Identifying abnormalities in red blood cells can provide important medical clues for the diagnosis, prognosis, and treatment of some health disorders. A common test used to examine the conditions of erythrocytes in humans is osmotic fragility. The standard technique to determine the osmotic fragility of red blood cells is laborious and time-consuming, and provides only approximate values with a few experimental data. In this work, we propose and investigate a way to measure the osmotic fragility of erythrocytes, rapidly and in a straightforward, more quantitative and less laborious way, with an optical sensor. The basic idea is monitoring in real time the refractive index of a suspension of erythrocytes to follow the kinetics of their lysis as the hemoglobin-rich cytosol is released from the erythrocytes in the surrounding hypotonic solution during hemolysis, resulting in an increase of the refractive index. We propose a device to measure in real time the refractive index and demonstrate the feasibility of monitoring the hemolysis process with high resolution. Our results show that the release of hemoglobin-rich cytosol during the progress of the hemolysis of erythrocytes in hypotonic media is generally an exponential-like function of time. We provide a mathematical model that reproduces experimental curves effectively. We show results for erythrocytes obtained from human blood stored in a blood bank and from volunteers diagnosed with two different types of anemia: hemolytic and drepanocytic. We define times, measurable from the kinetics of the refractive index signal, to quantify the progress of hemolysis as a possible new method of measuring osmotic fragility.

Background: Monitoring lysis in cell's suspensions can be a good indicator of cell's integrity and health. The current techniques to measure the number of lysed cells in some process are generally time consuming. Objective: In this paper, we investigate the viability of a simple method for sensing hemolysis in real time through the electrical impedance of a parallel-plate capacitor that holds a biological cell suspension. Materials and methods: The method is based on measuring variations of the effective dielectric function of an erythrocyte suspension during a lysis process induced by osmotic stress. Our sensor is strongly affected by the so-called electrode polarization phenomenon which prevents sensing variations of the real part of the effective dielectric function. However, measurements of the imaginary part, and thus of the electrical resistivity of the suspension, is straight forward. Results: We show that from resistivity measurements, it is possible to sense variations of the lysed erythrocytes' volume fraction with a resolution better than 0.1%. Conclusions: The proposed sensor can offer a high resolution in monitoring hemolysis and is a viable option for developing new rapid diagnostic medical tests.

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Light's internal reflectivity near a critical angle is very sensitive to the angle of incidence and the optical properties of the external medium near the interface. Novel applications in biology and medicine of subcritical internal reflection are being pursued. In many practical situations the refractive index of the external medium may vary with respect to its bulk value due to different physical phenomena at surfaces. Thus, there is a pressing need to understand the effects of a refractiveindex gradient at a surface for near-critical-angle reflection. In this work we investigate theoretically the reflectivity near the critical angle at an interface with glass assuming the external medium has a continuous depthdependent refractive index. We present graphs of the internal reflectivity as a function of the angle of incidence, which exhibit the effects of a refractive-index gradient at the interface. We analyse the behaviour of the reflectivity curves before total internal reflection is achieved. Our results provide insight into how one can recognise the existence of a refractive-index gradient at the interface and shed light on the viability of characterising it.

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