Focusing and enrichment of deep-submicron particles and exosomes in a viscoelastic fluid using a hybrid cruciform-square microchannel

The precise manipulation of biological nanovesicles such as exosomes is crucial for biomedical applications. However, their small size makes them difficult to handle with conventional viscoelastic microfluidics, as the key elastic forces diminish cubically with particle diameter. To overcome this challenge, we developed a novel hybrid microfluidic device featuring a 16-cm-long square channel for initial alignment, followed by a 4-cm-long cruciform channel for intensive final focusing. The unique geometry of the cruciform channel, with its four 270º reflex angles, generates strong, localized shear rate gradients. This effect dramatically amplifies the elastic forces responsible for migrating particles to the channel centerline. We demonstrated the device’s superior performance by focusing synthetic particles as small as 50 nm into a single, tight stream. Critically, we also achieved high-efficiency focusing and an approximately 29-fold enrichment of exosomes, which were subsequently collected at the central outlet with minimal diffusion. This robust, label-free platform represents a significant advancement in nanoparticle manipulation, holding great promise for high-throughput applications in diagnostics, nano-flow cytometry, and liquid biopsy.