全文获取类型
收费全文 | 3856篇 |
免费 | 2篇 |
专业分类
电工技术 | 3篇 |
化学工业 | 12篇 |
机械仪表 | 1篇 |
轻工业 | 26篇 |
水利工程 | 1篇 |
无线电 | 3篇 |
一般工业技术 | 2篇 |
冶金工业 | 3800篇 |
自动化技术 | 10篇 |
出版年
2019年 | 1篇 |
2018年 | 1篇 |
2017年 | 2篇 |
2013年 | 1篇 |
2012年 | 4篇 |
2011年 | 3篇 |
2010年 | 1篇 |
2009年 | 2篇 |
2008年 | 3篇 |
2007年 | 2篇 |
2006年 | 2篇 |
2005年 | 2篇 |
2004年 | 5篇 |
2003年 | 13篇 |
2000年 | 2篇 |
1999年 | 137篇 |
1998年 | 1252篇 |
1997年 | 779篇 |
1996年 | 475篇 |
1995年 | 232篇 |
1994年 | 216篇 |
1993年 | 219篇 |
1992年 | 19篇 |
1991年 | 31篇 |
1990年 | 35篇 |
1989年 | 46篇 |
1988年 | 31篇 |
1987年 | 26篇 |
1986年 | 22篇 |
1985年 | 24篇 |
1984年 | 1篇 |
1983年 | 5篇 |
1982年 | 13篇 |
1981年 | 23篇 |
1980年 | 28篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 57篇 |
1976年 | 133篇 |
1975年 | 2篇 |
1970年 | 1篇 |
排序方式: 共有3858条查询结果,搜索用时 15 毫秒
151.
BACKGROUND: Previous studies have shown that the maze operation can restore sinus rhythm and atrial transport function in patients with chronic atrial fibrillation. The purpose of this study was to test the feasibility of the application of radiofrequency and cryoablation as an alternative to the classic maze operation. METHODS: Twelve patients undergoing mitral valve procedures were included in this study. Radiofrequency and cryoablation were applied to create lesions in both atria to simulate the classic maze operation. RESULTS: There were two surgical deaths. At the mean follow-up of 10.25 months for the remaining 10 patients; 6 were in sinus rhythm, 2 in atrial rhythm, 1 in paroxysmal atrial tachycardia, and 1 in atrial fibrillation. Doppler echocardiography at 6-month follow-up showed emergence of biatrial transport function in 3 patients and right atrial contractility in 8. At 12-month follow-up of 5 patients, Doppler echocardiography showed biatrial transport function in 3 and right atrial contractility in 4. CONCLUSIONS: Our modified maze procedure during valvular operation is effective for achieving an acceptable success rate to restore sinus rhythm and atrial transport function in patients with chronic atrial fibrillation. 相似文献
152.
153.
KH Lee MC Panelli CJ Kim AI Riker MP Bettinotti MM Roden P Fetsch A Abati SA Rosenberg FM Marincola 《Canadian Metallurgical Quarterly》1998,161(8):4183-4194
Peptide vaccination against tumor Ags can induce powerful systemic CTL responses. However, in the majority of patients, no tumor regression is noted. To study this discrepancy, we analyzed CTL reactivity in a melanoma patient (F001) vaccinated with g209-2M peptide, a single residue variant of gp100(209-217). G209/g209-2M-reactive CTL were identified in post- but not prevaccination PBL. Limiting dilution analysis identified one predominant CTL clone (C1-35), with TCR Vbeta6s2, recognizing g209/HLA-A*0201-expressing targets. Additionally, two autologous melanoma lines (F001TU-3 and -4) and 20 separate tumor-infiltrating lymphocyte cultures were generated from a fine needle aspirate of a metastatic lesion progressing after initial response to vaccination. Both F001TU did not express gp100 and were not recognized by C1-35. Loss of gp100 by F001TU correlated with a marked reduction of gp100 expression in the same metastatic lesion compared with prevaccination. Thus, ineffectiveness of C1-35 and tumor progression could be best explained by loss of target Ag expression. Interestingly, 12 of 20 tumor-infiltrating lymphocyte cultures recognized F001TU, but none demonstrated g209/g209-2M reactivity, suggesting a functional dissociation between systemic and local immune response. This study suggests that vaccination effects must be analyzed in the target tissue, rather than in the systemic circulation alone. 相似文献
154.
We showed that the efficacy of the new 2'-deoxycytidine (2'-dCyd) analogue antimetabolite 2'-deoxy-2'-methylidenecytidine (DMDC) correlates well with tumor levels of cytidine (Cyd) deaminase in human cancer xenograft models. DMDC was highly effective in tumors with higher levels of Cyd deaminase, whereas lower levels yielded only slight activity. In contrast, gemcitabine (2',2'-difluorodeoxycytidine), which has action mechanisms similar to those of DMDC, is only slightly active in tumors with higher levels of the enzyme. In the present study, we investigated the roles of Cyd deaminase in the antitumor activity of the two 2'-dCyd antimetabolites in 13 human cancer cell lines. Tetrahydrouridine, an inhibitor of Cyd deaminase, reduced the antiproliferative activity of DMDC (P = 0.0015). Furthermore, tumor cells transfected with the gene of human Cyd deaminase become more susceptible to DMDC both in vitro and in vivo. These results indicate that Cyd deaminase is indeed essential for the activity of DMDC. In contrast, the antiproliferative activity of gemcitabine was increased to some extent by tetrahydrouridine (P = 0.0277), particularly in tumor cell lines with higher levels of Cyd deaminase. This suggests that higher levels of Cyd deaminase may inactivate gemcitabine. Among nucleosides and deoxynucleosides tested, only dCyd, a natural substrate of both Cyd deaminase and dCyd kinase, suppressed the antiproliferative activity of DMDC by up to 150-fold. Because the Vmax/Km of DMDC for dCyd kinase was 8-fold lower than that for dCyd, the activation of DMDC to DMDC monophosphate (DMDCMP) by dCyd kinase might be competitively inhibited by dCyd. In addition, the dCyd concentrations in human cancer xenografts were inversely correlated with levels of Cyd deaminase activity. It is therefore suggested that higher levels of Cyd deaminase reduce the intrinsic cellular concentrations of dCyd in tumors, resulting in efficient activation of DMDC to DMDCMP by dCyd kinase. These results indicate that the efficacy of DMDC may be predicted by measuring the activity of Cyd deaminase in tumor tissues before treatment starts and that DMDC may be exploited in a new treatment modality: tumor enzyme-driven cancer chemotherapy. 相似文献
155.
Nitrogen-starved diploid cells of the yeast Saccharomyces cerevisiae differentiate into a filamentous, pseudohyphal growth form. Recognition of nitrogen starvation is mediated, at least in part, by the ammonium permease Mep2p and the Galpha subunit Gpa2p. Genetic activation of the pheromone-responsive MAP kinase cascade, which is also required for filamentous growth, only weakly suppresses the filamentation defect of Deltamep2/Deltamep2 and Deltagpa2/Deltagpa2 strain. Surprisingly, deletion of Mep1p, an ammonium permease not previously thought to regulate differentiation, significantly enhances the potency of MAP kinase activation, such that the STE11-4 allele induces filamentation to near wild-type levels in Deltamep1/Deltamep1 Deltamep2/Deltamep2 and Deltamep1/Deltamep1 Deltagpa2/Deltagpa2 strains. To identify additional regulatory components, we isolated high-copy suppressors of the filamentation defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant. Multicopy expression of TEC1, PHD1, PHD2 (MSS10/MSN1/FUP4), MSN5, CDC6, MSS11, MGA1, SKN7, DOT6, HMS1, HMS2, or MEP2 each restored filamentation in a Deltamep1/Deltamep1 Deltamep2/Deltamep2 strain. Overexpression of SRK1 (SSD1), URE2, DAL80, MEP1, or MEP3 suppressed only the growth defect of the Deltamep1/Deltamep1 Deltamep2/Deltamep2 mutant strain. Characterization of these genes through deletion analysis and epistasis underscores the complexity of this developmental pathway and suggests that stress conditions other than nitrogen deprivation may also promote filamentous growth. 相似文献
156.
A durable bond between the end of skeletal muscles and prosthetic structures could, with appropriate linkage, allow circulatory support power by synchronous and/or sequential contraction of several in situ conditioned muscles. Potential advantages relative to a myoplasty wrap involve 1) less traumatic dissection, 2) efficient linear force development, 3) selectable contraction rate, 4) greater stroke work, 5) independent control of muscle pre-load and end diastolic pressure, and 6) independent control of duration of muscle tension and ejection time. However, no existing means of tissue-prosthetic bonding appears adequate. Practicality would demand that full tension bearing capacity by the bond take no longer than muscle conditioning. A prosthesis was developed to achieve those goals. As scaled for this study, it is made of 7,200-7,800 unspun, unplaited, 22 to 26 microns diameter polyester fibers swaged into four taper needles for weaving through distal muscle. The other end is formed into a polyurethane sheathed kernmantel cord for distal fixation. Devices were implanted in six 3 to 4 kg rabbits (unilateral posterior tibial tendon replacement, random side selection with contralateral dissection/closure controls), and their tensile strength was tested at 30 days. All healed well; leg movements were normal after 1 week. Limbs were frozen at -70 degrees C between death and testing. Control failure occurred at 243 +/- 94 N and experimental at 163 +/- 44 N (p = 0.065, t-test); highest estimated requirement was 17.2 N. Interface strength was adequate by 30 days. Continued investigations, addressing other questions, are warranted. 相似文献
157.
158.
MR imaging is an essential tool for the evaluation of musculoskeletal neoplasms in children. It provides useful information for diagnosis, determination of extent of disease, assessment of response to treatment, and detection of complications and effects of therapy. This article reviews the current role of MR imaging in each of these situations. A protocol for the evaluation of these lesions is suggested. Also addressed are issues related to differentiating pathologic conditions, normal marrow, and cartilaginous structures of the pediatric skeleton. 相似文献
159.
MC Mackey 《Canadian Metallurgical Quarterly》1998,11(3):19-32
The purpose of this study was to identify how women described and evaluated their labor and delivery experience and what factors were related to their responses. Sixty Lamaze-prepared, married multigravidae, aged 21 to 37 years, participated in this qualitative field study. Detailed, open-ended tape-recorded interviews were conducted on the postpartum unit of a community hospital or in the women's homes early during the postpartum period. Women evaluated their labor and delivery experience according to how well they perceived they had managed their own childbirth performance. Women who managed well viewed childbirth as positive, whereas women who had difficulty or managed poorly viewed it as both positive and negative. Women who managed well thought their own performance and the nature of labor and delivery (physical aspects) went well; women who had difficulty thought labor and delivery and the performance of others went well, but women who managed poorly had problems identifying anything that went well. There was overall agreement that the baby was the best part of the experience and that pain and pushing were the worst parts. Since women's evaluation of their labor and delivery experience may be related to the quality of their subsequent mothering, it is important to enhance their perceptions of their own performance, and thus their evaluation of the childbirth experience. 相似文献
160.
The increasing presence of genotoxic chemicals in the aquatic environment has led to the development of both in vivo and in vitro assays for target species. The fish population represents an important level of aquatic ecosystems that can be threatened by increased environmental pollution. The authors have studied the DNA pattern of the RTG-2 fish cell line, a fibroblast-like cell line, derived from rainbow trout (Oncorhynchus mikyss), to use this cell line as an in vitro system to study genotoxicity by means of random amplified polymorphic DNA primers (RAPDs). A constant pattern in the DNA band is essential when an organism or cell line is used to detect DNA alterations produced by genotoxic environmental chemicals. DNA fingerprints with RAPDs were obtained for RTG-2 by testing 26 single and 70 pairwise combinations of primers. Different methods of DNA extraction (chelating resin, salting out, and phenolization), the influence of spectrometric measures at 320 nm in the 260/280 quotient to quantify DNA extracts, genomic DNA and primer concentrations, annealing temperatures, and cell line passage were studied in the cell line characterization. RAPD products were identified by agarose gel electrophoresis. The good results obtained should allow the use of this system as a possible tool for detection of the genotoxicity of aquatic pollutants. 相似文献