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Open Access Publications from the University of California

Scholarly Works (9 results)

  • Thesis
  • Peer Reviewed
UCLA Electronic Theses and Dissertations (2020)

Metals have been widely used for biomedical applications, especially as bio-implants, due to their excellent mechanical properties that are suitable for load-bearing applications. Zn has recently attracted attention for being used as biodegradable materials for implants, especially for bioresorbable stent and biodegradable electrodes. The suitable corrosion rate and benign biocompatibility promote Zn as a potential candidate for bioresorbable cardiovascular stents. However, the lack of strength and thermal stability inhibited its practical application and commercialization. Alloying has been applied for strengthening but inevitably introducing side-effects, such as increased corrosion rate and decreased elongation. In this dissertation, a new class of materials, metal matrix nanocomposite, was investigated for biomedical applications, including biodegradable implants (e.g., bioresorbable cardiovascular stents) and medical devices (e.g., degradable nanowire biosensors). In the beginning, this work has developed and optimized the fabrication methods of Zn matrix nanocomposite, enabling efficient nanoparticle incorporation and homogeneous dispersion. Additionally, through the characterization in microstructure and mechanical properties, this work has thoroughly investigated the nanoparticle interaction with the grain boundaries and intermetallic interfaces during the solidification process and aging process. Furthermore, this work has characterized the mechanical properties of Zn-based nanocomposites and has fabricated the prototype of bioresorbable stents for evaluation. For the application of biodegradable electrodes consisting of metal nanowires, nanoparticles have enabled molten metal viscosity/interfacial energy modification and fluid instability suppression to realize the high aspect ratio nanowire fabrication. This dissertation successfully establishes fundamental knowledge for metal matrix nanocomposite as suitable biomaterials for various medical applications, bridging the advanced manufacturing of novel biometals to innovative biomedical devices and providing innovative solutions for human health.

    Cover page: The Fundamental Study of Nanoparticle Effects in Metal Matrix Nanocomposite for Biomedical Applications
    • Article
    • Peer Reviewed
    UCLA Previously Published Works (2019)
    This work presents the manufacturing and characterization of zinc-tungsten carbide (Zn-WC) nanocomposite as a potential biodegradable material. A highly homogeneous WC nanoparticle dispersion in a Zn matrix was achieved by molten salt assisted stir casting followed with hot rolling. The Vickers microhardness and ultimate tensile strength of zinc were enhanced more than 50% and 87%, respectively, with the incorporation of up to 4.4 vol. % WC nanoparticles. Additionally, Zn-WC nanocomposite retained high ductility (> 65%). However, the electrical and thermal conductivities were reduced by 12% and 21%, respectively. The significant enhancement in mechanical strength makes nanoparticle-reinforced zinc a promising candidate material for biodegradable metallic implants for a wide range of clinical applications, including orthopaedic and cardiovascular implants as well as bioresorbable electronics.
      Cover page: Manufacturing and Characterization of Zn-WC as Potential Biodegradable Material.
      • Article
      • Peer Reviewed
      UCLA Previously Published Works (2020)
      Ultra-long metal nanowires and their facile fabrication have been long sought after as they promise to offer substantial improvements of performance in numerous applications. However, ultra-long metal ultrafine/nanowires are beyond the capability of current manufacturing techniques, which impose limitations on their size and aspect ratio. Here we show that the limitations imposed by fluid instabilities with thermally drawn nanowires can be alleviated by adding tungsten carbide nanoparticles to the metal core to arrive at wire lengths more than 30 cm with diameters as low as 170 nm. The nanoparticles support thermal drawing in two ways, by increasing the viscosity of the metal and lowering the interfacial energy between the boron silicate and zinc phase. This mechanism of suppressing fluid instability by nanoparticles not only enables a scalable production of ultralong metal nanowires, but also serves for widespread applications in other fluid-related fields.
        Cover page: Nanoparticles suppress fluid instabilities in the thermal drawing of ultralong nanowires
        • Article
        • Peer Reviewed
        UCLA Previously Published Works (2020)
        There is a lack of bioabsorbable materials with adequate mechanical strength suitable for implant applications that provide temporary support while tissue integrity is restored, especially for pediatric applications. Bioabsorbable metals have emerged as an attractive choice due to their combination of strength, ductility, and biocompatibility in vivo. Zinc has shown great promise as a bioabsorbable metal, but the weak mechanical properties of pure zinc limit its application as an implant material. This study investigates zinc-tungsten carbide (Zn-WC) nanocomposite as a novel material for bioabsorbable metallic implants. Ultrasound-assisted powder compaction was used to fabricate Zn-WC nanocomposites. This study includes the material characterization of microstructure, microhardness, and degradability. Results showed that tungsten carbide nanoparticles enhanced the mechanical properties of Zn, and maintained the favorable corrosion rate of pure Zn. These results encourage further investigation of Zn-WC nanocomposites for biomedical applications with the ultimate goal of creating safe and efficacious bioabsorbable metallic implants for many clinical applications.
          Cover page: Novel Zinc / Tungsten Carbide Nanocomposite as Bioabsorbable Implant.
          • Article
          • Peer Reviewed
          UCLA Previously Published Works (2020)
          Zinc (Zn) has been widely investigated as a biodegradable metal for orthopedic implants and vascular stents due to its ideal corrosion in vivo and biocompatibility. However, pure Zn lacks adequate mechanical properties for load-bearing applications. Alloying elements, such as iron (Fe), have been shown to improve the strength significantly, but at the cost of compromised ductility and corrosion rate. In this study, tungsten carbide (WC) nanoparticles were incorporated into the Zn-2Fe alloy system for strengthening, microstructure modification, and ductility enhancement. Thermally stable WC nanoparticles modified the intermetallic ζ-FeZn13 interface morphology from faceted to non-faceted. Consequently, WC nanoparticles simultaneously enhance mechanical strength and ductility while maintaining a reasonable corrosion rate. Overall, this novel Zn-Fe-WC nanocomposite could be used as biodegradable material for biomedical applications where pure Zn is inadequate.
            Cover page: Highly Ductile Zn-2Fe-WC Nanocomposite as Biodegradable Material.
            • Article
            • Peer Reviewed
            UCLA Previously Published Works (2022)
            Bioabsorbable metals are increasingly attracting attention for their potential use as materials for degradable implant devices. Zinc (Zn) alloys have shown great promises due to their good biocompatibility and favorable degradation rate. However, it has been difficult to maintain an appropriate balance among strength, ductility, biocompatibility, and corrosion rate for Zn alloys historically. In this study, the microstructure, chemical composition, mechanical properties, biocompatibility, and corrosion rate of a new ternary zinc-iron-silicon (Zn-Fe-Si) alloy system was studied as a novel material for potential biodegradable implant applications. The results demonstrated that the in situ formed Fe-Si intermetallic phases enhanced the mechanical strength of the material while maintaining a favorable ductility. With Fe-Si reinforcements, the microhardness of the Zn alloys was enhanced by up to 43%. The tensile strength was increased by up to 76% while elongation to failure remained above 30%. Indirect cytotoxicity testing showed the Zn-Fe-Si system had good biocompatibility. Immersion testing revealed the corrosion rate of Zn-Fe-Si system was not statistically different from pure Zn. To understand the underlying phase formation mechanism, the reaction process in this ternary system during the processing was also studied via phase evolution and Gibbs free energy analysis. The results suggest the Zn-Fe-Si ternary system is a promising new material for bioabsorbable metallic medical devices.
              Cover page: Experimental study on novel biodegradable Zn-Fe-Si alloys.
              • Article
              • Peer Reviewed
              UCLA Previously Published Works (2022)
              Zinc (Zn) and Zn alloys have been studied as potential materials for bioresorbable stents (BRSs) in the last decade due to their favorable biodegradability and biocompatibility. However, most Zn alloys lack the necessary combination of strength, ductility, fatigue resistance, corrosion rate (CR), and thermal stability needed for such applications. In this study, nanoparticles made of tungsten carbide (WC) were successfully incorporated into Zn alloyed with 0.5 wt % magnesium (Mg) and evaluated for their suitability for BRS applications. Specifically, the resulting Zn-0.5Mg-WC nanocomposite's microstructure, mechanical properties, in vitro CR, and thermal stability were evaluated. The Zn-0.5Mg-WC nanocomposite had excellent mechanical strength [ultimate tensile strength (UTS) > 250 MPa], elongation to failure (>30%), and a suitable in vitro CR (∼0.02 mm/y) for this clinical application. Moreover, the Zn-0.5Mg-WC nanocomposite survived 10 million cycles of tensile loading (stress ratio, R = 0.053) when the maximum stress was 80% of the yield stress. Its ductility was also retained during a 90-day thermal stability study, indicating an excellent shelf life. Stent prototypes were fabricated using this composition and were successfully deployed during bench testing without fracture. These results show that the Zn-0.5Mg-WC nanocomposite is a promising material for BRS applications. In vivo studies are underway to validate both biocompatibility, stent function, and degradation.
                Cover page: Zn–Mg–WC Nanocomposites for Bioresorbable Cardiovascular Stents: Microstructure, Mechanical Properties, Fatigue, Shelf Life, and Corrosion
                • Article
                • Peer Reviewed
                UCLA Previously Published Works (2016)
                We developed a compact plane illumination plugin (PIP) device which enabled plane illumination and light sheet fluorescence imaging on a conventional inverted microscope. The PIP device allowed the integration of microscope with tunable laser sheet profile, fast image acquisition, and 3-D scanning. The device is both compact, measuring approximately 15 by 5 by 5 cm, and cost-effective, since we employed consumer electronics and an inexpensive device molding method. We demonstrated that PIP provided significant contrast and resolution enhancement to conventional microscopy through imaging different multi-cellular fluorescent structures, including 3-D branched cells in vitro and live zebrafish embryos. Imaging with the integration of PIP greatly reduced out-of-focus contamination and generated sharper contrast in acquired 2-D plane images when compared with the stand-alone inverted microscope. As a result, the dynamic fluid domain of the beating zebrafish heart was clearly segmented and the functional monitoring of the heart was achieved. Furthermore, the enhanced axial resolution established by thin plane illumination of PIP enabled the 3-D reconstruction of the branched cellular structures, which leads to the improvement on the functionality of the wide field microscopy.
                  Creative Commons 'BY' version 4.0 license
                  • Article
                  • Peer Reviewed
                  UC Irvine Previously Published Works (2019)
                  Cooling, nucleation, and phase growth are ubiquitous processes in nature. Effective control of nucleation and phase growth is of significance to yield refined microstructures with enhanced performance for materials. Recent studies reveal that ultrafine grained (UFG)/nanocrystalline metals exhibit extraordinary properties. However, conventional microstructure refinement methods, such as fast cooling and inoculation, have reached certain fundamental limits. It has been considered impossible to fabricate bulk UFG/nanocrystalline metals via slow cooling. Here, we report a new discovery that nanoparticles can refine metal grains to ultrafine/nanoscale by instilling a continuous nucleation and growth control mechanism during slow cooling. The bulk UFG/nanocrystalline metal with nanoparticles also reveals an unprecedented thermal stability. This method overcomes the grain refinement limits and may be extended to any other processes that involve cooling, nucleation, and phase growth for widespread applications.
                    Cover page: Bulk ultrafine grained/nanocrystalline metals via slow cooling
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