© 1999 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999

3.3.2 Dynamic Pore Sizing

A more efficient nanosieve³²⁵¹ system can be designed if pore size and shape can be actively modified during device operation, as for example by exchanging filters (from a membrane library stocking various pore sizes) each half cycle. Better, if pores can be reliably dilated or constricted in place during a period of time Dt << t_p, then filtration cascades can be more rapidly reconfigured to match changing input feedstock characteristics or to extract varying selections of desired molecules at will. Additionally, fully differentiating sieving cascades can be collapsed into a single unit, providing more compact devices especially useful in chemical sensor systems requiring preconcentration of sample (Section 4.2.1). Control of pore shape should also provide finer discrimination among molecules of similar size but different shape, such as some isomers of nonchiral molecules.

Two or more overlapping surfaces containing regular arrays of perforations of fixed size and shape can conveniently generate a wide variety of pore geometries. Control of pore geometry is achieved by sliding or rotating one surface relative to the other surface by a small increment, as suggested schematically by the examples in Figure 3.3.

Circular dilating apertures can also be constructed using a matched set of overlapping segments, which may be driven either radially or tangentially to enlarge or contract the hole like an irising camera diaphragm (Fig. 3.4), consistent with Akey's model of the nuclear pore complex present on the cell nucleus surface.¹⁴⁰⁹ Diaphragming mechanisms may be vertically staggered to maximize areal hole density in filtration surfaces (at the cost of increased vertical rugosity). Filters constructed of hydrogen-passivated diamondoid can have pores with <0.1 nm feature sizes, although H-free fullerene materials might avoid any possibility of dehydrogenation shearing.

Methods of positioning surfaces to accuracies of ~0.01 atomic diameter (~0.001 nm) are discussed in Section 3.5.6. Assuming pore sizing blades require ~25 nm² of diamondoid contact surface per pore and each blade travels 25 nm at 0.01 meter/sec during one cycle, sliding friction¹⁰ dissipates ~0.01 zJ/pore, or ~4 x 10^-18 watts/pore during each 2.5 microsec resizing cycle. Since fluid friction approaches kT for nanometer-size holes changing size in ~10^-9 sec, maximum blade speed is ~1 m/sec and the fastest resizing cycle is ~10^-8 sec.

A single sieving unit with controllable pores can be moderately efficient. Consider a design similar to that described in Section 3.3.1, except for a separate chamber and piston on either side of the filter block. Suppose that the particles desired to be extracted are of radius r, and the next smallest possible pore size is r - Dr. The device operates in two phases. In the first phase, the sample is placed in the first chamber, pore size is set equal to r, then the first piston forces the fluid through the membrane. Particles larger than r remain behind and are flushed from the first chamber. The pores then contract to r - Dr, and the second piston pushes the remaining filtrate back into the first chamber. After this second phase, particles of radius ~(r ± Dr) remain in the second chamber at significantly higher concentration and may be removed for further use. Analogous double-sieve editing paradigms are commonplace in biological systems.¹⁵²⁰

Other designs might work equally well, such as a 3-chamber flowthrough design using a variable pore membrane with pores of size r between the first and second chamber, a membrane with pores of size r - Dr between the second and third chamber, and a piston at either end (one pushing, one pulling), thus concentrating molecules of size r ± Dr in the central chamber. M. Krummenacker suggests fixed chambers with a moving sieve operated as a dragnet. Filtration processes may be most useful in performing complete separations of complex mixtures. But they are inefficient in the sense that the energy expended to orient molecules passing through pores is wasted if the molecules are allowed to randomize on the other side; a eutactic mill-like molecule handling system (Section 3.4.3) might preserve this order and greatly improve energy efficiency.

Last updated on 7 February 2003