A flexible, three-axis carbon nanotube (CNT)–polymer composite-based tactile sensor is presented. The proposed sensor consists of a flexible substrate, four sensing cells, and a bump structure. A CNT–polydimethylsiloxane (PDMS) composite is produced by a solvent evaporation method, and thus, the CNTs are well-dispersed within the PDMS matrix. The composite is directly patterned onto a flexible substrate using a screen printing technique to fabricate a sensor with four sensing cells. When a force is applied on the bump, the magnitude and direction of force could be detected by comparing the changes in electrical resistance of each sensing cell caused by the piezoresistive effect of the composite. The experimentally verified sensing characteristics of the fabricated sensor exhibit a linear relationship between the resistance change and the applied force, and the measured sensitivities of the sensor for the normal and shear forces are 6.67 and 86.7%/N for forces up to 2.0 and 0.5 N, respectively. Experiments to verify the load-sensing repeatability show a maximum 2.00% deviation of the resistance change within the tested force range.

various fabrics [7][8][9][10][11][12] and fibers [13][14][15] for fabricating triboelectric energy harvesters. Fabric-or fiber-based wearable energy harvesters facilitate lightweight energy generation systems that are comfortable for the wearer. [1] However, the output voltage of triboelectric energy harvesters is drastically reduced by adsorbed water molecules, [16] such as humidity originating from the body, rain, or other surrounding environmental conditions. In general, the output voltage of a triboelectric energy harvester is proportional to the charge density of the contact surface. [17] Under high relative humidity conditions, a thick adsorbed water layer increases the conductivity of the contact surface, causing dissipation of surface charges induced by triboelectrification to another material. [18,19] The decreased charge density of the contact surface results in the deterioration of the output voltage; therefore, there is motivation to develop a humidity-resistant, wearable, triboelectric energy harvester. To date, there have been some efforts to impart humidity resistance to triboelectric energy harvesters. [20][21][22][23][24][25][26] For example, Seol et al. reported a triboelectric vibrational energy harvester sealed in an acrylic tube to reduce the penetration of humidity; this device showed excellent resistance to ambient humidity. [20] Additionally, various methods for fabricating humidity-resistant triboelectric energy harvesters have been reported, including a hydrophobic microsponge structure, [21,22] nature-replicating micro-/nanostructure, [23] and micro/nanosurface morphologies [24,25] using polystyrene microbeads and natural materials with micro-/nanomorphologies as a template or mold. However, no further research has been undertaken to apply these techniques to fabric-based, wearable triboelectric energy harvesters.Kim et al. reported a fabric-based, wearable, humidityresistant triboelectric energy harvester by fabricating individual ZnO-polydimethylsiloxane core-shell fibers. [26] Each core-shell fiber was sealed at both sides by a polymer and then woven to prepare a fabric-based triboelectric energy harvester. This harvester showed excellent humidity resistance up to a relative humidity of 95%; however, the fabrication of the humidityresistant fabric is very complicated as it required the formation of nanostructures, fiber-combining, and sealing of each fiber.The development of fabric-based triboelectric energy harvesters is of great interest for converting human motion into electricity and is relevant for the development of wearable electronics. However, such harvesters exhibit significant degradation in performance under high humidity conditions. To solve this problem, a humidity-resistant, fabric-based triboelectric energy harvester by depositing self-assembled monolayers (SAM) to increase the hydrophobicity of the fabric surface is demonstrated. The SAM coating is compatible with various fabrics and a noticeable improvement in triboelectric performance under high humidity conditions...

A polymer-based tactile sensor with flexibility and multi-directional sensing capability is presented. The proposed sensor consists of a polydimethylsiloxane (PDMS) bump, a polyimide (PI) substrate, Cr/Au electrode lines for electrical connection, NiCr piezoresistors, and an SU-8 support structure. The sensing mechanism is based on piezoresistive effect, in which the resistance of NiCr changes under mechanical load. The PMDS bump positioned at the center of the sensor transfers an applied force to the PI film, and the piezoresistors are differently deformed depending on the magnitude and direction of the force. A diaphragm structure formed by the SU-8 support with a trench allows the piezoresistor to be effectively deformed. Simulation and experimental results confirm that magnitude and direction can be obtained from an arbitrarily applied force by comparing the change in resistance of each sensing element. Based on its compatibility with conventional microfabrication, the proposed sensor may be a promising candidate for a low-cost tactile sensing solution for human-machine interfaces.

Textile-based triboelectric nanogenerators (TENGs) have received considerable attention for wearable applications owing to their significant advantages, such as flexibility, lightness, and breathability. Recently, several studies based on the modification of friction surfaces for improving the triboelectric output performance have been reported. However, previously reported methods require complicated fabrication processes and may deteriorate the intrinsic properties of a textile. Herein, we present a wearable TENG utilizing a textile composed of pile-embroidered (rough-textured) fibers as a contact surface. The deformability of the fibers originating from the suspended structure provides a large contact area that can participate in triboelectrification. This significantly increases charge density induced on the surface in response to a compressive force, resulting in a high output voltage of 113 V. The TENG also exhibits a high output power, which is 24 times higher than that of the TENG based on satin-embroidered (flat-textured) fibers. More importantly, owing to the conventional textile manufacturing process based on three-dimensional embroidery, the fabrication of our TENG is significantly simpler and more cost-effective than previously reported techniques. Experimental demonstrations as a wearable energy harvester highlight the utility of the TENG for generating electricity from various human motions. Based on ease of manufacturing and high output performance, the proposed harvester is a promising candidate as a low-cost power source for nextgeneration electronics, such as Internet of Things devices and self-powered smart clothing.

We demonstrate a highly sensitive force sensor based on self-adjusting carbon nanotube (CNT) arrays. Aligned CNT arrays are directly synthesized on silicon microstructures by a space-confined growth technique which enables a facile self-adjusting contact. To afford flexibility and softness, the patterned microstructures with the integrated CNTs are embedded in polydimethylsiloxane structures. The sensing mechanism is based on variations in the contact resistance between the facing CNT arrays under the applied force. By finite element analysis, proper dimensions and positions for each component are determined. Further, high sensitivities up to 15.05%/mN of the proposed sensors were confirmed experimentally. Multidirectional sensing capability could also be achieved by designing multiple sets of sensing elements in a single sensor. The sensors show long-term operational stability, owing to the unique properties of the constituent CNTs, such as outstanding mechanical durability and elasticity.