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   6093篇
  免费   386篇
  国内免费   126篇
耳鼻咽喉   103篇
儿科学   128篇
妇产科学   64篇
基础医学   947篇
口腔科学   152篇
临床医学   594篇
内科学   1283篇
皮肤病学   132篇
神经病学   474篇
特种医学   485篇
外科学   772篇
综合类   114篇
预防医学   273篇
眼科学   62篇
药学   507篇
中国医学   79篇
肿瘤学   436篇
  2024年   4篇
  2023年   60篇
  2022年   180篇
  2021年   263篇
  2020年   128篇
  2019年   129篇
  2018年   168篇
  2017年   162篇
  2016年   228篇
  2015年   362篇
  2014年   406篇
  2013年   447篇
  2012年   585篇
  2011年   554篇
  2010年   320篇
  2009年   301篇
  2008年   348篇
  2007年   344篇
  2006年   261篇
  2005年   176篇
  2004年   127篇
  2003年   105篇
  2002年   92篇
  2001年   101篇
  2000年   78篇
  1999年   57篇
  1998年   101篇
  1997年   70篇
  1996年   66篇
  1995年   57篇
  1994年   49篇
  1993年   49篇
  1992年   22篇
  1991年   7篇
  1990年   17篇
  1989年   22篇
  1988年   25篇
  1987年   16篇
  1986年   21篇
  1985年   17篇
  1984年   9篇
  1983年   12篇
  1982年   9篇
  1981年   4篇
  1980年   8篇
  1979年   4篇
  1978年   6篇
  1977年   6篇
  1976年   12篇
  1975年   9篇
排序方式: 共有6605条查询结果,搜索用时 15 毫秒
51.

Introduction

Humans are widely exposed to estrogenically active phthalates, parabens, and phenols, raising concerns about potential effects on breast tissue and breast cancer risk. We sought to determine the association of circulating serum levels of these chemicals (reflecting recent exposure) with mammographic breast density (a marker of breast cancer risk).

Methods

We recruited postmenopausal women aged 55 to 70 years from mammography clinics in Madison, Wisconsin (N = 264). Subjects completed a questionnaire and provided a blood sample that was analyzed for mono-ethyl phthalate, mono-butyl phthalate, mono-benzyl phthalate, butyl paraben, propyl paraben, octylphenol, nonylphenol, and bisphenol A (BPA). Percentage breast density was measured from mammograms by using a computer-assisted thresholding method.

Results

Serum BPA was positively associated with mammographic breast density after adjusting for age, body mass index, and other potentially confounding factors. Mean percentage density was 12.6% (95% confidence interval (CI), 11.4 to 14.0) among the 193 women with nondetectable BPA levels, 13.7% (95% CI, 10.7 to 17.1) among the 35 women with detectable levels below the median (<0.55 ng/ml), and 17.6% (95% CI, 14.1 to 21.5) among the 34 women with detectable levels above the median (>0.55 ng/ml; Ptrend = 0.01). Percentage breast density was also elevated (18.2%; 95% CI, 13.4 to 23.7) among the 18 women with serum mono-ethyl phthalate above the median detected level (>3.77 ng/ml) compared with women with nondetectable BPA levels (13.1%; 95% CI, 11.9 to 14.3; Ptrend = 0.07). No other chemicals demonstrated associations with percentage breast density.

Conclusions

Postmenopausal women with high serum levels of BPA and mono-ethyl phthalate had elevated breast density. Further investigation of the impact of BPA and mono-ethyl phthalate on breast cancer risk by using repeated serum measurements or other markers of xenoestrogen exposure are needed.  相似文献   
52.
Anti-bone resorption properties of the Korean herbal formulation, Gami-Honghwain (HJ), which comprises Carthamus tinctorius L. seed and hominis placenta, were investigated. We demonstrate that the production of PGE2 is inhibited by 20-100 microg/ml HJ in nontransformed osteoblastic cells (MC3T3-E1 cells), indicating that HJ inhibits PGE2 production. The effect of HJ on the proliferation and osteoblastic differentiation in MC3T3-E1 was also studied. HJ dose-dependently increased DNA synthesis (significant at 20-100 microg/ml), and increased alkaline phosphatase (ALP) and prolyl hydroxylase activities of MC3T3-E1 cells (20-100 microg/ml), while anti-estrogen tamoxifen eliminated the stimulation of proliferation and ALP activity of MC3T3-E1 which was induced by HJ. These results indicate that HJ directly stimulates cell proliferation and differentiation of osteoblasts. Also, when we assessed the effects of HJ on osteoblastic differentiation in MC3T3-E1, HJ enhanced ALP activity and mineralization in a dose- and time-dependent fashion. This stimulatory effect of the HJ was observed at relatively low doses (significant at 20-100 microg/ml and maximal at 100 microg/ml). Northern blot analysis showed that the HJ (60 microg/ml) increased in bone morphogenetic protein-2 as well as ALP mRNA concentrations in MC3T3-E1 cells. HJ (100 microg/ml) slightly increased in type I collagen mRNA abundance throughout the culture period, whereas it markedly inhibited the gene expression of collagenase-1 between days 15 and 20 of culture. These results indicate that HJ has anabolic effect on bone through the promotion of osteoblastic differentiation, suggesting that it could be used for the treatment of common metabolic bone diseases.  相似文献   
53.
OBJECTIVES: To increase the yield of fetal nucleated red blood cells (NRBCs) from maternal blood using a discontinuous Percoll gradient and to determine the effects of osmolality on NRBC yield. METHODS: Fetal NRBCs were isolated from combined umbilical cord blood and adult female blood, or from maternal blood using single or double Percoll gradients with different osmolalities. Magnetic activated cell sorting was used to enrich isolated NRBCs, and morphological differentiation was performed with Kleihauer-Betke stain. We also isolated fetal NRBCs from 25 10 mL samples of maternal blood and determined fetal sex by fluorescence in situ hybridization (FISH), using X-Y probes. RESULTS: For single-density Percoll columns, the greatest number of NRBCs was isolated using 280 mOsm/kg H(2)O with 1.077 g/mL Percoll and 520 mOsm/kg H(2)O with 1.119 g/mL Percoll. Significantly more fetal NRBCs were isolated with double Percoll density gradients than with double-Histopaque gradients (p = 0.043). FISH analysis on NRBC in 25 cases correctly identified 15 male and 9 female euploid fetuses and one Trisomy 21 fetus. CONCLUSION: The NRBC enrichment method we present requires less maternal blood and yields more NRBCs compared to previous methods.  相似文献   
54.
0 引言 膝关节周围是骨原发恶性肿瘤的好发部位 .病变组织广泛侵袭 ,切除肿瘤造成骨与软组织缺损 ,各种重建方法都需最大限度地保留膝关节功能 .如何采取早期系统的康复治疗 ,是临床骨肿瘤保肢手术的重要内容 .1 对象和方法  1992 - 0 5 / 1999- 0 3,膝关节周围恶性骨肿瘤患者 6 4例接受保肢手术治疗 .男 38例 ,女 2 6例 ;骨肉瘤 5 2例 ,恶性骨巨细胞瘤 5例 ,尤文瘤 3例 ,母细胞瘤 2例 ,原发神经外胚层肿瘤 2例 .手术方式 :异体半关节移植术 37例 ;异体骨段移植术 16例 ;复合异体骨段的人工全膝关节表面置换术3例 ,动力旋转铰链式人工全…  相似文献   
55.
This work aims to fabricate a large-area ceramic substrate for the application of probe cards. Mullite (M) and cordierite (C), which both have a low thermal expansion coefficient, excellent resistance to thermal shock, and high durability, were selected as starting powders. The mullite–cordierite composites were produced through different composition ratios of starting powders (M:C = 100:0, M:C = 90:10, M:C = 70:30, M:C = 50:50, M:C = 30:70, and M:C = 0:100). The effects of composition ratio and sintering temperature on the density, porosity, thermal expansion coefficient, and flexural strength of the mullite–cordierite composite pellets were investigated. The results showed that the mullite–cordierite composite pellet containing 70 wt% mullite and 30 wt% cordierite sintered at 1350 °C performed exceptionally well. Based on these findings, a large-area mullite–cordierite composite substrate with a diameter of 320 mm for use in semiconductor probe cards was successfully fabricated. Additionally, the changes in sheet resistance and flexural strength were measured to determine the effect of the environmental tests on the large-area substrate such as damp heat and thermal shock. The results indicated that the mullite–cordierite composite substrate was extremely reliable and durable.  相似文献   
56.
This study demonstrates the utility of thermo-regulated phase separable alumina/camphene suspensions containing poly(methyl methacrylate) (PMMA) microspheres as porogens for the production of multi-scale porosity structures. The homogeneous suspension prepared at 60 °C could undergo phase separation during freezing at room temperature. This process resulted in the 3D networks of camphene crystals and alumina walls containing PMMA microspheres. As a consequence, relatively large dendritic pores with several tens of microns size could be created as the replica of frozen camphene crystals. In addition, after the removal of PMMA microspheres via heat-treatment, micron-sized small spherical pores could be generated in alumina walls. As the PMMA content with respect to the alumina content increased from 0 vol% to 40 vol%, while the camphene content in the suspensions was kept constant (70 vol%), the overall porosity increased from 45.7 ± 0.5 vol% to 71.4 ± 0.5 vol%. This increase in porosity is attributed to an increase in the fraction of spherical pores in the alumina walls. Thus, compressive strength decreased from 153 ± 18.3 MPa to 33 ± 7.2 MPa. In addition, multi-scale porosity alumina objects with a honeycomb structure comprising periodic hexagonal macrochannels surrounded by dual-scale porosity walls were constructed using a 3D plotting technique.  相似文献   
57.
The mechanical behaviors of two multi-materials, DP590 (steel sheet)–A356 (cast aluminum alloy) and SS330 (steel sheet)–A5052 (aluminum sheet), were studied. A structural adhesive was used for the joining of steel and aluminum at adhesion strengths of 10, 22, and 30 MPa. To demonstrate that the three-point bending properties depend on the difference in strength between steel and aluminum and adhesion strength, optical microscopy (OM), scanning electron microscopy (SEM), and finite-element analysis (FEA) were performed. According to the results of the bending tests on both multi-materials under the same stacking conditions, the flexural stress increased with the improvement in the adhesion strength until interface separation or aluminum fracture. At the same adhesion strength, the DP590 (lower)–A356 (upper) and SS330 (upper)–A5052 (lower) configurations exhibited a tendency to decrease in the sudden stress drop due to aluminum fracture and interface separation. The bending results were analyzed through the FEA and the stress distribution as a function of the stacking and adhesion strength was confirmed.  相似文献   
58.
Perchlorate contamination in groundwater poses a serious threat to human health, owing to its interference with thyroid function. The high solubility and poor adsorption of perchlorate ions make perchlorate degradation a necessary technology in groundwater contaminant removal. Here, we demonstrate the perchlorate degradation by employing nano zero-valent iron (nZVI) embedded in biocompatible silica alginate hybrid beads fabricated using calcium chloride (1 wt%) as a crosslinker. The concentration of precursors (sodium alginate, sodium silicate) for bead formation was standardized by evaluating the thermal stability of beads prepared at different sodium silicate and alginate concentrations. Thermal degradation of silica alginate hybrid samples showed a stepwise weight loss during the thermal sweep, indicating different types of reactions that occur during the degradation process. The formation of the silica alginate hybrid structure was confirmed by FT-IR spectroscopy. Scanning electron microscopy (SEM) data revealed the surface morphology of silica alginate hybrid changes by varying sodium silicate and alginate concentrations. nZVI-loaded alginate–silicate polymer bead (nZVI-ASB) exhibited excellent perchlorate degradation efficiency by degrading 20 ppm of perchlorate within 4 h. Our study also showed the perchlorate degradation efficiency of nZVI-ASB is maximum at neutral pH conditions.  相似文献   
59.
Sarcopenia and obesity are serious health problems that are highly related to several metabolic diseases. Sarcopenic obesity, a combined state of sarcopenia and obesity, results in higher risks of metabolic diseases and even mortality than sarcopenia or obesity alone. Therefore, the development of therapeutic agents for sarcopenic obesity is crucial. C57BL/6 mice were fed with a high-fat diet (HFD) for 9 weeks. Then, mice were administered with Panax ginseng berry extract (GBE) for an additional 4 weeks, with continuous HFD intake. GBE significantly decreased the food efficiency ratio, serum lipid and insulin levels, adipose tissue weights, and adipocyte size. It significantly increased the grip strength, muscle masses, and myofiber cross-sectional area. It deactivated the protein kinase C (PKC) theta and zeta, resulting in activation of the phosphoinositide 3-kinase (PI3K)/protein kinase B (Akt) pathway, which is known to regulate muscle synthesis and degradation. Furthermore, it inhibited the production of inflammatory cytokines in the muscle tissue. GBE attenuated both obesity and sarcopenia. Thus, GBE is a potential agent to prevent or treat sarcopenic obesity.  相似文献   
60.
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