COMPEX MOTION - NEW PORTABLE TRANSCUTANEOUS STIMULATOR FOR NEUROPROSTHETIC APPLICATIONS

Transcription

1 To appear in the Proceedings of the European Control Conference 2001, Porto, Portugal, September 4-7, 2001 COMPEX MOTION - NEW PORTABLE TRANSCUTANEOUS STIMULATOR FOR NEUROPROSTHETIC APPLICATIONS M.R. Popovic 1,4, T. Keller 1,2, I.P.I. Pappas 1, P.Y. Müller 3 1 Automatic Control Laboratory, Swiss Federal Institute of Technology Zurich, Switzerland 2 ParaCare - Institute for Rehabilitation and Research, University Hospital Balgrist, Switzerland 3 Compex SA, Switzerland 4 Institute of Biomaterials and Biomedical Engineering, University of Toronto, Canada fes@aut.ee.ethz.ch Keywords: FES, FNS, electric stimulator, neuroprosthesis Abstract A new portable and programmable transcutaneous electric stimulator Compex Motion that can be used for a wide range of Functional Electrical Stimulation (FES) applications is presented. The stimulator has four current regulated stimulation channels and has two input channels that can be configured as analog or digital input channels. The number of stimulation channels can be expanded in multiples of four (e.g. 8,12,16, ) by using 2,3,4, or more stimulators that are working in parallel. When the stimulators work in parallel their stimulation sequences and stimulation timings are fully synchronized. The Compex Motion stimulator can be programmed to generate arbitrary stimulation sequences. The programmed stimulation sequences are stored on readily exchangeable memory chip-cards. By replacing (exchanging) the chip-card the function of the stimulator is changed to provide another function or FES treatment. The stimulator can be controlled via any external sensor, sensory system or laboratory equipment with an output voltage in the range 0-5 V. The Compex Motion stimulator can be used to develop various custom-made neuroprostheses, neurological assessment devices, muscle exercise systems, and experimental set-ups for physiological studies. The Compex Motion stimulator is manufactured by the company Compex SA, located in Switzerland, and is currently undergoing clinical trials. 1 Introduction The existing portable electric stimulators that apply surface FES technology and are used for neuroprostheses development are seldom designed to carry out two or more unrelated FES tasks. [3]. Usually single electric stimulator is designed for one very specific application. Therefore if one wants to use an electric stimulator designed for one application to perform another FES task, one has to modify either the stimulator's hardware or its software. In general, such alterations are impractical and costly. As a result, many FES groups were forced to develop their own stimulators in order to meet their research and clinical needs. Despite that so many different stimulators were developed thus far a standardized, programmable, and portable electric stimulator that can be used for a wide range of transcutaneous FES applications does not exist yet [1]. In this article we are describing a new generation of transcutaneous electric stimulator Compex Motion. This stimulator represents further evolution and expansion of the already existing ETHZ-ParaCare FES system [2,3]. This new stimulator together with a Personal Computer (PC) can be used to rapidly prototype neuroprostheses or to carry out physiological experiments involving electric stimulation. At the same time the system can be used as a stand alone neuroprosthetic device that assists Spinal Cord Injured (SCI) patients to perform various functions in Activities of Daily Living (ADL), such as walking, standing, and grasping. In a hospital setting the system can be used during rehabilitation to treat muscle atrophy, and to assist incomplete SCI and stroke patients to relearn functions such as grasping and walking. 2 Compex Motion Hardware The Compex Motion stimulator has the identical hardware as the Compex 2 stimulator, which is manufactured by the company Compex SA. The Compex 2 stimulator is produced in large quantities, it is CE approved, and it is successfully used for various therapeutic and medical applications. The electrical stimulator Compex Motion is a microcontroller based system with four stimulation channels (see Table 1). The

2 stimulation channels are current regulated and have 2 µs rise time for pulses with 100 ma amplitudes (100 ma is the maximum pulse amplitude that can be generated by the stimulator). Self-adhesive stimulation electrodes that can be placed on the subject s skin are used to deliver desired stimulation to the selected muscles and muscle groups. The stimulator has two input channels A and B, and a special purpose port C. Two input channels A and B can be configured either as analog or digital input channels with the voltage range 0-5 V. The special purpose port C is used to interconnect the stimulators, to serially communicate with a PC, and to trigger the stimulator using a push button. By interconnecting two or more stimulators one can expand the number of stimulation channels from four to multiplies of four channels (8,12,16, ). In such an arrangement one stimulator is set to be a master stimulator and all other stimulators that are attached to the master stimulator are set as slave stimulators. The function of the master stimulator is to ensure that all interconnected stimulators operate synchronously. 3 Compex Motion Software The Compex Motion stimulator is programmed with a Graphical User Interface (GUI) software that is installed on a PC (see Figure 2). The GUI software applies drag-and-drop technique to program the stimulation sequences. This is done by sequentially placing icons called primitives on a time line that describes the chronology of the tasks that will be carried out by a single stimulation channel. There are four such time lines and each time line defines tasks that will be executed by a corresponding stimulation channel. There are 56 primitives that describe tasks that can be carried out by the stimulator (see Table 2). These 56 primitives are grouped into following types of primitives: (1) constant pulse width, (2) pulse width ramp-up, (3) pulse width ramp-down, (4) no stimulation, (5) delay, (6) change amplitude, (7) change frequency, (8) jump back, (9) synchronize, (10) user interaction, (11) user branch, (12) user interrupt, (13) end, (14) turn off, (15) display text, (16) generate sound, (17) random frequency, (18) random pulse width, and (19) random amplitude. These primitives are either global or local. The global primitives are marked with and represent tasks that affect all active stimulation channels. The local primitives are labeled with and represent tasks that affect only channels in which time lines they appear. Figure 1: Compex Motion electric stimulator and its hardware: stimulator, 3 memory chip-cards, 2 EMG sensors, and 2 stimulation electrodes The Compex Motion stimulator has a rechargeable NiMH battery that provides eight hours of continuous stimulation and requires less than two hours to charge the battery. An AC/DC adapter is used to charge the battery. The stimulator also has a dot matrix LED display that provides a visual interface between the user and the stimulator. The stimulator can be controlled via nine push buttons placed above the LED display (see Figure 1) or via various user interfaces that can be attached to A, B and C ports. Standard user interfaces that are currently provided with the stimulator are an EMG sensor and a push button. These interfaces can be used to control and regulate the stimulation sequences and stimulation parameters. Additional stimulator features, accessories, and hardware data are provided in Table 1. Figure 2: Main window of the Compex Motion GUI software. It shows four horizontal time lines associated with each stimulation channel (center and right), pulse amplitude and pulse width safety limits (left), pulse type settings (center bottom), memory chip-card functions (right bottom), and setup functions (left bottom). The user interaction primitives are functions that allow a subject to generate commands to trigger the stimulator. Triggering can be facilitated either using digital or analog input signals. In either case an arbitrary curve profile that represents the triggering condition and is defined with the GUI software is used to trigger the stimulator. When the desired curve profile is generated at the 2

3 specified input port A or B, the triggering is executed. The triggering control can be used, for example, to initiate the stimulation patterns, to terminate stimulation, and to choose between two different stimulation patterns. Sensors such as EMG sensors, force sensitive resistors, gyroscopes, foot switches, and push buttons were already successfully applied with the user interaction primitives. Continuous regulation of the stimulation intensity can be done using an analog input signal. In that case the pulse amplitude depends on the voltage level of the input signal. This dependence can be nonlinear and is defined with a lookup table that can be imported as an ASCII file and can be edited both graphically and numerically. Each stimulation channel has its own lookup table. This feature allows one to regulate the stimulation intensity in real time using analog inputs. Thus far sensors such as EMG sensors, sliding resistors, and potentiometers were already successfully implemented with this control strategy. Main features of the GUI software: The GUI software specifies the chronology of the stimulation sequences, stimulation parameters, interfaces the stimulator should support, and commands the user needs to issue via the user interfaces to evoke a desired stimulation sequence. Stimulation programs developed with the GUI software can be stored as files that can be up loaded. This feature allows one to create libraries of stimulation programs that can be used with various subjects. The software also allows one to use various pulse width ramp-up and ramp-down profiles. These profiles can be either linear or nonlinear. They can be imported as ASCII files and edited both graphically and numerically. Each stimulation channel can have two different ramp-up and ramp-down profiles. One can use these profiles to compensate for the nonlinear muscle recruitment property [1]. The software also allows one to select monophasic or biphasic pulses, monopolar or bipolar pulses, and alternating or nonalternating pulses [1]. A stimulation program developed with the GUI software is stored on a chip-card that is plugged in the stimulator s card read-and-write module. The stimulation program developed with the GUI software is downloaded via serial port C to the chip-card. The content of the chip-card can be also uploaded and displayed using the GUI software. Without the chip-card the stimulator is not able to perform any function. By exchanging the chip-card (which takes less than 5 s) one instantly changes the function of the stimulator. This feature allows one to apply the same stimulator for various FES applications and various subjects that require different FES treatments, without loosing any time. The simulator also facilitates real-time EMG signal processing with stimulation artifact suppression algorithm that can be used for EMG neuroprosthesis control [2]. The Compex Motion stimulator allows one to control the pulse amplitude and the pulse width independently. Conceptually, the pulse width is used to sequence the stimulation patterns, and the pulse amplitude is used to regulate the overall stimulation intensity. This feature allows one to decouple the control of the stimulation patterns and the stimulation intensity. For example, the sequence of hand opening and closing tasks in the case of a neuroprosthesis for grasping is defined with strings of primitives that control pulse widths in the stimulation channels. At the same time the intensity of grasp is regulated with the pulse amplitude. In such a case one can use a push button to trigger the stimulation sequences defined with the primitives while the stimulation intensity is regulated via sliding resistor (analog input) that controls the stimulation amplitude. 4 Examples of Stimulation Programs 4.1 Neuroprosthesis for Grasping - Lateral Grasp In Figure 3 it is shown how the GUI software can be used to program a stimulation sequence that generates simple lateral grasp. The output of Channel 1 is connected to the stimulation electrodes placed above the finger extensors, and the output of Channel 2 is connected to the stimulation electrodes placed above the finger flexors. The first primitive (icon) in the time lines of both Channels 1 and 2 is a user interaction primitive (marker 1 in Figure 3). Let us assume that this user interaction primitive is set such that in order to trigger the stimulation the patient has to voluntarily contract a deltoid muscle that is instrumented with an EMG sensor connected to the port A. Until the patient contracts the deltoid muscle the stimulator does not generate any stimulation. After the deltoid muscle is contracted stimulation is provided by the Channel 1 that has the following four primitives after user interaction primitive: rump up, constant pulse, ramp down, and no stimulation. These primitives generate the following stimulation sequence. The width of the stimulation pulse is increased from 0 µs to a predefined value (for example 250 µs), stays at that level for a certain time period, and then decreases back to 0 µs. While the stimulation pulse width increases and decreases the stimulation amplitude remains constant (for example 22 ma). While Channel 1 is active Channel 2 does not generate any stimulation since its second icon in the time line is no stimulation primitive. This stimulation sequence generates a hand opening function that typically lasts 2 or 3 seconds. 3

4 When the hand opening task is completed, the program reaches primitives marked with 2. This indicates that as the stimulation sequence in Channel 1 is completed, Channel 2 starts generating the stimulation sequence as shown in Figure 3. At this point in Channel 2 the following primitives appear: rump up, constant pulse, and user interaction. The onset of the stimulation in Channel 2 starts in the same way as it did previously with Channel 1 except that the stimulation does not terminate after the predefined time period. In this particular case the user interaction that comes after the constant pulse primitive ensures that the stimulation pulse with constant pulse width will be generated until the deltoid muscle is contracted for the second time (i.e. user interaction is activated). This ensures that the patient will have the fingers flexed (hand closed) as long as he/she desires. In the time lines in Figure 3 this is indicated with the marker 3. This type of stimulation sequence is typically used to train high lesion SCI patients to perform lateral grasp. Once the patients master this grasping strategy more sophisticated grasping strategies are applied [3] Figure 4: Time lines that are used to generate a walking in a disabled leg. Markers 1 and2 indicate primitives that are executed at the same time. 4.2 Neuroprosthesis for Walking Figure 3: Time lines that are used to generate a lateral grasp. Each icon in the time line represents a primitive. Markers 1,2, and 3 indicate primitives that are executed at the same time. When the patient contracts the deltoid muscle for the second time in Channel 2 the following primitives are executed: ramp down, no stimulation, jump to first, and end. In Channel 1 primitives that are executed after the user interaction is activated are: time delay, rump up, constant pulse, no stimulation, jump to first, and end. These two sequences of primitives cause the finger flexors to relax (Channel 1) and the finger extensors to contract (Channel 2) for a predefined period of time (typically 2 s). These stimulation sequences generate hand opening followed by the termination of all stimulation. Then the program in both stimulation channels jumps to the first primitives in both time lines. This effectively brings the stimulation sequences in both channels to the starting point and the stimulator is again ready to generate hand opening function after the patient voluntarily contracts the deltoid muscle. In Figure 4 a stimulation sequence that is used to generate a walking sequence in a disabled lag of an incomplete SCI patient is presented [3]. Channels 1, 2, 3 and 4 are stimulating extensor digitorum longus, tibialis anterior, vastus medialis, and hamstrings of the disabled leg, respectively. The other leg of the patient is in a good condition and the patient uses it to pace the gait, to maintain balance during walking and standing, and to support the body during walking and standing. The stimulated leg only generates the walking sequence and it carries a part of the body weight during walking. Besides the neuroprosthesis the patient uses crutches or a walker to maintain stability during walking. This stimulation sequence is also triggered with a user interaction primitive. In this particular case the stimulation sequences are triggered with a push button [3]. Each time the patient wants to make a step with the disable leg he/she has to push the button. Once the button is pushed the stimulation sequence shown in Figure 4 is generated causing a steping motion with the disable leg. This stimulation program is commonly used as a part of treadmill training for incomplete SCI and stroke patients who are trained to walk. 5 Clinical Trials Currently a number of patients are using the Compex Motion stimulator as a neuroprosthesis for grasping or walking at our 4

5 facilities at the ParaCare center. Besides walking and grasping the system is successfully used to treat shoulder subluxation and to strengthen muscles in some patients. Multi-center trials are expected to start by the end of this year. Acknowledgements This project was supported by a grant from the Federal Commission for Technology and Innovation, Switzerland - Project No Practical Guide, USA: Rehabilitation Engineering Program, Los Amigos Research and Education Institute, Rancho Los Amigos Medical Center, 3 ed. [2] Keller T., Curt A., Popovic M. R., Dietz V., and Signer A., Grasping in High Lesioned Tetraplegic Subjects Using the EMG Controlled Neuroprosthesis, The Journal of Neuro- Rehabilitation, vol.10, pp , [3] Popovic M.R., Keller T., Pappas I.P.I., Dietz V., and Morari M., Surface-Stimulation Technology for Grasping and Walking Neuroprostheses, IEEE Eng. in Medicine and Biology, January/February References [1] Baker L., McNeal D., Benton L., Bowman B., and Waters R., Neuromuscular Electrical Stimulation - A Feature Characteristics 4 stimulation channels Current regulated Pulse amplitude Range: ma resolution: 1 ma (8 Bit) Pulse width Range: 0-16 ms resolution: 500 ηs (14 Bit) Stimulation frequency Range: Hz resolution: 1 Hz (8 Bit) 2 digital input ch. (A & B) Range: 0-5 V TTL 2 analog input ch. (A & B) Max sampling frequency: 12 khz Range: 0-5 V resolution: 20 mv (8 Bit) 1 special purpose port (C) Push button, serial port communication, and stimulator interconnection Working regimes Master/slave Stimulation pulses Monophasic/biphasic; monopolar/bipolar; and alternating/nonalternating Microcontroller Motorola HC11 dot matrix LED display No. pixels: 165 x 64 dimensions: 72 mm x 30 mm chip-card Can store up to 255 primitives per channel and all relevant stimulation parameters NiMH battery Rechargeable, 8 h of continuous stimulation Stimulator dimensions 148 mm x 80 mm x 30 mm & 420 g Accessories AC/DC adapter, push button, 4 cables, self-adhesive electrodes, EMG sensor,... Table 1: Compex Motion data sheet Pulse Width Primitives: constant pulse width Generates a pulse with a constant width (4 different values are available per channel) pulse width ramp-up Profile for changing the pulse width (2 different profiles are available per channel; profiles are described with 16 values that can be edited) pulse width rampdown no stimulation Pulse width equal to 0 Profile for changing the pulse width (2 different profiles are available per channel; profiles are described with 16 values that can be edited) delay Keeps the actual pulse width at the previous level for the given time interval Table 2: List of GUI primitives part 1 5

6 Pulse Amplitude Primitives: change amplitude Changes the amplitude from previous to new value in a specified time period (change is linear) Pulse Frequency Primitives: change frequency Changes stimulation frequency (4 different values are available and they apply to all stimulation channels) Primitive Sequence Control: jump back Program jumps back n times in the sequence to the marker primitive, where n=1-255, or n is infinite (n=0). synchronize Synchronizes otherwise independent stimulation sequences in all 4 stimulation channels Human Interaction Primitives: user interaction This primitive waits for a specific user action to trigger a stimulation sequence. One can use an arbitrary triggering profile and a sensor. (7 different user interaction criteria can be used) user branch Two trigger criteria set in user interaction primitive are used to generate branching. If criterion 1 is fulfilled the program proceeds with the next primitive in the line. If criterion 2 is fulfilled the program jumps to a predefined marker and proceeds with the next primitive after the marker. (2 user branches are available) user interrupt One trigger criterion set in user interaction primitive is used to generate interrupt. If this criterion is fulfilled between markers ON and OFF the program jumps to a predefined marker and proceeds with the next primitive after the marker. (1 user interrupt is available) General Primitives: end Terminates stimulation in the specified channel time line turn off display text generate sound Turns off the stimulator Displays two text lines with 8 characters in each text line (4 different text primitives are available) Generates a melody (2 different short melodies are available) Special Primitives: random frequency Activates the stochastic variation of the frequency. The frequency varies randomly about the nominal value, within a specified range (± 0-100%), following a uniform probability distribution function. random pulse width Activates the stochastic variation of the pulse width in the specified channel(s). The pulse width varies randomly about the nominal value, within a specified range (± 0-100%), following a uniform probability distribution function random amplitude Activates the stochastic variation of the pulse amplitude in the specified channel(s). The actual amplitude varies randomly about the nominal value, within a specified range (± 0-100%), following a uniform probability distribution function Table 2: List of GUI primitives part 2 6