Biopotential sensor electrode system

Details Biopotential sensor electrode system Biopotential sensor electrode system

2000-03-21

2002-08-20

Shaver, Kevin (Department: 3736)

Surgery

Diagnostic testing

Measuring electrical impedance or conductance of body portion

C600S372000, C128S903000

Reexamination Certificate

active

06438413

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to the field of biopotential sensors. In particular, the invention relates to surface electrodes.
2. Description of the Related Art
Typical biopotential sensing/stimulating systems are used to gather a growing variety of biopotential signal types or information from subjects or patients. These sensing/stimulating systems are also used to stimulate the patient with a known signal such that skin impedance and other information can be sensed from the subjects or patients. Typical biopotential sensing/stimulating systems include two types of subsystems, biopotential sensors and the associated external monitoring systems or instrumentation. While the medical information capable of being extracted from this biopotential information has increased significantly with advances in medical science and technology, the usefulness of these systems remains as a limiting factor in patient treatment because of deficiencies of the typical biopotential sensing/stimulating systems.
Regarding the biopotential sensor/stimultor subsystem, biopotential sensors/stimulators can generally be categorized as being invasive or non-invasive. Invasive sensors are implanted surgically, and are used for accurate isolation of potential sources during sensing and/or delivery of a stimulating signal in to a specific target location. The invasive sensors/stimulators can usually be applied to peripheral nervous systems (i.e., axons or muscles) or to introcerebral sites as in brain research.
Non-invasive sensors/stimulators, also referred to as surface, skin, or scalp electrodes and/or sensors, are applied to the skin surface. These electrodes are typically connected to the surface of the skin via an electrolyte or gel, hence they are also referred to as wet surface electrodes, or wet electrodes. Wet surface electrodes are commercially available and are routinely used in the clinics and research labs. The preference for wet surface electrodes is due in part to the relatively low manufacturing cost of wet electrodes, and historically proven technology. Furthermore, the wet surface electrodes are passive devices that can be used for both sensing and stimulating, since all the necessary electronics and intelligence resides in the external monitoring systems or instruments.
One class of surface electrodes does not use electrolytes. These electrodes, referred to as active electrodes, employ an impedance transformation at the sensing site via active electronics. The active electrodes are subdivided into two electrode types, dry electrodes and insulated electrodes. The dry electrode has a metal in direct contact with the skin which is followed by an impedance converting amplifier. The insulated electrode is capacitively coupled to the skin via a dielectric which is followed by an impedance converting amplifier.
Research results for active electrodes have demonstrated that both dry and insulated electrodes are comparable to wet electrodes for sensing or receiving electrocardiogram (ECG or EKG) signals. However, typical active dry and insulated electrodes do not exhibit the same consistency and signal to noise ratio (SNR) as the wet electrodes. In addition, the typical non-invasive active electrodes have been used for signal sensing purposes only and not stimulating. While the research has focused on ECG signals, there are numerous other biopotential signal types to which the application of active electrodes would be desirable but has yet to be demonstrated.
Efforts to realize active insulated electrodes have included significant research and development in the area of sensor dielectrics. A number of materials have been investigated for thin-film capacitor fabrication in sensors of the active hybrid electrodes. Some of the materials typically considered for use include silicon monoxide (SiO), silicon dioxide (SiO
2
), silicon nitride (Si
3
N
4
), Diamond like Carbon (DLC), and tantalum pentoxide (Ta
2
O
5
). In practice, deposited dielectric films thinner than 500-700 Angstroms (Å) have a fairly high pinhole density and the yields are poor. Pinholes lead to resistive shorts between the electrodes (in the vicinity of each other) and increase the leakage current. Thick dielectric films, or films with a thickness greater than approximately 20,000 Å also may exhibit problems because of the high internal stress levels found in these films. High compressive forces cause the films to peel off; however, large tensile forces can be relieved by crazing, or the production of fine cracks in the film. These factors thus may limit the thickness of the dielectric material to between 800 Å and 10,000 Å.
While both silicon monoxide and silicon dioxide are good insulators for electrical isolation, their behavior as a barrier to sodium ions (Na+) is poor. In addition, these two materials require high temperatures in order to form high quality films with few pinholes. Tantalum pentoxide also can suffer from high-temperature deposition requirements and low breakdown voltage.
The insulated electrodes with dielectrics currently in use are not practical because of breakage, scratched surfaces, and inconsistency. Therefore, there is a need for an electrode dielectric material having a number of specific properties. The properties desired include: low reaction with sodium chloride (NaCl) for biocompatibility and sensor protection; low deposition temperature (approximately less than 500 degrees Celsius) to be compatible with electrode material; high dielectric constant for obtaining a large capacitance in a small area; high dielectric strength (resulting in high breakdown voltage) for electrostatic protection; moderate leakage resistance for impedance matching to the amplifier; and, oxidation rate 30 times slower than that of silicon.
Regarding electrode monitoring subsystems, a typical electrode monitoring system consists of the following components: (1) an array of wet electrodes attached to the monitoring environment; (2) electrode cables for coupling each of the wet electrodes to instrumentation; (3) a cable converter box for receiving the electrode cables; and, (4) a monitoring system connected to the cable converter box with a series of cables. The typical electrode signal path from the sensor to the monitoring system is through unshielded cables of approximately 3 to 6 feet in length. These cables typically degrade the signal-to-noise-ratio (SNR) of the recording system and increase motion artifacts. In addition, the cables confine the movement of the subject as well as impose a health hazard in monitoring systems. Consequently, there is a need for an electrode monitoring system that does not require the patient to be wired to the monitoring system, a system that eliminates the need for electrode cables, the cable converter box, and the monitoring cables.
Wireless telemetry systems in general are classified as active or passive. Active telemetry systems are used for telemetry over longer distances. Therefore, typical active telemetry systems require a power source in both transmitter and receiver sections. The power source is primarily used to operate active devices such as transistors that form the circuits for these systems. The factors that influence the distance of communications include the available power, frequency of operation, and antenna size.
Typical telemetry for monitoring systems use either infrared red (IR) or radio-frequency (RF) links. These systems consist of several wet electrodes mounted on a cap and connected via cables to a transmitter section. The transmitter section consists of transmitter circuitry, a power unit, IR light emitting diodes or a large antenna, voltage converters/multiplexers, and a microcontroller unit. The transmitter section requires very high bandwidth and additional signal processing circuitry in order to provide digitized and time multiplexed data for transmission. As such, the size and weight of the transmitter section in these telemetry systems prevents them from being mounted on the cap with or

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