## 1 Introduction  
In recent years, the demand for smart actuators based on flexible intelligent driving materials has increased [1; 2; 3; 4]. The role of flexible intelligent driving materials is to convert external stimuli (such as light, heat, humidity, magnetic field/electric field) into executable actions or signals needed by processors through rapid, reversible and controllable structural/morphological changes [5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16]. Flexible smart actuators driven by these stimuli have significant advantages in the soft robot, electronic skin, bionic technology and other fields [17; 18; 19; 20]. For example, inspired by the flicking finger motions for rapidly releasing elastic energy, a curled carbon nanotube (CNT)-polydimethylsiloxane (PDMS) bilayer composite film actuator was designed and fabricated to simulate gymnastics tumbling in the air under low voltage and light illumination [21]. The flexible material encapsulates the micro-fabricated gold skeleton to replicate the fish's morphology, driving the fish's swimming and steering with optical stimulation [22]. Reversible deformed graphene monolayer paper combined with polyvinylidene fluoride (PVDF) film can stretch the PVDF film under light stimulation and produce open-circuit voltage up to 4 V [23]. However, most flexible actuators have restrictions such as single stimulation,slow response and recovery. It is desirable to develop a kind of actuator with advantages such as multiple stimulus-driven, fast-response and large-scale deformation for the multifunctional devices. The flexible polymer Nafion has high ionic conductivity, and is commonly used to make ion exchanged membranes for chlorine gas and alkali production, as well as for proton exchanged membranes in fuel cell polymer electrodes [24; 25]. However, the sulfonic acid groups in Nafion are highly hydrating, absorb water and swell very quickly and efficiently, and the intrinsic connection of sulfonic acid groups allows water to be removed quickly. Hence, the Nafion film has emerged as an interesting candidate component for an actuator which exhibits fast response and large-scale deformation under varying humidity [26]. PDMS, a transparent non-toxic organic polymer material, has a high thermal expansion coefficient and excellent biocompatibility and hydrophobicity, good chemical stability and commercial availability; these characteristics make it often and preferrably used in flexible actuators, sensors, biomechanical systems, microfluidic systems, etc. [27; 28; 29; 30; 31]. Also, traditional carbon-based material graphite has become a popular actuator component due to its excellent optical, thermal, electrical and mechanical properties. It is able to convert light into heat effectively because it can strongly absorb a wide range of light from ultraviolet to near infrared (NIR) wavelengths [32; 33; 34; 35]. Thus, a new type soft actuator composed of Nafion-polyppropylene-PDMS/Graphite is proposed in this paper. The actuator has an excellent reversible response to humidity, optical and thermal stimulation, as shown in Figure 1a. The humidity-driven actuation is due to the hygroscopic property of the Nafion layer and the non-hygroscopic property of PDMS/Graphite. Once the relative humidity increased, the absorption of water molecules resulted in the swelling of Nafion, while PDMS/Graphite layer remained substantially, so the actuator bent towards the PDMS/Graphite side [36; 37]. The light-driven and thermal-driven actuations are a utilization of difference of thermal expansion coefficient between PDMS/Graphite and Nafion layer. Also, graphite is able to convert photonic energy into thermal energy, which can speed up the volume expansion of PDMS/Graphite [38; 39; 40; 41]. Meanwhile, the increase of temperature also leads to water desorption and shrinking of the Nafion layer.