In this study, we introduce what we believe is a novel holographic optical element called a chiral square Fresnel zone plate (CSFZP) The chirality is imposed on a square Fresnel zone plate (SFZP) using a nonclassical technique by rotating the half-period zones relative to one another The rotation of the half-period zones, in turn, twists the side lobes of the diffraction pattern without altering the focusing properties inherent to a SFZP As a consequence, the beam profile is hybrid, consisting of a strong central Gaussian focal spot with gradient force similar to that generated by a lens and twisted side lobes with orbital angular momentum The optical fields at the focal plane were calculated and found to possess a whirlpool-phase profile and a twisted intensity profile Analysis of the field variation along the direction of propagation revealed a spiraling phase and amplitude distribution Poynting vector plot of the fields revealed the presence of angular momentum in the regions of chiral side lobes The phase of the CSFZPs were displayed on a phase-only reflective spatial light modulator and illuminated using a laser The intensity patterns recorded in the experiment match the calculated ones, with a strong central focal spot and twisted side lobes The beam pattern was implemented in an optical trapping experiment and was found to possess particle trapping capabilities

Experimental demonstration of square Fresnel zone plate with chiral side lobes.

Cell Membrane The Red Blood Cell As A Model

Human red blood cells (RBCs) are approximately 8 μm in diameter, but must repeatedly deform through capillaries as small as 2 μm in order to deliver oxygen to all parts of the body. The loss of this capability is associated with the pathology of many diseases, and is therefore a potential biomarker for disease status and treatment efficacy. Measuring RBC deformability is a difficult problem because of the minute forces (∼pN) that must be exerted on these cells, as well as the requirements for throughput and multiplexing. The development of technologies for measuring RBC deformability date back to the 1960s with the development of micropipette aspiration, ektacytometry, and the cell transit analyzer. In the past 10 years, significant progress has been made using microfluidics by leveraging the ability to precisely control fluid flow through microstructures at the size scale of individual RBCs. These technologies have now surpassed traditional methods in terms of sensitivity, throughput, consistency, and ease of use. As a result, these efforts are beginning to move beyond feasibility studies and into applications to enable biomedical discoveries. In this review, we provide an overview of both traditional and microfluidic techniques for measuring RBC deformability. We discuss the capabilities of each technique and compare their sensitivity, throughput, and robustness in measuring bulk and single-cell RBC deformability. Finally, we discuss how these tools could be used to measure changes in RBC deformability in the context of various applications including pathologies caused by malaria and hemoglobinopathies, as well as degradation during storage in blood bags prior to blood transfusions.

Technologies for measuring red blood cell deformability.

We design and characterize compact phase-modulated axilens devices that combine efficient point focusing and grating selectivity within four-level phase mask configurations. Specifically, we select and characterize in detail two device configurations designed for long-wavelength infrared (LWIR) operation in the $ 6\,\,\unicode{x00B5}{\rm m}\! -\! 12\,\,\unicode{x00B5}{\rm m} $6µm−12µm wavelength range. These devices are ideally suited for monolithic integration atop the substrate layers of infrared focal plane arrays (IR-FPAs) for use in multiband LWIR photodetection. We systematically study their focusing efficiency, spectral response, and crosstalk ratio, and we demonstrate a single-component microspectrometer. Our design method leverages the Rayleigh–Sommerfeld (RS) diffraction theory that is validated numerically using the finite element method (FEM). The proposed devices are broadband and polarization insensitive and add fundamental spectroscopic capabilities to miniaturized optical components for a number of applications in LWIR detection and spectroscopy.

Phase-modulated axilenses for infrared multiband spectroscopy.

We investigate the biophysical characteristics of healthy human red blood cells (RBCs) traversing microfluidic channels with cross-sectional areas as small as 2.7 × 3 μm. We combine single RBC optical tweezers and flow experiments with corresponding simulations based on dissipative particle dynamics (DPD), and upon validation of the DPD model, predictive simulations and companion experiments are performed in order to quantify cell deformation and pressure–velocity relationships for different channel sizes and physiologically relevant temperatures. We discuss conditions associated with the shape transitions of RBCs along with the relative effects of membrane and cytosol viscosity, plasma environments, and geometry on flow through microfluidic systems at physiological temperatures. In particular, we identify a cross-sectional area threshold below which the RBC membrane properties begin to dominate its flow behavior at room temperature; at physiological temperatures this effect is less profound.

Combined Simulation and Experimental Study of Large Deformation of Red Blood Cells in Microfluidic Systems

Human red blood cells (RBCs) need to deform in order to pass through capillaries in human vasculature with diameter smaller than that of the RBC. An altered RBC cell membrane stiffness (CMS), thereby, is likely to have consequences on their flow rate. RBC CMS is known to be affected by several commonly encountered disease conditions. This study was carried out to investigate whether an increase in RBC CMS, to the extent seen in such commonly encountered medical conditions, affects the RBC flow rate through channels with diameters comparable to that of the RBC. To do this, we use RBCs extracted from a healthy individual with no known medical conditions and treated with various concentrations of Bovine Serum Albumin (BSA). We study their flow through polycarbonate membranes with pores of diameter 5μm and 8μm which are smaller than and comparable to the RBC diameter respectively. The studies are carried out at constant hematocrit and volumetric flow rate. We find that when the diameter of the capillary is smaller than that of the RBC, the flow rate of the RBCs is lowered as the concentration of BSA is increased while the reverse is true when the diameter is comparable to that of the RBC. We confirm that this is a consequence of altered CMS of the RBCs from their reorientation dynamics in an Optical Tweezer. We find that a treatment with 0.50mg/ml BSA mimics the situation for RBCs extracted from a healthy individual while concentrations higher than 0.50mg/ml elevate the RBC CMS across a range expected for individuals with a condition of hyperglycemia. Using a simple theoretical model of the RBC deformation process at the entry of a narrow channel, we extract the RBC membrane bending modulus from their flow rate.

/pdf/estimation-of-membrane-bending-modulus-of-stiffness-tuned-3hpvphtcmw.pdf

Estimation of membrane bending modulus of stiffness tuned human red blood cells from micropore filtration studies

The phase of a negative axicon is combined with that of a Fresnel zone lens (FZL) to obtain an element labelled as
conical FZL, which can generate a focused ring pattern at the focal plane of the FZL. The phase integration is achieved
by modifying the location and width of zones of FZL in accordance with the phase variation of the negative axicon. The
element was designed for a high power laser with a wavelength of 1064 nm, focal length and diameter of conical FZL of
30 mm and 8 mm respectively and for a ring diameter of 50 μm. The element was fabricated using photolithography. The
pattern was transferred from the resist layer to the borosilicate glass plates by dry etching to achieve an etch depth of
1064 nm. The etch depth measured using confocal microscope was 1034 nm at the central part and 930 nm for the
outermost part of the device with a maximum error of 12.5% at the outermost part and 3% at the central part. The
element was used in an optical trapping experiment. The ring pattern generated by the conical FZL was reimaged into the
trapping plane using a tightly focusing microscopic objective. Polystyrene beads with diameters of 3 μm were
suspended in deionized distilled water at the trapping plane. The element was found to trap multiple particles in to the
same trap.

Conical Fresnel zone lens for optical trapping

We report the trapping characteristics of semi metallic micro beads in an optical trap. We observe that the semi metallic beads are trapped at the focus like dielectric beads but with reduced corner frequency.

Trapping Characterization of Semi Metallic Magnetic Beads in Optical Tweezers

Optical Tweezers are capable of trapping individual particles of sizes that range from micrometers to sub micrometers. One can compute the trap strength experienced by a particle by analyzing the fluctuations in the position of the trapped particle with time. It is reported that the trap strength of a dielectric bead increases linearly with increase in the power of the trapping laser. The situation with metallic particles, however, is strongly dependent on the particle size. Available literature shows that metallic Rayleigh particles experience enhanced trap strengths when compared to dielectric particles of similar sizes due to a larger polarizability. On the contrary, micrometer sized metallic particles are poor candidates for trapping due to high reflectivity. We report here that commercially available micrometer sized metal oxide core - dielectric shell (core – shell) beads are trapped in a single beam optical tweezer in a manner similar to dielectric beads. However as the laser power is increased these core – shell beads are trapped with a reduced corner frequency, which represents a lowered trap strength, in contrast to the situation with ordinary dielectric beads. We attribute this anomaly to an increase in the temperature of the medium in the vicinity of the core – shell bead due to an enhanced dissipation of the laser power as heat. We have computed autocorrelation functions for both types of beads at various trapping laser powers and observe that the variation in the relaxation times with laser power for core - shell beads is opposite in trend to that of ordinary dielectric beads. This supports our claim of an enhanced medium temperature about the trapped core – shell bead. Since an increase in temperature should lead to a change in the local viscosity of the medium, we have estimated the ratio of viscosity to temperature for core – shell and dielectric beads of the same size. We observe that while for ordinary dielectric beads this ratio remains a constant with increasing laser power, there is a decrease for core – shell beads. We plan to extend this work towards studying the hydrodynamic correlations between a pair of trapped beads where one of the beads acts as a heat source. © (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

/pdf/autocorrelation-and-relaxation-time-measurements-on-metal-2jwfelien5.pdf

Rekha Selvan

Papers

Estimation of membrane bending modulus of stiffness tuned human red blood cells from micropore filtration studies

Conical Fresnel zone lens for optical trapping

Trapping Characterization of Semi Metallic Magnetic Beads in Optical Tweezers

Autocorrelation and relaxation time measurements on metal oxide core: dielectric shell beads in an optical trap