Polymer-waveguide-based low-cost photonic biosensors for quantitative tear fluid diagnostics at the point of care

Tear fluid analysis is a minimally invasive and easily accessible method for biomarker testing compared to blood analysis, enabling frequent monitoring without significant discomfort for the patient. Among available sensing strategies for tear fluid diagnostics, photonic biosensors provide a quantitative biomarker sensing platform for various markers ranging from pH to pathogens, offering high sensitivity and specificity. Sensor concepts which are currently under investigation include, among others, interferometric and surface plasmon resonance (SPR) sensors. In this work, we present the design of waveguide-based SPR sensors as well as their integration into a low-cost polymer chip for tear fluid sensing. For fabrication, the waveguide cladding and sensing elements are realized on a low-cost poly(methyl methacrylate) (PMMA) substrate. Waveguide structures are defined by thermal nanoimprint using laser-written stamps in negative photoresist. The resulting recesses are subsequently filled with a higher-refractive-index UV-curable adhesive that forms the waveguide core with a sufficiently low propagation loss for compact photonic sensor systems. Optical fibers were chemically coated with silver to show a proof-of-concept SPR measurement with the aim to implement this method on our polymer waveguides. The long-term goal of the current study is to design, fabricate, and characterize multivariate sensor chips with emphasis on refractive-index sensitivity, limit of detection and quantification of physiologically relevant biomarkers in tear fluid. Ultimately, this platform is intended as a compact, low-cost, and easy-to-use foundation for minimally invasive, point-of-care monitoring of various diseases.