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Progress in microelectrode-based technologies has facilitated the development of sophisticated methods for manipulating and separating cells, bacteria, and other bioparticles. For many of these various applications, the theoretical modeling of the electrical response of compartmentalized particles to an external field is important. In this paper we address the analysis of the interaction between cells immersed in rf fields. We use an integral formulation of the problem derived from a consideration of the charge densities induced at the interfaces of the particle compartments. The numerical solution by a boundary element technique allows characterization of their dielectric properties. Experimental validation of this theoretical model is obtained by investigating two effects: (1) The influence that dipolar "pearl chaining" has on the dielectrophoretic behavior of human T lymphocytes and (2) the frequency variation of the spin and orbital torques of approaching insulinoma beta-cells in a rotating field.
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BACKGROUND: In the Andean Region, HIV and sexually transmitted infections (STI) are most prevalent among men who have sex with men (MSM), but incidence estimates and associated factors have never been prospectively assessed. METHODS: A cohort of 1,056 high-risk HIV-negative MSM in Lima, Peru, were recruited during 1998-2000 (the ALASKA Cohort), and a nested case-control analysis was conducted between seroconverters and nonseroconverters, matched 1:3 by age and duration of follow-up for comparison of risk behaviors, acute retroviral symptoms, circumcision, and STI. RESULTS: During average follow-up of 335 days, 34 men seroconverted, providing a HIV incidence estimate of 3.5 per 100 person-years [95% confidence interval (CI): 2.3 to 4.7]. High syphilis (8.4 per 100 person-years, 95% CI: 6.7 to 10.1) and herpes simplex virus type 2 (HSV-2) infection (10.4 per 100 person-years, 95% CI: 8.6 to 11.9) incidence estimates were obtained. HIV seroconverters were more likely than men who remained seronegative to report fever >or=3 days (46% vs. 7%), to seek medical care (62% vs. 27%), and to have >or=1 casual partner (86.2% vs. 74.1%) since their last visit. HIV seroconverters also were more likely to have acquired syphilis or HSV-2 infection (31% vs. 8% among initially HSV-2-seronegative men) although they were less likely to be circumcised (4.2% vs. 20.6%, a nonsignificant difference). In multivariate analysis, incident syphilis or HSV-2 infection (odds ratio [OR]: 5.9, 95% CI: 1.5 to 22.7) and sex with any casual partner (OR: 4.8, 95% CI: 0.9 to 26.2) were associated with HIV seroconversion. CONCLUSIONS: STI that may cause anogenital ulcers are important risk factors for HIV acquisition among high-risk MSM in Lima, a population with a very high HIV incidence estimate. Synergistic interventions focusing in preventing both HIV and HSV-2, like male circumcision, are warranted to be assessed, especially in MSM populations with low levels of circumcision and high incidence estimates of ulcerative STI.