Photo-patternable and transparent films using cellulose nanofibers for stretchable origami electronics

Substantial progress in flexible or stretchable electronics over the past decade has extensively impacted various technologies such as wearable devices, displays and automotive electronics for smart cars. An important challenge is the reliability of these deformable devices under thermal stress. Different coefficients of thermal expansion (CTE) between plastic substrates and the device components, which include multiple inorganic layers of metals or ceramics, induce thermal stress in the devices during fabrication processes or long-term operations with repetitions of thermal cyclic loading-unloading, leading to device failure and reliability degradation. Here, we report an unconventional approach to form photo-patternable, transparent cellulose nanofiber (CNF) hybrid films as flexible and stretchable substrates to improve device reliability using simultaneous electrospinning and spraying. The electrospun polymeric backbones and sprayed CNF fillers enable the resulting hybrid structure to be photolithographically patternable as a negative photoresist and thermally and mechanically stable, presenting outstanding optical transparency and low CTE. We also formed stretchable origami substrates using the CNF hybrid that are composed of rigid support fixtures and elastomeric joints, exploiting the photo-patternability. A demonstration of transparent organic light-emitting diodes and touchscreen panels on the hybrid film suggests its potential for use in next-generation electronics.ope

Stretchable and transparent electrodes based on in-plane structures

Stretchable electronics has attracted great interest with compelling potential applications that require reliable operation under mechanical deformation. Achieving stretchability in devices, however, requires a deeper understanding of nanoscale materials and mechanics beyond the success of flexible electronics. In this regard, tremendous research efforts have been dedicated toward developing stretchable electrodes, which are one of the most important building blocks for stretchable electronics. Stretchable transparent thin-film electrodes, which retain their electrical conductivity and optical transparency under mechanical deformation, are particularly important for the favourable application of stretchable devices. This minireview summarizes recent advances in stretchable transparent thin-film electrodes, especially employing strategies based on in-plane structures. Various approaches using metal nanomaterials, carbon nanomaterials, and their hybrids are described in terms of preparation processes and their optoelectronic/mechanical properties. Some challenges and perspectives for further advances in stretchable transparent electrodes are also discussed. © 2015 The Royal Society of Chemistry.open0

Formation of Functional Nanofibers and Their Applications for Wearable Electronics

Department of Materials Science EngineeringThe electrospinning is facile method to fabricate nanofibers with high-throughput performance. However, this process is limited the use of materials such as polymer or polymer-mixed materials with low contents. To come close toward wearable, skin attachable, or injectable electronics, inorganic nanofibers are necessary for realization future electronics as main components, such as electrodes, semiconducting materials, or energy harvesting materials with high performances and mechanical properties. For this reason, we suggested the coaxial electrospinning process with inorganic nanoparticle inks as a core solution. The polymeric shell solution can be dragged out and guided core solution during the coaxial electrospinning process. After sintering process, polymeric parts can be removed various approaches and inorganic nanofibers can be obtained. First, we fabricated metal nanofibers such as silver nanofibers (AgNFs), copper nanofibers (CuNFs), and nickel nanofibers (NiNFs) using coaxial electrospinning. Combining with twodimensional graphene and random networks of electrospun silver nanofibers (AgNFs) has low sheet resistance (~4 Ω/sq), high total transmittance (93% in visible-ray), and superb mechanical properties. We are convinced that this hybrid nanostructures can be ubstituted ITO and occupied an important position in flexible electronics. For possibility for future electronics, we demonstrate wireless attachable sensor consisted AgNF-graphene on various surfaces. In addition, we demonstrated AgNF embedded structures. Specific applications, which are required relatively low surface roughness (below 10 nm), are limited due to metal NFs’ roughness. To overcome these issues, we demonstrate metal nanofiber embedded structures and organic light emitting device (OLED) is successfully demonstrated. Second, four different types of nanostructured carbon nanofibers (CNFs), plain, hollow, multichannel (MC), and hollowed MC, were fabricated using coaxial lectrospinning and thermal treatment for supercapacitor electrodes. The influence of the porosity on the specific surface area (SSA), pore volumes, and electrochemical propoerties of nanostructured CNFs were investigated. Also, their hybrid structures with multi-walled carbon nanotubes (MWCNTs) was analyzed in therms of their porosity, SSA, and electrochemical properties for supercapacitors (specific capacitance and long-term cycling). These hybrid structures can improve overall porosity and lectrochemical propoerties due to the extra mesoporous structures formed by entangling MWCNTs. In conclusion, these anostructured CNFs have a promising potential for various fields which need high porosity and SSA, and can be used as the platforms for catalysis, sensors, or energy devices. We demonstrate the nanofiber-based energy device and wearable devices, such as skinattachable gas sensor, flexible and transparent heater. Nanofiber is one of promising materials for future electronics and substantial progress towards producing wearable electronics.clos

Photo-Patternable and Transparent Films Using Cellulose Nanofibers for Stretchable, Origami Electronics

Substantial progress in flexible or stretchable electronics over the past decade has extensively impacted on various technologies such as wearable devices, displays, or automotive electronics for smart cars. An important challenge here is reliability of these deformable devices against thermal stress. Different coefficients of thermal expansion (CTE) between plastic substrates and the device components which include multiple inorganic layers of metals or ceramics induce thermal stress to the devices during fabrication processes or long-term operations with repetitions of thermal cyclic loading-unloading, lead to device failure and degrade their reliability. Here we report an unconventional approach to form photo-patternable, transparent cellulose nanofiber (CNF) hybrid films as flexible and stretchable substrates toward reliable devices, using simultaneous electrospinning and spraying. The electrospun polymeric backbones and sprayed cellulose nanofiber fillers enable the resulting hybrid structure to be patternable photolithographically as a negative photoresist, and stable thermally and mechanically, with presenting outstanding optical transparency (~ 89 %) and low CTE (< 10 ppm/K). We also formed stretchable, origami substrates using the CNF hybrid, which are composed of rigid support fixtures and elastomeric joints, exploiting the photo-patternability. Demonstrations of transparent organic light-emitting diodes and touch-screen panels on the hybrid film suggest a promise for next generation electronics

A Novel Vital-Sign Sensing Algorithm for Multiple Subjects Based on 24-GHz FMCW Doppler Radar

A novel non-contact vital-sign sensing algorithm for use in cases of multiple subjects is proposed. The approach uses a 24 GHz frequency-modulated continuous-wave Doppler radar with the parametric spectral estimation method. Doppler processing and spectral estimation are concurrently implemented to detect vital signs from more than one subject, revealing excellent results. The parametric spectral estimation method is utilized to clearly identify multiple targets, making it possible to distinguish multiple targets located less than 40 cm apart, which is beyond the limit of the theoretical range resolution. Fourier transformation is used to extract phase information, and the result is combined with the spectral estimation result. To eliminate mutual interference, the range integration is performed when combining the range and phase information. By considering breathing and heartbeat periodicity, the proposed algorithm can accurately extract vital signs in real time by applying an auto-regressive algorithm. The capability of a contactless and unobtrusive vital sign measurement with a millimeter wave radar system has innumerable applications, such as remote patient monitoring, emergency surveillance, and personal health care

Recent Advances in Transparent Electronics with Stretchable Forms

Advances in materials science and the desire for next-generation electronics have driven the development of stretchable and transparent electronics in the past decade. Novel applications, such as smart contact lenses and wearable sensors, have been introduced with stretchable and transparent form factors, requiring a deeper and wider exploration of materials and fabrication processes. In this regard, many research efforts have been dedicated to the development of mechanically stretchable, optically transparent materials and devices. Recent advances in stretchable and transparent electronics are discussed herein, with special emphasis on the development of stretchable and transparent materials, including substrates and electrodes. Several representative examples of applications enabled by stretchable and transparent electronics are presented, including sensors, smart contact lenses, heaters, and neural interfaces. The current challenges and opportunities for each type of stretchable and transparent electronics are also discussed

Nanomaterial-based stretchable and transparent electrodes

The recent advent of unprecedented wearable applications engendered the need for stretchable electronics, which can be realized by making the individual components stretchable. The transparent conducting electrode is one of the most important components of optoelectronic devices. Therefore, developing transparent electrodes in a stretchable form is essential for the implementation of stretchable electronics. In this paper, the recent efforts in the development of stretchable and transparent electrodes, particularly those using nanomaterials such as metal nanowires, metal nanofibers, and carbon nanotubes are introduced.close