.
Significant advances and cost reduction in imaging and additive manufacturing (AM) technologies have driven the development of patient-specific implants, which can improve recovery time and overall patient well-being. Additional considerations need to be made when evaluating these types of devices as well as the manufacturing processes. This article will focus on the testing of titanium as additively manufactured raw material and finished products for the medical industry using several common methods.
Traditional material procurement and machining have well-established procedures and quality processes for certification and inspection. Compared to traditional materials, additive manufactured materials require qualifying and validating of the powders, structures, individual printers and overall design, creating additional steps for characterization.
There are several existing publications and guidance documents for medical additive manufacturing processes. The FDA-issued guidance provides comprehensive information on the recommended steps to evaluate additive manufactured medical devices:
The document outlines overall device design, patient-matched device design, software workflow, material controls, post-processing, process validation and quality data analysis. It also provides commentary on device testing considerations.
Additive manufacturing evaluation methods
Raw material (Powder) – Raw material chemistry and morphology affect how the powder fuses under precisely-configured printer parameters. Inconsistency or deviation from powder settings for the machine can result in fluctuation of the mechanical properties of the end products. As such, it is common to test the powder for chemical composition and size and shape distribution before manufacturing. Also, due to material cost, it is often advantageous to reuse unfused powder for subsequent print operations. Even though the powder has not been fused, trace amounts of oxygen may have been introduced which could ultimately affect subsequent builds. It is important to understand how many times the raw material can be reintroduced in the process to reduce cost but not jeopardize patient safety.
Orientation and build volume placement – 3D printers can differ in functional performance depending on the location of the physical build inside the printers, and the layering process can have an impact on mechanical properties in various planes. Coupons are created and can be tested in X, Y, Z planes, combined planes, and in different build locations. Even though two printers may be the same model, different printers can exhibit varied behavior requiring individual qualification.
Post-processing validation – After all post-processing is complete, the mechanical properties of the printed item need to be evaluated to confirm they are within tolerance.
Final design evaluation – Once the three above items have been resolved for printer evaluation, the next step is to test the design according to functional testing specifications. Testing needs to be performed in line with the indications of the device. For example, an intervertebral body fusion device (IBFD) should be tested following ASTM F2077.
Production lot or batch build validation – Once the implants are cleared for manufacture via 510(k) or another regulatory pathway, each manufacturer has to determine their quality-driven acceptance criteria to evaluate whether the production build passes or fails. Coupons are printed alongside the product for lot validation.
Additive manufacturing test methods
The designated ASTM specifications for titanium devices are ASTM F3001 (additive manufacturing) or ASTM F136 (wrought titanium) and are used for defining the requirements for mechanical properties for Ti-6Al-4V ELI. Additive manufacturers who use full-melt powder bed fusion by either electron beam melting or laser melting will need to verify chemical composition, mechanical properties, and microstructure.
Identifying chemical composition will determine the levels of aluminum, vanadium, iron, carbon, yttrium and other elements within the material. Additionally, testing for oxygen, nitrogen and hydrogen is also required. Characterization can be performed on the powder or a built component, effectively evaluating pre and post AM processing. Standard test methods include ASTM E1941, E1447, E1409 and E2371. These analyses are performed using different equipment including inductively coupled plasma (ICP-OES), carbon analyzer (Combustion Infrared Detection), hydrogen analyzer (Inert Gas Fusion Thermal Conductivity Detection) and oxygen and nitrogen Analyzer (Inert Gas Fusion Thermal Conductivity Detection).
Evaluating mechanical properties includes testing for tensile strength, yield strength, elongation, and reduction of area. On a build platform, locations for test specimens are selected per ISO/ASTM Terminology 52921 and generally built in X, Y, and Z orientations. Once the specimens are printed near net shape, the machining and testing of these build specimens follow ASTM E8 and sometimes reference ASTM A370. The specifications outline many different geometries for testing which is important to understand due to the various sizes and shapes of the builds. Testing is performed at room temperature and has specific loading rates.
As additive manufacturing is routinely used for intervertebral body fusion devices (IBFD), static compression via ASTM F2077 – Test Methods for Intervertebral Body Fusion Devices is commonly performed on either custom coupons or complete products. ASTM D638 is also used for compression testing of a more standard geometry. Four point bend testing is another method used to evaluate coupon properties.
One factor to consider is the amount of support material and the removal process necessary to properly machine the samples. Samples for tensile testing are prepared either by CNC or manual machining. CNC machining is preferred as it is time- and cost-effective. However, coupons often have to be longer compared to the ones required for manual machining.
The microstructure requirements include an examination for alpha case, which is not permitted on final, net components at magnifications of 100x. Alpha case occurs when titanium is exposed to heated air or oxygen, resulting in an oxygen-enriched surface phase. This embrittled surface can promote a series of microcracks which can reduce the mechanical properties of the material. Standard methods for specimen preparation include ASTM E3 and Practice E407. The method is intended to reveal the structure of metals using light optical or scanning electron microscopy (SEM). The process includes cutting the specimen if needed; usually, 12 to 25mm of the available surface is acceptable. Specimens are then mounted in resins or plastics, ground to prepare the surfaces, and finally polished. Recommended preparation methods can differ by material and are outlined in the specification. In order to make the microstructure visible, an acid solution is applied to the surface, which etches the polished surface. Typical solutions for titanium etching are Kroll’s Reagent and Ammonium Bifluoride.
The test methods mentioned are generally performed on coupons of different geometries. If the printed materials have a strut or porous structure, it is common to consider evaluating the functional performance of the design using a representative structural feature. As such, custom coupons can be created that allow for design evaluation. These are usually small, but help support performance evaluation of the material mechanical properties and also the design confirmation. As such, it is also acceptable to use a final part for validation testing. These coupons can be used throughout the process to confirm printer performance and validate post-processing. It is best practice to continuously monitor the results of printed batches. However, continuous monitoring may not be required depending on the results from previous validation testing per the manufacturer’s quality system. Once the coupon geometry is identified, the design can be tested using a variety of test methods.
Full submission testing
If manufacturers replicate an existing PEEK or Ti geometry with additive manufacturing, the common test methods for that type of device still apply in addition to the above material characterization. Applicable test methods outlined in regulatory guidance documents for the type of device should also be followed. While AM devices may have adequate strength for the purpose, they may not perform the same as devices made from similar materials but different manufacturing processes. When comparing AM products to predicate devices for substantial equivalence, the manufacturer should take this into account when selecting predicates and developing acceptance criteria.
The test methods outlined support the validations of printing processes for both raw material considerations and final product evaluation. This article is not intended to be an exhaustive list of test methods and as printing technologies change, new methods may be developed to address findings. There is significant activity within ASTM to continue to address standards development for additively manufactured medical materials.
To learn more about the services we provide for medical additive manufacturing, contact us today.
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