Sunday, December 11, 2005

PDMS Microfluidic Mixer - Lateral Pneumatic Concept

12-11-05 At the same 12-8-05 Thursday Bay Area MEMS Journal Club Meeting, Narayan Sundararajan of Intel described in his presentation to the group, some interesting development work that Intel is persuing in the Digital Health R&D work. Fascinating stuff, microfluidics and single molecule detection methods and more.

Nara described several things that he has been working on, and there was emphasis on novel innovations - oriented towards building blocks for single moelcule detection with microfluidics.

There was description of a 3-d fluidic sheath confinement method which employs a lateral and more tricky vertical sheath flows to permit 2D confinement of a flowing analyte - to enchance S/N in flourescence detection of low concentration analyte flows and to avoid wall interactions by use of the flowing sheath protection.

Nara also talked about a unique simple to fabricate lateral pneumatic PDMS microfluidic actuator - that can rely on using only a single PDMS molded layer to implement pneumatically actuated fluidic mechanisms with a very simple microfab process. No complicated multi layer soft lithography required to build some pretty sophisticated devices. This was very innovative and Nara demonstrated a functioning peristalic pump implemented using the method, as well as a simpler variable flow restrictor.

Nara brought up the point that the smooth flat vertical sidewalls - in the thin membrane sidewall actuators - were not able to implement a lateral pneumatic actuated mixer. It was not stated so explicitly, but this was apparent from what was described in trying to observe mixing with the flat sidewall devices.

I saw this and had my interest piqued, since I remember that when fluid flows make sharp turns, the resultant lateral/radial gradient in flow velocity, generates a dispersive effect in the flow profile past the sharp turn.

Clearly this observation could result in a useful method to make a variation of the lateral pneumatic PDMS structure, to enable a multistage mixer for the channel flow, when surrounded by a PDMS lateral pneumatic structure. The sidewall shape just had to be modified to generate lateral flow velocity gradients.

When Nara said that the smooth sidewalls of the tested lateral PDMS pneumatic channel did not mix - the observation I had was to replace the smooth horizontal PDMS wall profile of the pneumatic actuator - with either a zig zag, or series of protrusions of some indeterminate shape periodicity and (a)symmetry ( 1 sided or 2 sides of the lateral pneumatic PDMS actuator )

So this is what we will attempt to prototype in the coming months to demonstrate that the simple cost effective lateral pnuematic PDMS channel actuator ( valve / peristaltic pump structure concepts ) might be extended to flow mixing with similar PDMS single layer process simplicity.

One of the challenges will be to ensure that the foot of the PDMS wall still adheres to the substrate to maintain a seal to the fluid channel, and the second will be to observe how the mixing efficiency varies with 1] the shapes of the tested sidewall lateral profile, 2] # of "mixing elements" and 3] asymmetry of the structures.

I remember several efforts at generating mixers in PDMS microfluidics and the challenges are not particularly trivial such as in DEP ( dielectrophoresis ) microfluidic mixers for example.....

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