Page 131 - Organic Electronics in Sensors and Biotechnology

108 Cha pte r T h ree

FIGURE 3.17 Cross section of human skin. 22

The dermis is tightly connected to the epidermis by a basement mem-
brane. It also contains many nerve endings that provide the sense of
touch and heat.
Somatic sensation consists of the various sensory receptors that
trigger the experiences labeled as touch or pressure, temperature
(warm or cold), pain (including itch and tickle), and the sensations of
muscle movement and joint position including posture, movement,
and facial expression.
In human beings, touch is in fact a combination of different feel-
ings, for instance, perception of pressure (hence shape, softness, tex-
ture, vibration, etc.), relative temperature, and sometimes pain. In
addition, complex actions based on touch (such as grasping) are the
result of a powerful sensory-motor integration which fully exploits
the wealth of information provided by the cutaneous and kinesthetic
neural afferent systems. A very accurate description of tactile units
22
is available in Ref. 23, where a classification of these units according
to receptive fields and response time is given.
Obviously, reproducing the human sense of touch with an artifi-
cial system is a very challenging task, first, because the term touch is
actually the combined term for several senses.
The tentative specifications for tactile sensors have been defined
in Ref. 23 as follows:
1. The sensor surface or its covering should combine compli-
ance with robustness and durability.
2. The sensor should provide stable and repeatable output sig-
nals. Loading and unloading hysteresis should be minimal.
3. Linearity is important, although only monotonic response is
absolutely necessary. Some degree of nonlinearity can be cor-
rected through signal processing.