Ubiquitous computing has a number of compelling applications ranging from biomedical sensing to environmental monitoring. These computing systems require low cost sensor nodes with volumes ≪1mm3and lifetimes on the order of months or years. We advocate the use of aggressively scaled supply voltages in such applications to maximize energy efficiency. This paper reviews our recent progress in mapping out the low energy design space. We explore the design and test of three low voltage systems targeting ubiquitous computing. We conclude with a survey of open research directions in the ultra-low energy design space.

Lithographically defined microelectrode arrays now permit high-density recording and stimulation in the brain and are facilitating new insights into the organization and function of the central nervous system. They will soon allow more detailed mapping of neural structures than has ever before been possible, and capabilities for highly localized drug-delivery are being added for treating disorders such as severe epilepsy. For chronic neuroscience and neuroprosthesis applications, the arrays are being used in implantable microsystems that provide embedded signal processing and wireless data transmission to the outside world. A 64-channel microsystem amplifies the neural signals by 60 dB with a user-programmable bandwidth and an input-referred noise level of 8$\mu{\hbox{V}}_{\rm rms}$before processing the signals digitally. The channels can be scanned at a rate of 62.5 kS/s, and signals above a user-specified biphasic threshold are transmitted wirelessly to the external world at 2 Mbps. Individual channels can also be digitized and viewed externally at high resolution to examine spike waveforms. The microsystem dissipates 14.14 mW from 1.8 V and measures 1.4$\times$1.55 cm$^2$.

Integrated microsystems merging embedded computing with sensing and actuation are poised to dramatically expand our ability to gather information from the nonelectronic world. Examples include a microassembled multichip electronic interface to the brain, an integrated electrofluidic gas chromatography system for environmental monitoring, and a wireless intra-arterial microsystem for pressure and flow measurements. In general, such microsystems will consist of a few chips, integrated in generic platforms that are customized for a given application by the sensors selected and by software. This paper illustrates this approach with a 0.15-cm/sup 3/ multisensor microsystem for autonomously sensing and storing environmental and biological data. The microsystem is formed using on-board pressure/temperature/humidity sensors, off-board strain gauges and neural/EMG electrodes, a custom sensor-interface chip, a mixed-signal microcontroller, and a nonvolatile memory. These components allow the acquisition and storage of multidomain data at low power levels (< 50 /spl mu/W reading capacitive sensors at 1 Hz). The system is programmable in gain (0.4-3.2 mV/fF), offset (10b), accuracy (14b), and sampling rate (0.1 Hz-10 kHz) and is integrated in a micromachined silicon platform that implements through-wafer interconnects, solder-based microconnectors, and recessed cavities for chip-stacking. The microsystem is realized in 9.5 mm/spl times/7.6 mm/spl times/2.0 mm (0.15 cm/sup 3/) (< 0.5 cm/sup 3/ with a lithium battery).

This paper describes the development of a high-density electronic interface to the central nervous system. Silicon micromachined electrode arrays now permit the long-term monitoring of neural activity in vivo as well as the insertion of electronic signals into neural networks at the cellular level. Efforts to understand and engineer the biology of the implant/tissue interface are also underway. These electrode arrays are facilitating significant advances in our understanding of the nervous system, and merged with on-chip circuitry, signal processing, microfluidics, and wireless interfaces, they are forming the basis for a family of neural prostheses for the possible treatment of disorders such as blindness, deafness, paralysis, severe epilepsy, and Parkinson's disease.

An open-architecture microsystem that can be populated with a variety of sensors and actuators is described. The microsystem is designed for low-power wireless applications where small size and high sensor accuracy are important. It consists of an in-module microcontroller connected to multiple front-end transducers through an intramodule sensor bus. An external interface allows internally processed data to be output through either a hard-wired input/output port or a radio-frequency transmitter. The present microsystem is configured for environmental monitoring and measured temperature, barometric pressure, relative humidity, and acceleration/vibration. It occupies less than 10 cc, consumes an average of 530 /spl mu/W from 6 V, and transmits data up to 50 m. System features such as active power management, the intramodule sensor bus, generic bus interface circuitry, and in-module sensor compensation based on bivariate polynomials are discussed.

The authors describe a high resolution monolithic current stimulating probe which is a multichannel microprobe capable of delivering precisely controlled charge to highly-localized regions of tissue (for example, the auditory nervous system) on a chronic basis. This chip has 8 stimulating sites and provides 7 bits of current level control, that is -126 mu A to +126 mu A with 2 mu A resolution. In order to improve controllability and reduce the cost of the circuitry, the integrated stimulating microprobe is addressable and self-testing. Implemented features include self-test capabilities, a bipolar current forcing scheme for the effective stimulation of the tissue and a user-specified time-out scheme which makes the microstimulator safe. The total number of pads is 14, eight of which are stimulating sites, with the others being used for overall circuit control and power supplies. The chip is designed in an n-epitaxial, p-well, 1.2 mu m, double metal CMOS process with an area of 0.24 mm/sup 2/ excluding bonding pads.>