High-performance, low-voltage electroosmotic pumps with molecularly thin silicon nanomembranes

Abstract

We have developed electroosmotic pumps (EOPs) fabricated from 15-nm-thick porous nanocrystalline silicon (pnc-Si) membranes. Ultrathin pnc-Si membranes enable high electroosmotic flow per unit voltage. We demonstrate that electroosmosis theory compares well with the observed pnc-Si flow rates. We attribute the high flow rates to high electrical fields present across the 15-nm span of the membrane. Surface modifications, such as plasma oxidation or silanization, can influence the electroosmotic flow rates through pnc-Si membranes by alteration of the zeta potential of the material. A prototype EOP that uses pnc-Si membranes and Ag/ AgCl electrodes was shown to pump microliter per minute-range flow through a 0.5-mm-diameter capillary tubing with as low as 250 mV of applied voltage. This silicon-based platform enables straightforward integration of low-voltage, on-chip EOPs into portable microfluidic devices with low back pressures.lectroosmotic flow results from the interaction between an electric field and the diffuse layer of ions at a charged surface. In capillaries or pores, the migration of the diffuse layer toward the oppositely charged electrode causes the bulk fluid within the channel to flow through viscous drag. Electroosmotic pumps (EOPs) are designed to generate high flow rates in microchannels using these principles (1, 2). EOPs present a number of advantages over mechanical pumps, including the lack of mechanical parts, pulse-free flows, and ease of control through electrode actuation. EOPs have been suggested as pumps for cooling circuits (3) and microfluidic devices that aid in drug delivery (4, 5) or diagnostics (2, 6). Microfluidic devices enable the miniaturization of multistep laboratory processes into small, low-cost, disposable units (6, 7). The inclusion of multiple steps into a single device increases the need for the precision pumping of fluids on-chip.High voltages (>1 kV) are often required for direct current (dc) EOPs to achieve sufficient flow rates in microchannels (8, 9). However, devices with high-voltage EOPs require bulky external power supplies and a skilled technician to operate, which defeats the ease of use and portability aims of a microfluidic diagnostic tool. For these reasons, the development of a low-voltage EOP is a current focus in the literature. Several recent low-voltage EOPs have been fabricated from porous silicon (10), alumina (11-13), track-etched polymer (14), and carbon nanotube membranes (15). These low-voltage EOPs are much thinner than their highvoltage predecessors (60-350 μm compared with >10 mm). Yao et al. suggest that further thinning of EOPs will enable better voltage-specific characteristics (16). Here, we examine the electroosmotic pumping by nanoporous membranes that are more than two orders of magnitude thinner than any membrane material previously used in an EOP.We have recently developed an ultrathin (15-30 nm), nanoporous membrane material called porous nanocrystalline silicon (pnc-Si) (17). pnc-Si membranes are fabricated on silicon wafers usin...

Electroosmotic Flow in Cell Built with Electrodes Having Two Redox Couples

“…The higher flux in the flow cells built with RGO compared to those with GO was attributed to the better conductivity of RGO and to the presence of redox functions, which were introduced in the hydrothermal reaction with ammonia, exemplified by aromatic amines reversibly oxidized to imines, as seen by XPS ( Figure S4 20 This flux was lower than the 93 ± 3 μL min −1 cm −2 flux at 1 V for Ag/silica frit/Ag 2 O 7 electrodes and 2 orders of magnitude lower than the 2600 μL min −1 cm −2 V −1 flux for an EOP built with a 15 nm thick porous silica membrane and Pt electrodes. 21 In an EOP built with Pt electrodes and an ion exchange membrane, 22 the flux is ≫6 μL min −1 mA −1 cm −2 at 30 V. Figure 5b shows the flow-opposing pressure dependence of the flux of flow cells built with GO-CeO 2−x electrodes. As in other EOPs, the stall pressure increases with the applied voltage.…”

Electroosmotic Flow Pump

“…Metal wire electrodes [33][34][35][36] are inserted into the inlet/outlet reservoirs of the pump channels in EOF pumps, as shown in Figure 13. These metal wire electrodes are the simplest type for EOF pumps, which can be bought easily.…”

Induced charge electroosmosis micropumps using arrays of Janus micropillars

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“…56,58,63 Recently, new designs of electroosmotic pumps have continued to optimize designs for high flow rate or pressure 64 or low voltage. 59,65 Nonlinear electrokinetic flows represent an alternative solution; the capacitive AC current used to drive such flows alleviates Faradaic reactions. However, the pressures achieved in ICEO pumps are typically orders of magnitude smaller than those achieved with DC electroosmosis through porous structures, largely due to the limited 'pore size' s in typical ACEO systems (ΔP max~1 /s 2 ) compared with the small s possible in DCEO through porous frits.…”