flexible organic or inorganic transistors. However, in many of the emerging applications, such as the Internet of Things and implantable electronic systems, in addition to the aforementioned basic building blocks, functional elements that include wireless RF electronic devices are also essential elements for wireless interconnection and data transmission. RF resonators, such as film bulk acoustic resonators (FBARs), which are traditionally used as the basic building blocks for modern RF filters [12] and oscillators, [13] are a natural candidate for wireless flexible electronics. FBAR-based electronic systems have also been extensively used in the biochemical sensing and actuating domains, [14] e.g., chemical vapors [15] and antibody-antigen reactions for immunosensors. [16] Although FBARs are small in size and light in weight, they have traditionally not been amenable to mechanical bending or stretching because of their rigid and brittle silicon substrate. Therefore, incorporating flexible electronic technology into FBARs is highly demanding and offers much more functionality for next-generation flexible electronic systems. However, since the fabrication process of these FBARs is intrinsically incompatible with the majority of plastic substrates, it is difficult to fabricate flexible FBARs with prominent electrical performance and mechanical flexibility. Previously, a flexible air-gap-type FBAR on a silicon substrate was demonstrated by thinning the silicon wafer to 50 µm. [17] By means of directly fabricating the resonator on polyimide, FBARs integrated on arbitrary substrates (polyimide, glass, and silicon) have also been achieved but with compromised electrical performance. [18] In our previous work, free-standing FBARs on polyethylene terephthalate substrates were demonstrated. [19] However, none of these devices is fully qualified for the rigorous requirements of satisfactory mechanical flexibility and uncompromised electrical performance.In this work, a highly bendable FBAR was introduced to meet these rigorous requirements by placing the FBAR at the mechanical neutral plane using two polyimide thin films. Furthermore, to achieve a high quality factor, the acoustic energy should be efficiently trapped in the material stack by a significant acoustic impedance difference between the FBAR and its surroundings. Therefore, air cavities were created both above

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