Fabrication of practical deformable displays: advances and challenges.

Display form factors such as size and shape have been conventionally determined in consideration of usability and portability. The recent trends requiring wearability and convergence of various smart devices demand innovations in display form factors to realize deformability and large screens. Expandable displays that are foldable, multi-foldable, slidable, or rollable have been commercialized or on the edge of product launches. Beyond such two-dimensional (2D) expansion of displays, efforts have been made to develop three dimensional (3D) free-form displays that can be stretched and crumpled for use in realistic tactile sensation, artificial skin for robots, and on-skin or implantable displays. This review article analyzes the current state of the 2D and 3D deformable displays and discusses the technological challenges to be achieved for industrial commercialization.

A wireless closed-loop system for optogenetic peripheral neuromodulation.

The fast-growing field of bioelectronic medicine aims to develop engineered systems that can relieve clinical conditions by stimulating the peripheral nervous system1-5. This type of technology relies largely on electrical stimulation to provide neuromodulation of organ function or pain. One example is sacral nerve stimulation to treat overactive bladder, urinary incontinence and interstitial cystitis (also known as bladder pain syndrome)4,6,7. Conventional, continuous stimulation protocols, however, can cause discomfort and pain, particularly when treating symptoms that can be intermittent (for example, sudden urinary urgency)8. Direct physical coupling of electrodes to the nerve can lead to injury and inflammation9-11. Furthermore, typical therapeutic stimulators target large nerve bundles that innervate multiple structures, resulting in a lack of organ specificity. Here we introduce a miniaturized bio-optoelectronic implant that avoids these limitations by using (1) an optical stimulation interface that exploits microscale inorganic light-emitting diodes to activate opsins; (2) a soft, high-precision biophysical sensor system that allows continuous measurements of organ function; and (3) a control module and data analytics approach that enables coordinated, closed-loop operation of the system to eliminate pathological behaviours as they occur in real-time. In the example reported here, a soft strain gauge yields real-time information on bladder function in a rat model. Data algorithms identify pathological behaviour, and automated, closed-loop optogenetic neuromodulation of bladder sensory afferents normalizes bladder function. This all-optical scheme for neuromodulation offers chronic stability and the potential to stimulate specific cell types.

Optogenetic silencing of nociceptive primary afferents reduces evoked and ongoing bladder pain.

Patients with interstitial cystitis/bladder pain syndrome (IC/BPS) suffer from chronic pain that severely affects quality of life. Although the underlying pathophysiology is not well understood, inhibition of bladder sensory afferents temporarily relieves pain. Here, we explored the possibility that optogenetic inhibition of nociceptive sensory afferents could be used to modulate bladder pain. The light-activated inhibitory proton pump Archaerhodopsin (Arch) was expressed under control of the sensory neuron-specific sodium channel (sns) gene to selectively silence these neurons. Optically silencing nociceptive sensory afferents significantly blunted the evoked visceromotor response to bladder distension and led to small but significant changes in bladder function. To study of the role of nociceptive sensory afferents in freely behaving mice, we developed a fully implantable, flexible, wirelessly powered optoelectronic system for the long-term manipulation of bladder afferent expressed opsins. We found that optogenetic inhibition of nociceptive sensory afferents reduced both ongoing pain and evoked cutaneous hypersensitivity in the context of cystitis, but had no effect in uninjured, naïve mice. These results suggest that selective optogenetic silencing of nociceptive bladder afferents may represent a potential future therapeutic strategy for the treatment of bladder pain.

Fully implantable, battery-free wireless optoelectronic devices for spinal optogenetics.

The advent of optogenetic tools has allowed unprecedented insights into the organization of neuronal networks. Although recently developed technologies have enabled implementation of optogenetics for studies of brain function in freely moving, untethered animals, wireless powering and device durability pose challenges in studies of spinal cord circuits where dynamic, multidimensional motions against hard and soft surrounding tissues can lead to device degradation. We demonstrate here a fully implantable optoelectronic device powered by near-field wireless communication technology, with a thin and flexible open architecture that provides excellent mechanical durability, robust sealing against biofluid penetration and fidelity in wireless activation, thereby allowing for long-term optical stimulation of the spinal cord without constraint on the natural behaviors of the animals. The system consists of a double-layer, rectangular-shaped magnetic coil antenna connected to a microscale inorganic light-emitting diode (µ-ILED) on a thin, flexible probe that can be implanted just above the dura of the mouse spinal cord for effective stimulation of light-sensitive proteins expressed in neurons in the dorsal horn. Wireless optogenetic activation of TRPV1-ChR2 afferents with spinal µ-ILEDs causes nocifensive behaviors and robust real-time place aversion with sustained operation in animals over periods of several weeks to months. The relatively low-cost electronics required for control of the systems, together with the biocompatibility and robust operation of these devices will allow broad application of optogenetics in future studies of spinal circuits, as well as various peripheral targets, in awake, freely moving and untethered animals, where existing approaches have limited utility.

Flexible Near-Field Wireless Optoelectronics as Subdermal Implants for Broad Applications in Optogenetics.

In vivo optogenetics provides unique, powerful capabilities in the dissection of neural circuits implicated in neuropsychiatric disorders. Conventional hardware for such studies, however, physically tethers the experimental animal to an external light source, limiting the range of possible experiments. Emerging wireless options offer important capabilities that avoid some of these limitations, but the current size, bulk, weight, and wireless area of coverage is often disadvantageous. Here, we present a simple but powerful setup based on wireless, near-field power transfer and miniaturized, thin, flexible optoelectronic implants, for complete optical control in a variety of behavioral paradigms. The devices combine subdermal magnetic coil antennas connected to microscale, injectable light-emitting diodes (LEDs), with the ability to operate at wavelengths ranging from UV to blue, green-yellow, and red. An external loop antenna allows robust, straightforward application in a multitude of behavioral apparatuses. The result is a readily mass-producible, user-friendly technology with broad potential for optogenetics applications.

Encapsulated, High-Performance, Stretchable Array of Stacked Planar Micro-Supercapacitors as Waterproof Wearable Energy Storage Devices.

We report the fabrication of an encapsulated, high-performance, stretchable array of stacked planar micro-supercapacitors (MSCs) as a wearable energy storage device for waterproof applications. A pair of planar all-solid-state MSCs with spray-coated multiwalled carbon nanotube electrodes and a drop-cast UV-patternable ion-gel electrolyte was fabricated on a polyethylene terephthalate film using serial connection to increase the operation voltage of the MSC. Additionally, multiple MSCs could be vertically stacked with parallel connections to increase both the total capacitance and the areal capacitance owing to the use of a solid-state patterned electrolyte. The overall device of five parallel-connected stacked MSCs, a microlight-emitting diode (µ-LED), and a switch was encapsulated in thin Ecoflex film so that the capacitance remained at 82% of its initial value even after 4 d in water; the µ-LED was lit without noticeable decrease in brightness under deformation including bending and stretching. Furthermore, an Ecoflex encapsulated oximeter wound around a finger was operated using the stored energy of the MSC array attached to the hand (even in water) to give information on arterial pulse rate and oxygen saturation in the blood. This study suggests potential applications of our encapsulated MSC array in wearable energy storage devices especially in water.

Soft, stretchable, fully implantable miniaturized optoelectronic systems for wireless optogenetics.

Optogenetics allows rapid, temporally specific control of neuronal activity by targeted expression and activation of light-sensitive proteins. Implementation typically requires remote light sources and fiber-optic delivery schemes that impose considerable physical constraints on natural behaviors. In this report we bypass these limitations using technologies that combine thin, mechanically soft neural interfaces with fully implantable, stretchable wireless radio power and control systems. The resulting devices achieve optogenetic modulation of the spinal cord and peripheral nervous system. This is demonstrated with two form factors; stretchable film appliqués that interface directly with peripheral nerves, and flexible filaments that insert into the narrow confines of the spinal epidural space. These soft, thin devices are minimally invasive, and histological tests suggest they can be used in chronic studies. We demonstrate the power of this technology by modulating peripheral and spinal pain circuitry, providing evidence for the potential widespread use of these devices in research and future clinical applications of optogenetics outside the brain.

Biaxially stretchable, integrated array of high performance microsupercapacitors.

We report on the fabrication of a biaxially stretchable array of high performance microsupercapacitors (MSCs) on a deformable substrate. The deformable substrate is designed to suppress local strain applied to active devices by locally implanting pieces of stiff polyethylene terephthalate (PET) films within the soft elastomer of Ecoflex. A strain suppressed region is formed on the top surface of the deformable substrate, below which PET films are implanted. Active devices placed within this region can be isolated from the strain. Analysis of strain distribution by finite element method confirms that the maximum strain applied to MSC in the strain suppressed region is smaller than 0.02%, while that on the Ecoflex film is larger than 250% under both uniaxial strain of 70% and biaxial strain of 50%. The all-solid-state planar MSCs, fabricated with layer-by-layer deposited multiwalled carbon nanotube electrodes and patterned ionogel electrolyte of poly(ethylene glycol) diacrylate and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide having high-potential windows, are dry-transferred onto the deformable substrate and electrically connected in series and parallel via embedded liquid metal interconnection and Ag nanowire contacts. Liquid metal interconnection, formed by injecting liquid metal into the microchannel embedded within the substrate, can endure severe strains and requires no additional encapsulation process. This formed MSC array exhibits high energy and power density of 25 mWh/cm(3) and 32 W/cm(3), and stable electrochemical performance up to 100% uniaxial and 50% biaxial stretching. The high output voltage of the MSC array is used to light micro-light-emitting diode (µ-LED) arrays, even under strain conditions. This work demonstrates the potential application of our stretchable MSC arrays to wearable and bioimplantable electronics with a self-powered system.

High-density, stretchable, all-solid-state microsupercapacitor arrays.

We report on the successful fabrication of stretchable microsupercapacitor (MSC) arrays on a deformable polymer substrate that exhibits high electrochemical performance even under mechanical deformation such as bending, twisting, and uniaxial strain of up to 40%. We designed the deformable substrate to minimize the strain on MSCs by adopting a heterogeneous structure consisting of stiff PDMS islands (on which MSCs are attached) and a soft thin film (mixture of Ecoflex and PDMS) between neighboring PDMS islands. Finite element method analysis of strain distribution showed that an almost negligible strain of 0.47% existed on the PDMS islands but a concentrated strain of 107% was present on the soft thin film area under a uniaxial strain of 40%. The use of an embedded interconnection of the liquid metal Galinstan helped simplify the fabrication and provided mechanical stability under deformation. Furthermore, double-sided integration of MSCs increased the capacitance to twice that of MSCs on a conventional planar deformable substrate. In this study, planar-type MSCs with layer-by-layer assembled hybrid thin film electrodes of MWNT/Mn3O4 and PVA-H3PO4 electrolyte were fabricated; when they are integrated into a circuit, these MSCs increase the output voltage beyond the potential of the electrolyte used. Therefore, various LEDs that require high voltages can be operated under a high uniaxial strain of 40% without any decrease in their brightness. The results obtained in this study demonstrate the high potential of our stretchable MSC arrays for their application as embedded stretchable energy storage devices in bioimplantable and future wearable nanoelectronics.

Design and fabrication of novel stretchable device arrays on a deformable polymer substrate with embedded liquid-metal interconnections.

Stretchable devices are fabricated on a newly designed deformable substrate. Active devices attached on the stiff islands are electrically connected by an embedded EGaIn interconnection, which ensures protection from external damage. In this structure, the local strain in the active device area is estimated to be less than 1% under applied strain of 30% by analysis of the strain distribution using the finite element method.

Fabrication of stretchable single-walled carbon nanotube logic devices.

The fabrication of a stretchable single-walled carbon nanotube (SWCNT) complementary metal oxide semiconductor (CMOS) inverter array and ring oscillators is reported. The SWCNT CMOS inverter exhibits static voltage transfer characteristics with a maximum gain of 8.9 at a supply voltage of 5 V. The fabricated devices show stable electrical performance under the maximum strain of 30% via forming wavy configurations. In addition, the 3-stage ring oscillator demonstrates a stable oscillator frequency of ∼3.5 kHz at a supply voltage of 10 V and the oscillating waveforms are maintained without any distortion under cycles of pre-strain and release. The strains applied to the device upon deformation are also analyzed by using the classical lamination theory, estimating the local strain of less than 0.6% in the SWCNT channel and Pd electrode regions which is small enough to keep the device performance stable under the pre-strain up to 30%. This work demonstrates the potential application of stretchable SWCNT logic circuit devices in future wearable electronics.

Controlling the electronic properties of SWCNT FETs via modification of the substrate surface prior to atomic layer deposition of 10 nm thick Al2O3 film.

We demonstrate the controllability of the electronic transport properties of single-walled carbon nanotube (SWCNT) field effect transistors (FETs) via the use of 10 nm thick atomic-layer-deposited aluminum oxide (Al2O3) gate dielectric films, where the substrate surfaces were modified with differently functionalized self-assembled monolayers (SAMs) prior to their growth, namely SAMs with hydrophobic (-CH3) or hydrophilic (-OH) groups. Al2O3 grown on a hydrophilic surface causes the SWCNT FETs to keep their intrinsic p-type transfer characteristics by alleviating the electron-doping effect originating from defects in the Al2O3 film. However, the SAM with methyl groups increases the defect density of the Al2O3 film, enhancing the n-type transfer characteristics and inducing ambipolar to n-type behavior in the SWCNT FETs. In this work, we find clues about the distribution of charged defects in the Al2O3 film, which strongly influences the transfer characteristics of the SWCNT FETs, by measuring the thickness-dependent flat band voltages.

High performance stretchable UV sensor arrays of SnO2 nanowires.

A high performance, stretchable UV sensor array was fabricated based on an active matrix (AM) device that combined field effect transistors of SWCNTs and SnO2 nanowires. The AM devices provided spatial UV sensing via the individual sensors in the array. SnO2 NW UV sensors showed an average photosensitivity of ∼10(5) and a photoconductive gain of ∼10(6) under very low UV (λ = 254 nm) power intensities of 0.02-0.04 mW cm(-2). The UV sensing performance was not deteriorated by a prestrain of up to 23% induced by radial deformation, consistent with the mechanical analysis.

p-n hetero-junction diode arrays of p-type single walled carbon nanotubes and aligned n-type SnO2 nanowires.

p-n hetero-junction diode arrays were fabricated using specific direct techniques for the transfer of p-type single walled carbon nanotubes (SWCNTs) and aligned n-type SnO2 nanowires (NWs) onto a patterned substrate surface. Their electronic and optoelectronic properties were characterized. Perpendicular crossings of the p- and the n-channels with each other were confirmed by transfer characteristics with respect to the bottom gate. The resulting diode showed a good rectifying behavior with a rectification ratio of over 10² at ±5 V, where the equivalent circuit model of a serially connected diode and resistor was used for analysis of the electrical properties. Both the forward and the reverse currents were observed to increase with the application of a positive gate bias, indicating an n-type gate dependence. Under a forward bias, the dominant contribution of the SnO2 NW channel to the total resistance of the equivalent model is attributed to the n-type gate dependence since the resistance of the n-channel increased with a negative gate bias, resulting in the decrease of the forward current. Under a reverse bias, positive gate increased the concentration of valence electrons in the SWCNTs, enhancing direct tunneling to the conduction band of the SnO2 NWs. High sensitivity to UV irradiation under the reverse bias was also demonstrated with a photosensitivity over 10², suggesting potential applicability of the hetero-junction diodes in optoelectronic devices.

p-n homo-junction arrays of aligned single walled carbon nanotubes fabricated by selective patterning of polyethyleneimine film.

We describe the fabrication and electrical performance of p-n homo-junction diode arrays of horizontally aligned single walled carbon nanotubes (SWCNTs). Horizontally aligned SWCNTs grown on stable temperature-cut quartz with a density of ∼ 6 SWCNTs µm(-1) were transferred onto a SiO(2)/Si substrate. After the electrical breakdown, aligned SWCNT field effect transistors (FETs) showed unipolar p-type characteristics with a large current on/off ratio of 10(6) at 1 V and a hole mobility per tube of 1500 cm(2) V(-1) s(-1). Spin-coating of polyethyleneimine (PEI) onto p-type SWCNT FETs showed the n-type transfer characteristics. Patterning of spin-coated PEI film enabled the fabrication of p-n homo-junction arrays of aligned SWCNTs in an easy way, where the rectifying behavior was observed with a rectification ratio of ∼ 10(4) at ± 2 V. A comparative study with a p-n homo-junction of random networks of SWCNTs confirmed the advantage of aligned SWCNTs for applications in high performance electronic devices.