Similarly, we often assume materials are isotropic (i.e., they have the same mechanical properties with respect to every direction within the material). One example illustrating the difference between material and structure flexibility is the assertion that the stiffness of an alloy is close to that of bone. However true this may be in the sense of material properties, it is only meaningful when the same amount of alloy is used to replace a like amount of bone. Less alloy in the same location would result in a less stiff structure than the bone replaced.
The distinction between material and structure gets even fuzzier with porous materials, especially when the pores are small. (Is porous titanium a material or a structure in which a web of titanium surrounds empty spaces?) Even the definition of porous is relatively vague. (Is it the relative volume of voids?) If so, many different porous structures could have the same relative volume. (How is pore size, and pore size distribution accounted for? What about open versus closed pores? What about the nature of the pathways through open pores, sometimes called tortuosity?) Pores may also be present only relatively close to the surface, or an entire structure might be porous. Another issue with open pores and bone ingrowth is the difference in structural properties between the more-or-less empty material and when it becomes a composite material (or structure) of metal and bone.
Nanomaterials—a topic that seems to have slipped from its earlier hype phase—also raised some unique questions, particularly as used in implanted medical devices. This includes whether adjacent cells have a physical relationship with a nanostructured surface different from the relationship of cells to a more traditional surface of the same materials. Such a relationship may occur over a range of geometric scales, although the cellular process is poorly understood. Reactive properties of a nanoized surface apart from cellular response have also been examined. Mechanical and structural issues associated with nanomaterials has not received significant attention. A regularity curiosity of nano surfaces is that some manufacturers have received 510(k) clearances for implants using this technology, meaning they were argued to be “substantially equivalent” to non-nano products, yet at the same time, these manufacturers claim unique properties.
Material and structural questions also arise in 3D printing (additive manufacturing or AM) of orthopedic and other implants, in which the process may create a solid structure or open or closed pores of different size, size distributions, and spatial distributions (e.g., coatings). These capabilities have been employed in orthopedics for complex designs and patient-specific devices.
AM has been addressed in a 2017 FDA Guidance Document (GD), which includes sections on material characterization and mechanical testing. Even a solid AM printed structure may have different properties parallel to the printed layers than across the printed layers, making the isotropic assumption no longer correct. This is described in the FDA’s GD as “Due to the nature of AM, devices will likely have an orientation (i.e., anisotropy) relative to the build direction and location within the build space.” The FDA also notes variation in properties within the build space may result in inherent inhomogeneity. These factors may make process quality control and validation particularly important. The fundamental nature of AM may result in a printed device having substantially different properties than a device of the same shape and size manufactured by another process. These differences may be beneficial, may be claimed to be beneficial, or may turn out not to be beneficial.
The bio-responsiveness of an AM device may also be of interest, as it is in nano. Here, the question is whether the microstructure can be manipulated to enhance a desired biological response, such as bone growth. In addition to geometry effects and mechanical properties, biological response can interact where it is believed controlled deformation can have a positive influence on tissue growth, or when cells are directly printed onto or in combination with other materials.
The program of a forthcoming ASTM meeting on Structural Integrity of Additive Manufactured Parts is instructive concerning the issues that will be addressed. These include:
- Fatigue, fracture, tensile, wear, corrosion, and creep behavior including the effects of surface roughness, build orientation, heat treatment, size and shape
- Applicability of existing mechanical test methods and the need for new tests
- Effects of process and design parameters on fatigue and fracture behavior
- Process optimization to improve structural integrity
- Defect and acceptance criteria
- Microstructure-property predictive models
- Multiscale modeling of the fatigue and fracture behavior
- Structural integrity of additively manufactured biomaterials (Cobalt Chromium, Titanium Alloys, Stainless Steels, NiTi, etc.)
The last topic focuses the discussion from materials in general to those of current interest in orthopedics. It also raises the question of the meaning of “biomaterial,” and whether that term should still be applied to a raw material, or only to the particular conditions that exist when that material is manufactured into a particular device for a specific purpose.
New fabrication methods and their capabilities are often exciting, and often trigger enthusiastic claims about benefits. However, they are not without technical challenges including proper material and structural characterization. In this regard, we should remember innovation—and even disruptive innovation—when it means new and different, may not always mean better, or even good.