全文获取类型
收费全文 | 15344篇 |
免费 | 1446篇 |
国内免费 | 4篇 |
出版年
2023年 | 81篇 |
2022年 | 168篇 |
2021年 | 434篇 |
2020年 | 265篇 |
2019年 | 297篇 |
2018年 | 346篇 |
2017年 | 302篇 |
2016年 | 488篇 |
2015年 | 846篇 |
2014年 | 906篇 |
2013年 | 993篇 |
2012年 | 1333篇 |
2011年 | 1410篇 |
2010年 | 879篇 |
2009年 | 738篇 |
2008年 | 956篇 |
2007年 | 944篇 |
2006年 | 891篇 |
2005年 | 718篇 |
2004年 | 785篇 |
2003年 | 683篇 |
2002年 | 674篇 |
2001年 | 136篇 |
2000年 | 78篇 |
1999年 | 110篇 |
1998年 | 150篇 |
1997年 | 86篇 |
1996年 | 75篇 |
1995年 | 77篇 |
1994年 | 79篇 |
1993年 | 97篇 |
1992年 | 59篇 |
1991年 | 54篇 |
1990年 | 47篇 |
1989年 | 34篇 |
1988年 | 37篇 |
1987年 | 39篇 |
1986年 | 28篇 |
1985年 | 36篇 |
1984年 | 43篇 |
1983年 | 28篇 |
1982年 | 44篇 |
1981年 | 37篇 |
1980年 | 33篇 |
1979年 | 22篇 |
1978年 | 33篇 |
1977年 | 17篇 |
1976年 | 21篇 |
1975年 | 15篇 |
1974年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 500 毫秒
991.
Jack C. Bridge Jonathan W. Aylott Christopher E. Brightling Amir M. Ghaemmaghami Alan J. Knox Mark P. Lewis Felicity R.A.J. Rose Gavin E. Morris 《Journal of visualized experiments : JoVE》2015,(101)
Electrospinning is a highly adaptable method producing porous 3D fibrous scaffolds that can be exploited in in vitro cell culture. Alterations to intrinsic parameters within the process allow a high degree of control over scaffold characteristics including fiber diameter, alignment and porosity. By developing scaffolds with similar dimensions and topographies to organ- or tissue-specific extracellular matrices (ECM), micro-environments representative to those that cells are exposed to in situ can be created. The airway bronchiole wall, comprised of three main micro-environments, was selected as a model tissue. Using decellularized airway ECM as a guide, we electrospun the non-degradable polymer, polyethylene terephthalate (PET), by three different protocols to produce three individual electrospun scaffolds optimized for epithelial, fibroblast or smooth muscle cell-culture. Using a commercially available bioreactor system, we stably co-cultured the three cell-types to provide an in vitro model of the airway wall over an extended time period.This model highlights the potential for such methods being employed in in vitro diagnostic studies investigating important inter-cellular cross-talk mechanisms or assessing novel pharmaceutical targets, by providing a relevant platform to allow the culture of fully differentiated adult cells within 3D, tissue-specific environments. 相似文献
992.
Effects of the Epichloë fungal endophyte symbiosis with Schedonorus pratensis on host grass invasiveness
下载免费PDF全文
![点击此处可从《Ecology and evolution》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Kruti Shukla Heather A. Hager Kathryn A. Yurkonis Jonathan A. Newman 《Ecology and evolution》2015,5(13):2596-2607
Initial studies of grass–endophyte mutualisms using Schedonorus arundinaceus cultivar Kentucky‐31 infected with the vertically transmitted endophyte Epichloë coenophiala found strong, positive endophyte effects on host‐grass invasion success. However, more recent work using different cultivars of S. arundinaceus has cast doubt on the ubiquity of this effect, at least as it pertains to S. arundinaceus–E. coenophiala. We investigated the generality of previous work on vertically transmitted Epichloë‐associated grass invasiveness by studying a pair of very closely related species: S. pratensis and E. uncinata. Seven cultivars of S. pratensis and two cultivars of S. arundinaceus that were developed with high‐ or low‐endophyte infection rate were broadcast seeded into 2 × 2‐m plots in a tilled, old‐field grassland community in a completely randomized block design. Schedonorus abundance, endophyte infection rate, and co‐occurring vegetation were sampled 3, 4, 5, and 6 years after establishment, and the aboveground invertebrate community was sampled in S. pratensis plots 3 and 4 years after establishment. Endophyte infection did not enable the host grass to achieve high abundance in the plant community. Contrary to expectations, high‐endophyte S. pratensis increased plant richness relative to low‐endophyte cultivars. However, as expected, high‐endophyte S. pratensis marginally decreased invertebrate taxon richness. Endophyte effects on vegetation and invertebrate community composition were inconsistent among cultivars and were weaker than temporal effects. The effect of the grass–Epichloë symbiosis on diversity is not generalizable, but rather specific to species, cultivar, infection, and potentially site. Examining grass–endophyte systems using multiple cultivars and species replicated among sites will be important to determine the range of conditions in which endophyte associations benefit host grass performance and have subsequent effects on co‐occurring biotic communities. 相似文献
993.
Parameter determination and validation for a mechanistic model of the enzymatic saccharification of cellulose‐Iβ
下载免费PDF全文
![点击此处可从《Biotechnology progress》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ambarish Nag Michael A. Sprague Andrew J. Griggs James J. Lischeske Jonathan J. Stickel Ashutosh Mittal Wei Wang David K. Johnson 《Biotechnology progress》2015,31(5):1237-1248
Cost‐effective production of fuels and chemicals from lignocellulosic biomass often involves enzymatic saccharification, which has been the subject of intense research and development. Recently, a mechanistic model for the enzymatic saccharification of cellulose has been developed that accounts for distribution of cellulose chain lengths, the accessibility of insoluble cellulose to enzymes, and the distinct modes of action of the component cellulases [Griggs et al. (2012) Biotechnol. Bioeng., 109(3):665–675; Griggs et al. (2012) Biotechnol. Bioeng., 109(3):676–685]. However, determining appropriate values for the adsorption, inhibition, and rate parameters required further experimental investigation. In this work, we performed several sets of experiments to aid in parameter estimation and to quantitatively validate the model. Cellulosic materials differing in degrees of polymerization and crystallinity (α‐cellulose‐Iβ and highly crystalline cellulose‐Iβ) were digested by component enzymes (EGI/CBHI/ ) and by mixtures of these enzymes. Based on information from the literature and the results from these experiments, a single set of model parameters was determined, and the model simulation results using this set of parameters were compared with the experimental data of total glucan conversion, chain‐length distribution, and crystallinity. Model simulations show significant agreement with the experimentally derived glucan conversion and chain‐length distribution curves and provide interesting insights into multiple complex and interacting physico‐chemical phenomena involved in enzymatic hydrolysis, including enzyme synergism, substrate accessibility, cellulose chain length distribution and crystallinity, and inhibition of cellulases by soluble sugars. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1237–1248, 2015 相似文献
994.
A purification process for heparin and precursor polysaccharides using the pH responsive behavior of chitosan
下载免费PDF全文
![点击此处可从《Biotechnology progress》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Ujjwal Bhaskar Anne M. Hickey Guoyun Li Ruchir V. Mundra Fuming Zhang Li Fu Chao Cai Zhimin Ou Jonathan S. Dordick Robert J. Linhardt 《Biotechnology progress》2015,31(5):1348-1359
The contamination crisis of 2008 has brought to light several risks associated with use of animal tissue derived heparin. Because the total chemical synthesis of heparin is not feasible, a bioengineered approach has been proposed, relying on recombinant enzymes derived from the heparin/HS biosynthetic pathway and Escherichia coli K5 capsular polysaccharide. Intensive process engineering efforts are required to achieve a cost‐competitive process for bioengineered heparin compared to commercially available porcine heparins. Towards this goal, we have used 96‐well plate based screening for development of a chitosan‐based purification process for heparin and precursor polysaccharides. The unique pH responsive behavior of chitosan enables simplified capture of target heparin or related polysaccharides, under low pH and complex solution conditions, followed by elution under mildly basic conditions. The use of mild, basic recovery conditions are compatible with the chemical N‐deacetylation/N‐sulfonation step used in the bioengineered heparin process. Selective precipitation of glycosaminoglycans (GAGs) leads to significant removal of process related impurities such as proteins, DNA and endotoxins. Use of highly sensitive liquid chromatography‐mass spectrometry and nuclear magnetic resonance analytical techniques reveal a minimum impact of chitosan‐based purification on heparin product composition. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1348–1359, 2015 相似文献
995.
A quantitative proteomic analysis of cellular responses to high glucose media in Chinese hamster ovary cells
下载免费PDF全文
![点击此处可从《Biotechnology progress》网站下载免费的PDF全文](/ch/ext_images/free.gif)
Zhenke Liu Shujia Dai Jonathan Bones Somak Ray Sangwon Cha Barry L. Karger Jingyi Jessica Li Lee Wilson Greg Hinckle Anthony Rossomando 《Biotechnology progress》2015,31(4):1026-1038
A goal in recombinant protein production using Chinese hamster ovary (CHO) cells is to achieve both high specific productivity and high cell density. Addition of glucose to the culture media is necessary to maintain both cell growth and viability. We varied the glucose concentration in the media from 5 to 16 g/L and found that although specific productivity of CHO‐DG44 cells increased with the glucose level, the integrated viable cell density decreased. To examine the biological basis of these results, we conducted a discovery proteomic study of CHO‐DG44 cells grown under batch conditions in normal (5 g/L) or high (15 g/L) glucose over 3, 6, and 9 days. Approximately 5,000 proteins were confidently identified against an mRNA‐based CHO‐DG44 specific proteome database, with 2,800 proteins quantified with at least two peptides. A self‐organizing map algorithm was used to deconvolute temporal expression profiles of quantitated proteins. Functional analysis of altered proteins suggested that differences in growth between the two glucose levels resulted from changes in crosstalk between glucose metabolism, recombinant protein expression, and cell death, providing an overall picture of the responses to high glucose environment. The high glucose environment may enhance recombinant dihydrofolate reductase in CHO cells by up‐regulating NCK1 and down‐regulating PRKRA, and may lower integrated viable cell density by activating mitochondrial‐ and endoplasmic reticulum‐mediated cell death pathways by up‐regulating HtrA2 and calpains. These proteins are suggested as potential targets for bioengineering to enhance recombinant protein production. © 2015 American Institute of Chemical Engineers Biotechnol. Prog., 31:1026–1038, 2015 相似文献
996.
Cheryl Carson Pichai Raman Jennifer Tullai Lei Xu Martin Henault Emily Thomas Sarita Yeola Jianmin Lao Mark McPate J. Martin Verkuyl George Marsh Jason Sarber Adam Amaral Scott Bailey Danuta Lubicka Helen Pham Nicolette Miranda Jian Ding Hai-Ming Tang Haisong Ju Pamela Tranter Nan Ji Philipp Krastel Rishi K. Jain Andrew M. Schumacher Joseph J. Loureiro Elizabeth George Giuliano Berellini Nathan T. Ross Simon M. Bushell Gül Erdemli Jonathan M. Solomon 《PloS one》2015,10(6)
Englerin A is a structurally unique natural product reported to selectively inhibit growth of renal cell carcinoma cell lines. A large scale phenotypic cell profiling experiment (CLiP) of englerin A on ¬over 500 well characterized cancer cell lines showed that englerin A inhibits growth of a subset of tumor cell lines from many lineages, not just renal cell carcinomas. Expression of the TRPC4 cation channel was the cell line feature that best correlated with sensitivity to englerin A, suggesting the hypothesis that TRPC4 is the efficacy target for englerin A. Genetic experiments demonstrate that TRPC4 expression is both necessary and sufficient for englerin A induced growth inhibition. Englerin A induces calcium influx and membrane depolarization in cells expressing high levels of TRPC4 or its close ortholog TRPC5. Electrophysiology experiments confirmed that englerin A is a TRPC4 agonist. Both the englerin A induced current and the englerin A induced growth inhibition can be blocked by the TRPC4/C5 inhibitor ML204. These experiments confirm that activation of TRPC4/C5 channels inhibits tumor cell line proliferation and confirms the TRPC4 target hypothesis generated by the cell line profiling. In selectivity assays englerin A weakly inhibits TRPA1, TRPV3/V4, and TRPM8 which suggests that englerin A may bind a common feature of TRP ion channels. In vivo experiments show that englerin A is lethal in rodents near doses needed to activate the TRPC4 channel. This toxicity suggests that englerin A itself is probably unsuitable for further drug development. However, since englerin A can be synthesized in the laboratory, it may be a useful chemical starting point to identify novel modulators of other TRP family channels. 相似文献
997.
998.
Emmanuel Cohen Jonathan Y. Bernard Amandine Ponty Amadou Ndao Norbert Amougou Rihlat Sa?d-Mohamed Patrick Pasquet 《PloS one》2015,10(11)
Background
The social valorisation of overweight in African populations could promote high-risk eating behaviours and therefore become a risk factor of obesity. However, existing scales to assess body image are usually not accurate enough to allow comparative studies of body weight perception in different African populations. This study aimed to develop and validate the Body Size Scale (BSS) to estimate African body weight perception.Methods
Anthropometric measures of 80 Cameroonians and 81 Senegalese were used to evaluate three criteria of adiposity: body mass index (BMI), overall percentage of fat, and endomorphy (fat component of the somatotype). To develop the BSS, the participants were photographed in full face and profile positions. Models were selected for their representativeness of the wide variability in adiposity with a progressive increase along the scale. Then, for the validation protocol, participants self-administered the BSS to assess self-perceived current body size (CBS), desired body size (DBS) and provide a “body self-satisfaction index.” This protocol included construct validity, test-retest reliability and convergent validity and was carried out with three independent samples of respectively 201, 103 and 1115 Cameroonians.Results
The BSS comprises two sex-specific scales of photos of 9 models each, and ordered by increasing adiposity. Most participants were able to correctly order the BSS by increasing adiposity, using three different words to define body size. Test-retest reliability was consistent in estimating CBS, DBS and the “body self-satisfaction index.” The CBS was highly correlated to the objective BMI, and two different indexes assessed with the BSS were consistent with declarations obtained in interviews.Conclusion
The BSS is the first scale with photos of real African models taken in both full face and profile and representing a wide and representative variability in adiposity. The validation protocol proved its reliability for estimating body weight perception in Africans. 相似文献999.
1000.
Christopher Paul Baker Bharati Kulkarni K. V. Radhakrishna M. S. Charyulu John Gregson Mika Matsuzaki Amy E. Taylor Dorairaj Prabhakaran Raja Sriswan Mamidi Jonathan Wells Ian Wilkinson Carmel McEniery Yasmin George Davey Smith Yoav Ben-Shlomo Hannah Kuper Sanjay Kinra 《PloS one》2015,10(6)