Flexible wearable chemical sensors are emerging tools which target diagnosis and monitoring of medical conditions. One of the potential applications of wearable chemical sensors is therapeutic drug monitoring for drugs that have narrow therapeutic range. The effective dose of these drugs is so close to the toxic dose that it requires frequent monitoring of drug plasma concentration during patient administration. One example of such drugs is lithium which is used to treat bipolar disorder and major depression. This thesis investigated the possibility of developing a fibre-based wearable chemical sensor for lithium drug monitoring. A flexible cotton-based Li+ selective sensor and carbon-based reference electrode were fabricated to be incorporated in a wearable dermal patch for potential lithium drug monitoring in patient with bipolar disorder. Cotton fibres were converted to conductive cotton fibres by dipping in single-walled carbon nanotube ink until their resistance decreased to 500 Ω. Conductive cotton fibres were coated with a Li+ selective membrane solution via dip-coating to fabricate the Li+ sensor. Carbon fibres were dip-coated in Ag/AgCl ink followed by a reference membrane solution to obtain the carbon-based reference electrode. Potentiometric measurements of the Li+ sensor and reference electrode were performed vs. a double junction reference electrode and vs. each other and comparable results were obtained. The potentiometric response of the cotton-based Li+ sensor was linear over the Li+ concentration range (0.1 - 63 mM) for Li+ sensors which spans the ineffective, clinically relevant analytical range (0.4 - 1.0 mM) and toxic range of Li+ serum concentration. These fibre-based sensors were capable of determining Li+ concentration in aqueous and plasma spiked samples. An in vitro reverse iontophoresis experiment was performed to extract Li+ from under porcine skin by applying a current density of 0.4 mA cm-2 via two electrodes. Carbon fibre-based reverse iontophoresis electrodes were fabricated and used instead of conventional silver wire-based version and comparable results were obtained. The fibre-based Li+ sensor and reference electrodes were capable of determining the Li+ concentration in samples collected via reverse iontophoresis. Furthermore, a pilot experiment was performed to determine the biocompatibility of the materials used to develop the fibre-based Li+ sensor and reference electrode with promising initial results.