Additive manufacturing inert gas flow path strategies for multi-laser powder bed fusion systems and their impact on lattice structure mechanical responses.

Multi-laser Additive Manufacturing systems hold great potential to increase productivity. However, adding multiple energy sources to a powder bed fusion system requires careful selection of a laser scan and inert gas flow strategy to optimize component performance. In this work, we explore four different laser scan and argon flow strategies on the quasi-static compressive mechanical response of Body Centered Cubic lattices. Three strategies employ a swim lane method where laser pathing tends to progress parallel to argon flow. Method one only uses a single laser while method two uses four, both with the laser path working against the argon flow. The third method uses four lasers, each operating in their own lane like the second method, but the laser pathing progresses with the argon flow. The fourth method has all four lasers operating in quadrants and the laser pathing trends against the argon flow.The single-laser strategy generally had the lowest mechanical responses compared to the other three strategies. A quadrant strategy generally had the highest quasi-static mechanical responses and was at least 25% greater in stiffness, yield force, ultimate force, and energy absorption when compared to the single laser strategy. However, the four-laser swim strategy where the laser pathing tends against the argon flow was found to be statistically similar to the quadrant strategy. It is hypothesized that spatter introduced onto the powder layer from the melt pool and particle entrainment may be worse for laser pathing which trends with the argon flow direction. Additionally, the additional energy added to the build volume helps to mitigate inter-layer cool time which reduces temperature gradients. This shows that multi-laser AM systems have an impact on part performance and potentially shows lattices built with multi-laser AM systems may have certain advantages over single-laser AM systems.

Build parameter influence on strut thickness and mechanical performance in additively manufactured titanium lattice structures.

Additively manufactured lattices have been adopted in applications ranging from medical implants to aerospace components. For solid AM components, the effect of build parameters has been well studied but comparably little attention has been paid to the influence of build parameters on lattice performance. For this project, the main aim was to evaluate static compressive mechanical performance of regular and stochastic lattices as a function of build parameters. The second aim was to compare strut dimensions of the metal lattice structures as build parameters were changed. Both regular and stochastic lattices were fabricated with a designed strut diameter of either 200 µm or 300 µm on a laser powder bed fusion machine. A range of laser power (140-180 W), scan speed (1700-2100 mm/s), and laser offset (0-45 µm) were used in fabricating each lattice. Compression tests were performed following the ISO 13314 (2011) standard to measure modulus, yield strength, and ultimate compressive strength values. Laser power adjustments produced the most significant effect on lattice performance. A change of 50 W resulted in roughly a 2X increase in maximum load and modulus for both regular and stochastic lattice structures. Regular lattice structures had a higher mechanical response during the mechanical evaluation. Internal strut diameters varied between build parameters as well, with laser offset adjustments producing the most noticeable change in strut geometry between lattice samples. These findings suggest that build parameter optimization, in lieu of using OEM parameters developed for solid structures, is necessary to ensure the optimum mechanical performance of AM lattice structures.

Dimensional variability characterization of additively manufactured lattice coupons.

BACKGROUND: Additive manufacturing (AM), commonly called 3D Printing (3DP), for medical devices is growing in popularity due to the technology's ability to create complex geometries and patient-matched products. However, due to the process variabilities which can exist between 3DP systems, manufacturer workflows, and digital conversions, there may be variabilities among 3DP parts or between design files and final manufactured products. The overall goal of this project is to determine the dimensional variability of commercially obtained 3DP titanium lattice-containing test coupons and compare it to the original design files. METHODS: This manuscript outlines the procedure used to measure dimensional variability of 3D Printed lattice coupons and analyze the differences in external dimensions and pore area when using laser and electron beam fabricated samples. The key dimensions measured were the bulk length, width, and depth using calipers. Strut thickness and pore area were assessed for the lattice components using optical imaging and µCT. RESULTS: Results show a difference in dimensional measurement between printed parts and the computer-designed files for all groups analyzed including the internal lattice dimensions. Measurements of laser manufactured coupons varied from the nominal by less than 0.2 mm and results show averages greater than the nominal value for length, width, and depth dimensions. Measurements of Electron Beam Melting coupons varied between 0.4 mm-0.7 mm from the nominal value and showed average lengths below the nominal dimension while the width and depths were greater than the nominal values. The length dimensions of Laser Powder Bed Fusion samples appeared to be impacted by hot isostatic press more than the width and depth dimension. When lattice relative density was varied, there appeared to be little impact on the external dimensional variability for the as-printed state. CONCLUSIONS: Based on these results, we can conclude that there are relevant variations between designed files and printed parts. However, we cannot currently state if these results are clinically relevant and further testing needs to be conducted to apply these results to real-world situations.

Nitinol Release of Nickel under Physiological Conditions: Effects of Surface Oxide, pH, Hydrogen Peroxide, and Sodium Hypochlorite.

Nitinol is a nickel-titanium alloy widely used in medical devices for its unique pseudoelastic and shape-memory properties. However, nitinol can release potentially hazardous amounts of nickel, depending on surface manufacturing yielding different oxide thicknesses and compositions. Furthermore, nitinol medical devices can be implanted throughout the body and exposed to extremes in pH and reactive oxygen species (ROS), but few tools exist for evaluating nickel release under such physiological conditions. Even in cardiovascular applications, where nitinol medical devices are relatively common and the blood environment is well understood, there is a lack of information on how local inflammatory conditions after implantation might affect nickel ion release. For this study, nickel release from nitinol wires of different finishes was measured in pH conditions and at ROS concentrations selected to encompass and exceed literature reports of extracellular pH and ROS. Results showed increased nickel release at levels of pH and ROS reported to be physiological, with decreasing pH and increasing concentrations of hydrogen peroxide and NaOCl/HOCl having the greatest effects. The results support the importance of considering the implantation site when designing studies to predict nickel release from nitinol and underscore the value of understanding the chemical milieu at the device-tissue interface.

Nylon lattice design parameter effects on additively manufactured structural performance.

Lattice structures are used in a multitude of applications from medical to aerospace, and their adoption in these applications has been further enabled by additive manufacturing. Lattice performance is governed by a multitude of variables and estimating this performance may be needed during various phases of the design and validation process. Numerical modeling and constitutive relationships are common methodologies to assess performance, address risks, lower costs, and accelerate time to market for innovative and potentially life altering products. These methods are usually accompanied by engineering rationales to justify the methods appropriateness. However, engineering analyses and numerical models should be validated using experimental data when possible to quantify the accuracy of their predictions under conditions relevant to their planned use. In this work, a set of lattice design parameters are evaluated using numerical modeling and experimental methods under quasi-static tensile, compressive, and shear modalities. Regular body centered cubic (BCC) and stochastic Voronoi Tessellation Method (VTM) lattices are constructed with three different cell lengths (2.5 mm, 4.0 mm, 5.0 mm) and various strut diameter thicknesses (ranging from 0.536 mm-1.3506 mm) while maintaining the lattice's relative density (0.2 and 0.3). Some strut diameters were selected to challenge the AM process limits. Specimens were fabricated in nylon 12 on a laser powder bed fusion system. Optical microscopy showed up to a 28.6% difference between as-designed and fabricated strut diameters. Simulated reaction loads revealed up to a 4.6% difference in BCC lattices within a constant relative density at a 1.4 mm displacement boundary condition while the VTM samples had up to a 19.5% difference. Errors between the experimental and simulated lattice reaction loads were as high as 97.0%. This error magnitude appears to strongly correlate with lattice strut diameter. These results showcase that a computational estimation, even one with reasonable assumptions, may erroneously characterize the performance of these lattice structures, and that these assumptions should be challenged by experimentally evaluating and validating critical quantities of interest.

A comparison of metal/metal and ceramic/metal taper-trunnion modular connections in explanted total hip replacements.

Corrosion and wear are commonly found at the taper-trunnion connection of modular total hip arthroplasty (THA) explanted devices. While metal/metal (M/M) modular taper-trunnion connections exhibit more wear/corrosion than ceramic/metal (C/M) modular taper-trunnion connections, damage is present in both, regardless of material. This study used a combination of assessment techniques including clinical data, visual scoring assessment, optical imaging, profilometry, and x-ray photoelectron microscopy (XPS), to investigate wear mechanisms and damage features at the modular taper-trunnion connection of 10 M/M and 8 C/M explanted THAs. No correlation was found between any demographic variable and corrosion wear and assessment scores. All assessment techniques demonstrated that the stem trunnions had more damage than head tapers for both explant groups and agreed that C/M explants had less corrosion and wear compared to M/M explants. However, visual assessment scores differed between assessment techniques when evaluating the tapers and trunnions within the two groups. Profilometry showed an increase (p <.05) in surface roughness for stem trunnions compared to head tapers for both explant groups. X-ray photoelectron spectroscopy performed on deposits from two M/M explants found chromium and molybdenum carbides beneath the surface while chromium sulfate and aged bone mineral were found on the surface suggesting that the debris is a result of corrosion rather than wear. These results indicate that taper-trunnion damage is more prevalent for M/M explants, but C/M explants are still susceptible to damage. More comprehensive analysis of damage is necessary to better understand the origins of taper-trunnion damage.

Temperature dependence of nickel ion release from nitinol medical devices.

Nitinol exhibits unique (thermo)mechanical properties that make it central to the design of many medical devices. However, nitinol nominally contains 50 atomic percent nickel, which if released in sufficient quantities, can lead to adverse health effects. While nickel release from nitinol devices is typically characterized using in vitro immersion tests, these evaluations require lengthy time periods. We have explored elevated temperature as a potential method to expedite this testing. Nickel release was characterized in nitinol materials with surface oxide thickness ranging from 12 to 1564 nm at four different temperatures from 310 to 360 K. We found that for three of the materials with relatively thin oxide layers, ≤ 87 nm nickel release exhibited Arrhenius behavior over the entire temperature range with activation energies of 80 to 85 kJ/mol. Conversely, the fourth ''black-oxide'' material, with a much thicker, complex oxide layer, was not well characterized by an Arrhenius relationship. Power law release profiles were observed in all four materials; however, the exponent from the thin oxide materials was approximately 1/4 compared with 3/4 for the black-oxide material. To illustrate the potential benefit of using elevated temperature to abbreviate nickel release testing, we demonstrated that a > 50 day 310 K release profile could be accurately recovered by testing for less than 1 week at 340 K. However, because the materials explored in this study were limited, additional testing and mechanistic insight are needed to establish a protective temperature scaling that can be applied to all nitinol medical device components.

Specimen-Specific Finite Element Models for Predicting Fretting Wear in Total Hip Arthroplasty Tapers.

Products from fretting wear and corrosion in the taper junction of total hip arthroplasty (THA) devices can lead to adverse local tissue reactions. Predicting damage as a function of design parameters would aid in the development of more robust devices. The objectives of this study were to develop an automated method for identifying areas of fretting wear on THA taper junctions, and to assess the predictive ability of a finite element model to simulate fretting wear in THA taper junctions. THA constructs were fatigue loaded, thus inducing damage on the stem taper. An automated imaging and analysis algorithm quantified fretting wear on the taper surfaces. Specimen-specific finite element models were used to calculate fretting work done (FWD) at the taper junction. Simulated FWD was correlated to imaged fretting wear. Results showed that the automated imaging approach identified fretting wear on the taper surface. Additionally, finite element models showed the greatest predictive ability for tapers exhibiting distal contact. Finite element models predicted an average of 30.3% of imaged fretting wear. With additional validation, the imaging and finite element techniques may be useful to manufacturers and regulators in the development and review of new THA devices.

Comparison of ASTM F2129 and ASTM F746 for Evaluating Crevice Corrosion.

Crevice corrosion is one of the major mechanisms that drives implant failure in orthopedic devices that have modular interfaces. Despite the prevalence of crevice corrosion in modular interfaces, very little is known with regards to the susceptibility of different material combinations to participate in crevice corrosion. In this study, we compare two electrochemical methods, ASTM F2129, Standard Test Method for Conducting Cyclic Potentiodynamic Polarization Measurements to Determine the Corrosion Susceptibility of Small Implant Devices, and a modified version of ASTM F746, Standard Test Method for Pitting or Crevice Corrosion of Metallic Surgical Implant Materials, in their ability to induce crevice corrosion. Four commonly used metals, 316 stainless steel, commercially pure titanium (Ti grade 2), Ti-6Al-4V (Ti grade 5), and cobalt-chromium-molybdenum per ASTM F1537, Standard Specification for Wrought Cobalt-28Chromium-6Molybdenum Alloys for Surgical Implants (UNSR31537, UNSR31538, and UNSR31539), were used to form crevices with a rod and washer combination. As a control, the metal rod materials were tested alone in the absence of crevices using ASTM F2129 and the modified ASTM F746 method. As another control to determine if crevices formed with polymeric materials would influence crevice corrosion susceptibility, experiments were also conducted with metal rods and polytetrafluorethylene washers. Our results revealed more visible corrosion after ASTM F2129 than ASTM F746. Additionally, ASTM F746 was found to falsely identify crevice corrosion per the critical pitting potential when visual inspection found no evidence of crevice corrosion. Hence, ASTM F2129 was found to be more effective overall at evaluating crevice corrosion compared to ASTM F746.

Regulating 3D-printed medical products.

Additive manufacturing [also known as three-dimensional (3D) printing] is the layer-wise deposition of material to produce a 3D object. This rapidly emerging technology has the potential to produce new medical products with unprecedented structural and functional designs. Here, we describe the U.S. regulatory landscape of additive manufactured (3D-printed) medical devices and biologics and highlight key challenges and considerations.

Current practices in corrosion, surface characterization, and nickel leach testing of cardiovascular metallic implants.

In an effort to better understand current test practices and improve nonclinical testing of cardiovascular metallic implants, the Food and Drug Administration (FDA) held a public workshop on Cardiovascular Metallic Implants: corrosion, surface characterization, and nickel leaching. The following topics were discussed: (1) methods used for corrosion assessments, surface characterization techniques, and nickel leach testing of metallic cardiovascular implant devices, (2) the limitations of each of these in vitro tests in predicting in vivo performance, (3) the need, utility, and circumstances when each test should be considered, and (4) the potential testing paradigms, including acceptance criteria for each test. In addition to the above topics, best practices for these various tests were discussed, and knowledge gaps were identified. Prior to the workshop, discussants had the option to provide feedback and information on issues relating to each of the topics via a voluntary preworkshop assignment. During the workshop, the pooled responses were presented and a panel of experts discussed the results. This article summarizes the proceedings of this workshop and background information provided by workshop participants. Published 2016. This article is a U.S. Government work and is in the public domain in the USA. J Biomed Mater Res Part B: Appl Biomater, 105B: 1330-1341, 2017.

The effect of fatigue on the corrosion resistance of common medical alloys.

The effect of mechanical fatigue on the corrosion resistance of medical devices has been a concern for devices that experience significant fatigue during their lifespan and devices made from metallic alloys. The Food and Drug Administration had recommended in some instances for corrosion testing to be performed on post-fatigued devices [Non-clinical tests and recommended labeling for intravascular stents and associated delivery systems: guidance for industry and FDA staff. 2005: Food and Drug Administration, Center for Devices and Radiological Health], although the need for this has been debated [Nagaraja S, et al., J Biomed Mater Res Part B: Appl Biomater 2016, 8.] This study seeks to evaluate the effect of fatigue on the corrosion resistance of 5 different materials commonly used in medical devices: 316 LVM stainless steel, MP35N cobalt chromium, electropolished nitinol, mechanically polished nitinol, and black oxide nitinol. Prior to corrosion testing per ASTM F2129, wires of each alloy were split into subgroups and subjected to either nothing (that is, as received); high strain fatigue for less than 8 min; short-term phosphate buffered saline (PBS) soak for less than 8 min; low strain fatigue for 8 days; or long-term PBS soak for 8 days. Results from corrosion testing showed that the rest potential trended to an equilibrium potential with increasing time in PBS and that there was no statistical (p > 0.05) difference in breakdown potential between the fatigued and matching PBS soak groups for 9 out of 10 test conditions. Our results suggest that under these nonfretting conditions, corrosion susceptibility as measured by breakdown potential per ASTM F2129 was unaffected by the fatigue condition. 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2019-2026, 2017.

Effect of wire fretting on the corrosion resistance of common medical alloys.

Metallic medical devices such as intravascular stents can undergo fretting damage in vivo that might increase their susceptibility to pitting corrosion. As a result, the US Food and Drug Administration has recommended that such devices be evaluated for corrosion resistance after the devices have been fatigue tested in situations where significant micromotion can lead to fretting damage. Three common alloys that cardiovascular implants are made from [MP35N cobalt chromium (MP35N), electropolished nitinol (EP NiTi), and 316LVM stainless steel (316LVM)] were selected for this study. In order to evaluate the effect of wire fretting on the pitting corrosion susceptibility of these medical alloys, small and large fretting scar conditions of each alloy fretting against itself, and the other alloys in phosphate buffered saline (PBS) at 37°C were tested per ASTM F2129 and compared against as received or PBS immersed control specimens. Although the general trend observed was that fretting damage significantly lowered the rest potential (Er ) of these specimens (p < 0.01), fretting damage had no significant effect on the breakdown potential (Eb , p > 0.05) and hence did not affect the susceptibility to pitting corrosion. In summary, our results demonstrate that fretting damage in PBS alone is not sufficient to cause increased susceptibility to pitting corrosion in the three common alloys investigated. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 2487-2494, 2017.

Effect of Applied Potential on Fatigue Life of Electropolished Nitinol Wires.

Nitinol is used as a metallic biomaterial in medical devices due to its shape memory and pseudoelastic properties. The clinical performance of nitinol depends on factors which include the surface finish, the local environment, and the mechanical loads to which the device is subjected. Preclinical evaluations of device durability are performed with fatigue tests while electrochemical characterization methods such as ASTM F2129 are employed to evaluate corrosion susceptibility by determining the rest potential and breakdown potential. However, it is well established that the rest potential of a metal surface can vary with the local environment. Very little is known regarding the influence of voltage on fatigue life of nitinol. In this study, we developed a fatigue testing method in which an electrochemical system was integrated with a rotary bend wire fatigue tester. Samples were fatigued at various strain levels at electropotentials anodic and cathodic to the rest potential to determine if it could influence fatigue life. Wires at potentials negative to the rest potential had a significantly higher number of cycles to fracture than wires held at potentials above the breakdown potential. For wires for which no potential was applied, they had fatigue life similar to wires at negative potentials.

Additively manufactured medical products - the FDA perspective.

Additive manufacturing/3D printing of medical devices is becoming more commonplace, a 3D printed drug is now commercially available, and bioprinting is poised to transition from laboratory to market. Despite the variety of technologies enabling these products, the US Food and Drug Administration (FDA) is charged with protecting and promoting the public health by ensuring these products are safe and effective. To that end, we are presenting the FDA's current perspective on additive manufacturing/3D printing of medical products ranging from those regulated by the Center for Devices and Radiological Health (CDRH), the Center for Drug Evaluation and Research (CDER), and the Center for Biologics Evaluation and Research (CBER). Each Center presents an overview of the additively manufactured products in their area and the specific concerns and thoughts on using this technology in those product spaces.

A comparison of polymer substrates for photolithographic processing of flexible bioelectronics.

Flexible bioelectronics encompass a new generation of sensing devices, in which controlled interactions with tissue enhance understanding of biological processes in vivo. However, the fabrication of such thin film electronics with photolithographic processes remains a challenge for many biocompatible polymers. Recently, two shape memory polymer (SMP) systems, based on acrylate and thiol-ene/acrylate networks, were designed as substrates for softening neural interfaces with glass transitions above body temperature (37 °C) such that the materials are stiff for insertion into soft tissue and soften through low moisture absorption in physiological conditions. These two substrates, acrylate and thiol-ene/acrylate SMPs, are compared to polyethylene naphthalate, polycarbonate, polyimide, and polydimethylsiloxane, which have been widely used in flexible electronics research and industry. These six substrates are compared via dynamic mechanical analysis (DMA), thermogravimetric analysis (TGA), and swelling studies. The integrity of gold and chromium/gold thin films on SMP substrates are evaluated with optical profilometry and electrical measurements as a function of processing temperature above, below and through the glass transition temperature. The effects of crosslink density, adhesion and cure stress are shown to play a critical role in the stability of these thin film materials, and a guide for the future design of responsive polymeric materials suitable for neural interfaces is proposed. Finally, neural interfaces fabricated on thiol-ene/acrylate substrates demonstrate long-term fidelity through both in vitro impedance spectroscopy and the recording of driven local field potentials for 8 weeks in the auditory cortex of laboratory rats.

Stretchable nanoparticle conductors with self-organized conductive pathways.

Research in stretchable conductors is fuelled by diverse technological needs. Flexible electronics, neuroprosthetic and cardiostimulating implants, soft robotics and other curvilinear systems require materials with high conductivity over a tensile strain of 100 per cent (refs 1-3). Furthermore, implantable devices or stretchable displays need materials with conductivities a thousand times higher while retaining a strain of 100 per cent. However, the molecular mechanisms that operate during material deformation and stiffening make stretchability and conductivity fundamentally difficult properties to combine. The macroscale stretching of solids elongates chemical bonds, leading to the reduced overlap and delocalization of electronic orbitals. This conductivity-stretchability dilemma can be exemplified by liquid metals, in which conduction pathways are retained on large deformation but weak interatomic bonds lead to compromised strength. The best-known stretchable conductors use polymer matrices containing percolated networks of high-aspect-ratio nanometre-scale tubes or nanowires to address this dilemma to some extent. Further improvements have been achieved by using fillers (the conductive component) with increased aspect ratio, of all-metallic composition, or with specific alignment (the way the fillers are arranged in the matrix). However, the synthesis and separation of high-aspect-ratio fillers is challenging, stiffness increases with the volume content of metallic filler, and anisotropy increases with alignment. Pre-strained substrates, buckled microwires and three-dimensional microfluidic polymer networks have also been explored. Here we demonstrate stretchable conductors of polyurethane containing spherical nanoparticles deposited by either layer-by-layer assembly or vacuum-assisted flocculation. High conductivity and stretchability were observed in both composites despite the minimal aspect ratio of the nanoparticles. These materials also demonstrate the electronic tunability of mechanical properties, which arise from the dynamic self-organization of the nanoparticles under stress. A modified percolation theory incorporating the self-assembly behaviour of nanoparticles gave an excellent match with the experimental data.

Transparent conductors from carbon nanotubes LBL-assembled with polymer dopant with π-π electron transfer.

Single-walled carbon nanotube (SWNT) and other carbon-based coatings are being considered as replacements for indium tin oxide (ITO). The problems of transparent conductors (TCs) coatings from SWNT and similar materials include poor mechanical properties, high roughness, low temperature resilience, and fast loss of conductivity. The simultaneous realization of these desirable characteristics can be achieved using high structural control of layer-by-layer (LBL) deposition, which is demonstrated by the assembly of hydroethyl cellulose (HOCS) and sulfonated polyetheretherketone (SPEEK)-SWNTs. A new type of SWNT doping based on electron transfer from valence bands of nanotubes to unoccupied levels of SPEEK through π-π interactions was identified for this system. It leads to a conductivity of 1.1 × 10(5) S/m at 66 wt % loadings of SWNT. This is better than other polymer/SWNT composites and translates into surface conductivity of 920 Ω/◻ and transmittance of 86.7% at 550 nm. The prepared LBL films also revealed unusually high temperature resilience up to 500 °C, and low roughness of 3.5 nm (ITO glass -2.4 nm). Tensile modulus, ultimate strength, and toughness of such coatings are 13 ± 2 GPa, 366 ± 35 MPa, and 8 ± 3 kJ/m(3), respectively, and exceed corresponding parameters of all similar TCs. The cumulative figure of merit, ∏(TC), which included the critical failure strain relevant for flexible electronics, was ∏(TC) = 0.022 and should be compared to ∏(TC) = 0.006 for commercial ITO. Further optimization is possible using stratified nanoscale coatings and improved doping from the macromolecular LBL components.