CapSense-Flex: A Self-Powered Capillary Lab-on-Chip for Universal Electrochemical Biosensing

Capillary microfluidics offer an attractive route to pump-free point-of-care (POC) diagnostics, yet most existing platforms remain constrained by continuous flow, short residence times (<2 min), and mandatory wash cycles that prevent integration with many affinity-based assays. We present CapSense-Flex, a laser-cut, roll-to-roll-manufacturable capillary chip that enables wash-free, diffusion-dominated molecular sensing with programmable incubation windows of 6-30 min, matching the binding kinetics required for molecularly imprinted polymer (MIP)-based electrochemical detection. The platform autonomously transports ≤25 μL of sweat, saliva, plasma, or whole blood without pumps, valves, or user actuation, achieving fluid-front arrival times within <5% of finite-element transport predictions while preventing bubble formation via fiber-assisted wicking. Embedded with a cortisol-selective MIP electrode, CapSense-Flex provides log-linear quantification in buffer (PBS) and human saliva from 1 to 1000 ng mL⁻¹ (R² = 0.9835), a 0.1 ng mL⁻¹ limit of detection, an imprinting factor of ≈5.0, and <3.1% RSD (n = 5). The sensor retained an ∼90% signal after 60 days at 4 °C and directly quantified cortisol in unprocessed saliva samples (1.6-5.0 ng mL⁻¹) with <12% signal loss relative to PBS standards. Each capillary chip is fabricated in <7 min at <$0.40 material cost and supports plug-and-replace electrode modules for alternative MIP targets. By decoupling capillary transport from binding kinetics and eliminating external pumps or wash steps, CapSense-Flex establishes a universal architecture for scalable MIP-based biosensing across handheld, wearable, and decentralized diagnostic formats.