Muscle Function Restoration by Bioelectrical Stimulation

Xiao Li

Imperial College London, London, SW7 2AZ, U.K.

Keywords: Muscle Rehabilitation, Functional Electrical Stimulation, Implantable FES.

Abstract: There are millions of people who are suffered from neurological diseases such as stroke and spinal cord

injury, and their nervous systems are damaged from rapid muscle denervation, caused by neurological

diseases, which results paralysis or muscle weakness. In order to solve these problems and meditate

inconvenience of patients, muscle refunction and rehabilitation are necessary to be introduced. Besides, as

human stepping into electrical age, bioelectrical stimulation is developed. This paper covers the effect of

bioelectrical stimulation on muscle refunction and rehabilitation, as well as different applications for

bioelectrical stimulation. In addition, different topologies for bioelectrical stimulation are also included.

1 INTRODUCTION

Nerve cells and muscle cells are significant to human

body and these two types of cells are deeply

correlated with each other. Nervous system is

combined by Central Nervous System (CNS) and

Peripheral Nervous System (PNS). Central Nervous

system includes brain and spinal cord, and Peripheral

Nervous System includes autonomic nervous system,

which is involuntary, and somatic nervous system,

which is voluntary. CNS receives the sensory signals

from PNS, and also sends motor signals to PNS to

trigger movements. Neuron has three main parts.

Dendrites receive chemical input from other neurons;

soma, the main body of neuron, integrates the

information received from dendrites; and Axon

conveys electrical signals out of the cell. Electrical

signals include the signal which triggers muscle

contraction (Malmivuo et al. 1995).

From the perspective of biology, Nerves always

work along with action potentials, and damaged

nerves are not able to perform action potentials

regularly by themselves. Thus, electrical stimulation

is introduced to depolarizes neuron cell membrane,

by creating a localized electric field, to reach a

critical threshold and generate action potential which

propagates in both directions away from where it

stimulated. Stimulation to motor nerves triggers

muscle contraction which can be used to rehabilitate

muscle functions. Luigi Galvani applied electrical

wires to leg muscles which cleaved from the frog

body in 1790 and the motion was observed. Michael

Faraday demonstrated that movement can be created

by using electrical currents to stimulate nerves in

1831 (Cambridge NA, 1997). Moreover, Electrical

stimulation for peroneal nerve in hemiplegia patients

attempted to correct foot drop during ambulation in

early clinical experiments (Liberson et al. 1961).

Thus, electrical stimulation has the great potential in

the field of rehabilitation recovery, including muscle

strength improvement, range of motion increment,

edema reducing, tissue healing, pain relieving, etc.

(Doucet et al. 2012).

In this paper, the topic is focused on how to use

bioelectrical stimulation to help people with mobility

dysfunction, such as patients with spinal cord

injuries, to reactivate muscle contraction and

partially restore their mobility. This review firstly

aims to introduce backgrounds for bioelectrical

stimulation, mainly functional electrical stimulation,

which pairing a functional task with electrical

stimulation, as well as implantable FES. Secondly,

this review explains how to stimulate tissues by

electrical input with examples of different topologies.

Moreover, prospects for the future of this technology

are also included in the conclusion aiming to provide

any constructive suggestions for future studies.

2 FUNCTIONAL ELECTRICAL

STIMULATION

Functional electrical stimulation (FES), which is

similar to neuromuscular electrical stimulation but

Li, X.

Muscle Function Restoration by Bioelectrical Stimulation.

DOI: 10.5220/0011207400003443

In Proceedings of the 4th International Conference on Biomedical Engineering and Bioinformatics (ICBEB 2022), pages 349-354

ISBN: 978-989-758-595-1

349

pairing the stimulation with a functional task.

Paralyzed muscle contractions, triggered by using

short electrical pulses, is programed to fit with

different tasks. For instance, multiple muscles,

flexor muscles and extensor muscles, connected

with the joint are stimulated with different

stimulation intensity delivered to achieve different

joint angles. Wrist extensors and finger flexors are

stimulated to contract the fingers around an object in

order to facilitate a grasping task. Flexion of the

shoulder and elbow extension is also triggered by

FES to produce a forward reaching motion. FES is

used most commonly for spinal cord injured (SCI)

individuals to improve their motor functions. For

example, FES can be used to reproduce the

activation pattern of lower extremity muscles to

produce human gait (Lynch et al. 2008).

Electrical pulses are delivered via electrodes.

Electrodes can be placed in four ways:

transcutaneous (on the skin surface), epimysial (on

the surface of the muscle), percutaneous (in the

muscle), or cuff (motor nerve surrounding) (Popovic

et al. 2000). Frequency and intensity, which is the

amount of charge input in muscle, of stimulation

decide the level of the contraction (Lynch et al.

2008).

The stimulus pulses for FES usually have both

cathodic phase and anodic phase. Cathodic phase

triggers the action potential along with harmful

electrochemical processes occurred at the interface

between electrode and tissue. Anodic phase

followed by cathodic phase reverses those damage

by neutralizing the charge accumulated during

cathodic phase, to avoid tissue damage, and the

charge of two phases are ideally equal. Figure 1

illustrated three types of stimulus waveforms.

Figure 1: Stimulus waveforms. (a) Monophasic. (b) Biphasic with active cathodic and active anodic phases. (c) Biphasic

with active cathodic phase and passive anodic phase (exponential decay) (Demosthenous, 2014).

People with spinal cord injury might not able to

complete fundamental behavior tasks such as

walking, standing, jumping, due to their insufficiency

of lower extremities strength. Thus, FES is

introduced to facilitated with finishing fundamental

tasks. Figure 2(b) shows how FES works to

compensate the nonfunction of muscle caused by

injury. For intact nervous system, each motor unit are

stimulated at a frequency of 6-8 Hz, and adjacent

units are sequentially stimulated, which overall

triggers muscles to produce tetanic contractions, and

finally triggers the change of knee angle. Previous

mechanism is called asynchronous recruitment. For

SCI individuals, motor units are stimulated by FES at

the same time, which what we called synchronous

recruitment, and different from the asynchronous

recruitment happens in complete nervous system.

Because FES has to stimulate the motor units

synchronously, a higher frequency is required which

ranges from 20 to 40 Hz to trigger tetanic

contractions.

ICBEB 2022 - The International Conference on Biomedical Engineering and Bioinformatics

350

Figure 2: Different mechanisms for the production of tension in intact nerve system individuals and spinal cord injury

individuals.

A further application for FES is FES facilitated

muscle rehabilitation. As mentioned previously, SCI

individuals might not able to complete fundamental

behavior tasks. Thus, immobility will directly cause

insufficiency of physical activity which is always a

problem along with multiple syndromes such as

muscular atrophy, cardiovascular disease, type 2

diabetes, etc. To increase the level of physical

activities of SCI individual, FES training machine,

which is the combination of typical training machine

and FES system, is introduced. FES triggers

paralyzed muscles to produce tetanic contractions,

which is crucial for SCI individuals to complete the

training tasks. One representative product for FES

training machine is motor driven FES rowing

machine. SCI individuals are not able to complete

normal rowing machine training due to their

insufficiency of low extremities strength. Thus,

motor-driven FES machine is able to support them

to complete the rowing. This machine is constructed

by a chair with inclination control, control program,

leg supporter, motor system, and the most

importantly, the four-channel FES system (MEGA

XP, Cybermedic Corp.) which is surrounding the

legs to stimulate hamstring and quadriceps muscles.

As mentioned previously, FES synchronously

stimulates the motor units which leads the frequency

ranging from 20 to 40 Hz. The FES rowing machine

stimulates through surface electrodes to the

hamstring and quadriceps muscles with a frequency

of 30 Hz. An optical encoder which senses the seat

position and controls the input of stimulation is used

to construct a closed-loop and feedback control FES

system (Kim et al. 2014).

3 STIMULATION CIRCUIT

To achieve stimulation, at least two electrodes are

needed in the circuit to produce current flow.

Current stimulation mode and voltage stimulation

mode, which is shown in Figure 3, are two main

stimulation modes. voltage stimulation use voltage

as its output and current stimulation use current. The

output of voltage is constant, thus the generated

current delivered to the tissue, depends on the

inter-electrode impedance. A sequence of voltage

steps has to be introduced to control the charge

supply, which requires large number of capacitors

and not able to be on-chip. Thus, voltage stimulators

usually use with surface electrodes. The magnitude

of the current delivered to the tissue is controlled by

current stimulation and not dependents with

inter-electrode impedance, and current stimulation is

suitable with implanted electrodes.

Muscle Function Restoration by Bioelectrical Stimulation

351

Figure 3: (a) current stimulation circuit. (b) voltage stimulation circuit (Suo et al. 2013).

As mentioned previously, biphasic stimulus

pulses are usually used in bioelectrical stimulation

because anodic phase followed by cathodic phase

neutralizing the accumulated charge and avoid tissue

damage. Thus, the total charge in cathodic phase and

anodic phase has to be balanced. To achieve charge

balancing, three classic biphasic current stimulators

topologies are introduced below in Figure 4.

Figure 4: Three main topologies for charge balancing. (a) Dynamic current balancer (Sit et al. 2007); (b) Active charge

balancer (Ortmanns et al. 2007); (c) H-bridge with multiple current sinks (Williams et al. 2013)

Dynamic current balancer includes two sampling

switches, and two more switches for cathodic phase

and anodic phase respectively. Two Ssamp close to

form a close circuit. Because of feedback, the

amplitude of M1 drain current is equals to the

current sink, and the bias voltage on M1 is sampled

and held. Then, two Ssamp open and only Scathodic

closes to form the current which stimulates the

tissue. Then Sanodic closes and Scathodic opens,

and the held bias voltage results the anodic current

has the same amplitude with cathodic current to

reach charge balancing. Active charge balancer

combines voltage monitor on electrodes and uses

two switches, Sanodic and Scathodic, to manipulate

the circuit to achieve charge balancing. only

Scathodic closes firstly and the current, get

measured by Voltage Monitor, reaches to the tissue

and go to VSS; then during anodic phase, Sanodic

closes and Scathodic opens, the monitored amount

of charges are coming out from VDD to the tissue

for the neutralization. “H-bridge” configuration uses

four switches in two groups to form cathodic current

and anodic current. S1 and S4 firstly close to form

cathodic current. Then, S1, S4 open and S2, S3 close

to form anodic current to neutralize the accumulated

charge.

Blocking capacitors are usually used in

stimulator circuits. When the circuit failed, blocking

capacitors avoid DC connection to supplies to

ensure safety. Secondly, they also ensure the

electrodes have no net charge. In addition, adding

blocking capacitors into stimulation circuit realize

active discharge function.

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4 IMPLANTABLE FUNCTIONAL

ELECTRICAL STIMULATION

Implantable functional electrical stimulation is

becoming a more popular direction of research due

to its high mobility. Moreover, because the FES

devices are implanted into human body, not

percutaneous, the impedance is lower which leads to

a lower power consumption directly. A typical

example for implantable FES is implantable device

for hand grasping which is shown in Figure 5. The

hand grasping device uses an external control unit,

which provides energy source and particular

program, that connects to an implantable stimulator.

The implantable stimulator connects with group of

electrodes used for neuromuscular stimulation to

trigger muscle contraction and finish tasks. The

stimulator also connects with electromyography

(EMG) recording electrodes to measure the

electrical activities of muscle in responding to nerve

stimulation. The connection between external

control unit and implant stimulator can be wireless.

The data and power transfer are via inductively

coupled power transfer. The efficiency of

Inductively coupled power transfer in its working

range is really high, especially for high power

delivery, which has a total efficiency to be greater

than 95%. Moreover, the range of its output power

levels is from milliwatts to tens of watts which

indicates that it can be used in a huge variety of

implantable devices (Schormans et al. 2018).

Figure 5: Diagram of implantable hand grasping FES

device (Hart et al. 1998).

5 CONCLUSIONS

In this paper, the fundamental principles and

applications about bioelectrical stimulation is

introduced. The development of bioelectrical

stimulation is boosted in these two decades.

However, the space of bioelectrical stimulation

development and application is big. For instance,

one problem for stimulator is power consumption.

Bioelectrical stimulation can be divided into

implanted and not implanted. The power

consumption for implanted stimulator is always a

breakthrough point because no direct power

transmission cable is connected. Thus, to achieve

wireless power transfer. Inductively coupled power

transfer (ICPT) is introduced as the most capable

way of power transfer for implanted stimulator

(Schormans et al. 2018). With the correct way of

power transfer, the efficiency of ICPT is the biggest

research field currently. The realization of wireless

fast charging safely is a meaningful goal. The

combination of bioelectrical stimulation with

close-loop control has brought bioelectrical

stimulation into the cross field of biotechnology,

electronic engineering, and programing. The

monitor can track different behaviors and muscle

movements and upload the feedback into computer,

and software engineers can analysis the big data and

development different programs which is able to

simulate the muscle movement. The simulation

program can be updated and transmitted back into

the stimulator to realize iteration.

In the future, big data analysis is able to

prognosis the next muscle movement and stimulate

with less lag to reduce delay of actions. Moreover, a

new innovation for bioelectrical stimulation is the

application of brain-computer interface (BCI) and

nanorobots. Trillions of nanorobots which controlled

and monitored by brain-computer interface is

launched into different part of human body. For

instance, when a SCI individual is willing to move

legs, electrical signal from brain is recorded and

decoded into binary language which controls

nanorobots, located on paralyzed leg muscle, to

stimulate the neuron and trigger muscle contraction,

and overall realize the leg movement. The

innovation sounds surrealistic, but with the rapid

development of scientific and technological level,

BCI and nanorobotic induced bioelectrical

stimulation is feasible and is able to be popularized

in several decades.

ACKNOWLEDGEMENTS

Grateful acknowledgement is made to professor

Andreas Demosthenous who gave me two weeks of

guidance and inspiration during the summer project:

Muscle Function Restoration by Bioelectrical Stimulation

353

Bioelectronics for Applications in Implantable and

Wearable Medical Devices.

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