The combination of biodegradability and biocompatibility makes chitosan a principal bioresourced material in biomedical engineering, wearable technology, and medical diagnostics, particularly for integration in human interfaces for soft electronic applications. However, this requires the introduction of soft electronic circuits with the capability of recycling the functional materials, while biodegrading the substrate. This paper presents the development and characterization of biodegradable soft circuits that are constructed using stretchable and flexible substrates from plasticized chitosan and conductive functional wiring from recyclable silver nanowires (AgNWs). The chitosan substrate demonstrates tunable mechanical properties with a maximum stretchability of ∼116%, in addition to desirable characteristics such as transparency, breathability, and controlled degradation. The plasticizing effect of glycerol reduces the rigidity associated with pure chitosan and imparts flexibility and stretchability to the AgNW-chitosan-glycerol (AgNW-Chi-Gly) composite. The AgNWs embedded in the Chi-Gly matrix are highly conductive, and their functionality in soft electronic devices such as strain sensors and electromyography (EMG) sensors is demonstrated. We show that the soft chitosan-based substrates can be subject to biodegradation at the end of their operational lifespan. The AgNWs can be recycled and reused, enhancing the overall sustainability of such soft electronic devices.

This study describes the production of biodegradable and recyclable flexible electronic devices created by screen-printing silver nanowires (AgNWs) onto regenerated cellulose films (RCFs). RCFs, derived from microcrystalline cellulose (MCC), are developed and further enhanced for flexibility with additives such as glycerol and poly(ethylene glycol) diglycidyl ether (PEGDE). The resulting cellulose films display relatively high tensile strength (up to 120 MPa), low Young's Modulus (down to 1500 MPa), and 90% optical transparency. Ink with AgNWs and poly(ethylene oxide) (PEO) as a binder is screen-printed on regenerated cellulose films. The printed AgNWs patterns exhibit high electrical conductivity, excellent electromechanical performance, and strong interfacial adhesion with RCFs. To demonstrate the application of printed AgNWs on RCFs for soft electronics, transparent conductive electrodes (TCEs) are fabricated. Grid and honeycomb structures are printed separately and evaluated in terms of sheet resistance and optical transparency. TCEs with approximate to 80% transparency and very low sheet resistance (0.045 Omega sq-1) are obtained. Furthermore, enzymatic hydrolysis of the cellulose substrate and the recovery for reuse of the AgNWs are demonstrated, showing the potential of integrating natural polymers and recyclable nanomaterials for eco-friendly and sustainable soft flexible electronics.

Surface electromyography (sEMG) is a widely used non-invasive diagnostic modality in rehabilitation, designed to detect muscle activation in response to neural signals at the skin’s surface. However, sEMG has limitations, including its inability to accurately measure deeper muscle activation, low spatial resolution, susceptibility to noise interference, and crosstalk from neighboring muscles. In contrast, ultrasound (US) imaging provides valuable insights into muscle thickness, stiffness, force, and fatigue with high spatial resolution and detection accuracy. Furthermore, the US can assess both superficial and deep muscle layers, addressing the shortcomings of sEMG. The integration of sEMG with US can provide comprehensive insights into both electrophysiological and morphological activities of muscle simultaneously. To facilitate this, we developed ultrasoundcompatible EMG (US-EMG) sensors and a 64-element wearable ultrasound (WUS) array. The US-EMG sensor was fabricated using silver-nanowires and polydimethylsiloxane (AgNWs/PDMS) composite. The fabricated US-EMG sensor performed comparably to the commercial EMG electrodes. Next, The WUS was electrically and acoustically characterized, demonstrating uniformity across the array. Successful acquisition of EMG signals and B-mode US imaging without significant signal loss was achieved during the vertical integration of the WUS array with the US-EMG sensors. On the contrary, the commercial EMG sensors did not allow for clear US imaging. This multi-modal sensing device shows great potential for enhancing motion prediction and muscle fatigue detection.

Functional electrical stimulation (FES) is a vital method in neurorehabilitation used to reanimate paralyzed muscles, enhance the size and strength of atrophied muscles, and reduce spasticity. FES often leads to increased muscle fatigue, necessitating careful monitoring of the patient’s response. Ultrasound (US) imaging has been utilized to provide valuable insights into FES-induced fatigue by assessing changes in muscle thickness, stiffness, and strain. Current commercial FES electrodes lack sufficient US transparency, hindering the observation of muscle activity beneath the skin where the electrodes are placed. US-compatible electrodes are essential for accurate imaging and optimal FES performance, especially given the spatial constraints of conventional US probes and the need to monitor muscle areas directly beneath the electrodes. This study introduces specially designed body-conforming US-compatible FES (US-FES) electrodes constructed with a silver nanowire/polydimethylsiloxane (AgNW/PDMS) composite. We compared the performance of our body-conforming US-FES electrode with a commercial hydrogel electrode. The findings revealed that our US-FES electrode exhibited comparable conductivity and performance to the commercial one. Furthermore, US compatibility was investigated through phantom and in vivo tests, showing significant compatibility even during FES, unlike the commercial electrode. The results indicated that US-FES electrodes hold significant promise for the real-time monitoring of muscle activity during FES in clinical rehabilitative applications.

Both liquid metal (LM) and metallic filler-based conductive composites are promising stretchable conductors. LM alloys exhibit intrinsically high deformability but present challenges for patterning on polymeric substrates due to high surface tension. On the other hand, conductive composites comprising metallic fillers undergo considerable decrease in electrical conductivity under mechanical deformation. To address the challenges, we present silver nanowire (AgNW)-LM-elastomer hybrid composite films, where AgNWs and LM are embedded below the surface of an elastomeric matrix, using two fabrication approaches, sequential and mixed. We investigate and understand the process-structure-property relationship of the AgNW-LM-elastomer hybrid composites fabricated using two approaches. Different weight ratios of AgNWs and LM particles provide tunable electrical conductivity. The hybrid composites show more stable electromechanical performance than the composites with AgNWs alone. In particular, 1:2.4 (AgNW:LMP w/w) sequential hybrid composite shows electromechanical stability similar to that of the LM-elastomer composite, with a resistance increase of 2.04% at 90% strain. The sequential approach is found to form AgIn2 intermetallic compounds which along with Ga-In bonds, imparts large deformability to the sequential hybrid composite as well as mechanical robustness against scratching, cutting, peeling, and wiping. To demonstrate the application of the hybrid composite for stretchable electronics, a laser patterned stretchable heater on textile and a stretchable circuit including a light-emitting diode are fabricated.

The dissemination of sensors is key to realizing a sustainable, ‘intelligent’ world, where everyday objects and environments are equipped with sensing capabilities to advance the sustainability and quality of our lives—e.g. via smart homes, smart cities, smart healthcare, smart logistics, Industry 4.0, and precision agriculture. The realization of the full potential of these applications critically depends on the availability of easy-to-make, low-cost sensor technologies. Sensors based on printable electronic materials offer the ideal platform: they can be fabricated through simple methods (e.g. printing and coating) and are compatible with high-throughput roll-to-roll processing. Moreover, printable electronic materials often allow the fabrication of sensors on flexible/stretchable/biodegradable substrates, thereby enabling the deployment of sensors in unconventional settings. Fulfilling the promise of printable electronic materials for sensing will require materials and device innovations to enhance their ability to transduce external stimuli—light, ionizing radiation, pressure, strain, force, temperature, gas, vapours, humidity, and other chemical and biological analytes. This Roadmap brings together the viewpoints of experts in various printable sensing materials—and devices thereof—to provide insights into the status and outlook of the field. Alongside recent materials and device innovations, the roadmap discusses the key outstanding challenges pertaining to each printable sensing technology. Finally, the Roadmap points to promising directions to overcome these challenges and thus enable ubiquitous sensing for a sustainable, ‘intelligent’ world.

Functional electrical stimulation (FES) is commonly utilized in rehabilitation to induce muscle contractions, yet its effectiveness is limited by the rapid onset of muscle fatigue. Spatially distributed sequential stimulation (SDSS) has recently emerged as a promising solution by temporally shifting FES pulses across multiple electrodes. This protocol activates motor units at a lower frequency than conventional FES, which minimizes FES-induced muscle fatigue while maintaining or even increasing the overall muscle power output. To optimize SDSS parameters and electrode placement, it is essential to assess muscle contractility in real time. Ultrasound (US) technology plays a pivotal role, offering deep wave penetration non-invasively, furnishing functional insights into musculoskeletal activities. Conventional US transducers suffer from a lack of flexibility and wearability, thereby constraining mobility. To overcome these limitations, we integrated a flexible/wearable sparse US (fWSUS) 2D array (4 by 4) with an SDSS electrode array. A center frequency of 6 MHz was chosen to achieve the sufficient penetration depth of US wave for monitoring the lower limb muscle activity. The SDSS array, fabricated using silver nanowire/polydimethylsiloxane (AgNW/PDMS) composite, was tested against the commercial FES electrodes. US-compatibility was investigated by collecting A-mode US signals with the fWSUS array placed vertically on the SDSS array. The results suggest that the proposed approach is promising to reduce FES-induced muscle fatigue while enabling real-time monitoring of targeted muscle activity, offering significant potential to advance rehabilitation technology.

Transcranial focused ultrasound (tFUS) is promising for non-invasive brain stimulation, due to its millimeter-level spatial resolution and centimeter-level penetration depth. To stimulate various regions within the human brain, including cortical and subcortical structures, it is necessary to adjust the focal depth of a single-element focused ultrasound transducer. Common approaches involve the utilization of an acoustic lens, altering the curvature of transducer surface, or controlling the distance between the transducer and the scalp using either a water bag or a gel pad. However, such methods require multiple interchanges of lenses, transducers, or bags/pads, rendering them impractical for clinical applications. In this paper, we introduce a novel ultrasound stimulation device comprising a flexible ultrasound transducer and a precise movable plunger to enable focus tunability. The focused effect was simulated through finite element analysis, while the adjustability of focal depth was gauged by controlling interface curvature of the flexible transducer, as measured with hydrophone. This novel apparatus has the potential to enhance ease of use of tFUS and efficiency of its clinical applications.

Functional electrical stimulation (FES) is a neurorehabilitation modality that helps improve the size and strength of atrophied muscles after paralysis and reduce spasticity. However, FES causes muscle fatigue, and monitoring the patient's response to avert muscle fatigue caused by FES becomes imperative. Ultrasound (US) imaging can elucidate valuable information about muscle thickness, stiffness, force, and fatigue in the muscles. However, if the FES electrodes do not match the acoustic properties of the surrounding tissues, it could result in the interference with other electronic devices, potentially affecting both US imaging and the performance of the FES system. This study evaluates a specially designed body-conforming FES customized electrode that is intended to be compatible with US. The electrode is made with silver-nanowires/ Polydimethylsiloxane (AgNW/PDMS). The performance of the body-conforming customized FES electrode was demonstrated in parallel to that of the commercial hydrogel electrode. Moreover, compatibility with US was established through tests employing a 3.5 MHz single-element US transducer. The customized FES electrode exhibited a comparably minor variance (< 8%) relative to the commercial hydrogel electrode.

Screen printing is a promising route towards high throughput printed electronics. Currently, the preparation of nanomaterial based conductive inks involves complex formulations with often toxic surfactants in the ink's composition, making them unsuitable as an eco-friendly printing technology. This work reports the development of a silver nanowire (AgNW) ink with a relatively low conductive particle loading of 7 wt%. The AgNW ink involves simple formulation and comprises a biodegradable binder and a green solvent with no toxic surfactants in the ink formulation, making it an eco-friendly printing process. The formulated ink is suitable for printing on a diverse range of substrates such as polydimethylsiloxane (PDMS), polyethylene terephthalate (PET), polyimide (PI) tape, glass, and textiles. By tailoring the rheological behaviour of the ink and developing a one-step post-printing process, a minimum feature size of 50 μm and conductivity as high as 6.70 × 106 S m-1 was achieved. Use of a lower annealing temperature of 150 °C makes the process suitable for plastic substrates. A flexible textile heater and a wearable hydration sensor were fabricated using the reported AgNW ink to demonstrate its potential for wearable electronic applications.

The COVID-19 pandemic has put extraordinary stress on medical systems and global society more broadly. The condition of infected patients may deteriorate rapidly due to overburdened hospital systems. This raises an urgent need for real-time and remote monitoring of physiological parameters to address the challenges associated with the COVID-19 pandemic. This review will present recent progress on soft wearable sensors that can potentially be used for monitoring respiratory diseases such as COVID-19. First, emerging monitoring devices and systems that can monitor key physiological parameters as suggested by the Centers for Disease Control and Prevention (e.g. body temperature, respiration rate, heart rate, oxygen saturation and body movement) are reviewed. Then, multimodal sensor systems consisting of two or more correlative sensors are presented. This review will conclude with challenges and future directions for wearable sensors for the diagnosis and therapy of respiratory diseases. While this review focuses on COVID-19, the sensing technologies reviewed can be applicable to other respiratory diseases such as H1N1 influenza.

Numerous fine coal processing technologies like froth flotation, enhanced gravity separation exist today, but their insularity have led to the adoption of oil agglomeration as a retrieval technique. In the present study, coal fines were recovered by the oil agglomerating process with the agglomerating oil as a waste vegetable oil - waste mustard oil. The effect of pulp density, oil dosage and agglomeration time were analysed on the responses – organic matter recovery (% OMR) and ash rejection (% AR). The responses were optimized using three–level Box Behnken experimental design in conjunction with the Response Surface Methodology. The resulting conditions were 99.69% for % OMR and 62.56% for % AR, showcasing that waste mustard oil has good agglomerating properties, despite of the heat treatment undergone by it during frying. Optimization studies were also performed by minimizing the oil dosage to countervail the poor economics of the oil agglomeration process. The observed conditions were investigated as 79.21% for % OMR and 65.41% for % AR obtained at an oil dosage of 8.39%. The comparative examination of these results with the contemporary studies of different oil types proved that waste mustard oil is a much better bridging liquid than the currently available oils.