**1. Introduction: functional biomimetic materials and devices**

Elegant and sophisticated structures and functionalities of nanoscale biological systems in nature have offered unique inspirations in the development of new concepts, diverse classes of nanomaterials, and various functional devices. The design and synthesis of bio-inspired materials with tailored properties for actuation, sensing, smart electronics, and highly efficient energy harvesting have enabled artificial devices to be endowed with bio-mimicking fea‐ tures, among which artificial muscle and electronic skin that can sense and respond to environmental changes by mimicking human ways have been widely considered as crucial‐ ly important for new-generation biomimetic devices. In this regard, newly emerged versa‐ tile and high-performance two-dimensional (2D) nanomaterials such as graphene and its derivatives have been explored and proven as promising candidates. In this chapter, we aim at highlighting the latest efforts toward design, synthesis, and applications of 2D nanomate‐ rial-based functional biomimetic systems.

#### **1.1. Key concepts of biomimetics**

The term "biomimetics" was *first* coined during the 1950s to describe a biological approach to the needs of engineering science. Lepora et al. have recently attempted to make a connected‐ ness of current popular terms in biomimetics (**Figure 1**) [1]. With such a connectedness, biomimetics now encompasses various disciplines of biomaterials that retain their strong connections to biomimetic. Biomimetic materials, or biomaterials, traditionally defined as materials used in medical devices, have been used since antiquity, but recently their degree of

**Figure 1.** Connectedness of popular terms in biomimetics [1].

sophistication has been enhanced significantly. Biomimetic materials made today are routinely information rich and incorporate biologically active components derived from nature, and have found use in a wide variety of nonmedical applications [2]. Advancements in materials science, manufacturing process, and continual miniaturization of components have enabled biomimetic materials to have the ability of sensing, actuation, communication, and even computation [3]. Engineers have employed such intelligent materials to fabricate precise, predictable robotic devices and systems, which learned from studying biological systems are now culminating in the definition of a new class of machines that researches refer to as soft robots [4], which connects strongly with biomimetics (**Figure 1**).

Similar to humans, such robots will, in addition to hard components such as bones, have soft bodies made of soft materials, and will be capable of soft movements and soft interactions with people (**Figure 2**). A recent trend in soft robotics is to simplify the typically computationally intensive, neutrally inspired control through smart morphological design and use of functional materials [5–7]. There is no doubt that soft biomimetic materials enable most of the automation of tasks beyond the capacities of current robotic technology. The full integration of biomimetic materials and devices into complete robotic systems is of significant interest in science and technologies, but is full of complex challenges.

**Figure 2.** An anthropomimetic humanoid robot ECCE (Embodied Cognition in a Compliantly Engineered Robot) [8].

### **1.2. Design and synthesis of 2D materials**

**1. Introduction: functional biomimetic materials and devices**

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

rial-based functional biomimetic systems.

**Figure 1.** Connectedness of popular terms in biomimetics [1].

**1.1. Key concepts of biomimetics**

Elegant and sophisticated structures and functionalities of nanoscale biological systems in nature have offered unique inspirations in the development of new concepts, diverse classes of nanomaterials, and various functional devices. The design and synthesis of bio-inspired materials with tailored properties for actuation, sensing, smart electronics, and highly efficient energy harvesting have enabled artificial devices to be endowed with bio-mimicking fea‐ tures, among which artificial muscle and electronic skin that can sense and respond to environmental changes by mimicking human ways have been widely considered as crucial‐ ly important for new-generation biomimetic devices. In this regard, newly emerged versa‐ tile and high-performance two-dimensional (2D) nanomaterials such as graphene and its derivatives have been explored and proven as promising candidates. In this chapter, we aim at highlighting the latest efforts toward design, synthesis, and applications of 2D nanomate‐

The term "biomimetics" was *first* coined during the 1950s to describe a biological approach to the needs of engineering science. Lepora et al. have recently attempted to make a connected‐ ness of current popular terms in biomimetics (**Figure 1**) [1]. With such a connectedness, biomimetics now encompasses various disciplines of biomaterials that retain their strong connections to biomimetic. Biomimetic materials, or biomaterials, traditionally defined as materials used in medical devices, have been used since antiquity, but recently their degree of

> In a biomimetic material, biomimetic behaviors refer to changing the material properties of the underlying base material to actuating, sensing, and communicating. Some possible mechanisms involve changes of stiffness, volume and shape, electronic properties, or color

(**Figure 3**). For example, inorganic nanowires such as single-walled carbon nanotube (SWCNT), ZnO, Cu, In2O3, and etc. offer new material basis and opportunities for flexible electronics that enables many biomimetic applications, including sensors, display devices, and logic gates [9]. All-dielectric meta-materials that can respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, have promising potential to be used in fabrication of biomimetic meta-surfaces [10]. Ionic polymer-metal composites show large deformation in the presence of low applied voltage and therefore have been widely used as highly active actuators and sensors [11]. Shape-memory polymers are an important class of stimuli-responsive soft materials for which shape-shifting behavior can be programmed, enabling the application as artificial muscles [12]. Overall, from inorganic to organic, from nanoscale to macroscale, various range of materials can be fabricated into designable biomi‐ metic devices including electronic skins [13, 14], artificial muscles [15, 16], etc.

**Figure 3.** Examples of a biomimetic material working toward changing the crucial properties of the base material. Changes in (a) stiffness and (b) shape/volume could enable shape-changing actuation materials. Robotic skins could utilize changes in (c) electronic responses and (d) appearance to sense and communicate.

Particularly, due to their inherent flexibility and novel properties, 2D-layered nanomaterials as the *in situ* unit of biomimetic materials hold great promise in flexible and versatile biomi‐ metic devices. Here we summarized the reported alternative 2D materials, and the description has been organized in an order according to their synthesis methods.

In addition to graphene, micromechanical exfoliation has been extended to prepare other 2D inorganic materials. In fact, following this approach, individual crystal sheets from a variety of layered materials have been isolated. Monolayers of BN, MoS2, NbSe2, and Bi2Sr2CaCu2Ox have been prepared by rubbing a layered crystal against a substrate and leaving random flakes on it [17]. Among the resulting flakes, single layers were always found. This method leads to high crystal quality and macroscopic continuity, and is considered as one of the easiest and the fastest ways, as in the case of well-known graphene. However, a serious drawback is noticed: monolayers obtained by micromechanical exfoliation are in a great minority among accompanying thicker flakes. Despite allowing the first example of characterization of oneatom-thick monolayers, this is not a feasible procedure for large-scale production of 2D materials for technological applications. Therefore, in the last few years, new methods have been developed to approach scalable synthesis of 2D inorganic materials.

(**Figure 3**). For example, inorganic nanowires such as single-walled carbon nanotube (SWCNT), ZnO, Cu, In2O3, and etc. offer new material basis and opportunities for flexible electronics that enables many biomimetic applications, including sensors, display devices, and logic gates [9]. All-dielectric meta-materials that can respond to both the electric and magnetic fields of light, support large optical chirality and anisotropy, have promising potential to be used in fabrication of biomimetic meta-surfaces [10]. Ionic polymer-metal composites show large deformation in the presence of low applied voltage and therefore have been widely used as highly active actuators and sensors [11]. Shape-memory polymers are an important class of stimuli-responsive soft materials for which shape-shifting behavior can be programmed, enabling the application as artificial muscles [12]. Overall, from inorganic to organic, from nanoscale to macroscale, various range of materials can be fabricated into designable biomi‐

**Figure 3.** Examples of a biomimetic material working toward changing the crucial properties of the base material. Changes in (a) stiffness and (b) shape/volume could enable shape-changing actuation materials. Robotic skins could

Particularly, due to their inherent flexibility and novel properties, 2D-layered nanomaterials as the *in situ* unit of biomimetic materials hold great promise in flexible and versatile biomi‐ metic devices. Here we summarized the reported alternative 2D materials, and the description

utilize changes in (c) electronic responses and (d) appearance to sense and communicate.

has been organized in an order according to their synthesis methods.

metic devices including electronic skins [13, 14], artificial muscles [15, 16], etc.

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

Vapor deposition techniques have been most extensively explored due to their potential for high scalability and morphological control. By balancing the production cost and the above prerequisites, chemical vapor deposition (CVD) is the most promising route to produce largearea device-grade graphene. Normally, the procedure involves two steps: *first*, pyrolysis of a precursor to form carbon and disassociation of carbon atoms, and then the formation of the graphene. The pyrolysis to disassociate carbon atoms must be carried out on selected substrates to prevent the precipitation of carbon clusters during the gas phase. This leads to a problem and metal catalysts must be used to reduce the reaction temperature required for pyrolytic decomposition of precursors. During the reaction, the metal substrate not only works as a catalyst to lower the energy barrier of the reaction, but also determines the graphene deposition mechanism, which ultimately affects the quality of graphene [18]. Graphene growth has been demonstrated on a variety of transition metals such Ni, Pd, Ru, Ir, or Cu, and is also achievable on insulating SiC [19]. In a similar process, metal containing precursors [e.g., MoO3, WO3, or (NH4)2MoS4] are vaporized and reacted with chalcogen elements through vapor-solid reac‐ tions, leading to the growth of 2D materials beyond graphene on a substrate downstream [20]. Alternatively, 2D material bulk powders can also be used as the precursor directly [21]. For example, Feng et al. [22] employed a tree-zone furnace to synthesize large-area 2D MoS2(1−*x*) Se2*x* semiconductor alloys.

Liquid-phase exfoliation is Low-cost and scalable method which has been widely used for preparing individual sheets of 2D materials. Typically, this method requires homogeneous dispersion of 2D materials in diverse solvents or aqueous solutions. With the assistance of sonication, the weak van der Waals bonds between the layers are broken and individual layers are obtained. Shen et al. [23] have reported an effective strategy to exfoliate 2D materials in a high yield. In addition to the total surface tension, efficient solvents for liquid-phase exfoliation were found to be those which have a similar ratio of polar components to dispersive compo‐ nents of surface tension to the 2D materials. Mono- to few-layer graphene, WS2, MoS2 h-BN, MoSe2, Bi2Se3, TaS2, and SnS2 were prepared with low-toxic and low-boiling point solvents, such as 1:1 IPA/water for graphene, WS2, h-BN, and MoSe2; 1:4 IPA/water for Bi2Se3, and SnS2; 7:3 IPA/water for MoS2, acetonitrile for TaS2.
