Introduction: We investigated the safety of, and glucose control by, the insulin-only configuration of the iLet Ò bionic pancreas delivering fast-acting insulin aspart (faster aspart), using the same insulin-dosing algorithm but different time to maximal serum drug concentration (t max ) settings, in adults with type 1 diabetes. Methods: We performed a single-center, singleblinded, crossover (two 7-day treatment peri-ods) escalation trial over three sequential cohorts. Participants from each cohort were randomized to a default t max setting (t 65 [t max = 65 min]) followed by a non-default t max setting (t 50 [t max = 50 min; cohort 1], t 40 [t max = 40 min; cohort 2], t 30 [t max = 30 min; cohort 3]), or vice versa, all with faster aspart. Each cohort randomized eight new participants if escalation-stopping criteria were not met in the previous cohort. Results: Overall, 24 participants were randomized into three cohorts. Two participants discontinued treatment, one due to reported 'low blood glucose' during the first treatment period of cohort 3 (t 30 ). Mean time in low sensor glu-Supplementary Information The online version contains supplementary material available at