Additive manufacturing of Ti-Ni bimetallic structures

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.

3D printed silicon nitride, alumina, and hydroxyapatite ceramic reinforced Ti6Al4V composites - Tailored microstructures to enhance bio-tribo-corrosion and antibacterial properties

This study utilized directed energy deposition (DED) as a metal additive manufacturing (AM) technique to create ceramic-reinforced composites of Ti6Al4V (Ti64) with hydroxyapatite (HA), alumina (Al2O3), and silicon nitride (Si3N4). The resulting composites had tailored microstructures designed to improve bio-tribological and antibacterial properties simultaneously. A total of 5-wt % ceramic reinforcement were used in Ti64 in four different composites - (1) only Si3N4 (5S), (2) only Al2O3 (5A), (3) 3 wt % Si3N4 and 2 wt% HA (32SH) and (4) 3 wt % Al2O3 and 2 wt% HA (32AH). Microstructural observations revealed that martensite transformation between α and β-Ti in composites resulted in compressive residual stress at the matrix. Coherency is observed between the ceramic particles and Ti64 matrix, preventing cracking, debonding, or porosity. Vicker's hardness of the composite samples increases by 50% over the Ti64 matrix. Various strengthening mechanisms are discussed in detail, representing the reason behind the reduction of compound wear in 5S and 5A composites. Si3N4-added composites demonstrated an antibacterial response against gram-positive Staphylococcus aureus. The multifunctional performance of ceramic-reinforced Ti64 composites makes them suitable for articulating biomedical devices such as femoral heads in hip implants.

Radial bimetallic structures via wire arc directed energy deposition-based additive manufacturing

Bimetallic wire arc additive manufacturing (AM) has traditionally been limited to depositions characterized by single planar interfaces. This study demonstrates a more complex radial interface concept, with in situ mechanical interlocking and as-built properties suggesting a prestressed compressive effect. A 308 L stainless core is surrounded by a mild steel casing, incrementally maintaining the interface throughout the Z-direction. A small difference in the thermal expansion coefficient between these steels creates residual stresses at their interface. X-ray diffraction analysis confirms phase purity and microstructural characterization reveals columnar grain growth independent of layer transitions. Hardness values are consistent with thermal dissipation characteristics, and the compressive strength of the bimetallic structures shows a 33% to 42% improvement over monolithic controls. Our results demonstrate that biomimetic radial bimetallic variation is feasible with improved mechanical response over monolithic compositions, providing a basis for advanced structural design and implementation using arc-based metal AM.

Additive Manufacturing: An Opportunity for the Fabrication of Near-Net-Shape NiTi Implants

Nickel–titanium (NiTi) is a shape-memory alloy, a type of material whose name is derived from its ability to recover its original shape upon heating to a certain temperature. NiTi falls under the umbrella of metallic materials, offering high superelasticity, acceptable corrosion resistance, a relatively low elastic modulus, and desirable biocompatibility. There are several challenges regarding the processing and machinability of NiTi, originating from its high ductility and reactivity. Additive manufacturing (AM), commonly known as 3D printing, is a promising candidate for solving problems in the fabrication of near-net-shape NiTi biomaterials with controlled porosity. Powder-bed fusion and directed energy deposition are AM approaches employed to produce synthetic NiTi implants. A short summary of the principles and the pros and cons of these approaches is provided. The influence of the operating parameters, which can change the microstructural features, including the porosity content and orientation of the crystals, on the mechanical properties is addressed. Surface-modification techniques are recommended for suppressing the Ni ion leaching from the surface of AM-fabricated NiTi, which is a technical challenge faced by the long-term in vivo application of NiTi.