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   3930篇
  免费   190篇
  国内免费   24篇
  2023年   76篇
  2022年   105篇
  2021年   292篇
  2020年   169篇
  2019年   194篇
  2018年   264篇
  2017年   162篇
  2016年   235篇
  2015年   286篇
  2014年   309篇
  2013年   324篇
  2012年   327篇
  2011年   272篇
  2010年   178篇
  2009年   157篇
  2008年   161篇
  2007年   143篇
  2006年   107篇
  2005年   104篇
  2004年   65篇
  2003年   56篇
  2002年   37篇
  2001年   6篇
  2000年   6篇
  1999年   9篇
  1998年   11篇
  1997年   6篇
  1996年   6篇
  1995年   8篇
  1994年   6篇
  1993年   8篇
  1992年   8篇
  1991年   4篇
  1990年   1篇
  1989年   5篇
  1988年   2篇
  1987年   5篇
  1986年   2篇
  1985年   3篇
  1984年   2篇
  1983年   2篇
  1982年   5篇
  1981年   4篇
  1980年   2篇
  1979年   1篇
  1978年   2篇
  1976年   1篇
  1974年   3篇
  1972年   1篇
  1968年   1篇
排序方式: 共有4144条查询结果,搜索用时 78 毫秒
21.
22.
Recent advances in non-invasively collected samples have opened up new and exciting opportunities for wildlife research. Different types of samples, however, involve different limitations and certain physiological markers (e.g., C-peptide, oxytocin) can only be reliably measured from urine. Common collection methods for urine to date work best for arboreal animals and large volumes of urine. Sufficient recovery of urine is thus still difficult for wildlife biologists, particularly for terrestrial and small bodied animals. We tested three collection devices (two commercially available saliva swabs, Salivette synthetic and cotton, and cotton First aid swabs) against a control to permit the collection of small volumes of urine from the ground. We collected urine samples from captive and wild macaques, and humans, measured volume recovery, and analyzed concentrates of selected physiological markers (creatinine, C-peptide, and neopterin). The Salivette synthetic device was superior to the two alternative devices. Concentrations of creatinine, absolute C-peptide, C-peptide per creatinine, absolute neopterin, and neopterin per creatinine measured in samples collected with this device did not differ significantly from the control and were also strongly correlated to it. Fluid recovery was also best for this device. The least suitable device is the First aid collection device; we found that while absolute C-peptide and C-peptide per creatinine concentrations did not differ significantly from the control, creatinine concentrations were significantly lower than the control. In addition, these concentrations were either not or weakly correlated to the control. The Salivette cotton device provided intermediate results, although these concentrations were strongly correlated to the control. Salivette synthetic swabs seem to be useful devices for the collection of small amounts of urine from the ground destined for the assessment of physiological parameters. They thus provide new opportunities for field studies to incorporate physiological markers, particularly on smaller bodied and terrestrial animals and where urine collection is difficult.  相似文献   
23.
24.
The present research was conducted in district Jhang, Pakistan, to evaluate the concentration of metals/metalloids in soil and pumpkin (Cucurbita maxima) irrigated with domestic wastewater. Data revealed that the levels of metals and metalloids in soil samples from two different sites were below the safe limits except Cd, whereas, in the vegetable, the concentrations of As, Se, Ni, Mo, Pb, Mn, and Cu were above the safe limits. The levels of 12 metals and metalloids in the soil were ranged between 0.14 to 22.76 mg/kg at site-I and 0.16 to 22.13 mg/kg at site-II. The levels of these metals in the vegetable were found 0.35 to 61.13 mg/kg at site-I and 0.31 to 53.63 mg/kg at site-II. The transfer factor at both sites was highest for As and Co. The pollution load index recorded for Se, Cu, Cd, Mo, Pb, and Co was greater than 1. The daily intake of As, Mn, and Mo was above the oral reference dose, which reflects that the intake of pumpkin is not safe for the inhabitants of the selected sites. The control measures should be taken to phytoextract heavy metals and metalloids from polluted sites so as to reduce the health risks.  相似文献   
25.
Recently soluble CD163 (sCD163), a cleaved form of the macrophage receptor CD163, was identified as a macrophage-specific risk-predictor for developing Type 2 Diabetes. Here, we investigate circulating levels of sCD163 in gestational diabetes mellitus (GDM). Furthermore, given the role of the placenta in the pathogenesis of GDM, we assessed placental contribution to sCD163 secretion. Paired maternal (venous) and umbilical vein blood samples from GDM (n = 18) and Body Mass Index (BMI) matched control women (n = 20) delivered by caesarean section at 39–40 week gestation were assessed for circulating levels of sCD163, Tumour necrosis factor alpha (TNF-α) and Interleukin 6 (IL-6). Media from explant culture of maternal subcutaneous fat and corresponding placental tissues were assayed for these same molecules. CD163 positive cell numbers were determined in placental and adipose tissues of GDM and control women. We found significantly elevated circulating sCD163 levels in GDM mothers (688.4±46.9 ng/ml vs. 505.6±38.6 ng/ml) and their offspring (418.2±26.6 ng/ml vs. 336.3±24.4 ng/ml [p<0.05 for both]) as compared to controls, together with elevated circulating TNF-α and IL-6 levels. Moreover, both GDM placentae (268.1±10.8 ng/ml/mg vs. 187.6±20.6 ng/ml/mg) and adipose explants (41.1±2.7 ng/ml/mg vs. 26.6±2.4 ng/ml/mg) released significantly more sCD163 than controls. Lastly, significantly more CD163 positive cells were observed in GDM placentae (25.7±1.1 vs. 22.1±1.2) and adipose tissue (19.1±1.1 vs 12.7±0.9) compared to controls. We describe elevated sCD163 levels in GDM and identify human placenta as a novel source of sCD163 suggesting that placental tissues might contribute to the increased levels of circulating sCD163 in GDM pregnancies.  相似文献   
26.
Abstract. Three different culture media, Ham's F-12, medium 199, and Eagle's minimal essential medium (MEM), were compared with respect to the expression of neuronal (choline acetyl transferase activity: CAT) and glial (hydrocortisone-induced glutamine synthetase activity; GSase) markers of normal differentiation in cultures of 9-day chick embryo neuroretinal cells, and also with respect to the accumulation of a lens marker (δ crystallin) during so-called 'transdifferentiation' in these cultures.
MEM allows transient expression of both CAT and GSase activities in early cultures, but also permits extensive δ crystallin accumulation at later stages. F-12 medium gives somewhat higher levels of CAT and GSase activities, the former being noticeably prolonged as compared with parallel MEM cultures; δ crystallin accumulation, however, is largely inhibited in F-12 cultures. By contrast, medium 199 permits only low levels of CAT and GSase activities, perhaps because the neuronal cells are distributed individually over the glial cell sheet in 199 cultures, rather than forming aggregates as in MEM or F–12 cultures. Medium 199 also blocks δ crystallin accumulation.
The results of medium changeover between 'transdifferentiation'-permissive (MEM) and non-permissive (199, F-12) conditions suggest: (a) that potential lens precursor cells (whatever their nature) survive in F-12 medium for prolonged periods without extensive expression of the lens phenotype; (b) that such precursor cells become committed to subsequent differentiation as lens cells between 10 and 20 days of culture in permissive MEM medium (as judged by the accumulation of δ crystallin following transfer into F-12); and (c) that medium 199 can block expression of the lens phenotype even in cells already committed (by the above criteria) to lens differentiation, as for instance after 30 days of preculture in MEM.  相似文献   
27.
Increasing markets for biopharmaceuticals, including monoclonal antibodies, have triggered a permanent need for bioprocess optimization. Biochemical engineering approaches often include the optimization of basal and feed media to improve productivities of Chinese hamster ovary (CHO) cell cultures. Often, l ‐tyrosine is added as dipeptide to deal with its poor solubility at neutral pH. Showcasing IgG1 production with CHO cells, we investigated the supplementation of three l ‐tyrosine (TYR, Y) containing dipeptides: glycyl‐l ‐tyrosine (GY), l ‐tyrosyl‐l ‐valine (YV), and l ‐prolyl‐l ‐tyrosine (PY). While GY and YV led to almost no phenotypic and metabolic differences compared to reference samples, PY significantly amplified TYR uptake thus maximizing related catabolic activity. Consequently, ATP formation was roughly four times higher upon PY application than in reference samples.  相似文献   
28.
Biomechanics and Modeling in Mechanobiology - Every group of microorganism utilizes a diverse mechanical strategy to propel through complex environments. These swimming problems deal with the...  相似文献   
29.
Characterizing the quasi-stiffness and work of lower extremity joints is critical for evaluating human locomotion and designing assistive devices such as prostheses and orthoses intended to emulate the biological behavior of human legs. This work aims to establish statistical models that allow us to predict the ankle quasi-stiffness and net mechanical work for adults walking on level ground. During the stance phase of walking, the ankle joint propels the body through three distinctive phases of nearly constant stiffness known as the quasi-stiffness of each phase. Using a generic equation for the ankle moment obtained through an inverse dynamics analysis, we identify key independent parameters needed to predict ankle quasi-stiffness and propulsive work and also the functional form of each correlation. These parameters include gait speed, ankle excursion, and subject height and weight. Based on the identified form of the correlation and key variables, we applied linear regression on experimental walking data for 216 gait trials across 26 subjects (speeds from 0.75–2.63 m/s) to obtain statistical models of varying complexity. The most general forms of the statistical models include all the key parameters and have an R2 of 75% to 81% in the prediction of the ankle quasi-stiffnesses and propulsive work. The most specific models include only subject height and weight and could predict the ankle quasi-stiffnesses and work for optimal walking speed with average error of 13% to 30%. We discuss how these models provide a useful framework and foundation for designing subject- and gait-specific prosthetic and exoskeletal devices designed to emulate biological ankle function during level ground walking.  相似文献   
30.
Biomechanical data characterizing the quasi-stiffness of lower-limb joints during human locomotion is limited. Understanding joint stiffness is critical for evaluating gait function and designing devices such as prostheses and orthoses intended to emulate biological properties of human legs. The knee joint moment-angle relationship is approximately linear in the flexion and extension stages of stance, exhibiting nearly constant stiffnesses, known as the quasi-stiffnesses of each stage. Using a generalized inverse dynamics analysis approach, we identify the key independent variables needed to predict knee quasi-stiffness during walking, including gait speed, knee excursion, and subject height and weight. Then, based on the identified key variables, we used experimental walking data for 136 conditions (speeds of 0.75–2.63 m/s) across 14 subjects to obtain best fit linear regressions for a set of general models, which were further simplified for the optimal gait speed. We found R2 > 86% for the most general models of knee quasi-stiffnesses for the flexion and extension stages of stance. With only subject height and weight, we could predict knee quasi-stiffness for preferred walking speed with average error of 9% with only one outlier. These results provide a useful framework and foundation for selecting subject-specific stiffness for prosthetic and exoskeletal devices designed to emulate biological knee function during walking.  相似文献   
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