**4.2 Fabrication of wearable layered electromechanical sensor**

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, and other methods.

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 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

**Figure 4.**

*Wearable electromechanical sensor fabricated with different techniques: (a) inkjet printing, (b) dropping casting, and (c) spray coating.*

another special method to fabricate fiber shape wearable electromechanical sensor. **Figure 5c** shows a fiber strain sensor fabricated with coaxial wet-spinning and posttreatment process. The spinning nozzle has the coaxial inner and outer channels, respectively. The inner spinning dope is SWCNT/CH3SO3H, and the outer spinning solution is the solution of thermoplastic elastomer (TPE) in CH2Cl2. The SWCNT/ CH3SO3H dope from the inner channel and the TPE/CH2Cl2 solution from the outer channel are introduced into the ethanol coagulation bath simultaneously. A single TPE-wrapped SWCNT coaxial fiber is then wetspun and collected successfully. The sensors attain high sensitivity (with a gauge factor of 425 at 100% strain), high stretchability, and high linearity.
