Half-Bridge Silicon Strain Gauges with Arc-Shaped Piezoresistors.

Half-bridge silicon strain gauges are widely used in the fabrication of diaphragm-type high-pressure sensors, but in some applications, they suffer from low output sensitivity because of mounting position constraints. Through a special design and fabrication approach, a new half-bridge silicon strain gauge comprising one arc gauge responding to tangential strain and another linear gauge measuring radial strain was developed using Silicon-on-Glass (SiOG) substrate technology. The tangential gauge consists of grid patterns, such as the reciprocating arc of silicon piezoresistors on a thin glass substrate. When two half-bridges are connected to form a full bridge with arc-shaped gauges that respond to tangential strain, they have the advantage of providing much higher output sensitivity than a conventional half-bridge. Pressure sensors tested under pressure ranging from 0 to 50 bar at five different temperatures indicate a linear output with a typical sensitivity of approximately 16 mV/V/bar, a maximum zero shift of 0.05% FS, and a span shift of 0.03% FS. The higher output level of pressure sensing gauges will provide greater signal strength, thus maintaining a better signal-to-noise ratio than conventional pressure sensors. The offset and span shift curves are quite linear across the operating temperature range, giving the end user the advantage of using very simple algorithms for temperature compensation of offset and span shift.

Reciprocating Arc Silicon Strain Gauges.

Currently, silicon-strain-gauge-based diaphragm pressure sensors use four single-gauge chips for high-output sensitivity. However, the four-single-gauge configuration increases the number of glass frit bonds and the number of aluminum wire bonds, reducing the long-term stability, reliability, and yield of the diaphragm pressure sensor. In this study, a new design of general-purpose silicon strain gauges was developed to improve the sensor output voltage while reducing the number of bonds. The new gauges consist grid patterns with a reciprocating arc of silicon piezoresistors on a thin glass backing. The gauges make handling easier in the bonding process due to the use of thin glass for the gauge backing. The pressure sensors were tested under pressure ranging from 0 to 50 bar at five different temperatures, with a linear output with a typical sensitivity of approximately 16 mV/V/bar and an offset shift of -6 mV to 2 mV. The new approach also opens the possibility to extend arc strain gauges to half-bridge and full-bridge configurations to further reduce the number of glass frit and Al wire bonds in the diaphragm pressure sensor.

Functionalization of Molybdenum Disulfide via Plasma Treatment and 3-Mercaptopropionic Acid for Gas Sensors.

Monolayer and multilayer molybdenum disulfide (MoS2) materials are semiconductors with direct/indirect bandgaps of 1.2-1.8 eV and are attractive due to their changes in response to electrical, physicochemical, biological, and mechanical factors. Since the desired electrical properties of MoS2 are known, research on its electrical properties has increased, with focus on the deposition and growth of large-area MoS2 and its functionalization. While research on the large-scale production of MoS2 is actively underway, there is a lack of studies on functionalization approaches, which are essential since functional groups can help to dissolve particles or provide adequate reactivity. Strategies for producing films of functionalized MoS2 are rare, and what methods do exist are either complex or inefficient. This work introduces an efficient way to functionalize MoS2. Functional groups are formed on the surface by exposing MoS2 with surface sulfur vacancies generated by plasma treatment to 3-mercaptopropionic acid. This technique can create 1.8 times as many carboxyl groups on the MoS2 surface compared with previously reported strategies. The MoS2-based gas sensor fabricated using the proposed method shows a 2.6 times higher sensitivity and much lower detection limit than the untreated device.

Enhancement of Withstand Voltage in Silicon Strain Gauges Using a Thin Alkali-Free Glass.

We present a cost-effective approach to produce silicon strain gauges that can withstand very high voltage without using any complex package design and without sacrificing any sensor performance. This is achieved by a special silicon strain gauge structure created on an alkali-free glass substrate that has a high breakdown voltage. A half-bridge silicon strain gauge is designed, fabricated, and then tested to measure its output characteristics. The device has a glass layer that is only 25-55 µm thick; it shows it is able to withstand a voltage of over 2000 V while maintaining a high degree of linearity with correlation coefficients higher than 0.9990 and an average sensitivity of 104.13. Due to their unique electrical properties, silicon strain gauges-on-glass chips hold much promise for use in advanced force and pressure sensors.

Enhanced Stability and Amplified Signal Output of Single-Wall Carbon Nanotube-Based NH3-Sensitive Electrode after Dual Plasma Treatment.

Pristine nanomaterials are normally prepared using finely controlled fabrication processes. Because no imperfect nanostructure remains, they cannot be used directly as electrode substrates of functional devices. This is because perfectly organized nanostructures or nanomaterials commonly require posttreatment to generate intentionally, the kinds of desirable defects inside or on their surfaces that enable effective functionalization. Plasma treatment is an easier, simpler and more widely used way (relative to other methods) to modify a variety of nanomaterials, although plasma-functionalized nano surfaces commonly have a short lifetime. We present herein a dual plasma treatment (DPT) that significantly enhances the degree and lifetime of plasma-induced surface functional groups on single-walled carbon nanotubes (SWCNTs). The DPT process consists of two individually optimized oxygen-plasma treatments. The DPT-modified SWCNT functioned as a sensing material for ammonia gas for more than a month. It also provided more than three times the degree of functionality for amplified signal output than with a single-plasma-treated SWCNT electrode.

A Micromachined Metal Oxide Composite Dual Gas Sensor System for Principal Component Analysis-Based Multi-Monitoring of Noxious Gas Mixtures.

Microelectronic gas-sensor devices were developed for the detection of carbon monoxide (CO), nitrogen dioxides (NO2), ammonia (NH3) and formaldehyde (HCHO), and their gas-sensing characteristics in six different binary gas systems were examined using pattern-recognition methods. Four nanosized gas-sensing materials for these target gases, i.e., Pd-SnO2 for CO, In2O3 for NOX, Ru-WO3 for NH3, and SnO2-ZnO for HCHO, were synthesized using a sol-gel method, and sensor devices were fabricated using a microsensor platform. Principal component analysis of the experimental data from the microelectromechanical systems gas-sensor arrays under exposure to single gases and their mixtures indicated that identification of each individual gas in the mixture was successful. Additionally, the gas-sensing behavior toward the mixed gas indicated that the traditional adsorption and desorption mechanism of the n-type metal oxide semiconductor (MOS) governs the sensing mechanism of the mixed gas systems.

A Fully Integrated Paper-Microfluidic Electrochemical Device for Simultaneous Analysis of Physiologic Blood Ions.

A fully integrated paper microfluidic electrochemical device equipped with three different cation permeable films is developed to determine blood ions (Cl-, Na⁺, K⁺, and Ca2+) at a time. These blood ions that are normally dissolved in the real human blood stream are essential for cell metabolisms and homeostasis in the human body. Abnormal concentration of blood ions causes many serious disorders. The optimized microfluidic device working without any external power source can directly and effectively separate human blood components, and subsequently detect a specific blood ion with minimized interference. The measured sensitivity to Cl-, K⁺, Na⁺, and Ca2+ are -47.71, 45.97, 51.06, and 19.46 in mV decade-1, respectively. Potentiometric responses of the microfluidic devices to blood serum samples are in the normal ranges of each cation, and comparable with responses from the commercial blood ion analyzer Abbott i-Stat.

Buckling Structured Stretchable Pseudocapacitor Yarn.

Cable-type stretchable electrochemical pseudocapacitors based on multi-walled carbon nanotube (MWCNT) sheets and two different metal oxide nanopowders (NP), i.e., MnO2 and RuO2 are developed using a newly-devised dry painting method to mechanically fix the NP to the elastic rubber-based MWCNT electrode substrate, resulting in a porous buckling structured pseudocapacitor yarn. Highly stretchable stylene-ethylene/butylene-stylene (SEBS) is used as the supporting elastomeric core for wrapping with the MWCNT sheets and the electroactive NP. The dry painting can successfully deposit NP on the soft SEBS surface, which is normally an unfavorable substrate for coating alien materials. The resulting yarn-type pseudocapacitor, composed of eight-layered MWCNT sheets, three-layered RuO2, and two-layered MnO2, showing a diameter of approximately 400 µm with a porous buckling structure, records a specific capacitance of 25 F g-1. After being stretched by 200% in strain with no sacrifice of the porous buckling structure, the cable-type stretchable electrochemical pseudocapacitor yarn retains its electrical capacity, and is potentially applicable to energy storage devices for wearable electronics.

Evaluation of Surface Cleaning Procedures in Terms of Gas Sensing Properties of Spray-Deposited CNT Film: Thermal- and O2 Plasma Treatments.

The effect of cleaning the surface of single-walled carbon nanotube (SWNT) networks by thermal and the O2 plasma treatments is presented in terms of NH3 gas sensing characteristics. The goal of this work is to determine the relationship between the physicochemical properties of the cleaned surface (including the chemical composition, crystal structure, hydrophilicity, and impurity content) and the sensitivity of the SWNT network films to NH3 gas. The SWNT networks are spray-deposited on pre-patterned Pt electrodes, and are further functionalized by heating on a programmable hot plate or by O2 plasma treatment in a laboratory-prepared plasma chamber. Cyclic voltammetry was employed to semi-quantitatively evaluate each surface state of various plasma-treated SWNT-based electrodes. The results show that O2 plasma treatment can more effectively modify the SWNT network surface than thermal cleaning, and can provide a better conductive network surface due to the larger number of carbonyl/carboxyl groups, enabling a faster electron transfer rate, even though both the thermal cleaning and the O2 plasma cleaning methods can eliminate the organic solvent residues from the network surface. The NH3 sensors based on the O2 plasma-treated SWNT network exhibit higher sensitivity, shorter response time, and better recovery of the initial resistance than those prepared employing the thermally-cleaned SWNT networks.

Effect of oxygen plasma treatment on carbon nanotube-based sensors.

We present the research results of the use of plasma modification for the fabrication of carbon nanotube-based devices for chemical and biological sensing. The oxygen plasma treatment of multiwalled carbon nanotubes (MWCNTs) effectively grafts oxygen atoms onto the CNT surface. For investigating the impact of plasma modification on the MWCNT-based sensor performance, three different sensors are fabricated: NH3 gas sensors, humidity sensors, and immunosensors. The plasma-modified MWCNTs (p-MWCNTs) exhibit a sensitivity to NH3 that is approximately twice that of the corresponding untreated sensor. The humidity sensor with a p-MWCNT top electrode exhibits a much faster response time compared with the untreated MWCNT electrodes. The p-MWCNT immunosensor exhibits a detection limit almost 1000 times lower than that of the standard ELISA assay, while the untreated MWCNTs exhibit no detectable signal. These results imply that the oxygen-containing functional groups on the CNT surface significantly affect the performance of the CNT-based chemical and biological sensors.

Percolated carbon nanotube networks for high-speed humidity sensors.

Improving the response time for polyimide (PI)-based capacitive humidity sensors is critical for real-time sensing. Multi-walled carbon nanotube (MWCNT) films were used to form the upper electrode of humidity sensor to realize an extremely short response times. MWCNT films were spray-deposited on a moisture-sensitive PI layer and subsequently patterned by oxygen plasma. Random-network MWCNT electrodes have a well-entangled and open porous structure that is almost impossible to obtain with conventional metal electrodes. Compared with porous metal electrode-based sensors as an upper electrode, the fabricated capacitive humidity sensors with MWCNT electrodes showed an exceptionally short response time of less than 2.5 s and a good linearity of 0.998. An analysis of the long-term (100 days) stability data revealed that the MWCNT electrode humidity sensors showed little drift even after 100 days aging, indicating that they are suitable for practical and reliable humidity measurements. These improvements in performance may stem from their interconnected microscopic porous structure, which is more accessible to water molecules through the conductive electrode.

Design and fabrication of vibration based energy harvester using microelectromechanical system piezoelectric cantilever for low power applications.

We fabricated dual-beam cantilevers on the microelectromechanical system (MEMS) scale with an integrated Si proof mass. A Pb(Zr,Ti)O3 (PZT) cantilever was designed as a mechanical vibration energy-harvesting system for low power applications. The resonant frequency of the multilayer composition cantilevers were simulated using the finite element method (FEM) with parametric analysis carried out in the design process. According to simulations, the resonant frequency, voltage, and average power of a dual-beam cantilever was 69.1 Hz, 113.9 mV, and 0.303 microW, respectively, at optimal resistance and 0.5 g (gravitational acceleration, m/s2). Based on these data, we subsequently fabricated cantilever devices using dual-beam cantilevers. The harvested power density of the dual-beam cantilever compared favorably with the simulation. Experiments revealed the resonant frequency, voltage, and average power density to be 78.7 Hz, 118.5 mV, and 0.34 microW, respectively. The error between the measured and simulated results was about 10%. The maximum average power and power density of the fabricated dual-beam cantilever at 1 g were 0.803 microW and 1322.80 microW cm(-3), respectively. Furthermore, the possibility of a MEMS-scale power source for energy conversion experiments was also tested.

Investigation of ultraviolet optical properties of semiconducting-enriched and metal-enriched single-walled carbon nanotube networks using spectroscopic ellipsometry.

The ultraviolet optical properties of semiconducting-enriched and metallic-enriched single-walled carbon nanotube (semi-enriched and m-enriched SWCNT) networks were studied using spectroscopic ellipsometry. According to calculated energy loss function, the energy loss peak assigned to the maximum intensity of π-plasmon energy was found to increase from 4.5 eV to 5.0 eV as SWCNT network composition was changed from m-SWCNT enriched to semi-SWCNT enriched. These results clearly demonstrate that the dielectric response in the 4-6 eV range is sensitive to changes in the surrounding dielectric environment depending on the semi-/m-SWCNT content. Therefore, the spectral shift of this energy loss is attributed to the enhanced electron confinement by the presence of the surface plasmon due to a small amount of m-SWCNT, which is an important phenomenon at the SWCNT network.

Covalent attachment of biomacromolecules to plasma-patterned and functionalized carbon nanotube-based devices for electrochemical biosensing.

The interface between biomacromolecules and carbon nanotubes (CNTs) is of critical importance in developing effective techniques that provide CNTs with both biomolecular recognition and signal transduction through immobilization. However, the chemical inertness of CNT surfaces poses an obstacle to wider implementation of CNTs in bioanalytical applications. In this paper, we present a review of our recent research activities related to the covalent attachment of biomacromolecules to plasma-patterned and functionalized carbon nanotube films and their application to the fabrication of electrochemical biosensing devices. The SWCNT films were spray-deposited onto a miniaturized three-electrode system on a glass substrate and activated using highly purified atomic oxygen generated in radiofrequency plasma; this introduced oxygen-containing functional groups into the SWCNT surface without fatal loss of the original physicochemical properties of the CNTs. The carboxylated SWCNT electrodes were then selectively modified via amidation or esterification for covalent immobilization of the biomacromolecules. The plasma-treated SWCNT-based sensing electrode had an approximately six times larger effective area than the untreated SWCNT-based electrode, which significantly amplified the amperometric electrochemical signal. Finally, the efficacy of plasma-functionalized SWCNT (pf-SWCNT) as a biointerface was examined by immobilizing glucose oxidase, Legionella pneumophila ( L. pneumophila)-specific antibodies, L. pneumophila-originated DNAs, and thrombin-specific aptamers on the pf-SWCNT-based three-electrode devices. The pf-SWCNT films were found to support direct covalent immobilization of the above-listed biomacromolecules on the films and to thereby overcome the many drawbacks typically associated with simple physisorption. Thus, pf-SWCNT sensing electrodes on which biomacromolecules were covalently immobilized were found to be chemically stable and have a long lifetime.

Micromachining of a bimorph Pb(Zr,Ti)O3 (PZT) cantilever using a micro-electromechanical systems (MEMS) process for energy harvesting application.

We designed and fabricated a bimorph Pb(Zr,Ti)O3 (PZT) cantilever with an integrated Si proof mass to obtain a low resonant frequency for an energy harvesting application. The cantilevers were fabricated on the micro-electromechanical systems (MEMS) scale. A mode of piezoelectric conversions were d31 and d33 mode in cantilever vibration Therefore, we designed and fabricated a single cantilever with d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes. Finally, we fabricated a device with beam dimensions of about 5,400 microm x 480 microm x 14 microm (< +/- 5%), and an integrated Si proof mass with dimensions of about 1,481 microm x 988 microm x 450 microm (< +/- 5%). In order to measure the d31 and d33 modes, we fabricated top and bottom electrodes. The distance between the top electrodes was 50 microm and the resonant frequency was 89.4 Hz. The average powers of the d31 unimorph, d31 bimorph, d33 unimorph, and d33 bimorph modes were 3.90, 9.60, 21.42, and 22.47 nW at 0.8 g (g = 9.8 m/s2) and optimal resistance, respectively.

Study on the micro-heater geometry in In,2O3 micro electro mechanical systems gas sensor platforms and effects on NO2 gas detecting performances.

Micro electro mechanical systems (MEMS) platforms for gas sensing devices with the co-planar type micro-heaters were designed, fabricated and its effects on the In2O3 gas sensors were investigated. Micro-heaters in MEMS gas sensor platforms were designed in the four-type heater patterns with different geometries. Electro-thermal characterizations showed that the designed platforms had highly thermal efficiency because the micro hot-plate structures were formed in the diaphragm and the thermal efficiencies were analyzed for all of 16 models and compared with each other, respectively. The designed micro-platforms were fabricated by MEMS process, and Indium oxide (In2O3) nanoparticles were synthesized by sol-gel process and dropped on the MEMS platforms for detecting the noxious oxide gas (NO2) Fabricated micro-platforms had a very low power consumption in the fabricated 16-type models, especially, the minimum power consumption was 41 mW at the operating temperature of 250 degrees C. After experiments on gas sensing characteristics to NO2 gases, fabricated In2O3 gas sensors had almost the same gas sensitivity (Rs) at the operation temperature of 250 degrees C. It is concluded that the micro-heater geometries, pattern shapes and sizes, can be influential on the power consumption of the devices and its gas sensing characteristics.

A fully microfabricated carbon nanotube three-electrode system on glass substrate for miniaturized electrochemical biosensors.

We present an integration process to fabricate single-walled carbon nanotube (SWCNT) three-electrode systems on glass substrate for electrochemical biosensors. Key issues involve optimization of the SWCNT working electrode to achieve high sensitivity, developing an optimal Ag/AgCl reference electrode with good stability, and process development to integrate these electrodes. Multiple spray coatings of the SWCNT film on glass substrate enabled easier integration of the SWCNT film into an electrochemical three-electrode system. O2 plasma etching and subsequent activation of spray-coated SWCNT films were needed to pattern and functionalize the SWCNT working electrode films without serious damage to the SWCNTs, and to remove organic residues. The microfabricated three-electrode systems were characterized by microscopic and spectroscopic techniques, and the electrochemical properties were investigated using cyclic voltammetry and chrono-amperometry. The fully-integrated CNT three-electrode system showed an effective working electrode area about three times larger than its geometric surface area and an improved electrochemical activity for hydrogen peroxide decomposition. Finally, the effectiveness of miniaturized pf-SWCNT electrodes as biointerfaces was examined by applying them to immunosensors to detect Legionella(L) pneumophila, based on a direct sandwich enzyme-linked immunosorbent assay (ELISA) format with 3,3',5,5'-tetramethylbenzidine dihydrochloride/hydrogen peroxide(TMB/H2O2) as the substrate/mediator system. The lower detection limit of the pf-SWCNT-based immunosensors to L. pneumophila is about 1500 times lower than that of the standard ELISA assay.

Plasma-activated carbon nanotube-based high sensitivity immunosensors for monitoring Legionella pneumophila by direct detection of maltose binding protein peptidoglycan-associated lipoprotein (MBP-PAL).

Transferred multi-walled carbon nanotube (MWCNT)-modified platinum thin-film immunosensing electrode material was engineered on a glass substrate and fabricated a fully-integrated electrochemical three-electrode system for monitoring Legionella pneumophila. The transferred MWCNT film was treated with oxygen plasma to improve its electrochemical response and electrical conductivity. We voltammetrically characterized and optimized the electrochemical performance of the fabricated electrode for direct detection of Legionella pneumophila-specific peptidoglycan-associated lipoprotein (PAL) and maltose binding protein (MBP) peptidoglycan-associated lipoprotein (MBP-PAL) fusion. The latter, as an intermediate product to yield the former, has important roles in the growth and purification of PAL, which commercial enzyme-linked immunosorbent assay (ELISA) kits require as a target substrate. Consequently, direct electrochemical detection of MBP-PAL compared to PAL by square-wave voltammetry showed a greater than 50% increase in sensitivity with a lower detection limit of 5 pg mL(-1). We also investigated the affinity properties by determining kinetic parameters of the PAL and the MBP-PAL in relation to polyclonal antibodies immobilized on transferred MWCNT substrates using Michaelis-Menten assumptions and a Hanes-Woolf plot. This new method presented herein could save the time and effort for the separation and purification of PAL form MBP-PAL fusions that are required for performing ELISA-based immunoassay.

Design and fabrication of a PZT cantilever for low frequency vibration energy harvesting.

In this study, a PZT cantilever with a Si proof mass is designed and fabricated for a low frequency energy harvesting application. A mathematical model of a multi-layer composite beam was derived and applied in a parametric analysis of the piezoelectric cantilever. Finally, the dimensions of the cantilever were determined for the resonant frequency of the cantilever. Our cantilever design was based on MATLAB and ANSYS simulations. For this simulation, the proof mass volumes were varied from 0 to 0.5 mm3 and resonant frequencies were calculated from 833.5 Hz to 125.5 Hz, respectively. Based on simulation, we fabricated a device with beam dimensions of about 4.10 mm x 0.48 mm x 0.012 mm, and an integrated Si proof mass with dimensions of about 0.481 mm x 0.48 mm x 0.45 mm. The resonant frequency, maximum peak voltage, and highest average power of the cantilever device were 224.8 Hz, 4.8 mV, and 2.24 nW, respectively.

Microfabrication and characterization of spray-coated single-wall carbon nanotube film strain gauges.

We present the design, fabrication, and characterization results of single-wall carbon nanotube (SWCNT) film strain gauges for potential applications as highly sensitive strain, weight, or pressure sensors on the macro-scale. A batch microfabrication process was developed for practical device construction and packaging using spray-coated SWCNTs and a conventional semiconductor process. The prototype was characterized using a commercial metal foil gauge with tensile and compressive testing on a binocular load cell. Our test results demonstrated that the proposed SWCNT film gauges have a linear relationship between resistance changes and externally applied strain. The gauge factor ranged from 7.0 to 16.4 for four different micro-grid configurations, indicating that the maximum strain sensitivity of the prototype was approximately eight times greater than that of commercial gauges.