Ultrathin high-resolution flexographic printing using nanoporous stamps

Since its invention in ancient times, relief printing, commonly called flexography, has been used to mass-produce artifacts ranging from decorative graphics to printed media. Now, higher-resolution flexography is essential to manufacturing low-cost, large-area printed electronics. However, because of contact-mediated liquid instabilities and spreading, the resolution of flexographic printing using elastomeric stamps is limited to tens of micrometers. We introduce engineered nanoporous microstructures, comprising polymer-coated aligned carbon nanotubes (CNTs), as a next-generation stamp material. We design and engineer the highly porous microstructures to be wetted by colloidal inks and to transfer a thin layer to a target substrate upon brief contact. We demonstrate printing of diverse micrometer-scale patterns of a variety of functional nanoparticle inks, including Ag, ZnO, WO[subscript 3], and CdSe/ZnS, onto both rigid and compliant substrates. The printed patterns have highly uniform nanoscale thickness (5 to 50 nm) and match the stamp features with high fidelity (edge roughness, ~0.2 μm). We derive conditions for uniform printing based on nanoscale contact mechanics, characterize printed Ag lines and transparent conductors, and achieve continuous printing at a speed of 0.2 m/s. The latter represents a combination of resolution and throughput that far surpasses industrial printing technologies.Massachusetts Institute of Technology. Department of Mechanical EngineeringNational Science Foundation (U.S.) (Grant CMMI-1463181)United States. Air Force Office of Scientific Research. Young Investigator Program (Grant FA9550-11-1-0089)National Institutes of Health (U.S.) (Grant 1R21HL114011-01A1

Wireless, battery-free optoelectronic systems as subdermal implants for local tissue oximetry

Monitoring regional tissue oxygenation in animal models and potentially in human subjects can yield insights into the underlying mechanisms of local O2-mediated physiological processes and provide diagnostic and therapeutic guidance for relevant disease states. Existing technologies for tissue oxygenation assessments involve some combination of disadvantages in requirements for physical tethers, anesthetics, and special apparatus, often with confounding effects on the natural behaviors of test subjects. This work introduces an entirely wireless and fully implantable platform incorporating (i) microscale optoelectronics for continuous sensing of local hemoglobin dynamics and (ii) advanced designs in continuous, wireless power delivery and data output for tether-free operation. These features support in vivo, highly localized tissue oximetry at sites of interest, including deep brain regions of mice, on untethered, awake animal models. The results create many opportunities for studying various O2-mediated processes in naturally behaving subjects, with implications in biomedical research and clinical practice.Center for Bio-Integrated Electronics at Northwestern University; Soft and Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF) [ECCS-1542205]; Materials Research Science and Engineering Center [DMR-1720139]; State of Illinois; Northwestern University; Developmental Therapeutics Core at Northwestern University; Robert H. Lurie Comprehensive Cancer Center [NCI CA060553]Open access journalThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Three-dimensional, multifunctional neural interfaces for cortical spheroids and engineered assembloids.

Three-dimensional (3D), submillimeter-scale constructs of neural cells, known as cortical spheroids, are of rapidly growing importance in biological research because these systems reproduce complex features of the brain in vitro. Despite their great potential for studies of neurodevelopment and neurological disease modeling, 3D living objects cannot be studied easily using conventional approaches to neuromodulation, sensing, and manipulation. Here, we introduce classes of microfabricated 3D frameworks as compliant, multifunctional neural interfaces to spheroids and to assembloids. Electrical, optical, chemical, and thermal interfaces to cortical spheroids demonstrate some of the capabilities. Complex architectures and high-resolution features highlight the design versatility. Detailed studies of the spreading of coordinated bursting events across the surface of an isolated cortical spheroid and of the cascade of processes associated with formation and regrowth of bridging tissues across a pair of such spheroids represent two of the many opportunities in basic neuroscience research enabled by these platforms

Liquid manipulation using engineered carbon nanotube surfaces

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2017.Cataloged from PDF version of thesis.Includes bibliographical references (pages 148-159).The understanding and control of liquid-surface interactions is of fundamental interest and practical relevance for applications including self-cleaning surfaces, microfluidic devices, and phase change energy conversion. Carbon nanotubes (CNTs) are well known for their outstanding properties, and can be manufactured over large areas by scalable chemical vapor deposition methods. In this thesis, the mechanical properties of CNT microstructures, and their wettability and porosity via coating and functionalization, are explored as a platform for engineering liquid-surface interactions. First, I study the capillary-driven liquid imbibition in nanoporous ceramic-coated vertically aligned CNT films. Deposition of a conformal ceramic coating prevents capillary-induced deformation of the CNTs, and tailors the nanoporosity. A model based on Darcy's law is found to accurately relate the effective pore size and surface wettability to the imbibition dynamics. I then demonstrate the use of ceramic-coated CNT microstructured surfaces for enhanced pool boiling. A critical heat flux of 235 W-cm 2 is measured on the nanoporous microstructure surface, which is 9% and 57% greater than measured on solid microstructures and smooth ceramic surfaces, respectively. Via in situ infrared imaging, faster bubble nucleation and departure are observed on the nanoporous microstructures; the additional imbibed volume contributes to the liquid supply for vaporization and delays the boiling crisis. Further enhancement could be achieved by optimizing the microstructure pattern and nanoscale porosity and wettability. Third, I present a compliant CNT-based scale surface architecture for tuning anisotropic droplet adhesion on hydrophobic surfaces. This was inspired by the superhydrophobicity and anisotropic droplet roll-off on Morpho aega butterfly wings. By video microscopy, we reveal that extreme deflections of the individual scales are responsible for directional adhesion. Inspired by this finding, I demonstrate a synthetic scale surface with stiffness-tunable anisotropic droplet adhesion, fabricated from curved CNT microstructures. A model considering the contact line forces and scale stiffness predicts the scaling of adhesion anisotropy for the natural and synthetic surfaces. The findings in this thesis demonstrate the versatility of CNT-based surfaces for manipulating liquids. The electrical conductivity and mechanical robustness of the CNTs, and the ability to fabricate complex 3D microarchitectures, suggest further opportunities for future work.by Hangbo Zhao.Ph. D

Vertical silicon nanowire arrays for gas sensing

Thesis: S.M., Massachusetts Institute of Technology, Department of Mechanical Engineering, February 2014.Cataloged from PDF version of thesis. "February 2014."Includes bibliographical references (pages 92-97).The goal of this research was to fabricate and characterize vertically aligned silicon nanowire gas sensors. Silicon nanowires are very attractive for gas sensing applications and vertically aligned silicon nanowires are preferred over horizontal nanowires for gas sensing due to the high density of nanowire arrays and the increased nanowire surface area per substrate area. However, the development of such devices has been limited by a number of challenges. Two of the key challenges in fabricating vertical silicon nanowire sensors are the difficulty of making electrical contact to the tops of the wires and the large serial resistance of the substrate. In this thesis, highly ordered, dense arrays of vertically aligned silicon nanowires in patterned areas have been fabricated utilizing metal assisted chemical etching (MACE) in combination with interference lithography. In addition, we report a novel and simple approach for making reliable top electrical contacts by using tilted electron beam evaporation with a custom-built rotation plate. A suspended metal top contact layer was formed on vertically aligned silicon nanowires using this approach. We have also systematically investigated the contact behavior between silicon nanowires and metal electrodes with different nanowire doping and contact materials. Ohmic contact was formed between the suspended top metal layer and the tips of silicon nanowires. We have also solved the serial resistance problem by using lightly doped epitaxial silicon films (needed for the sensors) on heavily doped substrates. Based on these techniques and design considerations, we have successfully fabricated vertically aligned silicon nanowire field effect gas sensors. Finally, we have demonstrated highly sensitive detection of hydrogen, oxygen, 10 ppm (parts-per-million, 10-⁶) ammonia and nitrogen dioxide gases using the fabricated sensor devices at room temperature. The sensors have exhibited the highest sensitivity per unit chip area for hydrogen, oxygen and 10 ppm NH₃ gases at room temperature, among other vertically aligned silicon nanowire based gas sensors reported. Further improvements of the current sensor devices can be made to accelerate response and recovery of gas sensing.by Hangbo Zhao.S.M

Fabrication of high aspect ratio AFM probes with different materials inspired by TEM “lift-out” method

The most commonly used materials in all commercially available high-aspect-ratio (HAR) nanowire's (NW) tips are made of silicon and carbon nanotube which limit their applications in other types of atomic force microscopy (AFM), such as conducting AFM and magnetic force microscope. Therefore, a simple process inspired by cross-sectional transmission electron microscopy sample preparation method was used to demonstrate the feasibility of fabricating HAR AFM probes, which can easily define the tilt angle of the NW tip with respect to the direction that is normal to the axis of the cantilever to which it is attached by simply tilting the sample stage where the cantilever is placed. This is very important as it enables precise control of the inclination angle of the NW tip and allows the tip to be made perpendicular to the probed surface for scanning with different AFM mounts. Two different tips were fabricated, one attached parallel and the other attached at an angle of 13° with respect to the normal of the cantilever axis. These tips were used to profile the topography of a silicon nanopillar array. Only the probe attached at an angle of 13° allowed mapping of the topography between nanopillars. This is the first successful demonstration of an HAR AFM tip being used to map the topography of a nanopillar array. In addition, the authors also demonstrated that this method can be extended to fabricate HAR AFM tips of different materials such as copper with a slightly modified approach. ©2016 American Vacuum Society

Liquid Imbibition in Ceramic-Coated Carbon Nanotube Films

Understanding of the liquid imbibition dynamics in nanoporous materials is important to advances in chemical separations, phase change heat transfer, electrochemical energy storage, and diagnostic assays. We study the liquid imbibition behavior in films of ceramic-coated vertically aligned carbon nanotubes (CNTs). The nanoscale porosity of the films is tuned by conformal ceramic coating via atomic layer deposition (ALD), enabling stable liquid imbibition and precise measurement of the imbibition dynamics without capillary densification of the CNTs. We show that the imbibition rate decreases as the ceramic coating thickness increases, which effectively changes the CNT-CNT spacing and therefore decreases the permeability. We derive a model, based on Darcy's law, that incorporates an expression for the permeability of nanoscale post arrays, and we show that the model fits the experimental results with high accuracy. The tailorable porosity, along with controllable surface wettability and mechanical stability of coated CNTs, suggest their suitability for application-guided engineering, and for further investigation of imbibition behavior at finer length scales

Soft nanocomposite electroadhesives for digital micro- and nanotransfer printing

Automated handling of microscale objects is essential for manufacturing of next-generation electronic systems. Yet, mechanical pick-and-place technologies cannot manipulate smaller objects whose surface forces dominate over gravity, and emerging microtransfer printing methods require multidirectional motion, heating, and/or chemical bonding to switch adhesion. We introduce soft nanocomposite electroadhesives (SNEs), comprising sparse forests of dielectric-coated carbon nanotubes (CNTs), which have electrostatically switchable dry adhesion. SNEs exhibit 40-fold lower nominal dry adhesion than typical solids, yet their adhesion is increased >100-fold by applying 30 V to the CNTs. We characterize the scaling of adhesion with surface morphology, dielectric thickness, and applied voltage and demonstrate digital transfer printing of films of Ag nanowires, polymer and metal microparticles, and unpackaged light-emitting diodes.National Science Foundation (U.S.) (CMMI-1463181)Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies (contract W911NF-13-D-0001