Showing papers in "Microsystems & Nanoengineering in 2017"

Hydrophilic surface modification of PDMS for droplet microfluidics using a simple, quick, and robust method via PVA deposition

Abstract: Polydimethylsiloxane (PDMS) is a dominant material in the fabrication of microfluidic devices to generate water-in-oil droplets, particularly lipid-stabilized droplets, because of its highly hydrophobic nature. However, its key property of hydrophobicity has hindered its use in the microfluidic generation of oil-in-water droplets, which requires channels to have hydrophilic surface properties. In this article, we developed, optimized, and characterized a method to produce PDMS with a hydrophilic surface via the deposition of polyvinyl alcohol following plasma treatment and demonstrated its suitability for droplet generation. The proposed method is simple, quick, effective, and low cost and is versatile with respect to surfactants, with droplets being successfully generated using both anionic surfactants and more biologically relevant phospholipids. This method also allows the device to be selectively patterned with both hydrophilic and hydrophobic regions, leading to the generation of double emulsions and inverted double emulsions.

State-of-the-art MEMS and microsystem tools for brain research

Abstract: Mapping brain activity has received growing worldwide interest because it is expected to improve disease treatment and allow for the development of important neuromorphic computational methods. MEMS and microsystems are expected to continue to offer new and exciting solutions to meet the need for high-density, high-fidelity neural interfaces. Herein, the state-of-the-art in recording and stimulation tools for brain research is reviewed, and some of the most significant technology trends shaping the field of neurotechnology are discussed. Understanding even the most basic brain functions will require considerable advances in the MEMS-based tools that are used in brain research. Sensors that are capable of monitoring single neurons or mapping the complex neural networks responsible for faculties such as memory or learning will be crucial for furthering our knowledge. As the human brain contains around 85 million neurons and 100 trillion synapses, the challenge is enormous. John Seymour and Euisik Yoon and colleagues at the University of Michigan, United States, review the state of the art in microsystem devices that are used to record and stimulate the brain. They highlight innovations in multimodal sensor arrays and illustrate the need for further innovation in packaging and microsystems to match the scale of the neuronal circuits under study. Ultimately the teamwork between neurotechnologists and neuroscientists will lead to critical breakthroughs in brain research over the next decade.

Nanoimprint lithography steppers for volume fabrication of leading-edge semiconductor integrated circuits.

Abstract: This article discusses the transition of a form of nanoimprint lithography technology, known as Jet and Flash Imprint Lithography (J-FIL), from research to a commercial fabrication infrastructure for leading-edge semiconductor integrated circuits (ICs) Leading-edge semiconductor lithography has some of the most aggressive technology requirements, and has been a key driver in the 50-year history of semiconductor scaling Introducing a new, disruptive capability into this arena is therefore a case study in a "high-risk-high-reward" opportunity This article first discusses relevant literature in nanopatterning including advanced lithography options that have been explored by the IC fabrication industry, novel research ideas being explored, and literature in nanoimprint lithography The article then focuses on the J-FIL process, and the interdisciplinary nature of risk, involving nanoscale precision systems, mechanics, materials, material delivery systems, contamination control, and process engineering Next, the article discusses the strategic decisions that were made in the early phases of the project including: (i) choosing a step and repeat process approach; (ii) identifying the first target IC market for J-FIL; (iii) defining the product scope and the appropriate collaborations to share the risk-reward landscape; and (iv) properly leveraging existing infrastructure, including minimizing disruption to the widely accepted practices in photolithography Finally, the paper discusses the commercial J-FIL stepper system and associated infrastructure, and the resulting advances in the key lithographic process metrics such as critical dimension control, overlay, throughput, process defects, and electrical yield over the past 5 years This article concludes with the current state of the art in J-FIL technology for IC fabrication, including description of the high volume manufacturing stepper tools created for advanced memory manufacturing

Monolithic ultrasound fingerprint sensor.

Abstract: This paper presents a 591×438-DPI ultrasonic fingerprint sensor. The sensor is based on a piezoelectric micromachined ultrasonic transducer (PMUT) array that is bonded at wafer-level to complementary metal oxide semiconductor (CMOS) signal processing electronics to produce a pulse-echo ultrasonic imager on a chip. To meet the 500-DPI standard for consumer fingerprint sensors, the PMUT pitch was reduced by approximately a factor of two relative to an earlier design. We conducted a systematic design study of the individual PMUT and array to achieve this scaling while maintaining a high fill-factor. The resulting 110×56-PMUT array, composed of 30×43-μm2 rectangular PMUTs, achieved a 51.7% fill-factor, three times greater than that of the previous design. Together with the custom CMOS ASIC, the sensor achieves 2 mV kPa-1 sensitivity, 15 kPa pressure output, 75 μm lateral resolution, and 150 μm axial resolution in a 4.6 mm×3.2 mm image. To the best of our knowledge, we have demonstrated the first MEMS ultrasonic fingerprint sensor capable of imaging epidermis and sub-surface layer fingerprints.

Ultrathin tunable terahertz absorber based on MEMS-driven metamaterial.

Abstract: The realization of high-performance tunable absorbers for terahertz frequencies is crucial for advancing applications such as single-pixel imaging and spectroscopy. Based on the strong position sensitivity of metamaterials’ electromagnetic response, we combine meta-atoms that support strongly localized modes with suspended flat membranes that can be driven electrostatically. This design maximizes the tunability range for small mechanical displacements of the membranes. We employ a micro-electro-mechanical system technology and successfully fabricate the devices. Our prototype devices are among the best-performing tunable THz absorbers demonstrated to date, with an ultrathin device thickness (~1/50 of the working wavelength), absorption varying between 60% and 80% in the initial state when the membranes remain suspended, and fast switching speed (~27 μs). The absorption is tuned by an applied voltage, with the most marked results achieved when the structure reaches the snap-down state. In this case, the resonance shifts by >200% of the linewidth (14% of the initial resonance frequency), and the absolute absorption modulation measured at the initial resonance can reach 65%. The demonstrated approach can be further optimized and extended to benefit numerous applications in THz technology. A material that can alter its interaction with long-wavelength radiation is constructed by researchers in Australia and the United States. Mingkai Liu et al. have created a structure with electrically tunable terahertz absorption. Terahertz light with wavelengths from 0.1 to 1 millimeter is useful in a variety of applications including security-screening. To improve current technologies, materials with a strong and tunable response are desired. The researchers used a microelectromechanical system (MEMS) to control terahertz absorption in a metamaterial — an array of sub-wavelength structures, the dimensions of which determine how the material interacts with electromagnetic waves. The geometry of the structure realized by the team led by Ilya Shadrivov (Australian National University) and Mariusz Martyniuk (University of Western Australia) is adaptively controlled by applying a voltage, rapidly changing the material terahertz absorption by up to 65%.

Organic transistor platform with integrated microfluidics for in-line multi-parametric in vitro cell monitoring.

Abstract: Future drug discovery and toxicology testing could benefit significantly from more predictive and multi-parametric readouts from in vitro models. Despite the recent advances in the field of microfluidics, and more recently organ-on-a-chip technology, there is still a high demand for real-time monitoring systems that can be readily embedded with microfluidics. In addition, multi-parametric monitoring is essential to improve the predictive quality of the data used to inform clinical studies that follow. Here we present a microfluidic platform integrated with in-line electronic sensors based on the organic electrochemical transistor. Our goals are two-fold, first to generate a platform to host cells in a more physiologically relevant environment (using physiologically relevant fluid shear stress (FSS)) and second to show efficient integration of multiple different methods for assessing cell morphology, differentiation, and integrity. These include optical imaging, impedance monitoring, metabolite sensing, and a wound-healing assay. We illustrate the versatility of this multi-parametric monitoring in giving us increased confidence to validate the improved differentiation of cells toward a physiological profile under FSS, thus yielding more accurate data when used to assess the effect of drugs or toxins. Overall, this platform will enable high-content screening for in vitro drug discovery and toxicology testing and bridges the existing gap in the integration of in-line sensors in microfluidic devices. An easy-to-manufacture microfluidic device that simultaneously monitors several parameters could lead to improved cell-based toxicology testing. Despite considerable advances in microfluidics, particularly in the organ-on-a-chip technology, there is still a need for tools that can perform real-time multi-parameter monitoring of live cells and be easily combined with a microfluidic system. To address this need, Roisin Owens and colleagues at the Ecole des Mines de Saint-Etienne, France, integrated an electronic sensor based on an organic electrochemical transistor (OECT) with a microfluidic device. Their platform provides high throughput real-time data for assessing the effects of drugs and toxins on cells for next generation of in vitro models.

High-fidelity replication of thermoplastic microneedles with open microfluidic channels.

Abstract: Development of microneedles for unskilled and painless collection of blood or drug delivery addresses the quality of healthcare through early intervention at point-of-care. Microneedles with submicron to millimeter features have been fabricated from materials such as metals, silicon, and polymers by subtractive machining or etching. However, to date, large-scale manufacture of hollow microneedles has been limited by the cost and complexity of microfabrication techniques. This paper reports a novel manufacturing method that may overcome the complexity of hollow microneedle fabrication. Prototype microneedles with open microfluidic channels are fabricated by laser stereolithography. Thermoplastic replicas are manufactured from these templates by soft-embossing with high fidelity at submicron resolution. The manufacturing advantages are (a) direct printing from computer-aided design (CAD) drawing without the constraints imposed by subtractive machining or etching processes, (b) high-fidelity replication of prototype geometries with multiple reuses of elastomeric molds, (c) shorter manufacturing time compared to three-dimensional stereolithography, and (d) integration of microneedles with open-channel microfluidics. Future work will address development of open-channel microfluidics for drug delivery, fluid sampling and analysis.

Particle focusing by 3D inertial microfluidics

Abstract: Three-dimensional (3D) particle focusing in microfluidics is a fundamental capability with a wide range of applications, such as on-chip flow cytometry, where high-throughput analysis at the single-cell level is performed. Currently, 3D focusing is achieved mainly in devices with complex layouts, additional sheath fluids, and complex pumping systems. In this work, we present a compact microfluidic device capable of 3D particle focusing at high flow rates and with a small footprint, without the requirement of external fields or lateral sheath flows, but using only a single-inlet, single-outlet microfluidic sequence of straight channels and tightly curving vertical loops. This device exploits inertial fluidic effects that occur in a laminar regime at sufficiently high flow rates, manipulating the particle positions by the combination of inertial lift forces and Dean drag forces. The device is fabricated by femtosecond laser irradiation followed by chemical etching, which is a simple two-step process enabling the creation of 3D microfluidic networks in fused silica glass substrates. The use of tightly curving three-dimensional microfluidic loops produces strong Dean drag forces along the whole loop but also induces an asymmetric Dean flow decay in the subsequent straight channel, thus producing rapid cross-sectional mixing flows that assist with 3D particle focusing. The use of out-of-plane loops favors a compact parallelization of multiple focusing channels, allowing one to process large amounts of samples. In addition, the low fluidic resistance of the channel network is compatible with vacuum driven flows. The resulting device is quite interesting for high-throughput on-chip flow cytometry.

Hexagonal boron nitride nanomechanical resonators with spatially visualized motion.

Abstract: Atomic layers of hexagonal boron nitride (h-BN) crystal are excellent candidates for structural materials as enabling ultrathin, two-dimensional (2D) nanoelectromechanical systems (NEMS) due to the outstanding mechanical properties and very wide bandgap (5.9 eV) of h-BN. In this work, we report the experimental demonstration of h-BN 2D nanomechanical resonators vibrating at high and very high frequencies (from ~5 to ~70 MHz), and investigations of the elastic properties of h-BN by measuring the multimode resonant behavior of these devices. First, we demonstrate a dry-transferred doubly clamped h-BN membrane with ~6.7 nm thickness, the thinnest h-BN resonator known to date. In addition, we fabricate circular drumhead h-BN resonators with thicknesses ranging from ~9 to 292 nm, from which we measure up to eight resonance modes in the range of ~18 to 35 MHz. Combining measurements and modeling of the rich multimode resonances, we resolve h-BN’s elastic behavior, including the transition from membrane to disk regime, with built-in tension ranging from 0.02 to 2 N m−1. The Young’s modulus of h-BN is determined to be E Y≈392 GPa from the measured resonances. The ultrasensitive measurements further reveal subtle structural characteristics and mechanical properties of the suspended h-BN diaphragms, including anisotropic built-in tension and bulging, thus suggesting guidelines on how these effects can be exploited for engineering multimode resonant functions in 2D NEMS transducers. Next-generation ultrathin sensors and ultralow-power signal processors could be a step closer thanks to single-atom layers of hexagonal boron nitride (h-BN) (aka ‘white’ graphene). Two-dimensional (2D) crystals of h-BN exhibit ultrawide bandgap, remarkable mechanical and optical properties and are chemically and thermally more stable than graphene — making them attractive building blocks for cutting-edge nanoelectromechanical systems (NEMS). By measuring the elasticity and optical properties of extremely thin flakes of h-BN, Philip Feng and his colleagues from Case Western Reserve University in Ohio, United States, were able to fabricate h-BN-based devices that function as highly sensitive nanomechanical resonators, with spatially visualized mutlimode motions. The team ’s work holds promise for the development of 2D devices for emerging applications such as nanoscale sensors and multiphysical transducers. It could also enable future investigations of piezoelectric effects in 2D electromechanical and optoelectromechanical devices made from atomic layers of h-BN and their heterostructures with other 2D materials.

Toward point-of-care management of chronic respiratory conditions: Electrochemical sensing of nitrite content in exhaled breath condensate using reduced graphene oxide

Abstract: We present a portable non-invasive approach for measuring indicators of inflammation and oxidative stress in the respiratory tract by quantifying a biomarker in exhaled breath condensate (EBC). We discuss the fabrication and characterization of a miniaturized electrochemical sensor for detecting nitrite content in EBC using reduced graphene oxide. The nitrite content in EBC has been demonstrated to be a promising biomarker of inflammation in the respiratory tract, particularly in asthma. We utilized the unique properties of reduced graphene oxide (rGO); specifically, the material is resilient to corrosion while exhibiting rapid electron transfer with electrolytes, thus allowing for highly sensitive electrochemical detection with minimal fouling. Our rGO sensor was housed in an electrochemical cell fabricated from polydimethyl siloxane (PDMS), which was necessary to analyze small EBC sample volumes. The sensor is capable of detecting nitrite at a low over-potential of 0.7 V with respect to an Ag/AgCl reference electrode. We characterized the performance of the sensors using standard nitrite/buffer solutions, nitrite spiked into EBC, and clinical EBC samples. The sensor demonstrated a sensitivity of 0.21 μA μM−1 cm−2 in the range of 20–100 μM and of 0.1 μA μM−1 cm−2 in the range of 100–1000 μM nitrite concentration and exhibited a low detection limit of 830 nM in the EBC matrix. To benchmark our platform, we tested our sensors using seven pre-characterized clinical EBC samples with concentrations ranging between 0.14 and 6.5 μM. This enzyme-free and label-free method of detecting biomarkers in EBC can pave the way for the development of portable breath analyzers for diagnosing and managing changes in respiratory inflammation and disease. Important biomarkers of respiratory stress that can be found inside exhaled breath can now be spotted with a graphene-powered portable device. Recent studies have shown that excess levels of nitrite ions in the breath of a patient are a strong indicator of inflamed airways and possible asthma. Now, Mehdi Javanmard and his colleagues from Rutgers University in New Jersey, United States, have developed a micro-electrochemical cell that analyses small samples of exhaled breath condensate for nitrites without the need for enzyme or labelling pretreatments. Central to the team’s device is a thin layer of reduced graphene oxide, fabricated onto electrodes through a custom drop-cast method, which is considerably more sensitive to nitrites than metals alone. Tests in complex biological matrices revealed that the instrument had precise quantification capabilities in clinically relevant concentration ranges and exhibited minimal fouling.

High-throughput physical phenotyping of cell differentiation

Abstract: In this report, we present multiparameter deformability cytometry (m-DC), in which we explore a large set of parameters describing the physical phenotypes of pluripotent cells and their derivatives. m-DC utilizes microfluidic inertial focusing and hydrodynamic stretching of single cells in conjunction with high-speed video recording to realize high-throughput characterization of over 20 different cell motion and morphology-derived parameters. Parameters extracted from videos include size, deformability, deformation kinetics, and morphology. We train support vector machines that provide evidence that these additional physical measurements improve classification of induced pluripotent stem cells, mesenchymal stem cells, neural stem cells, and their derivatives compared to size and deformability alone. In addition, we utilize visual interactive stochastic neighbor embedding to visually map the high-dimensional physical phenotypic spaces occupied by these stem cells and their progeny and the pathways traversed during differentiation. This report demonstrates the potential of m-DC for improving understanding of physical differences that arise as cells differentiate and identifying cell subpopulations in a label-free manner. Ultimately, such approaches could broaden our understanding of subtle changes in cell phenotypes and their roles in human biology.

A microscale optical implant for continuous in vivo monitoring of intraocular pressure

Abstract: Intraocular pressure (IOP) is a key clinical parameter in glaucoma management. However, despite the potential utility of daily measurements of IOP in the context of disease management, the necessary tools are currently lacking, and IOP is typically measured only a few times a year. Here we report on a microscale implantable sensor that could provide convenient, accurate, on-demand IOP monitoring in the home environment. When excited by broadband near-infrared (NIR) light from a tungsten bulb, the sensor’s optical cavity reflects a pressure-dependent resonance signature that can be converted to IOP. NIR light is minimally absorbed by tissue and is not perceived visually. The sensor’s nanodot-enhanced cavity allows for a 3–5 cm readout distance with an average accuracy of 0.29 mm Hg over the range of 0–40 mm Hg. Sensors were mounted onto intraocular lenses or silicone haptics and secured inside the anterior chamber in New Zealand white rabbits. Implanted sensors provided continuous in vivo tracking of short-term transient IOP elevations and provided continuous measurements of IOP for up to 4.5 months.

Characterization of forced localization of disordered weakly coupled micromechanical resonators.

Abstract: The mode localization phenomenon of disordered weakly coupled resonators (WCRs) is being used as a novel transduction scheme to further enhance the sensitivity of micromechanical resonant sensors. In this paper, two novel characteristics of mode localization are described. First, we found that the anti-resonance loci behave as a linear function of the stiffness perturbation. The anti-resonance behavior can be regarded as a new manifestation of mode localization in the frequency domain, and mode localization occurs at a deeper level as the anti-resonance approaches closer to the resonance. The anti-resonance loci can be used to identify the symmetry of the WCRs and the locations of the perturbation. Second, by comparing the forced localization responses of the WCRs under both the single-resonator-driven (SRD) scheme and the double-resonator-driven (DRD) scheme, we demonstrated that the DRD scheme extends the linear measurement scale while sacrificing a certain amount of sensitivity. We also demonstrated experimentally that the amplitude ratio-based sensitivity under the DRD scheme is approximately an order of magnitude lower than that under the SRD scheme, that is, the amplitude ratio-based sensitivity is −70.44% (N m−1)−1 under the DRD scheme, while it is −785.6% (N m−1)−1 under the SRD scheme. These characteristics of mode localization are valuable for the design and control of WCR-based sensors. Two sensing mechanisms, newly isolated in wave-trapping microresonators, can enhance the use of such devices in medical diagnostics. The standing waves generated by microresonators shift their resonant frequencies in the presence of foreign molecules. Honglong Chang and colleagues from Northwestern Polytechnical University in China now report that weakly coupled microresonator devices can access discreet energy modes with startling sensitivity properties. Fabrication of silicon-on-insulator devices helped the team to reveal that localized ‘anti-resonance’ minimums in oscillation amplitudes were linearly dependent on stiffness properties—a relationship that can characterize the localized energy modes on various resonators. By using a double-resonator-driven scheme to tune the microresonator device and force the localization modes to emerge, the team could then extend the linear measurement scales to a greater potential range of biosensing applications.

Multimodal epidermal devices for hydration monitoring.

Abstract: Precise, quantitative in vivo monitoring of hydration levels in the near surface regions of the skin can be useful in preventing skin-based pathologies, and regulating external appearance. Here we introduce multimodal sensors with important capabilities in this context, rendered in soft, ultrathin, 'skin-like' formats with numerous advantages over alternative technologies, including the ability to establish intimate, conformal contact without applied pressure, and to provide spatiotemporally resolved data on both electrical and thermal transport properties from sensitive regions of the skin. Systematic in vitro studies and computational models establish the underlying measurement principles and associated approaches for determination of temperature, thermal conductivity, thermal diffusivity, volumetric heat capacity, and electrical impedance using simple analysis algorithms. Clinical studies on 20 patients subjected to a variety of external stimuli validate the device operation and allow quantitative comparisons of measurement capabilities to those of existing state-of-the-art tools.

A 3D-printed platform for modular neuromuscular motor units.

Abstract: A complex and functional living cellular system requires the interaction of one or more cell types to perform specific tasks, such as sensing, processing, or force production. Modular and flexible platforms for fabrication of such multi-cellular modules and their characterization have been lacking. Here, we present a modular cellular system, made up of multi-layered tissue rings containing integrated skeletal muscle and motor neurons (MNs) embedded in an extracellular matrix. The MNs were differentiated from mouse embryonic stem cells through the formation of embryoid bodies (EBs), which are spherical aggregations of cells grown in a suspension culture. The EBs were integrated into a tissue ring with skeletal muscle, which was differentiated in parallel, to create a co-culture amenable to both cell types. The multi-layered rings were then sequentially placed on a stationary three-dimensional-printed hydrogel structure resembling an anatomical muscle-tendon-bone organization. We demonstrate that the site-specific innervation of a group of muscle fibers in the multi-layered tissue rings allows for muscle contraction via chemical stimulation of MNs with glutamate, a major excitatory neurotransmitter in the mammalian nervous system, with the frequency of contraction increasing with glutamate concentration. The addition of tubocurarine chloride (a nicotinic receptor antagonist) halted the contractions, indicating that muscle contraction was MN induced. With a bio-fabricated system permitting controllable mechanical and geometric attributes in a range of length scales, our novel engineered cellular system can be utilized for easier integration of other modular "building blocks" in living cellular and biological machines.

Controlled fabrication of nanoscale wrinkle structure by fluorocarbon plasma for highly transparent triboelectric nanogenerator

Abstract: In this paper, we report a novel nanoscale wrinkle-structure fabrication process using fluorocarbon plasma on poly(dimethylsiloxane) (PDMS) and Solaris membranes. Wrinkles with wavelengths of hundreds of nanometers were obtained on these two materials, showing that the fabrication process was universally applicable. By varying the plasma-treating time, the wavelength of the wrinkle structure could be controlled. Highly transparent membranes with wrinkle patterns were obtained when the plasma-treating time was 90% in the visible region, making it difficult to distinguish them from a flat membrane. The deposited fluorocarbon polymer also dramatically reduced the surface energy, which allowed us to replicate the wrinkle pattern with high precision onto other membranes without any surfactant coating. The combined advantages of high electron affinity and high transparency enabled the fabricated membrane to improve the performance of a triboelectric nanogenerator. This nanoscale, single-step, and universal wrinkle-pattern fabrication process, with the functionality of high transparency and ultra-low surface energy, shows an attractive potential for future applications in micro- and nanodevices, especially in transparent energy harvesters. A flat optical nanoantenna capable of beaming light emitted by a single molecule has been studied theorectically and experimentally. Current strategies for boosting the collection efficiency of quantum emitters such as single atoms and molecules are inflexible and fiddly to implement. Now, Mario Agio of the European Laboratory for Nonlinear Spectroscopy, Italy, and co-workers in Italy, Germany and the UK have theoretically proposed and experimentally demonstrated a planar optical nanoantenna that funnels light emitted from a single molecule into a narrow beam, thus boosting the light collection efficiency. Unlike previous approaches, their strategy does not depend stringently on the position of the emitter. Furthermore, it is straightforward to fabricate, broadband and scalable across the spectrum. The team anticipates it will be rapidly adopted in spectroscopy, quantum optics and sensing applications.

Rapid synthesis of transition metal dichalcogenide-carbon aerogel composites for supercapacitor electrodes.

Abstract: Transition metal dichalcogenide (TMD) materials have recently demonstrated exceptional supercapacitor properties after conversion to a metallic phase, which increases the conductivity of the network. However, freestanding, exfoliated transition metal dichalcogenide films exhibit surface areas far below their theoretical maximum (1.2 %), can fail during electrochemical operation due to poor mechanical properties, and often require pyrophoric chemicals to process. On the other hand, pyrolyzed carbon aerogels exhibit extraordinary specific surface areas for double layer capacitance, high conductivity, and a strong mechanical network of covalent chemical bonds. In this paper, we demonstrate the scalable, rapid nanomanufacturing of TMD (MoS2 and WS2) and carbon aerogel composites, favoring liquid-phase exfoliation to avoid pyrophoric chemicals. The aerogel matrix support enhances conductivity of the composite and the synthesis can complete in 30 min. We find that the addition of transition metal dichalcogenides does not impact the structure of the aerogel, which maintains a high specific surface area up to 620 m2 g−1 with peak pore radii of 10 nm. While supercapacitor tests of the aerogels yield capacitances around 80 F g−1 at the lowest applied currents, the aerogels loaded with TMD’s exhibit volumetric capacitances up to 127% greater than the unloaded aerogels. In addition, the WS2 aerogels show excellent cycling stability with no capacitance loss over 2000 cycles, as well as markedly better rate capability and lower charge transfer resistance compared to their MoS2-loaded counterparts. We hypothesize that these differences in performance stem from differences in contact resistance and in the favorability of ion adsorption on the chalcogenides. A fast and scalable technique for manufacturing nanoscale supercapacitors could lead to better performing energy-storage technologies. Supercapacitors made from transition metal dichalcogenides (TMDs) have the potential to replace lithium batteries as an energy-storage technology. However, they are prone to mechanical failure and their production often requires unstable, dangerous chemicals, which restricts their application. To address these limitations, Peter Pauzauskie and colleagues from the University of Washington in Seattle, United States, have developed a method for fabricating operationally stable nanocomposite supercapacitors that combines the high-capacitive properties of TMDs with the the electrically conductive, porous characteristics of pyrolyzed carbon aerogels. By encapsulating sheets of TMDs in a resorcinol-formaldehyde resin, the researchers demonstrate a safe, ultrafast, and inexpensive process that could lead to new devices for photovoltaic and catalytic applications.

Accurate nanoelectrode recording of human pluripotent stem cell-derived cardiomyocytes for assaying drugs and modeling disease.

Abstract: The measurement of the electrophysiology of human pluripotent stem cell-derived cardiomyocytes is critical for their biomedical applications, from disease modeling to drug screening. Yet, a method that enables the high-throughput intracellular electrophysiology measurement of single cardiomyocytes in adherent culture is not available. To address this area, we have fabricated vertical nanopillar electrodes that can record intracellular action potentials from up to 60 single beating cardiomyocytes. Intracellular access is achieved by highly localized electroporation, which allows for low impedance electrical access to the intracellular voltage. Herein, we demonstrate that this method provides the accurate measurement of the shape and duration of intracellular action potentials, validated by patch clamp, and can facilitate cellular drug screening and disease modeling using human pluripotent stem cells. This study validates the use of nanopillar electrodes for myriad further applications of human pluripotent stem cell-derived cardiomyocytes such as cardiomyocyte maturation monitoring and electrophysiology-contractile force correlation.

Photopatternable PEDOT:PSS/PEG hybrid thin film with moisture stability and sensitivity.

Abstract: A wearable humidity sensor that can analyse perspiration and breath rates has been devised by researchers in the United States. A conductive polymer known as poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) has been deployed with success in flexible electronic devices, but it can degrade when exposed to water or photolithography solvents. To mitigate these problems, Tingrui Pan from the University of California, Davis, and his colleagues coated PEDOT:PSS onto poly (ethylene glycol), a pliable substrate that is able to absorb water. They then developed a process that connects a photomask to the conductive polymer and patterns it in a single step, using ultraviolet light and a photosensitive cross-linking reagent. This strategy produced hybrid films in a capacitive sensing mode that show capacitive signals change with rising environmental moisture and that are resilient enough for real-time exercise monitoring.

Advances in diamond nanofabrication for ultrasensitive devices.

Abstract: This paper reviews some of the major recent advances in single-crystal diamond nanofabrication and its impact in nano- and micro-mechanical, nanophotonics and optomechanical components. These constituents of integrated devices incorporating specific dopants in the material provide the capacity to enhance the sensitivity in detecting mass and forces as well as magnetic field down to quantum mechanical limits and will lead pioneering innovations in ultrasensitive sensing and precision measurements in the realm of the medical sciences, quantum sciences and related technologies.

Fully inkjet-printed microwave passive electronics.

Abstract: Fully inkjet-printed three-dimensional (3D) objects with integrated metal provide exciting possibilities for on-demand fabrication of radio frequency electronics such as inductors, capacitors, and filters. To date, there have been several reports of printed radio frequency components metallized via the use of plating solutions, sputtering, and low-conductivity pastes. These metallization techniques require rather complex fabrication, and do not provide an easily integrated or versatile process. This work utilizes a novel silver ink cured with a low-cost infrared lamp at only 80 °C, and achieves a high conductivity of 1×107 S m−1. By inkjet printing the infrared-cured silver together with a commercial 3D inkjet ultraviolet-cured acrylic dielectric, a multilayer process is demonstrated. By using a smoothing technique, both the conductive ink and dielectric provide surface roughness values of <500 nm. A radio frequency inductor and capacitor exhibit state-of-the-art quality factors of 8 and 20, respectively, and match well with electromagnetic simulations. These components are implemented in a lumped element radio frequency filter with an impressive insertion loss of 0.8 dB at 1 GHz, proving the utility of the process for sensitive radio frequency applications. Adding a pinch of cellulose to a silver-based ink enables production of high-quality radio frequency components using 3D printing. Inkjet printers that build complex 3D objects layer-by-layer are usually restricted to using acrylic-based inks. Now, Garret McKerricher and his colleagues at King Abdullah University of Science and Technology in Saudi Arabia have developed a way to incorporate metallic layers into 3D polymer objects for use in devices. Mixing cellulose with liquid organometallic precursors yielded the correct viscosity for reliable jetting and improves adhesion properties of the ink. Once printed, the silver ink was transformed into a stable, smooth layer using quick infrared curing. Multilayer, high-definition printing generated insulator-metallic prototypes capable of acting as capacitors and inductors. Their subsequent assembly into a low-pass filter yielded a device with signal quality comparable to commercial filters.

Detecting cell-secreted growth factors in microfluidic devices using bead-based biosensors.

Abstract: Microfluidic systems provide an interesting alternative to standard macroscale cell cultures due to the decrease in the number of cells and reagents as well as the improved physiology of cells confined to small volumes. However, the tools available for cell-secreted molecules inside microfluidic devices remain limited. In this paper, we describe an integrated microsystem composed of a microfluidic device and a fluorescent microbead-based assay for the detection of the hepatocyte growth factor (HGF) and the transforming growth factor (TGF)-β1 secreted by primary hepatocytes. This microfluidic system is designed to separate a cell culture chamber from sensing chambers using a permeable hydrogel barrier. Cell-secreted HGF and TGF-β1 diffuse through the hydrogel barrier into adjacent sensing channels and are detected using fluorescent microbead-based sensors. The specificity of sensing microbeads is defined by the choice of antibodies; therefore, our microfluidic culture system and sensing microbeads may be applied to a variety of cells and cell-secreted factors.

Modular architecture for fully non-blocking silicon photonic switch fabric.

Abstract: Integrated photonics offers the possibility of compact, low energy, bandwidth-dense interconnects for large port count spatial optical switches, facilitating flexible and energy efficient data movement in future data communications systems. To achieve widespread adoption, intimate integration with electronics has to be possible, requiring switch design using standard microelectronic foundry processes and available devices. We report on the feasibility of a switch fabric comprised of ubiquitous silicon photonic building blocks, opening the possibility to combine technologies, and materials towards a new path for switch fabric design. Rather than focus on integrating all devices on a single silicon chip die to achieve large port count optical switching, this work shifts the focus towards innovative packaging and integration schemes. In this work, we demonstrate 1×8 and 8×1 microring-based silicon photonic switch building blocks with software control, providing the feasibility of a full 8×8 architecture composed of silicon photonic building blocks. The proposed switch is fully non-blocking, has path-independent insertion loss, low crosstalk, and is straightforward to control. We further analyze this architecture and compare it with other common switching architectures for varying underlying technologies and radices, showing that the proposed architecture favorably scales to very large port counts when considering both crosstalk and architectural footprint. Separating a switch fabric into functional building blocks via multiple photonic integrated circuits offers the advantage of piece-wise manufacturing, packaging, and assembly, potentially reducing the number of optical I/O and electrical contacts on a single die. Ring-shaped silicon photonic switches offer compact geometries at the micron-scale for superior on-chip integration and scalability. The resonant nature of these devices allows for excellent noise isolation down to a single wavelength. Integrated circuits with many ‘microrings’ have been developed recently. Dessislava Nikolova, David M. Calhoun, and researchers at Columbia University and Coriant ATG propose that coupling several such ‘microring integrated circuits’ into a combination of multiplexing and demultiplexing arrays could yield ultrafast optical switches. In the team’s architecture, each combination of two circuits filters a single optical wavelength. Keeping only two microrings per port on resonance through voltage adjustments enables scalable, port-to-port communication without detrimental crosstalk effects. A prototype 8×8 switching device featuring microring circuits with multiplexing and demultiplexing functionality demonstrates the feasibility of the team’s approach and its compatibility with existing semiconductor manufacturing processes.

Resonant pitch and roll silicon gyroscopes with sub-micron-gap slanted electrodes: Breaking the barrier toward high-performance monolithic inertial measurement units.

Abstract: This paper presents the design, fabrication, and characterization of a novel high quality factor (Q) resonant pitch/roll gyroscope implemented in a 40 μm (100) silicon-on-insulator (SOI) substrate without using the deep reactive-ion etching (DRIE) process. The featured silicon gyroscope has a mode-matched operating frequency of 200 kHz and is the first out-of-plane pitch/roll gyroscope with electrostatic quadrature tuning capability to fully compensate for fabrication non-idealities and variation in SOI thickness. The quadrature tuning is enabled by slanted electrodes with sub-micron capacitive gaps along the (111) plane created by an anisotropic wet etching. The quadrature cancellation enables a 20-fold improvement in the scale factor for a typical fabricated device. Noise measurement of quadrature-cancelled mode-matched devices shows an angle random walk (ARW) of 0.63° √h−1 and a bias instability of 37.7° h−1, partially limited by the noise of the interface electronics. The elimination of silicon DRIE in the anisotropically wet-etched gyroscope improves the gyroscope robustness against the process variation and reduces the fabrication costs. The use of a slanted electrode for quadrature tuning demonstrates an effective path to reach high-performance in future pitch and roll gyroscope designs for the implementation of single-chip high-precision inertial measurement units (IMUs). A low-cost technique for fabricating high-precision microscale gyroscopes could lead to personal navigation devices that do not require GPS (global positioning system). The small size and low cost of silicon microscale gyroscopes has enabled their use in various applications, from gaming to the automotive industry. However, the assembly of high-performance triaxial gyroscopes that are required for navigation systems drives up the cost of manufacturing and increases alignment inaccuracies. By using an anisotropic wet-etching technique to fabricate the planar pitch/roll gyroscope, Haoran Wen and Farrokh Ayazi at the Georgia Institute of Technology, and their colleagues avoided the need for expensive deep reactive-ion etching and three-dimensional chip assembly, reducing both manufacturing costs and the errors that result from the fabrication process. The team’s work facilitates the production of robust single-chip multiaxial gyroscope units that have potentially navigation-grade performance.

Optical screw-wrench for microassembly.

Abstract: For future micro- and nanotechnologies, the manufacturing of miniaturized, functionalized, and integrated devices is indispensable. In this paper, an assembly technique based on a bottom-up strategy that enables the manufacturing of complex microsystems using only optical methods is presented. A screw connection is transferred to the micrometer range and used to assemble screw- and nut-shaped microcomponents. Micro-stereolithography is performed by means of two-photon polymerization, and microstructures are fabricated and subsequently trapped, moved, and screwed together using optical forces in a holographic optical tweezer set-up. The design and construction of interlocking microcomponents and the verification of a stable and releasable joint form the main focus of this paper. The assembly technique is also applied to a microfluidic system to enable the pumping or intermixing of fluids on a microfluidic chip. This strategy not only enables the assembly of microcomponents but also the combination of different materials and features to form complex hybrid microsystems. An all-optical, bottom-up technique that uses optical tweezers to assemble complex microsystems has been developed by a team in Germany. There is a strong push to increasingly miniaturize and integrate microsystems such as lab-on-a-chip and micro total analysis systems, but using mechanical grippers to combine components is difficult on a microscale. Now, by employing optical tweezers as an optical screw wrench, Jannis Kohler and co-workers at Applied Laser Technologies have used optical forces to position and screw together microscale screws and nuts. They further demonstrate the effectiveness of this method by using it to assemble and actuate a microrotor in a microfluidic system. The method can be used to combine components made from different materials and having different functions to produce complex hybrid microsystems.

Large-scale microfluidic gradient arrays reveal axon guidance behaviors in hippocampal neurons.

Abstract: High-throughput quantitative approaches to study axon growth behaviors have remained a challenge. We have developed a 1024-chamber microfluidic gradient generator array that enables large-scale investigations of axon guidance and growth dynamics from individual primary mammalian neurons, which are exposed to gradients of diffusible molecules. Our microfluidic method (a) generates statistically rich data sets, (b) produces a stable, reproducible gradient with negligible shear stresses on the culture surface, (c) is amenable to the long-term culture of primary neurons without any unconventional protocol, and (d) eliminates the confounding influence of cell-secreted factors. Using this platform, we demonstrate that hippocampal axon guidance in response to a netrin-1 gradient is concentration-dependent-attractive at higher concentrations and repulsive at lower concentrations. We also show that the turning of the growth cone depends on the angle of incidence of the gradient. Our study highlights the potential of microfluidic devices in producing large amounts of data from morphogen and chemokine gradients that play essential roles not only in axonal navigation but also in stem cell differentiation, cell migration, and immune response.

High-aspect-ratio nanoimprint process chains

Abstract: Different methods capable of developing complex structures and building elements with high-aspect-ratio nanostructures combined with microstructures, which are of interest in nanophotonics, are presented. As originals for subsequent replication steps, two families of masters were developed: (i) 3.2 μm deep, 180 nm wide trenches were fabricated by silicon cryo-etching and (ii) 9.8 μm high, 350 nm wide ridges were fabricated using 2-photon polymerization direct laser writing. Both emerging technologies enable the vertical smooth sidewalls needed for a successful imprint into thin layers of polymers with aspect ratios exceeding 15. Nanoridges with high aspect ratios of up to 28 and no residual layer were produced in Ormocers using the micromoulding into capillaries (MIMIC) process with subsequent ultraviolet-curing. This work presents and balances the different fabrication routes and the subsequent generation of working tools from masters with inverted tones and the combination of hard and soft materials. This provides these techniques with a proof of concept for their compatibility with high volume manufacturing of complex micro- and nanostructures. New methods pave the way for the high-volume manufacture of complex micro- and nanostructures with a high aspect ratio (HAR). HAR microstructures have uses in a variety of applications, including gas chromatography and X-ray optics, but making such structures with height-to-width ratios of 10 or more is particularly challenging. To address this, Helmut Schift and his colleagues from the Paul Scherrer Institute, Switzerland, in partnership with German company micro resist technology, used two emerging technologies for manufacturing HAR microstructures to help fabricate nanoridge structures with aspect ratios of up to 28. The team’s work provides molding tools for fabricating HAR microstructures that combine hard and soft materials, and confirms that the emerging technologies are compatible with the replication of complex micro- and nanostructures, holding promise for the development of innovative nanophotonics applications.

Controllable alignment of elongated microorganisms in 3D microspace using electrofluidic devices manufactured by hybrid femtosecond laser microfabrication.

Abstract: This paper presents a simple technique to fabricate new electrofluidic devices for the three-dimensional (3D) manipulation of microorganisms by hybrid subtractive and additive femtosecond (fs) laser microfabrication (fs laser-assisted wet etching of glass followed by water-assisted fs laser modification combined with electroless metal plating). The technique enables the formation of patterned metal electrodes in arbitrary regions in closed glass microfluidic channels, which can spatially and temporally control the direction of electric fields in 3D microfluidic environments. The fabricated electrofluidic devices were applied to nanoaquariums to demonstrate the 3D electro-orientation of Euglena gracilis (an elongated unicellular microorganism) in microfluidics with high controllability and reliability. In particular, swimming Euglena cells can be oriented along the z-direction (perpendicular to the device surface) using electrodes with square outlines formed at the top and bottom of the channel, which is quite useful for observing the motions of cells parallel to their swimming directions. Specifically, z-directional electric field control ensured efficient observation of manipulated cells on the front side (45 cells were captured in a minute in an imaging area of ~160×120 μm), resulting in a reduction of the average time required to capture the images of five Euglena cells swimming continuously along the z-direction by a factor of ~43 compared with the case of no electric field. In addition, the combination of the electrofluidic devices and dynamic imaging enabled observation of the flagella of Euglena cells, revealing that the swimming direction of each Euglena cell under the electric field application was determined by the initial body angle. Devices for manipulating microorganisms have been made using a simple method involving a femtosecond laser by researchers in Japan and China. Electrofluidic devices that permit rapid 3D control of the orientation of microorganisms are important for exploring the functions of various microorganism structures. Koji Sugioka at the RIKEN Center for Advanced Photonics, Japan, and his co-workers have demonstrated that hybrid femtosecond-laser microfabrication is promising for making such devices because it enables electrodes of designable geometries to be integrated at desired locations. The researchers used the technique to make ‘nanoaquariums’ that can rapidly orientate the elongated unicellular microorganisms Euglena gracilis, allowing them to obtain desired images of Euglena cells much faster than when no electric field is applied. The team anticipates that the devices can be applied to the 3D alignment of other elongated cells.

Photomechanical meta-molecule array for real-time terahertz imaging

Abstract: Real-time terahertz (THz) imaging offers remarkable application possibilities, especially in the security and medical fields. However, most THz detectors work with scanners, and a long image acquisition time is required. Some thermal detectors can achieve real-time imaging by using a focal plane array but have the drawbacks of low sensitivity due to a lack of suitable absorbing materials. In this study, we propose a novel photomechanical meta-molecule array by conveniently assembling THz meta-atom absorbers and bi-material cantilevers together, which can couple THz radiation to a mechanical deflection of the meta-molecules with high efficiency. By optically reading out the mechanical deflections of all of the meta-molecules simultaneously, real-time THz imaging can be achieved. A polyimide sacrificial layer technique was developed to fabricate the device on a glass wafer, which facilitates the transmission of a readout light while the THz wave radiates onto the meta-molecule array directly from the front side. THz images and video of various objects as well as infrared images of the human body were captured successfully with the fabricated meta-molecule array. The proposed photomechanical device holds promise in applications in single and broadband THz as well as infrared imaging.

Inkjet printing technology for increasing the I/O density of 3D TSV interposers.

Abstract: Interposers with through-silicon vias (TSVs) play a key role in the three-dimensional integration and packaging of integrated circuits and microelectromechanical systems. In the current practice of fabricating interposers, solder balls are placed next to the vias; however, this approach requires a large foot print for the input/output (I/O) connections. Therefore, in this study, we investigate the possibility of placing the solder balls directly on top of the vias, thereby enabling a smaller pitch between the solder balls and an increased density of the I/O connections. To reach this goal, inkjet printing (that is, piezo and super inkjet) was used to successfully fill and planarize hollow metal TSVs with a dielectric polymer. The under bump metallization (UBM) pads were also successfully printed with inkjet technology on top of the polymer-filled vias, using either Ag or Au inks. The reliability of the TSV interposers was investigated by a temperature cycling stress test (-40 degrees C to + 125 degrees C). The stress test showed no impact on DC resistance of the TSVs; however, shrinkage and delamination of the polymer was observed, along with some micro-cracks in the UBM pads. For proof of concept, SnAgCu-based solder balls were jetted on the UBM pads.