SOLID-STATE ELECTROMETERS WITH INPUT-CAPACITANCE NEUTRALIZATION

An Insulated Flexible Sensor for Stable Electromyography Detection: Applicationto Prosthesis Control

Electromyography (EMG), the measurement of electrical muscle activity, is used in a variety of applications, including myoelectric upper-limb prostheses, which help amputees to regain independence and a higher quality of life. The state-of-the-art sensors in prostheses have a conductive connection to the skin and are therefore sensitive to sweat and require preparation of the skin. They are applied with some pressure to ensure a conductive connection, which may result in pressure marks and can be problematic for patients with circulatory disorders, who constitute a major group of amputees. Due to their insulating layer between skin and sensor area, capacitive sensors are insensitive to the skin condition, they require neither conductive connection to the skin nor electrolytic paste or skin preparation. Here, we describe a highly stable, low-power capacitive EMG measurement set-up that is suitable for real-world application. Various flexible multi-layer sensor set-ups made of copper and insulating foils, flex print and textiles were compared. These flexible sensor set-ups adapt to the anatomy of the human forearm, therefore they provide high wearing comfort and ensure stability against motion artifacts. The influence of the materials used in the sensor set-up on the magnitude of the coupled signal was demonstrated based on both theoretical analysis and measurement.The amplifier circuit was optimized for high signal quality, low power consumption and mobile application. Different shielding and guarding concepts were compared, leading to high SNR.

Preamplifiers for non-contact capacitive biopotential measurements

Non-contact biopotential sensing is an attractive measurement strategy for a number of health monitoring applications, primarily the ECG and the EEG. In all such applications a key technical challenge is the design of a low-noise trans-impedance preamplifier for the typically low-capacitance, high source impedance sensing electrodes. In this paper, we compare voltage and charge amplifier designs in terms of their common mode rejection ratio, noise performance, and frequency response. Both amplifier types employ the same operational-transconductance amplifier (OTA), which was fabricated in a 0.35 um CMOS process. The results show that a charge amplifier configuration has advantages for small electrode-to-subject coupling capacitance values (less than 10 pF--typical of noncontact electrodes) and that the voltage amplifier configuration has advantages for electrode capacitances above 10 pF.

Micropower non-contact EEG electrode with active common-mode noise suppression and input capacitance cancellation

A non-contact EEG electrode with input capacitance neutralization and common-mode noise suppression circuits is presented. The coin sized sensor capacitively couples to the scalp without direct contact to the skin. To minimize the effect of signal attenuation and channel gain mismatch, the input capacitance of each sensor is actively neutralized using positive feedback and bootstrapping. Common-mode suppression is achieved through a single conductive sheet to establish a common mode reference. Each sensor electrode provides a differential gain of 60 dB. Signals are transmitted in a digital serial daisy-chain directly from a local 16-bit ADC, minimizing the number of wires required to establish a high density EEG sensor network. The micropower electrode consumes only 600 microW from a single 3.3 V supply.

Membrane potentials recorded from the mucosa of the toad's tongue during chemical stimulation

An isolated stretch of tongue mucosa was obtained from the Chilean toad (Callyptocephalella gayi). The preparation was observed under transmitted illumination through a binocular microscope. The surface cells were impaled with micro-electrodes and different chemical agents were applied to the area. The following results were obtained.1. The surface cells had resting potentials of -6 to -40 mV (mean of -17.6 mV) with the preparation bathed in Ringer solution.2. The cells underwent depolarization by application of different salts (NaCl, NaF, KCl, Na(2)SO(4), CaCl(2) and MgCl(2)) in concentrations of 0.25-1.0 M. The potentials evoked by the salt solutions often overshot the zero membrane potential level by several millivolts. The positive-going potential change produced by application of salts was increased during hyperpolarization of the membrane by inward current and was decreased by current of the opposite sign. Application of salts during depolarization of the membrane to a certain positive level produced a negative-going potential change. The potentials evoked by different salts were about the same, qualitatively, when recordings were made from different areas of the tongue, i.e. top of the fungiform and filiform papillae, doughnut-shaped folds at the base of the fungiform papillae, areas between papillae and from the side of the tongue totally devoid of papillary structures.3. Quinine applied in concentrations of 2 x 10(-2)M did not change the resting polarization of the surface epithelial cells. However, quinine applied to cells already depolarized by outward currents induced further depolarization. When it was delivered to cells hyperpolarized by inward current the substance induced further hyperpolarization.4. Sucrose (0.5-1.0 M) did not change the membrane potential of lingual cells regardless of the level of cell polarization induced by injected currents.5. Hydrochloric, sulphuric, nitric and acetic acids produced minimal biphasic effects: a brief hyperpolarization followed by a slower secondary depolarization.6. Water increased the membrane potential of the surface cells. Salts applied at the peak of the water-evoked hyperpolarization induced cell depolarization which was much larger than that evoked during application of salts to cells bathed in Ringer solution.

Different types of extrafusal muscle fibres in snake costocutaneous muscles

1. Tonic and twitch muscle fibres were identified physiologically in m. costocutanei superiores and inferiores of garter snakes and grass snakes.2. Tonic fibres were multiterminally innervated and showed s.j.p.s in response to nerve stimulation. They did not show propagated A.P.s. They were innervated by motor axons with lower conduction velocities than those to twitch fibres, and often gave a contraction and developed tension in response to a single shock to the nerve. Intracellular square pulse analysis showed that C(m) = 1 muF/cm(2) and R(m) = 40,000 Omega cm(2).3. Twitch fibres showed a conducted action potential in response to nerve stimulation, and focal, as opposed to diffuse, innervation. They showed a variety of isometric twitch contraction times (times-to-peak of about 30-65 msec). Groups of similar motor units contained fibres of approximately similar contraction times. Slow twitch (and tonic) fibres often appeared silvery under dark field illumination, while faster twitch fibres appeared clear. No difference in C(m), R(m) or lambda was found between faster and slow twitch fibres. Values were approximately 3-4 muF/cm(2), 3000-4000 Omega cm(2) and 2 mm respectively.

Some properties of the denervated anterior gracilis muscle of the rat

1. Microdrops of acetylcholine (ACh) were topically applied to denervated, mammalian skeletal muscle in vivo, and ACh-evoked depolarization and contracture were recorded simultaneously.2. Contracture tension was directly proportional to the degree of ACh-elicited depolarization.3. Atropine and (+)-tubocurarine increased the amount of ACh required to produce a given amount of depolarization, but these anticholinergic agents did not alter the relationship between the degree of ACh-evoked depolarization and contracture tension.4. Topically applied catecholamines did not produce either depolarization or contracture, despite the fact that parenterally administered catecholamines elicited both responses.

Differences between the effects of saxitoxin (paralytic shellfish poison) and tetrodotoxin on the frog neuromuscular junction

1. End-plate potentials (e.p.p.) have been recorded from the neuromuscular junctions of frog sartorius and extensor longus dig. IV muscles, using intracellular micropipettes. Either curare or MgCl(2) were present in the Ringer solution, to keep the e.p.p. amplitude below the threshold for a muscle action potential and contraction.2. It has been shown that saxitoxin (paralytic shellfish poison) usually caused a progressive reduction in the amplitude of the e.p.p. Occasionally, when it was applied in the presence of MgCl(2), the e.p.p. disappeared abruptly.3. Tetrodotoxin usually caused the e.p.p. to disappear abruptly. Occasionally, when applied in the presence of curare, the e.p.p. declined progressively for a short time before disappearing abruptly.4. It is concluded that at the frog neuromuscular junction the preferential site of action of saxitoxin is at the nerve terminals, but tetrodotoxin preferentially blocks nerve conduction at a site proximal to the junction.5. It is suggested that this preparation would be a convenient and reliable test object for distinguishing saxitoxin from tetrodotoxin.

The relation of membrane changes ot contraction in twitch muscle fibres

1. Contractile responses in short twitch-type snake muscle fibres have been studied. These fibres are sufficiently short to allow fairly uniform changes in membrane potential along their length when current is passed through an intracellular micropipette. Active sodium permeability changes were blocked with tetrodotoxin (TTX), procaine, or by using solutions low in sodium. Current and voltage micropipettes were used to voltage-clamp these fibres. Depolarization steps to about -40 mV evoked contractile responses, maximal tension being developed between -10 and 0 mV. The relation between contraction and membrane potential was sigmoid.2. Depolarization beyond a critical threshold produced an increment of outward current which inactivated with time. The threshold for this delayed rectification was normally similar to the threshold for contractile activation. Fibres exposed to high potassium showed a reversal of this inactivating current to slightly super-threshold depolarizing pulses. At membrane potentials near 0 mV, no inactivating current was noted, while stronger depolarizing pulses produced an inactivating current in the normal direction. Fibres in high potassium show the same threshold for initiation of contraction as in normal solution.3. Thiocyanate, nitrate, and caffeine shifted the relation between membrane potential and contraction toward higher levels of membrane potential. The threshold for inactivating rectifying current failed to shift to a corresponding extent, although some shift in rectification which did not inactivate was evident.4. When depolarization was maintained, contractile tension was maximal for several seconds, then gradually disappeared. The rate of this contractile inactivation depended upon the level of depolarization.