118

౪

6 Skin, The Body’s Largest Organ

mechanically stiffer than the rest of the surface, as the epidermis does, and the pressure

sensing elements are placed in the same position relative to the ridges as the human

Merkel discs in skin. Additionally, the strain sensors show different strain values based

on the direction of the stress, which with the right algorithm can be translated into the

sensation of surface properties. The “skin” material is easily deformable, allowing for

strain transduction. Research results demonstrate that ridges enhance sensitivity and

add information about the three-dimensional structure of the surface as human touch

is able to do.

To mimic skin, pressure-sensing needs to be scaled up to larger areas. A sensor based

on carbon nanotubes is attempting this (Figure 6.8) [28]. The sensor contains single-

walled CNTs in a cellulose layer. This layer can be spray-coated, allowing for facile fab-

rication for larger sensors. Additionally, the electrodes that will transduce the pressure

into an electrical signal via resistive measurements can be ink-jet printed, which can

also be scaled-up easily.

Figure 6.8: Piezoresistive flexible cellulose pressure sensor containing single-walled CNTs (with permission

from [28]). (a) Schematic of the layers of the sensor. (b) Schematic of the fabrication process of spray coat-

ing the CNT/cellulose sensing film onto the protective polyimide film. (c) Schematic of inkjet printing the

interdigitated silver electrode.

Another group is connecting temperature sensing with actuation: at a low temperature,

the material sensor will bend [29]. Adding electrodes to an asymmetric graphene-coated

paper becomes a flexible piezoresistive strain sensor. When coating this film with a

porous hydrophilic cellulose fiber network, the adding/removing of water at different

temperatures makes the film bend, thus adding actuation to the sensor film.

Layering two sensor films on top of each other created a film that can measure both

temperature and pressure (Figure 6.9) [30]. A capacitive pressure sensor array in a di-

electric elastomer is placed between two electrodes. On top of that, a layer with a tem-