The ultimate goal of the investigators ongoing research is the development of a closed-loop system for insulin delivery, which can help people with type 1 diabetes (T1D) attain a tight glucose control avoiding the risk of hypoglycaemia. The main components of the system are a continuous glucose monitor (CGM), an insulin pump and a computer-based 'model-predictive algorithm', which computes the amount of insulin to be given by the insulin pump according to the CGM values. In the studies performed thus far the efficacy and safety of closed-loop glucose control was evaluated both overnight and over a prolonged period of time including the day-time in children and adolescents with T1D. The results showed that closed-loop improved control of blood glucose and prevented nocturnal hypoglycaemia, as compared to the conventional insulin pump therapy.

The objective of the current study is to test the performance of closed-loop further, by evaluating the system during common non-compliant behaviours in the administration of meal insulin doses in adolescents with T1D. This will pave the way for a more comprehensive use of closed loop systems to control glucose levels in T1D under various "real-life mimicking" common circumstances. The present study adopts an open-label, randomised, 2 period cross-over design whereby closed-loop insulin therapy will be compared with the conventional insulin pump therapy in 12 adolescents with T1D.

Participants aged 12 to 18 years will be randomised for two 24 hour studies in a clinical research facility, during which glucose levels will be controlled by either the computer-based closed-loop algorithm (intervention arm) or by conventional insulin pump therapy (control arm). On both occasions, participants will under-estimate and omit the meal-related insulin dose for the evening-meal and lunch, respectively. The study will take place at the Wellcome Trust Clinical Research Facility (WTCRF), Cambridge with participants recruited from paediatric diabetes clinics in England.

Inclusion Criteria:

• Aged 12-18 years
• Type 1 diabetes diagnosed for > 1 year
• Insulin pump treatment for at least 3 months
• HbA1c between 8 and 12%
• Subject willing to perform reduction/omission of meal insulin boluses during clinical studies

Exclusion Criteria:

• Non-type 1 diabetes mellitus
• Physical or psychological disease likely to interfere with the normal conduct of the study
• Current treatment with drugs known to interfere with glucose metabolism
• Known or suspected allergy against insulin
• Subjects with clinical significant nephropathy, neuropathy or proliferative retinopathy
• Total daily insulin dose >= 2 IU/kg/day
• Pregnancy, planned pregnancy, or breast feeding