Showing papers in "Journal of Micromechanics and Microengineering in 2019"

Flexible, transparent, sub-100 µm microfluidic channels with fused deposition modeling 3D-printed thermoplastic polyurethane

Abstract: The need for accessible and inexpensive microfluidic devices requires new manufacturing methods and materials that can replace traditional soft lithography and polydimethylsiloxane (PDMS). Here, we use fused deposition modeling (FDM) 3D printing to create transparent, flexible, and biocompatible microfluidic devices with channel dimensions consistently under 100 µm and as small as 40 µm. Channels consistently printed about 100 µm smaller than designed, but were repeatable and predictable. We demonstrate that thermoplastic polyurethane (TPU) has properties that may be useful for microfluidic applications, while remaining cost-efficient (~$0.01 per device) and optimal for rapid prototyping (fabrication time < 25 min). FDM printing of TPU was shown to be able to produce high aspect ratio channels. Methods to compensate for sagging of bridging layers are provided. The 3D printed TPU was shown to be 85% transparent, durable, flexible, robust, and capable of withstanding high pressures when compared with PDMS. 3D printed TPU was also found to be compatible with cell culture, suggesting its usefulness in many biological applications.

Numerical and experimental investigations into microchannel formation in glass substrate using electrochemical discharge machining

Abstract: Microchannels formed in non-conductive substrates like fused silica, glass and quartz, etc, have wide applications in the field of micro-fluidic and lab-on-chip applications due to their optical transparency, chemical inertness, and biocompatible nature. Electrochemical discharge machining (ECDM) has emerged as a potential low-cost fabrication method to fabricate microfeatures in these materials, compared to conventional laser etching techniques. In this paper, numerical simulation and experimental fabrication of microchannels in a glass substrate using the ECDM based micromilling technique is demonstrated. Stainless steel needle as tool electrode is used in alkaline electrolyte medium. The effects of process parameters viz. tool feed rate, pulse frequency and machining voltage on material removal rate (MRR) and surface roughness (SR) of the microchannels were analysed. The experimental results showed that the MRR and SR increases with an increase in machining voltage and tool feed rate but reduces with an increase in the pulse frequency. Simulations using FEM-based model showed similar trends in MRR with that of experiments. A comparison between the cross-section profiles obtained by the experimental work and predicted profile by the numerical simulation showed some deviation between them due to the Gaussian heat flux assumption in the numerical model. Optical images showed that KOH performance is comparatively better than NaOH with respect to thermal damage and width of cut. Further, multi-objective optimization was performed using utility theory coupled with Taguchi's method to optimize the process parameters. Moreover, the capability of the ECDM process was demonstrated in fabricating various other micro-features such as sinusoidal channel, letter engraving, etc in a glass substrate, which can be extended to other brittle materials like quartz, fused silica, ceramic, etc.

A 5 mg micro-bristle-bot fabricated by two-photon lithography

Abstract: A bristle-bot or vibrobot is a multi-legged robot made of bristles and an oscillating actuator that generates vibrations. This work presents the first demonstration of a micro-bristle-bot, with 3D-printed legs, fabricated by two-photon polymerization (TPP) lithography. The presented miniaturized bristle-bot has a weight of only 5 mg, in the size of 2 mm × 1.87 mm × 0.8 mm, and can achieve a speed up to 4 times the body length per second. A base structure with six legs is fabricated by TPP direct laser writing in a single fabrication step, allowing for rapid prototyping of various leg designs. The base is attached to a 0.3 mm thick lead zirconate titanite (PZT) actuator block. The vibrational energy is provided by an external piezoelectric shaker in this work, which mimics the ocillatory behavior of the on-board PZT block. This work demonstrates the locomotion of micro-bristle-bots with various leg designs that utilizes the resonant bending mode shape at small excitation voltages applied to the external piezoelectric shaker. The presented micro-bristle-bots show a resonant frequency around 6.3 kHz, which can be tailored based on their geometry. This feature allows for addressing individual micro-bristle-bots with various geometries based on their unique resonance frequency.

Performance enhance of CMOS-MEMS thermoelectric infrared sensor by using sensing material and structure design

Abstract: This study presents the micro thermoelectric infrared (IR) sensor consisting of the heat transduction absorber and the serpentine structure with embedded thermocouple using the TSMC 0.18 µm 1P6M standard CMOS process and the in-house post-CMOS MEMS process. The proposed IR absorber design has an umbrella-like structure with a post anchor to the serpentine suspension with embedded thermocouple. Compared to the reference design (IR sensor consisted of only the serpentine structure with embedded thermocouple), a better heat-flow path is achieved and the temperature difference between the hot and cold junctions is increased. Moreover, the umbrella-like structure has higher IR absorption area compared with the serpentine structure. In addition, the Seebeck coefficients of poly-Si films with and without silicide are respectively characterized. The poly-Si with no silicide has a much higher Seebeck coefficient (56-fold), and is employed in this study as the thermocouple material. Experiment results indicate the detectivity of proposed design is 2–2.6 fold higher than that of the reference one at 200 mTorr. Experiment also show that the responsivity enhancement of proposed design is further increased as the sensor size is reduced in area.

Dynamic bifurcation MEMS gas sensors

Abstract: A novel electrostatic MEMS gas sensor is demonstrated. It employs a dynamic-bifurcation detection technique. The sensor detects ethanol vapor in a binary mode, reporting ON-state (1) for concentrations above a preset threshold and OFF-state (0) for concentrations below the threshold. The sensing mechanism exploits the qualitative difference between the sensor state before and after the dynamic pull-in bifurcation.Experimental demonstration was carried out using a laser Doppler vibrometer to measure the sensor response before and after detection. The sensor was able to detect ethanol vapor concentrations as 100 ppb in dry nitrogen. A closed-form expression for the sensitivity of dynamic bifurcation sensors was derived. It captures the dependence of sensitivity on the sensor dimensions, material properties, and electrostatic field.

Extracellular matrix micropatterning technology for whole cell cryogenic electron microscopy studies.

Abstract: Author(s): Engel, Leeya; Gaietta, Guido; Dow, Liam; Swift, Mark; Pardon, Gaspard; Volkmann, Niels; Weis, William; Hanein, Dorit; Pruitt, Beth | Abstract: ABSTRACT Cryogenic electron tomography is the highest resolution tool available for structural analysis of macromolecular organization inside cells. Micropatterning of extracellular matrix (ECM) proteins is an established in vitro cell culture technique used to control cell shape. Recent traction force microscopy studies have shown correlation between cell morphology and the regulation of force transmission. However, it remains unknown how cells sustain increased strain energy states and localized stresses at the supramolecular level. Here, we report a technology to enable direct observation of mesoscale organization in epithelial cells under morphological modulation, using a maskless protein photopatterning method to confine cells to ECM micropatterns on electron microscopy substrates. These micropatterned cell culture substrates can be used in mechanobiology research to correlate changes in nanometer-scale organization at cell-cell and cell-ECM contacts to strain energy states and traction stress distribution in the cell.

Fabrication of infrared hexagonal microlens array by novel diamond turning method and precision glass molding

Abstract: Single-point diamond turning assisted with slow/fast tool servo is a viable technique for complex microlens arrays manufacturing. However, there are still challenges one must overcome when efficiently fabricating microlens arrays with discontinuous features. In this study, a novel slow-tool-servo diamond turning method, termed toroid-surface-based slow tool servo turning method, was proposed for generation of discontinuously structured microlens arrays. A discussion on the advantages for processing discontinuous features was presented and a hexagonal spherical microlens array over a large area was fabricated using the proposed method. This toolpath generation strategy, combined with a segmented turning approach, was adopted with consideration of both the geometry of the diamond tool and the profile of the microlens array. The surface roughness, form accuracy and geometry periodicity, were investigated. Results indicated that the entire microlens array has high homogeneous quality for optical elements. The fabricated microlens array was further utilized as a mold insert in a precision chalcogenide glass molding process. Compared with conventional fabrication methods, this novel technique method can be successfully implemented to fabricate various discontinuous microlens arrays with high accuracy and great efficiency.

Anisotropic etching of Si

Abstract: In this paper, the structure of silicon crystal and the course of anisotropic etching have been presented in a simple way. Connections between etching anisotropy and surface morphology along with these between etching anisotropy and solution composition are shown. Correlation between solution composition and both morphology of the etched surface and shape of structures etched anisotropically using an oxide mask is analyzed. A simple scheme for designing cross-sections of etched structures and a simplified geometrical model of etching concave and convex structures are demonstrated. KOH and TMAH etching solutions, along with solutions containing additions of tensioactive compounds are compared. Special attention is paid to differences among these solutions.

Vibrational modes in MEMS resonators

Abstract: The advances in microfabrication technology enabled micromechanical systems (MEMS) to become a core component in a manifold of applications. For many of these applications the vibrational eigenmodes of a MEMS resonator play a crucial role. However, despite this wide-spread use of the notion of vibrational modes, the general properties of vibrational modes are not well known. In this review, we aim to provide a general overview of various aspects of vibrational modes in MEMS resonators. Vibrational eigenmodes are a type of solution to deformation problems in linear elasticity which give rise to a spatial and a temporal eigenvalue problem coupled by a common eigenvalue. The spatial eigenvalue problem gives determines a mode shape while the temporal eigenvalue problem defines an eigenfrequency at which the structure vibrates. To reduce the modelling complexity, we introduce different simplified elastic models for different eigenmodes like flexural modes in beams and strings in one dimension or plates and membranes in two dimensions. Even though very different eigenmode solutions can be found in MEMS resonators, the dynamics of vibrational modes are governed by generic laws. In the linear regime these laws connect vibrational eigenmodes to the phenomenon of resonance. In the nonlinear regime also other phenomena like bifurcations, multi-stability or parametric amplification are observed. Besides general properties of vibrational eigenmodes, we discuss how different aspect of vibrational modes are utilized in different applications. MEMS timing devices can be optimized by exploring the damping in different eigenmodes and resonant amplification is utilized in the measurement of acceleration, mass or fluid properties. While measurements with MEMS resonators are often performed in the linear regime, the modal dynamics in atomic force microscopy are inherently nonlinear. Beyond classical mechanics, we discuss how vibrational modes in MEMS resonators recently entered the quantum world.

On-ratio PDMS bonding for multilayer microfluidic device fabrication

Abstract: Integrated elastomeric valves, also referred to as Quake valves, enable precise control and manipulation of fluid within microfluidic devices. Fabrication of such valves requires bonding of multiple layers of the silicone polymer polydimethylsiloxane (PDMS). The conventional method for PDMS-PDMS bonding is to use varied ratios of base to crosslinking agent between layers, typically 20:1 and 5:1. This bonding technique, known as "off-ratio bonding," provides strong, effective PDMS-PDMS bonding for multi-layer soft-lithography, but it can yield adverse PDMS material properties and can be wasteful of PDMS. Here we demonstrate the effectiveness of "on-ratio" PDMS bonding, in which both layers use a 10:1 base-to-crosslinker ratio, for multilayer soft lithography. We show the efficacy of this technique among common variants of PDMS: Sylgard 184, RTV 615, and Sylgard 182.

Cascadable Microelectromechanical Resonator Logic Gate

Abstract: Authors acknowledge Mr. Ren Li from Integrated Circuits and Systems Group, CEMSE Division, KAUST for his help with energy cost analysis for CMOS. This publication is based upon work supported by the King Abdullah University of Science and Technology (KAUST) office of sponsored research OSR under Award No. OSR-2016-CRG5-3001.

Characterization of polymer-based piezoelectric micromachined ultrasound transducers for short-range gesture recognition applications

Abstract: This paper deals with the design, simulation and characterization of polymer-based piezoelectric micromachined ultrasound transducers (PMUT) (arrays) intended for short-range gesture recognition applications. The presented process flow is fully compatible with existing flat-panel display fabrication. Finite element models were developed for the evaluation of the frequency response, deflection and acoustic pressure output of single PMUT as a function of the membrane diameter. A laser Doppler vibrometer was used to measure the frequency response, membrane velocity and displacement, as well as mode shapes of the microfabricated PMUT in air. An optical microphone was used to measure the pressure emitted by a single PMUT at various distances along the normal axis of the oscillating membrane. A strong correlation between simulations and measurement results is shown. The device geometries most suitable for short-range gesture recognition purposes are selected and the radiation pattern of square arrays is analyzed using simulations. The resonance properties of single PMUT in an array are determined using measurements. An optimized array is used to demonstrate pulse-echo measurements, and the requirements for a simple gesture recognition platform are elucidated.

Acoustic power transfer for biomedical implants using piezoelectric receivers: effects of misalignment and misorientation

Abstract: Implantable medical devices (IMDs) can be powered wirelessly using acoustics with no need for a battery. In an acoustic power transfer system, which consists of a transmitter, medium, and a receiver, the power that the receiver generates is a function of its position (depth, orientation, and alignment relative to the transmitter). The power delivered to the implant should remain stable and reliable even with possible uncertainties in the location of the implant. In this paper, we compare two common designs for piezoelectric ultrasonic transducers that can be used for acoustically powering IMDs, and study their generated power sensitivity to any change in their location. Although commercial off-the-shelf (COTS) transducers are widely being used in the literature, they may not be the best candidate for powering small implants since they may not be able to provide sufficient power in the presence of location uncertainties. Piezoelectric micromachined ultrasonic transducers (pMUTs) are diaphragm structures and are also suitable for wirelessly powering implants. We present a pMUT receiver and study the sensitivity of the generated power of the pMUT to changes in its position. We then perform a comparative study between power generation capability of our pMUT and a COTS transducer with the same lateral dimensions as the pMUT. We observed that the generated power from a pMUT structure is less sensitive to misorientation and misalignment of the device. The average percentage improvement in the generated power from pMUT compared to COTS are 86%, 917%, and 111% for depth, alignment, and orientation, respectively.

Piezoresistive strain sensor based on monolayer molybdenum disulfide continuous film deposited by chemical vapor deposition

Abstract: In this paper, a centimeter-scale monolayer molybdenum disulfide (MoS2) film deposition method has been developed through a simple low-pressure chemical vapor deposition (LPCVD) growth system. The growth pressure dependence on film quality is investigated in this LPCVD system. The layer nature, electrical characteristic of the as-grown MoS2 films indicate that high quality films have been achieved. In addition, a hydrofluoric acid treated SiO2/Si substrate is used to improve the quality of the MoS2 films. Piezoresistive strain sensor based on the monolayer MoS2 film elements is fabricated by directly patterning metal contact pads on MoS2 films through a silicon stencil mask. A gauge factor of 104 ± 26 under compressive strain is obtained by using a four-point bending method, which may inspire new possibilities for two-dimensional (2D) material-based microsystems and electronics.

Experimental analysis of a valve-less piezoelectric micropump with crescent-shaped structure

Abstract: This study presents a high-performance valve-less piezoelectric micropump with crescent-shaped structure This micropump was designed by finite element analysis and fabricated through three-dimensional printing technology Different configurations of the micropump with different distances from inlet to the crescent-shaped structure (δ = 03, 06, 09, 12, and 15 mm) and numbers of crescent-shaped structures (from one up to four stages) were tested The sound pressure level characteristics were experimentally investigated and evaluated at 06, 15, 20, 25, and 30 mm The results gave a maximum flow rate of 2194 mL/min at 06 mm under 220 V and 45 Hz Moreover, the flow rate for the quadruple crescent-shaped structure was 286 mL/min at 220 V and 82 Hz Besides, the difference of the sound pressure level was 32 dB at 06 m, making this pump better than others; the difference was kept at about 18 dB when the gap exceeded 20 mm The results suggest that the structure with the best configuration is that with δ = 06 mm and four stages, and the sound pressure level fluctuation is smaller as the output performance becomes better The study offers some valuable insights into improving its performance and practical application