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Flexible pressure sensing is required for the excellent sensing performance and dexterous manipulation of the measured objects in their potential applications. Particularly, the ability to measure and discriminate the direction of force, contact surface, and contact location in real time is crucial for robotics with tactile feedback. Herein, a three-dimensional elastic porous carbon nanotube (CNT) sponge is synthesized by chemical vapor deposition, which is successfully applied in the piezoresistive sensor. In situscanning electron microscopy study intuitively illustrates the characteristics that the microfibers of the CNT sponge distort and contact with each other under an external force. As a result, new conductive paths are created at the contact points between the CNT microfibers, which provides a basic sensing principle for a piezoresistive sensor. The CNT sponge-based sensor has an ultrahigh sensitivity in a wide pressure range (0–4 kPa for 4015.8 kPa–1), a rapid response time of 120 ms, and excellent durability over 5000 cycles. Moreover, a finlike flexible double-sided electronic skin (e-skin) is fabricated by a simple method to achieve force direction detection, which has potential applications in intelligent wearable devices and human–machine interaction.

As an indispensable and prevailing kind of flexible sensors, pressure sensors have been extensively used in the field of flexible electronics. To enhance the performance and broaden the applications of flexible pressure sensors, emerging nanomaterials with excellent electromechanical properties have been explored and developed over recent years. Herein, the latest progresses in flexible pressure sensors based on metal and carbonaceous nanomaterial are reviewed in this paper. Firstly, fundamental considerations of pressure sensors including transduction principles, evaluation parameters and nano-micro structures are presented. Furthermore, flexible pressure sensors based on carbonaceous nanomaterials, transition metal carbides and carbonitrides (Mxene), along with metal nanomaterials are reviewed and classified by different transduction principles. At last, a number of applications on electronic skin (E-skin) involving metal and carbonaceous nanomaterial-based flexible pressure sensors are introduced, including pressure distribution sensing, force direction detection, multifunctional sensing and system integration.

The highly stretchable, flexible and self-healing strain sensor is considered as a promising wearable device for motion detection in fields such as robotics and human-machine interface. Herein, a strain sensor with high stretchability and sensitivity is designed and fabricated based on the poly(acrylamide) (PAAm) hydrogel. The ionic conductive PAAm hydrogel shows an excellent self-healing property with fast electrical self-healing speed (within 1.8 s) and high self-healing efficiency (99%). The PAAm hydrogel based strain sensor exhibits excellent performance with large stretchability (>900%), high sensitivity (with maximum gauge factor of 6.44), fast response time (~150 ms), and good cycling durability (>3000 cycles). As the wearable device, the strain sensor can detect various human motions and is able to transmit the data to the smart phone combined with a readout and wireless system. Furthermore, a smart glove was fabricated by coupling multiple strain sensors and the corresponding circuit. The smart glove is capable of expressing and recognizing American Sign Language and can be used to control a robotic hand wirelessly through the hand gestures. Thus, the highly stretchable, self-healing hydrogel-based strain sensor shows a potential application for the human-machine interface, personal healthcare and sports training.

A flexible skin-like tactile sensor as one of the key components in the next generation for robots should have the ability to perform real-time feedback, continuous measurement, and quantization of weak target signals in the real applications (e.g. intelligent Braille recognition). Herein, inspired by human skin, a novel flexible piezoresistive tactile sensor with high sensitivity and linearity is designed and fabricated. It is composed of three main parts including the random Gaussian distribution (RGD) spinosum on polydimethylsiloxane as the top substrate, the multilayer Ti2C-MXene film as the intermediate conductive filler, and the commercial polyimide-based interdigital electrodes as the bottom substrate. The working mechanism of RGD spinosum and Ti2C-MXene films in the flexible tactile sensor are clarified by COMSOL Multiphysics simulations and density functional theory calculations, respectively. The assembled sensor demonstrates ultrahigh sensitivity, large linearity, excellent cycling stability, and fast response speed in the milliseconds. Moreover, the development of advanced neural network technology makes it possible to realize intelligent skin-like sensor. In this study, it is the first time to realize real-time Braille effective intelligent recognition by such a flexible skin-like tactile sensor with the random decision forests algorithm. This study is of great significance to solve the social and public issues of information exchange for the vision-impaired or even the blind that has been highly concerned in recent years.