Stretchability and self-healing ability are essential attributes of modern electronic sensors for wearable applications. Herein, intrinsically ultrastretchable, self-healing and transparent humidity sensors composed of ionic κ-carrageenan/polyacrylamide (PAM) double network (DN) organohydrogels are fabricated via a facile solvent-exchange strategy. Hygroscopic ethylene glycol (EG) and glycerol (Gly) are introduced in the organohydrogels to promote both anti-drying and humidity sensing capabilities. The organohydrogel sensors display unprecedented stretchability (1225% strain) and excellent sensing performance, including fast response (0.27 s) and recovery time (0.3 s), wide relative humidity (RH) detection range (4–90%), stability and linearity. The conductance of Gly-DN sensors increases more than 543 times with increasing RH from 4% to 90%. With sufficient sensitivity, the humidity sensors can monitor human respiration with stable and repeatable output. The enhanced sensitivity and moisture-holding ability are attributed to the ready formation of hydrogen bonds between water molecules and enormous hydrophilic groups, including –OH, SO₃ ⁻ and –NH₂, in the elaborate polymer networks and binary solvent. The hindering effect of polymer chains on the transport of conductive ions plays a key role in producing a transducing signal, which is proposed as the mechanism of the novel sensors.

3D printing has attracted a lot of attention in recent years. Over the past three decades, various 3D printing technologies have been developed including photopolymerization-based, materials extrusion-based, sheet lamination-based, binder jetting-based, power bed fusion-based and direct energy deposition-based processes. 3D printing offers unparalleled flexibility and simplicity in the fabrication of highly complex 3D objects. Tactile sensors that emulate human tactile perceptions are used to translate mechanical signals such as force, pressure, strain, shear, torsion, bend, vibration, etc. into electrical signals and play a crucial role toward the realization of wearable electronics and electronic skin. To date, many types of 3D printing technologies have been applied in the manufacturing of various types of tactile sensors including piezoresistive, capacitive and piezoelectric sensors. This review attempts to summarize the current state-of-the-art 3D printing technologies and their applications in tactile sensors for wearable electronics and electronic skin. The applications are categorized into five aspects: 3D-printed molds for microstructuring substrate, electrodes and sensing element; 3D-printed flexible sensor substrate and sensor body for tactile sensors; 3D-printed sensing element; 3D-printed flexible and stretchable electrodes for tactile sensors; and fully 3D-printed tactile sensors. Latest advances in the fabrication of tactile sensors by 3D printing are reviewed and the advantages and limitations of various 3D printing technologies and printable materials are discussed. Finally, future development of 3D-printed tactile sensors is discussed.