The role of water molecules in piezoelectric-like effects in chitosan-based biodegradable films

Abstract

Soft, flexible piezoelectric polymers have gained increased attention for powering wearable, implantable, and autonomous Internet-of-Things devices. However, the state-of-the-art flexible piezoelectric polymers are fluoropolymers, facing global bans due to environmental concerns. This has led to the development of natural piezoelectric materials. However, achieving reliable piezoelectric performance is challenging due to the limited output of these materials and the complexity of accurately measuring piezoelectric signals, often complicated by other charge-generation mechanisms. Here, flexible and robust chitosan-based films are produced using a simple solvent-casting method. The results show that substrate type and solvent evaporation temperature affect the piezoelectric-like output, with the highest apparent d₃₃ of 1.9 ± 0.3 pC N⁻¹ observed for chitosan cast on a hydrophilic polystyrene substrate at an evaporation temperature of 40 °C. Importantly, the water content of the films plays a critical role in both mechanical and piezoelectric-like properties. The fully dried films under vacuum exhibit no d₃₃ signal, while fully hydrated films show a significantly enhanced response of 11.3 ± 7.0 pC N⁻¹. This work demonstrates the fabrication of biodegradable, piezoelectric-like films and provides key insights into measurement reliability and often-overlooked parameters, such as hydration state, that are crucial for advancing the development of piezoelectric biological materials.