Lab-on-a-capillary: instrument-free compartmentalization using photopatterned hydrogel rings embedded inside a glass capillary for amplified bioassays.

Sensitive and low-cost protein biomarker detection is critical for disease diagnosis. Advanced microfluidic systems can generate miniature reaction compartments for a high-sensitivity assay. However, these platforms often require external instruments, skilled operators, and complex setups. Here, we develop a lab-on-a-capillary (LabCap) platform that integrates photopatterned hydrogel rings within a glass capillary using a reconfigurable stop-flow lithography system. During sample loading and unloading steps, nanoliter-scale aqueous droplets (torodrops) are spontaneously formed around the hydrogel rings, creating isolated reaction compartments without the need for external instruments or an immiscible oil phase. The LabCap platform enables detection of clinically relevant biomarkers, including C-reactive protein (CRP) and N-terminal pro-B-type natriuretic peptide (NT-proBNP). By adjusting the incubation protocol, the detection performance can be tuned to meet different analytical requirements. A 16 min periodic medium exchange protocol yielded calculated limits of detection of 1.3 ng mL-1 for CRP and 1.8 ng mL-1 for NT-proBNP, whereas static incubation yielded 0.5 ng mL-1 for both biomarkers after 1 h and 0.14 ng mL-1 for CRP at a matched 16 min incubation. In addition, LabCap offers practical advantages, including a low consumable material cost for fabricating one device (<€1), low reagent consumption (<100 μL per assay step), and minimal wash-buffer usage (1 mL). These results demonstrate that LabCap is a simple, cost-effective, and versatile platform for compartmentalized biomarker detection.