This study aims to suggest a simple
migratory cell monitoring method
in the Transwell system by utilizing retroreflective Janus microparticles
(RJPs) as an optical probe. The RJP could be internalized on cells
without compromising the cell viability and can be registered as bright
spots within the cell body by inducing retroreflection from nonspectroscopic
light sources. Conventional optical probes (e.g., fluorophores, chromogens,
and nanoparticles) have been extensively studied and applied across
diverse platforms (e.g., Boyden chamber, wound closing, and microfluidic
chips) for understanding in vitro kinetic cell behavior. However,
the complexities of running such platforms and setting up analytical
instruments are limiting. In this regard, we aimed to demonstrate
a modified Transwell migration assay by introducing the retroreflection
principle to the cell quantification procedures that ensure a simplified
optical setup, assure easy signal acquisition, and are compatible
with conventional platforms. To demonstrate retroreflection as a signaling
principle, a half-metal-coated silica particle that can induce interior
retroreflection was synthesized. Because the RJPs can concentrate
incident light and reflect it back to the light source, retroreflection
was distinctively recognizable and enabled sensitive visualization.
To verify the applicability of the developed migration assay, cell
quantification during the incremental progress of macrophage migration,
and cell quantification under gradients of chemoattractant monocyte
protein-1, was accomplished by obtaining phagocytosed RJP-mediated
retroreflection signals. Considering that conventional assays are
designed as endpoint measurements, we anticipate the proposed retroreflection-based
cell quantification technique to be a promising solution, bypassing
current limitations.