Microscale Devices Research Articles

MEMS microphones in commercial applications and beyond: Technology trends in the next decade

Since the initial introduction in the early 2000s, MEMS-based microphones have grown to become the preferred choice in a large range of commercial applications. This growth has been driven by continuous improvements in transduction performance, product consistency, and device ruggedness to meet ever more stringent application requirements. The emergence and rapid development/growth of small wearable devices, has given rise to a new set of possibilities and requirements for the acoustic transducer. In these applications power consumption, size, and ingress immunity are of major importance. While existing MEMS microphone topologies are being adjusted and adopted for this purpose, there are also opportunities for different structures and new operational schemes to further enhance product performance. In the area of acoustic performance, MEMS microphone technology has reached some fundamental barriers, particularly on size, at which entirely new approaches will be necessary to change the performance/size equation. In this presentation, a review will be given of MEMS microphone technology and how it got us to where we are today. In addition, projections will be shared on where the technology might go in the future where fully sealed, high performance, micro-scale devices are imaginable.

Subtractive patterning: High-resolution electrohydrodynamic jet printing with solvents

Advancements in 3D printing have initiated a paradigm in device fabrication. Electrohydrodynamic jet (e-jet) printing is a high-resolution 3D printing method that enables customizable patterning of thin-film structures, while reducing fabrication complexity and achieving high-resolution patterns with a wide variety of materials. However, to date, e-jet printing has focused on additive material deposition, rather than patterning through material subtraction. This work proposes displacement-based e-jet printing using solvent inks for subtractive patterning of polymer thin films, with microscale resolution in the x–y plane and nanoscale control in the z (dissolving) direction. The behavior of displacement-based e-jet printing is characterized using atomic force microscopy, and two methodologies are developed for controlling the linewidth and displaced depth. An example of area-selective thin film deposition on displacement-based e-jet patterns is provided to demonstrate the applicability of this patterning technique for printable microscale devices.

Thermoelectric response from grain boundaries and lattice distortions in crystalline gold devices

The electronic Seebeck response in a conductor involves the energy-dependent mean free path of the charge carriers and is affected by crystal structure, scattering from boundaries and defects, and strain. Previous photothermoelectric (PTE) studies have suggested that the thermoelectric properties of polycrystalline metal nanowires are related to grain structure, although direct evidence linking crystal microstructure to the PTE response is difficult to elucidate. Here, we show that room temperature scanning PTE measurements are sensitive probes that can detect subtle changes in the local Seebeck coefficient of gold tied to the underlying defects and strain that mediate crystal deformation. This connection is revealed through a combination of scanning PTE and electron microscopy measurements of single-crystal and bicrystal gold microscale devices. Unexpectedly, the photovoltage maps strongly correlate with gradually varying crystallographic misorientations detected by electron backscatter diffraction. The effects of individual grain boundaries and differing grain orientations on the PTE signal are minimal. This scanning PTE technique shows promise for identifying minor structural distortions in nanoscale materials and devices.

Inertial focusing of CTCs in a novel spiral microchannel

Circulating Tumor Cells (CTCs), which are rare types of cancer cells, detach from a primary tumor and enter the bloodstream, leading to the formation of metastatic cancer. One of the most effective methods to detect and separate CTCs from the bloodstream is inertial focusing using microfluidic devices. The present work proposed a novel microscale device for continuous flow separation of two types of CTC from blood cells. The MCF-7 (human breast cancer) cells and HeLa (human epithelial cervical cancer) cells, along with blood cells, enter a single-loop spiral microchannel. The cancer cells are separated at the outlet of the channel from each other and the white blood cells (WBCs) and red blood cells (RBCs) due to the inertial force. Different inlet flows of 38–150 ml/h corresponding to the Reynolds numbers of 30–120 are investigated to achieve the maximum separation efficiency of the device. Three-dimensional simulations are performed using the COMSOL Multiphysics 5.5 software and finite element method. It is demonstrated that the separation efficiency reaches 100% over a range of Reynolds numbers from 90 to 110 (throughput range of 113–139 ml/h).

Polymeric Microcuboids Programmable for Temperature‐Memory

AbstractMicroobjects with programmable mechanical functionality are highly desirable for the creation of flexible electronics, sensors, and microfluidic systems, where fabrication/programming and quantification methods are required to fully control and implement dynamic physical behavior. Here, programmable microcuboids with defined geometries are prepared by a template‐based method from crosslinked poly[ethylene‐co‐(vinyl acetate)] elastomers. These microobjects could be programmed to exhibit a temperature‐memory effect or a shape‐memory polymer actuation capability. Switching temperatures Tsw during shape recovery of 55 ± 2, 68 ± 2, 80 ± 2, and 86 ± 2 °C are achieved by tuning programming temperatures to 55, 70, 85, and 100 °C, respectively. Actuation is achieved with a reversible strain of 2.9 ± 0.2% to 6.7 ± 0.1%, whereby greater compression ratios and higher separation temperatures induce a more pronounced actuation. Micro‐geometry change is quantified using optical microscopy and atomic force microscopy. The realization and quantification of microparticles, capable of a tunable temperature responsive shape‐change or reversible actuation, represent a key development in the creation of soft microscale devices for drug delivery or microrobotics.

Nano- and Micro-Scale Simulations of Ge/3C-SiC and Ge/4H-SiC NN-Heterojunction Diodes

During the last decade, silicon carbide (SiC) and its heterostructures with other semiconductors have gained a significant importance for wide range of electronics applications. These structures are highly suitable for high frequency and high power applications in extremely high temperature environments. SiC exists in more than 200 different polycrystalline forms, called polytypes. Among these 200 types, the most prominent polytypes with exceptional physical and electrical attributes are 3C-SiC, 4H-SiC and 6H-SiC. Heterostructures of these SiC polytypes with other conventional semiconductors (like Si, Ge) can give rise to interesting electronic characteristics. In this article, Germanium (Ge) has been used to make heterostructures with 3C-SiC and 4H-SiC using a novel technique called diffusion welding. Microscale and nanoscale simulations of nn-heterojunction of Ge/3C-SiC and Ge/4H-SiC have been done. Microscale devices have been simulated with a commercially available semiconductor device simulator tool called Silvaco TCAD. Whereas nanoscale devices have been simulated with QuantumWise Atomistix Toolkit (ATK) software package. Current-voltage (IV) curves of all simulated devices have been calculated and compared. In nanoscale device, the effects of defects on IV-characteristics due to non-ideal bonding (lattice misplacement) at heterojunction interface have been analyzed. Our simulation results reveal that the proposed heterostructure devices with diffusion welding of wafers are theoretically possible. These simulations are the preparations of our near future physical experiments targeted to fabricate SiC based heterostructure devices using diffusion bonding technique.

Alternately Dipping Method to Prepare Graphene Fiber Electrodes for Ultra-high-Capacitance Fiber Supercapacitors.

SummaryFlexible fiber supercapacitors are promising candidate for power supply of wearable electronics. Fabrication of high-performance fibers is in progress yet challenging. The currently available graphene fibers made from wet-spinning or electro-deposition technologies are far away from practical applications due to their unsatisfactory capacitance. Here we report a facile alternately dipping (AD) method to coat graphene on wire-like substrates. The excellent mechanical properties of the substrate with greatly diverse choices can be carried over to the fiber supercapacitors. Under such guideline, the graphene fiber with a titanium core made by our AD method (AD:Ti@RGO) shows an ultra-high specific capacitance of up to 1,722.1 mF cm−2, which is ∼1,000 times that of wet-spinning- and electro-deposition-fabricated neat graphene fibers and presents the highest specific capacitance to date. With excellent mechanical properties and striking electrochemical performances, the AD:Ti@RGO-based supercapacitors light the path to the next-generation technologies for wearable devices.

3D Printing: Shaping Giant Membrane Vesicles in 3D‐Printed Protein Hydrogel Cages (Small 27/2020)

In article number 1906259, Petra Schwille and co-workers demonstrate 3D printing of a microscale device made of a biocompatible protein hydrogel that can be used to induce reversible shape changes in trapped vesicles by pH-stimulus. These soft material-based cages provide an artificial microenvironment, which may in the future allow us to investigate how synthetic cells react to and interact with external mechanical cues.

Synthesis and Magnon Thermal Transport Properties of Spin Ladder Sr14Cu24O41 Microstructures

AbstractThe spin ladder compound (Sr,Ca,La)14Cu24O41 exhibits an incommensurate layered structure with strong antiferromagnetic coupling. Besides intriguing superconducting behavior, recent experiments on bulk Sr14Cu24O41 single crystals have revealed a remarkable magnon thermal conductivity, which is the largest above 100 K among all known quantum magnets. Although bulk (Sr,Ca,La)14Cu24O41 crystals have been synthesized and studied extensively, there have been few reports on the synthesis and magnon thermal transport investigation of their microstructures. Here, the synthesis and thermal transport properties of Sr14Cu24O41 microrods are reported. Electron microscopy studies indicate that these microrods synthesized by a coprecipitation method are single crystals grown preferentially along the ladder axis. Based on a four‐probe thermal transport measurement, the thermal conductivity of the microrods reveals appreciable magnon transport in the microstructures. According to a kinetic model analysis, magnon transport in the microrods is suppressed mainly by increased point defect scattering compared to the bulk crystals, whereas surface scattering is negligible for anisotropic 1D magnon transport along the ladder. Moreover, the thermal conductivity is enhanced after annealing as a result of reduced oxygen vacancies. These results help to build the foundation for future heterogeneous integration of magnetic microstructures in microscale devices for the transport of energy and quantum information.

Efficient Spin Selectivity in Self-Assembled Superhelical Conducting Polymer Microfibers.

Chiral materials, natural or synthetic, have been widely studied since Pasteur's separation of enantiomers over a century ago. The connection between electron transmission and chirality was, however, established recently where one spin was preferably selected by the chiral molecules, displaying a typical chirality-induced spin selectivity (CISS) effect. Currently, this CISS effect was mainly demonstrated in the molecular-scale devices. Herein, we explored this effect in a microscale device where an efficient spin selectivity was found in the self-assembled superhelical conducting polyaniline (PANI) microfibers. A spin-selective efficiency up to 80% (not magnetoresistance) was achieved when spins traversed the ca. 2-6 μm-long helical channels at room temperature. Importantly, the long-range ordering of chiral PANI molecules is crucial to observe this efficient spin selectivity, whereas no selective transmission was found in the "amorphous" chiral PANIs. This efficient spin selectivity was subsequently rationalized by using an extended Su-Schrieffer-Heeger model where the Rashba spin-orbit coupling was considered. We expect these results could inspire the research of organic spintronics by using molecularly ordered, self-assembled, and chiral π-conjugated materials.

Uniaxial three-dimensional phase-shifting profilometry using a dual-telecentric structured light system in micro-scale devices

This paper presents a novel method for uniaxial microscopic three-dimensional (3D) profilometry using a structured light system with dual-telecentric lenses in micro-scale devices. Specifically, a telecentric lens can produce an orthographic view of an object and provide the exact size of objects in the x and y directions. An electronically focus-tunable lens attached to the projector rapidly and precisely changes the focal plane of the projected structured patterns, a camera captures the structured patterns from the same perspective and a computational framework analyzes the relationship between the captured structure images and the drive current of the electronically focus-tunable lens to obtain depth information for each pixel. By replacing the normal lens with telecentric lenses, the proposed method dodges perspective error and makes precision measurement easier. We developed a shadow-free coaxial 3D shape measurement system with dual-telecentric lenses that achieved a spatial resolution of approximately 5.86 µm and 4.08 µm root-mean-square error with a depth range of 1200 µm.

Instabilities, bifurcations, and transition to chaos in electrophoresis of charge-selective microparticle

Electro-hydrodynamic instabilities near a cation-exchange microgranule in an electrolyte solution under an external electric field are studied numerically. Despite the smallness of the particle and practically zero Reynolds numbers, in the vicinity of the particle, several sophisticated flow regimes can be realized, including chaotic ones. The obtained results are analyzed from the viewpoint of hydrodynamic stability and bifurcation theory. It is shown that a steady-state uniform solution is a non-unique one; an extra solution with a characteristic microvortex, caused by non-linear coupling of the hydrodynamics and electrostatics, in the region of incoming ions is found. Implementation of one of these solutions is subject to the initial conditions. For sufficiently strong fields, the steady-state solutions lose their stability via the Hopf bifurcation and limit cycles are born: a system of waves grows and propagates from the left pole, θ = 180°, toward the angle θ = θ0 ≈ 60°. Further bifurcations for these solutions are different. With the increase in the amplitude of the external field, the first cycle undergoes multiple period doubling bifurcation, which leads to the chaotic behavior. The second cycle transforms into a homoclinic orbit with the eventual chaotic mode via Shilnikov’s bifurcation. Santiago’s instability [Chen et al., “Convective and absolute electrokinetic instability with conductivity gradients,” J. Fluid Mech. 524, 263 (2005)], the third kind of instability, was then highlighted: an electroneutral extended jet of high salt concentration is formed at the right pole (region of outgoing ions, θ = 0°). For a large enough electric field, this jet becomes unstable; the perturbations are regular for a small supercriticality, and they acquire a chaotic character for a large supercriticality. The loss of stability of the concentration jet significantly affects the hydrodynamics in this area. In particular, the Dukhin–Mishchuk vortex, anchored to the microgranule at θ ≈ 60°, under the influence of the jet oscillations loses its stationarity and separates from the microgranule, forming a chain of vortices moving off the granule. This phenomenon strongly reminds the Kármán vortices behind a sphere but has another physical mechanism to implement. Besides the fundamental importance of the results, the instabilities found in the present work can be a key factor limiting the robust performance of complex electrokinetic bio-analytical systems. On the other hand, these instabilities can be exploited for rapid mixing and flow control of nanoscale and microscale devices.

Direct Fabrication of Freestanding and Patterned Nanoporous Junctions in a 3D Micro-Nanofluidic Device for Ion-Selective Transport.

In the field of micro-nanofluidics, a freestanding configuration of a nanoporous junction is highly demanded to increase the design flexibility of the microscale device and the interfacial area between the nanoporous junction and microchannels, thereby improving the functionality and performance. This work first reports direct fabrication and incorporation of a freestanding nanoporous junction in a microfluidic device by performing an electrolyte-assisted electrospinning process to fabricate a freestanding nanofiber membrane and subsequently impregnating the nanofiber membrane with a nanoporous precursor material followed by a solidification process. This process also enables to readily control the geometry of the nanoporous junction depending on its application. By these advantages, vertically stacked 3D micro-nanofluidic devices with complex configurations are easily achieved. To demonstrate the broad applicability of this process in various research fields, a reverse electrodialysis-based energy harvester and an ion concentration polarization-based preconcentrator are produced. The freestanding Nafion-polyvinylidene fluoride nanofiber membrane (F-NPNM) energy harvester generates a high power (59.87 nW) owing to the enlarged interfacial area. Besides, 3D multiplexed and multi-stacked F-NPNM preconcentrators accumulate multiple preconcentrated plugs that can increase the operating sample volume and the degree of freedom of handling. Hence, the proposed process is expected to contribute to numerous research fields related to micro-nanofluidics in the future.

The Effect of Streaming Potential and Viscous Dissipation in the Heat Transfer Characteristics of Power-Law Nanofluid Flow in a Rectangular Microchannel.

The non-Newtonian nanofluid flow becomes increasingly important in enhancing the thermal management efficiency of microscale devices and in promoting the exploration of the thermal-electric energy conversion process. The effect of streaming potential and viscous dissipation in the heat transfer characteristics of power-law nanofluid electrokinetic flow in a rectangular microchannel has been investigated to assist in the development of an energy harvesting system. The electroviscous effect caused by the streaming potential influences the hydrodynamical and thermal characteristics of flow. With the change in constitutive behavior of power-law nanofluid, the viscous dissipation effect is considered. The Poisson–Boltzmann equation, the modified Cauchy momentum equation, and the energy equation were solved. The temperature and heat transfer rate were analytically expressed for Newtonian nanofluid and numerically obtained for power-law nanofluid. The interactive influence of streaming potential, viscous dissipation, and hydrodynamical features of power-law nanofluid on the heat transfer characteristics were studied. The presence of streaming potential tends to reduce the dimensionless bulk mean temperature. The introduction of nanoparticles augments dimensionless temperature difference between channel wall and bulk flow, which decreases the heat transfer rate. The shear thinning nanofluid is more sensitive to the above effects. The temperature is a weak function of the flow behavior index.

Exploring fabrication methods to highly sensitive and selective InP nanowire biosensors

Fabrication methodologies for integration of nano-objects into microscale devices is still an active area of research. Here we analyze possible methods of incorporation of semiconductor nanowires into lithographically-defined electrode pads. Mechanically-transferred InP nanowires were metallized into Au and Pt pads using a electron-beam-induced Pt metallization. Atomic and Kelvin Probe Force Microscopies show that a contamination of Pt on the nanowire and the region around it can prevent application of this technique to biosensors in which surface functionalization protocols must be applied as part of the fabrication methodology. Other transfer methods with more controlled nanowire positioning, such as nanotweezers, may be necessary to overcome this problem.

Modeling and Simulation Software in MEMS Design

Micro-Electro-Mechanical Systems (MEMS) is a process technology that combines mechanical and electrical components to make micro-scale range devices. A considerable cost of the device can be reduced if we simulate the design. There are many available simulation software to choose from, which in turn is one of the major challenge. The paper explores the functional and technical features of some software used in MEMS designing. It further presents the keypoints which we should acknowledge while selecting software. Basic features are available in all MEMS Simulation software. However, if the design involves specific physics, geometry, material or meshing, the search must be done to find the appropriate software. If the user intends to fabricate the device then software with a virtual fabrication tool needs to be selected.

Shaping Giant Membrane Vesicles in 3D-Printed Protein Hydrogel Cages.

Giant unilamellar phospholipid vesicles are attractive starting points for constructing minimal living cells from the bottom-up. Their membranes are compatible with many physiologically functional modules and act as selective barriers, while retaining a high morphological flexibility. However, their spherical shape renders them rather inappropriate to study phenomena that are based on distinct cell shape and polarity, such as cell division. Here, a microscale device based on 3D printed protein hydrogel is introduced to induce pH-stimulated reversible shape changes in trapped vesicles without compromising their free-standing membranes. Deformations of spheres to at least twice their aspect ratio, but also toward unusual quadratic or triangular shapes can be accomplished. Mechanical force induced by the cages to phase-separated membrane vesicles can lead to spontaneous shape deformations, from the recurrent formation of dumbbells with curved necks between domains to full budding of membrane domains as separate vesicles. Moreover, shape-tunable vesicles are particularly desirable when reconstituting geometry-sensitive protein networks, such as reaction-diffusion systems. In particular, vesicle shape changes allow to switch between different modes of self-organized protein oscillations within, and thus, to influence reaction networks directly by external mechanical cues.

Achieving a superlubricating ohmic sliding electrical contact via a 2D heterointerface: a computational investigation.

Simultaneously achieving low friction and fine electrical conductance of sliding electrical contacts is a crucial factor but a great challenge for designing high-performance microscale and nanoscale functional devices. Through atomistic simulations, we propose an effective design strategy to obtain both low friction and high conductivity in sliding electrical contacts. By constructing graphene(Gr)/MoS2 two-dimensional (2D) heterojunctions between sliding Cu surfaces, superlubricity can be achieved with a remarkably lowered sliding energy barrier as compared to that of the homogeneous MoS2 lubricated Cu contact. Moreover, by introducing vacancy defects into MoS2 and substituting Cu with active metal Ti, the Schottky and tunneling barriers can be substantially suppressed without losing the superlubricious properties of the tribointerface. Consequently, a high conductivity ohmic contact with low sliding friction could be realized in our proposed Ti-MoS1.5-Gr-Ti system, which provides a potential strategy for tackling the well-known dilemma for high performance sliding electrical contacts.

Cyclic phase transformation behavior of nanocrystalline NiTi at microscale

Cuboidal micropillars of nanocrystalline superelastic NiTi shape memory alloys with an average grain size of 65 nm were fabricated by focused ion beam and then subjected to cyclic compression. It is found that the micropillars have maintained superelasticity for over 106 full-transformation cycles under a maximum compressive stress of 1.2 GPa. Functional degradation of the micropillars mainly occurs in the first 104 cycles where hysteresis loop area and forward transformation stress rapidly decrease from initial 11 MPa (MJ/m3) and 586 MPa to 6 MPa and 271 MPa. In the 104 ~ 106 cycles, stress-strain responses of the micropillars show asymptotic stabilization. Residual strain is accumulated to 3.3% and multiple ~50 nm wide extrusions are found at the surface of the micropillars after 106 cycles. SEM and TEM studies indicate that cyclic phase transformation results in formation and glide of transformation-induced dislocations that create surface steps and the extrusions. The dislocations inhibit reverse transformation and result in residual martensite and residual stresses. The dislocations and the residual martensite lead to the functional degradation. The role of the residual martensite in the functional degradation is further verified by 21% recovery of the residual strain and an increase of 278 MPa in the forward transformation stress after heating up the cyclically deformed micropillars to 100 °C. The recorded over 106 phase transformation cycles under a maximum stress of 1.2 GPa of the NiTi shape memory alloys at microscale open up new avenues for applications of the material in microscale devices and engineering.

A Fully Integrated In Vitro Diagnostic Microsystem for Pathogen Detection Developed Using a "3D Extensible" Microfluidic Design Paradigm.

Microfluidics is facing critical challenges in the quest of miniaturizing, integrating, and automating in vitro diagnostics, including the increasing complexity of assays, the gap between the macroscale world and the microscale devices, and the diverse throughput demands in various clinical settings. Here, a “3D extensible” microfluidic design paradigm that consists of a set of basic structures and unit operations was developed for constructing any application-specific assay. Four basic structures—check valve (in), check valve (out), double-check valve (in and out), and on–off valve—were designed to mimic basic acts in biochemical assays. By combining these structures linearly, a series of unit operations can be readily formed. We then proposed a “3D extensible” architecture to fulfill the needs of the function integration, the adaptive “world-to-chip” interface, and the adjustable throughput in the X, Y, and Z directions, respectively. To verify this design paradigm, we developed a fully integrated loop-mediated isothermal amplification microsystem that can directly accept swab samples and detect Chlamydia trachomatis automatically with a sensitivity one order higher than that of the conventional kit. This demonstration validated the feasibility of using this paradigm to develop integrated and automated microsystems in a less risky and more consistent manner.