Harvard Medical School plans to strengthen its conflict-of-interest rules for doctors and researchers. The move comes amid a U.S. Senate investigation into several faculty members and a new state law that will make public some of the payments doctors receive from pharmaceutical and medical-device companies. Many top medical schools have adopted stricter policies in the past two years. Last year, the American Medical Student Association graded Harvard with an F on its conflict-of-interest policy because it does not address issues like whether companies can provide gifts and meals for faculty.