Electrostatic Effects

Many materials have an anionic surface charge at physiologic pH. The net charge density on a microfluidic substrate in contact with an aqueous solution gives rise to an electrical double layer in the aqueous solution. This charge density creates an electric field, drawing oppositely charged ions towards it and driving like-charged ions away from it. This shielding layer is commonly known as the Debye layer or the electrical double layer [13]. For nanometer scale pores, the thickness of the electrical double layer can be the same order of magnitude as the pore size itself, and can contribute to rejection of charged solutes by the pore even for a theoretical solute of infinitesimal size (and thus no steric hindrance).

Initial Data

Silicon nanopore membranes (SNMs) with 10-100 nm X 45 (xm slit pores were designed and fabricated at the Cleveland Clinic Foundation and Case Western Reserve University. Initial hydraulic characterization of these membranes has been completed [6, 7]. Silicon chips bearing 1 X 1 mm arrays of approximately 104 slit pores were fabricated via sacrificial layer techniques (fig. 1) [14]. The pore structure is defined by deposition and patterning of a polysilicon film on the silicon wafer. The critical submicron pore dimension is defined by the thickness of a sacrificial SiO2 layer, which can be grown with unprecedented control to within 81 nm. The oxide layer is etched away in the final processing step to create the porous polysilicon membrane.

The hydraulic permeability to gas, deionized water, and phosphate-buffered saline (PBS) of these novel membranes was tested in a custom-engineered ultrafiltration cell. Gas flow data were well fitted by transitional flow models, consistent with pore sizes approximating the mean free path of the gas molecule. Liquid flow was almost perfectly predicted by conventional Navier-Stokes predictions (fig. 2).

Rejection of charged proteins in solution by SNMs was examined in the same ultrafiltration cell. Briefly, solutions of three proteins (carbonic anhydrase, 25 kD, bovine serum albumin, 66 kD, and sweet potato amylase, 200 kD) in 1X PBS (150 mM) and 10X PBS (1,500 mM) were subjected to ultrafiltration by SNMs. Even for membranes with pore sizes much larger than the molecular radii of the proteins, some rejection of the proteins by the membrane was observed, but this effect was abrogated in high ionic strength solutions, strongly suggesting an electrostatic barrier to passage (fig. 3). Further definition of steric and electrostatic barriers to passage is an area of active research.

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