## 1 Introduction  
Flapping-wing micro aerial vehicles (FWMAVs) have many unique advantages compared to fixed-wing and rotary-wing MAVs, and one of the most important of these is having the integrated functions of hovering, lifting, and propulsion at a low Reynolds number environment with less energy for a long-time flight [1, 2, 3]. As result, FWMAVs have attracted a lot of attention from research institutions and companies due to their brilliant application prospects in defense and civilian fields [1, 2]. Lots of flapping wing vehicles have been designed, characterized and modeled inspired by bats, insects and birds [4, 5, 6, 7, 8, 9]. The flying endurance of these flapping wing vehicles has been continuously growing from 8 s to several minutes, but the traditional way that improves the battery performance in its capacity and energy density to increase aircrafts' flying time is almost reaching a bottleneck. The flying performance of a flapping wing aircraft is affected by every component and every parameter. Adjustment of the battery may not bring obvious improvement in flying endurance, and may even impede the development of flapping wing vehicle performance because of the weight limit for the energy systems, that cannot be incorporated into the aircraft very well [10, 11]. There have been many attempts to extend the flight time of unmanned aerial vehicles (UAVs) by the way of collecting energy, including solar and vibration energy [12, 13, 14, 15]. With attached solar cells on the wing surface of an aircraft, the operational time obtained a maximum 18.7% increase. Normally, as the solar panels on aircraft wing surface increased, the energy would be more collected with the solar cell wing [16; 17; 18], which meant the potential of photovoltaic devices to harvest energy depends on the size of wing. Additionally, the illumination intensity would also have a large effect on energy collection and the solar panels could only harvest energy when exposed to light. For flapping wing micro air vehicles, solar panels were not appropriate for attachment on the flapping wing due to the aerodynamic requirement for the wing flexibility. Not to mention the rigid, heavy traditional solar panels, the thin film solar cell technology also had adversely effect on wing kinematics when applied on the wing surface [19; 20]. However, the volumetric energy harvested by the piezoelectric patches normally was orders of magnitude less than that of the solar panels. In summary, piezoelectric technology harvesting on FWMAV had more advantages than photovoltaic technology. Energy harvesting with piezoelectric materials has received more and more attentions in research communities throughout the past decade [21; 22; 23; 24; 25; 26]. Many studies have investigated the feasibility of polyvinylidene fluoride (PVDF) material for microelectronics applications, such as the PVDF glove, the self-powered insole for human motion recognition, as well as the piezoelectric wearable sensors for body motion energy harvesting [21; 22; 23; 24]. However, there are seldom relevant studies about the application of piezoelectric material on flapping-wing MAV. Researchers from Tamkang University proposed that a PVDF wing could modify the aerodynamic forces of a micro aerial vehicle. By comparing two kinds of data obtained in the experiment, there was a phase delay between the signal and load-cell signal. The research demonstrated that the PVDF wing has the promising capability to monitor aerodynamic information of flapping [25; 26]. In this paper, we aimed to investigate further applications of PVDF material for the FWMAVs. The feasibility of energy harvesting and wing deformation sensing without increasing extra load on aircrafts was developed for FWMAVs with the new type PVDF piezoelectric wing which could generate the electric potential energy caused by the wing deformation due to the characteristics of the PVDF material.