The review, authored by M. Mutharasu and J. Joel at the School of Mechanical Engineering, Vellore Institute of Technology in India, synthesizes a sprawling body of literature on additive manufacturing (AM) of metallic implants and proposes a unified design framework. Rather than optimizing biomaterials, architectures, printing processes, and bioactive coatings as separate silos — a fragmentation the authors identify as a persistent weakness in the field — the framework links patient anatomy, material selection, architected porosity, AM technology, and hybrid surface engineering into a single development pathway. The stakes are high: implants that are too stiff relative to bone can trigger stress shielding, a phenomenon in which the metal carries so much of the load that the surrounding bone resorbs and weakens over time, ultimately leading to loosening and revision surgery.

At the heart of the review is a deceptively simple engineering problem. Human bone is not a uniform material. Cortical bone, the dense outer shell, has an elastic modulus of roughly 15 to 20 gigapascals, while trabecular bone, the spongy interior, sits far lower at around 0.01 to 3 gigapascals. Fatigue strength follows a similar split, with cortical bone tolerating roughly 40 to 60 megapascals and trabecular bone only about 0.3 to 3 megapascals. Conventional dense metallic implants — titanium alloys, cobalt-chromium, stainless steel — are dramatically stiffer than any of these. Ti-6Al-4V, the workhorse of the industry, has an elastic modulus of approximately 130 gigapascals, more than six times that of cortical bone. The review’s proposed solution is to use topology-optimized lattice structures, printed via powder bed fusion, to bring the effective stiffness of the implant down into the physiological range of 0.01 to 20 gigapascals, targeting volumetric porosity levels between 50 and 85 percent.

But the review is candid about the trade-offs. Increasing porosity does reduce effective modulus and can improve tissue ingrowth, yet it simultaneously erodes fatigue resistance. At high porosity levels, the lattice relies on ultra-thin struts that shift the primary failure mechanism from bulk compression to localized elastoplastic bending. Under cyclic loading, micro-defects at nodal junctions act as severe stress concentrators, leading to rapid crack initiation, progressive strut rupture, and sudden macroscopic fatigue collapse long before the theoretical yield strength is reached. Retaining fatigue strength above the cortical bone threshold of 40 to 60 megapascals while simultaneously achieving bone-matching stiffness remains, in the authors’ assessment, a profound manufacturing challenge. The review emphasizes that compressive strength alone is insufficient to establish the mechanical suitability of a lattice implant; fatigue behavior is governed by a complex interplay of porosity, pore morphology, strut thickness, surface roughness, residual stress, internal defects, and loading mode.

The material landscape itself is evolving. Titanium alloys remain the most studied, prized for their low density, high specific strength, and corrosion resistance, but the review highlights a shift toward novel beta-phase titanium compositions engineered with non-toxic stabilizers such as niobium, tantalum, zirconium, and molybdenum. These alloys can drive the elastic modulus down toward 50 to 60 gigapascals without any porous structuring at all, and compositions like Ti-25Nb reach as low as 19 gigapascals — approaching cortical bone directly. Copper-containing titanium alloys, such as Ti-0.3Cu, add antibacterial functionality alongside high tensile strength. Cobalt-chromium-molybdenum alloys, meanwhile, remain the gold standard for wear resistance in articulating environments, thanks to high hardness and stable passive oxide films, though their high stiffness and susceptibility to micro-galvanic corrosion at grain boundaries present their own challenges. 316L stainless steel, when printed via laser powder bed fusion, develops a refined sub-micron cellular austenitic microstructure with dense dislocation networks that breaks the traditional strength-ductility tradeoff — a property the review notes is particularly valuable for porous scaffolds that must dissipate physiological loads through progressive plastic bending rather than catastrophic brittle fracture.

Magnesium alloys occupy a different niche entirely. Unlike titanium or cobalt-chromium, magnesium is biodegradable: it dissolves in the body over months to years, potentially eliminating the need for secondary removal surgery. The review catalogs a growing clinical portfolio, from BIOmag coronary stents launched in 2024 to WE43 bone screws and high-purity magnesium vascular closure devices. But the same reactivity that makes magnesium attractive also makes it dangerous. Rapid corrosion generates hydrogen gas faster than the body can transport it away, and accumulation can produce local cavities, alter tissue-implant contact, and interfere with healing. The review cites a frequently discussed reference level of approximately 0.01 milliliters per square centimeter per day for tolerable hydrogen evolution, while cautioning that this is not a universal clinical threshold — tolerance depends on anatomical site, vascularization, implant geometry, and degradation kinetics.

Surface engineering emerges as the third pillar of the integrated framework. Hydroxyapatite, a calcium phosphate ceramic that closely mimics the mineral component of natural bone, has long been the go-to bioactive coating, but the review details its limitations: inherent brittleness, thermal expansion mismatch with metallic substrates that causes residual stress and microcracking, and dissolution behavior that varies with crystalline phase. Ion-substituted variants — incorporating zinc, magnesium, or strontium — offer improved antibacterial properties, corrosion resistance, and osteogenic activity. Bioactive glasses, both silicate-based and the newer borate compositions, release therapeutic ions that promote bone regeneration; borate glasses like 13-93B3 can degrade 85 percent within 7 to 14 days, far outpacing silicate counterparts, though this rapid dissolution risks compromising mechanical integrity before bone has fully healed. Hybrid hierarchical coatings that combine ceramic, polymeric, and biomolecular layers — such as polydopamine, zinc oxide nanoparticles, chitosan, and nanocrystalline hydroxyapatite — represent the current frontier, achieving adhesion strengths exceeding 50 megapascals in some configurations.

The review also confronts a manufacturing reality that is easy to overlook: the machines themselves wear out. In directed energy deposition systems, the continuous flow of abrasive metallic powders induces significant wear on entry nozzles, altering powder focus and causing spatial inaccuracies in the melt pool. In laser powder bed fusion, abrasive wear of the re-coater blade against the powder bed can induce subtle variations in layer thickness. This machine-level tribological degradation translates directly into metallurgical defects, heterogeneous porosity, and diminished fatigue life in the final biomedical component — a feedback loop that the authors argue must be accounted for in any robust quality-control framework.

On the clinical side, the pathway from computed tomography or magnetic resonance imaging scans to a printed, patient-specific implant involves segmentation, computer-aided design, finite element analysis, and build preparation. The review notes that patient-specific implants currently cost five to ten times more than conventional standard implants, a premium driven by imaging software, prolonged CAD processing, specialized machine usage, and tailored post-processing. This economic reality confines their routine use to complex revision procedures, cancer resections, or critical trauma where conventional implants would fundamentally fail. Regulatory qualification, manufacturing reproducibility, and coating durability on complex lattice geometries are identified as the leading barriers to broader clinical translation.

Ultimately, the review’s central message is that no single material-architecture-coating combination can be considered universally optimal. The authors argue that implant performance emerges from the combined effects of alloy properties, architectural design, manufacturing-induced defects, surface characteristics, and post-processing conditions — and that future research must simultaneously consider mechanical compatibility, fatigue performance, corrosion and degradation, coating durability, biological response, powder removal from complex architectures, and economic and regulatory requirements. The era of treating the implant as a monolithic object, they suggest, is over; what replaces it is a design philosophy in which every layer, from the atomic arrangement of the alloy to the nanometer-scale chemistry of the outermost surface, is treated as a coupled variable in a single, interconnected system.

Subject of Research: Additive manufacturing of metallic implants integrating material selection, porous lattice architecture, and hybrid surface engineering

Article Title: Additive manufacturing of metallic implants: a comprehensive review of material selection, porous architecture, and hybrid surface engineering

Article References: Mutharasu, M., & Joel, J. (2026). Additive manufacturing of metallic implants: a comprehensive review of material selection, porous architecture, and hybrid surface engineering. Frontiers in Mechanical Engineering, 12, Article 1978898. https://doi.org/10.3389/fmech.2026.1978898

Image Credits: AI Generated

DOI: 10.3389/fmech.2026.1978898

Keywords: additive manufacturing, metallic implants, porous lattice, stress shielding, titanium alloys, cobalt-chromium, magnesium alloys, hydroxyapatite, bioactive glass, surface engineering, powder bed fusion, biodegradable implants

The study, led by Virginia Uralde and Fernando Veiga with colleagues Alfredo Suarez and Tomas Ballesteros, addresses a problem that has grown acute as metal 3D printing has matured. Unlike conventional subtractive manufacturing, in which milling, turning, grinding and drilling remove material from a solid workpiece, additive manufacturing builds parts layer by layer, unlocking geometries that would otherwise be impossible: hollow sections, intricate internal lattices and topology-optimised structures that slash weight. That design freedom, however, comes at the price of complexity. With each process offering a different balance of feedstock, energy source, deposition rate, accuracy, build volume and post-processing burden, the question of which technology to use no longer has an obvious answer.

The researchers’ first move is deceptively simple: rule out what cannot work. Before any ranking takes place, three design-phase criteria act as exclusion filters: the geometric complexity of the part, the material it must be made from, and its overall dimensions. A technology that cannot print a particular alloy, or whose build volume cannot accommodate a large component, is eliminated before the analysis begins. This pre-screening stage, the authors argue, is the study’s key novelty, because it tailors the subsequent comparison to the specific application rather than producing a generic, one-size-fits-all ranking. The underlying hypothesis is blunt: there is no universally optimal additive manufacturing technology for all materials, sizes and shapes.

Once the field has been narrowed, the framework deploys the Analytical Hierarchical Process, or AHP, to rank the surviving candidates. AHP decomposes a decision into a hierarchy: the overall objective at the top, criteria in the middle, and alternatives at the bottom. Decision-makers compare elements in pairs using Saaty’s famous 1-to-9 scale, where 1 means two items are equally preferable and 9 means one is extremely preferable to the other. These subjective judgments are converted into numerical weights through eigenvector calculations, and a consistency ratio, which must fall below 0.1, verifies that the comparisons are logically coherent rather than arbitrary. The method’s strength, the authors note, is precisely its ability to transform qualitative expert judgment into transparent mathematics, a valuable asset in a field where quantitative data can be scarce, unreliable or quickly outdated.

The team considered five technological alternatives spanning the main families of metal additive manufacturing. Powder Bed Fusion, or PBF, which fuses fine metal powders layer by layer, offers high geometric complexity, tight tolerances and smooth surface finishes, with typical layer thicknesses of just 0.02 to 0.06 millimetres, but limited productivity and build size. Material Extrusion, or MEX, uses metal-filled filaments followed by debinding and sintering, offering accessible equipment and low costs at the expense of mechanical performance. Directed Energy Deposition, or DED, which deposits material directly into a melt pool, was subdivided by energy source into laser-beam, electron-beam and arc-based variants, the last of which stands out for very high deposition rates of roughly 10 kilograms per hour and suitability for large components and repair work.

To weight the criteria, the researchers surveyed 260 respondents: 130 additive manufacturing experts, 70 designers and 60 customers of metal parts. The questionnaire posed 17 paired comparisons on a 1-to-9 scale, asking participants to trade off criteria against one another. Nine sub-criteria were organised into two phases of the additive workflow. The print phase covered machine price, manufacturing cost, productivity, reliability and part finish after printing; the final phase covered final part properties, post-processing complexity, process sustainability and user preferences. The results were striking: the print phase received a weight of 0.683, more than double the final phase’s 0.317, and reliability emerged as the single most important criterion overall, with a global weight of 0.304. Final part properties followed at 0.186, then manufacturing cost at 0.147, while sustainability, machine price and user preferences ranked lowest.

Interesting differences emerged between respondent profiles. Experts and designers both weighted the print phase heavily, drawn to factors such as productivity and process reliability, with the experts placing particular emphasis on reliability. Customers, by contrast, cared more about the finishing phase, because the final properties of the delivered part matter most to them. For criteria that could not be answered from the literature, such as sustainability perceptions and user preferences, a second survey of purely technical interviewees supplied the paired comparisons needed to score each alternative.

When all five technologies were evaluated against the full set of criteria, arc-based Directed Energy Deposition, or DED-Arc, came out on top with a final score of 0.249, narrowly ahead of Powder Bed Fusion at 0.231, with DED-EB, DED-LB and MEX trailing at 0.179, 0.172 and 0.170 respectively. The result tracks the underlying physics and economics. DED-Arc machines cost roughly 110,000 euros, the lowest of the group, and its arc-welding deposition achieves productivity around 10 kilograms per hour, roughly fifty to one hundred times that of PBF, which manages only 0.1 to 0.2 kilograms per hour. PBF, however, dominates on quality: surface roughness of Ra 6 to 12 micrometres, tensile strengths of 800 to 1100 megapascals in Ti6Al4V, and the highest survey scores for reliability and as-printed finish, reflecting the fine powder particles that yield smoother surfaces. Its weakness is cost, with the highest manufacturing cost of any alternative, driven by material waste and energy consumption.

The framework’s real power shows itself when the pre-selection filter is applied. In a worked example, the researchers considered a part of medium-high geometric complexity, medium size, made from aluminium. The filter excluded DED-EB and DED-Arc, leaving PBF, DED-LB and MEX as viable candidates. Re-running the AHP with only these three alternatives reshuffled the ranking dramatically: PBF surged to first place with a score of 0.415, followed by MEX at 0.306 and DED-LB at 0.279. The lesson is that the ‘best’ technology is not an intrinsic property of a process but a function of the part, the material and the constraints, and that a ranking computed without feasibility screening can be actively misleading.

The authors are candid about the method’s limits. The criterion weights reflect the preferences of the surveyed group rather than universal priorities, and different expert populations could produce different rankings. Technical characteristics were drawn from literature and expert assessment and vary with machine, material and process parameters, so the output should be read as decision support rather than a definitive verdict. AHP also assumes criteria are independent and uses deterministic comparisons, leaving uncertainty and interactions unmodelled; the researchers suggest fuzzy or probabilistic extensions as future work. And because metal additive manufacturing is evolving rapidly, the technical data underpinning the framework need periodic updating, though the two-stage structure itself can absorb new alternatives, screening conditions and weights without modification. For manufacturers weighing six-figure machine investments against part quality, throughput and sustainability, the study offers something rarer than a winner: a defensible, customisable way to find the right tool for each job.

Subject of Research: Multi-criteria decision-making for selecting optimal metal additive manufacturing technologies using the Analytical Hierarchical Process

Article Title: Method for selecting the optimal technology in metal additive manufacturing using an analytical hierarchical process

Article References: Uralde, V., Veiga, F., Suarez, A., & Ballesteros, T. (2026). Method for selecting the optimal technology in metal additive manufacturing using an analytical hierarchical process. Heliyon, 12(15), Article e45519. https://doi.org/10.1016/j.heliyon.2026.e45519

Image Credits: AI Generated

DOI: 10.1016/j.heliyon.2026.e45519

Keywords: metal additive manufacturing, analytical hierarchical process, multi-criteria decision making, powder bed fusion, directed energy deposition, material extrusion, DED-Arc, reliability, manufacturing cost, productivity, technology selection, 3D printing