Mechanisms of fatigue crack initiation and propagation in auxetic meta-biomaterials.

The fatigue performance of additively manufactured auxetic meta-biomaterials made from commercially pure titanium has been studied only recently. While certain assumptions have been made regarding the mechanisms underlying their fatigue failure, the exact mechanisms are not researched yet. Here, we studied the mechanisms of crack formation and propagation in cyclically loaded auxetic meta-biomaterials. Twelve different designs were subjected to compression-compression fatigue testing while performing full-field strain measurement using digital image correlation (DIC). The fatigue tests were stopped at different points before complete specimen failure to study the evolution of damage in the micro-architecture of the specimens using micro-computed tomography (micro-CT). Furthermore, finite element models were made to study the presence of stress concentrations. Structural weak spots were found in the inverted nodes and the vertical struts located along the outer rim of the specimens, matching the maximum principal strain concentrations and fracture sites in the DIC and micro-CT data. Cracks were often found to originate from internal void spaces or from sites susceptible to mode-I cracking. Many specimens maintained their structural integrity and exhibited no signs of rapid strain accumulation despite the presence of substantial crack growth. This observation underlines the importance of such microscale studies to identify accumulated damage that otherwise goes unnoticed. The potential release of powder particles from damaged lattices could elicit a foreign body response, adversely affecting the implant success. Finding the right failure criterion, therefore, requires more data than only those pertaining to macroscopic measurements and should always include damage assessment at the microscale. STATEMENT OF SIGNIFICANCE: The negative Poisson's ratio of auxetic meta-biomaterials makes them expand laterally in response to axial tension. This extraordinary property has great potential in the field of orthopedics, where it could enhance bone-implant contact. The fatigue performance of additively manufactured auxetic meta-biomaterials has only recently been studied and was found to be superior to many other bending- and stretch-dominated micro-architectures. In this study, we go beyond these macroscopic measurements and focus on the crack initiation and propagation. Full-field strain measurements and 3D imaging are used to paint a detailed picture of the mechanisms underlying fatigue. Using these data, specific aspects of the design and/or printing process can be targeted to improve the performance of auxetic meta-biomaterials in load-bearing applications.

Fatigue performance of auxetic meta-biomaterials.

Meta-biomaterials offer a promising route towards the development of life-lasting implants. The concept aims to achieve solutions that are ordinarily impossible, by offering a unique combination of mechanical, mass transport, and biological properties through the optimization of their small-scale geometrical and topological designs. In this study, we primarily focus on auxetic meta-biomaterials that have the extraordinary ability to expand in response to axial tension. This could potentially improve the longstanding problem of implant loosening, if their performance can be guaranteed in cyclically loaded conditions. The high-cycle fatigue performance of additively manufactured (AM) auxetic meta-biomaterials made from commercially pure titanium (CP-Ti) was therefore studied. Small variations in the geometry of the re-entrant hexagonal honeycomb unit cell and its relative density resulted in twelve different designs (relative density: ~5-45%, re-entrant angle = 10-25°, Poisson's ratio = -0.076 to -0.504). Micro-computed tomography, scanning electron microscopy and mechanical testing were used to respectively measure the morphological and quasi-static properties of the specimens before proceeding with compression-compression fatigue testing. These auxetic meta-biomaterials exhibited morphological and mechanical properties that are deemed appropriate for bone implant applications (elastic modulus = 66.3-5648 MPa, yield strength = 1.4-46.7 MPa, pore size = 1.3-2.7 mm). With an average maximum stress level of 0.47 σy at 106 cycles (range: 0.35 σyσy- 0.82 σyσy), the auxetic structures characterized here are superior to many other non-auxetic meta-biomaterials made from the same material. The optimization of the printing process and the potential application of post-processing treatments could improve their performance in cyclically loaded settings even further. STATEMENT OF SIGNIFICANCE: Auxetic meta-biomaterials have a negative Poisson's ratio and, therefore, expand laterally in response to axial tension. Recently, they have been found to restore bone-implant contact along the lateral side of a hip stem. As a result, the bone will be compressed along both of the implant's contact lines, thereby actively reducing the risk of implant failure. In this case the material will be subjected to cyclic loading, for which no experimental data has been reported yet. Here, we present the first ever study of the fatigue performance of additively manufactured auxetic meta-biomaterials based on the re-entrant hexagonal honeycomb. These results will advance the adoption of auxetic meta-biomaterials in load-bearing applications, such as the hip stem, to potentially improve implant longevity.

Additively manufactured space-filling meta-implants.

The unprecedented properties of meta-biomaterials could pave the way for the development of life-lasting orthopedic implants. Here, we used non-auxetic meta-biomaterials to address the shortcomings of the current treatment options in acetabular revision surgery. Due to the severe bone deficiencies and poor bone quality, it can be very challenging to acquire adequate initial implant stability and long-term fixation. More advanced treatments, such as patient-specific implants, do guarantee the initial stability, but are formidably expensive and may eventually fail due to stress shielding. We, therefore, developed meta-implants furnished with a deformable porous outer layer. Upon implantation, this layer plastically deforms into the defects, thereby improving the initial stability and homogeneously stimulating the surrounding bone. We first studied the space-filling behavior of additively manufactured pure titanium lattices, based on six different unit cells, in a compression test complemented with full-field strain measurements. The diamond, body-centered cubic, and rhombic dodecahedron unit cells were eventually selected for the design of the deformable porous outer layer. Each design came in three different relative density profiles, namely maximum (MAX), functionally graded (FG), and minimum (MIN). After their compression in bone-mimicking molds with simulated acetabular defects, the space-filling behavior of the implants was evaluated using load-displacement curves, micro-CT images, and 3D reconstructions. The meta-implants with an FG diamond infill exhibited the most promising space-filling behavior. However, the required push-in forces exceed the impact forces currently applied in surgery. Future research should, therefore, focus on design optimization, to improve the space-filling behavior and to facilitate the implantation process for orthopedic surgeons. STATEMENT OF SIGNIFICANCE: Ideally, orthopedic implants would last for the entire lifetime of the patient. Unfortunately, they rarely do. Critically sized defects are a common sight in the revision of acetabular cups, and rather difficult to treat. The permanent deformation of lattice structures can be used to create shape-morphing implants that would fill up the defect site, and thereby restore the physiological loading conditions. Bending-dominated structures were incorporated in the porous outer layer of the space-filling meta-implants for their considerable lateral expansion in response to axial compression. A functionally graded density offered structural integrity at the joint while enhancing the deformability at the bone-implant interface. With the use of a more ductile metal, CP-Ti, these meta-implants could be deformed without strut failure.