Abstract
This special issue of the IEEE Journal on Flexible Electronics (J-FLEX) presents ten papers submitted directly to the journal prior to the 2024 IEEE International Flexible Electronics Technology Conference (IFETC) held at the DAMSLab at the University of Bologna, Bologna, Italy, from September 15 to 18, 2024. Over three days, the IFETC 2024 featured a technical program of 37 invited talks, 57 contributed talks, and 48 poster presentations selected through a rigorous review process and covering a broad array of topics in flexible electronics over the 12 tracks: materials and manufacturing; energy harvesting and storage devices; imaging and lighting devices; sensors, biosensors, and actuators; biointerfaced, bioinspired, and neuromorphic electronics; transistors and circuits; simulation and modeling; reliability and lifetime; heterogeneous and hybrid integration; emerging applications and products; flexible and printable solutions in RFIDs and IoT; and sustainability and energy efficiency, and the four special sessions: biological and bioinspired smart materials; memristor technology: theory, design, and applications; printed biosensors and wearables for healthcare applications; and soft robotics meets soft electronics. The conference included three keynote talks and four plenary talks from world-renowned researchers in the field. Four tutorial sessions were held covering materials for green electronics, computational approaches for semiconductors, organic bioelectronic interfaces, and nanomaterials for security. Two panel discussions, one related to industrial collaborations and another related to diversity, were also included in the conference program. The concept behind this special issue was to enable a rigorous peer-review of conference contributions prior to the event and thus to enhance their impact on the scientific community. The outcome of this careful selection process are the following ten articles. Four of the papers report on important developments in sustainable and biocompatible electronic devices. In [A1], Bhagavathi et al. study and characterize a silk-aloe vera composite piezoelectric film and demonstrate pressure sensors based on this eco-friendly material that show enhanced sensitivity when compared with a silk-only film. The use of sustainable materials for an electronic device is also shown in [A2] where Annese et al. fabricate and test a negative temperature coefficient resistive thermistor based on food-grade materials including activated carbon, gelatin, gold, ethyl cellulose, and beeswax for use as an edible temperature sensor. These devices had a sensitivity of –0.32% °C−1 in the range of 5–50 °C, providing a promising path for applications such as GI tract monitoring, as well as generally demonstrating sustainable material options for environmental monitoring. Temperature sensors are also the subject of [A3] where Husain et al. investigated the use of biodegradable metals, including Mg, Mo, and Zn for thin-film resistance temperature detectors (RTDs). The temperature sensors showed a consistent response from 25–75 °C, and the authors studied the impact of humidity and mechanical strain on the device response. They also showed that the deposited metals could be dissolved in water, allowing the substrate to be reused. In [A4], Rao et al. present electrochemical sensors using mechanically flexible, biocompatible cellulose acetate (CA) substrates. Carbon electrodes were screen printed onto the CA, and the working electrode was modified with COOH-functionalized Au nanoparticles to create biosensors for the detection of Staphylococcus aureus. These devices showed a limit of detection of 0.13 CFU mL−1, with performance retained after repeated bending cycles. Further, flexible sensing is also the subject of three other papers in this collection. In [A5], Wu et al. report on an MXene-based dual network hydrogel (DNH) for a flexible wearable strain sensor to detect human actions. In this paper, a Ti3C2TX MXene and polydopamine are introduced into a polyacrylamide:PVA-borax DNH. The mechanical and electrical properties of the hydrogels were investigated and the MXene-DNH was used as a strain sensor, demonstrating a wide strain sensing range (0%–965%) with a gain factor (GF) of 0.38 in the 0%–100% strain range, 1.2 in the 100%–300% range, and 2.9 in the 300%–965% range. Wearable devices are also studied in [A6] where Ibba, et al. demonstrate a smart glove concept which combines textile thermoplastic polyurethane-carbon (TPU-CNF)-based printed electrodes with a portable impedance analyzer for on-site fruit monitoring applications. These highly stretchable electrodes (up to 250% strain) are robust, maintaining their electrical properties during mechanical cycling and damage, and provide similar results to commercial electrodes (up to frequencies of 500 kHz) when used for bioimpedance analysis of fruit to assess produce quality. In [A7] Faramarzi et al. design and characterize flexible electronics in low-temperature polysilicon (LTPS) thin-film technology. These circuit blocks are designed for use as components of read-out circuitry for sensor arrays implemented directly inside solar modules and batteries. A current source, low supply voltage (6 V) OpAmp, and a high-speed latched comparator with a clock frequency up to 1 MHz, and low input offset are all investigated in this study. Transistor circuits are also the topic of [A8], where Mehrolia, et al. study compact modelling of organic thin-film transistors (OTFTs) on both silicon and flexible substrates. Based on prior OTFT experimental results, inverter, 2-input NAND gate, and-half adder circuits are simulated. These data show the flexible OTFT configuration being 49% faster with a gain 1.5 times higher than OTFTs fabricated on a silicon substrate. The other two papers in the collection relate to print-based manufacturing of electronic materials and devices. In [A9], Philippin et al. describe free-form fabrication of flexible, stretchable electronics using high-resolution electrohydrodynamic printing of silver ink onto an TPU substrate. Hyperelastic material models were used to understand the TPU material behavior during deformation, and based on these results, the flat TPU substrates containing EHD-printed silver structures were converted to 3-D shapes using vacuum thermoforming. In [A10], Amarakonda et al. introduce a novel binder-free 1T MoS2 micro-supercapacitor fabricated using screen-printing onto a flexible paper substrate, which employs a quasi-solid polyvinylalcohol (PVA)-H2SO4 electrolyte. The device showed performance stability during cycling and provided an areal capacitance of 22.5 mF cm−2 and an energy density of 0.45 μ Wh cm−2 at a power density of 49.14 μ W cm−2. Serial integration of these devices enables performance suitable for application in powering portable and wearable electronics. Putting together this special edition of J-FLEX, particularly within a condensed timeframe, required considerable effort from many people, and the guest editors extend their heartfelt thanks to the reviewers for their prompt and meticulous evaluation of each manuscript and its revisions. We sincerely appreciate the authors’ contribution of their valuable research findings, and their responsiveness to requests from the editors and reviewers. The Guest Editors would specifically like to thank the Editor-in-Chief Paul Berger for valuable support. A special thanks is also due to Mansi Kukreti of the IEEE J-FLEX publications office. We have gained valuable insights while assembling this special issue, and we hope that the readers will find it both informative and enjoyable.