An implantable electrical interface for in vivo studies of the neuromuscular system
Introduction
Many studies focused on the in vivo behavior of the neuromuscular system utilize implanted devices to characterize electrical and mechanical signals associated with neuromuscular function (e.g. Prochazka et al., 1976; Hoffer et al., 1987; Gregor et al., 1988; Herzog et al., 1993a,b). These implanted devices include nerve cuff stimulating and recording electrodes, electromyography (EMG) electrodes, tendon force transducers, distensible tube length transducers and a variety of other transducers and electrodes. These devices typically require an electrical interface with external equipment such as power supplies, amplifiers, telemetry, and other signal conditioning and recording equipment.
The interface between implanted devices and external equipment is usually located external to the animal. Such connectors include backpacks and skull pedestals (Loeb and Gans, 1986). Leads from implanted transducers are routed to the back or head of the animal and exit through a small incision in the skin. The leads are then soldered to a connector which is sutured or glued to bone or fascia at the desired location. These connectors have been used successfully in a variety of studies (Prochazka et al., 1976; Hoffer et al., 1987; Gregor et al., 1988; Herzog et al., 1993a,b). However, the external location of the connector involves risk of (1) the animal accidentally or intentionally damaging the connector and (2) bacteria tracking down leads or sutures causing infection. The need for eliminating the external interface arose in our laboratory when many rabbits chewed on their own backpacks and holding sutures. The rabbits destroyed their backpacks despite antichew creams, sprays, and backpack covers.
One method for sending signals between implanted devices and external equipment without external connectors or percutaneous leads is radio-frequency telemetry. For example, telemetry has been used to monitor blood pressure (Truett and West, 1995) and spinal loads (Rohlmann et al., 1994) in vivo. However, a simpler and cheaper alternative to telemetry may be desirable in many situations.
The purposes of this study were to develop and test a simple, inexpensive, implantable interface that could be used for repeated temporary electrical connections between implanted devices and external equipment. A review of the literature revealed no mention of such an interface. Use of this interface could reduce the risk of damage by the animal and reduce the risk of infection associated with permanent percutaneous leads and sutures.
Section snippets
Description of interface
The implantable multi-use interface (TIMI) was designed to serve as an implantable electrical interface between implanted stimulating and recording devices and external equipment. The TIMI consists of an implanted set of connectors and temporary percutaneous leads. Fig. 1a shows a schematic of the parts used for a two-lead TIMI. The housing of the implanted connectors is made of acrylic. Connector wells (one per lead for the device(s) to be implanted) are formed by drilling part way into the
Saline tests
Connector resistance was low, and leak impedance was high, despite a small amount of saline that appeared in the connector wells after repeated insertions of the temporary leads. The DC resistance between the lead inserted into the connector well and the device lead was always within 2.5 Ω of the sum of the resistance of the individual leads. The leak impedance of the rubber seal, measured as the AC impedance (at 1 kHz) between the inserted lead and a lead immersed in the surrounding saline, was
Discussion
The TIMI provides an implantable interface for repeated temporary electrical connections. Such connections are required for many in vivo neuromuscular studies. The TIMI is an alternative to the standard backpack connector in situations where (1) the animal will not tolerate a backpack and/or (2) infection from permanent percutaneous leads and sutures is a problem. The TIMI was developed after many rabbits showed no tolerance for a backpack and has successfully provided an electrical interface
Acknowledgments
The authors would like to acknowledge Dr. Walter Herzog and Gordon Hamilton for discussions on a preliminary form of the technology, and the Alberta Heritage Foundation for Medical Research and NSERC of Canada for financial support.
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