Noninvasive electronic tattoo (e‐tattoo) attached to human skin surface for accurately obtaining various physiological information, has been widely used in wearable human‐machine interfaces (HMIs) for medical care, sports training, and artificial intelligence. The existing preparation technologies of e‐tattoos are difficult to satisfy the requirement of commercial mass production due to their high cost and low time efficiency. Here, inspired by the Chinese folk craftsmanship of dough figurines, a set of one‐step forming embossing process and a thermal‐mismatch‐induced transfer method are proposed for high‐efficient manufacture of low‐cost, large‐area, and multifunctional stretchable e‐tattoos. Benefited from the excellent flexibility and stretchability, the fabricated e‐tattoos can conformably follow the deformation of skin and collect high‐quality bioelectrical signals. Moreover, it can maintain good mechanical and electrical properties even when stretched to 40%. As a proof‐of‐concept, the 5‐µm‐thick e‐tattoo electrodes with hollow‐out and serpentine design are fabricated and demonstrated for the wearable HMI applications of electrocardiogram, electromyogram, brain training, epidermal heating, and flexible interconnection. Therefore, this cost‐effective and high‐fidelity e‐tattoo provides a potential path for the wearable HMI of widespread, non‐irritating, and multifunctional.

Flexible pressure sensors have been widely applied in wearable devices, e-skin, and the new generation of robots. However, most of the current sensors use connecting wires for energy supply and signal transmission, which presents an obstacle for application scenarios requiring long endurance and large movement, especially. Flexible sensors combined with wireless technology is a promising research field for realizing efficient state sensing in an active state. Here, we designed and fabricated a soft wireless passive pressure sensor, with a fully flexible Ecoflex substrate and a multi-walled carbon nanotube/polydimethylsiloxane (MWCNT/PDMS) bilayer pyramid dielectric structure. Based on the principle of the radio-frequency resonator, the device achieved pressure sensing with a changeable capacitance. Subsequently, the effect of the pyramid density was simulated by the finite element method to improve the sensitivity. With one-step embossing and spin-coating methods, the fabricated sensor had an optimized sensitivity of 14.25 MHz/kPa in the low-pressure range. The sensor exhibited the potential for application in limb bending monitoring, thus demonstrating its value for long-term wireless clinical monitoring. Moreover, the radio frequency coupling field can be affected by approaching objects, which provides a possible route for realizing non-contact sensing in applications such as pre-collision warning.

Continuous monitoring of physical motion, which can be successfully achieved via a wireless flexible wearable electronic device, is essential for people to ensure the appropriate level of exercise. Currently, most of the flexible LC pressure sensors have low sensitivity because of the high Young’s modulus of the dielectric properties (such as PDMS) and the inflexible polymer films (as the substrate of the sensors), which don’t have excellent stretchability to conform to arbitrarily curved and moving surfaces such as joints. In the LC sensing system, the metal rings, as the traditional readout device, are difficult to meet the needs of the portable readout device for the integrated and planar readout antenna. In order to improve the pressure sensitivity of the sensor, the Ecoflex microcolumn used as the dielectric of the capacitive pressure sensor was prepared by using a metal mold copying method. The Ecoflex elastomer substrates enhanced the levels of conformability, which offered improved capabilities to establish intimate contact with the curved and moving surfaces of the skin. The pressure was applied to the sensor by weights, and the resonance frequency curves of the sensor under different pressures were obtained by the readout device connected to the vector network analyzer. The experimental results show that resonant frequency decreases linearly with the increase of applied pressure in a range of 0–23,760 Pa with a high sensitivity of −2.2 MHz/KPa. We designed a coplanar waveguide-fed monopole antenna used to read the information of the LC sensor, which has the potential to be integrated with RF signal processing circuits as a portable readout device and a higher vertical readout distance (up to 4 cm) than the copper ring. The flexible LC pressure sensor can be attached to the skin conformally and is sensitive to limb bending and facial muscle movements. Therefore, it has the potential to be integrated as a body sensor network that can be used to monitor physical motion.

Flourishing in recent years, intelligent electronics is desirably pursued in many fields including bio-symbiotic, human physiology regulatory, robot operation, and human–computer interaction. To support this appealing vision, human-like tactile perception is urgently necessary for dexterous object manipulation. In particular, the real-time force perception with strength and orientation simultaneously is critical for intelligent electronic skin. However, it is still very challenging to achieve directional tactile sensing that has eminent properties, and at the same time, has the feasibility for scale expansion. Here, a fully soft capacitive omnidirectional tactile (ODT) sensor was developed based on the structure of MWCNTs coated stripe electrode and Ecoflex hemisphere array dielectric. The theoretical analysis of this structure was conducted for omnidirectional force detection by finite element simulation. Combined with the micro-spine and the hemispheric hills dielectric structure, this sensing structure could achieve omnidirectional detection with high sensitivity (0.306 ± 0.001 kPa−1 under 10 kPa) and a wide response range (2.55 Pa to 160 kPa). Moreover, to overcome the inherent disunity in flexible sensor units due to nano-materials and polymer, machine learning approaches were introduced as a prospective technical routing to recognize various loading angles and finally performed more than 99% recognition accuracy. The practical validity of the design was demonstrated by the detection of human motion, physiological activities, and gripping of a cup, which was evident to have great potential for tactile e-skin for digital medical and soft robotics.

between systems and objects. [1][2][3][4][5] Sensor arrays have been demonstrated to realize high-sensitivity haptic position sensing, [6] objects recognition, [7] force direction detection, [8,9] modulus measurement, [10] texture recognition, [11] and other comprehensive multisensory functions. [3,12,13] However, when numerous sensors are implemented in the human body or installed on machines with complex actions, power supplies and signal lines become intractable obstacles. [14,15] To solve this issue, passive and wireless radiofrequency (RF) resonant sensors, have received considerable attention as promising candidates for hard-to-wire scenarios. [16][17][18][19][20] These sensors can be flexibly mounted on the moving part or inside the sealing equipment because they use RF electromagnetic waves to exchange energy and signals directly, eliminating the need of wiring and batteries. Examples include the wind-pressure probes on the propellers, [18] noninvasive detection inside the pipelines, [21,22] and the longendurance monitoring of air chambers and tire pressure. [23,24] Moreover, the combination of RF technology and soft materials can build a bridge from the human body to machines, such applications involve nanowire contact lenses for intraocular pressure monitoring, [17] arterial-pulse sensors for bloodflow monitoring, [16] and biodegradable implanted devices for intracranial pressure measurement. [25,26] Although wireless RF sensing technology has demonstrated considerable application potential, the array construction of RF tactile sensors still faces several challenges. In typical sensor array based on split-ring resonator (SRR) arrays [27][28][29] or compact inductor-capacitor (LC) arrays, [30][31][32] sensor units are required to operate at various frequencies. According to the existing methods, different inductance geometries need to be constructed for this purpose, which requires a complex micromachining process owing to the limited device area. Simultaneously, different inductances result in an irregular and incalculable mutual inductance between neighbors, which causes clutter interference while reading. Moreover, the shift of the resonant peak in the scatter spectrum is typically read as a response to the applied pressure. A common drawback is that the frequency sensor array needs to provide an independent operating frequency range for each sensitive Intelligent soft robotics and wearable electronics require flexible, wireless radio frequency (RF) pressure sensors for human-like tactile perception of their moving parts. Existing devices face two challenges for array extension: the construction of sensitive units over a limited area and the handling of resonant peaks overlapping within the channel width. Herein, a simply adjustable RF-resonator-based tactile array (RFTA) is reported, in which the initial frequency of each resonator unit is regulated by doping polydimethylsiloxane (PDMS) dielectric layers with various concentrations of multiwalled carbon nanotubes (MWCNTs). An array is constructed usi...