Functionalization of metallic glasses through hierarchical patterning

Hierarchical Surface Pattern on Ni-Free Ti-Based Bulk Metallic Glass to Control Cell Interactions

Ni-free Ti-based bulk metallic glasses (BMGs) are exciting materials for biomedical applications because of their outstanding biocompatibility and advantageous mechanical properties. The glassy nature of BMGs allows them to be shaped and patterned via thermoplastic forming (TPF). This work demonstrates the versatility of the TPF technique to create micro- and nano-patterns and hierarchical structures on Ti40Zr10Cu34Pd14Sn2 BMG. Particularly, a hierarchical structure fabricated by a two-step TPF process integrates 400 nm hexagonal close-packed protrusions on 2.5 µm square protuberances while preserving the advantageous mechanical properties from the as-cast material state. The correlations between thermal history, structure, and mechanical properties are explored. Regarding biocompatibility, Ti40Zr10Cu34Pd14Sn2 BMGs with four surface topographies (flat, micro-patterned, nano-patterned, and hierarchical-structured surfaces) are investigated using Saos-2 cell lines. Alamar Blue assay and live/dead analysis show that all tested surfaces have good cell proliferation and viability. Patterned surfaces are observed to promote the formation of longer filopodia on the edge of the cytoskeleton, leading to star-shaped and dendritic cell morphologies compared with the flat surface. In addition to potential implant applications, TPF-patterned Ti-BMGs enable a high level of order and design flexibility on the surface topography, expanding the available toolbox for studying cell behavior on rigid and ordered surfaces.

Effect of Chemical Composition on the Thermoplastic Formability and Nanoindentation of Ti-Based Bulk Metallic Glasses

A series of Ti41Zr25Be34-xNix (x = 4, 6, 8, 10 at.%) and Ti41Zr25Be34-xCux (x = 4, 6, 8 at.%) bulk metallic glasses were investigated to examine the influence of Ni and Cu content on the viscosity, thermoplastic formability, and nanoindentation of Ti-based bulk metallic glasses. The results demonstrate that Ti41Zr25Be30Ni4 and Ti41Zr25Be26Cu8 amorphous alloys have superior thermoplastic formability among the Ti41Zr25Be34-xNix and Ti41Zr25Be34-xCux amorphous alloys due to their low viscosity in the supercooled liquid region and wider supercooled liquid region. The hardness and modulus exhibit obvious variations with increasing Ni and Cu content in Ti-based bulk metallic glasses, which can be attributed to alterations in atomic density. Optimal amounts of Ni and Cu in Ti-based bulk metallic glasses enhance thermoplastic formability and mechanical properties. The influence of Ni and Cu content on the hardness of Ti-based bulk metallic glasses is discussed from the perspective of the mean atomic distance.

Review of Thermoplastic Drawing with Bulk Metallic Glasses

This study summarizes the recent progress in thermoplastic drawing of bulk metallic glasses. The integration of drawing with templated embossing enables the fabrication of arrays of high-aspect-ratio nanostructures whereas the earlier drawing methodologies are limited to a single fiber. The two-step drawing can produce metallic glass structures such as, vertically aligned nanowires on substrates, nanoscale tensile specimens, hollow microneedles, helical shafts, and micro-yarns, which are challenging to fabricate with other thermoplastic forming operations. These geometries will open new applications for bulk metallic glasses in the areas of sensors, optical absorption, transdermal drug-delivery, and high-throughput characterization of size-effects. In this article, we review the emergence of template-based thermoplastic drawing in bulk metallic glasses. The review focuses on the development of experimental set-up, the quantitative description of drawing process, and the versatility of drawing methodology.

Thermoplastic embossing device to probe rheological changes of supercooled metallic liquids during rapid heating

The setup and working principle of a device designed for thermoplastic embossing and investigating rheological changes during fast-resistive heating of bulk metallic glasses (BMGs) is presented. The device was developed and custom-built at the Leibniz Institute for Solid State and Materials Research Dresden (Leibniz IFW Dresden) and is integrated into a universal testing machine. By varying the electrical-current pulse signal, the rate at which BMGs are resistively heated is controlled. Next to temperature and electrical resistance, the displacement of the punch, which penetrates the sample during rapid heating, is monitored. Additional pre-heating controlled by thermocouples allows for stable heating conditions to minimize heat extraction from the resistively heated specimen, which could eventually lead to damage of the device. The main focus of this device is to study fundamental phenomena under kinetic constraints evoked by fast heating conditions and to structure the surface of BMGs by thermoplastic embossing. A case study, which is carried out with Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ BMG, demonstrates the device's performance covering heating rate ranges of three magnitudes and the access to rheological changes during the devitrification and subsequent crystallization of the BMG during rapid heating. Furthermore, the present device was successfully used to nanostructure the surface of the BMG at elevated heating rates.

Nanomolding of Gold and Gold-Silicon Heterostructures at Room Temperature

Nanofabrication techniques are limited by at least one of the required characteristics such as choice of material, control over geometry, fabrication requirements, yield, cost, and scalability. Our previously developed method of thermomechanical nanomolding fulfills these requirements, although it requires high processing temperatures. Here, we demonstrate low-temperature molding where we utilize the enhanced diffusivity on "eutectic interfaces". Gold nanorods are molded at room temperature using Au-Si alloy as feedstock. Instead of using alloy feedstock, these "eutectic interfaces" can also be established through a feedstock-mold combination. We demonstrate this by using pure Au as feedstock, which is molded into Si molds at room temperature, and also the reverse, Si feedstock is molded into Au molds forming high aspect ratio Au-Si core-shell nanorods. We discuss the mechanism of this low-temperature nanomolding in terms of lower homologous temperature at the eutectic interface. This technique, based on enhanced eutectic interface diffusion, provides a practical nanofabrication method that eliminates the previous high-temperature requirements, thereby expanding the range of the materials that can be practically nanofabricated.

Open Problems in Wetting Phenomena: Pinning Retention Forces

We review existing explanations for drop pinning and the origin of the force required to initiate the sliding of a drop on a solid surface (depinning). Theories that describe these phenomena include de Gennes', Marmur's, Furmidge's, the related Furmidge-Extrand's, and Tadmor's theory. These theories are all well cited but generally do not address each other, and usually papers that cite one of them ignore the others. Here, we discuss the advantages and disadvantages of these theories and their applicability to different experimental systems. Thus, we link different experimental systems to the theories that describe them best. We describe the force laws that can be deduced should these theories be united and the major open problems that remain. We describe a physical meaning that can be extracted from retention force measurements, specifically, the interfacial modulus that describes the tendency of a solid to conform to the liquid. This has implications for various wetting phenomena such as adhesion robustness, drug penetration into biological tissues, and solid robustness/resilience versus solid degradation over time as a result of its contact with a liquid.

Biocompatibility of platinum-based bulk metallic glass in orthopedic applications

Bulk metallic glasses (BMGs) are a class of amorphous metals that exhibit high strength, ductility paired with wear and corrosion resistance. These properties suggest that they could serve as an alternative to conventional metallic implants that suffer wear and failure. In the present study, we investigated Platinum (Pt)-BMG biocompatibility in bone applications. Specifically, we investigated osteoclast formation on flat and nanopatterned Pt_57.5Cu_14.7Ni_5.3P_22.5(atomic percent) as well as titanium (control). Specifically, receptor activator of NF-κB (RANK) ligand-induced murine bone marrow derived mononuclear cell fusion was measured on multiple nanopatterns and was found to be reduced on nanorods (80 and 200 nm in diameter) and was associated with reduced tartrate-resistant acid phosphatase (TRAP) and matrix metalloproteinase (MMP9) expression. Evaluation of mesenchymal stem cell (MSC) to osteoblast differentiation on nanopatterned Pt-BMG showed significant reduction in comparison to flat, suggesting that further exploration of nanopatterns is required to have simultaneous induction of osteoblasts and inhibition of osteoclasts.Invivo studies were also pursued to evaluate the biocompatibility of Pt-BMG in comparison to titanium. Rods of each material were implanted in the femurs of mice and evaluated by x-ray, mechanical testing, micro-computed tomography (micro-CT), and histological analysis. Overall, Pt-BMG showed similar biocompatibility with titanium suggesting that it has the potential to improve outcomes by further processing at the nanoscale.

Viscous Flow of Supercooled Liquid in a Zr-Based Bulk Metallic Glass Synthesized by Additive Manufacturing

The constraint in sample size imposed by the critical cooling rate necessary for glass formation using conventional casting techniques is possibly the most critical limitation for the extensive use of bulk metallic glasses (BMGs) in structural applications. This drawback has been recently overcome by processing glass-forming systems via additive manufacturing, finally enabling the synthesis of BMGs with no size limitation. Although processing by additive manufacturing allows fabricating BMG objects with virtually no shape limitation, thermoplastic forming of additively manufactured BMGs may be necessary for materials optimization. Thermoplastic forming of BMGs is carried out above the glass transition temperature, where these materials behave as highly viscous liquids; the analysis of the viscosity is thus of primary importance. In this work, the temperature dependence of viscosity of the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ metallic glass fabricated by casting and laser powder bed fusion (LPBF) is investigated. We observed minor differences in the viscous flow of the specimens fabricated by the different techniques that can be ascribed to the higher porosity of the LPBF metallic glass. Nevertheless, the present results reveal a similar overall variation of viscosity in the cast and LPBF materials, which offers the opportunity to shape additively manufactured BMGs using already developed thermoplastic forming techniques.

Robust Fabrication of Polymeric Nanowire with Anodic Aluminum Oxide Templates

Functionalization of a surface with biomimetic nano-/micro-scale roughness (wires) has attracted significant interests in surface science and engineering as well as has inspired many real-world applications including anti-fouling and superhydrophobic surfaces. Although methods relying on lithography include soft-lithography greatly increase our abilities in structuring hard surfaces with engineered nano-/micro-topologies mimicking real-world counterparts, such as lotus leaves, rose petals, and gecko toe pads, scalable tools enabling us to pattern polymeric substrates with the same structures are largely absent in literature. Here we present a robust and simple technique combining anodic aluminum oxide (AAO) templating and vacuum-assisted molding to fabricate nanowires over polymeric substrates. We have demonstrated the efficacy and robustness of the technique by successfully fabricating nanowires with large aspect ratios (>25) using several common soft materials including both cross-linking polymers and thermal plastics. Furthermore, a model is also developed to determine the length and molding time based on nanowires material properties (e.g., viscosity and interfacial tension) and operational parameters (e.g., pressure, vacuum, and AAO template dimension). Applying the technique, we have further demonstrated the confinement effects on polymeric crosslinking processes and shown substantial lengthening of the curing time.

Accelerated discovery and mechanical property characterization of bioresorbable amorphous alloys in the Mg-Zn-Ca and the Fe-Mg-Zn systems using high-throughput methods

Ternary amorphous alloys in the magnesium (Mg)-zinc (Zn)-calcium (Ca) and the iron (Fe)-Mg-Zn systems are promising candidates for use in bioresorbable implants and devices. The optimal alloy compositions for biomedical applications should be chosen from a large variety of available alloys with best combination of mechanical properties (modulus, strength, hardness) and biological response (in situ degradation rates, cell adhesion and proliferation). As a first step towards establishing a database designed to enable such targeted material selection, amorphous alloy composition libraries were fabricated employing a combinatorial magnetron sputtering approach where Mg, Zn, and Ca/Fe are co-deposited from separate sources onto a silicon wafer substrate. Composition analysis using energy dispersive X-ray spectroscopy documented a composition range of ∼15-85 at% Mg, ∼6-55 at% Zn, and ∼5-60 at% Ca for the Mg-Zn-Ca library and ∼26-84 at% Mg, ∼10-61 at% Zn, and ∼7-55 at% Fe for the Fe-Mg-Zn library. X-ray diffraction measurements established that amorphous alloys (i.e., glasses) form in almost the entire range of composition at the high cooling rates during sputtering for both alloy libraries. Finally, the effective material modulus, the Oliver-Pharr hardness, and the yield strength values obtained using nanoindentation reveal a wide range of mechanical properties within both systems.

Flexible and stretchable metallic glass micro- and nano-structures of tunable properties

Flexible and stretchable nanostructures have broad technological applications. Although nanostructures synthesized with metallic glasses, the alloys being of amorphous atomic structure, exhibit superior properties, they are typically too rigid to be used as flexible materials with existing synthesis techniques. In this study we report periodic and crumpled metallic glass nanostructures that can accommodate a large amount of stretching. We demonstrate that their morphologies and characteristic length scale can be well controlled, and that feature sizes as small as ∼200 nm can be readily achieved. With their integrity maintained, the nanostructures can be stretched to a strain of ∼100%, leading to broadly tunable properties. The approach is not limited to specific metallic glasses, but is applicable to a wide range of glass-forming alloys. This not only enables metallic glasses to be used under extreme stretching conditions, but also helps in the exploration of new functionalities of glassy materials.

Nanoshaping of glass forming metallic liquids by stretching: evading lithography

Lithography-free nanomanufacturing by elongation and fracture of glass forming metallic liquid is presented. The viscous metallic liquid confined in a cavity is laterally downsized to nanoscale by stretching. The extent of size-reduction can be controlled by tuning the active volume of liquid and the viscous and capillary stresses. Very high aspect-ratio metal nanostructures can be fabricated without using lithography or expensive molds. A systematic study is performed using glass forming Pt-Cu-Ni-P alloy to understand the effects of viscosity, surface tension, pulling velocity, and cavity size on the evolution of cylindrical liquid column under tension. The results are quantitatively described using a phenomenological model based on lubrication theory and surface tension induced breakup of liquid filaments. A new manufacturing approach based on variable pulling velocity and/or spinning of metallic liquid is proposed for fabrication of complex geometries.

Hierarchical Nanoparticle Assemblies Formed via One-Step Catalytic Stamp Pattern Transfer

The one-step catalytic stamp pattern transfer process is described for producing arrays of hierarchical nanoparticle assemblies. The method simply combines in situ nanoparticle synthesis triggered by free residual Si-H groups on PDMS stamps and the lift-off pattern transfer technique. No additional nanoparticle synthesis procedure is required before the pattern transfer process. Exquisitely uniform and precisely spaced hierarchical nanoparticle assemblies with designed geometry can be rapidly produced using the catalytic stamp pattern transfer process. Sequential catalytic stamp pattern transfer also is described to generate multilayered, hierarchical nanoparticle assemblies with various geometries. The hierarchical nanoparticle assemblies catalytically transferred onto the surface are not just nanoparticles but nanoparticle-polydimethylsiloxane residue composites. The in situ-synthesized nanoparticles retain optical properties. The hierarchical nanoparticle assemblies with precisely controlled geometry further show potential in the application of surface-enhanced Raman scattering. The capability of one-step catalytic stamp pattern transfer allows the scalable and reproducible fabrication of well-defined hierarchical nanoparticle assemblies.

Micro-/Nano-Scales Direct Cell Behavior on Biomaterial Surfaces

Cells are the smallest living units of a human body's structure and function, and their behaviors should not be ignored in human physiological and pathological metabolic activities. Each cell has a different scale, and presents distinct responses to specific scales: Vascular endothelial cells may obtain a normal function when regulated by the 25 µm strips, but de-function if the scale is removed; stem cells can rapidly proliferate on the 30 nm scales nanotubes surface, but stop proliferating when the scale is changed to 100 nm. Therefore, micro and nano scales play a crucial role in directing cell behaviors on biomaterials surface. In recent years, a series of biomaterials surface with micro and/or nano scales, such as micro-patterns, nanotubes and nanoparticles, have been developed to control the target cell behavior, and further enhance the surface biocompatibility. This contribution will introduce the related research, and review the advances in the micro/nano scales for biomaterials surface functionalization.

Solid-Liquid Work of Adhesion

We establish a tool for direct measurements of the work needed to separate a liquid from a solid. This method mimics a pendant drop that is subjected to a gravitational force that is slowly increasing until the solid-liquid contact area starts to shrink spontaneously. The work of separation is then calculated in analogy to Tate's law. The values obtained for the work of separation are independent of drop size and are in agreement with Dupré's theory, showing that they are equal to the work of adhesion.

Bioinspired Hierarchically Structured Surfaces for Efficient Capture and Release of Circulating Tumor Cells

The development of novel bioinspired surfaces with hierarchical micro- and nanoscale topographic structures for efficient capture and release of circulating tumor cells (CTCs) is reported. The capture of CTCs, facilitated by surface-immobilized epithelial cell adhesion molecule antibodies (anti-EpCAM), was shown to be significantly enhanced in novel three-dimensional hierarchically structured surfaces that were fabricated by replicating the natural micro- and nanostructures of rose petals. Under static conditions, these hierarchical capture substrates exhibited up to 6 times higher cell capture ability at concentrations of 100 cells mL^-1 in contrast to flat anti-EpCAM-functionalized polydimethylsiloxane (PDMS) surfaces. As indicated by scanning electron microscopy (SEM) and immunofluorescent images, this enhancement can be in large part attributed to the topographical interaction between nanoscale cell surface components and nanostructures on the substrate. Similarly, the increased surface area affords a higher nominal coverage of anti-EpCAM, which increases the number of available binding sites for cell capture. By treating the substrates with the biocompatible reductant glutathione (GSH), up to 85% of the captured cells were released, which displayed over 98% cell viability after culturing on tissue culture polystyrene (TCP) for 24 h. Therefore, these bioinspired hierarchically structured and functionalized substrates can be successfully applied to capture CTCs, as well as release CTCs for subsequent analysis. These findings provide new prospects for designing cell-material interfaces for advanced cell-based biomedical studies in the future.

High-throughput drawing and testing of metallic glass nanostructures

Thermoplastic embossing of metallic glasses promises direct imprinting of metal nanostructures using templates. However, embossing high-aspect-ratio nanostructures faces unworkable flow resistance due to friction and non-wetting conditions at the template interface. Herein, we show that these inherent challenges of embossing can be reversed by thermoplastic drawing using templates. The flow resistance not only remains independent of wetting but also decreases with increasing feature aspect-ratio. Arrays of assembled nanotips, nanowires, and nanotubes with aspect-ratios exceeding 1000 can be produced through controlled elongation and fracture of metallic glass structures. In contrast to embossing, the drawing approach generates two sets of nanostructures upon final fracture; one set remains anchored to the metallic glass substrate while the second set is assembled on the template. This method can be readily adapted for high-throughput fabrication and testing of nanoscale tensile specimens, enabling rapid screening of size-effects in mechanical behavior.

Oxide-derived nanostructured metallic-glass electrodes for efficient electrochemical hydrogen generation

Nanostructured materials with large surface areas are continuing to be at the forefront of catalytic applications.

Tuning apparent friction coefficient by controlled patterning bulk metallic glasses surfaces

Micro-honeycomb structures with various pitches between adjacent cells were hot-embossed on Zr₃₅Ti₃₀Cu_8.25Be_26.75 bulk metallic glass surface. The effect of pitch geometry on the frictional behavior of metallic glass surface was systematically investigated. The results revealed that all textured metallic glass surfaces show a reduction in friction coefficient compared to smooth surface. More intriguingly, the friction coefficient first decreased and then increased gradually with increasing pitches. Such unique behavior can be understood fundamentally from the perspective of competing effects between contact area and local stress level with increasing pitches. This finding not only enhance the in-depth understanding of the mechanism of the significant role of surface topography on the frictional behavior of metallic glass surface, but also opens a new route towards other functional applications for bulk metallic glasses.

Designing a stronger interface through graded structures in amorphous/nanocrystalline ZrCu/Cu multilayered films

Many multilayered nano-structures appear to fail due to brittle matter along the interfaces. In order to toughen them, in this study, the microstructure and interface strength of multilayered thin films consisting of amorphous ZrCu and nanocrystalline Cu (with sharp or graded interfaces) are examined and analyzed. The interface possesses a gradient nature in terms of composition, nanocrystalline phase size and volume fraction. The bending results extracted from the nano-scaled cantilever bending samples demonstrate that multilayered films with graded interfaces would have a much higher interface bending strength/strain/modulus, and an overall improvement upgrade of more than 50%. The simple graded interface design of multilayered thin films with improved mechanical properties can offer much more promising performance in structural and functional applications for MEMS or optical coating.

Feasibility of using bulk metallic glass for self-expandable stent applications

Self-expandable stents are widely used to restore blood flow in a diseased artery segment by keeping the artery open after angioplasty. Despite the prevalent use of conventional crystalline metallic alloys, for example, nitinol, to construct self-expandable stents, new biomaterials such as bulk metallic glasses (BMGs) are being actively pursued to improve stent performance. Here, we conducted a series of analyses including finite element analysis and molecular dynamics simulations to investigate the feasibility of using a prototypical Zr-based BMG for self-expandable stent applications. We model stent crimping of several designs for different percutaneous applications. Our results indicate that BMG-based stents with diamond-shaped crowns suffer from severe localization of plastic deformation and abrupt failure during crimping. As a possible solution, we further illustrate that such abrupt failure could be avoided in BMG-based stents without diamond shape crowns. This work would open a new horizon for a quest toward exploiting superior mechanical and functional properties of metallic glasses to design future stents. © 2016 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1874-1882, 2017.

Shape-Controlled Metal-Metal and Metal-Polymer Janus Structures by Thermoplastic Embossing

We report the fabrication of metal-metal and metal-polymer Janus structures by embossing of thermoplastic metallic glasses and polymers. Hybrid structures with controllable shapes and interfaces are synthesized by template-assisted embossing. Different manufacturing strategies such as co-embossing and additive embossing are demonstrated for joining the materials with diverse compositions and functionalities. Structures with distinct combinations of properties such as hydrophobic-hydrophilic, opaque-transparent, insulator-conductor, and nonmagnetic-ferromagnetic are produced using this approach. These anisotropic properties are further utilized for selective functionalization of Janus structures.

Thermomechanical Behavior of Molded Metallic Glass Nanowires

Metallic glasses are disordered materials that offer the unique ability to perform thermoplastic forming operations at low thermal budget while preserving excellent mechanical properties such as high strength, large elastic strain limits, and wear resistance owing to the metallic nature of bonding and lack of internal defects. Interest in molding micro- and nanoscale metallic glass objects is driven by the promise of robust and high performance micro- and nanoelectromechanical systems and miniature energy conversion devices. Yet accurate and efficient processing of these materials hinges on a robust understanding of their thermomechanical behavior. Here, we combine large-scale thermoplastic tensile deformation of collections of Pt-based amorphous nanowires with quantitative thermomechanical studies of individual nanowires in creep-like conditions to demonstrate that superplastic-like flow persists to small length scales. Systematic studies as a function of temperature, strain-rate, and applied stress reveal the transition from Newtonian to non-Newtonian flow to be ubiquitous across the investigated length scales. However, we provide evidence that nanoscale specimens sustain greater free volume generation at elevated temperatures resulting in a flow transition at higher strain-rates than their bulk counterparts. Our results provide guidance for the design of thermoplastic processing methods and methods for verifying the flow response at the nanoscale.

Stretchable, Transparent Electrodes as Wearable Heaters Using Nanotrough Networks of Metallic Glasses with Superior Mechanical Properties and Thermal Stability

Mechanical robustness, electrical and chemical reliabilities of devices against large deformations such as bending and stretching have become the key metrics for rapidly emerging wearable electronics. Metallic glasses (MGs) have high elastic limit, electrical conductivity, and corrosion resistance, which can be promising for applications in wearable electronics. However, their applications in wearable electronics or transparent electrodes have not been extensively explored so far. Here, we demonstrate stretchable and transparent electrodes using CuZr MGs in the form of nanotrough networks. MG nanotroughs are prepared by electrospinning and cosputtering process, and they can be transferred to various desired substrates, including stretchable elastomeric substrates. The resulting MG nanotrough network is first utilized as a stretchable transparent electrode, presenting outstanding optoelectronic (sheet resistance of 3.8 Ω/sq at transmittance of 90%) and mechanical robustness (resistance change less than 30% up to a tensile strain of 70%) as well as excellent chemical stability against hot and humid environments (negligible degradation in performance for 240 h in 85% relative humidity and 85 °C). A stretchable and transparent heater based on the MG nanotrough network is also demonstrated with a wide operating temperature range (up to 180 °C) and excellent stretchability (up to 70% in the strain). The excellent mechanical robustness of these stretchable transparent electrode and heater is ascribed to the structural configuration (i.e., a nanotrough network) and inherent high elastic limit of MGs, as supported by experimental results and numerical analysis. We demonstrate their real-time operations on human skin as a wearable, transparent thermotherapy patch controlled wirelessly using a smartphone as well as a transparent defroster for an automobile side-view mirror, suggesting a promising strategy toward next-generation wearable electronics or automobile applications.

Imprinting bulk amorphous alloy at room temperature

We present investigations on the plastic deformation behavior of a brittle bulk amorphous alloy by simple uniaxial compressive loading at room temperature. A patterning is possible by cold-plastic forming of the typically brittle Hf-based bulk amorphous alloy through controlling homogenous flow without the need for thermal energy or shaping at elevated temperatures. The experimental evidence suggests that there is an inconsistency between macroscopic plasticity and deformability of an amorphous alloy. Moreover, imprinting of specific geometrical features on Cu foil and Zr-based metallic glass is represented by using the patterned bulk amorphous alloy as a die. These results demonstrate the ability of amorphous alloys or metallic glasses to precisely replicate patterning features onto both conventional metals and the other amorphous alloys. Our work presents an avenue for avoiding the embrittlement of amorphous alloys associated with thermoplastic forming and yields new insight the forming application of bulk amorphous alloys at room temperature without using heat treatment.

A Zr-based bulk metallic glass for future stent applications: Materials properties, finite element modeling, and in vitro human vascular cell response

Despite the prevalent use of crystalline alloys in current vascular stent technology, new biomaterials are being actively sought after to improve stent performance. In this study, we demonstrated the potential of a Zr-Al-Fe-Cu bulk metallic glass (BMG) to serve as a candidate stent material. The mechanical properties of the Zr-based BMG, determined under both static and cyclic loadings, were characterized by high strength, which would allow for the design of thinner stent struts to improve stent biocompatibility. Finite element analysis further complemented the experimental results and revealed that a stent made of the Zr-based BMG was more compliant with the beats of a blood vessel, compared with medical 316L stainless steel. The Zr-based BMG was found to be corrosion resistant in a simulated body environment, owing to the presence of a highly stable ZrO2-rich surface passive film. Application-specific biocompatibility studies were conducted using human aortic endothelial cells and smooth muscle cells. The Zr-Al-Fe-Cu BMG was found to support stronger adhesion and faster coverage of endothelial cells and slower growth of smooth muscle cells than 316L stainless steel. These results suggest that the Zr-based BMG could promote re-endothelialization and potentially lower the risk of restenosis, which are critical to improve vascular stent implantation integration. In general, findings in this study raised the curtain for the potential application of BMGs as future candidates for stent applications.

STATEMENT OF SIGNIFICANCE

Vascular stents are medical devices typically used to restore the lumen of narrowed or clogged blood vessel. Despite the clinical success of metallic materials in stent-assisted angioplasty, post-surgery complications persist due to the mechanical failures, corrosion, and in-stent restenosis of current stents. To overcome these hurdles, strategies including new designs and surface functionalization have been exercised. In addition, the development of new materials with higher performance and biocompatibility can intrinsically reduce stent failure rates. The present study demonstrates the advantages of a novel material, named bulk metallic glass (BMG), over the benchmarked 316L stainless steel through experimental methods and computational simulations. It raises the curtain of new research endeavors on BMGs as competitive alternatives for stent applications.