On the Morphological Deviation in Additive Manufacturing of Porous Ti6Al4V Scaffold: A Design Consideration

Abstract

Additively manufactured Ti scaffolds have been used for bone replacement and orthopaedic applications. In these applications, both morphological and mechanical properties are important for their in vivo performance. Additively manufactured Ti6Al4V triply periodic minimal surface (TPMS) scaffolds with diamond and gyroid structures are known to have high stiffness and high osseointegration properties, respectively. However, morphological deviations between the as-designed and as-built types of these scaffolds have not been studied before. In this study, the morphological and mechanical properties of diamond and gyroid scaffolds at macro and microscales were examined. The results demonstrated that the mean printed strut thickness was greater than the designed target value. For diamond scaffolds, the deviation increased from 7.5 μm (2.5% excess) for vertical struts to 105.4 μm (35.1% excess) for horizontal struts. For the gyroid design, the corresponding deviations were larger, ranging from 12.6 μm (4.2% excess) to 198.6 μm (66.2% excess). The mean printed pore size was less than the designed target value. For diamonds, the deviation of the mean pore size from the designed value increased from 33.1 μm (−3.0% excess) for vertical struts to 92.8 μm (−8.4% excess) for horizontal struts. The corresponding deviation for gyroids was larger, ranging from 23.8 μm (−3.0% excess) to 168.7 μm (−21.1% excess). Compressive Young’s modulus of the bulk sample, gyroid and diamond scaffolds was calculated to be 35.8 GPa, 6.81 GPa and 7.59 GPa, respectively, via the global compression method. The corresponding yield strength of the samples was measured to be 1012, 108 and 134 MPa. Average microhardness and Young’s modulus from α and β phases of Ti6Al4V from scaffold struts were calculated to be 4.1 GPa and 131 GPa, respectively. The extracted morphology and mechanical properties in this study could help understand the deviation between the as-design and as-built matrices, which could help develop a design compensation strategy before the fabrication of the scaffolds.

Mechanical Characterisation and Numerical Modelling of TPMS-Based Gyroid and Diamond Ti6Al4V Scaffolds for Bone Implants: An Integrated Approach for Translational Consideration

“…Overall, for satisfactory bone ingrowth and enhanced osseointegration, the porosity needs to be above 50% with a pore size range of 300–800 μm, which can benefit vascularisation and cell growth simultaneously [ 34 , 69 ]. A more detailed and comprehensive study on the morphological deviation of TPMS gyroid and diamond was undertaken by Naghavi et al [ 70 ] with a wider range of pore sizes and porosities.…”

Definition, measurement, and function of pore structure dimensions of bioengineered porous bone tissue materials based on additive manufacturing: A review

“…(D) The 3D pore structure was transformed into 2D planes using sample cutting, and the pore size data were measured indirectly by metallographic microscopy in 2D planes ( Wauthle et al, 2015 ). (E) Representative cross-sections were obtained by Micro-CT, which adequately represent the pore morphology and measurements of the distance between two points or lines that are representative of the pore size within the 2D cross-section ( Naghavi et al, 2022 ). (F,G) Using Micro-CT to obtain 2D cross-sectional images or 3D stereoscopic models of porous materials, the filling of different sizes of circular or spherical bodies was performed, as in Figure F ( Taniguchi et al, 2016 ).…”

“…However, since the pore in the 3D state is the same as the pore throat in the 3D state, the transformation of the 2D plane can be performed by changing the observation angle and cutting the material when the pore structure is regular, as shown in Figure 3D ( Wauthle et al, 2015 ; Zhang et al, 2020 ). However, in the 3D state, the pore has a more complex morphological structure compared to the pore throat, and sometimes it is only used to measure the pore size by considering the distance between two rods or walls that can roughly represent the pore size ( Gorgin Karaji et al, 2017 ; Ma et al, 2019 ; Naghavi et al, 2022 ). Nevertheless, there are some special structures, such as TPMS-Split p , lidinoid types, and bionic trabecular structures, where it is impossible to specify the morphology of the aperture, making it impossible to determine the specific dimensions from 2D or 3D morphology based on the relative relationship between points, lines, and surfaces ( Wang et al, 2018a ; Zhao et al, 2021 ; Zhu et al, 2021 ).…”

“…However, if planar switching of internal spatial structures is required, a highly accurate layer-by-layer scanning method of materials, such as Micro-CT or industrial CT, allows the acquisition of tomographic images for characterizing the layered morphology of materials and measuring local planes on tomographic 2D images ( Yavari et al, 2013 ; Li et al, 2018a ; Li et al, 2018b ; Schmidleithner et al, 2019 ; Li et al, 2020c ). The approach is, in essence, similar to that of direct measurement by optical instruments in the 2D plane, where the actual data measurement process is still highly dependent on the selection of 2D cross-section and view orientation, but its selection for 2D images are more accessible and accurate, as shown in Figure 3E ( Otsuki et al, 2006 ; Dong et al, 2020 ; Naghavi et al, 2022 ). Because of the limitations of this technical solution in terms of subjective judgment, the aperture diameter is measured using the maximal covering spheres (MCS) method after the 3D reconstruction of images based on techniques such as Micro-CT.…”

Stress Shielding and Bone Resorption of Press-Fit Polyether–Ether–Ketone (PEEK) Hip Prosthesis: A Sawbone Model Study

“…Krishna et al [ 20 ] proposed a porous titanium implant, with reduced stiffness (2–45 GPa) via laser engineered net shaping. Naghavi et al [ 21 , 22 ] performed a detailed mechanical (compression, tension, bending and torsion tensing) and morphological investigation on gyroid and diamond lattice structures suitable for development of porous hip stems to reduce stress shielding.…”