The vast amount of waste heat released into the environment, from body heat to factories and boilers, can be exploited for electricity generation. Thermoelectrics is a sustainable clean energy solution that converts a heat flux directly into electrical power and vice versa and therefore has the potential for both energy harvesting and cooling technologies. However, the usage of thermoelectrics for large-scale applications is restrained by its device topologies and energy conversion cost efficiency trade-offs. The increase in complex topological designs reported in literature shows a shift towards customizability and improvement of thermoelectric devices for maximum energy conversion efficiency. Increasing design complexity will require an innovative, cost-effective fabrication method with design freedom capabilities. In light of this, this review paper seeks to summarize various thermoelectric topological designs as well as how 3D Printing technology can be a solution to the fabrication of cost- and performance-efficient thermoelectric devices. Specifically, as a process category of 3D Printing technology, Materials Jetting will be elaborated for its usefulness in the fabrication of thermoelectric devices. With in-depth research in materials jetting of thermoelectrics, the gap between small-scale materials research and scaled-up industry applications for energy harvesting through thermoelectric devices is expected to be bridged.

Polyimide (PI), as an advanced polymer material, possesses the intrinsic merits of excellent resistance to extreme temperatures, good dielectric properties, flame resistance, strong processibility, biocompatibility, and flexibility. The outstanding performances of flexible PI have led to a wide range of applications in aerospace, medical, intelligent electronic devices, energy storage devices, and more. Notably, due to the swift progress of various flexible and soft devices, flexible PI has become ubiquitous in the form of thin films, fibers, and foam and gradually plays an indispensable role in all sorts of those devices. This review mainly focuses on the current advances in the usage of flexible PI for barrier, sensor, and functional purposes. Firstly, the key features of various methods for synthesizing and processing PI, as well as the relationship with their respective applications, are summarized. Secondly, to give readers a comprehensive view of the various applications of flexible PI materials, the applications are broken down into three categories: flexible barrier applications, flexible sensing applications, and flexible function applications, and the current research of each application is introduced in detail. Finally, a summary of the challenges and possible solutions in some flexible applications is present.

Soft hydrogels have become an important class of materials for mimicking and interfacing biological soft tissues with potential applications in drug delivery, tissue engineering and bioelectronics. Creative methods for integrating hydrogels with other materials such as organic conductors are highly desired. Here, we describe the single-step electrosynthesis of PEDOT/alginate into core-shell hybrid structures via an electrochemical-chemical-chemical mechanism. Using a pulsed electropolymerisation protocol, we generated PEDOT in either oxidized or reduced form. By-products of this electrochemical step trigger the chemical reactions for the concomitant assembly of alginate hydrogels. Characterization evidences that PEDOT (core) and alginate (shell) compartments form an electrochemically integrated interface. During growth, both can be loaded with useful cargo. We loaded a negatively charged small molecule and investigated passive and electroactive release mechanisms from the two compartments. Our electro-assisted assembly/crosslinking of integrated PEDOT/alginate hybrids contributes a promising approach to the design of functional interfaces for applications in controlled release and soft electronics.

Laser-induced graphene (LIG), which is directly fabricated by laser carbonization of polymers, has gained much attention in recent years since its first discovery in 2014. Specifically, featuring native porosity, good mechanical properties, and excellent electrical/electrochemical properties, it is considered a promising material for flexible electronic devices. Meantime, LIG can be processed in the atmosphere within a few seconds, thereby significantly reducing the fabrication cost of graphene. Facilitated by these features, this methodology has received great development with worldwide efforts in the following years, including the formation mechanism of LIG, the diversity of laser sources (from infrared laser to ultraviolet laser), the diversity of carbon sources (thermoset polymers, thermoplastic polymers, and natural polymers), and property modulation of LIG (porosity, electrical property, hydrophilic/hydrophobic property, electrochemical property), along with the broad applications of LIG in various flexible electronic devices. Here, the recent advances in the mechanism studies and preparation methods of LIG are comprehensively summarized. The various technologies for the modification of LIG are reviewed. A thorough overview of typical LIG-based flexible electronic devices is presented. Finally, the current challenges and future directions are discussed.

As soft active materials, shape-memory polymers (SMPs) and liquid crystal elastomers (LCEs) have attracted considerable research interest due to their potential applications in various areas. SMPs refer to polymeric materials that can return to their permanent shape in response to external stimuli, such as heat, light, and solvent. In this sense, LCEs can exhibit intrinsic shape-memory behaviors since LCEs can switch between two shapes with temperature change due to the order-disorder transition of liquid crystals. In this work, we fabricate both the polydomain and monodomain nematic LCEs through direct ink writing 3D printing. With increasing the temperature of the substrates, the printed LCEs change from the monodomain state to the polydomain state. For polydomain LCEs, a reversible shape change can occur upon constant loading, while the monodomain can switch the shape with temperature in the stress-free state. This two-way shape-memory behavior is caused by the nematic-isotropic phase transition. We further show that the printed LCEs exhibit a good one-way shape-memory effect due to glass transition. The shape recovery region increases with the programming temperature, which is a typical temperature memory effect. Finally, it is demonstrated that complex shape-memory performance can be designed by combining one-way and two-way shape-memory effects. Specifically, for the monodomain LCEs, with increasing temperature, the programmed shape first recovers, and a second shape change can further occur due to the nematic-isotropic transition.

Noninvasive monitoring of markers in biofluids is of paramount significance for health and welfare, which is being integrated into the next-generation consumable wearables. Movement-free sweat extraction and continuous monitoring of fresh sweat are two major challenges for wearable plasmonic sweat sensors. In this perspective, we highlight recent approaches that integrated an electronic sweat extraction system and a microfluidic system with plasmonic sensors to address the challenge. The future directions of systematic integration and miniaturization are discussed.

Biodegradable piezoelectrics represent an intriguing category of electroactive materials combining the mechanical-electrical coupling characteristics with a unique biodegradable feature that eliminates unnecessary materials retention and minimize associated infection risks. Here, we review the piezoelectric properties of representative organic biodegradable piezoelectric materials including amino acids, peptides, proteins, synthetic polymers and polysaccharides. Strategies to promote the piezoelectric activity are summarized, and recent progress in the utilization of biodegradable piezoelectric materials for bioelectronics is discussed, with perspectives and challenges provided at the end to enlighten possible future directions.

The robotic with integrated tactile sensors can accurately perceive contact force, pressure, vibration, temperature and other tactile stimuli. Flexible tactile sensing technologies have been widely utilized in intelligent robotics for stable grasping, dexterous manipulation, object recognition and human-machine interaction. This review presents promising flexible tactile sensing technologies and their potential applications in robotics. The significance of robotic sensing and tactile sensing performance requirements are first described. The commonly used six types of sensing mechanisms of tactile sensors are briefly illustrated, followed by the progress of novel structural design and performance characteristics of several promising tactile sensors, such as highly sensitive pressure and tri-axis force sensor, flexible distributed sensor array, and multi-modal tactile sensor. Then, the applications of using tactile sensors in robotics such as object properties recognition, grasping and manipulation, and human-machine interactions are thoroughly discussed. Finally, the challenges and future prospects of robotic tactile sensing technologies are discussed. In summary, this review will be conducive to the novel design of flexible tactile sensors and is a heuristic for developing the next generation of intelligent robotics with advanced tactile sensing functions in the future.

Ionic polymer metal composite (IPMC) transducers, as one of the typical electroactive polymers with excellent electromechanical coupling properties, have tremendous potential to achieve high-performance actuators and sensors for flexible electronic and soft robotics. In this survey, after briefly describing the energy conversion mechanism of IPMC, we divided the history of IPMC into three stages based on the published papers, and then introduced the preparation technologies of IPMC in detail, which mainly include the selection of ionomer membrane and formation of electrodes. From the point of view of optimization, we summarized and analyzed the performance improvement methods of IPMC and the problems when it is used as actuators and sensors, respectively. The latest and typical applications of IPMC are widely presented as actuators and sensors, such as actuation in robots, grippers, medical and wearable devices, underwater perception and energy harvesting. Moreover, the challenges and opportunities of IPMC were envisioned for future prosperity. This survey will provide an overall general outline for the categorization, mechanism, precursors, and preparation methods of IPMC, which is helpful in facilitating the rapid development and application of IPMC.

To alleviate the escalating global demands for electricity with a low carbon footprint, we can resort to a green energy source that is conveyed by tiny temperature or moisture gradients. A tremendous source of low-grade energy scatters around us and remains unutilized, which is why thermoelectric and hydrovoltaic devices were invented. Our review focuses on a growing trend of implementing hydrogel-based ionic thermoelectric systems and hydrovoltaic devices as they hold the promise of electric outputs that are several times higher than conventional solid-state inorganic counterparts. This is due to the molecular-level tailorable features of hydrogel polymers and their interactions with water and other functional additives, which provide an ideal platform for low-grade heat and water energy harvesting from fundamental and practical perspectives. This review is divided into three sections. We present working principles, engineering concepts, state-of-art designs, and urgent challenges for hydrogel-based (i) ionic thermoelectric systems; (ii) hydrovoltaic devices; and (iii) their hybrids.