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mechanoreceptors that are clustered nearby.
[ 166b ]
These ridges
are also essential for facilitating perception of slip, texture, and
hardness. Many essential aspects of life are mediated by the
multifunctional tactile sensing capabilities of skin, including:
• Normal force sensing for grasp control, object manipula-
tion,
[ 175 ]
and orientation determination;
[ 166a ]
• Tensile strain monitoring for proprioception (essential for
simple movements such as standing or walking);
[ 169 ]
• Shear force sensing for grasp control and friction determina
tion;
[ 166b,175b,176 ]
• Vibration detection for slip detection and texture
determination.
[ 166b,174 ]
The characteristics outlined above should be considered
as the minimum requirements for e-skin that would allow
for its interaction with the world in a human-like way. In the
1990’s, a survey of robotics developers outlined desired param-
eters for tactile sensing skins.
[ 163 ]
In most cases, these require-
ments agree well with the capabilities of human skin, and both
are summarized in Table 1 . However, e-skin with enhanced
capabilities, such as improved sensitivity,
[ 18 ]
higher receptor
density,
[ 23b ]
and faster response times, could collectively endow
robots and prosthetics with capabilities that surpass those of
our own skin. While the skin provides an impressive range
of tactile feedback capabilities, it is not a perfect system. For
example, the hysteretic nature of skin requires signifi cant cor-
rection through cognitive processing, and is undesirable in
electronic analogs.
[ 166 ]
Furthermore, its compliance and viscoe-
lasticity also complicate pressure distribution analyses
[ 163 ]
and
reduce spatial resolution.
[ 166b ]
Tactile sensing has attracted sig-
nifi cant interest, and development in the area has been rapid.
An exhaustive description of tactile sensing is not practical
within the limits of this review, and readers are directed to
other excellent reviews on the subject.
[ 166,177 ]
3.1. Transduction Methods
In order to measure the magnitude of a tactile stimulus, it
must be converted into an electrical signal. Methods for accom-
plishing this conversion are described in this section.
3.1.1. Piezoresistivity
Piezoresistive sensors transduce a change in the resistance of
a device into a measurement of strain and have been investi-
gated extensively because of their simple structure and readout
substrate, the OFETs could be isolated from the strain. Shin
et al. fabricated rigid islands of tin oxide (SnO
2
) transistors
connected with curvy metal interconnects.
[ 161 ]
The devices
could stretch up to 40% with negligible change in performance
but showed signifi cant device-to-device variation. Vanfl eteren
and coworkers have also reported impressive results in this
area.
[ 135d ]
3. Tactile Sensors for E-Skin
Robotic technologies have found enormous utility in improving
effi ciency and reducing the cost of repetitive, well-defi ned tasks
in manufacturing.
[ 162 ]
Recently, there has been intense interest
in designing robots that can work in less-structured environ-
ments by collecting information about their surroundings to
make appropriate responses.
[ 12,162 ]
Such capabilities would
allow them to work in close quarters with humans and com-
plete more complicated and dynamic tasks (e.g., providing basic
services to elderly people or undertaking dangerous rescue
missions).
[ 2,163 ]
However, highly functional tactile sensing will
be required to improve the operation speed and effectiveness
of current robotic technology.
[ 164 ]
In addition to robotic appli-
cations, effi cacious tactile sensing arrays could transform the
medical fi eld. Tactile sensors integrated into prosthetics could
allow amputees to regain considerable functionality, and touch-
sensitive sensor skins could be useful in augmenting sur-
gical gloves
[ 165 ]
or for continuously measuring the health of
patients.
[ 3 ]
The human tactile sensing system allows us to navigate our
surroundings amply, and knowledge of its working mechanisms
can be used as inspiration for effective design of electronic
substitutes. Tactile sensing within the skin relies on neuronal
sensor elements called mechanoreceptors, which are embedded
at different depths below the skin surface and respond to forces
on different timescales. Fast adapting (FA) mechanoreceptors
sense dynamic changes in force and respond with intense sig-
nals during the application or removal of forces.
[ 166 ]
By contrast,
slowly adapting (SA) mechanoreceptors are able to sense static
forces by responding continuously during prolonged stimula-
tion. The density of mechanoreceptors depends on the location
in the body and ranges from approximately 241 cm
−2
in fi n-
gertips to 58 cm
−2
in the palm,
[ 166b,167 ]
resulting in a resolution
of approximately 1 mm in the fi ngertips.
[ 168 ]
In humans, data
processing starts in the individual receptors, where the stim-
ulus is digitized.
[ 169 ]
Signals from groups of receptors are pro-
cessed en route to the brain,
[ 170 ]
where fi nal data interpretation
occurs.
[ 166b,169 ]
This breakdown of tactile signal processing helps
to reduce the brain’s data-processing requirements.
[ 166b,169 ]
With this signal processing pathway, the body can sense pres-
sures greater than ≈10 kPa
[ 171 ]
with a temporal resolution of
20–40 ms,
[ 172 ]
and vibrations can be sensed at frequencies up
to 800 Hz.
[ 166a ]
The mechanical properties of skin have a pronounced effect
on its tactile sensing capabilities. Mechanical compliance
improves grip by promoting an understanding of an object’s
shape and increasing contact area with the object.
[ 173 ]
The skin’s
morphology has evolved to benefi t tactile sensing, and the
skin’s ridges enhance sensitivity
[ 174 ]
by concentrating forces into
T a b l e 1 . Summary of the properties of human skin and corresponding
requirements for e-skin.
Parameter Human Skin Requirement for E-Skin
Spatial resolution 1 mm
[168]
1–2 mm
[163]
Temporal resolution 20–40 ms
[172]
1–10 ms
[163]
Working range >10 kPa
[171]
1–1000 g
[163]
Hysteresis High Low
[163]
Adv. Mater. 2013, 25, 5997–6038