Cuong M. Nguyen

Sol-Gel Iridium Oxide-Based pH Sensor Array on Flexible Polyimide Substrate

Iridium oxide pH sensing film is demonstrated with wide pH-sensing ranges, high durability, and small drifts in potentials. Using sol-gel process, a lower fabrication cost and less labor-intensive method, to deposit iridium oxide thin films for pH sensing is reported previously by our group with expected advantages. In this paper, we fabricate and test pH sensing characteristics of 4 × 4 anhydrous iridium oxide thin-film electrode arrays on flexible substrates. The sensors in arrays exhibit Nernstian potential responses in the range of 57.0-63.4 mV/pH. Stability, repeatability, and hysteresis effects of the pH sensor arrays are examined. A multichannel recording system is built to demonstrate the functionality of the pH sensor arrays in monitoring spatial and temporal pH changes across a surface.

An Integrated Flexible Implantable Micro-Probe for Sensing Neurotransmitters

In this work, we have developed an integrated flexible implantable probe on a polyimide-film substrate for sensing neurotransmitters. The flexibility of the probe helps to prevent scar forming in tissues aiming for long-term in vivo monitoring. A micro Ag/AgCl reference electrode was integrated in the same probe with the Au/Cr or Pt/Cr working electrodes providing measurements without the need of a separate reference probe. Several electrode configurations have been considered and designed for implantation at various locations in the central nervous system. The prototype device for proof of principle was an enzyme-based electrochemical L-glutamate sensor using L-glutamate oxidase. A comparison between Au and Pt thin films was conducted by cyclic voltammetry to evaluate their performance as working electrodes. The L-glutamate oxidase was deposited on the working electrodes followed by a meta-Phenylenediamine electropolymerization process to improve the selectivity. The self-referencing technique was also utilized to enhance both the limit of detection and selectivity. The assembled sensors were calibrated and tested at various concentrations of L-glutamate with and without the presence of interfering molecules. The results showed good sensitivity and selectivity. In vivo animal tests were conducted to show the capability of detecting changes of electrochemical signals responding to graded peripheral somatosensory stimuli.

Development and Characterization of a Novel Interdigitated Capacitive Strain Sensor for Structural Health Monitoring

Monitoring of structural health plays a crucial role in condition-based maintenance and degradation prediction of infrastructures. In this paper, we developed a novel interdigitated capacitive (IDC) strain sensor which could be integrated in a wireless monitoring system for structural health monitoring (SHM) applications. The IDC sensors were fabricated by laser-micromachining a 127-μm-thick brass sheet followed by encapsulation in deformable thermoset polymers. The wireless monitoring system was implemented using a commercial wireless module (eZ430-RF2500 from Texas Instruments) which could provide multi-modality monitoring simultaneously. A graphical user interface was developed to display and store data as well as perform real-time analysis remotely. The wireless communication distance was up to 35 m inside buildings. The sensitivity of the sensor was characterized in both stretching and bending aspects, yielding a limit of detection with respect to strain of 0.025%. The gauge factor was found in the range of 6-9 which is much higher than those of commercially available strain gauges. The bending detection is reliable up to 20°. Hysteresis and temperature dependence were also investigated, revealing predictable responses. Finally, the entire system was demonstrated with both single and multiple sensors for a real-time SHM case.

Micro pH Sensors Based on Iridium Oxide Nanotubes

Iridium oxide (IrOx) nanotubes have been grown on microelectrodes utilizing a patterned nanoporous aluminum oxide (AAO) template to form a micro pH sensor. A layer of 900-nm thick aluminum sputtered on a silicon wafer was locally anodized at various voltages to investigate the morphology of the patterned AAO templates. The electrode position of IrOx inside the supportive AAO template was carried out to yield IrOx nanotubes with a diameter ranging from 80 to 110 nm, corresponding to the AAO pore sizes. The fabrication processes produced free-standing IrOx nanotubes on top of the 50 × 100 μm2 micro-scale electrodes. Material characterization along with fabrication parameters was investigated. Potentiometric responses of the miniature sensors to hydrogen ions in terms of sensitivity, repeatability, response time, and reliability were performed. Super-Nernstian response was achieved. pH measurements for higher resolution targeting biological applications and sensor temperature dependence were also conducted.

Wireless sensor nodes for environmental monitoring in Internet of Things

Wireless networking sensor nodes for environment monitoring in Internet of Things (IOT) are reported in this work. The IOT network includes individual self-sustaining nodes wirelessly transmitting sensor signals to hubs that can be shared in the Internet Cloud. Each node consists of an optimized energy harvesting module, a System-on-Chip (SoC) integrated low-power Bluetooth Smart transceiver, and multi-functional sensor array to monitor environmental parameters. The energy harvesting module is able to adapt and collect energy from solar power, ambient radio waves, and direct wireless power transmission (WPT). The sensor arrays include pH sensor, temperature sensor, photo-detector, electromagnetic wave detector and acoustic noise detector. The SoC processes data and transmits compressed information about environmental conditions to the hub. This platform demonstrated the concepts of combining power harvesting techniques and low-power sensors for the IoT applications.

Wireless Power Transfer for Autonomous Wearable Neurotransmitter Sensors

In this paper, we report a power management system for autonomous and real-time monitoring of the neurotransmitter L-glutamate (L-Glu). A low-power, low-noise, and high-gain recording module was designed to acquire signal from an implantable flexible L-Glu sensor fabricated by micro-electro-mechanical system (MEMS)-based processes. The wearable recording module was wirelessly powered through inductive coupling transmitter antennas. Lateral and angular misalignments of the receiver antennas were resolved by using a multi-transmitter antenna configuration. The effective coverage, over which the recording module functioned properly, was improved with the use of in-phase transmitter antennas. Experimental results showed that the recording system was capable of operating continuously at distances of 4 cm, 7 cm and 10 cm. The wireless power management system reduced the weight of the recording module, eliminated human intervention and enabled animal experimentation for extended durations.

Flexible Sputter-Deposited Carbon Strain Sensor

In this letter, we report a piezoresistive amorphous carbon strain sensor fabricated on a flexible polyimide substrate. Amorphous carbon was sputter-deposited onto a 125-μm-thick polyimide film and the strain gauges were tailored using laser micromachining. The strain sensors were tested by using a cantilever setup to obtain sensitivity, hysteresis, repeatability, and dynamic response. Gauge factors in the range of 6-9 have been achieved.

Sol-gel deposition of iridium oxide for biomedical micro-devices.

Flexible iridium oxide (IrOx)-based micro-electrodes were fabricated on flexible polyimide substrates using a sol-gel deposition process for utilization as integrated pseudo-reference electrodes for bio-electrochemical sensing applications. The fabrication method yields reliable miniature on-probe IrOx electrodes with long lifetime, high stability and repeatability. Such sensors can be used for long-term measurements. Various dimensions of sol-gel iridium oxide electrodes including 1 mm × 1 mm, 500 μm × 500 μm, and 100 μm × 100 μm were fabricated. Sensor longevity and pH dependence were investigated by immersing the electrodes in hydrochloric acid, fetal bovine serum (FBS), and sodium hydroxide solutions for 30 days. Less pH dependent responses, compared to IrOx electrodes fabricated by electrochemical deposition processes, were measured at 58.8 ± 0.4 mV/pH, 53.8 ± 1.3 mV/pH and 48 ± 0.6 mV/pH, respectively. The on-probe IrOx pseudo-reference electrodes were utilized for dopamine sensing. The baseline responses of the sensors were higher than the one using an external Ag/AgCl reference electrode. Using IrOx reference electrodes integrated on the same probe with working electrodes eliminated the use of cytotoxic Ag/AgCl reference electrode without loss in sensitivity. This enables employing such sensors in long-term recording of concentrations of neurotransmitters in central nervous systems of animals and humans.

A wearable system for highly selective L-glutamate neurotransmitter sensing

The development of a wearable recording system to acquire L-glutamate (L-Glu) neurotransmitter signals from the central nervous system (CNS) was reported. The system utilized miniature (50×100 μm2) implantable amperometric sensors fabricated on a flexible polyimide substrate to minimize in vivo tissue damage. Selectivity to L-Glu was improved through implementation of self-referencing electrode and noise cancellation to minimize background noises and interference from other neurotransmitters. A microcontroller-based data acquisition system and a wireless transceiver were utilized to transmit measurements wirelessly to computer base stations and smart phones in real time. The experiments demonstrated the feasibility for monitoring neurotransmitters in freely-behaving animals and potentially human objects.

Lactate Sensors on Flexible Substrates.

Lactate detection by an in situ sensor is of great need in clinical medicine, food processing, and athletic performance monitoring. In this paper, a flexible, easy to fabricate, and low-cost biosensor base on lactate oxidase is presented. The fabrication processes, including metal deposition, sol-gel IrOx deposition, and drop-dry enzyme loading method, are described in detail. The loaded enzyme was examined by scanning electron microscopy. Cyclic voltammetry was used to characterize the sensors. Durability, sensibility, and selectivity of the biosensors were examined. The comparison for different electrode sizes and different sensing film materials was conducted. The sensor could last for four weeks with an average surface area normalized sensitivity of 950 nA/(cm2 mM) and 9250 nA/(cm2 mM) for Au-based electrodes, and IrOx-modified electrodes respectively, both with an electrode size of 100 × 50 μm. The self-referencing method to record noises simultaneously with the working electrode greatly improved sensor sensitivity and selectivity. The sensor showed little response to interference chemicals, such as glutamate and dopamine.