Rectangular Glass Micro-capillaries Research Articles

Spectral Interferometric Detection of NIR Optical Resonances of Rectangular Microcapillaries for Refractive Index Sensing

In this work, we propose a spectral interferometric technique for detecting the wavelength position of the resonances of low-cost optical resonators, consisting in rectangular glass microcapillaries. Moreover, we exploited these microdevices as core elements of a microoptofluidic sensing platform designed for the measurements of the refractive index (RI) of fluid samples, based on spectral phase shift interferometry. A detailed theoretical model was implemented to simulate the spectral shape of the interferometric signal in a wavelength range where cavity resonances of the capillary-based optical resonator occur. For experimental verification, the microdevice was placed at the end of the measurement arm of a Michelson interferometer and illuminated using a light source emitting broadband radiation in the near infrared region around 1.55 μm. The resonances of the empty capillary (i.e., filled with air) were successfully detected as steep phase jumps of the interferometric cosine signal recorded as a function of the wavelength, in good agreement with the theoretical predictions. Finally, further experimental measurements were carried out by filling the capillary channel with water-glucose dilutions with different concentrations to prove the suitability of the phase shift detection method to monitor RI variations with respect to a reference fluid.

Spectral Phase Shift Interferometry for Refractive Index Monitoring in Micro-Capillaries.

In this work, we demonstrate spectral phase-shift interferometry operating in the near-infrared wavelength range for refractive index (RI) monitoring of fluidic samples in micro-capillaries. A detailed theoretical model was developed to calculate the phase-sensitive spectral reflectivity when low-cost rectangular glass micro-capillaries, filled with samples with different refractive indices, are placed at the end of the measurment arm of a Michelson interferometer. From the phase-sensitive spectral reflectivity, we recovered the cosine-shaped interferometric signal as a function of the wavelength, as well as its dependence on the sample RI. Using the readout radiation provided by a 40-nm wideband light source with a flat emission spectrum centered at 1.55 µm and a 2 × 1 fiberoptic coupler on the common input-output optical path, experimental results were found to be in good agreement with the expected theoretical behavior. The shift of the micro-capillary optical resonances, induced by RI variations in the filling fluids (comparing saline solution with respect to distilled water, and isopropanol with respect to ethanol) were clearly detected by monitoring the positions of steep phase jumps in the cosine-shaped interferometric signal recorded as a function of the wavelength. By adding a few optical components to the instrumental configuration previously demonstrated for the spectral amplitude detection of resonances, we achieved phase-sensitive detection of the wavelength positions of the resonances as a function of the filling fluid RI. The main advantage consists of recovering RI variations by detecting the wavelength shift of “sharp peaks”, with any amplitude above a threshold in the interferometric signal derivative, instead of “wide minima” in the reflected power spectra, which are more easily affected by uncertainties due to amplitude fluctuations.

Flow-through micro-capillary refractive index sensor based on T/R spectral shift monitoring.

We present a flow-through refractive index sensor for measuring the concentration of glucose solutions based on the application of rectangular glass micro-capillaries, with channel depth of 50 µm and 30 µm. A custom designed and 3D printed polymeric shell protects the tiny capillaries, ensuring an easier handling and interconnection with the macro-fluidic path. By illuminating the capillary with broadband radiation centered at λ~1.55 µm, both the transmitted (T) and reflected (R) optical spectrum from the capillary are detected with an optical spectrum analyzer, exploiting an all-fiber setup. Monitoring the spectral shift of the ratio T/R in response to increasing concentration of glucose solutions in water we have obtained sensitivities up to 530.9 nm/RIU and limit of detection in the range of 10-5-10-4 RIU. Experimental results are in agreement with the theoretically predicted principle of operation. After the demonstration of amplitude detection at a single wavelength, we finally discuss the impact of the capillary parameters on the sensitivity.

Low-Coherence Reflectometry for Refractive Index Measurements of Cells in Micro-Capillaries.

The refractive index of cells provides insights into their composition, organization and function. Moreover, a good knowledge of the cell refractive index would allow an improvement of optical cytometric and diagnostic systems. Although interferometric techniques undoubtedly represent a good solution for quantifying optical path variation, obtaining the refractive index of a population of cells non-invasively remains challenging because of the variability in the geometrical thickness of the sample. In this paper, we demonstrate the use of infrared low-coherence reflectometry for non-invasively quantifying the average refractive index of cell populations gently confined in rectangular glass micro-capillaries. A suspension of human red blood cells in plasma is tested as a reference. As a use example, we apply this technique to estimate the average refractive index of cell populations belonging to epithelial and hematological families.

Refractive Index Sensing in Rectangular Glass Micro-Capillaries by Spectral Reflectivity Measurements

We report the application of rectangular glass micro-capillaries with cross-section of 20 μm × 200 μm and 50 μm × 500 μm as capillary-driven liquid-core optical cavities for fluid refractive index sensing. Reflected power spectra in two wavelength bands (around 1.3 μm and 1.55 μm) are detected with readout broadband radiation crossing the capillary in orthogonal direction with respect to the flat sides. Using a fiberoptic scheme for remote, non-contact measurements and inserting glucose or bovine serum albumin solutions in water at different concentrations as test fluids, we have revealed the shift of the wavelength position of the reflectivity minima due to refractive index variations. The combination of these low-cost devices with a spectral readout method allows achieving sensitivities higher than 250 nm/RIU and limits of detection better than 1.3 × 10−3 RIU.

Characterization of Rectangular Glass Microcapillaries by Low-Coherence Reflectometry

We report the functionality of optical low-coherence reflectometry (OLCR) to characterize glass micro-capillaries with 50- $\mu \text{m}$ deep rectangular cross section, in view of their application as microoptofluidic devices. We exploited infrared radiation generated by a tungsten lamp in a time-domain low-coherence interferometer based on a fiberoptic Michelson scheme. OLCR allowed us to easily detect the optical distance between in-depth interfaces of the capillary as well as the refractive index of ethylene glycol solutions in water at different concentrations, which were inserted into the channel by capillary action.

Surfactants Reduce Platelet–Bubble and Platelet–Platelet Binding Induced by In Vitro Air Embolism

The effect of gas bubbles on platelet behavior is poorly characterized. The authors assessed platelet-bubble and platelet-platelet binding in platelet-rich plasma in the presence and absence of bubbles and three surface-active compounds. Platelet-rich plasma was prepared from blood drawn from 16 volunteers. Experimental groups were surfactant alone, sparging (microbubble embolization) alone, sparging with surfactant, and neither sparging nor surfactant. The surfactants were Pluronic F-127 (Molecular Probes, Eugene, OR), Perftoran (OJSC SPC Perftoran, Moscow, Russia), and Dow Corning Antifoam 1510US (Dow Corning, Midland, MI). Videomicroscopy images of specimens drawn through rectangular glass microcapillaries on an inverted microscope and Coulter counter measurements were used to assess platelet-bubble and platelet-platelet binding, respectively, in calcium-free and recalcified samples. Histamine-induced and adenosine diphosphate-induced platelet-platelet binding were measured in unsparged samples. Differences between groups were considered significant for P < 0.05 using analysis of variance and the Bonferroni correction. Sixty to 100 platelets adhered to bubbles in sparged, surfactant-free samples. With sparging and surfactant, few platelets adhered to bubbles. Numbers of platelet singlets and multimers not adherent to bubbles were different (P < 0.05) compared both with unsparged samples and sparged samples without surfactant. No significant platelet-platelet binding occurred in uncalcified, sparged samples, although 20-30 platelets adhered to bubbles. Without sparging, histamine and adenosine diphosphate provoked platelet-platelet binding with and without surfactants present. Sparging causes platelets to bind to air bubbles and each other. Surfactants added before sparging attenuate platelet-bubble and platelet-platelet binding. Surfactants may have a clinical role in attenuating gas embolism-induced platelet-bubble and platelet-platelet binding.

Tetracaine modifies the fragmentation mode of heated human erythrocytes and can induce heated cell fusion

It is known that human erythrocytes in saline fragment by development of an unstable surface wave on the cell rim when cells are heated through the denaturation temperature of the structural protein, spectrin. Here the influence of tetracaine on the fragmentation process has been recorded and analysed by video microscopy of cells heated in rectangular glass microcapillaries. The number of waves per cell rim decreases with increasing tetracaine concentration until, at 0.5 mM tetracaine, wave growth on the cell rim is suppressed on most cells and the cells internalize membrane at the cell dimple. The rate constant for the change in the number of waves per cell with increasing tetracaine concentration is 9.6 mM −1 at a heating rate of 0.5 K/s. 50% of heated cells internalize membrane at 0.14 mM tetracaine. When cells are heated rapidly in suspension in test tubes the presence of tetracaine reduces the temperature for 50% haemolysis from 66°C for washed control cells to 60.5°C for cells in 2 mM tetracaine. Cells heated in microcapillaries in tetracaine concentrations of 3 mM and higher begin to swell before the spectrin denaturation temperature is reached. Cell fusion was observed at and above the spectrin denaturation temperature in cells heated in 3 and 4 mM tetracaine. It was also noted that the morphology of erythrocytes maintained in 3.6 mM tetracaine for times up to 30 min at 37°C or 20°C was strongly dependent on temperature and time.