**4. Outlook and perspective**

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

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.** (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

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

N/kg force output), and controllable mechanical

with reversible, fast (~0.3 s), powerful (7.5 × 105

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

and turning on a map driven by light irradiation [69].

(**Figure 14**) [69].

Over the past 5 years, advances in 2D material fabrication, 3D assembly, and biological analysis have accelerated development of soft materials in biomimetic applications. In the future, integration of other sensor components can be predicted using a similar platform to enable more sophisticated human-surface interfacing. Additional circuits, sensors (e.g., strain gauges, thermal flux sensors), and actuators (LEDs, pacing/ablation electrodes) have been designed with these design considerations in order to enable conformal integration with soft and curvilinear organs (**Figure 15**) [39, 79]. We believe that these 2D material-based biomimetic platforms would find a wide range of applications in automotive control panels, interactive input devices, robotics, and medical and health monitoring devices.

**Figure 15.** Bio-integrated flexible and stretchable systems: schematic illustration of bio-integrated electronics in devel‐ opment today across a broad range of biomedical applications. Minimally invasive and implantable devices include electrophysiological sensors (ECoG, ECG), angioplasty tools, prosthetic eye/skin, and optoelectronic nerve stimulator, etc. Wearable bioelectronics include physiological sensors (pressure, strain, temperature sensors) integrated with trans‐ dermal drug delivery devices and data storage devices. Continuous monitoring and real-time feedback therapy are performed in conjunction with the wireless communication. Energy supply module is an essential component to bioe‐ lectronics systems for mobile and personalized healthcare [34, 38, 73, 76, 77].
