Continuous monitoring of lithium concentration fluctuations in blood plasma is essential for patients with bipolar disorder and manic depression since lithium has a low therapeutic index. While blood plasma concentrations between 0.4 and 1.0 mmol/L are considered to be in the therapeutic zone, the concentrations exceeding 1.3 mmol/L are toxic to the patients. Most of the point-of-care devices for lithium monitoring have bulky peripherals and require extensive operator handling, yet simple-to-use devices are in demand for 2–5% of the worldwide population receiving lithium therapy. This paper aims to develop a self-contained microfluidic device to run colorimetric lithium assays without the need for dedicated personnel or equipment. In the developed microchip, the assay reagents are mixed in sequential order via custom-designed microfluidic capillary circuits with the aid of a finger pump. The operation of the finger pump was characterized mathematically and demonstrated experimentally. The finger-driven pump achieved 45.9 mm/s flow velocity when 8.3 µL liquid was placed in a 160 mm long channel with 200 µm height, as such rapid triggering was a requirement for the colorimetric lithium test. The final device is able to quantify the lithium concentrations between 0 and 2.0 mM using a smartphone camera. The detection limit of this microchip was calculated as 0.1 mM. This device presents a portable alternative to on-site quantitative detection techniques with bulky and expensive tools.
