The development of electronic interfaces for humans to quantitatively gain and store the information of things including hazardous gases, chemical and biological liquids and powders that are readily exposable to humans is of great importance for the emerging biomedical and health care applications based on Internet of Things technologies. The direct sensing, monitoring and storing of the information of liquids having different polarities are significant challenges, in particular, through means related to human senses such as vision and smell. In general, it is extremely difficult for human senses to distinguish two liquids that are mostly colourless and transparent, except through identification based on their unique smell, which is rarely recommendable owing to the possible toxicity of volatile liquids. Therefore, it is essential to develop a liquid sensing platform in which the information of a liquid can be stored in a non-volatile manner and extracted many times even after removal of the liquid from the platform.

We present an interactive sensing memory platform capable of sensing, monitoring, and storing the information of various liquids. Our platform utilises sound arising from liquid-interactive ferroelectric actuation, which is dependent upon the polarity of the liquid, allowing for direct sensing and storing of information of various liquids. Liquid-interactive ferroelectric sound (LIFS) is successfully developed when a liquid droplet is placed on a ferroelectric polymer layer across two in-plane electrodes underneath the ferroelectric layer under an external in-plane AC field, as shown in Figure 1. An AC field built up vertically between one of the electrodes and the liquid, varying with the polarity of the liquid, results in film actuation arising from the AC-field-dependent ferroelectric polarisation of the film. The sound pressure level (SPL) of a device, in turn, depends upon the polarity of the liquid, allowing for facile liquid sensing and identification. More importantly, as the SPL arising from LIFS of a liquid is correlated with the non-volatile ferroelectric remnant polarisation of the vibrating layer, the information of a liquid is readily stored and retrieved even after the liquid is removed, resulting in a sensing memory of the liquid. By employing a microfluidic channel on our platform, the flow of a human serum liquid through the channel is monitored and its velocity is obtained in terms of SPL.

We also demonstrate that LIFS is useful for identifying the 2-D position of a liquid droplet on a thin pad-type device with position-addressable LIFSs (Figure 2a). Furthermore, mechanically flexible tube-type LIFS AC devices allow for in situ sensing of a fluid passing through the tube in terms of SPL (Figure 2b). The utilization of sound, rather than electric signals, can also be beneficial because sound does not require physical contact of the detection components and can readily propagate in space. This can allow for wireless detection of the liquid information and thus make it potentially suitable potentially for on and in-body applications, where a detection microphone can be remotely placed.  

Looking into the future, we expect our mechanically flexible and microfluidic compatible sensing memory based on LIFS can provide great potentials for a variety of biomedical diagnosis, hazardous liquid detection such as toxic solvents and volatile organic compounds (VOCs), health monitor/care devices including biomarker-free detection sensors, microfluidic cell counting and sorting SPL devices, and non-volatile sound based touch pad for authentication.