Selective laser melted Ti6Al4V split-P TPMS lattices for bone tissue engineering

https://doi.org/10.1016/j.ijmecsci.2023.108353Get rights and content

Highlights

  • Split-P Ti6Al4V lattices successfully fabricated by selective laser melting.

  • Lattice morphology influences manufacturability and compressive behavior.

  • Anisotropy index was quantified based on cell morphology and relative density.

  • Mechanical properties of lattices are comparable to cortical or trabecular bone.

  • Split-P lattices are potential candidates for load-bearing applications.

Abstract

This study investigates Ti6Al4V Split-P triply periodic minimal surface (TPMS) structures produced by selective laser melting for the first time. The designs include two different cell morphologies (CM) with five different relative densities (RD). Scanning electron microscopy is utilized to assess the manufacturability and accuracy of the 3D printed Split-P lattice structures. The quasi-static mechanical responses are then studied to identify the failure mechanisms of each lattice type. Afterward, the stress-strain behavior and plateau stress responses are explored to evaluate the load-bearing capacity and energy absorption of five Split-P lattices. Furthermore, finite element analysis is performed to gain insight into the elasto-plastic behavior of the Split-P structures, and unit cell homogenization is employed to determine the equivalent stiffness tensor. The results demonstrate that the elastic modulus, yield strength, and ultimate strength of the 3D printed Split-P lattices range from 1.50 to 3.50 GPa, 57.95 to 152.74 MPa, and 93 to 170 MPa, respectively. Depending on RD and CM, load-bearing capacity range from 0.04 to 0.17. The energy absorption capacity and plateau stress of lattices vary from 22 to 61 MJ/m3 and 30 to 63 MPa, respectively. Gibson-Ashby power law represents the elastic modulus and yield stress of the Split-P lattices as a function of RD and reveals a stretch-dominated deformation behavior. Unit cell homogenization on Split-P structures with zero isovalue shows isotropy in mechanical properties and anisotropy for lattices with non-zero isovalue. Ti6Al4V Split-P lattices with the highest surface area and surface area-to-volume ratio, among other TPMS, can achieve mechanical properties close to those of trabecular and cortical bones, making them suitable for bone implants in load-bearing applications.

Introduction

Repairing bone defects traditionally involves autografts, allografts, and xenografts [1]. However, long-term outcomes may be compromised by severe risks associated with these treatments, including infection, potential morbidity at the donor site, and unpredictable functional outcomes [2]. To overcome the inherent limitations of autograft and allograft, bone-graft substitutes were developed as an alternative strategy, and biomaterials have also been used for bone tissue repair as a bone substitution [3]. Bone graft substitutes can be classified as synthetic, inorganic, or bio-organic [3,4]. Specially designed scaffolds produced by biomaterials can support three-dimensional (3D) tissue growth, maintain physical integrity, and promote bone ingrowth at the defect site [5]. A scaffold can facilitate the formation of uniform bone tissue by providing a guiding template. In general, scaffolds do not replace bone permanently as they resorb over time [4,6]. In contrast, bone implants remain permanently in the body to provide long-term structural support [3,4,6,7]. In tissue engineering (TE), the suitability of a bone construct is determined by four factors: biocompatibility, biodegradability, mechanical properties, and architecture [4,8]. Bone substitutes with relatively large surface area (SA) and surface area-to-volume ratio (SA/VR) with significant interconnectivity of pores promote osteoblast activity [9,10]. Therefore, pore size, pore shape, and porosity correlate with bone ingrowth [11], [12], [13], [14].

Human trabecular bone is a highly porous tissue with intricately interconnected plates and struts. Such complex architecture can be represented by lattices based on triply periodic minimal surfaces (TPMS). Fig. 1 shows a schematic representation of the Split-P TPMS lattice that resembles the morphology of trabecular bone.

TPMS-based lattices can provide the elastic modulus in the range of human trabecular and cortical bones and achieve the required compressive strength, energy absorption, and fatigue resistance, which are not easily obtained with strut-based structures [15], [16], [17]. Recently, TPMS structures have been explored as bone scaffolds and implants due to their smooth surfaces, high interconnectivity porous architectures, and mathematically controllable geometry [18], [19], [20], [21].

TPMS-based architectures have minimal surfaces with zero mean curvature at any point. Examples include Gyroid, Diamond, Neovius, Primitive, and Split-P [22], [23], [24]. Numerous studies have been conducted on the mechanical behavior of Gyroid, Diamond, and Primitive TPMS in orthopedic applications [20,[25], [26], [27]]. In contrast, less is known about the mechanical response of Split-P structures. Mubasher et al. [28] studied the mechanical behavior of seven different polymeric lattices and found that Split-P ranked third in energy absorption capacity. Miralbes et al. [29] investigated six different polymeric TPMS lattices under a quasi-static compression test; their results showed the remarkable energy absorption capacity of the Split-P lattice. Lehder et al. [30] presented a method to optimize the geometry of six TPMS-based lattices, including Split-P, made of nylon, to maximize cell growth rate while maintaining an elastic modulus equivalent to human bone. They found that the Split-P morphology is a strong candidate for bone scaffolds after the Lilinoid architecture, which has the highest cell growth rate due to its high pore size, large SA/VR, and high local curvature. Recently, extensive experiments and numerical simulations have been performed to investigate the mechanical behavior of cellular structures to achieve high strength and stability [31], [32], [33]. However, there is still limited information on the suitability of Split-P TPMS for bone substitutions.

Advances in additive manufacturing (AM) have enabled the fabrication of cellular solids with complex shapes, including porous materials and lattice structures [34,35]. Selective laser melting (SLM), one of the 3D printing technologies, is of high interest as it enables the customization of complex structures [36,37] and offers advantages over traditional manufacturing methods, such as time savings, lower material consumption, and fabrication of complex shapes [38]. SLM allows freedom in modulating the geometric specifications of complex metallic structures in orthopedic and other engineering applications.

SLM has demonstrated the ability to utilize various types of metal powders. One of the preferred metal materials for medical applications is titanium and its alloys [39,40]. Ti6Al4V is widely used for bone implants due to its good biocompatibility, predictable mechanical performance, and high corrosion resistance [41,42]. However, its elastic modulus ranges from 110 to 120 GPa [38], which is significantly larger than that of compact cortical bone (3–20 GPa) and porous trabecular bone (0.1–4.5 GPa) [43,44]. Such a high mismatch in the mechanical properties of a titanium alloy and human bone may result in stress shielding, leading to bone resorption.

This paper focuses on the experimental and numerical evaluation of Ti6Al4V Spilt-P TPMS lattices additively manufactured by SLM. The fabricated lattices have large SA/VR and high curvatures, characteristics that promote bone growth. Quasi-static uniaxial compression tests were used to investigate the mechanical behavior of the lattices. A quasi-static finite element analysis (FEA) was applied to simulate the elastic-plastic deformation behavior of the as-built lattices. To our knowledge, this is the first study investigating the mechanical behavior of 3D printed biocompatible Ti6Al4V alloy Split-P lattices using SLM.

The contents of the paper are as follows. Section 2 presents all methodology steps, including design parameters and printing procedures, followed by mechanical testing and numerical simulation procedures. Section 3 describes the structural response of the lattices, including the Gibson-Ashby relation and the energy absorption capacity. Homogenization of the unit cells is then performed to determine equivalent elastic moduli. Section 4 discusses the research findings, suggests possible areas of future work, and states the limitations of this study. Afterward, a comparison is made between the properties of human bone (trabecular and cortical parts) and fabricated lattices. Finally, conclusions are drawn in Section 5.

Section snippets

Materials and methods

This section discusses the designs, followed by the preparation process. After describing the mechanical testing procedure, a finite element analysis is presented, followed by the investigation of the anisotropy of the Split-P unit cells using numerical periodic homogenization.

Results

The first part of this section focuses on manufacturing imperfections and experimental surface morphology. After presenting the effect of the manufacturing process on the RD of the produced Split-P lattices, the structural response under the quasi-static compression test will be presented along with the Gibson-Ashby model. In the following sections, the effect of cell morphology on the mechanical response will be shown and all fabricated lattices will be evaluated for energy absorption

Discussion

This study investigates SLM fabricated Split-P TPMS lattices with different porosities and CM as potential bone substitutes for the first time. The SLM process was used to fabricate Ti6Al4V Split-P lattices demonstrating that AM technologies can fabricate Split-P lattices on specific structural supports, depending on RD and thickness. We found that for some porosities and CM, it is essential to consider support structures to secure overhanging features and parts of the build platform.

Conclusions

This study, for the first time, investigated experimentally and computationally the compressive behavior of Ti6AL4V SLM-manufactured Split-P TPMS lattices with different relative densities (RD) and cell morphologies (CM) for trabecular and cortical bone substitutes.

The present study found that the size of the unit cells and the thickness of the walls can affect the manufacturability of Split-P lattices. With the large size of the unit cell, low wall thickness, and specific porosities, the

CRediT authorship contribution statement

Mansoureh Rezapourian: Formal analysis, Investigation, Methodology, Software, Visualization, Writing – original draft. Iwona Jasiuk: Conceptualization, Data curation, Formal analysis, Methodology, Validation, Writing – review & editing. Mart Saarna: Investigation. Irina Hussainova: Conceptualization, Data curation, Formal analysis, Funding acquisition, Project administration, Resources, Supervision, Validation, Writing – review & editing.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by the Estonian Research Council grant (PRG643, I. Hussainova). The authors wish to express their gratitude to Dr. Le Liu for his technical assistance in the additive manufacturing process and to Dr. Olga Volobujeva for SEM images.

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