Multifunctional Energy-Integrated Devices

The era of Internet of Things has been driving the rapid development of multifunctional devices featuring the seamless integration of modules of energy, sensing, actuation, displays, etc., towards miniaturized devices and smart applications in fields as diverse as robotics, medicine, and space exploration. However, challenges such as incompatibilities between different functioning subunits, which inevitably cause compromised performance, distant or wire connections that lead to a poor level of integration and power loss, etc., demand immediate solutions. Moreover, core to any electronic applications, future power sources require new strategies to realize the combination of high energy density, security, ultralight weight, and small size.

This special issue is a collection of 12 research articles and 11 review articles, contributed by renowned researchers in the field of multifunctional energy-integrated devices. The articles can be sorted into three themes: 1) advanced energy storage devices, including batteries and supercapacitors; 2) energy harvesting devices, including photovoltaic cells, thermoelectric devices, and triboelectric nanogenerators; 3) multifunctional devices that integrate energy harvesting and storage for optoelectronic and biological sensory systems. The topics covered in this special issue tackle the aforementioned challenges from different angles spanning materials, mechanisms, devices, and systems as a whole, paving the way to the development of next-generation self-powered, wearable, and smart devices.

In this special issue, Djenizian and co-workers report coaxial wire-shaped Li-ion batteries by adopting the unidirectional helical winding method, which shows high energy storage capacity while maintaining high flexibility and stretchability without compromising the electrochemical performance upon mechanical deformation (article number 2302117). Lethien and co-workers demonstrate a new class of electrolytic micro-capacitors by miniaturizing an electrolytic capacitor based on tantalum materials, yielding good capacitance retention of over 90% upon cycling 300 000 times (article number 2400682). Wu and co-workers have designed a simple hydrothermal strategy to pre-intercalate gallium ions in vanadate electrodes for aqueous Zn batteries. The reported device shows high specific capacity and energy density, in addition to good cycling performance and stability upon bending (article number 2400125). These research results demonstrate effective strategies for advancing the frontiers of energy storage research, particularly of interest to portable and miniaturized applications. Park and co-workers review the prospects and challenges in the field of battery-integrated systems, highlighting the need for advancements in energy density, power output, and safety to meet the demands of modern electronics (article number 2302236).

Progress on energy harvesting devices is also a focus of this special issue. Dahiya and co-workers present 315 um²-sized miniaturized photovoltaic microcells made of roll-printed nanoscale photoactive electronic layers. By interconnecting 32 microcells in parallel, indoor light harvesting under white LED illumination has been realized (article number 2400728). Roy and co-workers report a composite-engineered thermoelectric module for high-efficiency waste heat recovery (article number 2301722). Chen and co-workers demonstrate a thermoelectric-water hybrid cooling garment that displays effective cooling along with a high coefficient of performance while maintaining thermal comfort (article number 2301069). Di and co-workers review advances in organic thermoelectric devices with particular focus on energy harvesting and sensing applications (article number 2302128). Hu and co-workers discuss progress in textile triboelectric nanogenerators which has enabled the rapid development of wearable energy-integrated technologies, leading to lightweight, low-cost, flexible, stretchable, washable, and smart energy-integrated devices (article number 2302012).

Six original research articles on sensory devices with diverse functionalities and their integrated systems are also collected in this special issue. Dong and co-workers report a mechanoluminescent material for accurate stress visualization and pioneer its use in security labels (article number 2301948). Nielsch and co-workers establish a thin film approach to develop photodetectors with improved sensitivity, showing great potential in optoelectronic applications (article number 2302049). Kim and co-workers demonstrate an implantable ultrasound-driven triboelectric nanogenerator of exceptional stability and acoustic impedance compatibility with human-like tissues. They have further validated consistent voltage generation and nerve stimulation capabilities (article number 2400317). Ni and co-workers report a flexible micro-supercapacitor made of anthracite-based porous carbon, showing consistent and stable signal output as a strain sensor (article number 2301523). Zhu and co-workers also report a supercapacitor for wearable self-powered smart sensors. The supercapacitor is made of Ni-Co layered double hydroxide, and can be used to monitor copper ions in sweat (article number 2400163). Apart from functional materials and devices, fabrication and architecture are equally important, especially for the applications of non-planar and wearable electronics. In this regard, Zheng et al. have developed an electrochemical replication and transfer method to realize 3D patterning, providing a high-throughput and low-cost approach to fabricating high Figure of Merit (over 30000) flexible transparent electrodes (article number 2301695).

The final eight articles are Reviews with a particular focus on function integration and self-powered systems, and comprehensively discuss advances and challenges in this field. Galstyan and co-workers review recent advances in self-powered electrochemical biosensors for early diagnosis of diseases (article number 2400395). Han and co-workers introduce recent progress in wearable self-powered biomechanical sensors towards cardiovascular system monitoring, acoustic signal detection, human motion tracking, etc. (article number 2301895). Lan and co-workers elaborate on progress in the integration of supercapacitors with sensors and energy-harvesting devices (article number 2301796). Yi and co-workers put further emphasis on flexible supercapacitor integrated systems (article number 2301931). Li and co-workers discuss the latest research on various methods of harvesting human body energy and related wearable parts, focusing on new materials, structures, and processes (article number 2302068). Mo and co-workers examine advanced electrochromic energy storage devices based on conductive polymers that merge the dual functions of energy storage and display, with great potential for use in wearable and portable devices (article number 2301969). Wen and co-workers report on recent progress in self-powered pressure sensors for the Internet of Healthcare (article number 2301480). Zhang and co-workers highlight advances in bioinspired soft robots from the point of view of fabrication, actuation, locomotion, and tracking, towards applications such as vessel recanalization, targeted drug delivery, and cargo manipulation (article number 2301862).

We thank all the authors in this special issue for their valuable contributions to this area of applied research. We also appreciate the input of other experts who have volunteered their expertise and time during the peer review process. We are especially grateful to the Advanced Materials Technologies Editorial Office for their efforts in making this special issue possible.

The papers in this special issue span a variety of important and interesting topics on multifunctional devices enabled by seamlessly integrating energy power sources with other functional components. We hope that this collection of articles informs and stimulates further research on this specific topic towards next-generation miniaturized and wearable electronics, promoting their practical use in important fields such as healthcare, biology, and robotics.

The authors declare no conflict of interest.