Nanomedicine, Volume I: Basic Capabilities

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

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

 

9.2.7.4 Nonmechanical Pumps

Nonmechanical pumps may induce fluid flow without using any moving parts in direct contact with the fluid. Electrohydrodynamic (EHD) pumps are one important class of these devices. Several different types of EHD micropumps have been fabricated.

For example, in the DC-charge injection pump1205-1207 or ion drag pump,1207-1209 ions are injected into the liquid at an emitting electrode under high field conditions and move under the influence of Coulombic forces toward a second collecting electrode where they are absorbed. Collisions between ions and fluid molecules transfer momentum from the ions to the fluid, producing fluid movement in the direction of ion flow. A prototype micropump has achieved a maximum pumping pressure of 0.01 atm (static) for ethanol flowing between two (2500 micron)2 grids spaced 10 microns apart at 300 volts, producing a volumetric flow rate of 'V = 2.7 x 10-11 m3/sec at a flow velocity of ~4 microns/sec.1206 Static pump pressure pstatic may be estimated1208,1209 as:

{Eqn. 9.39}

where cgeom is a correction factor for electrode geometry of order unity, e0 = 8.85 x 10-12 farad/m (permittivity constant), ke = dielectric constant, V is electric potential and d is electrode separation. Taking V/d ~ 107 volt/m and ke ~ 74.31 for deionized water at 310 K, pstatic ~ 0.7 atm. Pumping of different polar fluids with conductivities between 10-6­10-12 (ohm-m)-1 such as propanol, acetone, deionized water, and many organic solvents including several oils has been demonstrated, but aqueous solutions of electrolytes could not be pumped due to their high ionic conductivity.

In the traveling wave electroconvection voltage pump,1210-1212 phase-shifted rectangular voltages are applied across a series of parallel electrodes. Temperature induced conductivity gradients induce free electric charges which can interact with the traveling field due to charge relaxation processes in the volume of the fluid, causing fluid flow in a direction transverse to the electrodes. A small monotonic temperature gradient, which may be produced by the traveling waves themselves,1212 is also required. Early prototype devices could pump only nonconductive fluids,1211 but Fuhr et al1212 have demonstrated EHD pumping of conductive fluids with conductivities between 0.0001-0.1 (ohm-m)-1 (e.g., up to ~0.01M NaCl aqueous solutions) using high frequencies and a low voltage waveform. Typically, volumetric flow rates of 10-10­10-12 m3/sec and flow velocities of 50-1000 micron/sec were achieved using traveling wave frequencies of 0.1-30 MHz at ~ 40 volts across 30 micron-wide electrodes spaced 30 microns apart in 50 x 70 micron square channels ~4000 microns in length.1212 Flow velocity is a linear function of the square of the applied voltage and a complex function of the applied frequency. Further research is required to determine if unfavorable thermal scaling presents difficulties in submicron-size devices.

Other nonmechanical pumping methods have been studied. Electroosmotic pumping of electrolyte solutions has been observed in capillaries 50 microns in diameter or less, and separation of the components is also possible using electrokinetic phenomena with applied voltages up to 10,000 volts.1213,1217 Electrowetting pumps using electrical control of the interfacial surface tension have produced ~0.1 atm flow pressures in 10 micron radius channels.1221 Molecular flow can be induced by pulsed axial magnetic fields applied to carbon nanotubes placed in a transverse electric field. Ultrasonic pumps cause liquid to move in the direction of wave propagation with a speed proportional to the square of the acoustic amplitude;1214,1215 speeds of 130 micron/sec have been observed in water at 3.5 MHz.1214 Ferrofluids1241 containing ~10 nm particles in suspension and electrorheological fluids1242 allow electrostatic control of fluid flow via the Winslow effect,1243 although early reports that blood flow could be electrostatically regulated have not yet been confirmed experimentally.1244 Osmotic-pump motility across sugar gradients has been induced in micron-size phospholipid vesicles.3269

Optofluidic pumping is extremely energy inefficient and is feasible only at dangerously high beam intensities. Consider a beam of light of radius rbeam and intensity Ibeam that exerts a photon-pressure force of Fbeam = p rbeam2 ka Ibeam / c, where c = 3 x 108 m/sec (speed of light) and ka = 0 for total transmission, 1 for total absorption, and 2 for total reflection. Setting this equal to the Stokes law force (Eqn. 9.73) on a spherical target of radius rbeam in a fluid of absolute viscosity h gives the approximate magnitude of the fluid flow velocity as:

{Eqn. 9.40}

if we conservatively assume that dimensionless ka ~ rbeam sa where sa ~ 300 m-1 (optical absorption coefficient; Section 4.9.4). Taking Ibeam ~1011 W/m2 (~optical tweezer intensity; compare maximum "safe" in vivo intensity of <~105 watts/m2, Section 6.4.3.2) and rbeam = 200 nm giving ka ~6 x 10-5 (almost complete transmission), then vflow ~ 1 micron/sec but with a huge power dissipation of p rbeam2 Ibeam ~10 milliwatts while developing a pumping pressure of only pbeam ~ka Ibeam / c = 2 x 10-7 atm.

 

Last updated on 20 February 2003