Patterning of Metal Nanowire Networks: Methods and Applications.

With the advance in flexible and stretchable electronics, one-dimensional nanomaterials such as metal nanowires have drawn much attention in the past 10 years or so. Metal nanowires, especially silver nanowires, have been recognized as promising candidate materials for flexible and stretchable electronics. Owing to their high electrical conductivity and high aspect ratio, metal nanowires can form electrical percolation networks, maintaining high electrical conductivity under deformation (e.g., bending and stretching). Apart from coating metal nanowires for making large-area transparent conductive films, many applications require patterned metal nanowires as electrodes and interconnects. Precise patterning of metal nanowire networks is crucial to achieve high device performances. Therefore, a high-resolution, designable, and scalable patterning of metal nanowire networks is important but remains a critical challenge for fabricating high-performance electronic devices. This review summarizes recent advances in patterning of metal nanowire networks, using subtractive methods, additive methods of nanowire dispersions, and printing methods. Representative device applications of the patterned metal nanowire networks are presented. Finally, challenges and important directions in the area of the patterning of metal nanowire networks for device applications are discussed.

Nanomaterial-Enabled Flexible and Stretchable Sensing Systems: Processing, Integration, and Applications.

Nanomaterial-enabled flexible and stretchable electronics have seen tremendous progress in recent years, evolving from single sensors to integrated sensing systems. Compared with nanomaterial-enabled sensors with a single function, integration of multiple sensors is conducive to comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in robotics and prosthetics. Integration of sensors with other functional components promotes real-world applications of the sensing systems. Here, an overview of the design and integration strategies and manufacturing techniques for such sensing systems is given. Then, representative nanomaterial-enabled flexible and stretchable sensing systems are presented. Following that, representative applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions are provided. To conclude, perspectives on the challenges and opportunities in this burgeoning field are considered.

Gravure Printing of Water-based Silver Nanowire ink on Plastic Substrate for Flexible Electronics.

Gravure printing is a promising technique for large-scale printed electronics. However, gravure printing of silver nanowires (AgNWs) so far has been limited in terms of resolution and electrical conductivity. In this study, gravure printing of water-based AgNW ink on a flexible substrate is demonstrated. By tailoring the ink properties, printing conditions and post-printing treatment, gravure printing enables printing of high-resolution, highly conductive AgNW patterns in large areas, with resolution as fine as 50 µm and conductivity as high as 5.34 × 104 S cm-1. The printed AgNW patterns on the flexible substrate show excellent flexibility under repeated bending. All these characteristics demonstrate the excellent potential of gravure printing of AgNWs for developing large-area flexible electronics.

Electrohydrodynamic printing of silver nanowires for flexible and stretchable electronics.

A silver nanowire (AgNW) based conductor is a promising component for flexible and stretchable electronics. A wide range of flexible/stretchable devices using AgNW conductors has been demonstrated recently. High-resolution, high-throughput printing of AgNWs remains a critical challenge. Electrohydrodynamic (EHD) printing has been developed as a promising technique to print different materials on a variety of substrates with high resolution. Here, AgNW ink was developed for EHD printing. The printed features can be controlled by several parameters including AgNW concentration, ink viscosity, printing speed, stand-off distance, etc. With this method, AgNW patterns can be printed on a range of substrates, e.g. paper, polyethylene terephthalate (PET), glass, polydimethylsiloxane (PDMS), etc. First, AgNW samples on PDMS were characterized under bending and stretching. Then AgNW heaters and electrocardiogram (ECG) electrodes were fabricated to demonstrate the potential of this printing technique for AgNW-based flexible and stretchable devices.

Controlling the self-folding of a polymer sheet using a local heater: the effect of the polymer-heater interface.

Self-folding of a pre-strained shape memory polymer (SMP) sheet was demonstrated using local joule heating. Folding is caused by shrinkage variation across the thickness of the SMP sheet. The folding direction can be controlled by the interfacial interaction between the heater and the SMP sheet. When the heater is placed on the SMP sheet with no constraint (weak interface), the SMP sheet folds toward the heater. Temperature gradient across the SMP thickness gives rise to the shrinkage variation. By contrast, when the heater is fixed to the SMP sheet (strong interface), the SMP sheet can fold away from the heater. In this case shrinkage variation is dictated by the constraining effect of the heater. In either mode, 180 degrees folding can be achieved. The folding angle can be controlled by varying the heater width and folding time. This method is simple and can be used to fold structures with sharp angles in a sequential manner. A variety of structures were folded as demonstrations, including digital numbers 0-9, a cube, a boat, and a crane.

Highly thermostable, flexible, transparent, and conductive films on polyimide substrate with an AZO/AgNW/AZO structure.

Flexible transparent conductive films (TCFs) are used in a variety of optoelectronic devices. However, their use is limited due to poor thermostability. We report hybrid TCFs incorporation in both aluminum-doped zinc oxide (AZO) and silver nanowires (AgNWs). The layered AZO/AgNWs/AZO structure was deposited onto a transparent polyimide (PI) substrate and displayed excellent thermostability. When heated to 250 °C for 1 h, the change in resistivity (Rc) was less than 10% (Rc of pure AgNW film > 500) while retaining good photoelectric properties (Rsh = 8.6 Ohm/sq and T = 74.4%). Layering the AgNW network between AZO films decreased the surface roughness (Rrms < 8 nm) and enhances the mechanical flexibility of the hybrid films. The combination of these characteristics makes the hybrid film an excellent candidate for substrates of novel flexible optoelectronic devices which require high-temperature processing.

Preparation of solid silver nanoparticles for inkjet printed flexible electronics with high conductivity.

Silver nanoparticles (NPs) which could be kept in solid form and were easily stored without degeneration or oxidation at room temperature for a long period of time were synthesized by a simple and environmentally friendly wet chemistry method in an aqueous phase. Highly stable dispersions of aqueous silver NP inks, sintered at room temperature, for printing highly conductive tracks (∼8.0 µΩ cm) were prepared simply by dispersing the synthesized silver NP powder in water. These inks are stable, fairly homogeneous and suitable for a wide range of patterning techniques. The inks were successfully printed on paper and polyethylene terephthalate (PET) substrates using a common color printer. Upon annealing at 180 °C, the resistivity of the printed silver patterns decreased to 3.7 µΩ cm, which is close to twice that of bulk silver. Various factors affecting the resistivity of the printed silver patterns, such as annealing temperature and the number of printing cycles, were investigated. The resulting high conductivity of the printed silver patterns reached over 20% of the bulk silver value under ambient conditions, which enabled the fabrication of flexible electronic devices, as demonstrated by the inkjet printing of conductive circuits of LED devices.

[Changes of heat-shock protein 70 in the brain of rabbits with craniocerebral missile injury in hot and humid environment].

OBJECTIVE: To study the changes of heat shock protein 70 (HSP70) in the brain of rabbits with craniocerebral missile injury (CMI) and their impact on the injury in a hot and humid environment (HHE). METHODS: Twenty-four New Zealand white rabbits were randomized into 3 equal groups, namely normal temperature (NT) control group (subjected to treatment at temperature of 22.0+/-0.5 degrees C with relative humidity of 50%), NT gunshot group with CMI and HHE gunshot group with CMI. The rabbits in the latter two groups were subjected to normal temperature of 22.0+/-0.5 degrees C with relative humidity of 50% and environmental temperature of 39.0+/-0.5 degrees C with relative humidity of 80% to 85%, respectively, after establishment of CMI models. HSP70 in the brain tissues and lymphocytes was detected by Western blotting and visualized using chemoluminescence and X-ray films, followed by quantitative gel image analysis. RESULTS: Expressions of HSP70 in the cortex, hypothalamus and lymphocytes increased apparently in HHE gunshot group, and the changes of HSP70 in the lymphocytes were consistent with those in the cerebral cortex and hypothalamus. CONCLUSION: HSP70 expression in the brain tissues and the lymphocytes increase evidently after MCI in HHE.

[Changes of brain p38 MAPK in rabbits with craniocerebral gunshot injury in hot and humid environment].

OBJECTIVE: To study the activation of p38 mitogen-activated protein kinase (MAPK) in the brain of rabbits after craniocerebral gunshot injury in a hot and humid environment (HHE) and explore its possible mechanism. METHODS: Craniocerebral gunshot injury model was established in 30 New Zealand white rabbits, which were subsequently exposed to environment of normal temperature (at 22.0% +/- 0.5 degrees C; with relative humidity of 50%) or HHE at 39.0 +/- 0.5 degrees C; with relative humidity of 80%-85% for 10 min, 30 min, 1 h, 1.5 h, and 2 h groups, respectively, with 5 rabbits in each group. p38 MAPK activity in the brain tissues of the rabbits following the injury and environmental exposure were detected by Western blotting and analyzed semi-quantitatively by Bio-Profil gel image analysis system. RESULTS: p38 MAPK activity in the cortex and hypothalamus was significantly elevated following gunshot injury and HHE exposure, reaching the peak level at 1 h of HHE exposure and then decreased. p38 MAPK activity was significantly higher in the hypothalamus than in the cortex. CONCLUSION: p38 MAPK activity increases in the early stage following craniocerebral gunshot injury and HHE exposure in rabbits, the mechanism of which might involve the secondary brain insult.

[Effect of hyperthermia on tight junctions between endothelial cells of the blood-brain barrier model in vitro].

OBJECTIVE: To investigate the effect of hyperthermia in vitro on tight junctions between the endothelial cells of the blood-brain barrier. METHODS: An in vitro blood-brain barrier model was established by coculture of ECV304 cells with astrocytes. Transendothelial resistance (TER) of in vitro blood-brain barrier was determined by Millicell-ERS system, and the morphological change of tight junctions examined by silver staining. The expression level of zonula occluden-1(ZO-1) was analyzed by semi-quantitative reverse transcriptase-PCR. RESULTS: The integrity of tight junctions was disrupted and the expression level of ZO-1 decreased after treatment with hyperthermia. CONCLUSIONS: Tight junctions between the endothelial cells of the blood-brain barrier can be destroyed by hyperthermia, and decreased expression level of ZO-1 induced by hyperthermia is one of the most important molecular mechanisms.