**4. Fabrication technology of wearable electromechanical sensor**

The wearable electromechanical sensor usually consists of three basic components, which are substrate, active elements, and electrode/interconnect. They are usually fabricated with different materials. During the fabrication process, combining the substrate and active elements is the key step. Basically, there are two situations. One is that sensing material forms uniform composite with polymer substrate, the other is that sensing material is attached on substrate and a clear interface exists. In this part, we will focus on the combination strategies for substrates and sensing elements, and some key processes for performance enhancement are also concerned.

#### **4.1 Fabrication of wearable composite electromechanical sensor**

For the composite electromechanical sensor, the substrate and sensing materials should be fabricated into composite. The key process is how to mix them and prepare uniform composite. The sensing materials are usually mixed with polymers by magnetically or ultrasonically stirring, and then the dried elastic composites can be prepared in bulk or film forms. The mixed composites have complex electromechanical features that are induced by the diversity of sensing materials and polymer and significantly depend on concentration of sensing materials and its distribution state. For example, the electrical property of carbon black-silicone composite is mainly determined by carbon black concentration. The electrical resistance clearly increases with the applied uniaxial pressure when the concentration is about 0.08–0.09 wt%. By further increasing the concentration from 0.1 to 0.13 wt%, the change tendency of electrical resistance switches from increase to decrease. Finally, the electrical resistance starts to decrease with the uniaxial pressure with the concentration larger than 0.14 wt % [59].

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*Wearable Electromechanical Sensors and Its Applications DOI: http://dx.doi.org/10.5772/intechopen.85098*

and other methods.

**4.2 Fabrication of wearable layered electromechanical sensor**

and stable even after 20,000 cycles with loading/unloading test [47].

Novel techniques have been developed, such as laser scribed (LS) technique. Graphene oxide (GO) can be simultaneously reduced and patterned by laser [61]. Carbonating substrate material by one-step direct laser writing (DLW) has also been validated. Glassy and porous carbon structures have been produced from PI film via DLW. The DLW-based graphene possesses favorable electroconductibility, porousness, and superhydrophilic wettability. Directly drawing electronics with various instruments has recently become an alternative technique. This technique endows end-users the capability to design and realize sensors according to the "on-site, real-time" demands [62]. "Penciling it on" has been proved to be a simple, rapid, and solvent-free method for producing electronics [63]. Chinese brush pen is a possible more appealing writing instrument for sensor fabrication. Similarly, the animal hair bundle is first soaked into low-viscosity ink, and then the ink is uniformly coated on the substrate by well-controlled handwriting manner. Benefiting from excellent liquid manipulation of Chinese brush pen, sensing materials can be coated on different substrates without considering its rigidness and surface roughness. For example, a high-performance tattoo-like strain sensor has been fabricated with AuNWs/PANI ink writing by Chinese brush pen [64]. Various types of functional inks can be loaded in their reservoirs, including metal inks, liquid metals, and even organic mixtures. Sophisticated structures can be generated with controllable geometries on many substrates by using these two methods [65]. Wet spinning is

For the wearable layer electromechanical sensor, the substrate and sensing materials are assembled into film layer by layer. Many techniques have been developed to assemble active material on substrate, including printing, coating, casting,

Printing can simultaneously deposit and pattern many materials on various substrates without the need for sophisticated equipment and clean room. The wearable sensors can be printed with/without the help of masks, according to the specific implementation approach, as seen in **Figure 4a** [60]. The electrode pattern can directly be obtained by inkjet printing. Inkjet printing is an accurate, fast, and reproducible film preparation technique. Functional ink droplets are propelled onto different substrates by a nozzle. The functional inks should have proper solubility, viscosity, and surface tension. As a typical printing method, screen printing requires the help of mask and proper functional ink. During the process, screen openings are fully covered with functional by using fill blade or squeegee, and then it is transferred onto substrate surface. Finally, the mask is removed, and a patterned film is formed on the substrate by functional ink. This technique has been widely used in manufacturing sensing materials in electromechanical sensors. Lithography is a pattern transferring method to realize diverse and ingenious geometries. This process firstly deposits functional layer onto the substrate and then etches the undesired areas by reagent solutions with the help of photolithography. Since photolithography and wet etching has high accuracy, the devices with sophisticated geometries and rich functionality can be obtained. Coating technique is another popular method because of its low cost and simplicity. There are different advantages for different coating methods. Dip coating can be used to any kinds of substrate and can control the thickness by dipping time. Spin coating is easy to form uniform film and can control the thickness by time and spin speed. Compared with spin and dip coating, spray coating can fully utilize the functional inks. **Figure 4b** shows a buckled sheath-core fiber-based ultrastretchable sensor fabricated with spray coating methods. The fiber wearable strain sensor possesses excellent stretchability higher than 1135% and fast response time (≈16 ms). Moreover, the performance is very repeatable *Wearable Devices - The Big Wave of Innovation*

it possesses high sensitivity (2.04 kPa<sup>−</sup><sup>1</sup>

excellent bending and cycling stability [57].

0.1 Pa and has high sensitivity up to 1.1 V kPa<sup>−</sup><sup>1</sup>

sensitivity of ≈2% kPa<sup>−</sup><sup>1</sup>

also concerned.

porous tapes, using special molds (e.g., the surface of matte glass, a micromachined Si mold, or the surface of lotus leaf) to create microstructures in elastomers, using sugar cubes as the template to create porous elastomers and fabricating buckled structures through prestretching and releasing. As the dielectric constant of air is smaller than that of the dielectric material used for the sensor, the effective dielectric constant is increased under pressure when the air gap is compressed. For example, **Figure 3b** shows a flexible pressure sensor with high sensitivity been built, which is a typical sandwich structure by combining a microarrayed PDMS dielectric layer with PDMS substrates. The top/bottom electrode material is PDMS substrate coated with AgNWs, and the dielectric layer is a PDMS with microarray structure, which is used to improve the pressure sensitivity. The results show that

detection limits (<7 Pa), and fast response times (<100 ms). Meanwhile, it also has

In a representative work, a pressure-responsive triboelectric nanogenerator is used to gate the graphene transistors. Such graphene tribotronics showed a pressure

The wearable electromechanical sensor usually consists of three basic components, which are substrate, active elements, and electrode/interconnect. They are usually fabricated with different materials. During the fabrication process, combining the substrate and active elements is the key step. Basically, there are two situations. One is that sensing material forms uniform composite with polymer substrate, the other is that sensing material is attached on substrate and a clear interface exists. In this part, we will focus on the combination strategies for substrates and sensing elements, and some key processes for performance enhancement are

For the composite electromechanical sensor, the substrate and sensing materials should be fabricated into composite. The key process is how to mix them and prepare uniform composite. The sensing materials are usually mixed with polymers by magnetically or ultrasonically stirring, and then the dried elastic composites can be prepared in bulk or film forms. The mixed composites have complex electromechanical features that are induced by the diversity of sensing materials and polymer and significantly depend on concentration of sensing materials and its distribution state. For example, the electrical property of carbon black-silicone composite is mainly determined by carbon black concentration. The electrical resistance clearly increases with the applied uniaxial pressure when the concentration is about 0.08–0.09 wt%. By further increasing the concentration from 0.1 to 0.13 wt%, the change tendency of electrical resistance switches from increase to decrease. Finally, the electrical resistance starts to decrease with the uniaxial pressure with the

at a pressure of 10 kPa.

**4. Fabrication technology of wearable electromechanical sensor**

**4.1 Fabrication of wearable composite electromechanical sensor**

Progress has also been made on wearable piezoelectric and triboelectric pressure sensors. For example, it has been reported that a novel piezoelectric pressure sensor was fabricated through sandwiching freestanding electrospun polyvinyledenedifluoride-trifluoroethylene (PVDF-TrFE) nanofiber arrays [58] or electrospun PVDF-TrFE nanofiber between two electrodes. It can detect very tiny pressures as low as

) in low-pressure ranges (0–2000 Pa), low

for pressure range from 0.4–2 kPa.

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concentration larger than 0.14 wt % [59].
