1. Introduction
Recent advancements in biomedical titanium alloys have significantly enhanced the performance and applicability of medical implants, crucial for improving patient outcomes and quality of life. Biocompatibility remains a fundamental requirement, with current research focusing on incorporating elements such as molybdenum (Mo), tantalum (Ta), and chromium (Cr) to enhance the interaction between the implants and biological tissues [
1,
2,
3]. These elements improve cell adhesion and reduce inflammation, ensuring harmonious integration with human tissues. The development of specialized alloys like shape memory titanium alloys, β-type titanium alloys, and high-entropy titanium alloys has also progressed, offering them enhanced mechanical properties, including increased strength and improved fatigue resistance. Notably, the Ti-24Nb-4Zr-8Sn alloy, prepared through laser powder bed fusion (LPBF), exemplifies the advancements in alloy design, providing near-full density parts suitable for use in acetabular prostheses [
4]. Additionally, surface modification techniques such as anodization, micro-arc oxidation, and bioactive coatings have been extensively studied to enhance their biocompatibility, wear resistance, and corrosion resistance, contributing to the long-term effectiveness and safety of titanium-based implants [
5,
6,
7].
Additive manufacturing (AM) technologies, including LPBF and Electron Beam Melting (EBM), have revolutionized the fabrication of titanium-based biomedical implants, enabling precise and complex designs that enhance osseointegration and mechanical compatibility [
1,
8]. These technologies allow for the creation of tailored structures that meet individual patient needs, thereby improving implant stability and reducing the risk of complications. For instance, Ti-Ta alloy lattice structures produced using laser powder bed fusion (L-PBF) exhibit superior fracture energy and mechanical performance, making them highly suitable for orthopedic applications [
9]. Furthermore, Nitinol, a nickel–titanium alloy known for its superelasticity and shape memory effects, has emerged as a significant material for various medical devices, including orthopedic implants and vascular stents [
10]. Ongoing research aims to optimize Nitinol’s processing techniques to further improve its mechanical properties and corrosion resistance, ensuring its durability and functionality in clinical applications [
11]. These advancements in alloy design, surface modification, and additive manufacturing technologies are paving the way for the next generation of more effective, durable, and biocompatible medical implants.
Recent advancements in 3D printing, particularly in the creation of lattice structures, have revolutionized various industries through the development of sophisticated additive manufacturing (AM) technologies. Notable progress has been made in Powder Bed Fusion (PBF), Direct Energy Deposition (DED), and the integration of AM with traditional manufacturing methods. PBF technologies, including Selective Laser Sintering (SLS) and selective laser melting (SLM), have enabled the precise fabrication of complex geometries, improving mechanical properties by optimizing processing parameters. Similarly, DED technologies like Laser-Engineered Net Shaping (LENS) and Wire and Arc Additive Manufacturing (WAAM) facilitate the production and repair of large-scale components, although they still face challenges in terms of surface finish and accuracy [
12,
13]. Hybrid methods, combining rapid prototyping with powder metallurgy, have also emerged, allowing the creation of intricate lattice patterns previously unattainable with conventional techniques [
14]. Nevertheless, Perez’s exploration of stiffening near-net-shape functional parts through crystallographic texture control and laser beam shaping provides a future path to use AM with improved mechanical properties [
15,
16,
17]. For instance, their study found that non-Gaussian beam profiles significantly influence the mechanical properties of IN718 during laser powder bed fusion (LPBF) [
17]. Gaussian beams led to higher porosity and deeper melt pools with lamellar structures, whereas ring beams produced more uniform melt pools with a consistent crystalline orientation. Nevertheless, although IN718 was studied in the investigations of crystallographic texture control and laser beam shaping, other materials can be investigated using the same methodology. In addition to this, the post-processing and finishing of lattice structures have been investigated by electrical discharge machining (EDM) [
18]. That study showed the great promise of IN718 through-hole drilling in analyzing the variability in the dimensions of the features caused by the periodically open pores.
Collaborative innovations have further propelled the field, with partnerships such as Farsoon Technologies, Hyperganic, and BASF Forward AM developing the Ultrasim 3D Lattice Engine. This platform simplifies lattice structure production, democratizing access to advanced AM capabilities [
19]. In terms of biomedical applications, researchers at the City University of Hong Kong have enhanced the strength of 3D-printed polymeric lattice components through partial carbonization, making them suitable for critical applications like coronary stents [
20]. The aerospace and automotive industries benefit from technologies like the 3D Cocooner by Festo, which creates bionic lattice structures with high tensile strength and structural integrity [
20]. These advancements demonstrate the transformative potential of 3D printing in producing lightweight, robust components across various sectors.
Ti-6Al-4V is a widely used titanium alloy in biomedical applications due to its excellent mechanical properties, corrosion resistance, and biocompatibility. This alloy, composed of titanium with 6% aluminum and 4% vanadium, offers high tensile strength and fatigue resistance, making it ideal for load-bearing implants such as hip and knee replacements, dental implants, and spinal fixation devices [
21]. Its modulus of elasticity, closer to human bone, reduces stress shielding effects, while its stable oxide layer protects against corrosion, enhancing implant longevity [
21]. However, challenges remain, particularly in improving osseointegration and preventing postoperative infections [
22]. Research has focused on advanced surface modification techniques, such as sand-blasting large-grit acid-etching (SLA) and silver nanoparticle coatings via electron beam evaporation (EBE), to enhance the alloy’s bioactivity and antibacterial properties [
22]. While these modifications show promise in promoting better bone cell attachment and reducing bacterial colonization, optimizing these coatings for long-term stability and effectiveness while maintaining biocompatibility is crucial [
22]. Thus, ongoing research aims to overcome these limitations, developing next-generation implants with enhanced performance and reliability in clinical settings. While aluminum (Al) and vanadium (V) ions are widely accepted as industry standards, new research has raised serious concerns about their potential toxicity in the body [
23,
24]. Instead of these harmful elements, alloying with Niobium (Nb), Tantalum (Ta), or Zirconium (Zr) has been attempted recently [
25,
26,
27]. As a result, these materials have been found to perform better than Ti-6Al-4V, which has motivated researchers to create new biocompatible metals with different alloying components.
Tantalum (Ta) is gaining interest because of its exceptional biocompatibility and excellent resistance to corrosion; nonetheless, its applications are confined to mostly surface coatings due to its high cost and machining difficulty [
28]. For biomedical applications, it is a promising alloying complement to pure titanium. When standard irregular tantalum powders are utilized, mixing Ta and Ti powders is an effective technique to lower the material’s cost and increase the flowability of the powder mixture [
29]. Furthermore, tantalum can stabilize titanium’s β, which is advantageous for biomedical applications (i.e., creating a low modulus and high strength) [
30]. Sing et al. [
28] report that, compared to commercially pure titanium (cp-Ti), titanium–tantalum (Ti–Ta) alloys exhibit higher relative strength (for comparable stiffness) and a lower elastic modulus, as well as superior corrosion resistance and superior biocompatibility properties. However, instead of using pure Ti, there are also some upcoming investigations around Ti6Al4V-Ta attempting to improve its mechanical properties. There is an ongoing investigation on the creation of Ti-6Al-4V by adding Ta at different weight ratios (0%, 2%, 4%, and 6% of Ta) [
30]. The results showed that strength and ductility increase when Ta is added to the Ti-6Al-4V alloy. The alloy Ti-6Al-4V–4Ta has a weaker selection of α-variants and the most consistent basketweave microstructure. A four percent weight addition of Ta improves its mechanical performance significantly (Ultimate Tensile Strength (UTS)—1042 MPa, yield strength (YS)—956 MPa, and elongation—10.8%) compared to Ti-6Al-4V–0Ta without Ta. These improved mechanical properties are ascribed to a higher dislocation density, solid Ta element solution, weaker α-variants selection, and refined microstructure. This exploration of integrating tantalum into titanium alloys (Ti-6Al-4V-Ta) enhances their mechanical properties and biocompatibility. The addition of tantalum significantly improves the strength, ductility, and antibacterial properties of the alloy, making it promising for medical implants. As Perez’s contributions provide a comprehensive pathway for using SLM and LPBF to create advanced materials with tailored properties, improve material performance, and expand their application in critical fields, Ti-6Al-4V-Ta has a chance to be the future investigated material after IN718 [
15,
16,
17]. Nevertheless, improving the post-processing and finishing of samples made of Ti-6Al-4V-Ta, as in the study of Singh et al. [
18], is an advantage for future investigations.
This research aims to estimate the mechanical and biological response properties of the Ti-6Al-4V-Ta alloy. First of all, this study investigates the mechanical performance and energy absorption characteristics of different lattice structures made of Ti-6Al-4V and Ti-6Al-4V-Ta alloys. This study identifies the elastic moduli, yield strengths, energy absorption values, and energy absorption efficiencies of various stretch- and bending-dominated lattice structures under quasistatic compression. Finally, the biomedical compatibility of Ti-6Al-4V and Ti-6Al-4V-Ta lattice alloys will be assessed via the adhesion of bacteria on the LPBF-printed surface.
2. Materials, Methodology, and Equipment
This section covers the design and manufacturing processes of the chosen material, the mechanical analysis and characterization of printed and tested material, and the bacteria adhesion methodology for the Ti-6AL-4V and Ti-6AL-4V-Ta alloys.
2.1. Lattice Structures’ Design
The design of the lattice specimens was mainly implemented using MSLattice for triply periodic minimal surface (TPMS) structures. MSLattice uses Matlab’s code but simplifies the work: only the type of TPMS lattice structure, density, unit cell size, height, and width need to be entered. After that, it provided the .stl files of the needed TPMS structures for this research. A literature review showed that Diamond, Gyroid, and Primitive (
Figure 1) lattice structures perform best overall in energy absorption and contain porosity and stiffness similar to human bones [
23,
24,
25,
26,
27].
In this proposed research, 30–40% density and a unit cell size (UCS) of 3 mm for Ti-6Al-4V and Ti-6Al-4V-Ta structures were investigated.
Table 1 represents the density and size of the lattice structures generated in MSLattice.
2.2. LPBF Process Parameters and Manufacturing
The specimens used in this study were LPBF-printed Ti-6Al-4V ELI and Ti-6Al-4V-Ta alloys. The powder Ti-6Al-4V ELI, with a grade size of 20–53 μm, was acquired from Sino-Euro Materials of Xi’an Co, Ltd. (Sino-Euro, Xi’an, China), a Northwest Institute for Non-ferrous Metal Research division. Also, as an alloying option for this study, Ti-6Al-4V powder was mixed with Ta powder. It creates a mixture of 92 wt% Ti-6Al-4V and 8 wt% pure tantalum (Ta). The Ta powder has a grain size of 15-53 μm and was bought from Luoyang Tongrun Info Technology Co., Ltd. (Luoyang, Henan, China). A mixture machine Inversina 2L Tumbler Mixer which is bought from Bioengineering AG (Wald, Switzerland) was used to mix the powders. The Ti–6Al-4V-Ta powder was tumbled and mixed for 7.5 h at 60 rpm.
A Renishaw AM 400 (Renishaw plc, Staffordshire, United Kingdom) was utilized to produce the specimens using the LPBF principle. The process parameters selected were the default Renishaw suggestions, chosen from the software library for Ti-6Al-4V. Hence, all samples were manufactured with a laser power of 200 W, hatch spacing of 0.08 mm, scanning speed rate of 1112 mm/s, and layer thickness of 0.03 mm. These LPBF-printing process parameters were used for both alloying options made of Ti-6Al-4V and Ti-6Al-4V-Ta. All printing was carried out using the reduced build volume (RBV) platform.
2.3. Lattice Structure Characterization
Compression tests were performed on the TPMS lattice structures highlighted in
Table 1. An image of the test setup,
Figure A1, can be found in
Appendix A. During the test, a specimen is compressed from the up to the down position, as seen in
Figure A1, and a preload of up to 50 N is applied to fix the samples before the test run. The compression tests were conducted at a strain rate of 1 mm/min until the ultimate compression strength value was reached. After the test, other mechanical characteristics were investigated such as stress–strain, energy absorption, and its efficiency, plateau stress.
The values of the stress (represented by
σ) and strain (represented by
ε) are determined based on factors such as the force applied by the compression plate to the lattice (
F), the initial area of the lattice base (
A₀), the amount of deformation (Δ), and the original height of the structure (ℎ₀). These calculations can be expressed as follows:
The energy absorption was obtained as follows:
where ε
d is the densification strain.
The efficiency of energy absorption is provided by
where
σm is the maximum stress on the stress–strain curve and the efficiency of the bending-dominated lattice structures is higher than that of the stretch-based lattice structures due to the smooth stress–strain profile of the bending-dominated lattices [
31].
Plateau stress is calculated by
where ε
y is the strain when the first peak occurs.
2.4. Antibacterial Tests
The present study aimed to investigate the antibacterial efficacy of Ti-6Al-4V and Ti-6Al-4V + 8% Ta against various bacterial strains, namely Staphylococcus aureus (ATCC 6538-P),
Pseudomonas aeruginosa,
Escherichia coli (DH5α), and
Bacillus subtilis, by quantifying the amount of biofilm formed on the surface of different metal cubes after 48 h of inoculation in broth culture. The bacterial inoculum for the biofilm formation tests was prepared in this work using a method similar to our earlier work [
32].
All metal cubes were sterilized using a three-step process to ensure the removal of all potential contaminants. Initially, the cubes were treated with 70% ethanol for 30 min. Subsequently, they were plasma-cleaned for 5 min (Plasma Cleaner PDC-002-CE, Harrick Plasma, New York, NY, USA). The metal cubes were then sterilized under UV light for 30 min to ensure complete sterilization before culturing them with bacteria. To prepare the inoculum, a 50 mL Falcon tube was filled with Luria-Bertani (LB) broth, with the exception of S. aureus, which was grown in Tryptone Soy Broth (TSB). Each metal cube was then suspended in a separate 50 mL Falcon tube containing a single species of bacterium in the culture medium. These tubes were incubated under aerobic conditions in a shaker incubator at 37 °C and at a speed of 220 RPM for 48 h.
After 48 h of incubation, the broth was carefully decanted to eliminate non-adherent (planktonic) bacteria. The metal cubes harboring biofilms were then gently rinsed in sterile distilled water to remove loosely attached cells, ensuring the preservation of the biofilm structure. Subsequently, each metal cube was transferred into separate wells of a 24-well plate and allowed to air-dry for 30 min to ensure uniformity in subsequent staining procedures.
To visualize and quantify biofilm formation, 1.3 mL of 0.1% crystal violet solution was added to each well, covering the metal cube surface for 60 min. Excess crystal violet was carefully aspirated, and the metal cubes were washed thrice with distilled water to remove unbound dye. The samples were examined and imaged using a Zeiss AxioZoom V16 macroscope (Jena, Germany). Exposure times of 20 ms and 400 ms were used to capture both bright-field and red fluorescent images.
A 30% acetic acid solution was used for the quantitative evaluation of biofilm formation. To dissolve the crystal violet, 1.3 mL of the acetic acid solution was added to each well containing a metal cube in the 24-well plate. To facilitate the dissolution process, the 24-well plate was placed on a shaker and agitated for 30 min at 180 rpm. After this agitation process, the metal cubes were carefully removed from the wells to avoid disrupting the solubilized dye solution. To quantify the biofilm biomass, 100 µL of the acetic acid solution, now containing the dissolved crystal violet, was transferred from each well into a corresponding well of a 96-well plate. The absorbance (OD 590) values of the solutions in the 96-well plate were measured using a VICTOR Nivo multimode plate reader.