Grant A. Crawford

Security printing of covert quick response codes using upconverting nanoparticle inks

Counterfeiting costs governments and private industries billions of dollars annually due to loss of value in currency and other printed items. This research involves using lanthanide doped β-NaYF(4) nanoparticles for security printing applications. Inks comprised of Yb(3+)/Er(3+) and Yb(3+)/Tm(3+) doped β-NaYF(4) nanoparticles with oleic acid as the capping agent in toluene and methyl benzoate with poly(methyl methacrylate) (PMMA) as the binding agent were used to print quick response (QR) codes. The QR codes were made using an AutoCAD file and printed with Optomec direct-write aerosol jetting(®). The printed QR codes are invisible under ambient lighting conditions, but are readable using a near-IR laser, and were successfully scanned using a smart phone. This research demonstrates that QR codes, which have been used primarily for information sharing applications, can also be used for security purposes. Higher levels of security were achieved by printing both green and blue upconverting inks, based on combinations of Er(3+)/Yb(3+) and Tm(3+)/Yb(3+), respectively, in a single QR code. The near-infrared (NIR)-to-visible upconversion luminescence properties of the two-ink QR codes were analyzed, including the influence of NIR excitation power density on perceived color, in term of the CIE 1931 chromaticity index. It was also shown that this security ink can be optimized for line width, thickness and stability on different substrates.

A NIR-to-NIR upconversion luminescence system for security printing applications

A covert print-and-read system is demonstrated based on NIR-to-NIR upconversion luminescence. Inks activated with Yb3+/Tm3+ doped β-NaYF4 upconversion nanoparticles were used to print covert features on various substrates, including paper, epoxy resin, and circuit boards. The Yb3+/Tm3+ doping concentrations were optimized to maximize the brightness of 800 nm upconversion emission excited with 980 nm light, while simultaneously minimizing unwanted blue upconversion. Images printed with the NIR-optimized inks are invisible to the naked eye under ambient lighting or under 980 nm excitation. NIR-to-NIR images are easily captured, however, using an inexpensive, modified point-and-shoot CCD camera, even at modest excitation power densities (1.5 W cm−2). It is demonstrated that the latent images can also be read through select hard or soft coatings which are opaque to visible light, such as black inkjet print, or dyed epoxy resin, without significant attenuation of brightness. The ability to protect the printed images with durable, opaque coatings increases the tamper-resistance and the covertness of the system; removes the requirement that the print be invisible on the bare substrate; and blocks any visible emission that might be present, even under very high excitation power densities.

Multi-layered covert QR codes for increased capacity and security

Quick Response (QR) codes, used in marketing, warehouse management, product tracking, and other applications, are usually only comprised of visible black and white modules. The goal of this research is the creation of covert, color QR codes for increased data capacity, and security. This goal was met by layering QR codes, each of a unique color, printing the covert versions of the color QR codes, upconverting the covert codes into visible codes and unlayering them to take advantage of the various color channels present. It was shown that color layering of the QR codes effectively increased the data capacity by three times that of a traditional QR code, while the covert nature of the code provides added security. Finally, it was also demonstrated that a layered QR code with six base colors (using color intensity variations) could further increase QR code data capacity.

Structure–Properties Relations in High-Pressure Cold-Sprayed Deposits

In the cold spray (CS) process, deposits are produced by depositing powder particles at high velocity onto a substrate. Powders deposited by CS do not undergo melting before or upon impact with the substrate. This feature makes CS suitable for deposition of a wide variety of materials, most commonly metallic alloys but also ceramics and polymers or composites of those materials. During processing, the particles undergo severe plastic deformation, and both components of bonding, i.e., mechanical and metallurgical, are achieved with the underlying material, depending on the material type and impact velocity. The deformation behavior of powder particles depends on multiple material and process parameters. Changing to these parameters leads to the evolution of different microstructures and consequently changes the mechanical properties in the deposit. While CS technology has matured during the last decade, the process is inherently complex, and thus the effects of deposition parameters on particle deformation, deposit microstructure, and mechanical properties are not always clear. The purpose of this chapter is to describe the existing relationships between microstructure and mechanical properties of various CS deposits to illuminate what has been discovered to date.

Microstructural Evolution and Mechanical Behavior of Heat Treated Ti-6Al-4V Powders

Plasma-atomized Ti-6Al-4V powder is widely used in a variety of powder-based manufacturing technologies due to its attractive properties. In many cases there is a desire to modify the microstructure of the as-atomized powder via heat treatment, to improve manufacturing characteristics (e.g., deformability in the case of cold spray deposition). Microstructure characterization of as-received and heat treated plasma-atomized Ti-6Al-4V powder was performed using optical microscopy and x-ray diffraction. Additionally, heat treated powder was characterized using back-scatter electron imaging and energy-dispersive x-ray spectroscopy. The as-received powder was characterized by martensitic alpha grain structures, while the heat treated powder was characterized by either equiaxed alpha or lamellar alpha phase grains, with intergranular beta phase structures. Bulk Ti-6Al-4V was heat treated similar to powders to establish a control, for effective comparison. Microstructure characterization of bulk Ti-6Al-4V using optical microscopy revealed a bright oxygen-rich region, also known as alpha case, near the surface with microstructures resembling heat treated powder. Hardness of the powder and bulk sample was evaluated using Vickers microhardness testing. The hardness values for the alpha case in bulk Ti-6Al-4V correlated with the hardness of heat treated powder and were twice as hard when compared to as-received powder. Overall, this study demonstrated the conversion of powders to alpha case, even during heat treatment in argon with close oxygen control.

Transparent titanium dioxide nanotubes: Processing, characterization, and application in establishing cellular response mechanisms

The therapeutic applications of titanium dioxide nanotubes as osteogenic surface treatments for titanium-based implants are largely due to the finely tunable physical characteristics of these nanostructures. As these characteristics change, so does the cellular response, yet the exact mechanisms for this relationship remains largely undefined. We present a novel TiO2 NT imaging platform that is suitable for use with live-cell imaging techniques, thereby enabling, for the first time, dynamic investigation of those mechanisms. In this work, fabrication methods for producing transparent TiO2 NTs with diameters of 56 ± 6 nm, 75 ± 7 nm, 92 ± 9 nm, and 116 ± 10 nm are described. To demonstrate the diagnostic potential of these TiO2 NT imaging platforms, the focal adhesion protein vinculin and actin cytoskeletal filaments were fluorescently tagged in osteoblasts and real-time, high-resolution fluorescent microscopy of live-cell interactions with TiO2 NT substrates were observed. The scope of such a platform is expected to extend far beyond the current proof-of-concept, with great potential for addressing the dynamic response of cells interacting with nanostructured substrates. STATEMENT OF SIGNIFICANCE Titanium dioxide (TiO2) nanotubes are known to strongly enhance bone/mesenchymal stem cell behavior and, consequently, have gained attention as potential osteogenic surface treatments for titanium-bone implants. The exact mechanism by which TiO2 nanotubes influence cellular function remains controversial, partly due to limitations in existing cellular imaging methods with opaque substrates. This work identifies fabrication conditions for the successful production of transparent TiO2 nanotube arrays with tailorable diameters, as well as their functionality with pre-osteoblast mouse cells (MC3T3-E1) transfected with fluorescent focal adhesion protein vinculin and cytoskeletal filament actin. We demonstrate a means of recording live-cell, cell-substrate interaction mechanisms via high-resolution fluorescent microscopy and customizable, transparent TiO2 nanotubes to begin defining the relationship between TiO2 nanotube features and cell function.