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6 Skin, The Body’s Largest Organ

nerves, and other specialized cells for all the functions of skin. Over time, the system

no longer works perfectly, thus resulting in scars. In addition, when all layers of skin

have been destroyed by an injury (e. g., in the case of a third-degree burn), the scars still

allow for survival but a few of the functions of skin will not be perfectly recreated (such

as cooling or sensing). Even so, functional skin will be regrown automatically, in spite

of the possibility of wound and other complicating factors.

Let us summarize how our human temperature and touch sensors work: Both

senses are imbedded in nerves that include special ion channels that open in response

to a heat or touch signal. Temperature-sensitive channels open using a thermodynamics-

based mechanism whose details are not known yet. In the case of the mechano-sensitive

ion channels, the stretching of the skin-environment physically opens the channel, thus

creating a membrane potential that can directly or indirectly initiate an action potential

in the next nerve cell. This starts a path to the brain, where analysis takes place, which

includes the possible sensing of “pain” or either of these senses (in fact, pain can also

occur in hearing; however, this only takes place in rather extreme circumstances). Some

of the same channels that react to temperature or pressure also react to chemicals that

create a burning or cooling sensation. The sense of temperature or pressure leads to

some automatic avoidance movements or conscious decisions and behaviors.

At the same time, the organ with these sensing neurons happens to be skin, the

biggest organ that has additional functions, such as maintaining homeostasis, tempera-

ture control of the body, protection from injury, bacteria and viruses, and Vitamin D pro-

duction. It is involved in the large majority of adaptive behaviors of the body (which is

how skin maintains homeostasis), since it is the organ that borders the outside environ-

ment. In other words, skin is many different sensors embedded in a “smart”, adaptive

matrix. Could we use molecules and methods of the human senses to make an artificial,

smart, sensing material with similar function(s)?

6.2 Chemical, Thermal, and Pressure Sensors Using Biological

Cells, Molecules, and Methods

Remember how the two main sensors in skin worked: For temperature, an ion channel

in a nerve cell opened based on temperature via a (so far) unclear mechanism. No lit-

erature was found about using the nerve cells or ion channels in artificial temperature

sensors. This is likely due to the fact that a) the channel mechanism is not known, and

thus difficult to control outside its native environment and b) there are other highly de-

veloped temperature sensors already on the market, thermometers, with even more in

development, even on a very small scale.

Thermometers on a large scale use liquids that change their volume with temper-

ature. On the microscale (i. e., when they are small enough that they fit on a computer

chip), most temperature sensors are thermocouples [18]. A thermocouple combines two

different metals (often two small wires). When heated, a small voltage will be generated,