全文获取类型
收费全文 | 9734篇 |
免费 | 706篇 |
国内免费 | 607篇 |
出版年
2024年 | 15篇 |
2023年 | 108篇 |
2022年 | 258篇 |
2021年 | 420篇 |
2020年 | 281篇 |
2019年 | 341篇 |
2018年 | 351篇 |
2017年 | 283篇 |
2016年 | 361篇 |
2015年 | 534篇 |
2014年 | 651篇 |
2013年 | 715篇 |
2012年 | 819篇 |
2011年 | 749篇 |
2010年 | 436篇 |
2009年 | 388篇 |
2008年 | 440篇 |
2007年 | 439篇 |
2006年 | 376篇 |
2005年 | 364篇 |
2004年 | 297篇 |
2003年 | 304篇 |
2002年 | 245篇 |
2001年 | 197篇 |
2000年 | 195篇 |
1999年 | 162篇 |
1998年 | 109篇 |
1997年 | 102篇 |
1996年 | 104篇 |
1995年 | 102篇 |
1994年 | 81篇 |
1993年 | 78篇 |
1992年 | 127篇 |
1991年 | 101篇 |
1990年 | 72篇 |
1989年 | 73篇 |
1988年 | 67篇 |
1987年 | 49篇 |
1986年 | 41篇 |
1985年 | 53篇 |
1984年 | 33篇 |
1983年 | 25篇 |
1982年 | 19篇 |
1981年 | 10篇 |
1979年 | 10篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1971年 | 7篇 |
1970年 | 7篇 |
1966年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
Micro- and nanoparticulate drug-delivery systems (DDSs) play a significant role in formulation sciences. Most particulate DDSs are scaffold-free, although some particles are encapsulated inside other biomaterials for controlled release. Despite rapid progress in recent years, challenges still remain in controlling the homogenicity of micro-/nanoparticles, especially for two crucial factors in particulate DDSs: the size and shape of the particles. Recent approaches make use of microfabrication techniques to generate micro-/nanoparticles with highly controllable architectures free of scaffolds. This review presents an overview of a burgeoning field of DDSs, which can potentially overcome some drawbacks of conventional techniques for particle fabrication and offer better control of particulate DDSs. 相似文献
982.
Cell-based biosensors (CBBs) have emerged as promising biotechnical tools whereby various cell types can be used as basic sensing units to detect external stimuli. Specifically, CBBs have been applied in environmental monitoring, drug screening, clinical diagnosis and biosecurity. For these applications, CBBs offer several advantages over conventional molecular-based biosensors or living animal-based approaches, such as the capability to better mimic physiological situations, to enhance detection specificity and sensitivity, and to detect unknown compounds and toxins. On the other hand, existing CBBs suffer from several limitations, such as weak cell-substrate attachment, two-dimensional (2D) cell microenvironment, and limited shelf life. An emerging method for scaffold-free three-dimensional (3D) cell culture uses hydrogels to encapsulate cells. Advances in novel biomaterials and nano/microscale technologies have enabled encapsulation of cells in hydrogels to fabricate 3D CBBs, which hold great potential for addressing the limitation in existing 2D CBBs. Here, we present an overview of the emerging hydrogel-based CBBs, their applications in pathogen/toxin detection, drug screening and screening of cell-biomaterials interaction, and the associated challenges and potential solutions. 相似文献
983.
Spatial isolation is currently thought to represent one of the major factors resulting in bacteria genetic variation and population abundance. The bacterial diversity in a distinct environment Zoige Alpine Wetland located in the northeast of the Qinghai-Tibetan Plateau with the altitude 3400 m on average aroused our great attention. This area belongs to Qinghai-Tibetan cold climate zone with the mean annual temperature about 1 °C. Although several studies on bacterial diversity in Qinghai-Tibetan Plateau had been reported, there is no report on wetland water in this area. In this work, six water samples were collected and the water qualities including CODCr, NH4+-N, NO3--N, NO2--N, TN, TP, TOC were investigated, of which results indicated that more than 80% samples sorted as II–V class of surface water sources according to the National Water Quality Standard of China (GB3838-2002). Comparison of bacterial communities among the six samples was analyzed by DGGE of PCR-amplified 16S rDNA with universal bacterial primer sets. The profiles demonstrated that samples from the Flower Lake had more DNA bands than the Conservatory Station inferring higher diversity. In addition, the samples from the same environment shared similar compositions of bacterial communities. Bacterial community composition and predominant bacteria were analyzed by 16S rDNA clone library. The dominant group was Proteobacteria (51.6% of the total clones, which contained 24.2% alpha proteobacteria, 14.5% beta proteobacteria and 12.9% gamma proteobacteria). And the Bacteroidetes added to 17.7%, Verrucomicrobia to 4.8%. More than 24.2% of the total clones showed high similarity to uncultured bacteria. The above work provides some information on bacterial diversity for special site of spatial isolation. 相似文献
984.
985.
Zhang AM Bandelt HJ Jia X Zhang W Li S Yu D Wang D Zhuang XY Zhang Q Yao YG 《PloS one》2011,6(10):e26511
Mitochondrial transfer RNA (mt-tRNA) mutations have been reported to be associated with a variety of diseases. In a previous paper that studied the mtDNA background effect on clinical expression of Leber''s hereditary optic neuropathy (LHON) in 182 Chinese families with m.11778G>A, we found a strikingly high frequency (7/182) of m.593T>C in the mitochondrially encoded tRNA phenylalanine (MT-TF) gene in unrelated LHON patients. To determine the potential role of m.593T>C in LHON, we compared the frequency of this variant in 479 LHON patients with m.11778G>A, 843 patients with clinical features of LHON but without the three known primary mutations, and 2374 Han Chinese from the general populations. The frequency of m.593T>C was higher in LHON patients (14/479) than in suspected LHON subjects (12/843) or in general controls (49/2374), but the difference was not statistically significant. The overall penetrance of LHON in families with both m.11778G>A and m.593T>C (44.6%) was also substantially higher than that of families with only m.11778G>A (32.9%) (P = 0.083). Secondary structure prediction of the MT-TF gene with the wild type or m.593T>C showed that this nucleotide change decreases the free energy. Electrophoretic mobility of the MT-TF genes with the wild type or m.593T>C transcribed in vitro further confirmed the change of secondary structure in the presence of this variant. Although our results could suggest a modest synergistic effect of variant m.593T>C on the LHON causing mutation m.11778G>A, the lack of statistical significance probably due to the relatively small sample size analyzed, makes necessary more studies to confirm this effect. 相似文献
986.
Our recent studies have uncovered that aggregation-prone proinsulin preserves a low relative folding rate and maintains a homeostatic balance of natively and non-natively folded states (i.e., proinsulin homeostasis, PIHO) in β-cells as a result of the integration of maturation and disposal processes. Control of precursor maturation and disposal is thus an early regulative mechanism in the insulin production of β-cells. Herein, we show pathways involved in the disposal of endogenous proinsulin at the early secretory pathway. We conducted metabolic-labeling, immunoblotting, and immunohistochemistry studies to examine the effects of selective proteasome and lysosome or autophagy inhibitors on the kinetics of proinsulin and control proteins in various post-translational courses. Our metabolic-labeling studies found that the main lysosomal and ancillary proteasomal pathways participate in the heavy clearance of insulin precursor in mouse islets/β-cells cultured at the mimic physiological glucose concentrations. Further immunoblotting and immunohistochemistry studies in cloned β-cells validated that among secretory proteins, insulin precursor is heavily and preferentially removed. The rapid disposal of a large amount of insulin precursor after translation is achieved mainly through lysosomal autophagy and the subsequent basal disposals are carried out by both lysosomal and proteasomal pathways within a 30 to 60-minute post-translational process. The findings provide the first clear demonstration that lysosomal and proteasomal pathways both play roles in the normal maintenance of PIHO for insulin production, and defined the physiological participation of lysosomal autophagy in the protein quality control at the early secretory pathway of pancreatic β-cells. 相似文献
987.
BMI-1 is overexpressed in a variety of cancers, which can elicit an immune response leading to the induction of autoantibodies. However, BMI-1 autoantibody as a biomarker has seldom been studied with the exception of nasopharyngeal carcinoma. Whether BMI-1 autoantibodies can be used as a biomarker for cervical carcinoma is unclear. In this study,BMI-1 proteins were isolated by screening of a T7 phage cDNA library from mixed cervical carcinoma tissues. We analyzed BMI-1 autoantibody levels in serum samples from 67 patients with cervical carcinoma and 65 controls using ELISA and immunoblot. BMI-1 mRNA or protein levels were over-expressed in cervical carcinoma cell lines. Immunoblot results exhibited increased BMI-1 autoantibody levels in patient sera compared to normal sera. Additionally, the results for antibody affinity assay showed that there was no difference between cervical polyps and normal sera of BMI-1 autoantibody levels, but it was significantly greater in patient sera than that in normal controls (patient 0.827±0.043 and normal 0.445±0.023; P<0.001). What''s more, the levels of BMI-1 autoantibody increased significantly at stage I (0.672±0.019) compared to normal sera (P<0.001), and levels of BMI-1 autoantibodies were increased gradually during the tumor progression (stage I 0.672±0.019; stage II 0.775 ±0.019; stage III 0.890 ±0.027; stage IV 1.043±0.041), which were significantly correlated with disease progression of cervical carcer (P<0.001). Statistical analyses using logistic regression and receiver operating characteristics (ROC) curves indicated that the BMI-1 autoantibody level can be used as a biomarker for cervical carcinoma (sensitivity 0.78 and specificity 0.76; AUC = 0.922). In conclusion, measuring BMI-1 autoantibody levels of patients with cervical cancer could have clinical prognostic value as well as a non-tissue specific biomarker for neoplasms expressing BMI-1. 相似文献
988.
Rane AA Chuang JS Shah A Hu DP Dalton ND Gu Y Peterson KL Omens JH Christman KL 《PloS one》2011,6(6):e21571
Background
Several injectable materials have been shown to preserve or improve cardiac function as well as prevent or slow left ventricular (LV) remodeling post-myocardial infarction (MI). However, it is unclear as to whether it is the structural support or the bioactivity of these polymers that lead to beneficial effects. Herein, we examine how passive structural enhancement of the LV wall by an increase in wall thickness affects cardiac function post-MI using a bio-inert, non-degradable synthetic polymer in an effort to better understand the mechanisms by which injectable materials affect LV remodeling.Methods and Results
Poly(ethylene glycol) (PEG) gels of storage modulus G′ = 0.5±0.1 kPa were injected and polymerized in situ one week after total occlusion of the left coronary artery in female Sprague Dawley rats. The animals were imaged using magnetic resonance imaging (MRI) at 7±1 day(s) post-MI as a baseline and again post-injection 49±4 days after MI. Infarct wall thickness was statistically increased in PEG gel injected vs. control animals (p<0.01). However, animals in the polymer and control groups showed decreases in cardiac function in terms of end diastolic volume, end systolic volume and ejection fraction compared to baseline (p<0.01). The cellular response to injection was also similar in both groups.Conclusion
The results of this study demonstrate that passive structural reinforcement alone was insufficient to prevent post-MI remodeling, suggesting that bioactivity and/or cell infiltration due to degradation of injectable materials are likely playing a key role in the preservation of cardiac function, thus providing a deeper understanding of the influencing properties of biomaterials necessary to prevent post-MI negative remodeling. 相似文献989.
Background
Cardiac stem cells (CSCs) promote myocardial recovery following ischemia through their regenerative properties. However, little is known regarding the implication of paracrine action by CSCs in the setting of myocardial ischemia/reperfusion (I/R) injury although it is well documented that non-cardiac stem cells mediate cardioprotection via the production of paracrine protective factors. Here, we studied whether CSCs could initiate acute protection following global myocardial I/R via paracrine effect and what component from CSCs is critical to this protection.Methodology/Principal Findings
A murine model of global myocardial I/R was utilized to investigate paracrine effect of Sca-1+ CSCs on cardiac function. Intracoronary delivery of CSCs or CSC conditioned medium (CSC CM) prior to ischemia significantly improved myocardial function following I/R. siRNA targeting of VEGF in CSCs did not affect CSC-preserved myocardial function in response to I/R injury. However, differentiation of CSCs to cardiomyocytes (DCSCs) abolished this protection. Through direct comparison of the protein expression profiles of CSCs and DCSCs, SDF-1 was identified as one of the dominant paracrine factors secreted by CSCs. Blockade of the SDF-1 receptor by AMD3100 or downregulated SDF-1 expression in CSCs by specific SDF-1 siRNA dramatically impaired CSC-induced improvement in cardiac function and increased myocardial damage following I/R. Of note, CSC treatment increased myocardial STAT3 activation after I/R, whereas downregulation of SDF-1 action by blockade of the SDF-1 receptor or SDF-1 siRNA transfection abolished CSC-induced STAT3 activation. In addition, inhibition of STAT3 activation attenuated CSC-mediated cardioprotection following I/R. Finally, post-ischemic infusion of CSC CM was shown to significantly protect I/R-caused myocardial dysfunction.Conclusions/Significance
This study suggests that CSCs acutely improve post-ischemic myocardial function through paracrine factor SDF-1 and up-regulated myocardial STAT3 activation. 相似文献990.
Rust secreted protein Ps87 is conserved in diverse fungal pathogens and contains a RXLR-like motif sufficient for translocation into plant cells 总被引:3,自引:0,他引:3