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OBJECTIVES/GOALS: The goal of the study is to understand how changing the surface roughness of 3D printed Titanium either by processing printed samples or artificially printing rough topography impacts the mechanical and biological properties of the Titanium. METHODS/STUDY POPULATION: Titanium dog bones and discs were printed via laser powder bed fusion. groups were defined as 1. polished, 2.blasted, 4.as built, 4.sprouts and 5.rough sprouts. Roughness was measured with line measurement using a confocal microscope. Tensile testing of dog bones produced stress strain curves. MC3T3 preosteoblast were seeded on discs. Samples were analyzed at 0, 2, and 4 weeks. A cell viability assay and confocal fluorescent microscopy assessed cell growth. Alkaline Phosphatase (ALP) assay and Quantitative Polymerase Chain Reaction (qPCR) examined cell differentiation. Extracellular matrix (ECM) was stained for collagen and calcium. Scanning Electron Microcopy (SEM) was done on sputter coated discs. RESULTS/ANTICIPATED RESULTS: Measured roughness defined by Rz, maximum peak to valley distance of the sample profile ranged from 2.6-65.1 µm. The addition of printed roughness in the sprouts and rough sprouts group significantly diminished ductility resulting in early strain to failure during tensile testing. Cells adhered and proliferated on discs regardless of roughness group. There was no statistical difference in ALP activity, but qPCR showed that rough groups (sprouts and rough sprouts) had diminished Osteocalcin gene expression at week 2 and 4. The ECM in the rough groups was more resistant to repeated washes and was more extensive with SEM. DISCUSSION/SIGNIFICANCE OF IMPACT: Printing roughness diminished mechanical properties without clear benefit to cell growth. Roughness features were on mesoscale, unlike samples in literature on microscale that increase cell activity. Printed topography may aid in implant fixation and not osseous integration as hypothesized. CONFLICT OF INTEREST DESCRIPTION: Dr. Samual Adams, Dr. Ken Gall and Cambre Kelly own stock and/or stock options in restor3d, Inc.
The adoption of selective laser melting (SLM) for fabrication of porous titanium has resulted in many new investigations into the complex design parameters associated with porous architecture of high spatial resolution. The development of meta-materials has included research into the effects of unit cell architecture (strut versus sheet), porosity, pore size, and other factors on the performance of metallic scaffolds. The current study examined the interactive effects of varying the gyroid sheet unit cell size and overall specimen size on the compressive behavior of Ti–6Al–4V ELI porous scaffolds manufactured via SLM. The increasing unit cell size relative to specimen geometry was found to decrease the compressive strength and stiffness of the overall structure and shift the material fracture mode. The understanding of the relationship between unit cell size and specimen geometry can be used to optimize mechanical properties of implants with constrained volumes and pore/wall size requirements to optimize properties of porous titanium implants for strength and osseointegration.
The goal of this study is to investigate the fundamental relationship between the extent of crosslinking and shape-memory behavior of amorphous, (meth)acrylate-based polymer networks. The polymer networks were produced by copolymerization of tert-butyl acrylate (tBA) and poly(ethylene glycol) dimethacrylates of differing molecular weights (PEGDMA). Polymer compositions were tailored via the amount (weight percent (wt%)) and molecular weight of the PEGDMA crosslinking agents added to produce four materials with varying levels of crosslinking (0, 2, 10, and 40 wt% crosslinking agent corresponding to 0, 0.6, 3.2, and 16.6 mole%) and nearly equal glass transition temperatures (Tg). The effect of crosslinking on deformation limits and free-strain recovery is evaluated. Near complete strain recovery was demonstrated by all materials; however, absolute recovery strain decreased with increasing crosslinking due to a corresponding decrease in strain-to-failure. The results provide insights regarding the link between polymer structure, deformation limits, and strain-recovery capabilities of this class of shape-memory polymers. An improved understanding of this relationship is pivotal for optimizing system response for a wide range of shape-memory applications.
We studied the deformation mechanisms and mechanics during indentation of polycrystalline gold thin films at depths below 100 nm. The measured material hardness decreased from 2.1 ± 0.1 to 1.7 ± 0.1 GPa after annealing for 4 h at 177 °C. Upon closer inspection, the hardness trends in the gold thin films were discovered to vary according to the indentation depth. At nanometer depths, the material hardness was quantified using multiple parameters, some of which were independent of the area calibration for the tip. The annealed specimen was very “hard” at low indentation depths, relatively soft at moderate indentation depths, and finally harder until the grain-size limit was reached. The as-deposited specimen demonstrated a relatively continuous harness trend as function of indentation depth, exhibiting monotonic convergence to Hall–Petch limited behavior. Discrete displacement jump events (excursions or “pop-ins”) were frequently observed for the annealed specimen but not for the as-deposited specimen. Variation in hardness, excursion activity, and displacement during the hold at maximum load was observed according to the applied loading, which was parametrically varied at constant strain rates. Hardness results are explained in terms of the population and evolution of defects present within the specimens. The population of point defects is also influential, and critical thermal fluctuations, as well as the thermally activated process of diffusion, are believed to influence hardness at the specimen’s free surface and further into its volume. After converging to a monotonic trend (proper tip engagement), the modulus of the gold was measured to be 106.0 ± 12.9 and 101.3 ± 6.0 GPa for the respective Au/Cr/Si specimens. These values exceeded predictions from the aggregate polycrystalline material theory, a representation used to estimate results for anisotropic single crystals. Exaggerated modulus measurements are explained as the result of the contribution of modulus mismatch with the substrate, pileup at the indentor tip, residual stress in the films, and crystallographic anisotropy of the gold.
The shape-memory effect was examined in polymer stents intended for cardiovascular applications. Four polymer networks were synthesized from poly(ethylene glycol) dimethacrylate and tert-butyl acrylate with 10 wt% and 20 wt% crosslinker, and with glass transition temperatures (Tg) of 52°C and 55°C. Solid and 50% porous stents were manufactured and tested for free strain recoverability at temperatures at or just above 37°C. Stents with lower glass transition temperatures and a higher degree of crosslinking recovered faster than their counterparts. Lower deformation (packaging) temperatures and higher recovery temperatures induce more rapid recovery. The presence of geometrical features, such as pores, initiated recovery sooner, but had negligible influence on overall recovery.
Nickel-titanium (NiTi) shape memory alloys undergo relatively large recoverable inelastic deformations via a stress-induced martensitic phase transformation. The nanoindentation experimental results presented in this study are the first to show evidence of discrete forward and reverse stress-induced thermoelastic martensitic transformations in nanometer scaled volumes of material. Shape recovery due to indentation, followed by subsequent heating, is demonstrated for indents depths in the sub 10 nm range via atomic force microscopy. It is also shown that the local material structure can be utilized to modify transformation behavior at nanometer scales, yielding insight into the nature of stress-induced martensitic phase transformations at small scales and providing opportunity for the design of nanometer sized NiTi actuators.
The objective of this study is to examine the effect of heat treatment on polycrystalline Ti-50.9 at.%Ni subsequent to hot-rolling. In particular we examine microstructure, transformation temperatures and mechanical behavior of deformation processed NiTi. The results constitute a fundamental understanding of the effect of heat treatment on thermal/stress induced martensite, which is critical for optimizing mechanical properties. The high temperature of the hot-rolling process caused recrystallization, recovery, and hindered precipitate formation, essentially solutionizing the NiTi. Subsequent heat treatments were carried out at various temperatures for 1.5 hours. Transmission Electron Microscopy (TEM) observations revealed that Ti3Ni4 precipitates progressively increased in size and changed their interface with the matrix from being coherent to incoherent with increasing heat treatment temperature. Accompanying the changes in precipitate size and interface coherency, transformation temperatures were observed to systematically shift, leading to the occurrence of the R-phase and multiple-stage transformations. Room temperature stress-strain tests illustrated a variety of mechanical responses for the various heat treatments, from pseudoelasticity to shape memory. The changes in stress-strain behavior are interpreted in terms of shifts in the primary martensite transformation temperatures, rather then the occurrence of the R-phase transformation. The results confirm that Ti3Ni4 precipitates can be used to elicit a desired isothermal stress-strain behavior in polycrystalline NiTi.
Shape memory polymers (SMPs) have the capacity to store and recover relatively large strains when subjected to a unique thermomechanical cycle. In this study, the thermomechanics of strain storage and strain/stress recovery are investigated in a shape memory polymer deformed under uniaxial tension and compression. During heated recovery, three cases of constraint are examined: unconstrained (free) strain recovery, stress recovery under pre-strain constraint, and stress recovery under fixed-strain constraint. Based on the experimental results, a one-dimensional SMP constitutive model is developed, which is motivated by the shape memory mechanism of the polymer network. The foundation of the model is that the entropy change is gradually stored during cooling and released during reheating as free recovery strain or constrained recovery stress. When fit to free strain recovery data, the model can predict the trends of the stress evolution during shape fixation and constrained strain/stress recovery under various thermomechanical conditions.
First-principle, tight binding, and semi-empirical embedded atom calculations are used to investigate a tetragonal phase transformation in gold nanowires. As wire diameter is decreased, tight binding and modified embedded atom simulations predict a surface-stress-induced phase transformation from a face-centered-cubic (fcc) <100> nanowire into a body-centered-tetragonal (bct) nanowire. In bulk gold, all theoretical approaches predict a local energy minimum at the bct phase, but tight binding and first principle calculations predict elastic instability of the bulk bct phase. The predicted existence of the stable bct phase in the nanowires is thus attributed to constraint from surface stresses. The results demonstrate that surface stresses are theoretically capable of inducing phase transformation and subsequent phase stability in nanometer scale metallic wires under appropriate conditions.
Au/Cr/Si microcantilevers were studied in their as-deposited condition and annealed state, with emphasis on a thermal treatment of 225 °C for 24 hours. Change in beam curvature was monitored during isothermal hold as a function of time. Secondary grain growth was observed in the gold, which contained non-uniformly distributed twins and dislocation defects. Diffusional transport of the chromium layer was observed during annealing. Nodules arranged in the “rolling hill” topography were observed at the free surface, both before and after annealing. Nanometer thick coatings of alumina grown by atomic layer deposition improved the uniformity of both microstructure evolution and curvature evolution during high-temperature annealing.
The purpose of this study is to investigate the structure and properties of polycrystalline NiTi in its cast form. Although it is commonly stated in the literature that cast NiTi has poor shape-memory behavior, this study demonstrates that with appropriate nano/micro structural design, cast NiTi possesses excellent shape-memory properties. Cast NiTi shape-memory alloys may give rise to a new palette of low-cost, complex-geometry components. Results from two different nominal compositions of cast NiTi are presented: 50.1 at.%Ni and 50.9 at.%Ni. The cast NiTi showed a spatial variance in grain size and a random grain orientation distribution throughout the cast material. However, small variances in the thermo-mechanical response of the cast material resulted. Transformation temperatures were slightly influenced by the radial location from which the material was extracted from the casting, showing a change in Differential Scanning Calorimetry peak diffuseness as well as a change in transformation sequence for the 50.9 at.%Ni material. Mildly aged 50.9 at.%Ni material was capable of full shape-memory strain recovery after being strained to 5% under compression, while the 50.1 at.%Ni demonstrated residual plastic strains of around 1.5%. The isotropic and symmetric response under tensile and compressive loading is a result of the measured random grain orientation distribution. The favorable recovery properties in the cast material are primarily attributed to the presence of nanometer scale precipitates, which inhibit dislocation motion and favor the martensitic transformation.
We examine the shape-memory effect in polymer networks intended for biomedical applications. The polymers were photopolymerized from tert-butyl acrylate (tBA) with polyethyleneglycol dimethacrylate (PEGDMA) acting as a crosslinker. Three-point flexural tests were used to systematically investigate the thermomechanics of shape-storage deformation and shape recovery. The glass transition temperature (Tg) of the polymers varied over a range of 100°C and is dependent on the molecular weight and concentration of the crosslinker. The polymers show 100% strain recovery up to maximum strains of approximately 80% at low and high deformation temperatures (Td). Free strain recovery was determined to depend on the temperature during deformation; lower deformation temperatures (Td < Tg) decreased the temperature required for free strain recovery. Constrained stress recovery shows a complex evolution as a function of temperature and also depends on Td. The thermomechanical results are discussed in light of potential biomedical applications and a prototype stent that can be activated at body temperature is presented.
We present results on the thermomechanical behavior of bare and nanocoated gold/polysilicon (Au/Si) bilayer cantilever beams for microelectromechanical system applications. The cantilever beams have comparable thicknesses of the Au and Si layers and thus experience significant out-of-plane curvature due to a temperature change. The experiments focus on the inelastic behavior of the bilayer beams due to thermal holding and thermal cycling. In uncoated Au/Si beams, thermal holding directly after release or thermal cycling both lead to a curvature decrease as a function of time or cycle number, respectively. The drop in curvature during thermal cycling or thermal holding in uncoated beams was not accompanied by a change in the slope of the thermoelastic curvature–temperature relationship. The absolute change in curvature depends on the temperature and the holding time. When holding or cycling to a temperature of 175 °C, the curvature change in uncoated beams is minimal for hold times up to 4500 min or 15,000 cycles. When holding or cycling to temperatures of 200 or 225 °C, the curvature in uncoated beams drops by a factor of three for hold times up to 4500 min or 15,000 cycles. The surface structure induced by long-term holding of uncoated beams shows grooving at the grain boundaries while the surface structure induced by cycling of uncoated beams shows consolidation of the grain boundaries. The Au/Si beams with a conformal 40-nm atomic layer deposition Al2O3 coating show a considerably different response compared to identical Au/Si bare beams subjected to the same thermal histories. The coating completely suppresses decreases in curvature when the beams are held at 225 °C for 4500 min. On the contrary, the coating does not always suppress thermal ratcheting when the beam is cycled from a low temperature to 225 °C. In the coated beams, the drop in curvature due to thermal cycling was accompanied by a change in the thermoelastic slope of the curvature–temperature relationship. Negligible microstructural changes were detected on the Al2O3-coated Au surface after holding or cycling. The results are discussed in light of potential deformation mechanisms and a simple analysis linking the mismatch strain between the layers to the curvature in the beams.
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