Self-powered microfluidic device for the colorimetric detection of lithium via sequential reagent mixing

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.