Piezoelectric materials can directly transduce electrical and mechanical energy, making them attractive for applications such as sensors, actuators and energy harvesting devices. While often associated with ceramic materials, piezoelectric behaviour is also observed in many polymers. The flexibility, ease of processing and biocompatibility of piezoelectric polymers mean that they are often preferable for certain applications, despite their lower piezoelectric coefficients. This review will cover some theoretical and practical concepts of piezoelectricity in polymers, such as the symmetry requirements, the underlying mechanism and the necessary materials processing. A brief review of the applications of piezoelectric polymers is also presented. One of the main motivations of this review is to discuss the challenges and open questions in the field in an effort to highlight potential future research directions.

Piezoelectric polymers are promising energy materials for wearable and implantable applications, for replacing bulk batteries in small and flexible electronics. Therefore, many research studies are focused on understanding the behaviour of polymers at a molecular level and designing new polymer-based generators using polyvinylidene fluoride (PVDF). In this work, we investigated the influence of voltage polarity and ambient relative humidity in electrospinning of PVDF for energy harvesting applications. A multi-technique approach combining microscopy and spectroscopy was conducted to study the content of β-phase and piezoelectric properties of PVDF fibers. We shed new light on β-phase crystallization in electrospun PVDF and showed the enhanced piezoelectric response of PVDF fiber-based generator produced with the negative voltage polarity at a relative humidity of 60%. Above all, we proved that not only crystallinity but also surface chemistry is crucial for improving piezoelectric performance in PVDF fibers. Controlling relative humidity and voltage polarity, increased d33 piezoelectric coefficient for PVDF fibers more than 3 times and allowed to generate power density of 0.6 µW·cm-2 from PVDF membranes. This study showed that electrospinning technique can be used as a single-step process for obtaining a vast spectrum of PVDF fibers exhibiting different physicochemical properties with β-phase crystallinity reaching up to 73%. The humidity and voltage polarity are critical factors in respect of chemistry of material on piezoelectricity of PVDF fibers, which establishes a novel route to engineer materials for energy harvesting and sensing applications.

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a unique class of materials, with low-moduli high-strain capabilities and the ability to conform to surfaces of different shapes. These features make them attractive for applications such as wearable sensors and interfacing with soft tissues. Here, we review the major types of EAPs and their sensing mechanisms. These are divided into two classes depending on the main type of charge carrier: ionic EAPs (such as conducting polymers and ionic polymer–metal composites) and electronic EAPs (such as dielectric elastomers, liquid-crystal polymers and piezoelectric polymers). This review is intended to serve as an introduction to the mechanisms of these materials and as a first step in material selection for both researchers and designers of flexible/bendable devices, biocompatible sensors or even robotic tactile sensing units.

We report the discovery of serendipitous electrocaloric (EC) effects in commercial multilayer capacitors based on ferroelectric BaTiO₃. Direct thermometry records ∼0.5 K changes due to 300 kV cm⁻¹, over a wide range of temperatures near and above room temperature. Similar results are obtained indirectly, via thermodynamic analysis of ferroelectric hysteresis loops. We compare and contrast these two results. Optimized EC multilayer capacitors could find applications in future cooling technologies.