首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   4133篇
  免费   224篇
  国内免费   35篇
耳鼻咽喉   43篇
儿科学   54篇
妇产科学   51篇
基础医学   549篇
口腔科学   118篇
临床医学   303篇
内科学   1307篇
皮肤病学   88篇
神经病学   353篇
特种医学   98篇
外科学   598篇
综合类   22篇
一般理论   1篇
预防医学   192篇
眼科学   68篇
药学   248篇
中国医学   8篇
肿瘤学   291篇
  2023年   28篇
  2022年   56篇
  2021年   98篇
  2020年   61篇
  2019年   73篇
  2018年   109篇
  2017年   69篇
  2016年   83篇
  2015年   104篇
  2014年   157篇
  2013年   184篇
  2012年   276篇
  2011年   287篇
  2010年   168篇
  2009年   151篇
  2008年   285篇
  2007年   300篇
  2006年   263篇
  2005年   301篇
  2004年   279篇
  2003年   254篇
  2002年   210篇
  2001年   28篇
  2000年   24篇
  1999年   31篇
  1998年   35篇
  1997年   30篇
  1996年   33篇
  1995年   43篇
  1994年   31篇
  1993年   25篇
  1992年   28篇
  1991年   21篇
  1990年   18篇
  1989年   11篇
  1988年   12篇
  1987年   18篇
  1986年   10篇
  1985年   7篇
  1984年   20篇
  1983年   18篇
  1982年   19篇
  1981年   9篇
  1980年   7篇
  1979年   6篇
  1977年   9篇
  1976年   6篇
  1975年   10篇
  1957年   5篇
  1944年   5篇
排序方式: 共有4392条查询结果,搜索用时 265 毫秒
991.
Background and aimsThe WHO Global Action Plan for the Prevention of non-communicable diseases (NCDs) recommends a 30% relative reduction in mean population salt/sodium intake. The study assessed the trend in the habitual salt intake of the Italian adult population from 2008 to 2012 to 2018–2019 based on 24-h urinary sodium excretion, in the framework of the CUORE Project/MINISAL-GIRCSI/MENO SALE PIU’ SALUTE national surveys.Methods and resultsData were from cross-sectional surveys of randomly selected age and sex–stratified samples of resident persons aged 35–74 years in 10 (out of 20) Italian Regions distributed in North, Centre and South of the Country. Urinary sodium and creatinine measurements were carried out in a central laboratory. The analyses included 942 men and 916 women examined in 2008–2012, and 967 men and 1010 women examined in 2018–2019. The age-standardized mean daily population salt (sodium chloride) intake was 10.8 g (95% CI 10.5–11.1) in men and 8.3 g (8.1–8.5) in women in 2008–2012 and respectively 9.5 g (9.3–9.8) and 7.2 g (7.0–7.4) in 2018–2019. A statistically significant (p<0.0001) salt intake reduction was thus observed over 10 years for both genders, and all age, body mass index (BMI) and educational classes.ConclusionsThe average daily salt intake of the Italian general adult population remains higher than the WHO recommended level, but a significant reduction of 12% in men and 13% in women has occurred in the past ten years. These results encourage the initiatives undertaken by the Italian Ministry of Health aimed at the reduction of salt intake at the population level.  相似文献   
992.
993.
994.
The interfacial electronic structure and charge transfer dynamics of poly-3-hexylthiophene (P3HT) and multi-walled carbon nanotube (Fe-MWCNT) nanocomposites were investigated by near-edge X-ray absorption fine structure (NEXAFS) and resonant Auger (RAS) spectroscopies around the sulfur K-edge. Nanocomposites with 5 wt% (P3HT/Fe-MWCNT-5%) and 10 wt% (P3HT/Fe-MWCNT-10%) of Fe-MWCNT species were prepared and compared with pristine P3HT film. The quantitative NEXAFS analysis shows a strong π–π interchain interaction of the pristine P3HT polymer film, which is reduced by the presence of the Fe-MWCNT. S–KL2,3L2,3 RAS spectra were measured at photon energies corresponding to the main electronic transitions appearing in the S–K edge NEXAFS spectrum. Ultrafast charge transfer times were estimated from the RAS spectra using the core-hole clock approach with the S 1s core-hole lifetime as an internal clock. The π–π interchain charge transfer time increases from 4.7 fs on pristine P3HT polymer to 6.5 fs on the P3HT/Fe-MWCNT-5% nanocomposite. The electronic coupling between P3HT and Fe-MWCNT species occurs mainly through the P3HT π* molecular orbital. The increase of Fe-MWCNT concentration from 5 to 10 wt% reduces the charge transfer rate at the resonance maximum due probably to Fe-MWCNT aggregation, reducing the P3HT and Fe-MWCNT electronic coupling.

The electronic coupling between P3HT polymer and multi-walled carbon nanotubes was elucidated using NEXAFS and core hole clock approaches.  相似文献   
995.
BACKGROUNDMatrix metalloproteinases (MMPs) participate in the degradation of extracellular matrix compounds, maintaining the homeostasis between fibrogenesis and fibrolytic processes in the liver. However, there are few studies on the regulation of liver MMPs in fibrosis progression in humans. AIMTo assess the production activity and regulation of matrix metalloproteinases in liver fibrosis stages in chronic hepatitis C (CHC).METHODSA prospective, cross-sectional, multicenter study was conducted. CHC patients were categorized in fibrosis grades through FibroTest®and/or FibroScan®. Serum MMP-2, -7, and -9 were determined by western blot and multiplex suspension array assays. Differences were validated by the Kruskal-Wallis and Mann-Whitney U tests. The Spearman correlation coefficient and area under the receiver operating characteristic curve were calculated. Collagenolytic and gelatinase activity was determined through the Azocoll substrate and zymogram test, whereas tissue inhibitor of metalloproteinase-1 production was determined by dot blot assays.RESULTSSerum concentrations of the MMPs evaluated were higher in CHC patients than in healthy subjects. MMP-7 distinguished early and advanced stages, with a correlation of 0.32 (P < 0.001), and the area under the receiver operating characteristic displayed moderate sensitivity and specificity for MMP-7 in F4 (area under the receiver operating characteristic, 0.705; 95% confidence interval: 0.605-0.805; P < 0.001). Collagenolytic activity was detected at F0 and F1, whereas gelatinase activity was not detected at any fibrosis stage. Tissue inhibitor of metalloproteinase-1 determination showed upregulation in F0 and F1 but downregulation in F2 (P < 0.001). CONCLUSIONHigh concentrations of inactive MMPs were present in the serum of CHC patients, reflecting the impossibility to restrain liver fibrosis progression. MMPs could be good diagnostic candidates and therapeutic targets for improving novel strategies to reverse liver fibrosis in CHC.  相似文献   
996.
Bacterial adhesion to the surface of orthodontic materials is an important step in the formation and proliferation of plaque bacteria, which is responsible for enamel demineralization and periodontium pathologies. With the intent of investigating if adhesive resins used for bracket bonding are prone to bacteria colonization, the surface roughness of these materials has been analyzed, combining information with a novel methodology to observe the internal structures of orthodontic composites. Scanning electron microscopy, combined with focus ion bean micromachining and stylus profilometry analyses, were performed to evaluate the compositional factors that can influence specific pivotal properties facilitating the adhesion of bacteria to the surface, such as surface roughness and robustness of three orthodontic adhesive composite resins. To confirm these findings, contact angle measurements and bacteria incubation on resin slide have been performed, evaluating similarities and differences in the final achievement. In particular, the morphological features that determine an increase in the resins surface wettability and influence the bacterial adhesion are the subject of speculation. Finally, the focused ion beam technique has been proposed as a valuable tool to combine information coming from surface roughness with specific the internal structures of the polymers.  相似文献   
997.
Essential tremor (ET) is the most common pathological tremor disorder in the world, and post-mortem evidence has shown that the cerebellum is the most consistent area of pathology in ET. In the last few years, advanced neuroimaging has tried to confirm this evidence. The aim of the present review is to discuss to what extent the evidence provided by this field of study may be generalised. We performed a systematic literature search combining the terms ET with the following keywords: MRI, VBM, MRS, DTI, fMRI, PET and SPECT. We summarised and discussed each study and placed the results in the context of existing knowledge regarding the cerebellar involvement in ET. A total of 51 neuroimaging studies met our search criteria, roughly divided into 19 structural and 32 functional studies. Despite clinical and methodological differences, both functional and structural imaging studies showed similar findings but without defining a clear topography of neurodegeneration. Indeed, the vast majority of studies found functional and structural abnormalities in several parts of the anterior and posterior cerebellar lobules, but it remains to be established to what degree these neural changes contribute to clinical symptoms of ET. Currently, advanced neuroimaging has confirmed the involvement of the cerebellum in pathophysiological processes of ET, although a high variability in results persists. For this reason, the translation of this knowledge into daily clinical practice is again partially limited, although new advanced multivariate neuroimaging approaches (machine-learning) are proving interesting changes of perspective.  相似文献   
998.
999.
1000.
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号