Background: The association between copper level and risk of preeclampsia has produced inconsistent results. Thus, a meta-analysis was conducted to summarize the evidence from epidemiological studies for copper level and preeclampsia risk. Methods: Pertinent studies were identified by a search of PubMed and Web of Knowledge up to Apr 2016. Standardized mean difference (SMD) was performed to combine the results. Random-effect model was used. Publication bias was estimated using Egger's regression asymmetry test. Results: Twelve articles (10 case-control studies and 2 cross-sectional studies) involving 442 preeclampsia cases and 463 health controls were included in this meta-analysis. Our pooled results suggested that preeclampsia patients had a higher copper level compared with healthy pregnancy controls [summary SMD = 0.69, 95% CI: 0.54-0.84; I² = 96.7%; P< 0.001]. The association was also significant in Asian population [SMD= 0.73, 95% CI= 0.57-0.90, I² = 97.3%] and European populations [SMD= 0.50, 95% CI= 0.14-0.86, I² = 58.9%]. After conducting the subgroup analysis and sensitive analysis, the results showed consistent significant association with the one based on all studies. No publication biases were found. Conclusions: Our analysis indicated that plasma or serum copper level in preeclampsia patients was significantly higher than that in healthy pregnancy women.

To further study the discharge characteristics and machining mechanism of micro-electrical discharge machining (micro-EDM), the variation trends of the discharge energy and discharge crater size with actual discharge duration are discussed based on single-pulse experiments. The polarity effect of micro-EDM was analyzed according to the motion characteristics of electrons and ions in the discharge plasma channel. The results show that the discharge current and voltage of micro-EDM were independent of the discharge width and open-circuit voltage. The energy utilization rate of the short-pulse discharge was relatively high, and the energy utilization rate decreased gradually as the discharge duration increased. Even if the mass of the positive ion was much larger than that of the electron, the kinetic energy of the positive ion was still less than that of the electron when bombarding the surface of the electrode. The acceleration and speed of electrons were very high, and the number of times that electrons bombarded the surface of positive electrode was more than 600 times that of positive ions bombarding the surface of the negative electrode during the same time.