Triboelectric Nanogenerators as Active Tactile Stimulators for Multifunctional Sensing and Artificial Synapses

Abstract

The wearable tactile sensors have attracted great attention in the fields of intelligent robots, healthcare monitors and human-machine interactions. To create active tactile sensors that can directly generate electrical signals in response to stimuli from the surrounding environment is of great significance. Triboelectric nanogenerators (TENGs) have the advantages of high sensitivity, fast response speed and low cost that can convert any type of mechanical motion in the surrounding environment into electrical signals, which provides an effective strategy to design the self-powered active tactile sensors. Here, an overview of the development in TENGs as tactile stimulators for multifunctional sensing and artificial synapses is systematically introduced. Firstly, the applications of TENGs as tactile stimulators in pressure, temperature, proximity sensing, and object recognition are introduced in detail. Then, the research progress of TENGs as tactile stimulators for artificial synapses is emphatically introduced, which is mainly reflected in the electrolyte-gate synaptic transistors, optoelectronic synaptic transistors, floating-gate synaptic transistors, reduced graphene oxides-based artificial synapse, and integrated circuit-based artificial synapse and nervous systems. Finally, the challenges of TENGs as tactile stimulators for multifunctional sensing and artificial synapses in practical applications are summarized, and the future development prospects are expected.

Keywords: active tactile stimulators; artificial synapses; multifunctional sensing; triboelectric nanogenerators.

Figure 1

Brief timeline of tactile stimulator based on triboelectric nanogenerators. Reproduced with permission [41]. Copyright 2012, American Chemical Society. Reproduced with permission [21]. Copyright 2013, American Chemical Society. Reproduced with permission [22]. Copyright 2014, American Chemical Society. Reproduced with permission [33]. Copyright 2015, American Chemical Society. Reproduced with permission [43]. Copyright 2016, John Wiley and Sons. Reproduced with permission [44]. Copyright 2017, American Chemical Society. Reproduced with permission [48]. Copyright 2018, John Wiley and Sons. Reproduced with permission [49]. Copyright 2019, Elsevier. Reproduced with permission [50]. Copyright 2020, American Chemical Society. Reproduced with permission [51]. Copyright 2021, Springer Nature.

Figure 2

Pressure and temperature sensing. (a) Schematic diagram of the tactile stimulator. (b) Functional relationship between external pressure and the current. (c) Functional relationship between temperature and the current. Reproduced with permission [54]. Copyright 2019, Elsevier. (d) The working principle of the self-healing electronic skin. (e) Pressure response curve of the self-healing electronic skin. (f) Temperature response curve of the self-healing electronic skin. Reproduced with permission [55]. Copyright 2021, American Association for the Advancement of Science. (g) Schematic illustration of the tactile electronic skin. (h) Functional relationship between external pressure and output voltage at room temperature. (i) Functional relationship between reciprocal temperature and electrode resistance without deformation. Reproduced with permission [56]. Copyright 2020, Elsevier.

Figure 3

Pressure and temperature sensing. (a) Schematic diagram of pressure and temperature sensing mechanism based on multi-effect coupling. (b) The voltage response curve under the coupling of pressure and temperature stimuli. (c) Functional relationship between external pressure and output voltage. (d) Functional relationship between temperature and output voltage. Reproduced with permission [57]. Copyright 2019, Elsevier. (e) Schematic diagram of the multifunctional self-powered tactile stimulator. (f) Pressure response curve of the device. (g) Temperature response curve of the device. (h) The voltage peak amplitude generated by contact between the device and different materials. Reproduced with permission [58]. Copyright 2020, American Association for the Advancement of Science.

Figure 4

Pressure and proximity sensing. (a) Schematic illustration of the working mechanism of the triboelectric skins. (b) Functional relationship between pressure and the voltage of devices with different structures. (c) The variation characteristics of the voltage under different pressure and strain cycles. (d) Functional relationship between separation distance and the voltage. Reproduced with permission [59]. Copyright 2018, John Wiley and Sons. (e) Schematic diagram of the liquid-polymer tubular TENG. (f). Functional relationship between force and open-circuit voltage. (g). Functional relationship between distance and capacitance. Reproduced with permission [60]. Copyright 2019, Royal Society of Chemistry. (h) Schematic diagram of the stretchable dual-mode sensor array for a bionic robot. (i) Functional relationship between pressure and open-circuit voltage under different tensile strain. Reproduced with permission [61]. Copyright 2019, Elsevier.

Figure 5

Pressure and object recognition. (a) Schematic diagram of the triboelectric sensor matrix. (b) Functional relationship between pressure and the output voltage. (c) The output voltage is generated by contact between the device and different materials. Reproduced with permission [43]. Copyright 2019, John Wiley and Sons. (d) Schematic diagram of the tactile stimulator. (e) Functional relationship between pressure and the output voltage. (f) The current change is generated by contact between the device and different materials. Reproduced with permission [62]. Copyright 2017, American Chemical Society. (g) Schematic diagram of the triboelectric-photonic smart skin for a robotic finger. (h) The 3D normalized photoluminescence intensity map of a hand gesture “OK”. Reproduced with permission [48]. Copyright 2018, John Wiley and Sons. (i) Construction drawing of the TENG for soft gripper. (j) Actual gripping and virtual demonstration of object recognition. Reproduced with permission [63]. Copyright 2020, Springer Nature.

Figure 6

Electrolyte-gate synaptic transistors based on organic semiconductor material. (a) Schematic diagram of the SPST. (b) EPSC of the SPST at different touch frequencies. Reproduced with permission [49]. Copyright 2019, Elsevier. (c) Schematic diagram of the artificial sensory memory system. (d) Diagrams of the handwritten digit number and the tactile mapping. Reproduced with permission [70]. Copyright 2020, Elsevier.

Figure 7

Electrolyte-gate synaptic transistors based on two-dimensional semiconductor materials. (a) Schematic diagram of the triboiontronic synaptic transistor. (b) The output voltage of the TENG under the action of a displacement pulse. (c) Schematic diagram of a bio-neuron and the triboiontronic synaptic transistor. (d) The PSCs of the transistor under the action of a displacement pulse. (e) Functional relationship between time interval (Δt) and PPF index. Reproduced with permission [85]. Copyright 2020, American Chemical Society. (f) Schematic diagram of triboelectrification-activated artificial afferent neuron. (g) Functional relationship between displacement and energy dissipation. Reproduced with permission [51]. Copyright 2021, Springer Nature.

Figure 8

Mechano-photonic artificial synapse. (a) Schematic diagram of the mechano-photonic synapse device based on graphene/MoS₂ heterostructure. (i) Scanning electron microscope image of the top of the synapse device. (ii) Schematic diagram charge transfer/exchange for the graphene/MoS₂ heterostructure. (iii) Image recognition of the synapse device in response to mechano-photonic signal. (b) Functional relationship between displacement and real-time initial PSC in the dark. (c) Functional relationship between light intensity and real-time −ΔPSC. (d) Functional relationship between numbers of training samples and recognition accuracy. Reproduced with permission [91]. Copyright 2021, American Association for the Advancement of Science.

Figure 9

Artificial multisensory integration nervous system based on TENG and photosynaptic transistor. (a) Schematic diagram of the artificial multisensory integration nervous system. (b) The variation characteristics of the channel current under 30 continuously touch pulses and different light intensity. (c) The variation characteristics of the channel current under 30 continuously optical pluses and different pressure. (d) Functional relationship between numbers of training epochs and recognition accuracy. Reproduced with permission [93]. Copyright 2021, Elsevier.

Figure 10

Mechanoplastic tribotronic floating-gated synaptic transistor. (a) Schematic diagram of the floating-gated synaptic transistor. (b) The PSC of the transistor under the action of a displacement pulse. (c) Schematic diagram of the working mechanism of the transistor, (I–IV) are four different working states, respectively. (d) Functional relationship between displacement and paired pulse depression/facilitation index. (e) Functional relationship between pulse number and EPSC/IPSC. Reproduced with permission [94]. Copyright 2020, John Wiley and Sons.

Figure 11

Reduced graphene oxides-based artificial synapse. (a) Conceptual diagram of the artificial synapse. (b) Schematic structure of the artificial synapse. (c) Schematic diagram of the triboelectric electrons transfer process. (d) Functional relationship between press number and output voltage of the artificial synaptic device. Reproduced with permission [50]. Copyright 2019, American Chemical Society.

Figure 12

Integrated circuit-based artificial synapse and peripheral nervous system. (a) Schematic diagram of a bioinspired artificial tactile peripheral nervous system. (b) Spatial recognition of the two-tier artificial tactile peripheral nervous system. (I) Layout of the mechanoreceptor. (II) The mapping of the mechanoreceptors to artificial peripheral neurons. (III) The synaptic structures. Reproduced with permission [97]. Copyright 2021, Elsevier.