ERC for Wireless Integrated MicroSystems (WIMS) an NSF-University-Industry Collaboration

Electronic and micromechanical devices/circuits, such as resonant sensors, low-noise amplifiers, and micromechanical resonators, exhibit superior performance when operated at low temperatures. To take advantage of these potential performance gains, low-power microscale cooling systems must be developed that can be integrated easily with a wide variety of microfabricated devices. The goal of this project is to develop microthermoelectric coolers capable of cooling MEMS and electronic devices below 200K, while dissipating less than 100mW of power, all in a chip no larger than a few millimeters on a side.

The medical implant communications service (MICS) is an unlicensed, mobile radio system for transmitting data between an outside control unit and implanted medical devices. The MICS transceiver will enable important new applications such as auditory and visual prosthetics and bio-signal recording. In 1999, the FCC allocated the frequency band of 402–405MHz on a shared secondary use basis for medical implant communication services.

Vacuum packaging of micromachined gyroscopes for use in inertial navigation has been a major challenge. A wafer-level, environmentresistant package that can provide a stable, high-level vacuum has been developed and demonstrated with tuning-fork micromachined gyroscopes created at Georgia Tech.

Single-electron transistors (SET), operating on the principle of Coulomb blockade (CB) in nanostructures, are promising candidates for future ultralow-power and high-density integrated devices. SETs are similar to fieldeffect transistors, but the channel is replaced by a conducting island sandwiched between two tunnel junctions. Because the channel is reduced to a conducting island with dimensions of only a few nanometers, the energy levels in the island are discrete.

Thanks to advances in microtechnology, stand-alone subcutaneous operation of biomedical microsystems is now not only possible, but is facilitating important new applications, such as auditory and visual prostheses and bio-signal recording. To be implantable, a microsystem, in general, needs to receive its operating energy from the outside and be able to communicate with the external world bidirectionally. Implantable microsystems, independent of their application, require wireless interface modules with the general block diagram of Figure 1.

Microdroplets are an emerging trend in microscale biological and chemical analysis systems. Rather than the traditional approach of guiding reagents in microfluidic channels, samples and reagents are instead dispersed as droplets in a continuous oil phase. With volumes in the range of pico- and femtoliters, microdroplets can provide low reagent consumption, fast reaction times, and high throughput for biochemical analyses. The concept of digital microfluidics uses a PFP (Programmable Fluidic Processors) to manipulate droplets real-time; however, the difficulty with many PFPs is that they rely upon droplet interactions with surface electrodes, which results in contamination, sample loss, and limited reusability.

The hippocampus is a structure in the brain that is known to play an important role in the recording of information into long-term memory. The hippocampus is also one of the earliest and hardest hit areas of the brain in Alzheimer’s disease. Professor Gyorgy Buzsáki of Rutgers University, a long-time collaborator and ERC outreach faculty member, has been studying how the neuronal circuits of the brain, especially in the hippocampus region, support its cognitive capabilities. Because neural signals are meaningful only when considered in the context of signaling by a whole population of cells, this is a challenging problem requiring simultaneous recording from a large number of neurons in close proximity to each other.

Chemiresistors (CR) coated with thiolate-monolayer-protected gold nanoparticles (MPN) exhibit a highly sensitive resistance change in response to absorbed vapors and provide extremely low-detection limits. In practical applications, resolution is limited by the precision of measurement circuits and noise sources in the transducer and electronics. Theoretically, the capacitive response of a CR can be used to improve sensitivity, but this parameter has not been adequately explored due in large part to the absence of appropriate instrumentation circuits. The aim of this project is to develop a microelectronic instrumentation circuit that will elucidate, with high resolution, both the resistive and capacitive response of a CR within a platform suitable for monolithic integration of a gas sensor array microsystem.

In most applications of microfabricated sensor arrays to multi-vapor analysis, the devices employed operate on the same transduction principle. It stands to reason that arrays incorporating sensors that operate on different transduction principles should enhance response diversity by probing different aspects of the vapor-interface interaction, thereby improving vapor discrimination. This study explored and compared capabilities for vapor recognition and quantification with polymer-coated single-transducer (ST) and multi-transducer (MT) arrays.

Environmental tobacco smoke (ETS) is a complex mixture of compounds collectively classified by the International Agency for Research on Cancer as carcinogenic. The complexity of ETS and the presence of confounding sources in many environments have impeded accurate exposure assessments and have led to efforts to find surrogate measures of ETS contamination levels. Two such markers are 3-ethenylpyridine (3-EP) and 2,5-dimethylfuran (2,5-DMF). In a study completed this year, selective preconcentration, dual-column separation, and sensor-array detection were combined in a meso-scale gas chromatograph for the determination of these two markers in a complex matrix of indoor air pollutants.

The capability of gas chromatographic (GC) analyzers to separate and quantify the components of complex vapor mixtures renders them invaluable tools for chemical analysis. The WIMS µGC development program exemplifies efforts by several groups around the world to realize a high-performance gas analyzer small enough to fit in a shirt pocket or be deployed unobtrusively in the environment as part of a wireless sensing network. Among several unique features of the WIMS µGC that set it apart from contemporary alternatives is the incorporation of a MEMS vacuum pump to provide gas transport through the microsystem.

A fractional-N frequency synthesizer is a key building block of wireless systems because it can both generate a high-frequency signal with a well-defined frequency and modulate that signal, allowing an entirely wireless transmitter to be implemented with only a fractional-N frequency synthesizer and a power amplifier. Two limitations of this architecture have been overcome: the reliance on complex analog circuitry in deep sub-micron technology, and the trade-off between low-loop bandwidth for good noise rejection and high-loop bandwidth for fast modulation rates.

To improve the performance of sensors, harmful environmental effects must be eliminated. A new generic microsystem packaging technology comprising thermal and vibration isolation, as well as vacuum/hermetic encapsulation, has been developed. Microsystems containing sensors can be mounted on top of an isolation platform fabricated from 100µm-thick glass. The platform is supported and suspended using 100µm-thick glass tethers, which provide excellent thermal isolation and mechanical support. Using a temperature sensor and heater, the microsystem is maintained at constant temperature with minimal power.

Membrane proteins are excellent biological recognition elements that can be embedded with synthetic tethered bilayer lipid membranes (tBLM) to form nanostructured biomimetic interfaces. In this research, we are developing electrochemical impedance spectroscopy (EIS) circuitry that will enable monolithic implementation of biomimetic sensor array microsystems, providing significant improvements in measurement resolution and throughput and manufacturing cost.

Determining the composition of complex mixtures of gases and vapors in situ is critically important to effective security screening, human and ecological exposure assessment, industrial emission monitoring, and biomedical surveillance and diagnosis. The WIMS micro-GC (µGC) is a low-power integrated microsystem designed to meet the needs of all such applications.

Energy-efficient data processing remains one of the primary targets of WIMS research. Any mobile system, from medical implants to environmental sensors, requires robust digital components with a maximum battery lifetime. With the Subliminal Processor, we have recently made important advances in understanding the limits of energy-efficient digital computing.

The costs of petroleum exploration, field development, and long-term geological monitoring potentially can be reduced through the use of microholes and MEMS sensors. In the pursuit of this goal, the WIMS ERC has developed a system for gasphase chemical detection in harsh environments that uses three microplasma-based devices and operates at temperatures up to 200°C.

An important milestone was reached recently in the realization of a microfabricated preconcentrator for gas chromatography. The 44mm3 preconcentrator performs exhaustive sample extraction and injection in a single unit.

Our cortical microsystem continues to demonstrate increasing performance. Most recently, it was used to record wirelessly from the auditory cortex of two guinea pigs over a 30-day period. Two single-shank probes having sixteen 1250µm2 IrO sites on 400µm centers were implanted in each animal, with cables leading to a printed-circuit-board version of the penny-size circuit module.

A new wireless sensor platform, termed Narada, has been both designed and deployed by WIMS ERC researchers for a variety of monitoring and control applications.

The WIMS ERC has successfully fabricated a low-power, hybrid thermo-pneumatic and electrostatic microvalve that enables pressure programming of the WIMS micro gas chromatograph (µGC) columns; it is believed to be the first device of its kind.

A wireless interface chip has been designed and fabricated for a cochlear implant that receives power, data, and clock through an inductive link and in turn transmits neural response and position information prepared by the system to the external setup.

The WIMS ERC is developing an implantable MEMS-based cochlear prosthesis that provides high-density stimulation and embedded position sensing to achieve high-quality sound perception, minimize insertion damage, and optimize implant placement in restoring hearing to the profoundly deaf.

A new drug-delivery probe based on chemical-mechanical-polishing (CMP) and trench-refill techniques has been designed and fabricated as part of the ERC’s efforts to develop implantable neural microsystems. With these new techniques, the dimensions of the channel cross-section can be reduced to less than 50µm2 and channel formation yield is improved to above 90%.

Chronic tissue response induced by tethering is one of the major causes for implant failure in intracortical microelectrodes. We have explored the hypothesis that flexible interconnects can provide strain relief against forces of “micromotion” and hence can result in maintaining healthy tissue surrounding the implant.