Soft Robotic Actuation with Carbon Materials: History
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Subjects: Robotics
Contributor:
Xinyi Zhou
,
Wenhan Cao

Due to the superior softness and elasticity compared to conventional rigid devices, soft robotic actuators exhibit remarkable advantages in terms of their portability, power efficiency, and wearability, thus creating myriad possibilities of micro-/nano-soft actuation systems. Carbon-based materials, such as carbon nanotubes, carbon dots, and graphene, are integrated with polymers or elastomers in soft actuators for their excellent biocompatibility and conductivity.

  • flexible electronics
  • stretchable electronics
  • carbon nanotubes
  • graphene
  • actuators
  • soft robotics

1. Introduction

Actuators are devices that respond to various stimuli, including heat [1], pressure [2], light [3], humidity [4], electricity [5], and magnetism [6]. Carbon-based soft robotic actuators mainly consist of carbon nanomaterials and flexible substrates. For example, a multi-responsive actuator made up of a graphene oxide film and a layer containing mixed carbon nanotubes (CNTs) and polydimethylsiloxane (PDMS) was proposed in 2018, and it could produce reversible deformation under thermal, light, and humidity conditions [7]. Another actuator had displacement in response to voltage stimulation or light irradiation, in which CNTs were blade-coated to achieve orderly direction. It demonstrated bending deformation larger than 10 mm under external stimulus, having great inspiration for bionic soft robotics [8].
Thanks to their flexibility and stretchability, soft robotic actuators behave better than their rigid peers in fields of biomimicry and soft robotics [9][10], as do the actuators containing carbon nanomaterials. The following sections present actuation reacting to heat, light irradiation, and piezoelectricity.
Although these reported soft robotic actuators present an outstanding and obvious response to external stimuli, most of them are still on the centimeter or millimeter length scale, which remains to be greatly improved in the future. The internal structural design, use of materials, and the actuating mechanisms are among the factors that could be taken into account. Further, the minimization of soft actuators can benefit their applications in bio-related areas, especially under in vivo circumstances.

2. Thermal Actuation

Considering that the actuating mechanism of thermal actuation is converting thermal energy into kinetic energy and creating motion, materials involved should demonstrate expansion or contraction in response to thermal stimulation. For instance, hydrogel is regarded as a promising candidate for wearable electronics due to its reaction to thermal change [11]. In thermal actuation, the stimulation is usually provoked by change in ambient temperature or heat generated by electricity [12].
According to the introduced actuating mechanism, when applying voltage to conductive films, thermal-induced expansion or contraction will take place as the current flow heats up the films. The degree of deformation is closely related to the coefficient of thermal expansion (CTE), which is a material property. In a multi-layer structure, the mismatch of CTEs between the layers will result in bending. The carbon-based film is often treated as the conductive layer because of its excellent electrical conductivity, as well as being responsible for transferring heat between multiple layers due to its extreme sensitivity to heat. With relatively lower CTE, the carbon layer is less stretched than polymeric layers, leading to the bending deformation of carbon-based flexible films [13][14]. For instance, a typical electro-thermal actuator can be made up of three layers, namely the CNT layer, Kapton® layer, and shape memory polymer (SMP) layer. The applied voltage heats the CNT layer and heat transfers, making the SMP layer become flexible and finally inducing the bending [15]. Electro-thermal actuation excels in terms of the low applied voltage since a composite layer of CNTs and PDMS can achieve a large deformation at as low as 8 V bias [16]. In addition, the bending radius is influenced and controlled by multiple factors, involving the applied voltage, the layer thickness, and the CTE of different materials. In this case, the actuator can accomplish specific robotic motions, such as grasping and releasing objects [13].

3. Photo-Actuation

Photo-actuation is an easily implemented actuation method and basically relies on either the photochemical reactions or the photothermal effects of materials [17][18]. Although photothermal actuators similarly make use of the thermal sensitivity of materials with electro-thermal ones, they behave better at enabling contactless manipulation [19].
The fabrication of photo-actuators can be carried out by merging two films with different CTEs together, such as single-walled carbon nanotubes (SWCNTs) and polycarbonate. The photo energy absorbed by SWCNT film converts to thermal energy, resulting in the deformation of different extents [20]. Similarly, composites of liquid crystal elastomer and CNTs can be constructed into a light-powered soft robot. When the composite film was exposed to light irradiation, the surface facing the light had higher temperature than the opposite side, prompting the film to contract unevenly and therefore bend toward the light source. On the basis of the bending motion, the tiny robot can perform various locomotion under different illumination modes like a worm, including crawling, contraction, and jumping [21]. The carrying capacity has also been exceedingly enhanced in 2022, being reported to uphold loads more than 4600 times of the actuator's own weight. The composite film created by depositing CNTs onto a liquid crystal elastomer fiber displayed excellent photo-actuating behavior, and therefore it can be employed as artificial muscles [22].
It has also been proposed that precise functions other than functional movements, such as printing, can be realized by properly designing and utilizing photo-actuators. For example, a type of photo-actuated pen array made from CNT-PDMS composite realized massively parallel molecular dip-pen nanolithography. The pen would locally expand and print ink on the substrate when specific ones were exposed to light [23].

4. Piezoelectricity

Piezoelectric materials, by nature, are able to convert mechanical strain into electricity energy. This capability of energy transduction can be utilized in piezo-resistive strain sensors, nanogenerators, supercapacitors, and applications revolving around energy harvesting. Interestingly, inverse piezoelectric materials geometrically respond to voltages applied, which can be used to build piezo-actuators for soft robots [24][25].
Piezo-actuators are established on voltage-induced motion or the deformation of inverse piezoelectric materials. Advances have been made to improve their actuating performance. Merging the multi-walled carbon nanotube solution into polyvinylidene fluoride and distributing in the axial direction contribute to better bending capability, measured as bending to 24 μm in response to an electric field of 4 V/μm [26]. Types and quantities of materials can make a difference; for instance, adding a different amount of few-layer graphene into silicone rubber with different tensile modulus will change the conductivity and thus influence the actuating motion [27].
Due to the energy conversion allowed by the piezoelectric effect, piezoelectric batteries can also realize self-charging simply through bending or patting themselves. In nanogenerators, carbon-based materials are significant components of electrodes. Taking the one published in 2019 as an example, both the cathode material and anode material were carbon treated for higher electronic conductivity [28]. In addition to nanogenerators, carbon materials can also be used as flexible electrodes in supercapacitors, one of the energy harvesting applications. Graphene oxide-based electrodes were proved to have high rate capability in a supercapacitor proposed in 2020 [29]. Another flexible self-chargeable supercapacitor proposed in 2021 adopted electrodes fabricated by directly growing Co-Fe2O3 particles on activated carbon cloth. The assembled supercapacitor can charge itself with great durability of bending for 420 cycles [30].

This entry is adapted from the peer-reviewed paper 10.3390/nano13020316

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