A PORTABLE 24 CHANNEL FUNCTIONAL ELECTRICAL STIMULATION SYSTEM FOR UPPER AND LOWER EXTREMITY APPLICATIONS

B.T. Smith, B. McGee, M. Ignatoski, J. Douglas, M.J. Mulcahey, R.R. Betz Research Department, Shriners Hospitals Philadelphia, PA 19152 ABSTRACT

A 24 channel functional electrical stimulation (FES) system has been developed for use in both upper and lower extremity applications for children and adolescents with spinal cord injury (SCI) or cerebral palsy (CP). The system consists of a PC-based modular stimulator unit and battery pack. The stimulator can process both Hall Effect sensor inputs used to transduce joint position for proportional control of hand and arm movement and force sensing resistors used as foot contact switches during gait. The stimulator produces an asymmetrical, balanced biphasic waveform for intramuscular stimulation. The stimulus period, amplitude and pulse duration can be individually adjusted for each of the 24 channels. The stimulator is programmed from a Pentium-based host computer via a Windows-based interface program.

BACKGROUND

Research in upper and lower extremity functional electrical stimulation (FES) requires a stimulation system that is relatively small, lightweight and simple for the user to operate and yet contains the power and versatility to investigate novel applications of FES. Historically, these two goals have been at odds because it has been difficult to house the necessary computing power within an acceptably sized stimulator for clinical investigation. With the evolution of personal computer (PC) technology, however, PC's can now deliver tremendous computational abilities within a small area which has made them an important component of FES research devices [1-4].

STATEMENT OF PROBLEM

Our laboratory required a FES system that could be used for both upper and lower extremity applications for children and adolescents with spinal cord injury (SCI) or cerebral palsy (CP). The system was required for both laboratory investigation and for use by subjects at home and school. Thus, the system needed to be portable, light, battery-operated and easy to use outside the laboratory. We also required a FES system with sufficient stimulation channels and programming flexibility so that complex movements such as stepping or arm motion could be developed. In addition, the system had to process user-generated command signals to control either upper or lower extremity stimulated movement. Control of the stimulus period, amplitude, and pulse duration of the stimulus waveform was also required.

RATIONALE

While most FES systems are designed specifically for upper or lower extremity stimulation, our approach was to design one system to support both applications. Our system's control is centered around a single board PC module. There are several advantages to using a PC over custom designed microprocessor circuitry. There is extensive PC software support available. Off-the- shelf PC technology costs less, decreases design time, and increases reliability over custom designed modules. PC technology is relatively inexpensive to service or replace, and conforms to industry standards which allows a system to be upgraded easily.

SYSTEM DESIGN

A block diagram of the system design is shown in Figure 1. A 24 channel portable stimulator unit was designed to process user- generated command signals and, based on that input, output pre-programmed patterns of stimulation. The unit is programmed via a host computer and is operated as a stand alone system. The following paragraphs describe the major system components.

Host Computer

A Pentium-based host computer is used to control the stimulator during electrode characterization sessions and to develop the stimulation patterns and download them to the portable stimulator unit via serial port communication. The interface program is Windows- based, written in C++ programming language and is operated in a Windows NT environment.

Stimulator Unit

The unit consists of three modules - an off- the-shelf PC computer board based on PC/104 standards; a custom-designed board that contains the circuits that process sensor signals, produce the stimulation waveforms and regulate power; and an off-the-shelf analog and digital input-output board that coordinates information flow between the two other boards. The unit is 4.5"L x 4.5"W x 2.5"H and weighs approximately 1 pound. A hybrid circuit chip located on the custom module contains the circuitry to generate the stimulus waveforms for intramuscular stimulation. The circuit generates an asymmetrical, balanced biphasic waveform that can be programmed to provide amplitudes of up to 20 mA in 1 mA increments, stimulus periods from 0-33 milliseconds in 1 millisecond increments and pulse durations from 0-255 microseconds with a 1 microsecond resolution. All three parameters can be programmed independently for each channel.

Thumb Switch

When the stimulator is operated as a stand alone unit, a thumb switch is used to navigate a menu system shown on a liquid crystal display on the stimulator. The thumb switch consists of a four button keypad with two light emitting diodes (LEDs) mounted on a plastic ring that is physically connected to the stimulator via a standard phone cord. The ring slides over the index finger so that the keys can be activated with the thumb. The thumb switch is typically used by a subject to chose and initiate stimulation patterns for standing and stepping. The LEDs relay system status to the user. A hidden menu that can be accessed only by the researcher is also manipulated via the thumb switch for minor adjustments to stimulation patterns.

Control Sensors

The stimulator is designed to interpret signals from two types of sensors: a dual axis Hall Effect joint position transducer used for proportional control of stimulated hand and arm movement; and up to eight force sensing resistors (FSRs) which are typically placed in the soles of the shoes to determine foot-to-floor contact to coordinate stimulation for stepping. Four unassigned buffered, unity gain analog-to-digital (A/D) inputs are available to accept pre-processed analog signals. These are designed to investigate other control sources such as electrogoniometers, electromyography or accelerometers.

Battery Pack

The battery pack provides power for the system. It consists of 8 4/3 A size nickel metal hydride cells arranged in a 5.75"L x 3.6'W x 1.1"H enclosure and weighs 1.2 pounds. A cable with interlocking connectors on both ends couples the battery pack to the stimulator. A charger allows the batteries to be recharged from a standard 120V electrical outlet. The stimulator can operate for approximately 7 hours from fully charged batteries. Recharging the batteries requires about 5 hours. Because the battery packs are modular, they can be rotated so that one can be used while another is charging. In this way, stimulator down time is eliminated.

EVALUATION

This FES system is planned to be used to investigate the following FES applications for children and adolescents: arm and hand function for those with C3, C4, or C5 SCI; upright mobility for complete thoracic level SCI; and ambulation for those with incomplete SCI or spastic CP.

DISCUSSION

Our research laboratory has developed a 24 channel portable FES system for upper and lower extremity pediatric applications in SCI and CP. It's PC-based, modular design satisfies our need for both a small, lightweight, battery-powered unit that can be used by children in their home environments and a versatile, powerful research stimulator. For investigations with children, size and simplicity of operation are major concerns. Our device weighs slightly more than 2 pounds which is evenly distributed between the stimulator and modular battery pack. Both units are relatively small. Hence, the system should not burden the child while ambulating using FES. Since we have successfully used the Hall Effect transducer [5], thumb switch [6] and FSRs as control sources in the past with children and adolescents, we feel that control of the system for both applications is sufficiently simple and effective. For research purposes, a versatile system is important. With our system, the PC module can be replaced with a faster, more efficient module such as a 386 or 486-based computer without modification to the stimulator unit. This would allow us to investigate real time closed loop control schemes. With the unassigned A/D inputs the system can accommodate new control signals. Because the custom board that produces the stimulus waveforms is modular, it can be replaced or modified with circuitry to produce, under the same PC platform, either surface stimulation waveforms or a telemetric signal to control an implanted stimulator. The Windows-based interface program on the host computer provides a user- friendly environment in which to develop complex stimulation patterns and control schemes. This is important for our laboratory since development is performed by both the therapist and engineer.

REFERENCES

[1] Meadows, P.M., D. McNeal, "A Four-Channel IBM PC/AT Compatible Biphasic Pulse Generator for Nerve Stimulation," IEEE Trans. Biomed. Eng. 36(7):802-804, 1989.

[2] Broberg, R., A. Hubbard. "A Custom-Chip- Based Functional Electrical Stimulation System," IEEE Trans. Biomed. Eng. 41(9):909-912, 1994.

[3] Chrisensen, P.R., J.A. Hoffer, "An Eight- Channel Biphasic Stimulator for Functional Electrical Stimulation," Proc. RESNA Conf. 375- 377, 1995.

[4] Ilic, M., D Vasiljevic, D.B. Popovic, "A Programmable Electronic Stimulator for FES Systems," IEEE Trans. Rehab. Eng. 2(4):234-239, 1994.

[5] Smith, B.T., M.J. Mulcahey, R.J. Triolo, R.R. Betz. "The Application of a Modified Neuroprosthetic Hand System in a Child with a C7 Spinal Cord Injury," Paraplegia, 30:598-606, 1992.

[6] Hunt M., C. Mullin, M. Moynahan. "A Pilot Study of Functional Neuromuscular Stimulation for Standing by Adolescents with Paraplegia," J. Amer. Para. Soc. 17(2):122, 1994.

ACKNOWLEDGEMENTS

This research was funded by Shriners Hospitals Grant #15953. The authors acknowledge the engineering efforts of Gary Zinzell, Mike Nitowski, Mike Lawrence and Dr. Ronald Triolo in the design of this FES system.

Brian T. Smith Research Department, Shriners Hospitals 8400 Roosevelt Blvd., Philadelphia, PA 19152 Phone: 215-332-4500 FAX: 215-332-5766 E-mail: bsmith00@astro.ocis.temple.edu