Aging, diseases, and injuries often lead to irreversible tissue damage, especially in organs with limited self-healing capabilities like the central nervous system (CNS) and cartilage. Electrical stimulation has demonstrated the ability to promote cell regeneration by influencing cellular responses for repair. This has been observed on conductive materials powered by batteries. However, the need for surgical battery replacement once they reach the end of their lifespan poses health risks to patients, presenting a challenge for the long-term use of such devices. The key obstacle is finding a reliable power source capable of generating electricity from human-induced thermal and vibrational energy.
Ferroelectric materials, with inherent polarization generating surface charge variations through piezoelectricity and pyroelectricity in response to pressure and temperature changes, offer a promising solution. Yet, commonly used ferroelectric materials like lead zirconate titanate (PZT) and polyvinylidene difluoride (PVDF) contain toxic or non-biodegradable components, making them unsuitable for implantation. In addressing this issue, our work presents the successful fabrication and characterization of biocompatible ferroelectric polymer composites in the form of thin membranes and 3D porous scaffolds.
Thin ferroelectric membranes, composed of Polydimethylsiloxane (PDMS) and potassium sodium lithium niobate (KNLN), were created through in-situ poling-dielectrophoresis to achieve high piezoelectric sensitivity while maintaining flexibility. Significantly improved piezoelectric properties were achieved in quasi 1-3 composites through a combination of dielectrophoretic alignment and poling processes. Meanwhile, 3D porous scaffolds made of cellulose, poly(lactic acid) (PLA), and chitosan KNLN were developed using the freeze-casting technique, creating aligned porosity for guided cell outgrowth. Integration of KNLN piezoelectric particles enhanced average pore size, interconnectivity, and piezoelectricity, making these composites ideal for soft tissue regeneration applications. This research marks the first report on the fabrication and material characterization of highly porous Cellulose-KNLN and PLA-chitosan-KNLN composites through freeze casting, with morphological characterization linking microstructures to processing parameters and final properties. A poling study was conducted to achieve desirable piezoelectric properties, showcasing the potential of these structures for soft tissue applications.

(K,Na)NbO3 (KNN) based ceramics are among the most promising candidates to replace lead containing Pb(Zr,Ti)O3 based ceramics in various application. Over the past decades, phase composition design and domain engineering by chemical modification and sophisticated synthesizing led to excellent piezoelectric properties. One of the most challenging applications are multilayer actuators e.g., for precise positioning. In this technical environment the strain must not be affected by the temperature over a broad range, to achieve the needed accuracy and reliability. In recent years, attempts were undertaken that focus on a diffuse phase transition and at the same time a multiphase co-existence over a wide temperature range. Thus, stable and temperature independent strains are achieved in literature. However, most published investigations were performed on laboratory scale and the important assessment of their relevance for a possible industrial processing is lacking. In this contribution we investigate the sintering behavior, strain performance and difficulties during scaling attempts for different phase engineered compositions. A moderately increased sample size often results in e.g., tremendous porosity and degradation of piezoelectric performance. In many cases, the ideal sintering temperatures for best performance are much higher than expected and too high for multilayer processing. Thus, the impact of different sintering additives on the performance of the complex materials is also investigated.

Lead-based single crystals are well known for their superior electrical properties. However, environmental concerns about the effects of the continued use of Pb-based materials worldwide are encouraging research into alternatives to their use. Single crystals of (K, Na)NbO3-based compositions are one of the most promising materials for the substitution of Pb-based ones. In this work, the growth and the electrical properties of a Li, Ta, and Sb modified (K, Na)NbO3 lead-free single crystals grown by the Bridgman-Stockbarger method has been reported. The single crystals were obtained directly from the melt, and a polynucleation behavior was observed in the as-grown crystal. The harvested single crystals were mechanically polished before the characterization. The results of the XRD patterns showed high-quality single crystals of an orthorhombic structure, which differs from the polycrystalline form due to the segregation of the alkali elements, K and Na. The orientation of each crystal was confirmed by the LAUE diffraction with the (001), (100) and (111) planes identified for the different samples. The temperature dependence of the dielectric constant showed two dielectric anomalies, around 90 °C and 300 °C which corresponds to the ferroelectric-ferroelectric transition and ferroelectric-paraelectric transition respectively, which is in agreement with XRD and the thermal measurements performed in the crystals.

Acknowledgments: The authors would like to thank the financial support from FAPESP (2019/26807-4 and 2017/13769-1).

The tolerance factor for perovskites is a well-established and recognised tool to predict stable perovskite phases and inspired the formulation of a similar tool for the tetragonal tungsten bronzes (A2₄A1₂B₁₀X₃₀, TTBs). Despite the convenience of the tolerance factor for filled TTBs, difficulties arise when faced with unfilled TTBs, i.e., compositions that include A-cation vacancies. There are two “simple” ways to describe the size of a cation vacancy; the first one is to assume the vacancy to be zero dimensional, while the other one is to assume that the vacancy has approximately the same size as the average size of the present A-cations. Both assumptions lead to different tolerance factors and could tip the scale whether the studied TTB composition is predicted to be stable or not, especially since the stability window of TTBs is narrower than that of perovskites. Moreover, literature studies revealed that cation vacancies are larger than both these estimates. To be able to predict the stability of TTBs with cation vacancies, a mathematical model to determine the size of the cation vacancies has been developed and the results are compared to literature data for lead-free unfilled TTBs. Structural trends caused by the presence of cation vacancies in TTBs were identified and the impact of cation vacancy concentration on the TTB framework will be reported.