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排序方式: 共有311条查询结果,搜索用时 15 毫秒
51.
Nanostructures of different sizes, shapes and material properties have many applications in biomedical imaging, clinical diagnostics and therapeutics. In spite of what has been achieved so far, a complete understanding of how cells interact with nanostructures of well-defined sizes, at the molecular level, remains poorly understood. Here we show that gold and silver nanoparticles coated with antibodies can regulate the process of membrane receptor internalization. The binding and activation of membrane receptors and subsequent protein expression strongly depend on nanoparticle size. Although all nanoparticles within the 2-100 nm size range were found to alter signalling processes essential for basic cell functions (including cell death), 40- and 50-nm nanoparticles demonstrated the greatest effect. These results show that nanoparticles should no longer be viewed as simple carriers for biomedical applications, but can also play an active role in mediating biological effects. The findings presented here may assist in the design of nanoscale delivery and therapeutic systems and provide insights into nanotoxicity. 相似文献
52.
Beran Michael J.; Harris Emily H.; Evans Theodore A.; Klein Emily D.; Chan Betty; Flemming Timothy M.; Washburn David A. 《Canadian Metallurgical Quarterly》2008,122(1):52
Ordinal learning was investigated in capuchin monkeys (Cebus apella) and rhesus monkeys (Macaca mulatta). In Experiment 1, both species were presented with pairings of the Arabic numerals 0 to 9. Some monkeys were given food rewards equal to the value of the numeral selected and some were rewarded with a single pellet only for choosing the higher numeral within the pair. Both species learned to select the larger numeral, but only rhesus monkeys that were differentially rewarded performed above chance levels when presented with novel probe pairings. In Experiment 2, the monkeys were first presented with arrays of 5 familiar numerals (from the range 0 to 9) and then arrays of 5 novel letters (from the range A to J) with the same reward outcomes in place as in Experiment 1. Both species performed better with the numerals, suggesting that an ordinal sequence of all stimuli had been learned during Experiment 1, rather than a matrix of two-choice discriminations. (PsycINFO Database Record (c) 2010 APA, all rights reserved) 相似文献
53.
New didactics for university instruction: why and how? 总被引:1,自引:0,他引:1
Betty Collis 《Computers & Education》1998,31(4):373-393
Why should instructors in traditional higher education institutions consider the re-design of their courses? The paper begins with a defense of four compelling reasons for changes in didactics: a review of some key principles of good teaching and learning, the increasing diversity of student characteristics and associated needs, the strategic and educational rationales for five types of flexibility increase in our current courses, and the particular need for instructors in faculties of education to provide leadership and models. Given the why, the next question is how? The remainder of the paper discusses the how in terms of a conceptually grounded approach to the pedagogical re-engineering of existing courses so that they become more flexible, with more student engagement, more-targetted communication, and more-attuned instructor scaffolding of increased student responsibility for his or her own learning. These concepts are made concrete through the extension of six standard sets of course-related tasks into redesigned sets involving the new didactics and WWW-based course-support functionalities. Examples from current practice at the Faculty of Educational Science and Technology at the University of Twente are used to illustrate the new didactical categories and their use of WWW-based course environments. The paper concludes with a consideration of key challenges that will confront the implementation of such new didactics in practice. 相似文献
54.
Y. Paul Handa Betty Wong Zhiyi Zhang Vipin Kumar Sharon Eddy Kishan Khemani 《Polymer Engineering and Science》1999,39(1):55-61
The solubility of CO2 in PETG, a glycol-modified PET, was measured at different temperatures and over a broad pressure range, and diffusion coefficients were derived at the corresponding conditions. The solubility of CO2 is quits high. For example, almost 15 wt% CO2 can be dissolved in PETG at 35°C and 6.0 MPa. Consequently, CO2 is good blowing agent for PETG. Cellular foams in the density range of about 0.04 to 1.2 g/cm3 and diameters in the range of about 10 to 150 µm were produced. The foam density and the cell size were found to depend on the foaming temperature and time, with larger cells obtained at higher temperatures or when the sample was foamed for a longer time. The foam density decreased with an increase in the foaming temperature to about 90°C, beyond which the density tended to increase slightly due to the cell collapse or coalescence. The density reduction also depended on the pressure at which the polymer was saturated with CO2; the higher the saturating pressure at a given temperature, the greater the density reduction. 相似文献
55.
Spiro D. Alexandratos Robert Beauvais Joseph R. Duke Betty S. Jorgensen 《应用聚合物科学杂志》1998,68(12):1911-1916
Foams prepared from vinylbenzyl chloride and crosslinked with divinylbenzene were functionalized with trialkylphosphite and tetralkylvinylidene diphosphonate. It was determined that the foams could be uniformly functionalized. Batch studies with the functionalized foams show that high levels of metal ion complexation can be achieved. Foams may therefore offer an important alternative to beads for rapid complexation reactions due to their highly porous structure. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 68: 1911–1916, 1998 相似文献
56.
Three different semi-interpenetrating polymer network (semi-IPN) polyimide systems were prepared through blending in solution by using 2 different polyimides, BPDA–PDA and PMDA–ODA (E), and 2 different oligomers, bismaleimide (MDAB) and phenylthynyl-terminated BPDA–PDA (BPDA–PDA–PEPA) oligomers. The oligomers are used as crosslinkers to modify the morphology of polyimides. The results show that both MDAB and PEPA are miscible with BPDA–PDA, but MDAB is immiscible with PMDA–ODA (E). Fourier transform infrared spectrum, dynamic mechanical thermal analysis data, and calculated crosslinking density indicate that there are crosslinking networks in these semi-IPN polyimide systems. In addition, the density and wide-angle X-ray diffraction results confirm that the molecular ordering and packing order are reduced by the addition of oligomers for these semi-IPN polyimide systems. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 261–272, 1998 相似文献
57.
58.
Javiera Cornejo Ekaterina Pokrant Carolina Carvallo Aldo Maddaleno Betty San Martín 《Food additives & contaminants. Part A, Chemistry, analysis, control, exposure & risk assessment》2018,35(3):448-457
Tylosin is one of the most commonly used antimicrobial drugs from the macrolide family and in broiler chickens it is used specially for the treatment of infectious pathologies. The poultry industry produces several by-products, among which feathers account for up to 7% of a chicken’s live weight, thus they amount to a substantial mass across the whole industry. Feathers have been repurposed as an animal feed ingredient by making them feather meal. Therefore, the presence of high concentrations of residues from antimicrobial drugs in feathers might pose a risk to global public health, due to re-entry of these residues into the food chain. This work aimed to characterise the depletion behaviour of tylosin in feather samples, while considering its depletion in muscle and liver tissue samples as a reference point. To achieve this goal, we have implemented and validated an analytical methodology suitable for detecting and quantifying tylosin in these matrices. Sixty broiler chickens, raised under controlled conditions, received an oral dose of 32 mg kg-1 of tylosin for 5 days. Tylosin was quantified in muscle, liver and feathers by liquid chromatography coupled with a photodiode array detector (HPLC-DAD). High concentrations of tylosin were detected in feather samples over the whole experimental period after completing both the therapy and the recommended withdrawal time (WDT). On the other hand, tylosin concentrations in muscle and liver tissue samples fell below the limit of detection of this method on the first sampling day. Our results indicate that the WDT for feather samples is 27 days, hence using feather meal for the formulation of animal diets or for other agricultural purposes could contaminate with antimicrobial residues either other livestock species or the environment. In consequence, we recommend monitoring this matrix when birds have been treated with tylosin, within the context of poultry farming. 相似文献
59.
Roberto Quevedo Oscar Díaz Betty Ronceros Franco Pedreschi Jos Miguel Aguilera 《Food research international (Ottawa, Ont.)》2009,42(9):1309-1314
A novel methodology “fractal browning indicator” (FBI) is presented, that describes the enzymatic browning kinetic based on the use of irregular color patterns from banana slice images. It uses the fractal Fourier texture image value in a selected area, to calculate a fractal dimension (FD), which represents the complexity of color distribution. During the procedure, colors from digital images were first transformed to L*a*b* space color using a transformation function (quadratic model), in order to derivate three color channels, lightness (L*), redness (a*), and yellowness (b*). In the results, lightness and yellowness parameters decreased during the browning kinetic, when their respective FD values increased, indicating major color distribution complexity in a selected area analyzed during the kinetic. The redness color (a*) did not show any statistical variation. The empirical power law model was suitable to correlate enzymatic browning kinetic data both for FBI and for the traditional method (when an L* mean was used). However, enzymatic browning rates using the FBI method, were between 8.5 and 35 times higher than rates calculated with the traditional method. 相似文献
60.
Modeling secondary organic aerosol formation via multiphase partitioning with molecular data 总被引:1,自引:0,他引:1
A new model for atmospheric secondary organic aerosol (SOA) is presented for biogenic compounds. It is based to the extent possible on experimental molecular SOA data, and it is compatible with any existing gas-phase chemical kinetic mechanism. Six SOA precursors or groups of precursors are used to represent biogenic monoterpenes and sesquiterpenes. SOA formation is modeled using five SOA surrogates to represent classes of compounds with different partitioning properties, e.g., hydrophobicity, aqueous solubility, acid dissociation, and saturation vapor pressure. Model simulations are evaluated against smog chamber data for SOA yields and some adjustments are made to uncertain stoichiometric coefficients and saturation vapor pressure parameters to improve model performance. The model is applied undertypical atmospheric conditions to exemplify the effect of relative humidity on SOA formation and the relative contributions of hydrophilic and hydrophobic SOA. 相似文献