Additively manufactured Ti-Ta-Cu alloys for the next-generation load-bearing implants

Bacterial colonization of orthopedic implants is one of the leading causes of failure and clinical complexities for load-bearing metallic implants. Topical or systemic administration of antibiotics may not offer the most efficient defense against colonization, especially in the case of secondary infection, leading to surgical removal of implants and in some cases even limbs. In this study, laser powder bed fusion was implemented to fabricate Ti3Al2V alloy by a 1:1 weight mixture of CpTi and Ti6Al4V powders. Ti-Tantalum (Ta)-Copper (Cu) alloys were further analyzed by the addition of Ta and Cu into the Ti3Al2V custom alloy. The biological, mechanical, and tribo-biocorrosion properties of Ti3Al2V alloy were evaluated. A 10 wt.% Ta (10Ta) and 3 wt.% Cu (3Cu) were added to the Ti3Al2V alloy to enhance biocompatibility and impart inherent bacterial resistance. Additively manufactured implants were investigated for resistance against Pseudomonas aeruginosa and Staphylococcus aureus strains of bacteria for up to 48 h. A 3 wt.% Cu addition to Ti3Al2V displayed improved antibacterial efficacy, i.e. 78%-86% with respect to CpTi. Mechanical properties for Ti3Al2V-10Ta-3Cu alloy were evaluated, demonstrating excellent fatigue resistance, exceptional shear strength, and improved tribological and tribo-biocorrosion characteristics when compared to Ti6Al4V. In vivo studies using a rat distal femur model revealed improved early-stage osseointegration for alloys with 10 wt.% Ta addition compared to CpTi and Ti6Al4V. The 3 wt.% Cu-added compositions displayed biocompatibility and no adverse inflammatory response in vivo. Our results establish the Ti3Al2V-10Ta-3Cu alloy's synergistic effect on improving both in vivo biocompatibility and microbial resistance for the next generation of load-bearing metallic implants.

Additively Manufactured SiO2 and Cu-Added Ti Implants for Synergistic Enhancement of Bone Formation and Antibacterial Efficacy

Commercially pure titanium (CpTi), a bioinert metal, is used as an implant material at low load-bearing sites and as a porous coating on Ti6Al4V at high load-bearing sites. There is an unmet need for metallic biomaterials to improve osseointegration and inherent antimicrobial resistance. In this study, we have added 1 wt % SiO2 and 3 wt % Cu to the CpTi matrix and processed via metal additive manufacturing (AM). Si4+ ions promote angiogenesis and osteogenesis. CpTi-SiO2 composition exhibited 4.5 times higher bone formation at the bone-implant interface over CpTi in an in vivo study with a rat distal femur model. In vitro bacterial studies with Gram-positive Staphylococcus aureus bacterium revealed 85% antibacterial efficacy by CpTi-SiO2-3Cu than CpTi. CpTi-SiO2-3Cu did not show any inflammatory markers in vivo, indicating the absence of cytotoxicity, but displayed delayed osseointegration compared to CpTi-SiO2. CpTi-SiO2-3Cu displayed 3-fold higher mineralized bone formation than CpTi. Our results emphasize the synergistic effect of SiO2 and Cu addition in CpTi, promoting enhanced early stage osseointegration and inherent antibacterial efficacy, contributing toward implant longevity and stability in vivo.

Enhanced osteogenesis and bactericidal performance with additively manufactured MgO and Cu-added CpTi for load-bearing implants

Titanium, being the ultimate choice of metallic material for implant applications, its bio-inertness causes delayed bone-tissue integration at the implant site and prevents expedited healing for the patient. This can cause a severe issue for patients with immunocompromised bone health. Infections at the implant site are another concern; titanium does not offer inherent antimicrobial properties. Current strategies addressing the issues above include using cemented implants as a coating on Ti6Al4V bulk material for orthopedic applications. Roadblock arises with coating failure due to weak interfacial bond at the Ti-cement interface, resulting in revision surgeries. We have added osteogenic MgO and antibacterial Cu to CpTi and processed them using metal additive manufacturing (AM) to address these issues. Mg, an essential trace element in the body, has been proven to enhance osseointegration in vivo. Cu has been popular for its bactericidal capabilities. With 1 wt.% of MgO addition in the CpTi matrix, we have observed a four-fold increase in the mineralized bone formation at the bone-implant interface in vivo. The presence of 3 wt.% of Cu showed no cytotoxicity markers, and adding Cu to CpTi-MgO chemical makeup showed similar in vivo performance to CpTi-MgO. In vitro bacterial studies with gram-positive Staphylococcus aureus bacteria showed 81% bacterial efficiency displayed by CpTi-MgO-Cu at the end of 72 h of culture. Our findings highlight the synergistic benefits of CpTi-MgO-Cu, which exhibit superior early-stage osseointegration and antimicrobial capabilities.

Fatigue behavior of additively manufactured Ti3Al2V alloy

This study measured the tensile, compression, and fatigue behavior of additively manufactured Ti3Al2V as a function of build orientation. Ti3Al2V alloy was prepared by mixing commercially pure titanium (CpTi) and Ti6Al4V in 1:1 wt. ratio. Laser powder bed fusion (L-PBF) based additive manufacturing (AM) technique was used to fabricate the samples. Tensile tests resulted in an ultimate strength of 989 ± 8 MPa for Ti3Al2V. Ti6Al4V 90° orientation samples showed a compressive yield strength of 1178 ± 33 MPa, and that for Ti3Al2V 90° orientation was 968 ± 24 MPa. Varying the build orientation to account for anisotropy, Ti32-45° and Ti32-0° displayed similar compressive yield strength values of 1071 ± 16 and 1051± 18 MPa, respectively, higher than Ti32-90°. Fatigue loading revealed an endurance limit (10 million cycles) of 250 MPa for Ti6Al4V and 219 MPa for Ti3Al2V built at 90° orientations. The effect of the build orientation was significant under fatigue loading; Ti3Al2V built at 45° displayed an endurance limit of 387.5 MPa, and 0° showed 512 MPa; more than two-fold increment in endurance limit was observed. Our results show the potential of Ti3Al2V alloy for orthopedic devices, replacing Ti6Al4V alloy, particularly in load-bearing applications.

Influence of strut-size and cell-size variations on porous Ti6Al4V structures for load-bearing implants

Mechanical properties of porous metal coatings in load-bearing implants play a critical role in determining the in vivo lifetime. However, there is a knowledge gap in measuring the shear strength of porous metal coatings at the porous-dense interface. This study evaluated pore morphology dependence and strut-size on compression, shear deformation, and in vitro response of additively manufactured porous Ti6Al4V structures. Selective laser melting (SLM)-based additive manufacturing (AM) technique was used to process two types of structures with honeycomb cell design-one with constant cell-size of ∼470 μm with mean strut-size varying from 92 to 134 μm, and denoted as strut-size variation (SSV); and the other with a constant strut-size of ∼135 μm with mean cell-size varying from 580 to 740 μm, denoted as cell-size variation (CSV). It was observed that under compressive loading, changes in elastic modulus were more sensitive to variations in strut-size over cell-size. Under shear loading at the porous-dense interface, strength enhancement and material hardening were observed in both SSV and CSV samples due to pore-collapsing. Our results show that for hexagonal cell designs, shear behavior is more sensitive to variations in cell-size over strut-size, although elastic modulus is more sensitive to changes in strut-size for porous metallic structures. From in vitro hFOB analysis, it was observed that pore size of 670 μm demonstrated the highest osteoblast cell viability among porous structures with evidence of pore-bridging by cells. P. aeruginosa bacterial culture showed that bacterial cell viability was higher for porous structures than dense Ti, with evidence of pore-bridging by bacterial cells.