Widefield frequency domain fluorescence lifetime imaging microscopy (FD-FLIM) for accurate measurement of oxygen gradients within microfluidic devices

An oxygen gradient is a key variable influencing various biological activities in vivo, such as tissue repair and tumor growth. To study the phenomenon, in vitro cell studies using microfluidic devices capable of generating oxygen gradients have been developed recently. However, it is challenging to accurately measure the gradient profiles in devices. The traditional fluorescence intensity-based method suffers from the difficulty of accurate measurement due to background fluorescence artefacts. In addition, it is hard to obtain accurate calibration conditions because of the difficulties to achieve a fully depleted and saturated oxygen concentrations in the devices. To overcome these difficulties, a widefield frequency domain fluorescence imaging microscopy (FD-FLIM) system was constructed and utilized to accurately measure oxygen gradient profiles in a microfluidic device in this paper. Since lifetime-based measurements do not solely depend on intensity variations, oxygen calibration processes are amiable and the measured oxygen concentrations can be more accurate. The performance of the FD-FLIM system was validated by comparing the experimental and simulation results in microfluidic devices with different geometries. The experimental results show that the oxygen gradients generated from the chemical reaction method can provide more hypoxic oxygen conditions compared to the gradients created by the gas flowing method. Owing to the advantages provided by the widefield microscopy technique, the image acquisition time can be significantly reduced resulting in less photobleaching for time-lapsed imaging applications. Consequently, the measurement technique developed in this paper is an efficient tool, which can greatly help scientists to better study biological activities under various oxygen conditions.