Active learning, including the promotion of student interactivity in lectures, has been found to improve student engagement and performance in university science classes. This letter describes the use of Pearson's Learning Catalytics to run regular, formatively-assessed problem-solving sessions as part of the semi-flipped redesign of an introductory level university bioscience course. Students found the problem solving sessions more engaging than a traditional lecture, and felt that they were receiving better feedback on their progress in the course. Their participation in the problem solving sessions was strongly associated with their performance in the course's summative assessments, making it possible to identify and assist probable poor performers early in the course. Other measures of student engagement with the course were not improved, and neither were their average exam grades when compared with their grades in a course which had not been redesigned. Possible reasons for this are discussed.