全文获取类型
收费全文 | 7307篇 |
免费 | 490篇 |
国内免费 | 1篇 |
出版年
2023年 | 67篇 |
2022年 | 101篇 |
2021年 | 202篇 |
2020年 | 162篇 |
2019年 | 167篇 |
2018年 | 222篇 |
2017年 | 216篇 |
2016年 | 300篇 |
2015年 | 418篇 |
2014年 | 428篇 |
2013年 | 536篇 |
2012年 | 626篇 |
2011年 | 615篇 |
2010年 | 378篇 |
2009年 | 306篇 |
2008年 | 434篇 |
2007年 | 395篇 |
2006年 | 319篇 |
2005年 | 324篇 |
2004年 | 306篇 |
2003年 | 243篇 |
2002年 | 239篇 |
2001年 | 64篇 |
2000年 | 52篇 |
1999年 | 48篇 |
1998年 | 77篇 |
1997年 | 51篇 |
1996年 | 33篇 |
1995年 | 34篇 |
1994年 | 34篇 |
1993年 | 31篇 |
1992年 | 24篇 |
1991年 | 20篇 |
1990年 | 26篇 |
1989年 | 17篇 |
1988年 | 19篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 31篇 |
1984年 | 20篇 |
1983年 | 17篇 |
1982年 | 17篇 |
1981年 | 12篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1977年 | 7篇 |
1975年 | 7篇 |
1973年 | 11篇 |
1972年 | 7篇 |
1968年 | 11篇 |
排序方式: 共有7798条查询结果,搜索用时 46 毫秒
121.
122.
María Mena Alberto López-Luna Keiko Shirai Alberto Tecante Miquel Gimeno Eduardo Bárzana 《Bioprocess and biosystems engineering》2013,36(3):383-387
Hyperbranched poly-l-lactides have been synthesized by eROP in [C4MIM][PF6] media. The bis(hydroxymethyl)butyric acid molecule was used as the AB2 core co-monomer and immobilized lipase B from Candida antarctica as biocatalyst. The degree of branching could be controlled by the reaction conditions, with the maximum achieved being 0.21. The successful achievement of the hyperbranched structure is attributed to the high solvent power of substrates and products in the ionic liquid besides sustained lipase activity. 相似文献
123.
María Victoria Martín Ayelén Mariana Distefano Eduardo Julián Zabaleta Gabriela Carolina Pagnussat 《Plant signaling & behavior》2013,8(10)
Previously considered as toxic by-products of aerobic metabolism, reactive oxygen species (ROS) are emerging as essential signaling molecules in eukaryotes. Recent evidence showed that maintenance of ROS homeostasis during female gametophyte development is crucial for embryo sac patterning and fertilization. Although ROS are exclusively detected in the central cell of mature embryo sacs, the study of mutants deficient in ROS homeostasis suggests that controlled oxidative bursts might take place earlier during gametophyte development. Also, a ROS burst that depends on pollination takes place inside the embryo sac. This oxidative response might be required for pollen tube growth arrest and for sperm cell release. In this mini-review, we will focus on new insights into the role of ROS during female gametophyte development and fertilization. Special focus will be made on the mitochondrial Mn-Superoxide dismutase (MSD1), which has been recently reported to be essential for maintaining ROS homeostasis during embryo sac formation. 相似文献
124.
125.
126.
127.
Vijayalakshmi Sridharan Preeti Tripathi Sunil Sharma Peter M. Corry Eduardo G. Moros Awantika Singh Cesar M. Compadre Martin Hauer-Jensen Marjan Boerma 《PloS one》2013,8(7)
Radiation-induced heart disease (RIHD) is a long-term side effect of radiotherapy of intrathoracic, chest wall and breast tumors when radiation fields encompass all or part of the heart. Previous studies have shown that pentoxifylline (PTX) in combination with α-tocopherol reduced manifestations of RIHD in rat models of local heart irradiation. The relative contribution of PTX and α-tocopherol to these beneficial effects are not known. This study examined the effects of PTX alone or in combination with tocotrienols, forms of vitamin E with potential potent radiation mitigation properties. Rats received localized X-irradiation of the heart with an image-guided irradiation technique. At 3 months after irradiation rats received oral treatment with vehicle, PTX, or PTX in combination with a tocotrienol-enriched formulation. At 6 months after irradiation, PTX-treated rats showed arrhythmia in 5 out of 14 animals. PTX alone or in combination with tocotrienols did not alter cardiac radiation fibrosis, left ventricular protein expression of the endothelial markers von Willebrand factor and neuregulin-1, or phosphorylation of the signal mediators Akt, Erk1/2, or PKCα. On the other hand, tocotrienols reduced cardiac numbers of mast cells and macrophages, but enhanced the expression of tissue factor. While this new rat model of localized heart irradiation does not support the use of PTX alone, the effects of tocotrienols on chronic manifestations of RIHD deserve further investigation. 相似文献
128.
Leiz M. C. Véras Vanessa R. R. Cunha Filipe C. D. A. Lima Maria A. Guimar?es Marianne M. Vieira Yuri D. M. Campelo Vanessa Y. Sakai David F. Lima Paulo S. Carvalho Jr Javier A. Ellena Paulo R. P. Silva Luciene C. Vasconcelos Markus Godejohann Helena M. Petrilli Vera R. L. Constantino Yvonne P. Mascarenhas José Roberto de Souza de Almeida Leite 《PloS one》2013,8(6)
This paper presents an industrial scale process for extraction, purification, and isolation of epiisopiloturine (EPI) (2(3H)-Furanone,dihydro-3-(hydroxyphenylmethyl)-4-[(1-methyl-1H-imidazol-4-yl)methyl]-, [3S-[3a(R*),4b]]), which is an alkaloid from jaborandi leaves (Pilocarpus microphyllus Stapf). Additionally for the first time a set of structural and spectroscopic techniques were used to characterize this alkaloid. EPI has shown schistomicidal activity against adults and young forms, as well as the reduction of the egg laying adult worms and low toxicity to mammalian cells (in vitro). At first, the extraction of EPI was done with toluene and methylene chloride to obtain a solution that was alkalinized with ammonium carbonate. The remaining solution was treated in sequence by acidification, filtration and alkalinization. These industrial procedures are necessary in order to remove impurities and subsequent application of the high performance liquid chromatography (HPLC). The HPLC was employed also to remove other alkaloids, to obtain EPI purity higher than 98%. The viability of the method was confirmed through HPLC and electrospray mass spectrometry, that yielded a pseudo molecular ion of m/z equal to 287.1 Da. EPI structure was characterized by single crystal X-ray diffraction (XRD), 1H and 13C nuclear magnetic resonance (NMR) in deuterated methanol/chloroform solution, vibrational spectroscopy and mass coupled thermal analyses. EPI molecule presents a parallel alignment of the benzene and the methyl imidazol ring separated by an interplanar spacing of 3.758 Å indicating a π-π bond interaction. The imidazole alkaloid melts at 225°C and decomposes above 230°C under air. EPI structure was used in theoretical Density Functional Theory calculations, considering the single crystal XRD data in order to simulate the NMR, infrared and Raman spectra of the molecule, and performs the signals attribution. 相似文献
129.
Kátia Regina da Silva Roberto Costa Elizabeth Sartori Crevelari Marianna Sobral Lacerda Caio Marcos de Moraes Albertini Martino Martinelli Filho José Eduardo Santana Jo?o Ricardo Nickenig Vissoci Ricardo Pietrobon Jacson V. Barros 《PloS one》2013,8(7)
Background
The ability to apply standard and interoperable solutions for implementing and managing medical registries as well as aggregate, reproduce, and access data sets from legacy formats and platforms to advanced standard formats and operating systems are crucial for both clinical healthcare and biomedical research settings.Purpose
Our study describes a reproducible, highly scalable, standard framework for a device registry implementation addressing both local data quality components and global linking problems.Methods and Results
We developed a device registry framework involving the following steps: (1) Data standards definition and representation of the research workflow, (2) Development of electronic case report forms using REDCap (Research Electronic Data Capture), (3) Data collection according to the clinical research workflow and, (4) Data augmentation by enriching the registry database with local electronic health records, governmental database and linked open data collections, (5) Data quality control and (6) Data dissemination through the registry Web site. Our registry adopted all applicable standardized data elements proposed by American College Cardiology / American Heart Association Clinical Data Standards, as well as variables derived from cardiac devices randomized trials and Clinical Data Interchange Standards Consortium. Local interoperability was performed between REDCap and data derived from Electronic Health Record system. The original data set was also augmented by incorporating the reimbursed values paid by the Brazilian government during a hospitalization for pacemaker implantation. By linking our registry to the open data collection repository Linked Clinical Trials (LinkedCT) we found 130 clinical trials which are potentially correlated with our pacemaker registry.Conclusion
This study demonstrates how standard and reproducible solutions can be applied in the implementation of medical registries to constitute a re-usable framework. Such approach has the potential to facilitate data integration between healthcare and research settings, also being a useful framework to be used in other biomedical registries. 相似文献130.
Luciano Pereira Paulo R.L. Bittencourt Vinícius S. Pacheco Marcela T. Miranda Ya Zhang Rafael S. Oliveira Peter Groenendijk Eduardo C. Machado Melvin T. Tyree Steven Jansen Lucy Rowland Rafael V. Ribeiro 《Plant, cell & environment》2020,43(1):131-142
Xylem vulnerability to embolism represents an important trait to determine species distribution patterns and drought resistance. However, estimating embolism resistance frequently requires time-consuming and ambiguous hydraulic lab measurements. Based on a recently developed pneumatic method, we present and test the “Pneumatron”, a device that generates high time-resolution and fully automated vulnerability curves. Embolism resistance is estimated by applying a partial vacuum to extract air from an excised xylem sample, while monitoring the pressure change over time. Although the amount of gas extracted is strongly correlated with the percentage loss of xylem conductivity, validation of the Pneumatron was performed by comparison with the optical method for Eucalyptus camaldulensis leaves. The Pneumatron improved the precision of the pneumatic method considerably, facilitating the detection of small differences in the (percentage of air discharged [PAD] < 0.47%). Hence, the Pneumatron can directly measure the 50% PAD without any fitting of vulnerability curves. PAD and embolism frequency based on the optical method were strongly correlated (r2 = 0.93) for E. camaldulensis. By providing an open source platform, the Pneumatron represents an easy, low-cost, and powerful tool for field measurements, which can significantly improve our understanding of plant–water relations and the mechanisms behind embolism. 相似文献