1. Department of Biomedical Engineering, Columbia University, 351 Engineering Terrace, 1210 Amsterdam Avenue, New York, New York 10027, Department of Electrical Engineering, Columbia University, 1300 South West Mudd 500, West 120th Street, New York, New York 10027, and International Center for AIDS Care and Treatment Programs, Mailman School of Public Health, Columbia University, New York, New York 10032