Hygroscopic actuation is an important material function, which enables a broad range of applications such as self-healing devices, soft robotics, and catheter implantation. With the current paradigm of implantable devices shifting toward soft and tissue-mimicking systems, this function however, is particularly weak in soft- and bio-materials due to the rapid loss of intermolecular interactions upon water incorporation. Here, a chitosan-based bio-composite is developed, which sustains the intermolecular repulsive force during water absorption through synergistic effects of hydrogen bonding, plasticization, and nano-confinement. When interact with body fluids, this material provides a stable and strong tensile force throughout its volume expansion process. Therefore, it serves as a functional coating that self-flattens a thin film-based device which holds a tubular shape needed for catheter delivery, and then degrades naturally. This capability is further demonstrated in vivo using a rolled triboelectric nanogenerator (TENG) for intracardiac implantation. The TENG device recovers its original shape after being placed inside the heart left ventricle and restores its regular energy harvesting function, evidencing the feasibility for minimally invasive implantation of flexible film-based devices.