Table 1.

Summary of the imaging methods discussed in this review and their strengths and weaknesses when used to image microfluidic devices and OOC platforms.

Method	Sub-Method	Strengths for Microfluidic Devices	Weaknesses for Microfluidic Devices	Strengths for OOCs	Weaknesses for OOCs	Ref.
Transillumination	Brightfield	Ability to automatically count cells	Blur in images; need for traditional microscope; less cost-effective; not transportable	Preliminary method before higher resolution imaging	Low resolution	[26,27,30,31,32,33]
	Phase-contrast	Extended field depth; Ability to measure phase changes	Low resolution	Ability to measure phase and morphological changes	Low resolution	[39,40]
	Holographic optofluidic	Phase images can be obtained; Allow portability to device	High signal-to-noise ratio; complicated; decent but not the best resolution	None at this time	Not currently demonstrated in OOC platforms	[42,43]
Fluorescence	Confocal	High resolution	Photobleaching; phototoxicity; not label-free	High resolution; ability to measure cell–cell interactions	Photobleaching; phototoxicity; not label-free	[32,33,45,46,47,48,49,66]
	LSFM	High acquisition rate; reduced phototoxicity	Some phototoxicity still exists	High acquisition rate; reduced phototoxicity	Some phototoxicity still exists	[23,81,83,84,85,86,87,88,91]
Smartphone-based	Microscope attachment-based	Low-cost; portability	Compromised resolution compared to benchtop microscopy	Low cost; in situ monitoring	Compromised resolution compared to benchtop microscopy	[4,111]
	Quantitative phase microscopy	Ability to measure phase and morphological changes; label-free; high resolution; reduced cost	Only suitable for imaging phase changes	Ability to measure phase and morphological changes; label-free	Focus drift can occur	[114,115]
	Lens-free microscopy	Reduced cost and size; high resolution	Slow processing speed	Low cost; can monitor cell activity in response to biomolecules	Lower resolution if image sensor is not close to cells	[118,120,122]