The glucose sensor that will be used throughout the clinical validation studies is a CE marked, MHRA approved device manufactured by Medtronic. It is a subcutaneous sensor which sits just under the skin and samples interstitial fluid using an enzyme electrode. A small voltage is applied across the sensor and a current is fed back to the sensor instrumentation. This current is proportional to the glucose concentration in interstitial fluid and is calibrated against blood glucose a minimum of 12 hourly.