DEVELOPMENT OF AN INTEGRATED ELECTRICAL

STIMULATION SYSTEM WITH FEEDBACK FOR PHYSICAL

REHABILITATION

E. Durana

1,2

, V. Santos

, A. Lopes

and F. Vistulo de Abreu

Department of Physics and

Department of Mechanical Engineering, University of Aveiro, 3810-193 Aveiro, Portugal

Centro de Medicina e Reabilitação da Região Centro - Rovisco Pais, Quinta da Fonte Quente 3060-908 Tocha, Portugal

Keywords: Electrical stimulation, Rehabilitation, Functional Electric Stimulation (FES), Feedback, Spinal Cord Injury

(SCI), Stroke.

Abstract: In physical rehabilitation, electrical stimulation is widely used as a therapeutic method. However, as it is not

common to find portable devices, capable of integrating information from different sensors, and also with

flexibility in signal generation and triggering. This paper presents an integrated electrostimulation system

that encompasses all those facilities. The system integrates feedback signals coming from an accelerometer

and is capable of adapting electrostimulation depending on motor performance. The device uses a

microcontroller for the waveform generation, and allows controlled waveforms to be produced in response

to signals read from feedback sensors. Besides this high versatility, the principle of the power generation

employed by the device and additional hardware circuitry also provides mechanisms to ensure patient safety

in the unlikely cases of malfunction of the microcontroller. Here we also present an example of application

of the device that uses real time feedback information to control electrical stimulation.

1 INTRODUCTION

The latest technological development in electronic

miniaturization opens new perspectives in the

development of more sophisticated systems

integrating multiple components. Electrical

stimulation (ES) devices for medical rehabilitation

could benefit from these advances. Computational

power has also been made widely accessible, with

the appearance of highly portable inexpensive

computers. ES devices could use evolved

computational models to dynamically control the

electrical stimulation delivered by several

independent channels, responding to multiple

sensors, all working synchronously and according to

patient specific rehabilitation programmes.

Medical rehabilitation uses electric stimulation

intensively (Hennings et al., 2006). Usually, patients

with mobility limitations suffer from muscle

atrophy, which hinders their recovery. Electric

stimulation is an artificial way of inducing motor

movement and to prevent atrophy (Buckley et al.,

1987; Langzam et al., 2006; Durfee, 1999). It can

lead to synergistic gains, as the recovery of some

muscle fibbers allows the recovery of many others.

Indeed, clinicians believe that considerable

functional recoveries can be induced through

exercise (Doucet and Griffin, 2008). One main

problem, however, has always been in how to induce

physiological movements. This can be extremely

relevant as otherwise inadequate reinervation may

be stimulated leading to poor motor functional

recovery (Al-Majed et al., 2000; Franz et al., 2008).

Inducing physiological movements through ES has

been difficult for two main reasons. The first is that

electrical stimulation tends to produce non-

physiological muscle activation. Secondly, most

equipments do not use feedback information to

produce stimulation.

The minimisation of these difficulties motivates

the present work. Here we discuss the development

of a small, light and low power consumption circuit

that provides a closed loop electrical stimulation.

The circuit is capable of acquiring data from sensors

(like accelerometers, force sensors and alike),

communicating with a computer which can respond

through a versatile real time activation of ES stimuli.

User performances are registered and can be used in

later clinical evaluation.

Durana E., Santos V., Lopes A. and Vistulo de Abreu F. (2010).

DEVELOPMENT OF AN INTEGRATED ELECTRICAL STIMULATION SYSTEM WITH FEEDBACK FOR PHYSICAL REHABILITATION.

In Proceedings of the Third International Conference on Biomedical Electronics and Devices, pages 88-93

DOI: 10.5220/0002717900880093

 SciTePress

This article will focus on the development of an

ES circuit capable of producing symmetric biphasic

pulsed current stimuli, with real time control over

ES parameters (pulse phase, frequency and current

modulation). The circuit uses a microcontroller

(PIC) to integrate ES with inertial sensors data

acquisition and communication with a computer.

Before discussing the circuit, it is important to

introduce the current terminology used to

characterize electrical pulses in ES devices, and also

to discuss what their impact is in ES therapies. This

is done in the next section.

2 ELECTROTHERAPEUTICAL

REQUIREMENTS

Electrical stimulation is an artificial way to trigger

Action Potentials through the application of rapidly

changing electric fields on excitable tissues.

Transcutaneous Electrical Stimulators are the most

common (Nelson et al., 1999). They apply currents

through the skin to excite peripheral nerves.

Figure 1: Parameters defining a symmetrical biphasic

pulse.

Pulsed current waveforms are classified in

monophasic or biphasic. In monophasic pulse

signals current flows only in one direction (one

phase) between the electrodes. In biphasic signals

each pulse has two opposing phases. Biphasic pulses

can be symmetric or asymmetric, being symmetric

variants more appropriate for clinical applications

since they leave no residual charges in the

stimulated tissues.

Biphasic pulses are characterised by several

parameters (Figure 1): the pulse amplitude in each

phase, the interphase interval separating each phase,

the interpulse interval and the pulse duration.

Pulsed current signals can be modulated, varying

pulse duration, peak amplitude or pulse rate. This

modulation can be in the form of bursts,

interruptions and ramping. Bursts are created by

trains of pulses that flow for some milliseconds and

then stop for another period of time in a periodic

way. Interrupted pulses are a sequence of pulses

interrupted for a period of time for resting.

Interrupted pulses promote only twitch (brief)

contractions. For higher ES frequencies, bursts fuse

leading to tetanic contractions. In this case the

period between pulses is shorter than the muscle

contraction-relaxation cycle, summating the forces

produced by each impulse (Kitchen, 2002).

The muscle contraction strength can be

augmented by engaging more motor units or by

increasing the frequency at which each motor unit

fires (Vrbová et al, 2008). A higher number of

activated motor units can be achieved by applying

pulses with higher amplitude. The force produced by

increasing the ES frequency depends on the fibre

type. There are two main fibre types in a muscle,

type I and type II. Their proportion varies from

muscle to muscle and depends on function. Fibres

can change their type with exercise, and this is

particularly important in the medical rehabilitation

context. Indeed, in patients without active voluntary

movement, typically they loose type I fibres, a

process that can be reversed and/or prevented by ES.

Type I fibres are slow twitch muscle fibres that

produce relatively low forces but are also the less

fatigable. They are particularly important in

maintaining posture. As a result of their slow

contraction and relaxation they fuse at lower

frequencies when compared with type II fibres. The

later, are responsible for fast movements,

contracting and relaxing faster and consequently

fusing at higher frequencies. It is typically the aim of

the therapist to select the ES programme that best

selects the fibre type to be enhanced.

3 DEVELOPMENT OF AN

INTEGRATED ES DEVICE

The literature on ES circuits is not abundant

although a few examples can be found (Cheng et al.,

2004; McPartland and Mook, 1995). To deliver an

ES stimulus, any ES device has a voltage elevation

stage required to overcome the high skin and other

tissues impedance.

Some methods to increase voltage use

transformers, charge pumps, or switch mode power

supplies. Transformers are costly and large and on

high frequencies, inductive reactance grows

decreasing their efficiency. Charge pumping is

DEVELOPMENT OF AN INTEGRATED ELECTRICAL STIMULATION SYSTEM WITH FEEDBACK FOR

PHYSICAL REHABILITATION

another way to step-up voltage with capacitors and

switching elements. There are several charge pump

configurations, like the Cockcroft-Walton voltage

multipliers and the Dickson charge pump (Pan et al.,

2006). The strategy works by charging capacitors in

parallel and then, using switches to rearrange the

circuit, discharge the capacitors now in a serial

configuration.

Switch mode power supply (SMPS) is a power

conversion where high frequency switching is used.

Intrinsic spurious capacitors/inductors in some

electrical components may be used as more effective

means to reduce size and take advantage of their

effects. A particular interesting topology is the

quasi-resonant converter, which has the advantage of

reducing switching losses (Ye et al., 2004;

Pressman, 1998). This is performed by adding a

resonance inductor in series with a switch, a catch

diode and a resonance capacitor, as illustrated in

Figure 2. When the switch is on, the inductor stores

energy in its magnetic field. When the switch is

turned off the stored energy resonates with the

capacitor. The capacitor is charged with a sinusoidal

waveform, as shown in the V

SWITCH

curve in

Figure 2 (right). The diode stops resonance after half

period. This is valid for high switching rates (within

the half resonance period) as the resonance does not

have time to resume. In this case, there is zero

voltage across the switch (ZVS). Switching loss is

thus reduced as the voltage across the semiconductor

device is set to zero before switching (Neacsu,

2006). This circuit requires an on-off time control

that can be easily handled using a microcontroller.

Microcontrollers have many advantages, such as

the availability of analogue to digital converters and

acquisition modules that allow following in real time

the stimulus amplitude delivered to the patient and

acquire feedback information from sensors. Also,

communication modules allow information display,

remote control of peripheral components such as

digital potentiometers, read information from digital

sensors and interact with personal computers or

other processor units. Timer modules help designing

the waveform signal for the ES program.

The microcontroller improves the ES

functionality as it allows programming and

reprogramming, to change parameters according to a

pre-established algorithm, to give instructions to the

therapist and, of uttermost relevance, they can be

used to control safety levels. These were strong

motivations to use a Peripheral Interface Controller

(PIC), commonly abbreviated as “microcontroller”

(microchip PIC model 18f25K20), in the

development of the presented electrical stimulator

system.

Figure 2: Zero Voltage Switch circuit and waveforms.

Figure 3: Circuit diagram of the switching resonant

voltage elevator.

Figure 4: Simplified equivalent circuit for a MOSFET.

The proposed ES circuit uses a switching

resonant voltage elevator (SRVE) to generate

voltage pulses, as shown in Figure 3. The resonance

tank is composed by an inductor L

and the

MOSFET parasitic capacitance shown in more detail

in Figure 4. Hence, the resonating capacitor is

already built-in in the MOSFET. The circuit in

Figure 3 applies the ZVS method, in a similar way to

the circuit displayed in Figure 2. The MOSFET

performs the switching, but it also includes the diode

and the capacitor, so that the circuit indeed performs

the ZVS as the one in Figure 2.

The voltages across the capacitor and inductor

depend on the current supplied to the inductor

BIODEVICES 2010 - International Conference on Biomedical Electronics and Devices

during the switch-on period, and also on the

switching frequency and on the inductance and

capacitance values. Large voltages can thus be

generated with low power consumption and a

relatively small voltage power supply.

The high voltage generated during the switching

is then stored in the large capacitor, C, in Figure 3,

which can then be applied to the patient. Naturally,

the charge stored at each switching moment is

limited by the supplied power. The voltage is

sustained through the continuous generation of high

frequency voltage spikes controlled by the

microcontroller.

Another advantage of using high frequency

switching is that it minimizes components size and

cost. One example is the advantage taken from

MOSFET parasitic capacitances.

On the other side, it could be argued that high

frequency signals (the best results were obtained

with 1MHz switching frequencies) could suffer from

electromagnetic interferences. However, resonant

circuits produce sinusoidal currents which have

smooth changes, and hence are less prone to this

type of problems.

It should be noted that there are already some

circuits in the literature that use similar strategies for

voltage elevation (Cheng et al., 2004). However, the

solutions presented do not consider biphasic pulses

and furthermore, require using two independent

switching elements. This makes the circuit more

complex, possibly with no functional advantages.

In the present circuit, output voltage is not

directly applied to the patient. Instead, it is directed

onto a subsequent circuit (the waveform driver) as

depicted in Figure 5. This circuit is based on an H-

bridge, and was designed to control the duration and

polarity of the current that passes through the load

resistance, R

LOAD

. Control is made by two

microcontroller output signals, G

and G

. At all

moments, only one of the G

or G

signals can be in

a high state. With this procedure, biphasic pulses can

be generated since the current flows on different

senses through R

LOAD

, depending on which signal G

or G

is active. It is also possible to generate

symmetric or asymmetric pulses by changing the

duration of each G signal in the high state. The PIC

timer modules control pulse duration, pulse

frequency, and activation/rest periods of the stimulus

application.

Figure 5: Scheme of the ES circuitry. It includes the

SRVE, the waveform driver, the current sensing unit and

the injected charge safety control circuits.

To stimulate excitable fibbers in a selective way

and to prevent damaging tissues in the stimulated

area, the charge flow must be controlled. As living

tissues impedance is variable, the most effective and

safe strategy, is to control waveforms in current

rather than in voltage (Merrill et al., 2005).

The circuit that controls phase amplitude

operates in a closed loop. The signals from G

and

are voltage divided by a resistor and a digital

potentiometer, as represented in Figure 5 by

DigiPot1 and DigiPot2, respectively. These digital

potentiometers are adjusted in real-time by the PIC

microcontroller, controlling the voltage at the

MOSFET gate and limiting the maximal current

through R

LOAD

. This current is measured by an

analogue input to the PIC, after amplifying the

voltage drop across the small R

sense

resistor.

The presented ES system incorporates a circuit to

limit the charge injected for safety purposes. The

charge increases with the amplitude of the current

and the phase duration. The maximal current

allowed was designed taking into account the

electrode-tissue interface area to avoid hazardous

charge densities (Shannon, 1992). This depends on

DEVELOPMENT OF AN INTEGRATED ELECTRICAL STIMULATION SYSTEM WITH FEEDBACK FOR

PHYSICAL REHABILITATION

the size of the electrodes used. The capacitor C

safe

Figure 5 is charged by the small voltage drop across

sense

. The comparator module of the microcontroller

used is programmed to automatically shutdown the

PWM signal used in the SRVE when the voltage

across C

safe

reaches V

safe

. At the same time, the

comparator output pin pulls down the gates of the

waveform driver circuit to values below the

conduction threshold. In case of undesired charge

densities, the voltage elevation and the current flow

across R

load

stops immediately.

Feedback information is measured by a 3-axis

digital accelerometer MMA7456L manufactured by

Freescale Semiconductor. This sensor is applied in

functional electrical stimulation.

4 EXAMPLE OF APPLICATION

In this section we present an example of application

of the feedback ES device described above. The

prototype used is shown in Figure 6. The circuit is

Figure 6: Image of the electrostimulator prototype.

Figure 7: Snapshot of the oscilloscope monitor displaying

a biphasic pulse as obtained experimentally with the

prototype. 10x amplified probes were used in this picture.

Figure 7 shows a biphasic pulse obtained

experimentally with this prototype. For the purpose

of this figure a load resistance of 10kΩ was used.

The pulse is symmetric and biphasic. It has a pulse

duration of 300μs, an interphase interval of 50μs and

12mA of phase amplitude.

For a matter of illustration of the prototype at

work we designed an experimental protocol that

consisted of applying ES stimuli on the biceps of

healthy subjects to produce non-voluntary

movements. It was defined that the amplitude of the

stimulation should never exceed 17mA.

Furthermore, the amplitude of the ES should

decrease anytime the arm reached a maximal angle

of 83º (see Figure 8) and it should increase again if

the arm fell below 41.5º.

Figure 8: The ES exercise applied to the biceps of a

healthy subject. In this figure we also show how the

amplitude of the arm movement was measured has a result

of the applied electrical stimulation.

Typical examples of the results obtained are

shown in Figure 9. In the two exercises reported

different ES frequencies were applied. It can be

observed that the amplitude of the electrical

stimulation never exceeds the pre-defined threshold

value and that the evolution of the electrical

stimulation changed whenever the arm overcame the

maximal predefined amplitude of the movement.

These results clearly show a frequency dependent

muscle response that can be characteristic of muscle

fibre type composition which can be valuable for

clinical purposes.

BIODEVICES 2010 - International Conference on Biomedical Electronics and Devices

Figure 9: The amplitude of the arm movement, θ (in stars),

and of the applied electrical stimulation (triangles) as a

function of time. The stimulation frequencies were 30Hz

(Top) and 50Hz (Bottom).

5 CONCLUSIONS

This article presents an inexpensive small-sized

transcutaneous electrical stimulator unit capable of

integrating feedback information arising from a

digital accelerometer with electrostimulation. The

delivered pulsed currents can be controlled in real

time and according to a programmed protocol, on a

number of parameters - pulse duration and

amplitude, pulse rate, the type of current modulation

(burst, interrupt or ramp modulation).

In the future more elaborate ES programs should

be developed to deliver optimal ES adjusted to a

patient’s needs. These programmes can use higher

computational power and sophisticated theoretical

models of muscle function, to design the most

efficient ES programmes.

ACKNOWLEDGEMENTS

This work was supported by FCT project RIPD /

SAU-OBS / 63630 / 2005. ED is supported by FCT

SFRH / BD / 31218 / 2006.

REFERENCES

Al-Majed, Abdulhakeem A., Neumann, Catherine M.,

Brushart, Thomas M., Gordon, Tessa, 2000. Brief

Electrical Stimulation Promotes the Speed and

Accuracy of Motor Axonal Regeneration, The Journal

of Neuroscience.

Buckley, D. C., Kudsk, K. A., Rose B., Koetting C. A.,

Schlatter M., Miller C. A., 1987. Trans-cutaneous

Muscle Stimulation Promotes Muscle Growth ni

Immobilized Patients, Journal of Parenteral and

Enteral Nutrition.

Cheng, Eric K.W., Tong, Lu, Yan, Tong, Kai-Yu, Rad,

A.B., Chow, Daniel H.K., Sutanto, Danny, 2004.

Development of a Circuit for Functional Electrical

Stimulation, IEEE Transactions on Neural Systems

and Rehabilitation Engineering, Vol. 12, No.1, March.

Doucet, Barbara M, Griffin, Lisa, 2008. Maximal versus

submaximal intensity stimulation with variable

patterns, Muscle and Nerve.

Durfee, W. K., 1999. Electrical stimulation for restoration

of function, Neurorehabilitation.

Franz, C. K., Rutishauser, U., Rafuse, U. F., 2008.

Intrinsic neuronal properties control selective

targeting of regenerating motoneurons, Oxford Univ

Press.

Hennings, Kristian, Kamavuako, Ernest Nlandu, Farina,

Dario, 2006. The recruitment order of electrically

activated motor neurons investigated with a novel

collision technique, Clinical Neurophysiology.

Kitchen, Sheila, 2002. Electrotherapy: Evidence-Based

Practice, Churchill Livingstone, 11

edition.

Langzam, E., Nemirovsky, Y., Isakov, E., Mizrahi, J.,

2006. Muscle enhancement using closed-loop

electrical stimulation: Volitional versus induced

torque, Journal of Electromyography and Kinesiology.

McPartland, M. D., Mook, D. J., 1995. A Robust

Transcutaneous Electro-Muscle Stimulator (RTES) - A

Multimodality Tool, Medical Engineering & Physics.

Merrill, Daniel R., Bikson, Marom, Jefferys, John G.R.,

2005. Electrical stimulation of excitable tissue: design

of efficacious and safe protocols, Journal of

Neuroscience Methods.

Neacsu, Dorin O., 2006. Power-Switching Converters

Medium and High Power, CRC Press.

Nelson, Roger M., Hayes, Karen W., Currier, Dean P.,

1999. Clinical Electrotherapy, Appleton & Lange.

Stamford, Connecticut, 3

edition.

Pan, Feng, Samaddar, Tapan, 2006. Charge Pump Circuit

Design, McGraw-Hill.

Pressman, Abraham I., 1998. Switching Power Supply

Design, McGraw-Hill, 2

edtition.

Shannon, Robert V., 1992. A Model of Safe Levels for

Electrical Stimulation, IEEE Transactions on

Biomedical Engineering, Vol. 39, No. 4, April

Vrbová, Gerta, Hudlicka, Olga, Centofanti, Kristin

Schaefer, 2008. Application of Muscle/Nerve

Stimulation in Health and Disease, Springer Science +

Business Media B.V.

Ye, Hong, Luo, Fang Lin, 2004. Advanced DC/DC

Converters, CRC Press LLC.

DEVELOPMENT OF AN INTEGRATED ELECTRICAL STIMULATION SYSTEM WITH FEEDBACK FOR

PHYSICAL REHABILITATION