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.

Porous metal implants: processing, properties, and challenges

Porous and functionally graded materials have seen extensive applications in modern biomedical devices-allowing for improved site-specific performance; their appreciable mechanical, corrosive, and biocompatible properties are highly sought after for lightweight and high-strength load-bearing orthopedic and dental implants. Examples of such porous materials are metals, ceramics, and polymers. Although, easy to manufacture and lightweight, porous polymers do not inherently exhibit the required mechanical strength for hard tissue repair or replacement. Alternatively, porous ceramics are brittle and do not possess the required fatigue resistance. On the other hand, porous biocompatible metals have shown tailorable strength, fatigue resistance, and toughness. Thereby, a significant interest in investigating the manufacturing challenges of porous metals has taken place in recent years. Past research has shown that once the advantages of porous metallic structures in the orthopedic implant industry have been realized, their biological and biomechanical compatibility-with the host bone-has been followed up with extensive methodical research. Various manufacturing methods for porous or functionally graded metals are discussed and compared in this review, specifically, how the manufacturing process influences microstructure, graded composition, porosity, biocompatibility, and mechanical properties. Most of the studies discussed in this review are related to porous structures for bone implant applications; however, the understanding of these investigations may also be extended to other devices beyond the biomedical field.

Biotribocorrosion of 3D-Printed silica-coated Ti6Al4V for load-bearing implants

Laser-based 3D Printing was utilized to deposit a silica (SiO₂) coating on the surface of Ti6Al4V (Ti64) alloy for implementation onto articulating surfaces of load-bearing implants. The surface laser melting (SLM) technique was implemented in 1, and 2 laser passes (1LP and 2LP) after SiO₂ deposition to understand the influence of remelting on the coating's hardness and tribological performance. It was observed that compositional and microstructural features increased the cross-sectional hardness. Wear rate was observed to decrease from 2.9×10^-4 in the Ti64 to 5.2 ×10^-6, 3.8×10^-6, and 2.1×10^-7 mm³/Nm for the as-processed or zero laser-pass (0LP), 1LP, and 2LP, respectively. Coated samples displayed a positive shift in open-circuit potential (OCP) during linear wear by displaying a 368, 85, and 613 mV increase compared to Ti64 for 0LP, 1LP, and 2LP, respectively. Our results showed promising tribological performance of SiO₂ coated Ti6Al4V for articulating surfaces of load-bearing implants.

Zirconia-Toughened Alumina Coated Ti6Al4V via Additive Manufacturing

CoCr alloy-based femoral heads have failed prematurely due to galvanic-induced corrosion when coupled with a titanium hip stem. Coupling a titanium based-femoral head with the titanium hip stem is ideal in addressing this failure mode. Ti6Al4V (Ti64) alloy was reinforced with zirconia-toughened alumina (ZTA) by directed-energy deposition (DED)-based additive manufacturing (AM) to address that concern. Preliminary materials processing work resulted in failed samples due to cracking, porosity, and delamination. After careful parameter optimization, a Ti64+5wt.%ZTA (5ZTA) composition produced a metallurgically sound and coherent interface, minimal porosity, and bulk structures. Hardness was observed to increase by 27%, normalized wear rate reduced by 25%, and contact resistance increased during in vitro tribological testing along with faster surface re-passivation.

Hydroxyapatite reinforced Ti6Al4V composites for load-bearing implants

Titanium has been used in various biomedical applications; however, titanium exhibits poor wear resistance, and its bioinert surface slows osseointegration in vivo. In this study, directed energy deposition (DED)-based additive manufacturing (AM) was used to process hydroxyapatite (HA) reinforced Ti6Al4V (Ti64) composites to improve biocompatibility and wear resistance simultaneously. Electron micrographs of the composites revealed dense microstructures where HA was observed at the β-phase grain boundaries. Hardness increased by 57% and 71% for 2 and 3 wt.% HA in Ti64 composites, respectively. XRD analysis revealed no change in the phases with the addition of HA, when compared to the control. Tribological studies displayed an increase in contact resistance (CR) due to an in situ formed HA-based tribofilm, reduction in wear rate when testing in Dulbecco's Modified Eagle Medium (DMEM) with a ZrO₂ counter wear ball, <1% wear ball volume loss, and suppression of cohesive shear failure of the Ti matrix. Histomorphometric analysis from a rat distal femur study revealed an increase in the osteoid surface over the bone surface (OS/BS) for 3 wt.% HA composite over the control Ti64 from 9 ± 1% to 14 ± 1%. Additionally, from push-out testing, the shear modulus was observed to increase from 17 ± 3 MPa for control Ti64 to 32 ± 5 MPa for the 3 wt.% HA composite after 5-weeks in vivo. The present study demonstrates that the addition of HA in Ti64 can simultaneously improve bone tissue-implant response and wear resistance.

Alumina and tricalcium phosphate added CoCr alloy for load-bearing implants

Cobalt-chromium (CoCr) alloys are used in load-bearing implants due to their excellent wear resistance. However, poor tissue-material interactions can originate due to the release of wear and corrosion-induced Co and Cr ions, motivating the use of surface modification to reduce such phenomena. Premixed feedstock powders of CoCrMo + 2 wt.% tricalcium phosphate (TCP, CoCr_TCP) and CoCrMo + 2 wt.% tricalcium phosphate + 4 wt.% Al₂O₃ (CoCr_TCP+Al) were used to surface coat CoCr alloy via Laser Engineered Net Shaping (LENS™) with the objective of increasing CoCr alloy's wear resistance. Electron micrographs of the microstructure revealed the dissociation of intergranular globular carbide phases and reprecipitation into a finer network-like microstructure with homogeneous distribution of Co and Cr. X-ray diffraction (XRD) spectra revealed texturing or preferential crystallographic orientation amongst the LENS™ processed materials, with the TCP added CoCr displaying some ε-phase stabilization. Tribological testing resulted in an 82.3% and 71.6% decrease in wear rate and wear coefficient, respectively, for CoCr_TCP when compared to commercially available CoCr alloy. Additionally, in situ tribofilm development was observed for the fabricated samples via an increase in contact resistance. The current study resulted in not only a decrease in wear volume but also a decrease in the overall degradation of the coated CoCr alloy.

Titanium-Silicon on CoCr Alloy for Load-Bearing Implants Using Directed Energy Deposition-Based Additive Manufacturing

Cobalt-chromium (CoCr) alloys offer outstanding wear resistance when compared to other biocompatible metallic materials and are extensively used in articulating surfaces of total hip and knee arthroplasty. However, CoCr alloys' biocompatibility is known to be inferior to titanium (Ti). Wear- and corrosion-induced metal-ion release from CoCr alloys has been reported to cause cancer and negative physiological impacts. In this study, CoCr alloy was coated with commercially pure Ti (CpTi) and CpTi-Silicon (CoCr_Ti-Si) with the specific objective of reducing Co and Cr ion release during articulation, without degrading the excellent wear resistance of the CoCr alloy. Directed energy deposition (DED), a blown powder-based laser additive manufacturing technique, was utilized to process CpTi- and CpTi-Si-based coatings on Stellite 6B commercial CoCr alloy. Scanning electron microscopy (SEM), X-ray diffraction (XRD) analyses, and hardness testing found that refined carbides and titanium silicides increased the hardness from 321 ± 13 to 758 ± 48 HV_0.5. Tribological studies determined a comparable wear rate between Stellite 6B alloy and CoCr_Ti-Si in DI water but a statistically significant reduction in Dulbecco's Modified Eagle Medium (DMEM). The wear rates for Stellite 6B were 8.5 ± 0.8 × 10^-5 and 12.9 ± 0.4 × 10^-5 mm³/Nm in DI water and DMEM, respectively. While the wear rates for CoCr_Ti-Si were 9.1 ± 0.5 × 10^-5 and 8.9 ± 0.8 × 10^-5 mm³/Nm in DI water and DMEM, respectively. Contact resistance acquisition displayed the presence of a passive film formation during tribological testing. ICP-MS results for Stellite 6B and CoCr_Ti-Si concluded a reduction of Co ions release in DI water from 149.8 ± 66.7 to 17.5 ± 0.7 ppb and a reduction in Cr ions release from 66.7 ± 32.4 to 18.0 ± 0.5 ppb, respectively. In DMEM media, Co ion release for Stellite 6B and CoCr_Ti-Si reduced from 10.1 ± 1.4 to 4.1 ± 0.2 ppb and Cr ion release for Stellite 6B and CoCr_Ti-Si reduced from 8.7 ± 0.2 to 5.0 ± 0.7 ppb, respectively. The current study revealed a new mode of manufacturing for CoCr alloy-based load-bearing implants that can reduce toxic metal ions release due to wear- and corrosion-induced damages.

Additively Manufactured Ti6Al4V-Si-Hydroxyapatite composites for articulating surfaces of load-bearing implants

Directed-energy deposition (DED)-based additive manufacturing (AM) was explored for composite development using silicon (Si) and hydroxyapatite (HA) in Ti-6Al-4V (Ti64) matrix for articulating surfaces of load-bearing implants. Specifically, laser engineered net shaping (LENS™), a commercially available DED-based AM technique, was used to fabricate composites from premixed-feedstock powders. The AM'd composites proved to not only improve upon Ti64's mechanical properties but also produced an in-situ Si-based tribofilm during tribological testing that minimized wear induced damage. Additionally, it was found that with the introduction of Si, titanium silicides and vanadium silicides were formed; allowing for 114% increased hardness, decreased coefficient of friction (COF) and a reduction of wear rate of 38.1% and 48.7%, respectively. The produced composites also displayed a positive shift in open-circuit potential (OCP) during linear wear, along with a reduction in the change of OCP from idle to linear wear conditions. Additionally, contact resistance (CR) values increased with a maximum value of 1500 ohms due to the formation of Si-based tribofilm on the wear surface. Such composite development approach using DED-based AM can open up the possibilities of innovating next-generation implants that are designed and manufactured via multi-material AM.

Additively manufactured calcium phosphate reinforced CoCrMo alloy: Bio-tribological and biocompatibility evaluation for load-bearing implants

Cobalt-chromium-molybdenum (CoCrMo) alloys are widely used in load-bearing implants; specifically, in hip, knee, and spinal applications due to their excellent wear resistance. However, due to in vivo corrosion and mechanically assisted corrosion, metal ion release occurs and accounts for poor biocompatibility. Therefore, a significant interest to find an alternative to CoCrMo alloy exists. In the present work we hypothesize that calcium phosphate (CaP) will behave as a solid lubricant in CoCrMo alloy under tribological testing, thereby minimizing wear and metal ion release concerns associated with CoCrMo alloy. CoCrMo-CaP composite coatings were processed using laser engineered net shaping (LENS™) system. After LENS™ processing, CoCrMo alloy was subjected to laser surface melting (LSM) using the same LENS™ set-up. Samples were investigated for microstructural features, phase identification, and biocompatibility. It was found that LSM treated CoCrMo improved wear resistance by 5 times. CoCrMo-CaP composites displayed the formation of a phosphorus-based tribofilm. In vitro cell-material interactions study showed no cytotoxic effect. Sprague-Dawley rat and rabbit in vivo study displayed increased osteoid formation for CoCrMo-CaP composites, up to 2 wt.% CaP. Our results show that careful surface modification treatments can simultaneously improve wear resistance and in vivo biocompatibility of CoCrMo alloy, which can correlate to a reduction of metal ion release in vivo.

Influence of boron nitride reinforcement to improve high temperature oxidation resistance of titanium

Influence of boron nitride (BN) addition in commercially pure titanium (Cp-Ti) was characterized for their microstructural variation, hardness and oxidation kinetics. Feedstock powders Cp-Ti with 3 wt.% BN (3BN) and 6 wt.% BN (6BN) were prepared by roller mill followed by additive manufacturing using laser engineered net shaping (LENS™). Rate of oxidation was measured from thermogravimetric analysis (TGA) at 1000°C for 50 h. Average instantaneous parabolic constants (k _p ) for Cp-Ti, 3BN and 6BN were 41.2±12.0, 28.6±2.8 and 18.2±9.2 mg²·cm^-4·h^-1, respectively. Cp-Ti displayed acicular α-Ti microstructure. After TGA, large equiaxed grains along with TiO₂ formation at the grain boundaries was observed, which increased the hardness. With BN addition, plate-like TiN and needle-like TiB secondary phases were also observed. Hardness for Cp-Ti, 3BN and 6BN were 256.9, 424.0 and 548.3 HV_0.2, respectively. Overall, a small addition of BN was effective in improving the oxidation resistance of Cp-Ti.