Nanogenerators capable of converting energy from mechanical sources to electricity with high effective efficiency using low-cost, nonsemiconducting, organic nanomaterials are attractive for many applications, including energy harvesters. In this work, near-field electrospinning is used to direct-write poly(vinylidene fluoride) (PVDF) nanofibers with in situ mechanical stretch and electrical poling characteristics to produce piezoelectric properties. Under mechanical stretching, nanogenerators have shown repeatable and consistent electrical outputs with energy conversion efficiency an order of magnitude higher than those made of PVDF thin films. The early onset of the nonlinear domain wall motions behavior has been identified as one mechanism responsible for the apparent high piezoelectricity in nanofibers, rendering them potentially advantageous for sensing and actuation applications.

A near-field electrospinning (NFES) process has been developed to deposit solid nanofibers in a direct, continuous, and controllable manner. A tungsten electrode with tip diameter of 25 µm is used to construct nanofibers of 50−500 nm line width on silicon-based collectors while the liquid polymer solution is supplied in a manner analogous to that of a dip pen. The minimum applied bias voltage is 600 V, and minimum electrode-to-collector distance is 500 µm to achieve position controllable deposition. Charged nanofibers can be orderly collected, making NFES a potential tool in direct write nanofabrication for a variety of materials.Electrically driven liquid jets and the stability of electrically charged droplets have been studied for hundreds of years, 1,2 while the practical apparatus of electrospinning, in which a charged jet of polymer solution is deposited onto a collector under the influence of an electrical field, dated back in 1934. 3 The feasibility to construct long and continuous polymeric, 4-6 ceramic, 7 and composite 8 nanofibers as well as nanotubes 9 with diameters less than 100 nm has been demonstrated using electrospinning. Typical applications include bioscaffolding, 10 wound dressing, 11 and filtrations 12 to name a few. Researchers have further explored the possibilities of using electrospun nanofibers in fabricating micro-and nanodevices such as field effect transistors, 13 gas 14 and optical sensors, 15 and deposition of DNA on functional chips. 16 In these and other applications, the controllability of the electrospinning process is critical. Unfortunately, current setup of electrospinning is unstable in nature as it relies on the chaotic whipping of liquid jets to generate nanofibers. Limited works toward the control of electrospinning have emerged, including aligning nanofibers by electrical field 17 and using rotational mechanical mandrels. 18,19 Furthermore, numerous investigations by means of analytical and experimental methodologies have been conducted to study the fundamental physics and chemistry of electrospinning for further improvement and control, such as the effects of polymer solution concentration, applied voltage, and electrode-to-collector distance. 4,[20][21][22] Here we report experiments of controllable electrospinning based on a new type of "near-field" electrospinning (NFES). Figure 1A illustrates the schematic setup of NFES that merges several disparate concepts. First, the electrode-to-collector distance, h, is in the range of 500 µm to 3 mm to utilize the stable liquid jets region for controllable deposition. Second, a solid tungsten spinneret of 25 µm tip diameter as illustrated in Figure 1B is used in NFES to achieve nanofibers with sub-100-nm resolution. Third, the applied electrostatic voltage is reduced due to the short electrode-to-collector distance while the electrical field in the tip region maintains the strength in the range of 10 7 V/m as those used in conventional electrospinning to activate the process. Fourth, discrete droplets of polymer solu...

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A continuous near-field electrospinning (NFES) process has been developed to deposit solid nanofibers with orderly patterns over large areas. Before the onset of electrospinning, a bias voltage is applied to a semispherical shaped polymer droplet outside of a syringe needle, and a probe tip mechanically draws a single fiber from the droplet to initiate continuous NFES. Contrary to the conventional electrospinning process, we show that decreasing electrical field in continuous NFES results in smaller linewidth deposition, and nanofibers can be assembled into controlled complex patterns such as circular shapes and grid arrays on large and flat areas.

Although diffractive optics have played a major role in nanoscale soft X-ray imaging, highresolution and high-efficiency diffractive optics have largely been unavailable for hard X-rays where many scientific, technological and biomedical applications exist. This is owing to the long-standing challenge of fabricating ultra-high aspect ratio high-resolution dense nanostructures. Here we report significant progress in ultra-high aspect ratio nanofabrication of high-resolution, dense silicon nanostructures using vertical directionality controlled metalassisted chemical etching. The resulting structures have very smooth sidewalls and can be used to pattern arbitrary features, not limited to linear or circular. We focus on the application of X-ray zone plate fabrication for high-efficiency, high-resolution diffractive optics, and demonstrate the process with linear, circular, and spiral zone plates. X-ray measurements demonstrate high efficiency in the critical outer layers. This method has broad applications including patterning for thermoelectric materials, battery anodes and sensors among others.

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CONTEXT: There is an increasing concern about chronic low-level pesticide exposure during childhood and its influence on childhood cancers. OBJECTIVE:In this meta-analysis, we aimed to examine associations between residential childhood pesticide exposures and childhood cancers. DATA SOURCES:We searched all observational studies published in PubMed before February 2014 and reviewed reference sections of articles derived from searches.STUDY SELECTION: The literature search yielded 277 studies that met inclusion criteria.DATA EXTRACTION: Sixteen studies were included in the meta-analysis. We calculated effect sizes and 95% confidence intervals (CIs) by using a random effect model with inverse variance weights. RESULTS:We found that childhood exposure to indoor but not outdoor residential insecticides was associated with a significant increase in risk of childhood leukemia (odds ratio [OR] = 1.47; 95% CI, 1.26-1.72; I 2 = 30%) and childhood lymphomas (OR = 1.43; 95% CI, 1.15-1.78; I 2 = 0%). A significant increase in risk of leukemia was also associated with herbicide exposure (OR = 1.26; 95% CI, 1.10-1.44; I 2 = 0%). Also observed was a positive but not statistically significant association between childhood home pesticide or herbicide exposure and childhood brain tumors. LIMITATIONS:The small number of studies included in the analysis represents a major limitation of the current analysis. CONCLUSIONS:Results from this meta-analysis indicated that children exposed to indoor insecticides would have a higher risk of childhood hematopoietic cancers. Additional research is needed to confirm the association between residential indoor pesticide exposures and childhood cancers. Meanwhile, preventive measures should be considered to reduce children's exposure to pesticides at home.

Neonicotinoids have become the most widely used insecticides in the world since introduced in the mid 1990s, yet the extent of human exposure and health impacts is not fully understood. In this study, the residues were analyzed of seven neonicotinoids in fruit and vegetable samples collected from two cross-sectional studies: the U.S. Congressional Cafeteria study (USCC) and the Hangzhou China (HZC) study. We then employed a relative potency factor method to integrate all neonicotinoids in each food sample using the respective reference dose values as the basis for summation. The findings were compared with data published by the U.S. Department of Agriculture Pesticide Data Program (USDA/PDP). Imidacloprid and thiamethoxam were the most commonly detected neonicotinoids in fruits and vegetables with 66 and 51% detection in the HZC study and 52 and 53% detection in the USCC study, respectively. The overall frequency of detection for neonicotinoids in the USDA/PDP samples was much lower than those reported here for the USCC or HZC studies, with imidacloprid being the most frequently detected neonicotinoid at 7.3%. The high frequencies of neonicotinoid detection in fruits and vegetables in the USCC and HZC studies give us a snapshot of the ubiquity of neonicotinoid use in global agriculture and make it clear that neonicotinoids have become part of the dietary staple, with possible health implications for individuals.