Impedance based urine analysis for early diagnosis of chronic kidney disease

Biosensors for chronic disease biomarkers are needed for monitoring disease progression and early diagnosis of the disease. Label-free biosensing methods are well suited as direct application of the sample is possible without preparation steps for biomarker labelling. Electrical biosensors offer the required low-cost, label-free solution for disease biomarkers. This work describes the design, fabrication, characterization and functionalization of interdigitated electrodes (IDE) for an impedance spectroscopy-based biosensor.

The process of designing the IDE involves the analytical and finite element modelling of the IDE geometries. The influence of the geometrical parameters on the capacitance of the IDE is discussed. The capacitance of the IDE is directly proportional to the metallization ratio. The finite element simulation suggested that the thickness of the passivation layer should be about 0.8 μm from the sensor substrate. The electric fields are highly concentrated at this thickness.

The IDE designs suggested are fabricated using inkjet printing technology. Silver nanoparticle ink is printed to form the IDE fingers and the passivation layer is printed using SU-8 ink. The printing process for both ink types is individually optimized for repeatable print results. IDE with metallization ratio of 0.8 is achieved, which is the highest metallization ratio achieved by this fabrication process.

The non-passivated and the passivated IDE are characterized with different analytes to observe their impedance response. The impedance spectra are fitted to an equivalent circuit and the changes in the components of the equivalent circuit due to the analytes are observed. The passivated IDE shows a more capacitive response as compared to the non-passivated IDE. The impedance response of the passivated IDE is not affected by the temperature of the analytes.

Finally, functionalization protocols for the non-passivated and the passivated IDE established to detect proteins. The non-passivated IDE functionalized to immobilize streptavidin on the sensor. The decrease in impedance due to the immobilization of the protein proved that printed IDE can be used as a protein biosensor. The passivated IDE is functionalized to immobilize uromodulin which is a urine biomarker for chronic kidney disease. Uromodulin concentrations were measured successfully from artificial urine samples with a sensitivity of 4 pF/(μg/ml) of uromodulin. This concept of urine-based biosensor can be further used to early diagnose chronic kidney disease due to tubular damage of the nephrons.



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