Direct Metal Fabrication Research Articles

Non-thermal atmospheric pressure plasma treatment increases hydrophilicity and promotes cell growth on titanium alloys in vitro

Most medical implants are made of titanium. When titanium is exposed to air for a long time, hydrocarbons are deposited and the surface becomes hydrophobic. Cell attachment is important for bone ingrowth to occur on the implant surface, and hydrophilicity can enhance this. We examined whether non-thermal atmospheric pressure plasma treatment could increase the hydrophilicity of the titanium surface. Samples coated with four widely used coating types [grit blasting, micro arc oxidation (MAO), titanium plasma spray (TPS), and direct metal fabrication (DMF)] were treated with plasma. Each of the four surface-treated samples was divided into groups with and without plasma treatment. We analysed wettability by surface analysis and evaluation of contact angles, cell proliferation, and adhesion using scanning electron microscopy (SEM), confocal laser scanning microscopy, absorbance tests, and alkaline phosphatase (ALP) activity assay; four different Ti6Al4V surface types were compared. After plasma treatment, the contact angle was reduced on all surfaces, and the carbon content was reduced on all surfaces based on X-ray photoelectron spectroscopy (XPS) surface analysis. Under confocal laser scanning, the cell layer was thicker on the plasma-treated samples, especially in groups TPS and DMF. Cell proliferation was 41.8%, 17.7%, 54.9%, and 83.8% greater for the plasma- than non-plasma-treated grit blasting, MAO, TPS, and DMF samples, respectively. Hydrophilicity increased significantly under plasma treatment, and biological responsivity was also improved.

Improved Biological Responses of Titanium Coating Using Laser-Aided Direct Metal Fabrication on SUS316L Stainless Steel.

Direct metal fabrication (DMF) coatings have the advantage of a more uniform porous structure and superior mechanical properties compared to coatings provided by other methods. We applied pure titanium metal powders to SUS316L stainless steel using laser-aided DMF coating technology with 3D printing. The purpose of this study was to determine the efficacy of this surface modification of stainless steel. The capacity of cells to adhere to DMF-coated SUS316L stainless steel was compared with machined SUS316L stainless steel in vitro and in vivo. Morphological in vitro response to human osteoblast cell lines was evaluated using scanning electron microscopy. Separate specimens were inserted into the medulla of distal femurs of rabbits for in vivo study. The distal femurs were harvested after 3 months, and were then subjected to push-out test and histomorphometrical analyses. The DMF group exhibited a distinct surface chemical composition, showing higher peaks of titanium compared to the machined stainless steel. The surface of the DMF group had a more distinct porous structure, which showed more extensive coverage with lamellipodia from osteoblasts than the machined surface. In the in vivo test, the DMF group showed better results than the machined group in the push-out test (3.39 vs. 1.35 MPa, respectively, p = 0.001). In the histomorphometric analyses, the mean bone-to-implant contact percentage of the DMF group was about 1.5 times greater than that of the machined group (65.4 ± 7.1% vs. 41.9 ± 5.6%, respectively; p < 0.001). The porous titanium coating on SUS316L stainless steel produced using DMF with 3D printing showed better surface characteristics and biomechanical properties than the machined SUS316L.

Comparative analysis of titanium coating on cobalt-chrome alloy in vitro and in vivo direct metal fabrication vs. plasma spraying

sBackgroundTitanium surface coating on cobalt-chromium (CoCr) alloy has characteristics desirable for an orthopedic implant as follows: strength, osteointegrative capability, and biocompatibility. Creating such a coated surface takes a challenging process and two dissimilar metals are not easily welded. In our study, we utilized additive manufacturing with a 3D printing called direct metal fabrication (DMF) and compared it to the plasma spraying method (TPS), to coat titanium onto CoCr alloy. We hypothesized that this would yield a coated surface quality as acceptable or better than the already established method of plasma spraying. For this, we compared characteristics of titanium-coated surfaces created by direct metal fabrication method (DMF) and titanium plasma spraying (TPS), both in vitro and in vivo, for (1) cell morphology, (2) confocal microscopy images of immunofluorescent assay of RUNX2 and fibronectin, (3) quantification of cell proliferation rate, (4) push-out biomechanical test, and (5) bone histomorphometry.MethodFor in vitro study, human osteoblast cells were seeded onto the coated surfaces. Cellular morphology was observed with a scanning electron microscope. Cellular proliferation was validated with ELISA, immunofluorescent assay. For in vivo study, coated rods were inserted into the distal femur of the rabbit and then harvested. The rods were biomechanically tested with a push-out test and observed for histomorphometry to evaluate the microscopic bone to implant ratio.ResultFor cell morphology observation, lamellipodia and filopodia, a cytoplasmic projection extending into porous structure, formed on both surfaces created by DMF and TPS. The proliferation of the osteoblasts, the DMF group showed a better result at different optic density levels (p = 0.035, 0.005, 0.001). Expression and distribution of fibronectin and Runx-2 genes showed similar degrees of expressions. The biomechanical push-out test yielded a similar result (p = 0.714). Histomorphometry analysis also showed a similar result (p = 0.657).ConclusionIn conclusion, DMF is a method which can reliably create a proper titanium surface on CoCr alloy. The resulting product of the surface shows a similar quality to that of the plasma spraying method, both in vivo and in vitro, in terms of biological and mechanical property.

Titanium Powder Coating Using Metal 3D Printing: A Novel Coating Technology for Cobalt-Chromium Alloy Implants.

Three-dimensional (3D) printing with a direct metal fabrication (DMF) technology has been innovatively introduced in the field of surface treatment of prostheses. The purpose of this study was to determine whether such modifications on the surface of cobalt-chromium (CoCr) alloy by titanium powder coating using DMF improves the osseointegration ability of CoCr alloy. We compared the in vitro and in vivo ability of cells to adhere to DMF-coated CoCr alloy with machining. Biological and morphological responses to human osteoblast cell lines were examined by measuring cell proliferation rate and observing expression of actin filament. For in vivo study, we inserted different specimens in each medulla of the distal femurs of rabbit. After 3months, the distal femurs were harvested, and a push-out test and histomorphometric analyses were performed. The cell proliferation rate and cell adhesion in the DMF group were higher compared with those in the machined group. Human osteoblast cells on the DMF-coated surface were more strongly adhered and well-proliferated compared with those on the other surface. In the in vivo test, there was a significant difference in the ultimate shear strength between the DMF and machined groups (2.49MPa vs. 0.87MPa, respectively, p = 0.001). In the histomorphometric analysis, there was a significant difference in the mean bone-to-implant contact percentages between the DMF and machined groups (72.3 ± 6.2% vs. 47.6 ± 6.9%, respectively, p < 0.001). Titanium coating of CoCr alloy with 3D metal printing provides optimal surface characteristics and a good biological surface both in vitro and in vivo.

The biological response to laser-aided direct metal-coated Titanium alloy (Ti6Al4V).

ObjectivesLaser-engineered net shaping (LENS) of coated surfaces can overcome the limitations of conventional coating technologies. We compared the in vitro biological response with a titanium plasma spray (TPS)-coated titanium alloy (Ti6Al4V) surface with that of a Ti6Al4V surface coated with titanium using direct metal fabrication (DMF) with 3D printing technologies.MethodsThe in vitro ability of human osteoblasts to adhere to TPS-coated Ti6Al4V was compared with DMF-coating. Scanning electron microscopy (SEM) was used to assess the structure and morphology of the surfaces. Biological and morphological responses to human osteoblast cell lines were then examined by measuring cell proliferation, alkaline phosphatase activity, actin filaments, and RUNX2 gene expression.ResultsMorphological assessment of the cells after six hours of incubation using SEM showed that the TPS- and DMF-coated surfaces were largely covered with lamellipodia from the osteoblasts. Cell adhesion appeared similar in both groups. The differences in the rates of cell proliferation and alkaline phosphatase activities were not statistically significant.ConclusionsThe DMF coating applied using metal 3D printing is similar to the TPS coating, which is the most common coating process used for bone ingrowth. The DMF method provided an acceptable surface structure and a viable biological surface. Moreover, this method is automatable and less complex than plasma spraying.Cite this article: T. Shin, D. Lim, Y. S. Kim, S. C. Kim, W. L. Jo, Y. W. Lim. The biological response to laser-aided direct metal-coated Titanium alloy (Ti6Al4V). Bone Joint Res 2018;7:357–361. DOI: 10.1302/2046-3758.75.BJR-2017-0222.R1.

Strength analysis and modeling of cellular lattice structures manufactured using selective laser melting for tooling applications

Additive manufacturing is rapidly developing and gaining popularity for direct metal fabrication systems like selective laser melting (SLM). The technology has shown significant improvement for high-quality fabrication of lightweight design-efficient structures such as conformal cooling channels in injection molding tools and lattice structures. This research examines the effect of cellular lattice structures on the strength of workpieces additively manufactured from ultra high-strength steel powder. Two commercial SLM machines are used to fabricate cellular samples based on four architectures— solid, hollow, lattice structure and rotated lattice structure. Compression test is applied to the specimens while they are deformed. The analytical approach includes finite element (FE), geometrical and mathematical models for prediction of collapse strength. The results from the the models are verified with experimental data and it is shown that they agree well. The results from this research show that using lattice structures significantly reduces the strength of material with respect to solid samples while indicating no serious increase of strength compared to hollow structures. In combination with an analysis of microstructures, a description of strength analysis is obtained with respect to process parameters.

Powder Removal from Ti-6Al-4V Cellular Structures Fabricated via Electron Beam Melting

Direct metal fabrication systems like electron beam melting (EBM) and direct metal laser sintering (also called selective laser melting) are gaining popularity. One reason is the design and fabrication freedom that these technologies offer over traditional processes. One specific feature that is of interest is mesh or lattice structures that can be produced using these powder-bed systems. One issue with the EBM process is that the powder trapped within the structure during the fabrication process is sintered and can be hard to remove as the mesh density increases. This is usually not an issue for the laser-based systems since most of them work at a low temperature and the sintering of the powder is less of an issue. Within the scope of this project, a chemical etching process was evaluated for sintered powder removal using three different cellular structures with varying mesh densities. All meshes were fabricated via EBM using Ti6Al4V powder. The results are promising, but the larger the structures, the more difficult it is to completely remove the sintered powder without affecting the integrity of the mesh structure.

In vitro biocompatibility of titanium alloy discs made using direct metal fabrication

Custom orthopedic implants may be generated using free-form fabrication methods (FFF) such as electron beam melting (EBM). EBM FFF may be used to make solid metal implants whose surface is often polished using CNC machining and porous scaffolds that are usually left unpolished. We assessed the in vitro biocompatibility of EBM titanium–6 aluminum–4 vanadium (Ti6Al4V) structures by comparing the cellular response to solid polished, solid unpolished, and porous EBM discs to the cellular response to discs made of commercially produced Ti6Al4V. The discs were seeded with 20,000 human adipose-derived adult stem cells (hASCs) and assessed for cell viability, proliferation, and release of the proinflammatory cytokines interleukin-6 (IL-6) and interleukin-8 (IL-8). Cell viability was assessed with Live/Dead staining 8 days after seeding. Cell proliferation was assessed using alamarBlue assays at days 0, 1, 2, 3, and 7. The hASCs were alive on all discs after 8 days. Cellular proliferation on porous EBM discs was increased at days 2, 3, and 7 compared to discs made of commercial Ti6Al4V. Cellular proliferation on porous EBM discs was also increased compared to solid polished and unpolished EBM discs. IL-6 and IL-8 releases at day 7 were lower for porous EBM discs than for other discs. Solid polished, unpolished, and porous EBM Ti6Al4V discs exhibited an acceptable biocompatibility profile compared to solid Ti6Al4V discs from a commercial source. EBM FFF may be considered as an option for the fabrication of custom orthopedic implants.

Applications of structural optimization in direct metal fabrication

PurposeOptimization techniques can be used to design geometrically complex components with a wide variety of optimization criteria. However, such components have been very difficult and costly to produce. Layered fabrication technologies such as electron beam melting (EBM) open up new possibilities though. This paper seeks to investigate the integration of structural optimization and direct metal fabrication process.Design/methodology/approachMesh structures were designed, and optimization problems were defined to improve structural performance. Finite element analysis code in conjunction with nonlinear optimization routines were used in MATLAB. Element data were extracted from an STL‐file, and output structures from the optimization routine were manufactured using an EBM machine. Original and optimized structures were tested and compared.FindingsThere were discrepancies between the performance of the theoretical structures and the physical EBM structures due to the layered fabrication approach. A scaling factor was developed to account for the effect of layering on the material properties.Practical implicationsStructural optimization can be used to improve the performance of a design, and direct fabrication technologies can be used to realise these structures. However, designers must realize that fabricated structures are not identical to idealized CAD structures, hence material properties much be adjusted accordingly.Originality/valueIntegration of structural optimization and direct metal fabrication was reported in the paper. It shows the process from design through manufacturing with integrated analysis.

Direct metal fabrication of titanium implants with tailored materials and mechanical properties using electron beam melting technology

The design of custom or tailored implant components has been the subject of research and development for decades. However, the economic feasibility of fabricating such components has proven to be a challenge. New direct metal fabrication technologies such as Electron Beam Melting (EBM) have opened up new possibilities. This paper discusses the design and fabrication of titanium implant components having tailored mechanical properties that mimic the stiffness of bone to reduce stress shielding and bone remodeling. Finite Element Analysis was used to design the tailored structures, and results were verified using mechanical testing.

Direct metal part forming of 316L stainless steel powder by electron beam selective melting

Electron beam selective melting (EBSM) is a new type of rapid manufacturing of direct metal fabrication technology, which combines electron beam machining with rapid prototyping. It makes use of some advantages of an electron beam, such as high energy density, high absorption rate, and vacuum environment. In this study, direct metal part forming of gas and water-atomized 316L stainless steel powder were researched in detail. The one-dimensional experiments showed that the gas-atomized 316L stainless steel powder could be formed in a solid line at slow scanning velocity and low beam current while the water-atomized powder had serious balling and break-up phenomena at most conditions. The powder mixture containing 40–60 wt % of the gas-atomized powder has the optimum forming ability. The forming technology of the mixture powder must be divided into three steps: placing step, heating step, and melting step. After research on the technology parameters, some two-dimensional facets and three-dimensional parts were produced by EBSM. The microstructure of the three-dimensional forming parts was fine; the interfaces between the layers were metallurgically bonded.

Development of a direct metal freeform fabrication technique using CO 2 laser welding and milling technology

Since the first introduction of rapid prototyping in 1986, several techniques have been developed and successfully commercialized in the market. However, most commercial systems currently use resins or waxes as the raw materials. Thus, the limited mechanical strength for functional testing is regarded as an obstacle towards broader application of rapid prototyping techniques. To overcome this problem, direct metal deposition methods are being investigated worldwide for rapid prototyping and even for rapid tooling applications. As a contribution to this development, a fundamental study on a process combination of wire welding technology using CO 2 laser radiation with milling was carried out and is reported in this paper. Laser welding enables accurate deposition of metals and the subsequent milling increases the surface quality and accuracy to machining standard. Compared to powder, the use of wire is of advantage in terms of a simple feeding mechanism as well as a higher deposition rate. The main focus of the experimental investigation is to find the basic process characteristics. For this purpose, basic parts were fabricated as a function of process parameters such as laser power, welding speed and bead distance. The microstructure, hardness and tensile strength are then examined as a function of these process parameters. In conclusion, the advantages and disadvantages of this process are discussed in comparison with other direct metal fabrication techniques.

Taking a Powder

This article focuses on the use of laser in conjunction with metal powder. The use of the laser in conjunction with metal powder could be regarded as an extension of rapid prototyping, which has involved plastic parts almost exclusively. However, terms such as ‘direct metal fabrication’ and ‘rapid manufacturing’ suggest that developers have much more ambitious goals in mind for powder metals. Lockheed Martin Tactical Aircraft Systems in Fort Worth, Texas, is operating a research facility based on the laser engineered net shaping (LENS) process on the same factory floor where the F-16 fighter is assembled for the US Air Force. Optomec Design Co. in Albuquerque builds machines for use with the LENS process. Tests of Sandia National Laboratories' laser engineered net shaping process take place in a tank at Lockheed Martin Tactical Aircraft Systems. Much can be done in direct metal fabrication to improve the thermal conductivity of tooling for injection molding and die casting. The result can be increases in cycle times of as much as 80 percent.