We survey progress over the past 25 years in the development of microscale devices for pumping fluids. We attempt to provide both a reference for micropump researchers and a resource for those outside the field who wish to identify the best micropump for a particular application. Reciprocating displacement micropumps have been the subject of extensive research in both academia and the private sector and have been produced with a wide range of actuators, valve configurations and materials. Aperiodic displacement micropumps based on mechanisms such as localized phase change have been shown to be suitable for specialized applications. Electroosmotic micropumps exhibit favorable scaling and are promising for a variety of applications requiring high flow rates and pressures. Dynamic micropumps based on electrohydrodynamic and magnetohydrodynamic effects have also been developed. Much progress has been made, but with micropumps suitable for important applications still not available, this remains a fertile area for future research.

https://microfluidics.stanford.edu/Publications/Micropumps_Cooling/Laser%20Review%20of%20Micropumps%20in%20JMM.pdf

A review of micropumps

Electroosmotic pumping is receiving increasing attention in recent years owing to the rapid development in micro total analytical systems. Compared with other micropumps, electroosmotic pumps (EOPs) offer a number of advantages such as creation of constant pulse-free flows and elimination of moving parts. The flow rates and pumping pressures of EOPs matches well with micro analysis systems. The common materials and fabrication technologies make it readily integrateable with lab-on-a-chip devices. This paper reviews the recent progress on EOP fabrications and applications in order to promote the awareness of EOPs to researchers interested in using micro- and nano-fluidic devices. The pros and cons of EOPs are also discussed, which helps these researchers in designing and constructing their micro platforms.

/pdf/electroosmotic-pumps-and-their-applications-in-microfluidic-5500yjhi3z.pdf

Electroosmotic pumps and their applications in microfluidic systems

In this paper, we present a low-cost, PDMS-membrane micropump with two one-way ball check-valves for lab-on-a-chip and microfluidic applications. The micropump consists of two functional PDMS layers, one holding the ball check-valves and an actuating chamber, and the other covering the chamber and holding a miniature permanent magnet on top for actuation. An additional PDMS layer is used to cover the top magnet, and thereby encapsulate the entire device. A simple approach was used to assemble a high-performance ball check-valve using a micropipette and heat shrink tubing. The micropump can be driven by an external magnetic force provided by another permanent magnet or an integrated coil. In the first driving scheme, a small dc motor (6 mm in diameter and 15 mm in length) with a neodymium–iron–boron permanent magnet embedded in its shaft was used to actuate the membrane-mounted magnet. This driving method yielded a large pumping rate with very low power consumption. A maximum pumping rate of 774 µL min−1 for deionized water was achieved at the input power of 13 mW, the highest pumping rate reported in the literature for micropumps at such power consumptions. Alternatively, we actuated the micropump with a 10-turn planar coil fabricated on a PC board. This method resulted in a higher pumping rate of 1 mL min−1 for deionized water. Although more integratable and compact, the planar microcoil driving technique has a much higher power consumption.

/pdf/a-magnetically-driven-pdms-micropump-with-ball-check-valves-2dg9njmu5p.pdf

A magnetically driven PDMS micropump with ball check-valves

We review fundamental and applied acid-base equilibrium chemistry useful to microfluidic electrokinetics. We present elements of acid-base equilibrium reactions and derive rules for pH calculation for simple buffers. We also present a general formulation to calculate pH of more complex, arbitrary mixtures of electrolytes, and discuss the effects of ionic strength and temperature on pH calculation. More practically, we offer advice on buffer preparation and on buffer reporting. We also discuss "real world" buffers and likely contamination sources. In particular, we discuss the effects of atmospheric carbon dioxide on buffer systems, namely, the increase in ionic strength and acidification of typical electrokinetic device buffers. In Part II of this two-paper series, we discuss the coupling of acid-base equilibria with electrolyte dynamics and electrochemistry in typical microfluidic electrokinetic systems.

https://microfluidics.stanford.edu/Publications/ElectrokineticFlows/Persat%20-%20Basic%20principles%20of%20electrolyte%20chemistry%20for%20microfluidic%20electrokinteics%20Part%201%20-%20acid%20abse%20equilibria%20and%20pH%20buffers.pdf

Basic principles of electrolyte chemistry for microfluidic electrokinetics. Part I: Acid–base equilibria and pH buffers

We investigate the DNA–silica binding mechanism using molecular dynamics simulations. This system is of technological importance, and also of interest to explore how negatively charged DNA can bind to a silica surface, which is also negatively charged at pH values above its isoelectric point near pH 3. We find that the two major binding mechanisms are attractive interactions between DNA phosphate and surface silanol groups and hydrophobic bonding between DNA base and silica hydrophobic region. Umbrella sampling and the weighted histogram analysis method (WHAM) are used to calculate the free energy surface for detachment of DNA from a binding configuration to a location far from the silica surface. Several factors explain why single-stranded DNA (ssDNA) has been observed to be more strongly attracted to silica than double-stranded (dsDNA): (1) ssDNA is more flexible and therefore able to maximize the number of binding interactions. (2) ssDNA has free unpaired bases to form hydrophobic attachment to silica ...

DNA Binding to the Silica Surface.

N-type porous silicon can be used to realize electroosmotic pumps with high flow rates per applied potential difference. The porosity and pore size of porous silicon membranes can be tuned, the pore geometry has near-unity tortuosity, and membranes can be made thin and with integrated support structures. The size of hexagonally packed pores is modified by low-pressure chemical vapor deposition (LPCVD) polysilicon deposition, followed by wet oxidation of the polysilicon layer, resulting in a pore radius varying from 1 to 3 /spl mu/m. Pumping performance of these devices is experimentally studied as a function of pore size and compared with theory. These 350-/spl mu/m-thick silicon membranes exhibit a maximum flow rate per applied field of 0.13 ml/min/cm/sup 2//V. This figure of merit is five times larger than previously demonstrated porous glass EO pumps.

http://microfluidics.stanford.edu/Publications/Micropumps_Cooling/Yao%20Porous%20Silicon%20JMEMS%202006.pdf

Electroosmotic pumps fabricated from porous silicon membranes

We are developing a class of electroosmotic micropumps fabricated from silicon substrates that can be used for integrated circuit thermal management applications. Prototype micropumps with 0.15 cm/sup 3/ packages produce a maximum flow rate of 170 /spl mu/L min/sup -1/ and a maximum pressure of 10 kPa operating at 400 V. These specifications approach the requirements for single-phase forced-convective cooling of small IC hot spots. The micropumps operate on less than 200 mW and, having no moving structural elements, offer inherent reliability advantages. The impact on pump performance of geometry, surface treatment, and choice of working fluid has been characterized.

http://micromachine.stanford.edu/~dlaser/docs/laser_trans03.pdf

A.M. Myers

Papers

Electroosmotic pumps fabricated from porous silicon membranes

Silicon electroosmotic micropumps for integrated circuit thermal management