Nature News and Views - 15 Feb 2007
Not only do the authors' porous nanocrystalline silicon (pncSi) membranes combine small membrane thickness and pore sizes, but they are also robust, their pore size can be controlled, and they are simple to produce. Earlier attempts to make ultrathin nanoporous membranes used either sophisticated nanolithography or were based on colloidal templates. The first method is expensive; and although the second makes elegant use of self-organization principles, very small, controlled pore sizes are difficult to achieve.
At the core of SiMPore's proprietary technology portfolio is a remarkable silicon-based porous membrane less than 50 nm thick that is the focus of a research publication in the February 15, 2007 issue of the journal Nature. Despite this nanoscale thickness, arrays of porous nanocrystalline silicon (pnc-Si) membranes are extremely strong and robust, supporting well over 1 atmosphere of differential pressure. This stability enables a wide range of bioprocess applications and creates numerous opportunities in broad fields requiring size-based or charge-based separation of ions, molecules, or other nanoscale objects.
The image at the left is a transmission electron micrograph of a pnc-si membrane that is 15 nm thick. In addition to the white pores, some of the silicon nanocrystals can be observed as black objects. The pores form naturally when a thin film of amorphous silicon is crystallized by heating it beyond the crystallization temperature. The pore size and distribution can be controlled using the final annealing temperature, the film thickness, and the deposition conditions of the initial amorphous silicon film as variables.
Most commercial filters for biological materials use membranes made from sintered ceramic particles or netted polymers. Separations occur in these materials because some components of a mixture are too large to enter the filter or because components have different degrees of solubility in the membrane material. These membranes can be made more restrictive by increasing the density of the filter material. Typically these membranes are used to separate species that are orders-of-magnitude different in size. Although some polymer membranes can be designed to exclude passage of materials at molecular weights ranging from 1kD to 100 kD, the lack of defined pore sizes means the molecular weight exclusion profiles are gradual declines rather than sharp cut-offs. Track eteched, microsieve and nanoporous alumina membranes have more defined pore sizes in the nanometer range. These membranes, however, are still typically tens of microns in thickness resulting in a thickness to pore size ratio of 1000 to 1. These "pores" can be thought of more simply as tunnels rather than holes in a membrane filter.
Because of flow resistance and long diffusion times created by thick membranes, the ideal membrane for molecular or nanoparticle separations would have a molecular thickness as well as pore size. SiMPore's pnc-Si membranes have typical thickness to pore size ratios on the order of 1 to 1. Additionally, porosity and pore size are controllable with a well defined upper cutoff allowing separation of closely sized molecules or particles. These characteristics in combination with its strength create a truly unique material that can revolutionize separation of small molecules in a variety of applications. Please see our Product Areas page for additional information.
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