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Open Access Publications from the University of California

Scalable Manufacturing of Metal Micro/Nanowires and Applications by Thermal Fiber Drawing Method

  • Author(s): Hwang, Injoo
  • Advisor(s): Li, Xiaochun
  • et al.

The objective of this study is to better understand the fundamental principal of the thermal fiber drawing process with metal-core preforms. This would enable us to overcome the fundamental limits of current thermal drawing techniques by tuning material properties of core metals and interactions between core and cladding materials using nanoparticles. Metal micro/nanowires with controlled size, aspect ratio and spatial configurations of core and cladding materials exhibit extraordinary mechanical, thermal, electrical and optical properties. These metal micro/nanowires can be utilized for widespread applications such as: thermoelectric, conductive electrode and plasmonic photonic crystal fibers. Thermal fiber drawing method has emerged as an advanced scalable manufacturing technique for micro/nanowires production due to its unique characteristics that allow mass production of continuous and arbitrary designed wires. It is of tremendous scientific and technical interests to conduct a fundamental study on thermal fiber drawing methods and to break the current limits of the crystalline metal core thermal fiber drawing process.

In this study, metal core was fabricated by cold compaction of the Zinc (Zn)–Tungsten Carbide (WC) nanopowders. Our characterizations through scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) showed that WC nanoparticle are uniformly dispersed in Zn matrix. The effects of WC nanoparticles on the mechanical properties and degradation rate in Zn–WC nanocomposites were carefully analyzed by tensile, compressive, hardness, degradation and viscosity tests.

Metallic stents are commonly used to expand blood vessels that have been narrowed by plaque buildup (atherosclerosis). Fabrication difficulty and other constrains of metallic stents result in high cost. Zn–WC nanocomposite microwires were controllably drawn for stent struts with a diameter of 200 μm. Characterizations by the tensile and degradation tests of Zn–WC nanocomposite microwires validate the eligibility for stent fabrication. Single cell Zn–WC nanocomposite stents were fabricated by braiding thermally drawn Zn–WC nanocomposite microwires on a weaving stage built by 3D printing. Zn–WC nanocomposite stents with an inner diameter of 2 mm was expanded up to 10 mm without recoil by a catheter, which is thin tube inserted into human body serving in a broad range of functions. For the purpose of in vivo test, Zn–WC nanocomposite stents were deployed in a pig by percutaneous coronary intervention method (angioplasty with stent). The surgery under fluoroscopy that continuous X–ray beam is passed through the body part being examined. X–ray opaque Zn–WC nanocomposite stents were distinctly shown to be expanded by a catheter and remained without bounce back through the whole procedure. The Zn–WC nanocomposite stents were extracted from the pig a month later and studied for the degradability by SEM and EDS mapping analysis. SEM images of Zn-WC stents showed that the degradation of the stents was uniformly proceeded on the surface without fractures. While the Zn–WC nanocomposite stents stayed inside the vessel, good endothelializations between the Zn–WC stents and surrounding cell tissues as well as no acute pathological problems were discovered from this study.

One of the current challenges of thermal fiber drawing process for crystalline metal nanowires is low aspect ratio (< 10,000). A molten metal nanowire in a cladding material breaks up into shorter nanowires or smaller droplets due to Plateau–Rayleigh instability. It was experimentally and theoretically shown that molten liquid tends to minimize their surface area by virtue of surface tensions. The Tomotika model introduced the relation among instability time, viscosities of core and cladding materials, the wavelength and diameter of the core fluid, and interfacial energy between core and cladding materials as specifying the Plateau–Rayleigh instability [1]. The instability time was impeded by high viscosity of the Zn–WC nanocomposite core material while the preform of Zn–WC nanocomposite was thermally drawn by the stack-and-draw method. Consequently, high aspect ratio (> 1,500,000) of Zn–WC nanocomposite nanowires that are 200 nm in diameter and up to 31 cm length were achieved. Herein, we present that WC nanoparticles decreased interfacial energy between metal and glass due to its inherent characteristic such as partly metallic bonding. As a result, the nanoparticle can play the role of anchors to prevent breakage by capillary instability in nanoscale thermal fiber drawing process. Zn–WC nanocomposite nanowires surrounded by borosilicate glass were shown through the TEM (transmission electron microscope) diffraction patterns. By the electrical resistance test, not onlythe electrical resistance and but also the continuity of the Zn–WC nanocomposite nanowires was presented.

In summary, a high volume fraction of Zn–WC nanocomposite materials by cold compact method were fabricated to overcome capillary instability in nanoscale thermal fiber drawing process. WC nanoparticles enhanced viscosity of the liquid state Zn–WC nanocomposite materials and played as the anchors to prevent wire breakage due to fluid instability. Finally, Zn–WC nanocomposite stent was shown to be promising material for application biomedical field such as stent. The in vivo test further confirmed that the Zn-WC stent was biocompatible.

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