**3. Functional biomimetic devices based on 2D materials**

As described above, the development of integrated intelligent devices is essential to the realization of biomimetic systems. In this section, we discuss the two most promising classes of biomimetic devices: e-skins and artificial muscles based on 2D materials.

#### **3.1. e-Skins**

The skin, as the largest organ in the human body, is mechanically self-healing and can provide a variable degree of touch sensitivity. Mimicking the functions of natural skins is therefore widely accepted to be very important in the future for robots used by humans in daily life for numerous applications. Thus, the development of an artificial skin, also known as electronic skin (e-skin) that is flexible and stretchable [51–54], sensitive enough to perceive touch [55– 57], and yet able to heal itself following damage [58, 59] is in high demand in robotic applica‐ tions.

Design, Assembly, and Fabrication of Two-Dimensional Nanomaterials into Functional Biomimetic Device Systems http://dx.doi.org/10.5772/64127 259

**Figure 11.** Real-time current curves of the sensor pad during a finger touch/remove cycle on its surface [60].

**Figure 10.** (a) Cross-sectional schematic showing one pixel of the interactive e-skin device, consisting of various com‐ ponents.(b) Schematic of the e-skin circuit matrix.(c) Photograph of an integrated device showing that light is locally emitted when the device surface is touched. (d) PDMS slabs with C, A, and L shapes are prepared and used to apply pressure onto the sensor array.(e) Green, blue, and red color interactive e-skins are used to spatially map and display

As described above, the development of integrated intelligent devices is essential to the realization of biomimetic systems. In this section, we discuss the two most promising classes

The skin, as the largest organ in the human body, is mechanically self-healing and can provide a variable degree of touch sensitivity. Mimicking the functions of natural skins is therefore widely accepted to be very important in the future for robots used by humans in daily life for numerous applications. Thus, the development of an artificial skin, also known as electronic skin (e-skin) that is flexible and stretchable [51–54], sensitive enough to perceive touch [55– 57], and yet able to heal itself following damage [58, 59] is in high demand in robotic applica‐

**3. Functional biomimetic devices based on 2D materials**

258 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

of biomimetic devices: e-skins and artificial muscles based on 2D materials.

the pressure [52].

**3.1. e-Skins**

tions.

**Figure 12.** (a) Schematic of the hybrid e-skin and its pressure sensitivity.(b–e) Cross-sectional FESEM images of the hybrid e-skin showing microscopic structures and the boundary between functional layers. Scale bars are 30 μm, 5 μm, 3 μm, and 500 nm, respectively.(f)Photographs of the hybrid e-skin. Scale bar is 2 cm [59].

Hou et al. [60] designed and fabricated a novel reduced graphene oxide foam (rGOF) that is free-standing, flexible, and elastic. As shown in **Figure 11**, the rGOF shows temperature sensitivity based on thermoelectric effects in the graphene with assistance of its good electrical properties. The rGOF can respond rapidly to finger pressure owing to the finger heating effects. As a proof of concept, the authors also produced rGOF pressure sensor pad that can locate finger pressure points and measure the pressure level. Most importantly, all sensing abilities of this device can operate by itself without the need of any additional power supply.

Hou et al. [59] also presented the first self-healing, mechanically strong and stretchable, selfactivated pressure-sensing device based on rGO. The device is composed of various functional components including piezoelectric and electrically conductive layers as well as a healing substrate **Figure 12**. Poly(N,N-dimethylacrylamide), poly(vinyl alcohol), rGO, and polyviny‐ lidene difluoride are employed in this hybrid device. It is suggested that preparing flexible and porous hybrid foams with interconnected 3D networks is a practical way to fabricate stretchable and self-healing thin film e-skin.

In order to improve the texture resolution of e-skin, FET technology is usually involved in the design and fabrication of sensors. Mannsfeld et al. [51] reported an organic thin film pressure sensor, which employs a key organic FET structure consisting of a thin, regularly structured rubber. The dielectric capacitance in organic FET devices directly depends on the output current, which enables the sensing of an applied pressure. This device provides high sensitivity in both medium- and low-pressure regimes. Besides, unprecedentedly fast response and relaxation times (≪1 s) are also reported.

Sun et al. [61] reported a piezopotential-powered active matrix strain sensor arrays which combined coplanar gate graphene transistors and piezoelectric nanogenerators. The strain sensor demonstrated excellent performances including a high sensitivity (gauge factor = 389) and a minimum detectable strain as low as 0.008%. Excellent device durability was also observed after 3000 bending-releasing cycles. This transparent and conformal strain sensor could be mounted onto a human hand for continuous monitoring of hand movements.

In addition to graphene, other 2D nanomaterials, such as MoS2, WS2, and vanadium disulfide (VS2), have also been recently explored for effective conversion of environmental stimuli into electric signals.

For example, The VS2 nanosheets with a quasi-two-dimensional electronic structure are very promising building block material for high moisture responsiveness. Intriguingly, the structural characteristics and calculation results indeed revealed theoretical feasibility to achieve VS2 material in ultrathin structures with only a few atomic layers. Feng et al. [62] synthesized ultrathin VS2 nanosheets and assembled them into a highly cooriented structure, which had a maximum resistance response of almost two orders of magnitude toward moisture. Using VS2 nanosheets as the sole functional material, a new concept, flexible touchless positioning interface based on the spatial mapping of moisture distribution was demonstrated (**Figure 13**). This moisture-based positioning interface provides a new concep‐ tual approach to practical real-time moisture mapping matrix or noncontact control interfaces for advanced man-made interactive systems.

Design, Assembly, and Fabrication of Two-Dimensional Nanomaterials into Functional Biomimetic Device Systems http://dx.doi.org/10.5772/64127 261

**Figure 13.** Flexible touchless positioning interface based on the spatial mapping of moisture distribution [62].

#### **3.2. Artificial muscles**

Hou et al. [60] designed and fabricated a novel reduced graphene oxide foam (rGOF) that is free-standing, flexible, and elastic. As shown in **Figure 11**, the rGOF shows temperature sensitivity based on thermoelectric effects in the graphene with assistance of its good electrical properties. The rGOF can respond rapidly to finger pressure owing to the finger heating effects. As a proof of concept, the authors also produced rGOF pressure sensor pad that can locate finger pressure points and measure the pressure level. Most importantly, all sensing abilities

Hou et al. [59] also presented the first self-healing, mechanically strong and stretchable, selfactivated pressure-sensing device based on rGO. The device is composed of various functional components including piezoelectric and electrically conductive layers as well as a healing substrate **Figure 12**. Poly(N,N-dimethylacrylamide), poly(vinyl alcohol), rGO, and polyviny‐ lidene difluoride are employed in this hybrid device. It is suggested that preparing flexible and porous hybrid foams with interconnected 3D networks is a practical way to fabricate

In order to improve the texture resolution of e-skin, FET technology is usually involved in the design and fabrication of sensors. Mannsfeld et al. [51] reported an organic thin film pressure sensor, which employs a key organic FET structure consisting of a thin, regularly structured rubber. The dielectric capacitance in organic FET devices directly depends on the output current, which enables the sensing of an applied pressure. This device provides high sensitivity in both medium- and low-pressure regimes. Besides, unprecedentedly fast response and

Sun et al. [61] reported a piezopotential-powered active matrix strain sensor arrays which combined coplanar gate graphene transistors and piezoelectric nanogenerators. The strain sensor demonstrated excellent performances including a high sensitivity (gauge factor = 389) and a minimum detectable strain as low as 0.008%. Excellent device durability was also observed after 3000 bending-releasing cycles. This transparent and conformal strain sensor could be mounted onto a human hand for continuous monitoring of hand movements.

In addition to graphene, other 2D nanomaterials, such as MoS2, WS2, and vanadium disulfide (VS2), have also been recently explored for effective conversion of environmental stimuli into

For example, The VS2 nanosheets with a quasi-two-dimensional electronic structure are very promising building block material for high moisture responsiveness. Intriguingly, the structural characteristics and calculation results indeed revealed theoretical feasibility to achieve VS2 material in ultrathin structures with only a few atomic layers. Feng et al. [62] synthesized ultrathin VS2 nanosheets and assembled them into a highly cooriented structure, which had a maximum resistance response of almost two orders of magnitude toward moisture. Using VS2 nanosheets as the sole functional material, a new concept, flexible touchless positioning interface based on the spatial mapping of moisture distribution was demonstrated (**Figure 13**). This moisture-based positioning interface provides a new concep‐ tual approach to practical real-time moisture mapping matrix or noncontact control interfaces

of this device can operate by itself without the need of any additional power supply.

260 Two-dimensional Materials - Synthesis, Characterization and Potential Applications

stretchable and self-healing thin film e-skin.

relaxation times (≪1 s) are also reported.

for advanced man-made interactive systems.

electric signals.

Artificial muscles or biomimetic actuators are a type of devices that can operate on a certain source of energy, such as electric current, pressure and chemical energy, and transform the energy into motion. To build a self-folding structure, active materials that convert other forms of energy into mechanical work to enable folding and unfolding operations are required. Previous research on active materials has mainly focused on polymers, including gels [63], liquid crystalline polymers [64], shape memory polymers (SMPs) [65], and conjugated polymers [66]. These materials can respond to environmental stimuli including pH, tempera‐ ture, solvent, humidity, electricity, and light to change their shapes or/and other physical properties.

Although polymer-based actuators have value in certain context, they still face a number of practical challenges. For example, the actuation behavior of shape memory polymers is restricted by the number of temporary shapes that can be memorized in each cycle, and the ability to tune the transition temperature for shape changes. A disadvantage of another example, polymer multilayers, is their poor stability, because the multiple components do not expand/shrink uniformly which can cause interface problems. Some other demos have typically employed a circuit connection, but they are not favored for remote control applica‐ tions.

In order to address the above-mentioned issues, Mu et al. [67] developed a series of graphene monolayer (GM) papers with a gradient-reduced graphene oxide/graphene oxide structure. In the gradient GM paper, the GO region could readily desorb/adsorb water molecules in response to environment stimuli including changes in humidity, temperature, or light, leading to shrinking/swelling of the GO sheets. On the contrary, the rGO is inert to water molecules. Considering the excellent photothermal properties of rGO and GO [68], as well as its high flexibility and mechanical robustness, a rGO/GO hybrid paper holds great potential for photothermoresponsive actuator applications. Mu et al. exploited these properties to yield GM paper with reversible, fast (~0.3 s), powerful (7.5 × 105 N/kg force output), and controllable mechanical deformation and recovery, in response to moisture, heat, and light. The response of this waterdriven actuator to multiple stimuli allows the artificial muscles and electric generators to be fabricated. Furthermore, it was found that with a programmed dual-gradient (vertical and lateral) structure, a self-folding all-graphene origami was also developed to demonstrate three types of capability: (i) producing predesigned shapes, (ii) walking, and (iii) turning a corner (**Figure 14**) [69].

**Figure 14.** (a) I. Illustration of the mask-assisted filtration process, scale bar is 2 cm; II. Cross-sectional SEM images of different regions of the actuator, scale bar is 1 μm; III. CA measurement of the opposite surfaces of the actuator.(b) Schematic representations of the structures and mechanisms of the actuator paper.(c) Time profiles of self-folding movements of a cross-shaped piece of paper with and without NIR light irradiation.(d) Optical images showing artifi‐ cial/robotic hand holding an object driven by light irradiation.(e) Optical images showing the "micro robot" walking and turning on a map driven by light irradiation [69].

The hydration-triggered actuation of GO materials opens up a new possibility to synthesize graphene-based actuators responsive to changes in environmental water and/or relative humidity. Cheng et al. [70] designed and fabricated the region-asymmetric graphene/graphene oxide (G/GO) fiber actuators in virtue of the laser positioning reduction of the freshly spun GO fibers. The graphene-based fiber actuators display complex and well-controlled motion and deformation in a predetermined manner in response to moisture changes. This work offers a strategy for producing region-asymmetric G/GO fibers which can be deformed deliberately and can walk as a single-fiber robot along a channel. The fiber-based actuator with such a unique structure provides a new platform for the development of the wearable devices such as smart textiles.

Differently, the mechanisms based on ionic liquid (IL) electrolyte immersion have also been reported for graphene-based actuators. Lu et al. [71] showed that IL could be inserted to separate the layers of paralleled graphene nanosheets. The rGO–IL with 66.7 wt% of IL displayed a 98% variation in the thickness upon a 2 V electrical voltage stimulation.

Alternatively, graphene can be used as electrodes [73], fillers [74, 75], and conductive substrates [72] for actuators rather than being as the solo functional material. These have been well reviewed elsewhere. We next focus on other 2D materials themselves in biomimetic actuation applications. However, the successful examples are still very few up to date.

Yang et al. [76] tailored magnetic, optical, and electrical properties of ReSe2 nanosheets by local strain engineering through formation of ReSe2 wrinkles on elastomeric substrates. Local strain induced by generation of wrinkles could perform several actions: (1) to modulate the optical gap as evidenced by red-shifted photoluminescence peaks, (2) to enhance light emission, (3) to induce magnetism, and (4) to adjust electrical properties. Their work not only shows how to create materials with vastly different properties at the nanoscale, but also enables a wide range of applications based on 2D materials, including strain sensors, stretchable electrodes, flexible field-effect transistors, artificial-muscle actuators, solar cells, and other spintronic, electromechanical, piezoelectric, photonic devices.

Yang et al. [77] analyzed electromechanical coupling effects in suspended doubly clamped single-layer MoS2 structures, by which they designed suspended-channel FETs and vibratingchannel nanoelectromechanical resonators. In direct current gating scenario, signal transduc‐ tion processes (such as deflection, electrostatic actuation, mobility, straining on bandgap, carrier density, and their intricate cross-interactions) have been analyzed with considering the strain-enhanced mobility (by up to 4 times), in order to determine the transfer characteristics.

Yuan et al. [78] reported synthesis of monolayer perovskite-like KCa2Nb3O10 nanosheets, and therefore they were able to study the size-dependent properties of these nanosheets as monolayer nanosheet seed layers to grow functional thin films for piezo-microelectromechan‐ ical systems (piezo-MEMS). Their results implied that larger Ca2Nb3O10 nanosheets can be useful for constructing scale-up piezo-MEMS devices, such as microactuators.
