**2. 2D materials enable sensing, actuation, and communication**

Smart advanced materials that are flexible, adaptable, multifunctional, and meanwhile "green" are essential for biomimetic approaches. Since it was *first* isolated in 2004 [24], graphene has attracted tremendous attention because of its extraordinary electrical, thermal, and mechanical properties [25–27]. It is not surprising that graphene leads to 2D material research and has developed *rapidly* to several important applications such as energy technol‐ ogies [28, 29], electronics [30, 31], and biomedicine [32, 33]. Among these applications, graphene-based intelligent devices that can spontaneously detect and respond to external stimuli are of broad practical interest and importance. Inspired by the great success achieved through the research of graphene-based materials, the similar ideas and methodologies have also been extended to study other layered materials. This is well indicated by the number of new 2D inorganic nanomaterials blossomed in the past few years. In this section, we offer a brief overview of the successes reported on the biomimetic performance of 2D materials.

**Figure 4.** Experimental setup for measurements performed using the graphene CO2 gas sensor [35].

#### **2.1. Sensing**

2D materials are usually good candidates for gas sensors due to their large surface-to-volume ratio and the associated charge transfer between gas molecules and the substrates [34]. Graphene has very high electron mobility at room temperature, and hence, its gas sensitivity

is very high. Yoon et al. [35] fabricated a graphene-based CO2 gas sensor by mechanical cleavage and micromachining. The graphene sensor shows significant conductance changes when exposed to various concentrations of CO2 in air. The response time of the sensor is less than 10 s. The overall system is illustrated in **Figure 4**. They have shown the principle idea, but the sensing systems might need to be upgraded to wearable devices in order to meet the fashion applications. Late et al. [36] reported a comprehensive suite of sensing behavior of atomically thin-layered MoS2 structures in a transistor-like configuration. MoS2-based field emission transistor (FET) showed outstanding sensitive response to NH3, NO2, as well as water vapor at room temperature and atmospheric pressure. 2D material-based FET showed improved mobility and wearable capability, due to its nanoscale size and facile and precise testing systems. Kou et al. [34] reported first-principles calculations that examine the adsorp‐ tion of several typical molecules, CO, CO2, NH3, NO, and NO2 on phosphorene. They deter‐ mined their preferential binding positions and the corresponding binding energy. Their results show that the strength of binding is highly dependent on the extent of charge transfer between the adsorbed molecules and the phosphorene layer, which is similar to that observed in graphene and MoS2. However, the adsorption of gas molecules on phosphorene is notably stronger resulting in a more pronounced effect on the sensitivity.

**2. 2D materials enable sensing, actuation, and communication**

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

**Figure 4.** Experimental setup for measurements performed using the graphene CO2 gas sensor [35].

2D materials are usually good candidates for gas sensors due to their large surface-to-volume ratio and the associated charge transfer between gas molecules and the substrates [34]. Graphene has very high electron mobility at room temperature, and hence, its gas sensitivity

**2.1. Sensing**

Smart advanced materials that are flexible, adaptable, multifunctional, and meanwhile "green" are essential for biomimetic approaches. Since it was *first* isolated in 2004 [24], graphene has attracted tremendous attention because of its extraordinary electrical, thermal, and mechanical properties [25–27]. It is not surprising that graphene leads to 2D material research and has developed *rapidly* to several important applications such as energy technol‐ ogies [28, 29], electronics [30, 31], and biomedicine [32, 33]. Among these applications, graphene-based intelligent devices that can spontaneously detect and respond to external stimuli are of broad practical interest and importance. Inspired by the great success achieved through the research of graphene-based materials, the similar ideas and methodologies have also been extended to study other layered materials. This is well indicated by the number of new 2D inorganic nanomaterials blossomed in the past few years. In this section, we offer a brief overview of the successes reported on the biomimetic performance of 2D materials.

**Figure 5.** MoS2-based FET biosensors, which provides high sensitivity and at the same time offers possibility for facile patterning and device fabrication [38].

Biosensors based on 2D material FETs have also attracted much attention, as they offer rapid, inexpensive, and label-free detection of biologically related signals. Development of 2D material FETs may bridge the technological gap between signal transduction, conditioning, processing, and wireless transmission in a wearable biosensing device, by merging plasticbased sensors that interface the skin with silicon-integrated circuits on a flexible circuit board for complex signal processing [37]. Sarkar et al. [38] demonstrated a FET biosensor based on MoS2. This sensor shows ultrahigh sensitivity (713 for a pH change of 1 unit) and wide operation range (pH of 3–9) (**Figure 5**). It also demonstrates specific detection of protein as well as an extremely high sensitivity of 196 even at 100 femtomolar concentration.

**Figure 6.** Schematic of device structure and fundamental characteristics of the all-elastomeric transparent stretchable gated sensor [42].

**Figure 7.** (a) Optical image of the single-atomic layer MoS2 flake. Blue and yellow spheres represent Mo and S atoms, respectively.(b) Polar plot of the second harmonic intensity from single-layer MoS2 as a function of the crystal's azimu‐ thal angle θ.(c) A typical flexible device with single-layer MoS2 flake and electrodes at its zigzag edges.(d) Operation scheme of the single-layer MoS2 piezoelectric device [43].

In addition to the examples described above, sensing platforms equipped or integrated with temperature, strain, and humidity sensors have attracted more and more attention due to their natural skin-like biomimetic sensing behavior [39–41]. Trung et al. [42] developed a very simple fabrication process to realize the all-elastomeric temperature sensor array. They integrated a strain sensor on a platform which can be attached as a patch to objects or human skin (**Figure 6**). Reduced graphene oxide (rGO) nanosheets embedded in a elastomeric polyurethane matrix were used as the temperature sensing layer. Notably, most function layers of the device are intrinsically transparent and stretchable.

Wu et al. [43] reported an experimental observation of piezoelectricity in single-atomic-layer 2D MoS2 and explored its application in mechanical energy harvesting and piezotronic sensing (**Figure 7**). Cyclic stretching and releasing of odd-layer MoS2 flakes produced oscillating electrical outputs, which could convert mechanical energy into electricity. The strain-induced polarization charges in single-layer MoS2 can also modulate charge carrier transport at the MoS2–metal barrier and enables enhanced strain sensing. This study has demonstrated the potential of 2D nanomaterials for powering nanodevices, adaptive bioprobes, and tunable/ stretchable electronics/optoelectronics.

In short, these sensors have shown great potential for their adaptability to wearable skin electronics for recognition of human activity and environmental changes.
