Stimuli-responsive microgel-based etalons are promising optical and bio-sensors. These sensors play a pivotal role in modern healthcare by enabling rapid biomolecule detection, contributing to organ-on-chip applications and early disease diagnosis. This study investigates the suitability of poly(N-isopropylacrylamide) (pNIPAm)-based microgels for inkjet printing, focusing on optimizing their properties for effective deposition. Key parameters, including surface tension, viscosity, and particle size, are characterized to ensure compatibility with inkjet-printing requirements. The addition of surfactants tunes the suspensions’ properties to be in line with the requirements of inkjet printing. Jetting of pNIPAm-based microgels on gold-coated substrates forms a cohesive drop in a range of a few millimeters. Optical and scanning electron microscopy confirm the formation of a uniform microgel layer. The optical reflectance spectroscopy results indicate that inkjet-printed microgel-based etalons can effectively respond to changes in temperature and glucose concentration. In-liquid atomic force microscopy demonstrates the swelling dynamics of the microgels in different glucose concentrations, shedding light on their response dynamics. Our work demonstrates, for the first time, the feasibility of printing microgels in a controlled way, fabricating biocompatible inkjet-printed microgel-based etalon sensors with precise dimensions. The size precision and the sensitive monitoring capabilities of biomolecules hold great promise for in situ and continuous sensing in a wide range of biological and organ-on-chip applications.

Lateral flow assays (LFAs) are widely favored for on-site analysis due to their simplicity and cost-effectiveness. However, their limited quantitative capabilities constrain them to qualitative testing. In contrast, etalon sensors offer high sensitivity and enable quantitative detection. They operate by producing interference-based optical signals through multiple reflections of light within a cavity formed by two parallel reflective surfaces. This requires a stable optical path length, traditionally limiting their use to smooth substrates.

This study presents an integrated separation-sensing microfluidic platform (EtLFA). By fabricating etalons on commercial membranes and evaluating sensor's sensitivity and surface roughness, we determined that membrane surface roughness must meet two criteria — Sa < 0.5 μm and Smr > 90% — to support functional etalons. Capillary and permeability remain intact after etalon integration, ensuring membrane's purification performance. We further functionalized the etalon to respond specifically to glucose, to demonstrate the quantitative detection of glucose levels in a mimic blood sample. A glucose-responsive etalon@nylon served as the sensor module, while regenerated cellulose membrane enabled separation. This dual-module configuration filtered PDMS particles mimicking red blood cells and produced a 25 nm shift for 100 mg/dL glucose, enabling linear quantification via portable spectrometry. By incorporating etalon sensor onto rough membrane substrates, our platform transforms conventional LFAs into a quantitative analytical tool, offering novel avenues for enhancing analytical capabilities and broadening the applications of lateral flow assays.

We introduce Microgel-based Etalon Membranes (MEMs), based on the combination of stimuli-responsive microgels with an etalon, which is an optical device consisting of two reflecting plates and is used to filter specific wavelengths of light. The microgels are sandwiched between two reflective layers and, in response to a stimulus (e.g., temperature, pH, or biomarker concentration), swell or de-swell, thereby changing the distance between the two reflective layers and generating multiple peaks in the reflectance spectra. This property gives a MEM the unique capability of simultaneous separation and tunable responses to environmental changes and/or biomarker concentrations. We propose a design based on gold layers on a silicon nitride wafer membrane. Our comprehensive characterization, employing permeability experiments, in situ optical reflectance spectroscopy, in-liquid atomic force microscopy (AFM) analysis, and captive bubble contact angle measurements, elucidates the dynamic response of MEM to pH, temperature, and glucose stimuli and the corresponding effect of microgel swelling/de-swelling on the membrane properties, e.g., permeability. The AFM results confirm the dynamic changes of the microgel layer’s thickness on the membrane surface in response to the stimuli. Although the microgel’s swelling/de-swelling influences the effective pore radius, the decrease in the membrane’s permeance is limited to less than 10%. In the swollen state of the microgels, the etalon membranes show a prominent hydrophilic behavior, while they become less hydrophilic in the microgels’ de-swollen state. This work introduces MEM and provides novel insights into their behavior. The fundamental understanding that we reveal opens the way to applications ranging from point-of-care testing to continuous environmental monitoring.