Inclusion Criteria:

• Willing to sign an informed consent form
• Age ≥ 18 years
• Willing to comply to study protocol requirements (study visits with frequent venous blood sampling and deliberate insulin, glucose, ketone and lactate challenges)
• Willing to charge the SENSOR on daily basis
• Willing not to insert their standard CGM and/or insulin pump in and around the SENSOR implant site (± radius 5 cm)[TS| XR1.1] Diagnosed with type 1 diabetes mellitus according to the World Health Organization criteria, for at least 12 months prior to screening, on CSII therapy

Exclusion Criteria:

• Known allergy to polydimethylsiloxane (PDMS)
• A contraindication to undergo challenging tests (i.e., ischemic heart disease, epilepsy, panhypopituitarism, hypoadrenalism, untreated hypothyroidism)
• History of hepatitis C or HIV or other disease transmissible by blood
• A condition requiring or likely to require magnetic resonance imaging during the study duration
• Female participants who are pregnant, planning on becoming pregnant or nursing
• Coagulation disorder, wound healing and bleeding disorder or taking anticoagulant medication (oral anticoagulants; antiplatelets (such as aspirin, ticagrelor are allowed).
• Anemia identified by a haemoglobin <14 g/dL for men or <12 g/dL for women
• The presence of any other active implanted device
• The presence of any other CGM sensor/pump located in the lower abdomen (other location is acceptable)
• Waist circumference of >120 cm
• Any disorder, which in the investigator's opinion might jeopardise participants' safety or compliance with the protocol.