Showing papers in "Microsystems & Nanoengineering in 2016"

Emerging flexible and wearable physical sensing platforms for healthcare and biomedical applications

Abstract: There are now numerous emerging flexible and wearable sensing technologies that can perform a myriad of physical and physiological measurements. Rapid advances in developing and implementing such sensors in the last several years have demonstrated the growing significance and potential utility of this unique class of sensing platforms. Applications include wearable consumer electronics, soft robotics, medical prosthetics, electronic skin, and health monitoring. In this review, we provide a state-of-the-art overview of the emerging flexible and wearable sensing platforms for healthcare and biomedical applications. We first introduce the selection of flexible and stretchable materials and the fabrication of sensors based on these materials. We then compare the different solid-state and liquid-state physical sensing platforms and examine the mechanical deformation-based working mechanisms of these sensors. We also highlight some of the exciting applications of flexible and wearable physical sensors in emerging healthcare and biomedical applications, in particular for artificial electronic skins, physiological health monitoring and assessment, and therapeutic and drug delivery. Finally, we conclude this review by offering some insight into the challenges and opportunities facing this field.

A toolkit of thread-based microfluidics, sensors, and electronics for 3D tissue embedding for medical diagnostics.

Abstract: Threads, traditionally used in the apparel industry, have recently emerged as a promising material for the creation of tissue constructs and biomedical implants for organ replacement and repair. The wicking property and flexibility of threads also make them promising candidates for the creation of three-dimensional (3D) microfluidic circuits. In this paper, we report on thread-based microfluidic networks that interface intimately with biological tissues in three dimensions. We have also developed a suite of physical and chemical sensors integrated with microfluidic networks to monitor physiochemical tissue properties, all made from thread, for direct integration with tissues toward the realization of a thread-based diagnostic device (TDD) platform. The physical and chemical sensors are fabricated from nanomaterial-infused conductive threads and are connected to electronic circuitry using thread-based flexible interconnects for readout, signal conditioning, and wireless transmission. To demonstrate the suite of integrated sensors, we utilized TDD platforms to measure strain, as well as gastric and subcutaneous pH in vitro and in vivo. Implantable and wearable diagnostic devices could integrate more smoothly into living tissue through 3D thread-based platforms. Such devices will transform the diagnosis and treatment of diseases by facilitating continuous, in situ monitoring of an individual’s health. However, as well as requiring costly and highly specialized manufacturing procedures, existing substrates are limited to two dimensions, which restricts their ability to penetrate multiple layers of tissue. In their quest for suitable alternatives, Sameer Sonkusale at Tufts University, United States, and his co-workers have developed a microfluidic platform that uses threads as substrates and functional constituents. The threads exhibit different physical, chemical and biological functions, producing a network of sensors, microfluidic channels and electronic components. The platform can measure both pH and strain in vitro and in vivo, which demonstrates its potential for implementation in clothing and implants.

Recent advances in nanorobotic manipulation inside scanning electron microscopes.

Abstract: A scanning electron microscope (SEM) provides real-time imaging with nanometer resolution and a large scanning area, which enables the development and integration of robotic nanomanipulation systems inside a vacuum chamber to realize simultaneous imaging and direct interactions with nanoscaled samples. Emerging techniques for nanorobotic manipulation during SEM imaging enable the characterization of nanomaterials and nanostructures and the prototyping/assembly of nanodevices. This paper presents a comprehensive survey of recent advances in nanorobotic manipulation, including the development of nanomanipulation platforms, tools, changeable toolboxes, sensing units, control strategies, electron beam-induced deposition approaches, automation techniques, and nanomanipulation-enabled applications and discoveries. The limitations of the existing technologies and prospects for new technologies are also discussed.

Multi-analyte biosensor interface for real-time monitoring of 3D microtissue spheroids in hanging-drop networks.

Abstract: Microfluidics is becoming a technology of growing interest for building microphysiological systems with integrated read-out functionalities. Here we present the integration of enzyme-based multi-analyte biosensors into a multi-tissue culture platform for ‘body-on-a-chip’ applications. The microfluidic platform is based on the technology of hanging-drop networks, which is designed for the formation, cultivation, and analysis of fluidically interconnected organotypic spherical three-dimensional (3D) microtissues of multiple cell types. The sensor modules were designed as small glass plug-ins featuring four platinum working electrodes, a platinum counter electrode, and an Ag/AgCl reference electrode. They were placed directly into the ceiling substrate from which the hanging drops that host the spheroid cultures are suspended. The electrodes were functionalized with oxidase enzymes to enable continuous monitoring of lactate and glucose through amperometry. The biosensors featured high sensitivities of 322±41 nA mM−1 mm−2 for glucose and 443±37 nA mM−1 mm−2 for lactate; the corresponding limits of detection were below 10 μM. The proposed technology enabled tissue-size-dependent, real-time detection of lactate secretion from single human colon cancer microtissues cultured in the hanging drops. Furthermore, glucose consumption and lactate secretion were monitored in parallel, and the impact of different culture conditions on the metabolism of cancer microtissues was recorded in real-time. Microsensors in ‘hanging-drop’ devices can track the metabolism of three-dimensional arrangements of human colon cancer cells in real time. When cells turn cancerous, their internal machinery often changes the consumption of nutrients, such as glucose. Disrupting the chemical pathways involved could lead to potent anticancer therapeutics. Patrick Misun and Olivier Frey at ETH Zurich in Switzerland and their colleagues have developed a more-realistic cell-culture platform with which to investigate drug-development strategies. The team grew human colon carcinoma microtissues inside liquid droplets that were suspended underneath a microfluidic substrate. Plug-in electrodes functionalized with either glucose- or lactose-sensitive enzymes into the microfluidic network enabled near-instantaneous detection of the consumption or secretion of metabolic molecules by the spherical microtissues. The high sensitivity and multi-tissue analytical capabilities of the platform bodes well for future ‘body-on-a-chip’ applications.

Rapid assembly of multilayer microfluidic structures via 3D-printed transfer molding and bonding

Abstract: A critical feature of state-of-the-art microfluidic technologies is the ability to fabricate multilayer structures without relying on the expensive equipment and facilities required by soft lithography-defined processes. Here, three-dimensional (3D) printed polymer molds are used to construct multilayer poly(dimethylsiloxane) (PDMS) devices by employing unique molding, bonding, alignment, and rapid assembly processes. Specifically, a novel single-layer, two-sided molding method is developed to realize two channel levels, non-planar membranes/valves, vertical interconnects (vias) between channel levels, and integrated inlet/outlet ports for fast linkages to external fluidic systems. As a demonstration, a single-layer membrane microvalve is constructed and tested by applying various gate pressures under parametric variation of source pressure, illustrating a high degree of flow rate control. In addition, multilayer structures are fabricated through an intralayer bonding procedure that uses custom 3D-printed stamps to selectively apply uncured liquid PDMS adhesive only to bonding interfaces without clogging fluidic channels. Using integrated alignment marks to accurately position both stamps and individual layers, this technique is demonstrated by rapidly assembling a six-layer microfluidic device. By combining the versatility of 3D printing while retaining the favorable mechanical and biological properties of PDMS, this work can potentially open up a new class of manufacturing techniques for multilayer microfluidic systems. A 3D printing technique for fabricating multilayered microfluidic devices promises to overcome the limitations of conventional fabrication. Advances in microfluidic technology are proving invaluable for disease diagnosis, DNA analysis, and drug discovery, but lithography-based device construction is time-consuming, reliant on costly infrastructure, and restricted to rectangular features. To address these limitations, Casey Glick at the University of California, Berkeley, United States, and his colleagues developed a versatile 3D printed transfer molding technique enabling them to mold flexible polymers into arbitrary configurations, such as thin membranes and controllable microvalves. They also rapidly assembled multiple layers into a single device; by using custom alignment marks and 3D printed stamps, they selectively applied adhesives without clogging the hair-width microfluidic channels. The team's work paves the way for the cheap and speedy manufacture of sophisticated multilayer microfluidic systems.

A fully electronic microfabricated gas chromatograph with complementary capacitive detectors for indoor pollutants

Abstract: This paper reports a complete micro gas chromatography (μGC) system in which all the components are lithographically microfabricated and electronically interfaced. The components include a bi-directional Knudsen pump, a preconcentrator, separation columns and a pair of capacitive gas detectors; together, these form the iGC3.c2 system. All the fluidic components of the system are fabricated by a common three-mask lithographic process. The Knudsen pump is a thermomolecular pump that provides air flow to the μGC without any moving parts. The film heaters embedded in the separation columns permit temperature programming. The capacitive detectors provide complementary response patterns, enhancing vapor recognition and resolving co-eluting peaks. With the components assembled on printed circuit boards, the system has a footprint of 8×10 cm2 . Using room air as the carrier gas, the system is used to experimentally demonstrate the analysis of 19 chemicals with concentration levels on the order of parts per million (p.p.m.) and parts per billion (p.p.b.). The tested chemicals include alkanes, aromatic hydrocarbons, aldehydes, halogenated hydrocarbons and terpenes. This set of chemicals represents a variety of common indoor air pollutants, among which benzene, toluene and xylenes (BTX) are of particular interest.

Voltage-tunable dual-layer terahertz metamaterials

Abstract: This paper presents the design, fabrication, and characterization of a real-time voltage-tunable terahertz metamaterial based on microelectromechanical systems and broadside-coupled split-ring resonators. In our metamaterial, the magnetic and electric interactions between the coupled resonators are modulated by a comb-drive actuator, which provides continuous lateral shifting between the coupled resonators by up to 20 μm. For these strongly coupled split-ring resonators, both a symmetric mode and an anti-symmetric mode are observed. With increasing lateral shift, the electromagnetic interactions between the split-ring resonators weaken, resulting in frequency shifting of the resonant modes. Over the entire lateral shift range, the symmetric mode blueshifts by ~60 GHz, and the anti-symmetric mode redshifts by ~50 GHz. The amplitude of the transmission at 1.03 THz is modulated by 74%; moreover, a 180° phase shift is achieved at 1.08 THz. Our tunable metamaterial device has myriad potential applications, including terahertz spatial light modulation, phase modulation, and chemical sensing. Furthermore, the scheme that we have implemented can be scaled to operate at other frequencies, thereby enabling a wide range of distinct applications. Researchers in the United States have created an artificial material with optical properties that can be changed using an electrical voltage. Metamaterials consist of repeating arrays of subwavelength elements that are designed to interact with light in ways that go beyond the abilities of naturally occurring materials. Xin Zhang and Richard Averitt at Boston University and UC San Diego and their colleagues have fabricated such a structure that can manipulate long-wavelength light called terahertz radiation. The metamaterial comprises two arrays of incomplete rings — one on a silicon nitride thin film and the other on a movable frame. The team could shift the frame using a microelectromechanical system that was integrated into the substrate. This relative motion changed the intensity and phase of the terahertz radiation being transmitted through the metamaterial.

A flexible three-dimensional electrode mesh: An enabling technology for wireless brain-computer interface prostheses.

Abstract: The neural interface is a key component in wireless brain-computer prostheses. In this study, we demonstrate that a unique three-dimensional (3D) microneedle electrode on a flexible mesh substrate, which can be fabricated without complicated micromachining techniques, is conformal to the tissues with minimal invasiveness. Furthermore, we demonstrate that it can be applied to different functional layers in the nervous system without length limitation. The microneedle electrode is fabricated using drawing lithography technology from biocompatible materials. In this approach, the profile of a 3D microneedle electrode array is determined by the design of a two-dimensional (2D) pattern on the mask, which can be used to access different functional layers in different locations of the brain. Due to the sufficient stiffness of the electrode and the excellent flexibility of the mesh substrate, the electrode can penetrate into the tissue with its bottom layer fully conformal to the curved brain surface. Then, the exposed contact at the end of the microneedle electrode can successfully acquire neural signals from the brain.

Epidermal radio frequency electronics for wireless power transfer.

Abstract: Epidermal electronic systems feature physical properties that approximate those of the skin, to enable intimate, long-lived skin interfaces for physiological measurements, human-machine interfaces and other applications that cannot be addressed by wearable hardware that is commercially available today. A primary challenge is power supply; the physical bulk, large mass and high mechanical modulus associated with conventional battery technologies can hinder efforts to achieve epidermal characteristics, and near-field power transfer schemes offer only a limited operating distance. Here we introduce an epidermal, far-field radio frequency (RF) power harvester built using a modularized collection of ultrathin antennas, rectifiers and voltage doublers. These components, separately fabricated and tested, can be integrated together via methods involving soft contact lamination. Systematic studies of the individual components and the overall performance in various dielectric environments highlight the key operational features of these systems and strategies for their optimization. The results suggest robust capabilities for battery-free RF power, with relevance to many emerging epidermal technologies.

Heterogeneous 2D/3D photonic integrated microsystems.

Abstract: The continuing trend of exponential growth in data communications and processing are driving the need for large-scale heterogeneous integration. Similar to the trend we have observed in electronic integrated circuit development, we are witnessing a growing trend in 3D photonic integrated circuits (PICs) development in addition to that in 2D PICs. There are two main methods for fabricating 3D PICs. The first method, which utilizes ultrafast laser inscription (ULI), offers freeform shaping of waveguides in arbitrary contours and formations. The second method, which utilizes multilayer stacking and coupling of planar PICs, exploits relatively mature 2D PIC fabrication processes applied to each layer sequentially. Both the fabrication methods for 3D PICs have advantages and disadvantages such that certain applications may favor one method over the other. However, a joining of 2D PICs with 3D PICs can help develop integrated microsystems with new functionalities such as non-mechanical beam steering, space-division multiplexing (SDM), programmable arbitrary beam shaping, and photonic signal processing. We discuss examples of 3D PICs and 2D/3D integrated PICs in two applications: SDM via orbital-angular-momentum (OAM) multiplexing/demultiplexing and optical beam steering using optical phased arrays. Although a 2D PIC by itself can function as an OAM multiplexer or demultiplexer, it has limitations in supporting both polarizations. Alternatively, a 3D PIC fabricated by ULI can easily support both polarizations with low propagation loss. A combination of a 3D PIC and a 2D PIC designed and fabricated for OAM applications has successfully multiplexed and demultiplexed 15 OAM states to demonstrate polarization-diversified SDM coherent optical communications using multiple OAM states. Coherent excitation of multi-ring OAM states can allow highly scalable SDM utilizing Laguerre–Gaussian modes or linearly polarized (LP) modes. The preliminary fabrication of multi-ring OAM multiplexers and demultiplexers using the multilayer 3D PIC method and the ULI 3D PIC method has also been pursued. Large-scale (for example, 16×16 optical phased array) 3D PICs fabricated with the ULI technique have been demonstrated. Through these examples, we show that heterogeneous 2D/3D photonic integration retains the advantages of 2D PICs and 3D waveguides, which can potentially benefit many other applications. Researchers in the United States have demonstrated a combination of 2D and 3D microstructures that control the flow of light for ultrafast data processing. One common technique for fabricating these integrated photonic circuits is to use ultrafast lasers to write arbitrarily shaped optical circuitry directly into a dielectric material. A less-flexible alternative is to stack more-easily created 2D layers to form a 3D structure. S. J. Ben Yoo and his co-workers from the University of California, Davis, investigated how combining the 2D and 3D approaches could provide the best devices for certain applications. They applied this idea to build a multiplexer that combines beams with different orbital angular momentum and to demonstrate an optical beam steering module. The newly combined 2D/3D microsystem with optical path length matching enables it to support optical signal processing and transmission in the spatial, temporal, and spectral domains in both polarizations.

Substrate-decoupled, bulk-acoustic wave gyroscopes: Design and evaluation of next-generation environmentally robust devices.

Abstract: This paper reports on a new type of high-frequency mode-matched gyroscope with significantly reduced dependencies on environmental stimuli such as temperature, vibration, and shock. A novel stress-isolation system is used to effectively decouple an axis-symmetric bulk-acoustic wave (BAW) vibratory gyro from its substrate, minimizing the effect that external sources of error have on the offset and scale factor of the device. Substrate-decoupled (SD) BAW gyros with a resonance frequency of 4.3 MHz and Q values near 60 000 were implemented using the high aspect ratio poly and single-crystal silicon (HARPSS) process to achieve ultra-narrow capacitive gaps. Wafer-level packaged sensors were interfaced with a customized application-specific integrated circuit (ASIC) to achieve low variations in the offset across temperature (±26° s−1 from −40 to 85 °C), supreme random-vibration immunity (0.012° s−1 gRMS−1) and excellent shock rejection. With a scale factor of 800 μV (°s−1)−1, the SD-BAW gyro system attains a large full-scale range (±1250° s−1) with a non-linearity of less than 0.07%. A measured angle-random walk (ARW) of 0.39°/√h and a bias instability of 10.5°h−1 are dominated by the thermal and flicker noise of the integrated circuit (IC), respectively. Additional measurements using external electronics show bias-instability values as low as 3.5°h−1, which are limited by feed-through signals coupled from the drive loop to the sense channel, which can be further reduced through proper re-routing of the gyroscope pin-out configuration. Scientists in the USA have developed a device for measuring rotation that is resilient against shock and vibration. A team led by Diego Serrano at Qualtre Inc. fabricated a micrometer-scale gyroscope that is decoupled from motion in the substrate on which it is built. Micromachined gyroscopes usually comprise a low-frequency resonant cantilever or membrane, and rotation is detected through a corresponding change in the resonance pattern of the resonator. However, such devices are susceptible to external sources of vibrations. Serrano’s team created a device known as a bulk acoustic wave gyroscope that is made of a disk of silicon and supports high frequency resonance inherently resistant to random external vibrations. The disk is surrounded by a series of electrodes that can fine tune the properties of the disk to further improve performance.

Integrated optical switch matrices for packet data networks

Abstract: Integrated circuit technologies are enabling intelligent, chip-based, optical packet switch matrices. Rapid real-time re-configurability at the photonic layer using integrated circuit technologies is expected to enable cost-effective, energy-efficient, and transparent data communications. InP integrated photonic circuits offer high-performance amplifiers, switches, modulators, detectors, and de/multiplexers in the same wafer-scale processes. The complexity of these circuits has been transformed as the process technologies have matured, enabling component counts to increase to many hundreds per chip. Active–passive monolithic integration has enabled switching matrices with up to 480 components, connecting 16 inputs to 16 outputs. Integrated switching matrices route data streams of hundreds of gigabits per second. Multi-path and packet time-scale switching have been demonstrated in the laboratory to route between multiple fibre connections. Wavelength-granularity routing and monitoring is realised inside the chip. In this paper, we review the current status in InP integrated photonics for optical switch matrices, paying particular attention to the additional on-chip functions that become feasible with active component integration. We highlight the opportunities for introducing intelligence at the physical layer and explore the requirements and opportunities for cost-effective, scalable switching. Devices that can quickly redirect incoming packets of optically encoded data could substantially increase the speed of the Internet. Kevin Williams and colleagues have reviewed recent developments in large optical switch matrices with a particular focus on the work performed using Indium phosphide integrated photonics at the Eindhoven University of Technology in the Netherlands. Indium phosphide is a semiconducting material that can efficiently produce and amplify light. Optically active switches made from a single material can improve operation speed and reduce the optical and electrical energy consumption of the device. Thus, indium phosphide integrated photonic circuits can combine advanced routing and signal processing functions all on one chip. So far this technology has created switching matrices with up to 480 components, connecting sixteen inputs to sixteen outputs and routed data streams at rates of a few hundred Gigabits per second.

PDMS/MWCNT-based tactile sensor array with coplanar electrodes for crosstalk suppression.

Abstract: The severe crosstalk effect is widely present in tactile sensor arrays with a sandwich structure. Here we present a novel design for a resistive tactile sensor array with a coplanar electrode layer and isolated sensing elements, which were made from polydimethylsiloxane (PDMS) doped with multiwalled carbon nanotubes (MWCNTs) for crosstalk suppression. To optimize its properties, both mechanical and electrical properties of PDMS/MWCNT-sensing materials with different PDMS/MWCNT ratios were investigated. The experimental results demonstrate that a 4 wt% of MWCNTs to PDMS is optimal for the sensing materials. In addition, the pressure-sensitive layer consists of three microstructured layers (two aspectant PDMS/MWCNT-based films and one top PDMS-based film) that are bonded together. Because of this three-layer microstructure design, our proposed tactile sensor array shows sensitivity up to −1.10 kPa−1, a response time of 29 ms and reliability in detecting tiny pressures. A skin-like sensor with millisecond response times, enough sensitivity to detect water droplets and extremely low crosstalk has been created by researchers in China. Tactile sensors for robotics and other applications often use ‘sandwich’ devices in which a pressure-sensitive piezoelectric crystal is squeezed between two conducting electrodes. Now, Jingquan Liu at Shanghai Jiao Tong University and his colleagues report that arrays of a three-layer structure with microscaled pyramids, fabricated through the lithographic patterning of siloxane polymers and multiwalled carbon nanotube composites, can improve the performance of tactile sensors. The bumpy geometry of the flexible, conductive micropyramids offers improved pressure detection and stress distribution in comparison to devices without mircopyramids. Isolating the pressure sensing elements by placing them apart from each other and enabling the electrodes coplanar on the underside of the micropyramid array suppresses damaging electronic interference that normally limits the accuracy of artificial skin sensors.

Graphene-aluminum nitride NEMS resonant infrared detector.

Abstract: The use of micro-/nanoelectromechanical resonators for the room temperature detection of electromagnetic radiation at infrared frequencies has recently been investigated, showing thermal detection capabilities that could potentially outperform conventional microbolometers. The scaling of the device thickness in the nanometer range and the achievement of high infrared absorption in such a subwavelength thickness, without sacrificing the electromechanical performance, are the two key challenges for the implementation of fast, high-resolution micro-/nanoelectromechanical resonant infrared detectors. In this paper, we show that by using a virtually massless, high-electrical-conductivity, and transparent graphene electrode, floating at the van der Waals separation of a few angstroms from a piezoelectric aluminum nitride nanoplate, it is possible to implement ultrathin (460 nm) piezoelectric nanomechanical resonant structures with improved electromechanical performance (>50% improved frequency×quality factor) and infrared detection capabilities (>100× improved infrared absorptance) compared with metal-electrode counterparts, despite their reduced volumes. The intrinsic infrared absorption capabilities of a submicron thin graphene–aluminum nitride plate backed with a metal electrode are investigated for the first time and exploited for the first experimental demonstration of a piezoelectric nanoelectromechanical resonant thermal detector with enhanced infrared absorptance in a reduced volume. Moreover, the combination of electromagnetic and piezoelectric resonances provided by the same graphene–aluminum nitride-metal stack allows the proposed device to selectively detect short-wavelength infrared radiation (by tailoring the thickness of aluminum nitride) with unprecedented electromechanical performance and thermal capabilities. These attributes potentially lead to the development of uncooled infrared detectors suitable for the implementation of high performance, miniaturized and power-efficient multispectral infrared imaging systems. Researchers in the United States have developed an ultrathin nanoelectromechanical resonant thermal detector that could outperform standard microbolometers. The team, led by Matteo Rinaldi at Northeastern University, Boston, used a monolayer graphene sheet, in lieu of a bulky metal film, as an electrode on top of a piezoelectric aluminum nitride nanoplate resonator. This virtually massless graphene electrode, floating a few angstroms above the nanoplate, not only boosts the vibration frequency and electromechanical performance of the resonator but it also enables effective infrared absorption in such a vibrating structure with reduced volume and enhanced thermal sensitivity. The achievement of high infrared absorptance, in nanomechanical vibrating structures with reduced volume and improved electromechanical performance, addresses one of the most fundamental challenges in the field enabling the development of fast and high-resolution uncooled nanoelectromechanical resonant infrared detectors.

Precision in harsh environments

Abstract: Microsystems are increasingly being applied in harsh and/or inaccessible environments, but many markets expect the same level of functionality for long periods of time. Harsh environments cover areas that can be subjected to high temperature, (bio)-chemical and mechanical disturbances, electromagnetic noise, radiation, or high vacuum. In the field of actuators, the devices must maintain stringent accuracy specifications for displacement, force, and response times, among others. These new requirements present additional challenges in the compensation for or elimination of cross-sensitivities. Many state-of-the-art precision devices lose their precision and reliability when exposed to harsh environments. It is also important that advanced sensor and actuator systems maintain maximum autonomy such that the devices can operate independently with low maintenance. The next-generation microsystems will be deployed in remote and/or inaccessible and harsh environments that present many challenges to sensor design, materials, device functionality, and packaging. All of these aspects of integrated sensors and actuator microsystems require a multidisciplinary approach to overcome these challenges. The main areas of importance are in the fields of materials science, micro/nano-fabrication technology, device design, circuitry and systems, (first-level) packaging, and measurement strategy. This study examines the challenges presented by harsh environments and investigates the required approaches. Examples of successful devices are also given.

Chemical-sensitive graphene modulator with a memory effect for internet-of-things applications.

Abstract: Modern internet of things (IoTs) and ubiquitous sensor networks could potentially take advantage of chemically sensitive nanomaterials and nanostructures. However, their heterogeneous integration with other electronic modules on a networked sensor node, such as silicon-based modulators and memories, is inherently challenging because of compatibility and integration issues. Here we report a novel paradigm for sensing modulators: a graphene field-effect transistor device that directly modulates a radio frequency (RF) electrical carrier signal when exposed to chemical agents, with a memory effect in its electrochemical history. We demonstrated the concept and implementation of this graphene-based sensing modulator through a frequency-modulation (FM) experiment conducted in a modulation cycle consisting of alternating phases of air exposure and ethanol or water treatment. In addition, we observed an analog memory effect in terms of the charge neutrality point of the graphene, Vcnp, which strongly influences the FM results, and developed a calibration method using electrochemical gate-voltage pulse sequences. This graphene-based multifunctional device shows great potential for use in a simple, low-cost, and ultracompact nanomaterial-based nodal architecture to enable continuous, real-time event-based monitoring in pervasive healthcare IoTs, ubiquitous security systems, and other chemical/molecular/gas monitoring applications. A graphene-based transistor that detects chemical agents and lengths of exposure could make 24-hour healthcare easier to achieve. Embedding wireless sensors into devices such as infection monitors is an important part of Internet-of-Things applications. But nanomaterials that are well suited to sensing environmental particles are difficult to integrate into conventional silicon circuits. Haiyu Huang from the University of Texas at Austin in the United States, Pai-Yen Chen from the Wayne State University in the United States and their colleagues —show that graphene field-effect transistors can offer simultaneous radio-frequency modulation, chemical sensing and memory effects in a single component. The team’s large-area, back-gated device demonstrates distinct shifts in voltage when introduced to reducing or oxidizing chemical gases, or to more complex substances such as protein biomarkers. These molecular-induced shifts in transistor operation also depend on earlier chemical exposures — an electrochemical history not retained by typical semiconductors.

Electron-beam lithography for polymer bioMEMS with submicron features.

Abstract: We present a method for submicron fabrication of flexible, thin-film structures fully encapsulated in biocompatible polymer poly(chloro-p-xylylene) (Parylene C) that improves feature size and resolution by an order of magnitude compared with prior work. We achieved critical dimensions as small as 250 nm by adapting electron beam lithography for use on vapor deposited Parylene-coated substrates and fabricated encapsulated metal structures, including conducting traces, serpentine resistors, and nano-patterned electrodes. Structures were probed electrically and mechanically demonstrating robust performance even under flexion or torsion. The developed fabrication process for electron beam lithography on Parylene-coated substrates and characterization of the resulting structures are presented in addition to a discussion of the challenges of applying electron beam lithography to polymers. As an application of the technique, a Parylene-based neural probe prototype was fabricated with 32 recording sites patterned along a 2 mm long shank, an electrode density surpassing any prior polymer probe. Flexible, polymer-coated electrodes with features as narrow as 250 nm have been produced using electron-beam lithography. The polymer Parylene C is widely used in implantable devices such as neural probes as a biocompatible and insulating coating for electrodes. However, it is challenging to pattern this polymer with electron beams because of its sensitivity to heat and charge. Ellis Meng and Kee Scholten from the University of Southern California, United States, overcame these limitation with a chromium-capped methacrylate resist mask. Depositing this mask onto a Parylene C-encapsulated titanium thin film helped to reduce thermal stress effects and electric charge build-up, which improved feature resolution by an order of magnitude beyond that of existing approaches. A prototype neural probe with 32 data recording sites along a 2-mm span—an electrode density that greatly exceeds previous polymer implants—demonstrated the potential of the technique.

Multifrequency excitation of a clamped-clamped microbeam: Analytical and experimental investigation.

Abstract: Using partial electrodes and a multifrequency electrical source, we present a large-bandwidth, large-amplitude clamped-clamped microbeam resonator excited near the higher order modes of vibration. We analytically and experimentally investigate the nonlinear dynamics of the microbeam under a two-source harmonic excitation. The first-frequency source is swept around the first three modes of vibration, whereas the second source frequency remains fixed. New additive and subtractive resonances are demonstrated. We illustrated that by properly tuning the frequency and amplitude of the excitation force, the frequency bandwidth of the resonator is controlled. The microbeam is fabricated using polyimide as a structural layer coated with nickel from the top and chromium and gold layers from the bottom. Using the Galerkin method, a reduced order model is derived to simulate the static and dynamic response of the device. A good agreement between the theoretical and experimental data are reported.

Implementation of soft microfingers for a hMSC aggregate manipulation system.

Abstract: This paper describes a pneumatic balloon actuator (PBA) composed of polydimethylsiloxane (PDMS) for cellular aggregate manipulation. We evaluated the ability of the microdevice to manipulate a tiny and sensitive cellular aggregate without causing serious damage. We used human mesenchymal stem cells (hMSCs) for the cellular aggregate. We describe the design, fabrication, characterization and operation of the soft microfingers to pinch and release a spherical hMSC aggregate (φ200 μm), and we employed a PBA to serve as an artificial muscle to drive the microfingers. A design of the microfingers in terms of dimensions, generated force and contact conditions was accomplished. The designed dimensions of a single finger were 560 μm×900 μm. In summary, we demonstrate the utility of the surface modification of a fingertip for pinching and releasing a cellular aggregate and describe a manipulation system that was constructed to drive and control the microfingers. The implemented manipulation system, which is composed of microfingers and a positioning mechanism, was tested and verified in a series of operations. Small soft ‘fingers’ that safely manipulate cells will ensure the quality and precise control of biological products. The successful progression of regenerative medicine and drug delivery relies on the engineering of lifelike tissues. Specifically, 3D scaffolds provide artificial tissues that reproduce natural processes in the body. However, these fragile biological materials require careful handling — a challenge when using conventional techniques. In their search for precise and gentle alternatives, Satoshi Konishi from Ritsumeikan University and co-workers in Japan generated microscopic polymer-based fingers that pinch and release cell aggregates without damage. The fingers comprise two bonded pneumatic balloons that act as synthetic muscles. Normally closed, their extremities open when the balloons are inflated. Combined with a microscope and a 3D positioning system, the fingers can pick up the aggregates and transfer them to a specific location.

SU8 etch mask for patterning PDMS and its application to flexible fluidic microactuators

Abstract: Over the past few decades, polydimethylsiloxane (PDMS) has become the material of choice for a variety of microsystem applications, including microfluidics, imprint lithography, and soft microrobotics. For most of these applications, PDMS is processed by replication molding; however, new applications would greatly benefit from the ability to pattern PDMS films using lithography and etching. Metal hardmasks, in conjunction with reactive ion etching (RIE), have been reported as a method for patterning PDMS; however, this approach suffers from a high surface roughness because of metal redeposition and limited etch thickness due to poor etch selectivity. We found that a combination of LOR and SU8 photoresists enables the patterning of thick PDMS layers by RIE without redeposition problems. We demonstrate the ability to etch 1.5-μm pillars in PDMS with a selectivity of 3.4. Furthermore, we use this process to lithographically process flexible fluidic microactuators without any manual transfer or cutting step. The actuator achieves a bidirectional rotation of 50° at a pressure of 200 kPa. This process provides a unique opportunity to scale down these actuators as well as other PDMS-based devices. A method to etch structures in silicone rubber could prove useful for microfluidics, lithography and even soft robotics applications. Michael De Volder at the University of Cambridge, United Kingdom, and his colleagues at the KU Leuven, Belgium used a dual-layer mask consisting of a lift-off resist underneath an SU-8 photoresist layer to protect selected areas of the silicone polydimethylsiloxane (PDMS). Next, the team used reactive-ion etching with a specific gas composition to remove the exposed PDMS. The entire protective mask was then easily removed by etching the lift-off layer through exposure to a developer fluid. The researchers used their method to make simple, flexible fluidic actuators. The fabrication process removes the need for imprecise manual cutting or metal masks that introduce defects into PDMS structures, which should enable even greater scaling down of PDMS devices.

Functional imaging of neuron-astrocyte interactions in a compartmentalized microfluidic device

Abstract: Traditional approaches in cultivating neural cells in a dish without orienting their interactions have had only limited success in revealing neural network properties. To enhance the experimental capabilities of studying neural circuitry in vitro, we designed an experimental system combining concepts of micropatterned surfaces, microfluidic devices and genetically encoded biosensors. Micropatterning was used to position neurons and astrocytes in defined locations and guide interactions between the two cell types. Microfluidic chambers were placed atop micropatterned surfaces to allow delivery of different pharmacological agents or viral vectors to the desired cell types. In this device, astrocytes and neurons communicated through grooves molded into the floor of the microfluidic device. By combining microfluidics with genetically encoded calcium indicators as functional readouts, we further demonstrated the utility of this device for analyzing neuron-neuron and neuron-astrocyte interactions in vitro under both healthy and pathophysiological conditions. We found that both spontaneous and evoked calcium dynamics in astrocytes can be modulated by interactions with neurons. In the future, we foresee employing the microdevices described here for studying mechanisms of neurological disorders.

Nanoelectromechanical resonant narrow-band amplifiers.

Abstract: This study demonstrates amplification of electrical signals using a very simple nanomechanical device. It is shown that vibration amplitude amplification using a combination of mechanical resonance and thermal-piezoresistive energy pumping, which was previously demonstrated to drive self-sustained mechanical oscillation, can turn the relatively weak piezoresistivity of silicon into a viable electronic amplification mechanism with power gains of >20 dB. Various functionalities ranging from frequency selection and timing to sensing and actuation have been successfully demonstrated for microscale and nanoscale electromechanical systems. Although such capabilities complement solid-state electronics, enabling state-of-the-art compact and high-performance electronics, the amplification of electronic signals is an area where micro-/nanomechanics has not experienced much progress. In contrast to semiconductor devices, the performance of the proposed nanoelectromechanical amplifier improves significantly as the dimensions are reduced to the nanoscale presenting a potential pathway toward deep-nanoscale electronics. The nanoelectromechanical amplifier can also address the need for ultranarrow-band filtering along with the amplification of low-power signals in wireless communications and certain sensing applications, which is another need that is not efficiently addressable using semiconductor technology. A nanoscale vibrating beam can produce 100-fold gains of power in weak electronic signals by ‘self-amplifying’ its mechanical resonance. Selectively amplifying a narrow band of frequencies is important for wireless communication. However, it is difficult to achieve sufficient levels of control over bandwidths as chip dimensions are scaled down. Alireza Ramezany and co-workers at the University of Texas at Dallas, United States, constructed a nanomechanical device that overcomes these problems through resonant vibrations and the piezoresistivity of silicon, which causes changes in current under mechanical stress. The team designed an ‘energy-pumping’ setup that captures the heat generated by an expanding and contracting 70-nanometer-wide beam. This was then used to enhance the device’s vibrational resonance – a self-sustained oscillation that offers improved amplification as the resonator's dimensions are shrunk into the deep nanoscale range.

Thermoluminescent microparticle thermal history sensors.

Abstract: While there are innumerable devices that measure temperature, the nonvolatile measurement of thermal history is far more difficult, particularly for sensors embedded in extreme environments such as fires and explosions. In this review, an extensive analysis is given of one such technology: thermoluminescent microparticles. These are transparent dielectrics with a large distribution of trap states that can store charge carriers over very long periods of time. In their simplest form, the population of these traps is dictated by an Arrhenius expression, which is highly dependent on temperature. A particle with filled traps that is exposed to high temperatures over a short period of time will preferentially lose carriers in shallow traps. This depopulation leaves a signature on the particle luminescence, which can be used to determine the temperature and time of the thermal event. Particles are prepared—many months in advance of a test, if desired—by exposure to deep ultraviolet, X-ray, beta, or gamma radiation, which fills the traps with charge carriers. Luminescence can be extracted from one or more particles regardless of whether or not they are embedded in debris or other inert materials. Testing and analysis of the method is demonstrated using laboratory experiments with microheaters and high energy explosives in the field. It is shown that the thermoluminescent materials LiF:Mg,Ti, MgB4O7:Dy,Li, and CaSO4:Ce,Tb, among others, provide accurate measurements of temperature in the 200 to 500 °C range in a variety of high-explosive environments. Micrometer-scale particles offer a means of better understanding explosions by tracking changes in temperature through time. Joseph Talghader at the University of Minnesota, United States, and his colleagues review the use of thermoluminescent materials in the creation of sensors that provide a thermal history of high-temperature events. Such sensors record the magnitude and duration of temperature excursions. Thermoluminescent materials represent one possible approach to these sensors because the population of filled traps in the materials have a strong relationship with the temperatures to which they have been exposed. After an explosion, the remaining filled traps can be excited to emit light that indicates the thermal history. Thermoluminescent microparticles are cheap to produce and can be fabricated in large quantities and embedded throughout the test area. They are also robust enough to withstand destruction in harsh environments. A summary of the properties of fluoride- and oxide-based thermoluminescent materials, which can supply accurate measurements of temperatures between 200 °C and 500 °C, is provided.

Scalable fabrication of microneedle arrays via spatially controlled UV exposure

Abstract: This paper describes a theoretical estimation of the geometry of negative epoxy-resist microneedles prepared via inclined/rotated ultraviolet (UV) lithography based on spatially controlled UV exposure doses. In comparison with other methods based on UV lithography, the present method can create microneedle structures with high scalability. When negative photoresist is exposed to inclined/rotated UV through circular mask patterns, a three-dimensional, needle-shaped distribution of the exposure dose forms in the irradiated region. Controlling the inclination angles and the exposure dose modifies the photo-polymerized portion of the photoresist, thus allowing the variation of the heights and contours of microneedles formed by using the same mask patterns. In an experimental study, the dimensions of the fabricated needles agreed well with the theoretical predictions for varying inclination angles and exposure doses. These results demonstrate that our theoretical approach can provide a simple route for fabricating microneedles with on-demand geometry. The fabricated microneedles can be used as solid microneedles or as a mold master for dissolving microneedles, thus simplifying the microneedle fabrication process. We envision that this method can improve fabrication accuracy and reduce fabrication cost and time, thereby facilitating the practical applications of microneedle-based drug delivery technology.

Design and fabrication of silicon-tessellated structures for monocentric imagers

Abstract: Compared with conventional planar optical image sensors, a curved focal plane array can simplify the lens design and improve the field of view. In this paper, we introduce the design and implementation of a segmented, hemispherical, CMOS-compatible silicon image plane for a 10-mm diameter spherical monocentric lens. To conform to the hemispherical focal plane of the lens, we use flexible gores that consist of arrays of spring-connected silicon hexagons. Mechanical functionality is demonstrated by assembling the 20-μm-thick silicon gores into a hemispherical test fixture. We have also fabricated and tested a photodiode array on a silicon-on-insulator substrate for use with the curved imager. Optical testing shows that the fabricated photodiodes achieve good performance; the hemispherical imager enables a compact 160 ° field of view camera with >80% fill factor using a single spherical lens. US researchers have developed a curved photodetector array that could lead to miniature monocentric imagers. Silicon imagers in cell-phone cameras are planar grids of photodetectors, requiring flat imaging planes and complex lens designs to correct for aberrations. The curved imager would allow a simple, compact, and wide field-of-view spherical lens to be used instead of conventional lens stacks. Roger Howe and co-workers from Stanford University, California, have created a tessellated thin silicon array of optoelectronic devices that can conform to a 10-mm-diameter ball lens. The team constructed the photodetectors on a silicon-on-insulator substrate. The 20-μ-thick device layer was etched into gore segments consisting of arrays of spring-connected hexagons. These interconnected, compliant segments were released from the substrate and assembled onto the hemispherical mounting surface for the ball lens.

Printed unmanned aerial vehicles using paper-based electroactive polymer actuators and organic ion gel transistors

Abstract: We combined lightweight and mechanically flexible printed transistors and actuators with a paper unmanned aerial vehicle (UAV) glider prototype to demonstrate electrically controlled glide path modification in a lightweight, disposable UAV system. The integration of lightweight and mechanically flexible electronics that is offered by printed electronics is uniquely attractive in this regard because it enables flight control in an inexpensive, disposable, and easily integrated system. Here, we demonstrate electroactive polymer (EAP) actuators that are directly printed into paper that act as steering elements for low cost, lightweight paper UAVs. We drive these actuators by using ion gel-gated organic thin film transistors (OTFTs) that are ideally suited as drive transistors for these actuators in terms of drive current and frequency requirements. By using a printing-based fabrication process on a paper glider, we are able to deliver an attractive path to the realization of inexpensive UAVs for ubiquitous sensing and monitoring flight applications. Researchers in the United States have developed an electronically controlled actuator that is light enough to steer a paper airplane. Gerd Grau and co-workers from the University of California, Berkeley, printed a flexible transistor onto paper to manipulate simple flaps on a lightweight glider. The team's actuator was fabricated by infusing a paper with an electroactive polymer so that it bends under the influence of a current from the battery-powered organic thin-film transistor. The entire glider, including actuator and battery, weighs less than 8 grams. Although the actuator was fabricated separately and then attached to the prototype, all of the glider's electronic elements can be printed onto a single large sheet of paper for subsequent cutting and folding. The approach could enable the low-cost manufacture of unmanned aerial vehicles for sensing and monitoring applications.

Real-time mechanical characterization of DNA degradation under therapeutic X-rays and its theoretical modeling.

Abstract: The killing of tumor cells by ionizing radiation beams in cancer radiotherapy is currently based on a rather empirical understanding of the basic mechanisms and effectiveness of DNA damage by radiation. By contrast, the mechanical behaviour of DNA encompassing sequence sensitivity and elastic transitions to plastic responses is much better understood. A novel approach is proposed here based on a micromechanical Silicon Nanotweezers device. This instrument allows the detailed biomechanical characterization of a DNA bundle exposed to an ionizing radiation beam delivered here by a therapeutic linear particle accelerator (LINAC). The micromechanical device endures the harsh environment of radiation beams and still retains molecular-level detection accuracy. In this study, the first real-time observation of DNA damage by ionizing radiation is demonstrated. The DNA bundle degradation is detected by the micromechanical device as a reduction of the bundle stiffness, and a theoretical model provides an interpretation of the results. These first real-time observations pave the way for both fundamental and clinical studies of DNA degradation mechanisms under ionizing radiation for improved tumor treatment.

Vertical, capacitive microelectromechanical switches produced via direct writing of copper wires.

Abstract: Three-dimensional (3D) direct writing based on the meniscus-confined electrodeposition of copper metal wires was used in this study to develop vertical capacitive microelectromechanical switches. Vertical microelectromechanical switches reduce the form factor and increase the area density of such devices in integrated circuits. We studied the electromechanical characteristics of such vertical switches by exploring the dependence of switching voltage on various device structures, particularly with regard to the length, wire diameter, and the distance between the two wires. A simple model was found to match the experimental measurements made in this study. We found that the electrodeposited copper microwires exhibit a good elastic modulus close to that of bulk copper. By optimizing the 3D structure of the electrodes, a volatile electromechanical switch with a sub-5 V switching voltage was demonstrated in a vertical microscale switch with a gap distance as small as 100 nm created with a pair of copper wires with diameters of ~1 μm and heights of 25 μm. This study establishes an innovative approach to construct microelectromechanical systems with arbitrary 3D microwire structures for various applications, including the demonstrated volatile and nonvolatile microswitches. A procedure that directly writes intricate copper microwires onto device surfaces could aid development of inexpensive microswitches. The need for high-density integration, ultralow power consumption in computer circuitry has led the chip designers to explore unconventional microelectromechanical system (MEMS) architectures. For example, applying a voltage between two vertically aligned microwires can induce an electrostatic force that pulls the wires together, forming the ‘on’ state of a MEMS switch. Jianjun Guo from the Chinese Academy of Sciences and co-workers used meniscus-confined electrodeposition from a computer-controlled micropipette to grow three-dimensional (3D) copper-wire MEMS switches for the first time. Normally, synthesizing 3D copper structures is challenging because of oxidation issues, but the team’s approach led to successful fabrication of vertical microwires separated by a few hundred nanometers. The devices demonstrated volatile switching with minimal, sub-5 V power requirements.

Crystal orientation-dependent fatigue characteristics in micrometer-sized single-crystal silicon.

Abstract: Repetitive bending fatigue tests were performed using five types of single-crystal silicon specimens with different crystal orientations fabricated from {100} and {110} wafers. Fatigue lifetimes in a wide range between 100 and 1010 were obtained using fan-shaped resonator test devices. Fracture surface observation via scanning electron microscope (SEM) revealed that the {111} plane was the primary fracture plane. The crack propagation exponent n was estimated to be 27, which was independent of the crystal orientation and dopant concentration; however, it was dependent on the surface conditions of the etched sidewall. The fatigue strengths relative to the deflection angle were orientation dependent, and the ratios of the factors obtained ranged from 0.86 to 1.25. The strength factors were compared with those obtained from finite element method stress analyses. The calculated stress distributions showed strong orientation dependence, which was well-explained by the elastic anisotropy. The comparison of the strength factors suggested that the first principal stress was a good criterion for fatigue fracture. We include comparisons with specimens tested in our previous report and address the tensile strength, initial crack length, volume effect, and effects of surface roughness such as scallops.

Off-stoichiometry improves the photostructuring of thiol–enes through diffusion-induced monomer depletion

Abstract: Thiol-enes are a group of alternating copolymers with highly ordered networks used in a wide range of applications. Here, “click” chemistry photostructuring in off-stoichiometric thiol-enes is show ...