MINIATURIZED WIRELESS CONTROLLED

ELECTROSTIMULATOR

Tiago Ara

ujo

1,2

, Neuza Nunes

and Hugo Gamboa

Faculdade de Ci

encias e Tecnologia, Universidade Nova de Lisboa, Lisbon, Portugal

PLUX - Wireless Biosignals, Lisbon, Portugal

Keywords:

Electrostimulation, Wireless, Portability, Hardware development, Closed loop.

Abstract:

This project introduces a new approach to hardware and software controlled solutions in the electrical stimula-

tion ﬁeld. A miniaturized, portable and wireless electrostimulator was designed and its development steps and

also a new perspective to control the stimulation parameters in real time are exposed in this paper. Our system

allows the control and automation of the stimulation session with high ﬂexibility and easiness, using a user-

friendly interface for a computer or an Android platform, which communicates with the portable and wireless

device. The hardware performance was tested with a skin electric model, achieving the expected results. The

presented solutions have high applicability in the scientiﬁc and ambulatory electrostimulation context.

1 INTRODUCTION

The electrical stimulation is a technique widely used

in research with contributions of high applicability in

the clinical environment and the sports ﬁeld. Com-

bined with surface electromyography, this technique

enables a better understanding of neuromuscular re-

cruitment, exploring opportunities for study and de-

velopment of protocols for performance evaluation

and physical rehabilitation (Lake, 1992).

The electrical stimulation (ES) is a nervous acti-

vation generated by the application of low frequency

electric current, which will produce a recruitment ef-

fect in the stimulated muscle group. The electri-

cal stimulation can be applied directly on the mus-

cle’s surface, designed by electrical muscle stimula-

tion (EMS), or in the nerve structure, designed by

functional electrical stimulation (FES) (Bajd et al.,

1999; Thrasher and Popovic, 1999).

The stimulation of a particular muscle group is

controlled by frequency, amplitude and pulse width

of the electrical current applied (Robertson et al.,

2006; Cheng et al., 2004). Currently, the electros-

timulator systems can be classiﬁed into two types:

the open-loop system and closed-loop system. The

ﬁrst is characterized only by the stimulus control ap-

plied through the electrostimulator. The latter stands

out not only for controlling the intensity, frequency

and pulse width but also by the outcome evaluation

of muscle response to stimulation. This assessment

is made by processing the signal from sensors such

as accelerometers, goniometers, gyroscopes and elec-

tromyography sensors (Zhang et al., 2007). The sig-

nals obtained with these sensors will enable the anal-

ysis and assessment of the muscle’s recruitment re-

sponse and, at the same time, provide feedback for

the stimuli generation, adjusting the stimulus to its

response. Despite the characteristics reported above,

current systems that integrate electrostimulation usu-

ally have limitations in terms of usability and portabil-

ity or in terms of control ﬂexibility and synchroniza-

tion. This paper exposes a solution to overcome these

limitations by developing a miniaturized and portable

electrostimulation device, capable of controlling the

stimulation parameters with high ﬂexibility and en-

abling synchronization with external devices.

In the following section the development of the

electrostimulation system’s hardware and control in-

terface is described and we also depict the speciﬁca-

tions and characteristics of this system. Section 3 will

report and discuss the performance results of the sys-

tem in a skin electric model. Section 4 concludes the

work by stating its main contributions and exposing

the advantages of the developed system.

337

Araújo T., Nunes N. and Gamboa H..

MINIATURIZED WIRELESS CONTROLLED ELECTROSTIMULATOR.

DOI: 10.5220/0003772103370340

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2012), pages 337-340

ISBN: 978-989-8425-91-1

 2012 SCITEPRESS (Science and Technology Publications, Lda.)

2 SYSTEM DEVELOPMENT

2.1 System Control

The hardware was designed and developed to allow

greater ﬂexibility in the stimulation control param-

eters. This implementation delegates the real-time

control in an external application where it’s possible

to manipulate the entire stimulation protocol. The

hardware architecture accepts the input commands

from a computer or an Android Smartphone via blue-

tooth, raising new possibilities of portability and pro-

cessing performance in the electrostimulation ﬁeld.

Currently, both designed interfaces allow the user to

choose the frequency, duty cycle, amplitude in volt-

age and in current of the stimuli, as shown in ﬁgures

1 and 2. It’s also possible to choose the channel and

time of stimulation. In the customizable conﬁgura-

tions of the application the user can choose a pulsed

stimuli session.

Figure 1: Computer interface scheme for stimulation ses-

sion control.

Figure 2: Android interface scheme for stimulation session

control.

2.2 Hardware

In addition to the high control ﬂexibility, the elec-

trostimulator is developed in a real-time wireless

data communication, low power consumption and

high electromagnetic interference immunity guide-

line. The system hardware is internally divided into

three modules, a digital, an analogue and a power

module (as shown in Figure 3):

i) The digital module integrates the bluetooth commu-

nication module and the microcontroller. This mod-

ule establishes communication with the control soft-

ware, generates and controls the parameters of electri-

cal stimulation - as the total time of stimulation, fre-

quency, duty cycle and amplitude;

ii) In the analogue module the stimulation signal

is processed and ampliﬁed to guarantee the desired

power voltage and current;

iii) The power module supplies the analogue and dig-

ital modules. This module also controls the battery

level and charging cycle. Whenever it is connected to

supply voltage produces the decoupling between the

supply and the stimulation circuit, protecting the user.

2.3 Speciﬁcations and Characteristics

The device presented is a miniaturized and portable

system with 6.5 x 10.5 x 1.9 cm dimensions. It has

two independent channels for stimuli output and in

order to be able to work as a closed-loop system, it

also has a synchronization port that allows real time

communication with a signal acquisition system, such

as the bioPLUX system (PLUX, 2011), which enables

the measurement of the muscle response to the stim-

ulus induced. Figure 4 presents a scheme of the sys-

tems communication.

The current version of this electrostimulator en-

ables the generation of 0-250Hz frequencies with

a resolution of 16 bits. The ampliﬁcation module

ranges from 0-6V in voltage and 0-4mA in current.

The hardware performs the automatic adjustment of

the voltage value if it is in constant current mode, or

current if it is in constant voltage mode. The rang-

ing values that the system allows are still too low for

the application of surface electrical stimulation in hu-

mans but have high applicability in invasive applica-

tions with, for example, experimental rats where the

supra-maximal tension of their nerve is 3mA. The

next development step is the adaptation of the exist-

ing ampliﬁcation module for human non-invasive ap-

plication.

BIODEVICES 2012 - International Conference on Biomedical Electronics and Devices

338

Figure 3: Hardware schematics.

Figure 4: Illustration of the system communication and

control ﬂexibility. Example with invasive and non-invasive

electrostimulator.

3 SYSTEM PERFORMANCE

To evaluate the current and voltage automatic regu-

lation, we tested the circuit with a skin impedance

model (Figure 5). In this model, the resistor R1 mod-

els the constant resistance components of the skin and

deep tissue resistance. The R2 resistor, with C1 ca-

pacitor in parallel, represents the nonlinear dynamics

of the skin impedance (Dorgan and Lake, 1999). The

advantage of a voltage and current regulation is that

we surpass the variations of the tissues galvanic resis-

Figure 5: Skin impedance model. From (Dorgan and Lake,

1999).

Table 1: Current variation vs load variation in constant cur-

rent mode.

Resistance (kΩ) Current (mA) Tension (V)

1.72 3.00 5.20

1.29 3.00 3.90

0.99 3.00 2.90

0.72 3.00 2.20

0.25 3.00 0.80

tance which are being stimulated, ensuring the same

current or tension.

To test the constant current mode we programmed

a unipolar square wave stimulation pulse with 3mA

and variate the model resistor, monitoring the tension

and current variations. In table 1 we report the results

obtained in the constant current mode.

The same approach was used to test the constant

voltage mode. A unipolar square wave stimulation

pulse with 3V of amplitude was programmed and

we varied the model resistor while the peak-to-peak

voltage and current values were acquired. In table 2

MINIATURIZED WIRELESS CONTROLLED ELECTROSTIMULATOR

339

Table 2: Voltage variation vs load variation in constant volt-

age mode.

Resistance (kΩ) Current (mA) Tension (V)

1.80 1.67 3.01

1.50 2.00 2.97

1.31 2.30 3.05

1.19 2.53 3.01

0.82 3.64 2.93

Figure 6: Effect of the skin nonlinear dynamics represented

by the C1 capacitor: a) With C1=1µF; b) Without C1.

we report the results obtained in the constant voltage

mode.

As it was expected, in constant current mode the

voltage varies to equalize the variation in the resis-

tance, keeping the applied current constant.

The same effect, but now applied in the current is

veriﬁed in the constant voltage mode were the current

compensate the resistance variations keeping the ap-

plied voltage in the desired value. In constant voltage

mode, the system can’t keep the voltage regulation for

load values bellow 750Ω, because the system is cur-

rently limited to 4mA.

The implementation of this model enabled the

evaluation of the effect of the skin impedance non-

linear dynamics. For values of C1 higher than 1 µF

we observed an overshooting effect which can exceed

about 20% the programmed current value. This effect

is exposed in Figure 6, which shows the signal of cur-

rent corresponding to a square wave of 2Hz and 3mA

of amplitude with and without the C1 effect.

With this study we evaluated the constant current

and constant voltage mode and guaranteed the correct

operation of the developed device.

4 CONCLUSIONS

The main contributions of this work are related with

the high ﬂexibility in the control of the stimulation

session in association with the portability and low di-

mensions of the device.

With this device is possible to automate a stimu-

lation session and change it in real time, allowing, for

example, an external evaluator to analyse the differ-

ences in gait according to the protocol that is applied.

The high portability of this system and its user-

friendly characteristics may allow its usage in a pa-

tient home through ambient assisted living with a real-

time protocol controlled in a web based environment

by the physician.

5 FUTURE WORK

As a continuation of this work, we are now testing the

device in experimental animals, with good results to

the date.

We are also implementing a stimuli wave form

control, which will enable the study of the effect of

different wave forms for stimuli application in the

muscular structures.

Another advance to this work resides with the

adaptation of the hardware’s ampliﬁcation module for

surface electrostimulation application.

REFERENCES

Bajd, T., Kralj, A., Stefancic, M., and Lavrac, N. (1999).

Use of functional electrical stimulation in the lower

extremities of incomplete spinal cord injured patients.

In Artif Organs, 23(5):403-9.

Cheng, K., Lu, Y., Tong, K., Rad, A., Chow, D., and Su-

tanto, D. (2004). Development of a circuit for func-

tional electrical stimulation. In IEEE Transactions

on Neural Systems and Rehabilitation Engineering,

vol.12, no.1.

Dorgan, S. and Lake, R. (1999). A model for human skin

impedance during surface functional neuromuscular

stimulation. In IEEE Transactions on Rehabilitation

Engineering, vol. 7, no. 3.

Lake, D. (1992). Neuromuscular electrical stimulation. an

overview and its application in the treatment of sports

injuries. In Sports Med 13 320-36.

PLUX (2011). PLUX - Wireless Biosignals S.A.

www.plux.info.

Robertson, V., Ward, A., Low, J., and Reed, A. (2006).

Electrotherapy Explained: Principles and Practice.

Elsevier Health Sciences, 4th edition.

Thrasher, T. and Popovic, M. (1999). Funcional electrical

stimulation of walking: Function, exercise and reha-

bilitation. In Ann Readapt Med Phys. 51(6):452-60.

Zhang, D., Guan, T., Widjaja, F., and Ang, W. (2007). Func-

tional electrical stimulation in rehabilitation engineer-

ing: A survey. In Proceedings of the 1st international

convention on Rehabilitation engineering and assis-

tive technology.

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