(131f) Pico-Force Optical Exchange (pico-FOX): Separation of Particles From Molecular Components Utilizing Optical Forces With Orthogonal Fluid Flow With Applications to Malaria

Presented here are the results of a combined pico-Newton optical force applied orthogonally to a hydrodynamically pinched mixed sample of molecular and particulate components. The pico-FOX microfluidic platform successfully combines optical forces and electrokinetic forces to promote higher resolution separations by capitalizing on the different properties of a complex sample mixture. The combined portfolio of pooled separation vectors of the integrated system increases the diversity of exploitable particle properties, including: size, shape, refractive index, charge, charge distribution, charge mobility, permittivity, and deformability. The pico-FOX microfluidic platform consists of a simple 2-D fused silica constructed microfluidic device with a symmetric inlet/outlet system that hydrodynamically pinches the mixed sample flow. The simplicity of this geometry enables the separation of particulates into resolved sub-groups that are simultaneously removed from contaminating molecular components. Particle separations were achieved by addressing particles to specific fluid lamina that flowed into different outlet effluents. The pico-FOX device has been applied to a variety of both abiotic and biotic particulate types.

Experiments utilizing analog abiotic particles have demonstrated non-invasive separations based on single property differences such as size, refractive index, and electrophoretic mobility. Particle separations tested the different separation modes available, from purely optical force to combined optical and electrophoretic forces. These experiments utilized different combinations of polystyrene, PMMA, and silica particles with a commercially available dye. The particle properties varied in size (2, 6, 10 µm), refractive index, and electrophoretic mobility. Sized based separations of 6 and 10 µm polystyrene particles from a mixed component sample led to only 3% particle contamination in the original sample stream and inter-particle type enrichment levels >80% in their respective outlet effluents.

Separations exploiting differences in refractive index and electrophoretic mobility were able to uniquely address three different particle materials (polystyrene, PMMA, and silica) of the same size (2 µm) to a unique outlet without measureable contamination of the other particle types. Particles with the largest refractive indices experience the largest amount of displacement due their interaction with the laser. This was observed in the separation of PMMA, ƞ=1.49, from silica, ƞ=1.43, where PMMA was separated to a more distal outlet than the silica. [1] Although polystyrene has the highest refractive index, ƞ=1.59, and would normally be expected to experience the largest optical force, under the experimental conditions utilized, it never interacted with the laser due to its electrophoretic mobility pulling it in the opposite direction. Given that all of the particles are the same size and experience the same experimental conditions, the variation in electrophoretic mobility of the different particles in the system can be reduced to differences in zeta potential (ζ), or the electrical potential of the interfacial surface double layer. Polystyrene has the largest absolute ζ of the three different particle types, causing the polystyrene particles to be the first to respond in an electrophoretic manner. The reported ζ of 2 µm polystyrene particles is -70.8 mV, 2 µm silica particles is -60 mV, and 2 µm PMMA particles is +34.9 mV.[2-4] Thus, the pico-FOX separation method combines electrophoretic and optical forces to distinguish the three different particle types.

In the testing of analog abiotic particles, pico-FOX has demonstrated a separating size resolution of approximately 4 µm. Also promising is that although the particle flow densities were not optimized for maximum particle processing in these tests, the average processing rate reached over 5,500 particles per hour. In addition to particle separation, the device was able to minimize dye diffusion, leading to > 95% dye recovery. For all sample types examined, the resolution of the separation is dependent on the width of the hydrodynamically pinched sample flow along the wall at the point that the sample flow interacts with the laser beam. Given that the particles travel the same distance when displaced by the optical force, the width of the pinched flow is preserved in the final separated sample. Balancing dilution rate from increased pinch flow and separation resolution, the width of the pinched sample flow was ~50 µm. Modeling and optimization of the pico-FOX system, its separation potential, and theoretical underpinnings have been described and formed the basis for this experimental investigation. Theoretical calculations were conducted using COMSOL, a finite element multiphysics model, for geometric optimization and prediction of separation performance. Experimental separation performance for particulates and molecular component are in good agreement with modeled performance.[5]

Applications of pico-FOX on the separation of biological particulates were evaluated using malaria infected red blood cells and other blood components. Separation of blood components were investigated primarily through size variations in the main classes of blood components. However, the separation of red blood cells infected with Plasmodium falciparum, one species of protozoan that causes malaria, explores separating infected versus non-infected cells via differences in birefringence. Malaria infected red blood cells respond to linearly polarized laser light via intrinsic birefringence by rotating axially along the propagation of the beam. This is in contrast to non-infected red blood cells which are only form birefringent, causing them to simply align along the beam. Pursuing separations utilizing variations in birefringence in an orthogonal flow through system would enable the high throughput necessary to process sufficient quantities of red blood cells to address the low levels of parasitemia found in the different types of malaria that can range from 2 to >50% of red blood cells.[6]

Potential applications of this technology in the fields of biological and environmental sciences are promising. Pico-FOX is capable of removing particulates from potentially damaging molecular components in a mixed sample. This ability could prove useful in biomedical applications that require sample pretreatment prior to analysis steps, such as total sample analysis on a microfluidic platform. In settings where duplicative sampling would be prohibitive, such as medical cell sorting, bio-threat agent detection, water management and pollution detection, this technology could be used to treat a single, complex sample in a rapid and efficient manner. This sample could then be mated to a variety of different analytical detection techniques to analyze the various constituents. The capability for efficiently separating molecular components from particulates is especially valuable for samples containing a mixture of bacteria, protozoan, etc. and molecular components, e.g. chemical warfare agents, pollutants, and toxins. Two such common water-borne pathogenic protozoa are Cryptosporidium parvum and Giardia intestinalis, nominally 5 and 10 µm, respectively, which are comparable to the sizes tested here.[7] A non-invasive and non-destructive testing device tailored to particular analytes could find broad applications in water management and malaria detection, considering that annual deaths worldwide for all types of waterborne pathogens total more than 3.4 million and for malaria number around 660,000.[6, 8] In future applications, modeling of the desired particulates would aid in tailoring the geometry of the chip to offer the appropriate outlet configurations to maximize enrichment of the different sub-samples via their optical and electrophoretic properties. The combination of the non-invasive optical forces and electrokinetics is a rich area for improving complex sample separation and manipulation.

References:

1.            Hart, S. J.; Terray, A. V., Refractive-index-driven separation of colloidal polymer particles using optical chromatography. Applied Physics Letters 2003, 83, (25), 5316-5318.

2.            Tabata, Y.; Ikada, Y., Effect of the size and surface charge of polymer microspheres on their phagocytisis by macrophoage. Biomaterials 1988, 9, 356-362.

3.            Kim, J.; Lawler, D. F., Characteristics of Zeta Potential Distribution in Silica Particles. Bulletin to the Korean Chemical Society 2005, 26, (7), 1083-1089.

4.            Pham, V. H.; Dang, T. T.; Hur, S. H.; Kim, E. J.; Chung, J. S., Highly Conductive Poly(methyl methacrylate) (PMMA)-Reduced Graphene Oxide Composite Prepared by Self-Assembly of PMMA Latex and Graphene Oxide through Electrostatic Interaction. Applied Materials and Interfaces 2012, 4, 2630-2636.

5.            Staton, S. J. R.; Terray, A.; Collins, G. E.; Hart, S. J., Orthogonal optical force separation simulation of particle and molecular species mixtures under direct current electroosmotic driven flow for applications in biological sample preparation. Electrophoresis 2013, 34, 1175-1181.

6.            Trampuz, A.; Jereb, M.; Muzlovic, I.; Prabhu, R. M., Clinical review: Severe malaria. Critical Care 2003, 7, 315-323.

7.            Foodborne Parasites. Springer: New York, 2006.

8.            Berman, J. WHO: Waterborne Disease is World's Leading Killer. (Feb. 6), 2009.