## 3 Results and Discussion
### Multilayer Membrane of Optical Responsive Deforming  
To enable the multilayer membrane to respond to a wider range of electromagnetic spectrum, graphite was selected as a light-absorbing material because of its broad-spectrum absorption properties. The SEM image of the multilayer membrane's cross-section in Figure 3a shows that in the layered structure, graphite particles are uniformly dispersed in the PDMS layer, which is helpful to transfer the photothermal more quickly and evenly. When the multilayer membrane was illuminated, the PDMS/Graphite layer absorbed light efficiently, and the volume of the PDMS/Graphite layer began to expand. Simultaneously, the NIR light will also made the ambient temperature around the membrane rise and caused the local humidity to decrease, which finally led to the shrink of the Nafion layer by losing water which enhances the bending tendency towards the Nafion layer as illustrated in Figure 3b. When the light disappeared, the graphite particles began to dissipate heat. The PDMS/Graphite layer  
Figure 2: Humidity-responsive behavior of the multilayer membrane (the test ambient temperature was 25 \({}^{\circ}\)C). (**a**) Schematic diagram of the humidity deformation response testing. (**b**) Schematic illustration of the reversible deformation behaviors when humidity changes. (**c**) Photo of water droplet CA for Nafion film and **(d**) PDMS/Graphite film. (**e**) The maximum bending angle of the multilayer membrane under different relative humidity. (**f**) The time-dependent bending angle of the multilayer membrane under 95% (blue area) and 30% humidity stimulation. began to return to its original state, and then the multilayer membrane returned to its original state quickly. At the same time, local humidity around the membrane increased rapidly to the level of environment humidity after the light off, and the Nafion layer began to absorb moisture and swelled, which enhanced the expanding tendency of the device towards the PDMS/Graphite side. For the purpose of investigating the effect of the light absorption efficiency on the deformation of the multilayer membrane under illumination, we changed the graphite concentrations (0%, 5%, 10%, 15%, 20%) in the PDMS layer, as shown in Figure 3c. Under the same NIR light intensity of 0.88 W-cm\({}^{-2}\), the sample with 0% graphite concentration needed the longest time to reach its peak bending angle 47.64\({}^{\circ}\) within 20 s, which means its photothermal conversion efficiency is the lowest. When the graphite concentrations increased, the maximum bending angle and response speed of the multilayer membrane also rose. When the graphite concentration was 15%, the multilayer membrane could quickly reach its maximum bending angle of 251.97\({}^{\circ}\). However, it is worth noting that when the graphite concentration rose to 20%, the stress of the multilayer membrane increased and the strain decreased, resulting in a decrease of multilayer membrane in responding speed and the maximum bending angle. Although the photothermal conversion efficiency of the multilayer membrane benefits from the increase of the graphite concentrations, the bending trend is limited by the flexibility of the device as well. Within a certain range, in order to obtain the best overall response, a multilayer membrane with a graphite concentration of 15% was selected for further use. Figure 3d shows the response process of the multilayer membrane with 15% graphite concentration under NIR light intensity of 0.88 W-cm\({}^{-2}\). Figure 3: Optical-responsive behavior of the multilayer membrane (the test ambient relative humidity was 30%). (**a**) SEM image of the cross-section of the Nafion-polyppropylene-PDMS/Graphite multilayer membrane. (**b**) Schematic illustration of the reversible deformation behaviors under illumination. (**c**) Bending angle of the actuator with different graphite particle concentrations under the NIR light intensity of 0.88 W-cm\({}^{-2}\).