The most common methods of biomedical diagnosis are blood-based and therefore require invasive methods for acquiring a sample causing trauma and presenting a risk of infection. This has led to the development of sensors targeting minimally invasive bio-fluids such as interstitial fluid and non-invasive bio-fluids including saliva and sweat. The monitoring of sweat in electrolytes, for example, has a number of applications including determining the efficacy of drugs on the chloride and sodium levels in cystic fibrosis patients as well as the monitoring of hydration levels in athletes to detect non-symptomatic overhydration, dehydration and fatigue. This work presents the development of a platform for harvesting and analysing the sodium content of sweat in real time. In chapter 2 miniaturised solid-state sodium-selective and reference electrodes are developed, characterized and optimised. In chapter 3 these sensors are then incorporated into a custom designed sampling and fluidic system allowing passive transfer of the sweat from a point of contact on the skin to a fluidic channel across the electrodes for analysis before transport to a sampling reservoir. Chapter 4 presents the integration and real-time on-body trials of the sensing and fluidic components with a custom-designed electronic circuit board and lithium battery. Two platforms were developed, a ‘watch’ type format for wrist based analysis and a ‘pod’ format, arranged horizontally for adaptable monitoring in various regions of the body. The devices can be easily disassembled to replace the electrodes and the high capacity adsorbent material. The storage sweat is available for subsequent measurement of the total volume of sweat harvested and the average concentration of sodium over the period of use. Results obtained during trials over a period of up to 60 minutes controlled exercise are consistent with previously published data, showing a gradual increase of the sodium concentration in the sweat during this period.