Daniel Laser

A review of micropumps

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.

Silicon electroosmotic micropumps for integrated circuit thermal management

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.

Experimental investigation of flow transition in microchannels using micron-resolution particle image velocimetry

Microchannel heat sinks are promising for cooling applications in advanced electronic systems. More research is needed to understand microchannel flow regimes. Recent pressure drop data in microchannels with hydraulic diameters between 50 and 300 /spl mu/m suggest that the transition to turbulence may occur at lower than expected values of the Reynolds number. This work studies turbulent transition in microchannels using micron-resolution particle imaging velocimetry (/spl mu/PIV) with epifluorescent microscopy of 950 nm particles. Silicon channels with dimensions 150 /spl mu/m/spl times/100 /spl mu/m/spl times/1 cm are fabricated using deep reactive ion etching and sealed using a glass plate. Velocity field data for 200Re>1600, which is lower than values near 2200 measured previously for larger channels with similar shapes. This discrepancy may be caused by wall roughness, viscous heat generation, or electrokinetic effects. The experimental approach developed here provides the groundwork for a detailed study of turbulence transition in microchannels.

A Micromachined Silicon Low-Voltage Parallel-Plate Electrokinetic Pump

Interest is increasing in the development of electrokinetic pumps capable of generating high flow rates (> 1 ml/min). These pumps are suitable for applications such as microcooler systems. Electrokinetic pumps offer the considerable advantages of simple architecture, no moving parts, low power consumption, and robust operation. Large electric potentials are typically involved in the operation of electrokinetic devices, limiting the use of silicon in such devices. We have devised a pump design that addresses this problem. The performance of test devices of this new design suggests that electrokinetic pumping is a viable option for fluid transport and fluidic actuation in silicon micromachined devices.

Targeted microbubbles: a novel application for the treatment of kidney stones

Kidney stone disease is endemic. Extracorporeal shockwave lithotripsy was the first major technological breakthrough where focused shockwaves were used to fragment stones in the kidney or ureter. The shockwaves induced the formation of cavitation bubbles, whose collapse released energy at the stone, and the energy fragmented the kidney stones into pieces small enough to be passed spontaneously. Can the concept of microbubbles be used without the bulky machine? The logical progression was to manufacture these powerful microbubbles ex vivo and inject these bubbles directly into the collecting system. An external source can be used to induce cavitation once the microbubbles are at their target; the key is targeting these microbubbles to specifically bind to kidney stones. Two important observations have been established: (i) bisphosphonates attach to hydroxyapatite crystals with high affinity; and (ii) there is substantial hydroxyapatite in most kidney stones. The microbubbles can be equipped with bisphosphonate tags to specifically target kidney stones. These bubbles will preferentially bind to the stone and not surrounding tissue, reducing collateral damage. Ultrasound or another suitable form of energy is then applied causing the microbubbles to induce cavitation and fragment the stones. This can be used as an adjunct to ureteroscopy or percutaneous lithotripsy to aid in fragmentation. Randalls plaques, which also contain hydroxyapatite crystals, can also be targeted to pre‐emptively destroy these stone precursors. Additionally, targeted microbubbles can aid in kidney stone diagnostics by virtue of being used as an adjunct to traditional imaging methods, especially useful in high‐risk patient populations. This novel application of targeted microbubble technology not only represents the next frontier in minimally invasive stone surgery, but a platform technology for other areas of medicine.

Experimental observations and numerical modeling of lipid-shell microbubbles with stone targeting moieties for minimally-invasive treatment of urinary stones

A novel treatment modality incorporating calcium-adhering microbubbles has recently entered human clinical trials as a new minimally-invasive approach to treat urinary stones. In this treatment method, lipid-shell gas-core microbubbles can be introduced into the urinary tract through a catheter. Lipid moities with calcium-adherance properties incorporated into the lipid shell facilitate binding to stones. The microbubbles can be excited by an extracorporeal source of quasi-collimated ultrasound. Alternatively, the microbubbles can be excited by an intraluminal source, such as a fiber-optic laser. With either excitation technique, calcium-adhering microbubbles can significantly increase rates of erosion, pitting, and fragmentation of stones. We report here on new experiments using high-speed photography to characterize microbubble expansion and collapse. The bubble geometry observed in the experiments was used as one of the initial shapes for the numerical modeling. The modeling showed that the bubble dynamics strongly depends on bubble shape and stand-off distance. For the experimentally observed shape of microbubbles, the numerical modeling showed that the collapse of the microbubbles was associated with pressure increases of some two-to-three orders of magnitude compared to the excitation source pressures. This in-vitro study provides key insights into the use of microbubbles with calcium-adhering moieties in treatment of urinary stones.