全文获取类型
收费全文 | 20134篇 |
免费 | 2004篇 |
国内免费 | 1583篇 |
出版年
2023年 | 454篇 |
2022年 | 396篇 |
2021年 | 490篇 |
2020年 | 673篇 |
2019年 | 840篇 |
2018年 | 838篇 |
2017年 | 743篇 |
2016年 | 722篇 |
2015年 | 715篇 |
2014年 | 1074篇 |
2013年 | 1383篇 |
2012年 | 881篇 |
2011年 | 990篇 |
2010年 | 763篇 |
2009年 | 920篇 |
2008年 | 943篇 |
2007年 | 1002篇 |
2006年 | 844篇 |
2005年 | 739篇 |
2004年 | 653篇 |
2003年 | 576篇 |
2002年 | 491篇 |
2001年 | 361篇 |
2000年 | 385篇 |
1999年 | 360篇 |
1998年 | 299篇 |
1997年 | 297篇 |
1996年 | 273篇 |
1995年 | 258篇 |
1994年 | 216篇 |
1993年 | 237篇 |
1992年 | 197篇 |
1991年 | 177篇 |
1990年 | 170篇 |
1989年 | 144篇 |
1988年 | 151篇 |
1987年 | 133篇 |
1986年 | 133篇 |
1985年 | 213篇 |
1984年 | 344篇 |
1983年 | 279篇 |
1982年 | 274篇 |
1981年 | 270篇 |
1980年 | 296篇 |
1979年 | 230篇 |
1978年 | 174篇 |
1977年 | 159篇 |
1976年 | 132篇 |
1975年 | 120篇 |
1974年 | 115篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
851.
852.
《Biocatalysis and Biotransformation》2013,31(5-6):313-319
AbstractTo prepare a smart biocatalyst, cellulase was immobilized on the reversibly soluble matrix Eudragit L-100 by non-covalent and covalent methods. Covalent immobilization using carbodiimide coupling exhibited superior enzyme loading and reusability compared with non-covalent immobilization, and the covalent loading was increased by almost 20% through the addition of N-hydroxysuccinimide. The temperature optimum of the cellulase was not improved apparently by immobilization but the pH optimum increased from 4.75 to 5.25. Immobilized cellulase was more active than free cellulase above pH 5.0. Immobilized cellulase was more stable than free cellulase during storage at 4°C, room temperature and 50°C. Km values of immobilized and free cellulase were 85.55 and 73.84 g L?1, respectively. About 50% productivity was retained after five cycles for hydrolysis of steam-exploded straw. 相似文献
853.
Yonglin Su Peiran Yang Zengliang Fu Zhongmin Jin Chengtao Wang 《Computer methods in biomechanics and biomedical engineering》2013,16(6):539-548
This work is concerned with the lubrication analysis of artificial knee joints, which plays an increasing significant role in clinical performance and longevity of components. Time-dependent elastohydrodynamic lubrication analysis for normal total knee replacement is carried out under the cyclic variation in both load and speed representative of normal walking. An equivalent ellipsoid-on-plane model is adopted to represent an actual artificial knee. A full numerical method is developed to simultaneously solve the Reynolds and elasticity equations using the multigrid technique. The elastic deformation is based on the constrained column model. Results show that, under the combined effect of entraining and squeeze-film actions throughout the walking cycle, the predicted central film thickness tends to decrease in the stance phase but keeps a relatively larger value at the swing phase. Furthermore, the geometry of knee joint implant is verified to play an important role under its lubrication condition, and the length of time period is a key point to influence the lubrication performance of joint components. 相似文献
854.
《Biocatalysis and Biotransformation》2013,31(1):8-21
AbstractDuring directed evolution to functionally express the high redox potential laccase from the PM1 basidiomycete in Saccharomyces cerevisiae, the characteristic maximum absorption at the T1 copper site (Abs610T1Cu) was quenched, switching the typical blue colour of the enzyme to yellow. To determine the molecular basis of this colour change, we characterized the original wild-type laccase and its evolved mutant. Peptide printing and MALDI-TOF analysis confirmed the absence of contaminating protein traces that could mask the Abs610T1Cu, while conservation of the redox potential at the T1 site was demonstrated by spectroelectrochemical redox titrations. Both wild-type and evolved laccases were capable of oxidizing a broad range of substrates (ABTS, guaiacol, DMP, synapic acid) and they displayed similar catalytic efficiencies. The laccase mutant could only oxidize high redox potential dyes (Poly R-478, Reactive Black 5, Azure B) in the presence of exogenous mediators, indicating that the yellow enzyme behaves like a blue laccase. The main consequence of over-expressing the mutant laccase was the generation of a six-residue N-terminal acidic extension, which was associated with the failure of the STE13 protease in the Golgi compartment giving rise to alternative processing. Removal of the N-terminal tail had a negative effect on laccase stability, secretion and its kinetics, although the truncated mutant remained yellow. The results of CD spectra analysis suggested that polyproline helixes were formed during the directed evolution altering spectral properties. Moreover, introducing the A461T and S426N mutations in the T1 environment during the first cycles of laboratory evolution appeared to mediate the alterations to Abs610T1Cu by affecting its coordinating sphere. This laccase mutant is a valuable departure point for further protein engineering towards different fates. 相似文献
855.
Hongbo Zhang Michael J. Agnew 《Computer methods in biomechanics and biomedical engineering》2013,16(15):1693-1703
The critical time interval (CTI) is a parameter that has been used to distinguish open-loop from closed-loop control during upright stance. The aim of this study was to develop a new method to determine CTIs. The new approach, termed the intermittent critical time interval (ICTI) method, was motivated from evidence that upright standing is an intermittent rather than an asymptotic stability control process. For this ICTI method, center-of-pressure time series are first transformed to the time–frequency domain with a wavelet method. Subsequently, the CTI is assumed equal to the time span between two local maxima in the time–frequency domain within a distinct frequency band (i.e., 0.5–1.1 Hz). This new method may help facilitate better estimates of the transition time interval between open and closed-loop control during upright stance and can also be applied in future work such as in simulating postural control. In addition, this method can be used in future work to assess temporal changes in CTIs. 相似文献
856.
Gong Zhang Witold Kinsner Bin Huang 《Computer methods in biomechanics and biomedical engineering》2013,16(6):701-707
This paper presents an electrocardiogram (ECG) data mining scheme based on the ECG frame classification realised by a dynamic time warping (DTW) matching technique, which has been used successfully in speech recognition. We use the DTW to classify ECG frames because ECG and speech signals have similar non-stationary characteristics. The DTW mapping function is obtained by searching the frame from its end to start. A threshold is setup for DTW matching residual either to classify an ECG frame or to add a new class. Classification and establishment of a template set are carried out simultaneously. A frame is classified into a category with a minimal residual and satisfying a threshold requirement. A classification residual of 1.33% is achieved by the DTW for a 10-min ECG recording. 相似文献
857.
In electromagnetic dosimetry, anatomical human models are commonly obtained by segmentation of magnetic resonance imaging or computed tomography scans. In this paper, a human head model extracted from thermal infrared images is examined in terms of its applicability to specific absorption rate (SAR) calculations. Since thermal scans are two-dimensional (2D) representation of surface temperature, this allows researchers to overcome the extensive computational demand associated with 3D simulation. The numerical calculations are performed using the finite-difference time-domain method with mesh sizes of 2 mm at 900 MHz plane wave irradiation. The power density of the incident plane wave is assumed to be 10 W/m2. Computations were compared with a realistic anatomical head model. The results show that although there were marked differences in the local SAR distribution in the various tissues in the two models, the 1 g peak SAR values are approximately similar in the two models. 相似文献
858.
Xuejun Zhang Shengliang Ye Xi Du Jing Yuan Chaoming Zhao Changqing Li 《Biologicals》2013,41(6):364-367
In 2007, the Chinese State Food and Drug Administration (SFDA) implemented a management system for lot release of all plasma-derived products. Since then, there have been only a few systematic studies of the blood supply, which is a concern when considering the small amount of plasma collected per capita (approximately 3 L/1000 people). As a result, there may be a threat to the safety of the available blood supply. In this study, we examined the characteristics of the supply of Chinese plasma-derived products. We investigated the reports of lot-released biological products derived from all 8 national or regional regulatory authorities in China from 2007 to 2011. The market supply characteristics of Chinese plasma-derived products were analyzed by reviewing the changes in supply varieties, the batches of lot-released plasma-derived products and the actual supply. As a result, the national regulatory authorities can more accurately develop a specific understanding of the production and quality management information provided by Chinese plasma product manufacturers. The implementation of the lot release system further ensures the clinical validity of the plasma-derived products in China and improves the safety of using plasma-derived products. This work provides an assessment of the future Chinese market supply of plasma-derived products and can function as a theoretical basis for the establishment of hemovigilance. 相似文献
859.
860.
M. Djavaheri-Mergny C. Marsac C. Mazière R. Santus L. Michel L. Dubertret 《Free radical research》2013,47(6):583-594
UV-A irradiation caused a dose-dependent decrease in cellular oxygen consumption (56%) and ATP content (65%) in human NCTC 2544 keratinocytes, one hour after treatment. This effect was partially reversed by maintaining the irradiated cells in normal culture conditions for 24h. Using malate/glutamate or succinate as substrates for mitochondrial electron transport, the oxygen uptake of digitoninpermeabilised cells was greatly inhibited following UV-A exposure. These results strongly suggest that UV-A irradiation affects the state 3 respiration of the mitochondria. However, under identical conditions, UV-A exposure did not reduce the mitochondrial transmembrane potential. The antioxidant, vitamin E inhibited UV-A-induced lipid peroxidation, but did not significantly prevent the UV-A-mediated changes in cellular respiration nor the decrease in ATP content, suggesting that these effects were not the result of UV-A dependent lipid peroxidation. UV-A irradiation also led to an increase in MnSOD gene expression 24 hours after treatment, indicating that the mitochondrial protection system was enhanced in response to UV-A treatment. These findings provide evidence that impairment of mitochondrial respiratory activity is one of the early results of UV-A irradiation for light doses much lower than the minimal erythemal dose. 相似文献