Metal Lattices for Medical Implants: Fatigue Performance Optimization

blank blank Feb 16, 2021

Metal additive manufacturing (or metal 3D printing) is now a well-established and mature technology, used globally for the production of end-use metal parts in various industries. In medical applications, the major advantage of metal additive manufacturing is found for custom and patient-specific implants and medical devices. The patient-specific geometry ensures the best possible fit for the patient, improving the healing time and improving the success rate. A second major advantage is the possibility to create porous metal structures as a part or whole of the implant. These porous structures are also called cellular or lattice structures.  These intentionally-designed or “architected” cellular structures are beneficial for implants for a few reasons: 1) They can be designed to better match the required mechanical properties (to have effective properties more similar to that of the adjacent bone, compared to solid metal). 2) Their porous nature is beneficial to the flow of nutrients and vascularization, hence enhancing the bone growth process. And over the longer term, they allow bone ingrowth into the open porous structure, for a better bone-implant attachment. In this article, we will focus on an important concept in metal 3D printing, fatigue performance.  When an object is subjected to continued cyclic loads with maximum loads below the yield strength of the material, eventually cracks form and the material fails – this is fatigue. The fatigue strength refers to the load maximum at which the material withstands a few million cycles (high cycle fatigue) or several millions of cycles (very high cycle fatigue). The fatigue life refers to the number of cycles at a specific load before failure occurs.  Fatigue performance refers to the combination of these at various load conditions and therefore refers to the general ability of a material to withstand such failures. 

metal 3d printing
Figure 1: A series of lattice cubes printed in titanium at the Central University of Technology in Bloemfontein, South Africa.

Metal 3D printing of cellular structures are increasingly used with great success, but many questions remain: 

Which design and architecture of lattice should be used? 

Which pore size is the best? 

Should the entire implant be lattice or only a small section? 

performance fatique
Figure 2: Cellular structures in titanium implants produced in South Africa by LRS implants and Executive Engineering, Cape Town, South Africa.

More importantly, what is the cyclic performance of such structures? Most implants are expected to be subjected to cyclic loads and the fatigue performance of these structures is therefore highly important. Cracks may initiate and propagate in materials subjected to cyclic loads at stress levels much less than their yield strength, which is not desirable in applications such as for medical implants.

To provide a deep insight into the current state of the art on this topic, a comprehensive review paper of the mechanical properties and fatigue performance of cellular structures was recently published in the journal Materials Science and Engineering R, which can be found here for further reading [1]. This blog provides a short overview and discussion of this topic.

The mechanical properties of additively manufactured metallic lattices have been studied for years by many researchers, with early studies showing large discrepancies between designed and actual mechanical properties. Over the last 10 years, this situation has significantly improved with a better understanding of the influencing factors and large improvements in metal additive manufacturing technology. The main influences on the elastic modulus and strength of the cellular structures are the design parameters, i.e. a lattice with higher designed porosity has lower yield strength and elastic modulus. However, as the porosity increases, the feature sizes move closer to the manufacturing limits of the typical metal additive manufacturing technologies, which results in relatively large discrepancies between the actual lattice geometry and the designed geometry. This includes thicker and thinner struts and irregular surface conditions, varying with orientation (e.g. horizontal struts thicker and downward-facing surfaces roughest). 

performance fatique
Figure 3: Finite element modelling of actual lattice geometries showing locations of highest Von Mises stress in effective notches in the rough surface [1].

These deviations from the intended geometry affect the mechanical properties. Other factors are also important such as unexpected porosity inside the struts due to manufacturing process errors, residual stress due to high-temperature gradients, and unique microstructures due to the fast solidification and repeated thermal cycles during manufacturing. This is now all well understood and additive manufacturing processes can be optimized to obtain good mechanical properties for cellular structures over a wide range of design parameters, but fatigue performance is not guaranteed and not as well understood yet.

What this review showed is that widely varying fatigue performance has been found in different studies, with some key influencing factors being identified: lack of fusion porosity, rough surfaces, and designs with too thin features are the obvious “killers” of fatigue performance. In strut-based lattices, the nodes or junctions between struts are often the locations of fatigue failure, indicating thicker nodes or filleted nodes might be beneficial to fatigue performance. What was also found is that some metallic alloys have higher fatigue performance, and that appropriate heat treatment or hot isostatic pressing (HIP) improves the fatigue performance of typical medical grade metals considerably. It is also shown that some architectures achieve higher fatigue performance.

Designing and manufacturing cellular materials with adequate fatigue resistance and the ability to reliably predict their safe lifetimes is the ultimate goal, leading to their successful application in medical implants. Fully understanding the fatigue performance makes it possible to optimize the design, manufacturing, and quality control processes, mitigating potential problems, and allowing safe, reliable, and long-lasting implants.

performance fatique
Figure 4 A comparison of normalized fatigue strength for different lattice architectures and materials, from [1].

Reference:

[1] M. Benedetti, A. du Plessis, R.O. Ritchie, M. Dallago, S.M.J. Razavi, F. Berto, Architected cellular materials: A review on their mechanical properties towards fatigue-tolerant design and fabrication, Mater. Sci. Eng. R Reports. 144 (2021) 100606. https://doi.org/10.1016/j.mser.2021.100606.

About the Author:

Prof. Anton Du Plessis

Prof. Anton Du Plessis is an Associate Professor at Stellenbosch University, South Africa, and is also affiliated with Nelson Mandela University, South Africa. He is an experienced scholar in the field of additive manufacturing, with specific interests in quality control and process optimization, X-ray tomography and biomimicry applied to additive manufacturing. His interests and expertise range across a wide range of disciplines in the field, and he is an associate editor of the leading journal in the field, Elsevier’s Additive Manufacturing journal.

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