Flexible and Stretchable Carbon-Based Sensors and Actuators for Soft Robots

Abstract

In recent years, the emergence of low-dimensional carbon-based materials, such as carbon dots, carbon nanotubes, and graphene, together with the advances in materials science, have greatly enriched the variety of flexible and stretchable electronic devices. Compared with conventional rigid devices, these soft robotic sensors and actuators exhibit remarkable advantages in terms of their biocompatibility, portability, power efficiency, and wearability, thus creating myriad possibilities of novel wearable and implantable tactile sensors, as well as micro-/nano-soft actuation systems. Interestingly, not only are carbon-based materials ideal constituents for photodetectors, gas, thermal, triboelectric sensors due to their geometry and extraordinary sensitivity to various external stimuli, but they also provide significantly more precise manipulation of the actuators than conventional centimeter-scale pneumatic and hydraulic robotic actuators, at a molecular level. In this review, we summarize recent progress on state-of-the-art flexible and stretchable carbon-based sensors and actuators that have creatively added to the development of biomedicine, nanoscience, materials science, as well as soft robotics. In the end, we propose the future potential of carbon-based materials for biomedical and soft robotic applications.

Keywords: actuators; carbon nanotubes; flexible electronics; graphene; sensors; soft robotics; stretchable electronics.

Figure 1

Schematic of typical fabrication techniques for 0D, 1D, and 2D carbon–based materials. (top) Carbon dots. (a) Figure for laser ablation: Reproduced from [75] with permission, copyright 2016, Springer Nature. (b) Figure for microwave–assisted method: (A) Schematic of the preparation process and purification of carbon dots; (B) Images of the crude product under different irradiation conditions; (C) Images of purified carbon dots after ceasing UV irradiation. Reproduced from [94] with permission, copyright 2018, John Wiley and Sons. (c) Figure for template method: Reproduced from [81] with permission, copyright 2019, Elsevier. (middle) Carbon nanotubes. (d) Figure for arc discharge: Reproduced from [78] with permission, copyright 2014, Elsevier. (e) Figure for chemical vapor deposition: Reproduced from [82] with permission, copyright 2018, Elsevier. (f) Figure for laser ablation: Reproduced from [76] with permission, copyright 2015, John Wiley and Sons. (bottom) Graphene. (g) Figure for liquid–phase exfoliation: Reproduced from [83] with permission, copyright 2020, ACS. (h) Figure for mechanical exfoliation: Reproduced from [84] with permission, copyright 2019, Elsevier. (i) Figure for unzipping of carbon nanotubes: Reproduced from [86] with permission, copyright 2021, ACS.

Figure 2

Fabrication techniques for merging carbon–based materials with soft substrate: (a) Use PECVD for direct graphene growth on flexible substrates. Reproduced from [136] with permission, copyright 2019, ACS. (b) Spray–coating for MWCNT–embedded Ecoflex films: (i) Using petri dish as the substrate; (ii) Spray–coating of CNT–IPA solution on petri dish; (iii) Pouring of Ecoflex on the substrate; (iv) Schematic of the MWCNT–embedded Ecoflex film. Reproduced from [133] with permission, copyright 2019, ACS. (c) Drop–casting for preparing flexible rGO/CNT/AgNW films. Reproduced from [135] with permission, copyright 2021, Elsevier. (d) Use printing technologies for fabricating pressure sensors: (A) Preparation process of MWCNT–PDMS ink; (B) Electrode formulation by printing; (C) Sensing formulation by printing; (D) Images of printed electrode and sensing layer. Reproduced from [137] with permission, copyright 2020, John Wiley and Sons.

Figure 3

Applications of carbon–based sensors: (a) N–doped carbon dots for Fe(III) sensing and imaging in living cells. Reproduced from [153] with permission, copyright 2021, Elsevier. (b) Flexible polymer–CNT thermal sensor. Reproduced from [154] with permission, copyright 2018, ACS. (c) Highly sensitive and flexible pressure sensor built with carbon black. Reproduced from [155] with permission, copyright 2019, ACS. (d) Flexible capacitive pressure sensor with carbon fiber electrodes. Reproduced from [156] with permission, copyright 2021, ACS. (e) Flexible strain sensor based on carbonized conductive crepe paper. Reproduced from [32] with permission, copyright 2018, John Wiley & Sons. (f) Flexible carbon–based strain sensor with faster response. Reproduced from [157] with permission, copyright 2021, Springer Nature. (g) Flexible temperature sensor made of reduced graphene oxide. Reproduced from [158] with permission, copyright 2018, MDPI. (h) Flexible CNT–based temperature sensor with bio–compatibility. Reproduced from [159] with permission, copyright 2020, Elsevier. (i) Highly sensitive and flexible NH₃ sensor. Reproduced from [160] with permission, copyright 2016, Elsevier. (j) Flexible MWCNT–based NH₃ sensor, showing the sensing mechanism. Reproduced from [161] with permission, copyright 2021, Elsevier.

Figure 4

Typical actuation mechanisms for carbon–based soft robots: (left) (a) Flexible robotic hand based on graphene nanoplates and PI, demonstrating the bending mechanism of electro–thermal actuators. Reproduced from [210] with permission, copyright 2019, Elsevier. (b) Graphene–based electro–thermal actuator. Reproduced from [211] with permission, copyright 2022, ACS. (middle) (c) CNT–based light–driven fiber actuator. Reproduced from [212] with permission, copyright 2021, Elsevier. (d) Worm–like photo–actuated soft robot that can realize crawling, squeezing, and jumping. Reproduced from [213] with permission, copyright 2019, John Wiley & Sons. (right) (e) Flexible self–charging supercapacitor based on carbon cloth, showing the self–charging process: (A) Schematic of the supercapacitor composition; (B) Reaction to external force and generation of piezoelectric field; (C) Migration of electrolyte ions and charging; (D) Continuous migration of electrolyte ions and charging after external force is ceased; (E) New equilibrium after migration; (F) Piezoelectric field disappears and the supercapacitor returns to initial state. Reproduced from [214] with permission, copyright 2020, Elsevier. (f) Piezo–actuator containing aligned CNTs. Reproduced from [215] with permission, copyright 2021, Elsevier.