全文获取类型
收费全文 | 3028篇 |
免费 | 236篇 |
专业分类
3264篇 |
出版年
2022年 | 18篇 |
2021年 | 18篇 |
2019年 | 22篇 |
2018年 | 31篇 |
2017年 | 19篇 |
2016年 | 55篇 |
2015年 | 95篇 |
2014年 | 93篇 |
2013年 | 116篇 |
2012年 | 193篇 |
2011年 | 187篇 |
2010年 | 103篇 |
2009年 | 91篇 |
2008年 | 152篇 |
2007年 | 126篇 |
2006年 | 144篇 |
2005年 | 145篇 |
2004年 | 137篇 |
2003年 | 144篇 |
2002年 | 138篇 |
2001年 | 36篇 |
2000年 | 37篇 |
1999年 | 29篇 |
1998年 | 58篇 |
1997年 | 31篇 |
1996年 | 28篇 |
1995年 | 38篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 22篇 |
1991年 | 28篇 |
1990年 | 18篇 |
1989年 | 24篇 |
1988年 | 33篇 |
1987年 | 28篇 |
1986年 | 17篇 |
1985年 | 15篇 |
1984年 | 36篇 |
1983年 | 27篇 |
1982年 | 30篇 |
1981年 | 32篇 |
1980年 | 31篇 |
1979年 | 26篇 |
1978年 | 27篇 |
1977年 | 25篇 |
1976年 | 28篇 |
1975年 | 19篇 |
1973年 | 19篇 |
1972年 | 19篇 |
1965年 | 13篇 |
排序方式: 共有3264条查询结果,搜索用时 15 毫秒
111.
112.
Two-tier vessels, developed for culturing of microalgae and cyanobacteria at high cell density on a shaken platform, were assembled from a flat lower chamber to be filled with a CO2 buffer and an upper flat sterile chamber for the culture that was separated from the lower chamber by a porous polypropylene membrane. Diffusive gas exchange with the atmosphere was controlled by the O2 outlet channel. Referred to surface area, rates of CO2 transfer to a shaken weakly alkaline buffer solution across the membrane were higher than those reached on the conventional pathway through the free upper liquid surface. Membrane-mediated CO2 supply enabled rapid growth of Synechocystis sp. PCC 6803 and Synechococcus sp. PCC 7002 up to ultrahigh cell density. The biomass (dry weight) concentration of Synechococcus cultures reached more than 30 g L?1 on a buffered medium with adequate concentrations of mineral nutrients. An increase of 15 to 20 g L?1 was observed during repeated two-day cycles. Separate pathways for CO2 supply and oxygen outlet prevented significant loss of CO2. Convective gas flow through the oxygen outlet channel enabled the estimation of the O2 generation rate. The permeability of the channel for diffusive O2/N2 exchange limited the O2 concentration to a moderate value. It is concluded that shaken flat cultures using CO2 supply through a porous hydrophobic membrane and diffusive release of O2 through a separate pathway are promising for research on microalgae and cyanobacteria. 相似文献
113.
Falko Schmidt Martin Eberhard Geiger Rudolf Jäger Bernd Georg Lapatki 《Computer methods in biomechanics and biomedical engineering》2016,19(15):1673-1682
In orthodontic treatment, the locations of the centre of resistance (CR) of individual teeth and the applied load system are the major determinants for the type of tooth movement achieved. Currently, CR locations have only been specified for a relatively small number of tooth specimen for research purposes. Analysing cone beam computed tomography data samples from three upper central incisors, this study explores whether the effort to establish accurate CR estimates can be reduced by (i) morphing a pre-existing simplified finite element (FE) mesh to fit to the segmented 3D tooth-bone model, and (ii) individualizing a mean CR location according to a small parameter set characterising the morphology of the tooth and its embedding. The FE morphing approach and the semi-analytical approach led to CR estimates that differ in average only 0.04 and 0.12 mm respectively from those determined by very time-consuming individual FE modelling (standard method). Both approaches may help to estimate the movement of individual teeth during orthodontic treatment and, thus, increase the therapeutic efficacy. 相似文献
114.
115.
Ralf Müller-Rath Stefan Andereya Sascha Gravius Dieter Christian Wirtz Rudolf Marx Torsten Mumme 《Biomedizinische Technik》2007,52(6):391-397
Cemented femoral stems have shown decreased longevity compared to cementless implants in hip revision arthroplasty. The aim of this study was to evaluate the effect of an amphiphilic bonder on bone cement stability in a biomechanical femur expulsion test. A simplified hip simulator test setup with idealised femur stem specimens was carried out. The stems were implanted into bovine femurs (group 1: no bonder, n=10; group 2: bonder including glutaraldehyde, n=10; group 3: bonder without glutaraldehyde, n=10). A dynamic loading (maximum load: 800 N; minimum load: 100 N; frequency: 3 Hz; 105 cycles) was performed. Subsequently, the stem specimens were expulsed axially out of their implant beds and maximum load at failure was recorded. The static controls showed a mean maximum load to failure of 4123 N in group 1, 8357.5 N in group 2 and 5830.8 N in group 3. After dynamic loading, the specimens of group 2 reached the highest load to failure (8191.5 N), followed by group 3 (5649.5 N) and group 1 (3462 N), respectively. In group 2, we observed nine periprosthetic fractures at a load of 8400 N without signs of interface loosening. Application of an amphiphilic bonder led to a significant improvement of bonding stability, especially when glutaraldehyde was added to the bonder. This technique might offer an increased longevity of cemented femur revision stems in total hip replacement. 相似文献
116.
Siegmund KD Connor CM Campan M Long TI Weisenberger DJ Biniszkiewicz D Jaenisch R Laird PW Akbarian S 《PloS one》2007,2(9):e895
The role of DNA cytosine methylation, an epigenetic regulator of chromatin structure and function, during normal and pathological brain development and aging remains unclear. Here, we examined by MethyLight PCR the DNA methylation status at 50 loci, encompassing primarily 5' CpG islands of genes related to CNS growth and development, in temporal neocortex of 125 subjects ranging in age from 17 weeks of gestation to 104 years old. Two psychiatric disease cohorts--defined by chronic neurodegeneration (Alzheimer's) or lack thereof (schizophrenia)--were included. A robust and progressive rise in DNA methylation levels across the lifespan was observed for 8/50 loci (GABRA2, GAD1, HOXA1, NEUROD1, NEUROD2, PGR, STK11, SYK) typically in conjunction with declining levels of the corresponding mRNAs. Another 16 loci were defined by a sharp rise in DNA methylation levels within the first few months or years after birth. Disease-associated changes were limited to 2/50 loci in the Alzheimer's cohort, which appeared to reflect an acceleration of the age-related change in normal brain. Additionally, methylation studies on sorted nuclei provided evidence for bidirectional methylation events in cortical neurons during the transition from childhood to advanced age, as reflected by significant increases at 3, and a decrease at 1 of 10 loci. Furthermore, the DNMT3a de novo DNA methyl-transferase was expressed across all ages, including a subset of neurons residing in layers III and V of the mature cortex. Therefore, DNA methylation is dynamically regulated in the human cerebral cortex throughout the lifespan, involves differentiated neurons, and affects a substantial portion of genes predominantly by an age-related increase. 相似文献
117.
118.
119.
Greta Giljan Carol Arnosti Inga V. Kirstein Rudolf Amann Bernhard M. Fuchs 《Environmental microbiology》2022,24(5):2333-2347
Marine heterotrophic bacteria contribute considerably to global carbon cycling, in part by utilizing phytoplankton-derived polysaccharides. The patterns and rates of two different polysaccharide utilization modes – extracellular hydrolysis and selfish uptake – have previously been found to change during spring phytoplankton bloom events. Here we investigated seasonal changes in bacterial utilization of three polysaccharides, laminarin, xylan and chondroitin sulfate. Strong seasonal differences were apparent in mode and speed of polysaccharide utilization, as well as in bacterial community compositions. Compared to the winter month of February, during the spring bloom in May, polysaccharide utilization was detected earlier in the incubations and a higher portion of all bacteria took up laminarin selfishly. Highest polysaccharide utilization was measured in June and September, mediated by bacterial communities that were significantly different from spring assemblages. Extensive selfish laminarin uptake, for example, was detectible within a few hours in June, while extracellular hydrolysis of chondroitin was dominant in September. In addition to the well-known Bacteroidota and Gammaproteobacteria clades, the numerically minor verrucomicrobial clade Pedosphaeraceae could be identified as a rapid laminarin utilizer. In summary, polysaccharide utilization proved highly variable over the seasons, both in mode and speed, and also by the bacterial clades involved. 相似文献
120.
Michael A. Rudnicki Rudolf Jaenisch 《BioEssays : news and reviews in molecular, cellular and developmental biology》1995,17(3):203-209
Gene targeting has allowed the dissection of complex biological processes at the genetic level. Our understanding of the nuances of skeletal muscle development has been greatly increased by the analysis of mice carrying targeted null mutations in the Myf-5, MyoD and myogenin genes, encoding members of the myogenic regulatory factor (MRF) family. These experiments have elucidated the hierarchical relationships existing between the MRFs, and established that functional redundancy is a feature of the MRF regulatory network. Either MyoD or Myf-5 is sufficient for the formation or survival of skeletal myoblasts. Myogenin acts later in development and plays an essential in vivo role in the terminal differentiation of myotubes. 相似文献