The ability to continuously monitor key blood parameters such as glucose and lactate would benefit a number of patient groups including diabetics and patients in Intensive Care Units. Although many sampling methods are under investigation, few have been found capable of simultaneously and continuously sampling both glucose and lactate minimally-invasively. Therefore it is the aim of this thesis to develop such a device capable of continuously monitoring glucose and lactate. The study and development of such a device provides an important insight into how patients might be monitored in future for a range of parameters. The method selected was transdermal extraction of glucose and lactate by reverse iontophoresis.