Edgar A. Brown

Biologically Inspired Joint Stiffness Control

Biological systems are able to perform movements in unpredictable environments more elegantly than traditionally engineered robotic systems. A current limitation of robotic systems is their inability to simultaneously and independently control both joint angle and joint stiffness without electromechanical feedback loops, which can reduce system stability. In this paper, we describe the development and physical implementation of a servo-actuated robotic joint that uses antagonistic, series-elastic actuation with novel nonlinear spring mechanisms. These mechanisms form a real-time mechanical feedback loop that provides the joint with angle and stiffness control through differential and common-mode actuation of the servos, respectively. This approach to joint control emulates the mechanics of antagonistic muscle groups used by animals, and we experimentally show that it is capable of independently controlling both joint angle and joint stiffness using a simple open-loop control algorithm.

Stimulus-Artifact Elimination in a Multi-Electrode System

To fully exploit the recording capabilities provided by current and future generations of multi-electrode arrays, some means to eliminate the residual charge and subsequent artifacts generated by stimulation protocols is required. Custom electronics can be used to achieve such goals, and by making them scalable, a large number of electrodes can be accessed in an experiment. In this work, we present a system built around a custom 16-channel IC that can stimulate and record, within 3 ms of the stimulus, on the stimulating channel, and within 500 mus on adjacent channels. This effectiveness is achieved by directly discharging the electrode through a novel feedback scheme, and by shaping such feedback to optimize electrode behavior. We characterize the different features of the system that makes such performance possible and present biological data that show the system in operation. To enable this characterization, we present a framework for measuring, classifying, and understanding the multiple sources of stimulus artifacts. This framework facilitates comparisons between artifact elimination methodologies and enables future artifact studies.

An Integrated System for Simultaneous, Multichannel Neuronal Stimulation and Recording

Precision electronics that provide multi-electrode stimulation and recording capabilities are an important tool for the experimental study of neuronal development and plasticity. Towards this end, we present a custom analog integrated circuit (IC), fabricated in a 0.35-mum process, incorporating stimulation buffers and recording preamplifiers for multiple electrodes onto a single die. The architecture of the IC allows for arbitrary, independent configuration of electrodes for stimulation or recording, and the IC includes artifact-elimination circuitry that returns the stimulation electrode to its previous voltage following stimulation, minimizing the interference with recording. We analyze the thermal noise levels in the recording preamplifiers and experimentally measure input-referred noise as low as 4.77 muVrms in the frequency range of 30 Hz-3 kHz at a power consumption of 100 muW from a total power supply of 3.8 V. We also consider the temporal response and stability of the artifact elimination circuitry. We demonstrate that the use of the artifact-elimination circuitry with a 30-mum diameter stimulation electrode permits a return to recording mode in les 2 ms after stimulation, facilitating near-simultaneous stimulation and recording of neuronal signals. (Patent applied for, U.S. No. 2007/0178579.)

Nonlinear antenna technology

Nonlinear antennas combine advances in nonlinear dynamics, active antenna design, and analog microelectronics to generate beam steering and beam forming across an array of nonlinear oscillators. Nonlinear antennas exploit two phenomena typically shunned in traditional designs: nonlinear unit cells and interelement coupling. The design stems from nonlinear coupled differential equation analysis that by virtue of the dynamic control is far less complex than the linear counterparts by eliminating the need for phase shifters and beam forming computers. These advantages arise from incorporating nonlinear dynamics rather than limiting the system to linear quasisteady state operation. A theoretical framework describing beam shaping and beam forming by exploiting the phase, amplitude, and coupling dynamics of nonlinear oscillator arrays is presented. Experimental demonstration of nonlinear beam steering is realized using analog microelectronics.

Models of stimulation artifacts applied to integrated circuit design

The goal of this research is to develop a monolithic stimulation and recording system capable of simultaneous, multichannel stimulation and recording. Monolithic systems are advantageous for large numbers of recording sites because they scale better than systems composed of discrete amplifiers. A major problem in recording systems is the stimulation artifact, a transient distortion present after stimulation. In order to improve recording systems, we analyze models of the stimulation artifact. Comparisons between model predictions and physical measurements verify the models. We show that the linear model, suitable for inclusion in circuit simulators, can assist in the design of an integrated recording system capable of artifact removal. The proposed design occupies 18,000 /spl mu//sup 2/ and is suitable for monolithic integration.

Multielectrode impedance tuning: reducing noise and improving stimulation efficacy

Multielectrode arrays (MEAs) have emerged as a leading technology for extracellular, electrophysiological investigations of neuronal networks. The study of biological neural networks is a difficult task that is further confounded by mismatches in electrode impedance. Electrode impedance plays an important role in shaping incoming signals, determining thermal noise, and influencing the efficacy of stimulation. Our approach to optimally reduce thermal noise and improving the reliability of stimulation is twofold minimize the impedance and match it across all electrodes. To this aim, we have fabricated a device that allows for the automated, impedance-controlled electroplating of micro-electrodes. This device is capable of rapidly (minutes) producing uniformly low impedances across all electrodes in an MEA. The need for uniformly low impedances is important for controlled studies of neuronal networks; this need will increase in the future as MEA technology scales from tens of electrodes to thousands.

A custom multielectrode array with integrated low-noise preamplifiers

Multielectrode arrays (MEAs) have emerged as a leading technology for extracellular neural recording and stimulation. Their large number of recording sites promises to yield important insight into neural systems. As the density of recording sites increases, interfacing to each electrode becomes increasingly difficult. Introducing electronics onto the MEA substrate provides a technique for preliminary signal conditioning to take place at the MEA itself, reducing the complexity of off-package electronics. This paper introduces a custom MEA system with integrated preamplifiers. MEA fabrication, cell-culturing, and electrical performance are discussed.

A parameter-space search algorithm tested on a Hodgkin–Huxley model

We demonstrate a parameter-space search algorithm using a computational model of a single-compartment neuron with conductance-based Hodgkin–Huxley dynamics. To classify bursting (the desired behavior), we use a simple cost function whose inputs are derived from the frequency content of the neural output. Our method involves the repeated use of a stochastic gradient descent-type algorithm to locate parameter values that allow the neural model to produce bursting within a specified tolerance. We demonstrate good results, including those showing that the utility of our algorithm improves as the pre-defined allowable parameter ranges increase and that the initial approach to our method is computationally efficient.

A retrofitted neural recording system with a novel stimulation IC to monitor early neural responses from a stimulating electrode

Extracellular electrical stimulation is increasingly used for in vitro neural experimentation, including brain slices and cultured cells. Although it is desirable to record directly from the stimulating electrode, relatively high stimulation levels make it extremely difficult to record immediately after the stimulation. We have shown that this is feasible by a stimulation system (analog IC) that includes the feature of active electrode discharge. Here, we piggybacked the new IC onto an existing recording amplifier system, making it possible to record neural responses directly from the stimulating channel as early as 3 ms after the stimulation. We used the retrofitted recording system to stimulate and record from dissociated hippocampal neurons in culture. This new strategy of retrofitting an existing system is a simple but attractive approach for instrumentation designers interested in adding a new feature for extracellular recording without replacing already existing recording systems.

Stimulation and recording of neural tissue, closing the loop on the artifact

To achieve the full potential of neural prostheses, such as those used for the treatment of epilepsy, some means to close the loop around the electrical stimulation protocol has to be provided. To be able to close the loop, it is necessary to record the effect of stimulation itself on the surrounding tissue. Such recording is made impossible by the presence of a long-lasting stimulation artifact, which is due to the presence of residual charge in the electrode. Custom electronics, implementing a novel feedback scheme, can reduce or eliminate such charge and, by making the electronics scalable, a large number of electrodes can be simultaneously accessed. In this work we present such an artifact elimination system.