Separation of Multiscale Particles with Pore Array Membrane

Abstract

Particles are governed by different physical laws in different scale. In small scale, the effects of the environment and other unexpected factors which are not significant in macro scale, become more dominant. Size of particles determines the intensity of governing forces and the behavior of the particle. For example, gravity is one of the dominating forces in the movement of macro particle such as dune sand. However, temperature and pH become more significant factors in molecular particles such as DNA and protein. Recently applications in healthcare and environment use the separation of particles as a crucial process. Examples include pharmaceutical manufacturing/analysis, removal of air pollutants, wastewater treatment, and alternative energy production and storage. Based on mechanical, chemical, electromagnetic, and thermal properties of particles, various particle separation technologies have been developed over a period of time for different applications. Among these separation technologies, membrane separation is an intuitive and effective method because of direct particle-pore interaction. Also membrane method has been well-developed and widely used in the field of microbiology. However more fundamental study is required to use the membrane-based separation for modern biological applications such as molecular binary separation and red blood cell filtering for point of care diagnosis. In this study, separation processes of molecular particle such as DNA and micro particle such as blood cell by pore array membrane have been investigated. DNA as an aptamer is separated from a complex with protein by the nanoporous anodized aluminium oxide (AAO) membrane that has well controlled pores under 100 nm in diameter. For an effective separation, virtual pore size which is determined by electrical double layer (EDL) is controlled with different buffer condition based on ionic concentration. Electrophoretic (EP) force is applied to the negatively charged DNA through nano pores of AAO for translocation. Size exclusive binary separation process is achieved due to the size difference between DNA and DNA/protein complex. To verify the transport of DNA, fluorescence intensity change is measured, and the detail of the interaction process is analyzed. Conventional AAO membranes are already used for various applications such as bio/chemical sensors, pre-treatment filters, and nanoscale templates. These AAO membranes are made from Al-foil substrate of thickness 50 to 100 µm, and then usually assembled in to the other device. On the other hand, AAO membrane in this work is directly fabricated from deposited Al (< 2 µm thickness) on silicon substrate, followed by a MEMS fabrication process to complete an integrated device. This direct AAO fabrication process can reduce the length of AAO nanochannel and improve the compatibility with microfluidic devices. Blood cell is separated by micro pore array membrane with pore size of 2 to 10 µm. For effective blood cells separation, the size of pores, the number of pores, and the spacing between pores are investigated. The blood cells which might block the pores are repelled from the pore by DEP force. DEP parameters such as electrode shape and applied voltage of frequency/amplitude, are studied for enhanced purity and recovery. In this analysis, it is crucial to compare hydrodynamic drag and DEP forces because the parametric complexity including pore array designs and DEP conditions is not sufficient to estimate only by experimental results. Point-of-care (POC) diagnosis requires the blood plasma separation for the monitoring of cholesterol, glucose, and hepatic function in blood. Recent approaches include microfluidics-based blood separation by components. However, conventional microfluidic approaches used continuous process with large volume of sample through a syringe device. In POC application usually a small volume of analyte is needed in a single-shot processing on a compact platform. Our approach with the pore array membrane device satisfies such requirements for the single-shot separation with small droplet sample in a sample batch. The understanding of characteristics of multiscale particles is applied to the separation of particles in two different domains such as DNA and blood cell with pore array membrane. Each of these particles has different size scale and characteristic properties such as density, elasticity, conductivity, etc. Basic principle of separation for nano pore is the combination of physical pore diameter and thickness of electrical double layer. On the other hand, basic principle of separation for micro pore is the physical dimension of pore with respect to the particle size interacting with drag and DEP forces. Though EDL is negligible in micro pore because of relatively large pore diameter, EDL is an important factor in nano pore because of relatively small pore diameter. Also the driving forces for nano particle (DNA) and micro particle (Blood cell) are electrophoresis and dielectrophoresis, respectively. Through this study the effect of various factors that control for pore transport physics in different dimension is evaluated. This report presents a theoretical and engineering analysis of separation technique based on particle size, including experimental results of separation of DNA and blood cell from the blood using two different types of membrane devices.