Abstract Studies involving sensory substitution or feed-back date back to the 1960s. These seek to apply stimuli tobody regions with full sensitivity to replace the informationlost due to damaged, deficient, or absent sensory pathway.The main techniques applied are electrotactile stimulation(ETS) and vibrotactile stimulation. The objective of this arti-cle is to present a synopsis of current knowledge regardingtechnologies that employ electrical, mechanical, and pneu-matic resources in the sensory system as tools for physicalrehabilitation. ETS can be applied as a therapeutic interven-tion in the treatment of equilibrium disorders, as systemsof myoelectric prostheses in amputees whose mechanore-ceptors were removed together with the limb, as artificialskins developed prior to the application of prostheses andas prophylactic treatments for pressure sores. Vibrotactileor mechanotactile sensory substitutions that employ electro-mechanical and piezoelectric devices or inert gas jets facili-tate the training of equilibrium maintenance in patients withvestibular alterations, as well as the development of alterna-tive and expanded communication technologiesin signaling
E. Krueger · J. C. da Cunha · E. M. Scheeren · P. Nohama (B)Laboratory of Rehabilitation Engineering, UniversidadeTecnológica Federal do Paraná (UTFPR), Curitiba, PR, Brazile-mail: [email protected]
J. C. da CunhaDepartment of Computer Engineering,Universidade Positivo (UP), Curitiba, PR, Brazile-mail: [email protected]
E. M. ScheerenSchool of Health and Biosciences and Polytechnic School,Pontifícia Universidade Católica do Paraná (PUCPR),Curitiba, PR, Brazile-mail: [email protected]
and augmented reality for people with auditory and/or visualdisabilities. The cybernetic resources developed for physi-cal rehabilitation described herein were shown to efficientlygenerate sensory stimuli and the artificial control of tactileperception. Notwithstanding the greater functional simplic-ity of mechanotactile systems, the majority of the reviewedstudies focused on ETS systems. The results of this studydemonstrate the progress and potential of these technolo-gies, as well as the results of evaluations of their use andcurrent limitations in the electrical and mechanical devicesemployed in functional sensory system rehabilitation.
Resumo As pesquisas envolvendo substituição ou realimen-tação sensorial remontam à década de 1960. Visam a apli-cação de estímulos em regiões do corpo com sensibilidadeintacta para suprir as informações perdidas devido a um viasensorial lesado, deficiente ou ausente. As principais técnicasaplicadas são a estimulação eletrotátil (EET) e a vibrotátil.O objetivo deste artigo é apresentar uma síntese do conheci-mento atual sobre tecnologias que empregam recursos elétri-cos, mecânicos e pneumáticos no sistema sensorial comoferramenta para a reabilitação física. A EET pode ser apli-cada em: (a) intervenção terapêutica no tratamento de distúr-bios de equilíbrio; (b) sistemas de próteses mioelétricas, empacientes amputados que tiveram a extirpação dos mecanor-receptores juntamente com o membro; (c) peles artificiaispreviamente desenvolvidas à aplicação de próteses e (d) trata-mento de profilaxia de úlceras de pressão. A substituiçãosensorial vibrotátil ou mecanotátil, empregando dispositivoseletromecânicos, piezoelétricos ou jatos de gás inerte, con-tribui na facilitação do treino à manutenção de equilíbrio empacientes com alteração vestibular, bem como no desenvolvi-
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mento de tecnologias de comunicação alternativa e ampliadade pessoas com deficiência auditiva e/ou visual, em sinal-ização e em realidade aumentada. Os recursos cibernéticosdesenvolvidos para reabilitação física descritos neste artigomostram-se eficientes na geração de estímulos sensoriais econtrole artificial da percepção tátil. Apesar da maior natural-idade funcional contida nos sistemas mecanotáteis, a maioriados trabalhos revisados concentra-se no sistema de estimu-lação eletrotátil. Os resultados dessa pesquisa demonstrama evolução e as potencialidades dessas tecnologias, assimcomo os resultados da avaliação de seu uso e as limitaçõesatuais dos dispositivos elétricos e mecânicos empregados nareabilitação funcional do sistema sensorial.
As a person has a large area of skin, even when one of theprimary senses (as sight, hearing, and proprioception) is lost,the tactile sensation may serve as an alternative input sensorychannel to supplement or replace information that would beintended for other senses. A tactile display can be placedunder the clothes to avoid cosmetic changes, rejection by theuser, and social prejudices, it can be light and small, and canprovide large amounts of information tightly packed withinan useful spatiotemporal response range containing signalsrelatively free of environmental noises.
The substitution systems based on tactile function canbe used as sensory feedback in neural prosthetics, artificiallimbs, hands, and feet due to Hansen’s disease, artificial voicecoders for speech perception, reading for blind applicationsin telepresence and virtual environments and even in secretcommunications aimed at military applications, or astronautsin space facing the lack of feeling when in contact withobjects because their thick and pressurized gloves which alsoreduce tactile sensation. Moreover, when performing robot-assisted surgery, they can provide information to the surgeonsabout force and cutaneous feedback.
Sensory substitution is used daily in mobile phone devicesthrough mechanical vibration alert systems. Such technologyallows the user to substitute auditory information by tactilestimulation. Moreover, mobile devices could provide tactilesensory feedback for posture maintenance in addition to sub-stituting for hearing (Vuillerme et al. 2011).
The main techniques of sensory substitution are elec-trotactile (ETS) (Barros et al. 2010, Tyler et al. 2009),vibrotactile (Dobrzynski et al. 2012), and mechanotactilestimulation through gas jets (Cunha and Nohama 2012).Electrical stimulation is a widely used technique in phys-ical rehabilitation because it provides physiological bene-fits, such as electroanalgesia (Biggs et al. 2012), neuromus-
cular activation (Krueger-Beck et al. 2011b), tissue regen-eration (Krueger-Beck et al. 2011a), and motor control offunctional movements, among others. The main advantagesof current developed electrotactile systems are: low-power,small size devices, well- controlled parameters, reproduciblesensations, large useful bandwidth, and great number of mod-ulating codes. Its principal limitations are: the great numberof electrodes, difficulty to place the electrodes in exactlythe same place, inconveniences of placing and removal ofelectrodes every day, possibility of generating uncomfortablesensations and even pain, and keeping the same skin hydra-tion and same pressure against the skin. For vibrotactile andmechanotactile systems, the major advantage is the evoca-tion of more natural sensations; and the problems are: largesize, breakdown of moving parts, reduced useful bandwidth,difficulty to control and reproduce the perceived sensation,and high power consumption (Cunha and Nohama 2012).
The concept of sensory substitution was pioneered by Dr.Bach-y-Rita in the 1960s when he began the investigationon the mechanisms of tactile sensation and neural process-ing for visual information (Bach-Y-Rita et al. 1969) in orderto proportionate a certain kind of vision for blind peoplethrough the skin. At that time, the main limitations werethe lack of knowledge on the conversion of visual to tactileimages and neural plasticity, the size of the actuators andthe electronic circuitry, and its heating and power consump-tion.
For obtaining sensory substitution, the original informa-tion must be acquired and converted on a different kindof stimulus to be applied to sensitive regions on the body;for example, the application of electrical current by meansof a matrix of electrodes to the tongue may create (1) a2D tactile image in order to represent the original opticalimage and, consequently, substitute the natural visual sys-tem (Chekhchoukh et al. 2011), (2) balance (Wood et al.2009) generating dynamic tactile images for representingbody position, or (3) the kinesthetic features of hands duringan activity such as the artificial prehension in a spinal cordinjured individual using a neural prosthesis.
When ETS applications are intended to replace posturalinformation, as in the case of people with vestibular deficits orolder individuals, the resulting process for bringing back bal-ance is known as electrotactile vestibular substitution system(Vuillerme et al. 2011). Therefore, ETS can be used as a ther-apeutic intervention for balance disorders, such as those thatoccur post-stroke (Keshner et al. 2011). The wide versatilityof ETS makes applications possible in myoelectric prosthe-ses systems, because the mechanoreceptors were removed inamputees along the limb (Cipriani et al. 2012); in artificialskins (Yau et al. 2008), because the patients will perceiveprosthetic contact with the ground through the skin (Damianet al. 2011); and in the prophylactic treatment of pressuresores (Chenu et al. 2012).
273 abstracts were read 32 duplicates were eliminated
198 papers were excluded
(43 included papers)33 main content papers
10 complementary papers
Fig. 1 Flow chart of papers included in this study. *: it was includeda historical article on the subject from 1969
The progress of physical rehabilitation tools dependson the development of technologies such as computationalmodeling, which permits improvements to the quality ofdevices that provide postural feedback, because the equip-ment requires personalized information for each user basedon the current position, velocity and acceleration (Loughlinet al. 2011).
The objective of this article is to summarize the currentknowledge and thus delineate the progress and prospects oftechnologies that use electrical, mechanical, or pneumaticactuators in the sensory and neural system as a means ofphysical rehabilitation.
2 Materials and Methods
The English language was selected for the subject searchwhere similar studies to the present context were used. Thedata searching was performed combining the keywords ran-domly. Information about the applications of electrical andmechanical resources in the sensory system for physical reha-bilitation was extracted from the selected studies as inclusivecriteria as complementary papers to introduce the state of theart. Papers with the same technologies although with differentgoal, e.g., pain treatment or electrical stimulation to createartificial movements were excluded. A total of 43 articlesrelevant to the subject were selected, as shown in Fig. 1.
3 Results and Discussion
Table 1 summarizes studies from the last seven years (2007–2013) with regard to electrotactile stimulation (ETS) applica-
tions, researchers, number of volunteers and number of elec-trodes used, associated techniques, and desired objectives.Table 2 summarizes studies from the last six years (2008–2013) with regard to mechanotactile stimulation applica-tions, researchers, stimulation sites, modes of stimulation,and desired objectives.
The summary of studies from the last seven years (2007–2013) reflects the scientific and technological developmentsin sensory system-associated physical rehabilitation throughelectrotactile and mechanotactile actuators. The studies dis-cussed herein are organized into groups according to theirobjectives and the instruments used.
3.1 Sensory Substitution
Several studies involving sensory substitution were devel-oped to be used by people with physical disabilities, e.g.,Okada et al. (2007) tested an ETS system with the goal oforienting people with hearing losses relative to sound, inboth open and closed environments. The study intended thesystem to be capable of offering information with respectto the detection of sound occurrences, the direction of thesound source (front, back and sides) and the sound category(door bell, baby’s cry, fire alarm, telephone, tea kettle boiling,among others). Two microphones were positioned antero-posteriorly to the right sides of the volunteers. The electrodeswere positioned on the left hand, with channel 1 in the lateralregion (radial) and channel 2 in the medial region (ulnar).Each electrical stimulatory profile was associated to the typeof sound stimulus (43 dBA) that was occurring. Three con-secutive stimuli were triggered: the first stimulus alerted thepresence of a sound stimulus (“attention arousal”), the secondone informed the direction of the stimulus, and the third onedetermined the category of the stimulus (seven types), for atotal of twelve burst profiles. The parameters of the electricalstimulating current included a biphasic wave with an activepulse period of 150 µs and a frequency of 350 Hz (2.85 ms oftotal pulse period), with pulse amplitude modulation (PAM)that increased to 95 % of the amplitude of the pain thresh-old. In testing, 91 % of volunteers could correctly identifythe sound direction with a reaction time (volunteer interpre-tation time) of 1.21 s, and identification of the distinct soundinformation by the ETS achieved a correct recognition rateof 89 % with a reaction time of 1.62 s. Therefore, ETS appli-cation has been shown to be effective for the recognition ofsound direction and type in individuals with hearing losses.The Okada et al. (2007) sound categories tested have directapplication to the safety or wellness of the hearing lossespeople generating a social impact in their life. One safetyapplication is the residential smoke detectors that have anaudible fire alarm system. These people should be able tohear it (or better, acquire it by alternative means) to take anyaction. The technology upgrade must be improved to allow
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Tabl
e1
App
licat
ions
ofel
ectr
otac
tile
stim
ulat
ion
Aut
hors
Vol
Ele
ctro
des
App
lied
with
Obj
ectiv
e
Poin
tsPo
sitio
ning
Woo
det
al.(
2009
)10
(HV
)14
4A
SRT
Gal
vani
cve
stib
ular
stim
ulat
ion
and
ET
SPE
Tyle
ret
al.(
2009
)6
(HV
)9
ASR
TE
TS
onth
eto
ngue
Stim
ulat
ion
atdi
ffer
ent
elec
tric
alvo
ltage
leve
lsB
arro
set
al.(
2010
)7
(BV
L)
Bra
inPo
rt�
ASR
TE
TS
onth
eto
ngue
PE
Vui
llerm
eet
al.(
2008
)8
(HV
)B
rain
Port�
ASR
TE
TS
onth
eto
ngue
PEon
firm
and
soft
surf
ace
(6-c
mfo
amla
yer)
Vui
llerm
eet
al.(
2011
)4
(UV
L)
Bra
inPo
rt�
ASR
TE
TS
onth
efe
etPE
onfir
man
dso
ftsu
rfac
e(2
-cm
foam
laye
r)V
uille
rme
etal
.(20
11)
10(H
V)
Bra
inPo
rt�
ASR
TV
ibra
tion
syst
emon
the
Ach
illes
tend
onPE
Rob
inso
net
al.(
2009
)1
(BV
H)
Bra
inPo
rt�
ASR
TE
TS
onth
eto
ngue
PE/G
ait
Oka
daet
al.(
2007
)4
(HV
)2
chan
nels
(8po
ints
)M
edia
l-la
tera
lreg
ion
ofth
ele
ftha
ndD
etec
tor
ofdi
rect
ion
and
type
ofso
und
stim
ulus
Rec
ogni
tion
ofdi
rect
ion
and
type
ofso
und
byE
TS
Vui
llerm
ean
dB
oisg
ontie
r(2
009)
11(H
V)
36A
SRT
Forc
ede
tect
orof
the
plan
tarfl
exor
sFo
rce
corr
ectio
n
Yau
etal
.(20
08)
16(H
V)
16In
dex
finge
rtip
Bio
met
ric
iden
tifier
Iden
tifica
tion
offa
kefin
gers
War
ren
etal
.(20
08)
5(H
V)
4In
dex
finge
rtip
�=E
lect
rode
san
dsp
acin
gD
iscr
imin
atio
nof
two
poin
ts
Mar
cus
and
Fugl
evan
d(2
009)
45(H
V)
8In
dex
finge
rtip
/Pos
teri
orre
gion
ofth
ene
ckM
echa
nica
lstim
ulat
ion
Perc
eptio
nof
stim
ulus
vari
atio
nD
iscr
imin
atio
nbe
twee
ntw
opo
ints
Mat
teau
etal
.(20
10)
8(b
lind)
9(H
V)
144
ASR
TM
RI
Cer
ebra
lrec
ord
thro
ugh
fMR
I
Vui
llerm
eet
al.(
2008
)14
(HV
)36
ASR
TE
TS
onth
eto
ngue
PE
Sato
etal
.(20
08a)
4(H
V)
15T
ipof
the
inde
xfin
ger
and
thum
bof
the
righ
than
dR
obot
icha
ndId
entifi
catio
nof
sens
atio
nstr
ansm
itted
bya
robo
ticha
ndSa
toet
al.(
2008
b)9
(HV
)15
Inde
xfin
gert
ipV
irtu
alst
imul
usId
entifi
catio
nof
shap
es
Che
nuet
al.(
2012
)8
(HV
)14
4A
SRT
Vis
uala
ndso
und
stim
uli
Prev
entio
nof
pres
sure
sore
s
Ghu
lyan
-Bed
ikia
net
al.(
2013
)71
(BV
A;B
VL
;U
VA
;ULV
)B
rain
Port�
ASR
TE
TS
onth
eto
ngue
Che
ckw
heth
erE
TS
onth
eto
ngue
isap
plie
dfo
rse
veri
tyof
vest
ibul
arlo
ssan
dol
d-ag
e(>
65)
Ger
man
ieta
l.(2
013)
20(H
V)
256
pin
elec
trod
esar
rang
edas
a32
×8
grid
Dom
inan
tfing
erV
irtu
alst
imul
usPr
opos
ea
new
mod
eto
appl
yvi
rtua
lsim
ulat
ion
toid
entif
ydi
ffer
entt
extu
res
(pap
er,w
ood,
rubb
eran
dte
xtile
)W
ebb
etal
.(20
12)
13(H
V)
8E
quid
ista
ntly
spac
edin
atr
ansv
erse
ring
arou
ndth
eth
igh
Lay
ing
supi
nean
ddu
ring
trea
dmill
wal
king
task
Com
fort
able
rang
eof
elec
tro-
tact
ilese
nsat
ions
tofu
ture
trea
tmen
ton
ampu
tees
Yu
etal
.(20
13)
6(H
V)
Vib
ratio
nta
ctor
sO
npa
lmsu
rfac
eul
tras
onic
sens
ors
and
vibr
atio
nm
otor
sTa
ctile
disp
lay
toth
ew
alki
nggu
ide
Volv
olun
teer
,HV
heal
thy
volu
ntee
r,B
rain
Port
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alan
ceD
evic
e),W
icab
Inc.
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-mm
diam
eter
elec
trod
esw
ith10
0po
ints
(10
×10
),B
VH
bila
tera
lves
tibul
arhy
pofu
nctio
nan
dbi
late
ralt
rans
tibia
lam
puta
tion,
BV
Lbi
late
ralv
estib
ular
loss
,UV
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ilate
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estib
ular
loss
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teri
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peri
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ural
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libri
um,M
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mag
netic
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J Control Autom Electr Syst
Table 2 Applications of mechanotactile stimulation
Damian et al. (2011) Various regions of the body Mechanotactile and vibrotactilestimulation
Study of multi-modal tactileinformation
Lee et al. (2012) Hand Vibrotactile stimulation with amobile phone
Facilitate equilibrium training
Rahal et al. (2009) Various regionsof the body
Vibrotactile stimulation Study the response of stimulationin different parts of the body
Gwilliam et al. (2012) Index fingertip Stimulation by compressed air jet Tactile biofeedback
Bianchi et al. (2011) Index fingertip Stimulation by compressed air jet Tactile biofeedback
Furuya et al. (2009) Forearm Stimulation by compressed air jet Transfer acoustic informationthrough tactile stimulation
Cunha et al. (2010) Abdomen Fluxtactile stimulation by CO2 jet Recognition of graphic patternsthrough fluxtactile stimulation
Cunha and Nohama (2012) Abdomen Fluxtactile stimulation by CO2 jet Recognition of graphic patternsthrough fluxtactile stimulation
Tsalamlal et al. (2013) Hand Stimulation by compressed air jet Characterization of humanperception for the air jet tactilestimulation
Gwilliam et al. (2013) Right index finger Stimulation by compressed air jet Characterization andpsychophysical studies of anair-jet lump display
Janssen et al. (2012) Around the waist Vibrators N = 20 (bilateralvestibular areflexia)
Gait analysis to score individualbalance
the perception of more categories such as the graphesthesia,i.e., the comprehension of phrases through the skin.
Although a direct application to physical rehabilitation isabsent, the study by Yau et al. (2008) is relevant because itshows a proposed methodology for the identification of fin-gerprint authenticity with ETS. The authors researched bio-metric recognition with ETS for the identification of artificialfingers with fake fingerprints. The fake fingerprints (Fig. 2)were made with gelatin layers that varied between 4 and 1mm. The applied pressures in the biometric identifier wereclassified as light, normal, and high. For the real fingers, therespective required intensities of the initial electrotactile sen-sation for each pressure level were 0.60, 0.56, and 0.68 mA.The respective percentages of success for the real fingers foreach pressure level were 56, 69, and 46 %. The fingers cov-ered by the fake fingerprints had obtained no success rates(0 %) for the three tested pressure levels. It was concludedthat ETS could be used in conjunction with biometric iden-tification to identify counterfeits. It can support the devel-opment of new sophisticated processing techniques of theassessment to private devices minimizing the unscrupulousunauthorized access.
Altinsoy and Merchel (2012) developed a touch-sensitivedisplay with an ETS fingertip application (N = 14), whichsimulated different textures on the finger surface such asthe roughness shown in Fig. 3. The test protocol was pre-pared with electric current magnitude variations (10, 15,20, 25, 30, 35, and 40 mA) and frequency variations (30,
Fig. 2 Detection of the fake fingerprint [from Yau et al. (2008)]
50, 75, or 100 Hz) in an ETS application. The researchersfound that a reasonably high current intensity with a low fre-quency better represented the sensation of roughness to theskin. This type of study opened a range of possibilities forthe applications of touch-sensitive technology to the field ofsomatosensory rehabilitation, including touch (size, shape,texture, and movement), proprioception (static and movingposition), nociception (signaling of tissue damage throughpain), and temperature (cold and hot variations). These possi-bilities are broad and range from daily conditions commonlyperformed in different scopes. Currently, blind people can-
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Fig. 3 Schematic representation of a touch-sensitive system with anelectrotactile display. From Altinsoy and Merchel (2012)
not enjoy touchscreen technologies; however, this researchbrings these possibilities.
Warren et al. (2008) evaluated different electrode positionsand sizes for the discrimination of two points. The electricalstimuli were configured with a rectangular wave that had acurrent intensity of 2 mA and two frequency configurations(25 and 75 Hz). The ETS application was through a pulsewidth modulation (PWM) with a variable active pulse periodthat ranged from 0.25 to 3.00 ms, in which the smaller elec-trode (active) was positive (anode). Two electrode sizes weretested in three different geometrical arrangements as illus-trated in Fig. 4. The percentage of correct discrimination oftwo points was greater for the larger electrodes with greaterdistances between them as well as for the frequency of 25Hz. Their results indicate the best parameter for determiningthe fine resolution tactile by the human fingertips, provingequipment effectiveness for ETS discrimination. The worksof Warren et al. (2008) with Altinsoy and Merchel (2012)can be mixed to improve new features of tactile perceptionfor blind population.
Marcus and Fuglevand (2009) evaluated tactile percep-tion using ETS by PAM, PWM, and mechanical stimulation.All three techniques were applied to two participant bodyregions in order to confirm which region provided better per-ception. ETS (n = 24) and mechanical stimulation (n = 21)were applied to the posterior region of the neck and the indexfingertip. ETS was monophasic, with intensities ranging from0 to 15 mA and a maximum voltage of 150 V applied through
Fig. 4 Distances and sizes of the electrodes used [from Warren et al.(2008)]
Ag/AgCl electrodes. The voltage amplitude was approxi-mately 80 % of the pain threshold. The frequencies variedfrom 5 to 800 Hz. Mechanical stimulation was performedwith amplitudes ranging from 0.04 to 6.83 N. The two-pointsdiscrimination threshold for the posterior region of the neckwas similar for both ETS (37 ± 14 mm) and mechanical stim-ulation (39 ± 10 mm). The distinction of intensity variationwas better for the PAM ETS than for either the PWM ETSor mechanical stimulation. The results showed that tactileperception presents more sensitivity to electrical amplitudevariation, contributing to new ones systems involving artifi-cial sensations.
In neuroscience field, usually it is applied magnetic res-onance image to register the physiological modification duethe neural plasticity. In this sense, Matteau et al. (2010)evaluated the use of ETS in sighted and blind people withfunctional magnetic resonance imaging. The ETS trainingoccurred in ten sessions of approximately 15 min each, giventwice daily. The applications consisted of the activation of an8 × 8 electrode array to create a sensation of movement onthe tongue. The sighted volunteers were blindfolded duringthe test application. The use of functional magnetic resonanceimaging with ETS indicated a difference in the activated brainregions. The blind participants showed strong activation ofthe medial temporal cortex and the occipital cortex (mainlythe visual region). The results indicated the existence of plas-ticity in the occipital cortex (visual area) in blind people, whoresponded to sensory stimuli in a different manner from thatof sighted people, which therefore requires a period of train-ing and learning.
Sato et al. (2008a) evaluated ETS on the right hand forthe telemetric manipulation of a virtual hand (Fig. 5). ETSwas performed with a 3 × 5 electrode array. The pulse widthand frequency parameters were 40 µs and 60 Hz, respec-
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Fig. 5 Interface with the virtual hand through electrotactile stimulation[from Sato et al. (2008a)]
Fig. 6 Sensory information with movement from the electrotactilestimulation [from Sato et al. (2008a)]
tively. The stimulation current intensity varied between 1.25and 2.75 mA to maintain a comfortable sensation for theparticipant. ETS offers pressure sensations (varied inten-sity) and sliding sensations (activation of a specific electrodesequence) as illustrated in Fig. 6. The results denoted theviability of ETS in the virtual mimicking of tactile sensa-tions. The same research group studied ETS for discrimina-tion of objects with flat, edged, and convex shapes (Sato etal. 2008b). The stimuli were applied to the index fingertipwith frequencies of 20, 30, 60, 90, and 120 Hz, and anodicstimulation was applied with a 3 × 5 electrode array. Thebest results for object shape identification occurred through
stimulation frequency variations. Their greatest contributionis the improvement of accuracy during surgery tasks throughrobotic devices giving biofeedback to the user. Unfortunately,physiological effects as sweat (decrease the skin impedance)can alter electrotactile perception, jeopardizing the biofeed-back.
Chenu et al. (2012) evaluated the use of ETS in conjunc-tion with visual and sound stimuli as a biofeedback systemfor the prevention of pressure sores. Through the stimuli,the system informs the volunteer of the gluteal muscle pres-sure distribution. The stimuli (visual, sound, and tactile) weretested separately while indicating to which side the volun-teer should move to avoid pressure increases and consequentreductions in local circulation. The pressure map was con-structed with an array of 1,024 (32 × 32) sensors, each withresolution of less than 1 mmHg. The results from the visual,sound, and tactile stimuli were positive (greater than the con-trol group). Therefore, ETS can facilitate the prophylaxis ofpressure sores through visual and sound stimuli. This sensoryinformation could be beneficial to individuals with spinalcord injuries who remain seated for long periods of time. Itis well known that the pressure sore can lead to death (i. e.,infections). So, this study brings a new tool for prevention ofbed rest or wheel chair conditions.
Rahal et al. (2009) analyzed the development of electro-mechanical tactile displays in sensory replacement and/orcomplementation by analyzing psychophysical responses asa function of mechanical stimuli. These authors used the“funneling illusion” principle of tactile illusion to dynam-ically produce tactile sensations with discrete mechanicalactuators. Funneling allows one to overcome the low resolu-tion observed with mechanotactile devices. In that study, theauthors evaluated the effects of temporal changes in the stim-ulus intensity that were produced by the vibrotactile actuator(linear vs. logarithmic) as functions of orientation, distance,temporal order, limb site, and gender to obtain a sensation ofcontinuous smooth movement on the skin. The results indi-cate that these procedures are suitable for dynamically pro-duce tactile sensations. The equipment developed is effectivein producing the smooth tactile feeling. Table 3 shows somecurrently available vibrotactile devices.
Gwilliam et al. (2012) and Bianchi et al. (2011) developeda system to mimic the tactile sensations of a surgeon for min-imally invasive surgeries or those that use surgical robots;these incorporated an air jet directed at the fingertip of thehealth professional, thus facilitating the location of nodulesin soft tissues without direct contact by the surgeon. In thatstudy, the pressure and the opening of the compressed airjet were independently controlled as functions of the nodulesize and density. In this work, any subject reported physi-cal limitations that would affect their ability to perform theexperiment tasks, showing a good correlation with the nat-ural tactile perception of the nodules. The main limitation of
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Table 3 Comparison between the currently available vibrotactile devices
Tactor Motor Solenoid Piezoelectric Voice coil
Frequencyresponse (Hz)
40–1,000 (peakat 250)
40–1,000 40–1,000 40–1,000 40–1,000
Maximum amplitude Low High High High High
Control of amplitudeand frequency
Independent Dependent Only frequency Independent Independent
Type of control Audio signal PWM PWM Audio signal Audio signal
this method is related to the distance between the air jet out-put and the finger surface, since the gas develops a conicalshape with a jet spread rate, with the jet radius as a functionof the distance from the outlet and the aperture diameter. Asfar the skin surface is form the output, as bigger is the area ofstimulation and, consequently, smaller the perception, sincethe force over the skin is inversely proportional to the areaof contact of the jet over the skin. This happens because theair jet density is the same as the ambient air density. As apossible solution, or evolution, Cunha and Nohama (2012)proposed the use of the CO2, whose density is greater thanthe ambient air, producing a gas jet with cylindrical shape(coherent), minimizing the problem of increasing the con-tact area of the jet gas with the distancing from the surfacestimulated.
Furuya et al. (2009) evaluated the possibility of transfer-ring acoustic information to individuals with hearing lossesthrough tactile sensations via an air-jet stimuli. For the tests,percussion instruments were used to activate and controlthe tactile stimulation parameters in the transfer of acousticcharacteristics. The study results were based on the analy-sis of psychological impressions of the study volunteers andshowed promise for the use of this technology in sensorysubstitutions for such disabilities. Indeed, the equipment sizemust be compact to can be used daily.
Cunha et al. (2010); Cunha and Nohama (2012) studied thepsychophysical and cognitive responses of fluxtactile com-munication through mechanotactile stimulation, which wasproduced by a CO2 jet applied to the abdominal regions ofvolunteers; this occurred in a plotter system in which imageswere transferred over this region (Fig. 7). In the study, let-ters and geometric shapes were used to evaluate the imagerecognition rate in blind tests. The plotter used a needle withan internal diameter of 1 mm, applied at an average distanceof 10 mm from the cutaneous surface with a flow of 4 L/min
Fig. 7 Fluxtactile plotter positioned on the abdominal region of a vol-unteer [from Cunha and Nohama (2012)]
and a pressure of 70 kPa, to produce a stimulation force of1.8 mN, which was equivalent to three times the thresholdof perception of force determined in this study. For the tac-tile plotting, the authors used needle displacement speeds of20, 40, 67, and 100 mm/s and evaluated the effects of thesespeeds on the image recognition rate. They also observed thatthe stimulating force produced by the gas jet on the skin waskept constant for distances between the tip of the needle andthe cutaneous surface, which varied between 0 and 25 mm.The average success rate was 60 %; the best displacementspeed was observed at 40 mm/s, in which the average rate
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of correct image recognition by the volunteers was 68 %.The advantage of this technique is related to the replacementof the air jet by a CO2 jet, which delivers coherent jets atlarger distances than the air, allowing greater flexibility inthe development of equipment and stimulation techniques.At this moment, the CO2 plotter′s disadvantage is its lowspeed for images drawing.
Germani et al. (2013) proposed a new mode to apply vir-tual simulation to identify textures as paper, wood, rubber,and textile by a tactile display. The main question of the paperwas whether the system proposed was able to simulate differ-ent materials. For to test real tactile sensing, three tests wereexploited: detection, dissimilarity ratings and material classdiscrimination by real and virtual material textures. Accord-ing to statistical tests, there were no differences between thereal and virtual experiences.
Tsalamlal et al. (2013) carried out psychophysical experi-ments in order to characterize the human perception of the airjet tactile stimulation related to the air jet flow rate, the noz-zle geometry, and the distance between the nozzle and thehand palm. In this work, two psychophysical studies werecarried out, one related to the determination of the absolutethreshold of the air flow rate on the hand as a function of thedistance between the nozzle and the hand and other to deter-minate the just noticeable difference (JND) of the air flowrate according to three referential stimuli. They concludedthat the absolute threshold of the air flow rate on the handpalm increases linearly with increasing distance between thenozzle and the palm of the hand, as shown in Fig. 8, andthe differential threshold or the JND increases as the refer-ential flow rate increases, as indicated in Fig. 9. These airjet characteristics limits the application of this stimulus onlonger distances, which may reduce their possible applica-tions in other areas such as augmented reality, entertainment,
Fig. 8 Mean absolute threshold flow rate for the three distances to thenozzle [from Tsalamlal et al. (2013)]. JND: just noticeable difference
Fig. 9 Mean JND according to the three flow rate references [fromTsalamlal et al. (2013) ]. JND: just noticeable difference
and security. Again, as showed in the works of Gwilliam etal. (2012) and Bianchi et al. (2011) , the main problem isrelated to the air jet that produces a conical shape, due to thesame density between the air jet and the ambient air.
Gwilliam et al. (2013) describes the quantitative and psy-chophysical assessment of an air-jet tactile display that cre-ates a lump percept by directing pressurized air jet throughan aperture onto the finger pad to be used in haptic feedbackrobot-assisted minimally invasive surgery (RMIS) systems asan augmented reality system. In this work, they investigatedthe influence of the aperture size of the air injector and its airpressure related to the psychophysical response in perceivingthe shape and dimensions of a lamp. They concluded that thepsychophysical perception of lump size is indeed dictated bythe width of the Gaussian pressure profile over the finger pad,suggesting that perceived lump size could be controlled onlyby adjusting the aperture size, that in this work ranged fordiameters from 1.0–4.5 mm. To measure the pressure profileproduced by the jet impingement pressure, they used capac-itive tactile sensors and to measure the air flow mass flowmeters at varying aperture sizes and pressures.
3.2 Physical Rehabilitation
3.2.1 Stimulation on Tongue Surface
Electrical and mechanical tools are employed in physicaltherapy sessions to restore the sensorial and motor conditionof patients. The literature also includes studies of posturalequilibrium with sensory biofeedback systems, administeredthrough ETS applications on the tongue. Tyler et al. (2009)evaluated the required stimulatory magnitudes for ETS on thetongue with a 3×3 electrode array. The authors found that theback region of the tongue required higher voltage than otherregions of the tongue to achieve the same sensation threshold.In this sense, Wood et al. (2009) applied galvanic vestibularstimulation in conjunction with ETS on the tongue surface.
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These researchers evaluated postural equilibrium with a forceplatform. Anterior-posterior and medial-lateral body move-ments were measured with a 2-axis accelerometer positionedon the torso. The volunteers were trained by practicing move-ments while blindfolded and by equilibrium maintenance.The movements performed were measured at the baseline(without biofeedback), with galvanic vestibular stimulationand with galvanic vestibular stimulation and ETS. The perfor-mance of the volunteers improved when the ETS was active.These results showed the efficacy of ETS in the maintenanceof postural stability in individuals with vestibular disorders.
Barros et al. (2010)evaluated the efficiency of the electro-tactile tongue biofeedback (BrainPort�) technique in indi-viduals with bilateral vestibular loss who did not showimprovements with traditional vestibular rehabilitation. Stim-ulation on the tongue surface was created through a dynamicpattern of electrical pulses, wherein the patient controlledthe intensity and centralization of the stimuli on the array.The patients had to adjust their head orientation so that theETS was in the center of the electrode array. Training con-sisted of 12–15-min sessions applied during the same daywith an interval of 3–4 h, repeated three times per week fortwo weeks. The postures became more challenging as theindividuals achieved familiarity with the apparatus. In thesensory organization test, there was an increase of 38.3 ±8.7–59.9 ± 11.3 (p = 0.01) between the pre-training andpost-training measurements. The results showed that ETSbiofeedback improved the postural equilibrium of patientswith bilateral vestibular losses when the traditional vestibularrehabilitation was unsatisfactory. In spite of their promisingresults, the limitations of their study were the small samplesize and the short duration of the follow-up.
Vuillerme et al. (2008) studied ETS applications for pos-tural correction. ETS was applied to the tongue, and par-ticipants were submitted to different situations that requiredequilibrium maintenance. Postural stability was comparedwith and without the ETS biofeedback system on firm andsoft (6-cm foam layer) surfaces. Postural equilibrium wasmeasured through accelerometers that activated the ETSelectrode array. Postural stability was evaluated with a forceplatform. The results showed that the support area (centerof the foot) was more stable and smaller when the volun-teers used the ETS, indicating that ETS increased stabilityon both surfaces. These positive results corroborate Barroset al. (2010) research.
Vuillerme et al. (2011) evaluated alterations of posturalequilibrium in patients with unilateral vestibular losses. Pos-tural equilibrium was measured with accelerometers and aforce platform, which activated the ETS electrode. The testswere performed on firm and soft (2-cm foam layer) sur-faces with the participants’ eyes closed, both with and with-out ETS (applied to the feet). The ETS had pulse width of25 µs and pulse frequency of 200 Hz modulated by bursts of
50 Hz. The pressure centers of the feet were smaller whenthe ETS system was used, regardless of the surface. The pre-liminary results from Vuillerme et al. (2011) suggested thatpatients with unilateral vestibular losses could take advantageof electrotactile biofeedback by attenuating the postural dis-turbances induced by alterations in the deficient somatosen-sory signals of the foot and ankle.
Vuillerme and Cuisinier (2008) used mechanical vibrationon the Achilles tendon to generate postural destabilizationin healthy volunteers. Postural equilibrium was analyzed ona force platform with and without the use of biofeedbackfrom ETS on the tongue. The vibration system comprised amotor with an eccentric load configured at 80 Hz with 3 mmof displacement. Postural equilibrium was measured withaccelerometers, which activated the ETS electrode array. Theresults were similar to those reported by Vuillerme et al.(2008) such that the support area (center of the foot) wasmore stable and smaller when the volunteers used ETS, evenwhen vibrational stimulation was applied. Beyond studiesabout capacities and viability of the ETS, Ghulyan-Bedikianet al. (2013) investigated whether ETS applied to the tonguehas a positive influence on the balance restoration for peo-ple with severity of vestibular loss and old-age (>65 years).With this approach, they proposed training sessions of 1-hduration during four consecutive days. They have observedenhancement on the postural performance of the patients.However, as the effects perceived were acute, longitudinalstudies are needed to determine if the positive effects canalso be chronic. These contributions show that the ETS sys-tems can be incorporated in programs for balance dyscontroland falls, mainly in elderly people.
With this line of reasoning, Robinson et al. (2009) eval-uated the commercial BrainPort� ETS equipment (Fig. 10)in bilateral vestibular hypofunction and bilateral transtibialamputation to facilitate gait and balance. The case report fea-tured a 69-year-old man who had suffered from neurologicallesions for 28 months. The training period was 12 months,and during this period, the balance was corrected through theactivation of an electrode array on the tongue, as shown inFig. 11. The volunteer did not use ETS during the battery oftests. The benefit of using the ETS apparatus can be observedusing the following results: in the sensory organization test,there was an increase from 23 to 48; the dynamic gait indexincreased from 11/24 to 21/24; the distance walked during6 min increased from 212 to 363 m and the time in the ortho-static position increased from less than 2 s to 20 min. Evenwith positive results, the study was a case report, withoutstatistical power content due the sample (N = 1).
Vuillerme and Boisgontier (2009) studied the efficiency ofETS on the tongue as a biofeedback mechanism with which toimprove ankle force with the plantar-flexor muscles relaxedand fatigued. The feedback informed the force level of theplantar flexors through the ETS as illustrated in Fig. 12. The
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Fig. 10 BrainPort�
Electrotactile stimulationequipment [from Robinson et al.(2009)]
Fig. 11 Regions of electrodes activated during head movement [fromRobinson et al. (2009) ]
muscle force response of the evaluated limb was comparedagainst the plantar flexors of the opposite foot, which wasnot associated with the ETS system. The protocol was per-formed with the musculature of the right foot relaxed, andafter 5 min, with the musculature fatigued. The tests wereevaluated through the force differences between the refer-ence musculature (left foot) and the evaluated musculature(right foot) with and without the use of ETS. The results indi-cated much smaller force differences when ETS was used,both with the musculature relaxed as well as fatigued. Thisinformation is important to patients that reach fatigue stateeasily by decreases on ETS rehabilitation efficiency.
Vuillerme et al. (2008) evaluated postural balance with aforce platform and ETS on the tongue (Fig. 13). The pro-
cedure consisted of body posture maintenance with the eyesclosed while on a force platform; plantar pressure was moni-tored. The distance from the pressure center was indicated byETS, as illustrated in Fig. 14. The tests were performed withand without ETS and with the head in a neutral position orextended. The results showed a better performance for ETSwith regard to postural control, as determined by the smallerdistance in plantar pressure relative to the center.
3.2.2 Electrical and Mechanical Stimulation on OthersSensory Fields
Myoelectric hand prostheses can mimic prehensile move-ments; however, in spite of the lost ability to exert phys-iological force, kinesthetic and proprioceptive informationabout force and speed are still fed back through the cuta-neous mechanoreceptors in the skin. One challenge in theconception of tactile biofeedback devices for individuals withprosthetic hands is the simultaneous exhibition of multipletypes of tactile information (Cipriani et al. 2012). With thisissue in mind, Damian et al. (2011) developed a preliminarydevice for multi-modal tactile information. The device movesone set of contact points tangentially over the skin at a con-trolled speed and with a controlled normal force. The resultsindicated that under certain conditions, different forces andspeeds could be discriminated through vibrations. Precisespeed identification was provided by a series of vibrationevents that depended on the spatial distribution of the contactpoints. Damian et al. (2011) stated that this study encouragedfurther research into displays of artificial tactile perceptionfor prostheses that were based on this concept.
The stimulation on tongue brings great and positive resultsinvolving biofeedback to rehabilitation. However, tonguefunctions as deglutition and speech are jeopardized duringthe treatment. Researches are seeking for other techniquesin order to perform biofeedback to physical rehabilitation,e.g., Lee et al. (2012) who adapted a vibrational system in
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Fig. 12 Biofeedback of theforce differences between theleft and right feet [fromVuillerme and Boisgontier(2009)]
Fig. 13 Electrotactile stimulation electrodes (6 × 6) within the mouthregion [from Vuillerme et al. (2008)]
a commercial mobile phone device that was fitted with anaccelerometer (Fig. 15). In order to achieve balance on train-ing session, the system generated a 200 Hz vibration in twogroups, healthy volunteers (N = 5) and those with vestibu-lar alteration (N = 4). In spite of performed with healthyvolunteers, these showed the viability of real-time biofeed-back in balance training. The same research group, Lee etal. (2012), detected postural responses by vibrotactile stim-ulation applied to different torso locations with healthy vol-unteers. The authors observed that the postural trajectory is
shifted on the direction of the vibrotactile stimulation duringthe vibration period. And the postural trajectory was oppo-site when the stimulation was ceased. These studies bringencouraging perspectives for the design of vibrotactile dis-plays for postural equilibrium application indicating that thistechnology is a strong candidate for therapy in patients withvestibular alterations. However, there is a gap between theprototype device and the commercial version.
Janssen et al. (2012) using vibrotactile biofeedback (12uncentered motors at 300 Hz and displacement amplitudeat 0.5 mm) as shown in Fig. 16, during gait performance in20 volunteers with bilateral vestibular areflexia. The proto-col used two tilt sensor positions, head and trunk, to detecthead or body tilt. Comparing the gait task with and with-out biofeedback, the authors found significant improvementin balance using vibrotactile biofeedback with sensor on thetrunk.
Webb et al. (2012) used an eight-channel electrical stim-ulator with the electrodes spaced equidistant in a transversering around the thigh to evaluate the perception and discom-fort elicited by electrical stimulation to hereafter use to treat-ment in amputees patients. As the authors did not appliedstatistical tests to verify the results, the research needs to beimproved and augmented. They stated that the skin sensationof traumatic amputees is unaffected by surgery, particularlyat the mid-stump area. In this sense, the results could begeneralized to the amputees’ population and are important
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Fig. 14 Activation of the electrotactile stimulation electrodes due tovariations in plantar pressure. The black dots represent activated elec-trodes [from Vuillerme et al. (2008)]
Fig. 15 Mobile phone apparatus with an implanted equilibrium train-ing system [modified from Lee et al. (2012)]
because electrical stimulation showed to be useful to phan-tom limb/pain treatment.
Yu et al. (2013) assessed the array of six ultrasonic sensorsillustrated in Fig. 17 to detect the obstacles and inform thetactile stimulator matrix put on hand surface shown in Fig. 18as a tridimensional walking guide. The authors achieved arecognition rate of until 95 % and they estimate that obstacle
Fig. 16 Ambulatory vibrotactile biofeedback belts developed byJanssen et al. (2012)
Fig. 17 Tridimensional walking guide sensor array (input) to detectobstacles developed by Yu et al. (2013)
Fig. 18 Tridimensional walking guide tactile stimulator (output) todetect obstacles developed by Yu et al. (2013)
distributions using the proposed tactile stimulation would beproperly recognized.
4 Conclusion
The electrical and mechanical technologies developed forphysical rehabilitation are proving to be more appropriate for
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the substitution and control of lost sensory, proprioceptive,and kinesthetic functions.
The main applications of the sensory substitution tactileactuators are concentrated in neurological disorders resultingfrom vestibular lesions that affect postural equilibrium.
Notwithstanding the use of mechanical vibration systemsto obtain balance or gas jet stimulation, there are several stud-ies of sensory substitution in alternative communication oraugmented realities in the professional, entertainment, andsignaling fields, among others, for people without visualand/or hearing disabilities.
The results of this study demonstrate the progress andpotential of electrical and mechanical technologies in sen-sory system substitution and control, as well as the resultsfrom evaluations of the application of these technologies incontrolled experiments and the current limitations of the actu-ators that are employed in functional sensory system rehabil-itation. However, numerous devices are at the stage of proto-typing, lacking at the moment, unfortunately, a commercialversion.
It must be understood that these new cybernetic approa-ches are not only technical and engineering solutions; rather,they are more valuable as they are applied in ways that pos-itively influence the user’s quality of life.
Acknowledgments The authors thank SETI-PR (Secretaria da Ciên-cia, Tecnologia e Ensino Superior – Estado do Paraná / Ministry ofHigher Education, Science and Technology – Paraná state), CAPES(Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Coor-dination for the Improvement of Higher Education Personnel) and CNPq(Conselho Nacional de Desenvolvimento Científico e Tecnológico /National Council of Technological and Scientific Development) for theresources and grants provided to conduct this study.
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