Soft Electronics
-The Nexus of Energy, Healthcare, and Human-Machine Interactions
Flexible/Stretchable Perovskite Optoelectronic Devices
Printed perovskite light-emitting diodes (PeLEDs) on elastomer substrates
Zhao, Junyi, et al. “High‐Speed Fabrication of All‐Inkjet‐Printed Organometallic Halide Perovskite Light‐Emitting Diodes on Elastic Substrates.” Advanced Materials 33.48 (2021): 2102095. Link
Abstract
Halide perovskites have great potential for use in high-performance light-emitting diodes (LEDs) and displays. Here, a perovskite LEDs (PeLEDs) fabricated directly on an elastomer substrate, in which every single layer in the device from bottom anode to top cathode is patterned solely using a highly scalable inkjet printing process, is reported. Compared to PeLEDs made using conventional microfabrication processes, the printing process significantly shortens the fabrication time by at least tenfold (from over 5 h to less than 25 min). The all-printed PeLEDs have a novel 4-layer structure (bottom electrode, perovskite emissive layer, buffer layer, top electrode) without separate electron or hole transporting layers. For flexible PeLEDs printed directly in ambient conditions, a turn-on voltage, maximum luminance intensity, and maximum current efficiency of 3.46 V, 10227 cd m−2, and 2.01 cd A−1, respectively, is achieved. The devices also exhibit excellent robustness and stability even when bent to a curvature radius of 2.5 mm. The reported device structure and fabrication processes can enable high-performance flexible PeLEDs to be manufactured over a larger area at extremely low cost and fast speed, which can facilitate the adoption of the promising PeLED technology in the emerging foldable displays, smart wearables, and many other applications.
Handwriting perovskite optoelectronic devices on diverse substrates
Zhao, Junyi, et al. “Handwriting of perovskite optoelectronic devices on diverse substrates.” Nature Photonics, in press, 2023. Link
Abstract
Paper and textiles that are commonly used in our daily lives hold great potential as platforms for next-generation flexible and wearable electronics. However, strategies for fabricating light-emitting diodes and photodetectors on different substrates are restricted in terms of their quantity and variety as strict flatness and smoothness are often required. Here we develop a highly versatile, scalable and eco-friendly handwriting approach to draw multicolour perovskite light-emitting diodes and perovskite photodetectors on various substrates, including paper, textiles, plastics, elastomers, rubber and three-dimensional objects. Our method uses common ballpoint pens filled with newly formulated inks of conductive polymers, metal nanowires and multiple perovskites for a wide range of emission colours. Just like writing with multicoloured pens, writing layer-by-layer with these functional inks enables perovskite optoelectronic devices to be realized within minutes. This process can be carried out by individuals without specialized training. The handwritten perovskite light-emitting diodes can exhibit a brightness as high as 15,225 cd m−2, a current efficiency of 6.65 cd A−1 and a turn-on voltage of 2.4 V. The perovskite photodetectors exhibit an on/off ratio of over 10,000 and a responsivity of up to 132 mA W−1. This work offers a route to the integration of perovskite optoelectronics in low-cost and large-area application scenarios such as electronic textiles, electronic paper, smart packaging and other disposable electronics and wearables.
Soft Sensors and Wearable Healthcare Systems
3D E-textile system for exercise physiology & clinical maternal monitoring
Zhao, Junyi, et al. ” Waterproof and gel-free 3D E-textile System for exercise physiology and clinical maternal health monitoring”. In preparation.
Abstract
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Soft sponge sensor for multimodal distinguishable sensing of pressure, strain, and temperature
LW Lo, J. Zhao, H. Wan, Yong Wang, S. Chakrabartty, and C. Wang. “A soft sponge sensor for multimodal sensing and distinguishing of pressure, strain, and temperature.” ACS Applied Materials & Interfaces 14, no. 7 (2022): 9570-9578. Link
Abstract
Soft wearable sensors are essential components for applications such as motion tracking, human−machine interface, and soft robots. However, most of the reported sensors are either specifically designed to target an individual stimulus or capable of responding to multiple stimuli (e.g., pressure and strain) but without the necessary selectivity to distinguish those stimuli. Here we report an elastomeric sponge-based sensor that can respond to and distinguish three different kinds of stimuli: pressure, strain, and temperature. The sensor utilizes a porous polydimethylsiloxane (PDMS) sponge fabricated from a sugar cube sacrificial template, which was subsequently coated with a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate(PEDOT:PSS) conductive polymer through a low-cost dip-coating process. Responses to different types of stimuli can be distinguished by simultaneously recording resistance and capacitance changes. Because pressure, tensile strain, and temperature change result in different trends in resistance and capacitance change, those stimuli can be clearly distinguished from each other by simultaneously measuring the resistance and capacitance of the sensor. We have also studied the effect of the pore size on the sensor performance and have found that the sponge sensor with smaller pores generally offers greater resistance change and better sensitivity. As a proof-of-concept, we have demonstrated the use of the porous sponge sensor on an artificial hand for object detection, gesture recognition, and temperature sensing applications.
Stretchable sponge electrodes for high-quality electrophysiologic signals
LW. Lo, J. Zhao, K. Aono, W. Li, Z. Wen, S. Pizzella, Y. Wang, S. Chakrabartty, and C. Wang. “Stretchable sponge electrodes for long-term and motion-artifact-tolerant recording of high-quality electrophysiologic signals.” ACS Nano 16, no. 8 (2022): 11792-11801. Link
Abstract
Soft electronic devices and sensors have shown great potential for wearable and ambulatory electrophysiologic signal monitoring applications due to their light weight, ability to conform to human skin, and improved wearing comfort, and they may replace the conventional rigid electrodes and bulky recording devices widely used nowadays in clinical settings. Herein, we report an elastomeric sponge electrode that offers greatly reduced electrode–skin contact impedance, an improved signal-to-noise ratio (SNR), and is ideally suited for long-term and motion-artifact-tolerant recording of high-quality biopotential signals. The sponge electrode utilizes a porous polydimethylsiloxane sponge made from a sacrificial template of sugar cubes, and it is subsequently coated with a poly(3,4-ethylenedioxythiophene) polystyrenesulfonate (PEDOT:PSS) conductive polymer using a simple dip-coating process. The sponge electrode contains numerous micropores that greatly increase the skin–electrode contact area and help lower the contact impedance by a factor of 5.25 or 6.7 compared to planar PEDOT:PSS electrodes or gold-standard Ag/AgCl electrodes, respectively. The lowering of contact impedance resulted in high-quality electrocardiogram (ECG) and electromyogram (EMG) recordings with improved SNR. Furthermore, the porous structure also allows the sponge electrode to hold significantly more conductive gel compared to conventional planar electrodes, thereby allowing them to be used for long recording sessions with minimal signal degradation. The conductive gel absorbed into the micropores also serves as a buffer layer to help mitigate motion artifacts, which is crucial for recording on ambulatory patients. Lastly, to demonstrate its feasibility and potential for clinical usage, we have shown that the sponge electrode can be used to monitor uterine contraction activities from a patient in labor. With its low-cost fabrication, softness, and ability to record high SNR biopotential signals, the sponge electrode is a promising platform for long-term wearable health monitoring applications.
Printed PEDOT:PSS-Based Stretchable Conductor for Wearable Healthcare
LW. Lo, J. Zhao, H. Wan, Y. Wang, S. Chakrabartty, C. Wang. “An inkjet-printed PEDOT: PSS-based stretchable conductor for wearable health monitoring device applications.” ACS Applied Materials & Interfaces 13, no. 18 (2021): 21693-21702. Link
Abstract
A stretchable conductor is one of the key components in soft electronics that allows the seamless integration of electronic devices and sensors on elastic substrates. Its unique advantages of mechanical flexibility and stretchability have enabled a variety of wearable bioelectronic devices that can conformably adapt to curved skin surfaces for long-term health monitoring applications. Here, we report a poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS)-based stretchable polymer blend that can be patterned using an inkjet printing process while exhibiting low sheet resistance and accommodating large mechanical deformations. We have systematically studied the effect of various types of polar solvent additives that can help induce phase separation of PEDOT and PSS grains and change the conformation of a PEDOT chain, thereby improving the electrical property of the film by facilitating charge hopping along the percolating PEDOT network. The optimal ink formulation is achieved by adding 5 wt % ethylene glycol into a pristine PEDOT:PSS aqueous solution, which results in a sheet resistance of as low as 58 Ω/□. Elasticity can also be achieved by blending the above solution with the soft polymer poly(ethylene oxide) (PEO). Thin films of PEDOT:PSS/PEO polymer blends patterned by inkjet printing exhibits a low sheet resistance of 84 Ω/□ and can resist up to 50% tensile strain with minimal changes in electrical performance. With its good conductivity and elasticity, we have further demonstrated the use of the polymer blend as stretchable interconnects and stretchable dry electrodes on a thin polydimethylsiloxane (PDMS) substrate for photoplethysmography (PPG) and electrocardiography (ECG) recording applications. This work shows the potential of using a printed stretchable conducting polymer in low-cost wearable sensor patches for smart health applications.
Synaptic Transistors for Neurological E-skin Applications
Flexible CNT synaptic transistor for neurological E-skin
H. Wan, Y. Cao, LW. Lo, J. Zhao, N. Sepulveda, C. Wang. “Flexible carbon nanotube synaptic transistor for neurological electronic skin applications.” ACS Nano 14, no. 8 (2020): 10402-10412. Link
Abstract
There is an increasing interest in the development of memristive or artificial synaptic devices that emulate the neuronal activities for neuromorphic computing applications. While there have already been many reports on artificial synaptic transistors implemented on rigid substrates, the use of flexible devices could potentially enable an even broader range of applications. In this paper, we report artificial synaptic thin-film transistors built on an ultrathin flexible substrate using high carrier mobility semiconducting single-wall carbon nanotubes. The synaptic characteristics of the flexible synaptic transistor including long-term/short-term plasticity, spike-amplitude-dependent plasticity, spike-width-dependent plasticity, paired-pulse facilitation, and spike-time-dependent plasticity have all been systematically characterized. Furthermore, we have demonstrated a flexible neurological electronic skin and its peripheral nerve with a flexible ferroelectret nanogenerator (FENG) serving as the sensory mechanoreceptor that generates action potentials to be processed and transmitted by the artificial synapse. In such neurological electronic skin, the flexible FENG sensor converts the tactile input (magnitude and frequency of force) into presynaptic action potential pulses, which are then passed to the gate of the synaptic transistor to induce change in its postsynaptic current, mimicking the modulation of synaptic weight in a biological synapse. Our neurological electronic skin closely imitates the behavior of actual human skin, and it allows for instantaneous detection of force stimuli and offers biological synapse-like behavior to relay the stimulus signals to the next stage. The flexible sensory skin could potentially be used to interface with skeletal muscle fibers for applications in neuroprosthetic devices.
Multimodal artificial neurological sensory-memory system
H. Wan, J. Zhao, LW. Lo, Y. Cao, N. Sepúlveda, C. Wang. “Multimodal artificial neurological sensory–memory system based on flexible carbon nanotube synaptic transistor.” ACS Nano 15, no. 9 (2021): 14587-14597. Link
Abstract
As the initial stage in the formation of human intelligence, the sensory–memory system plays a critical role for human being to perceive, interact, and evolve with the environment. Electronic implementation of such biological sensory–memory system empowers the development of environment-interactive artificial intelligence (AI) that can learn and evolve with diversified external information, which could potentially broaden the application of the AI technology in the field of human–computer interaction. Here, we report a multimodal artificial sensory–memory system consisting of sensors for generating biomimetic visual, auditory, tactile inputs, and flexible carbon nanotube synaptic transistor that possesses synapse-like signal processing and memorizing behaviors. The transduction of physical signals into information-containing, presynaptic action potentials and the synaptic plasticity of the transistor in response to single and long-term action potential excitations have been systematically characterized. The bioreceptor-like sensing and synapse-like memorizing behaviors have also been demonstrated. On the basis of the memory and learning characteristics of the sensory–memory system, the well-known psychological model describing human memory, the “multistore memory” model, and the classical conditioning experiment that demonstrates the associative learning of brain, “Pavlov’s dog’s experiment”, have both been implemented electronically using actual physical input signals as the sources of the stimuli. The biomimetic intelligence demonstrated in this neurological sensory–memory system shows its potential in promoting the advancement in multimodal, user-environment interactive AI.