Previous Projects in MEMS Design
Silicon is a great material for many things. However for BioMEMS applications it is often not well suited because of biocompatibility issues. Our group has worked with various polymers including SU-8 Epoxy, Teflon and PDMS.
The image demonstrates a process for making microlenses using Teflon and SU-8. The polymer sticks only to the photolithographically defined hydrophilic regions of a Teflon substrate.
Fluid control on a chip will require valves. Our group has been working on microvalves for almost eight years starting with bubble-valves and then moving to creating valves that use moving structures. Working with Al Pisano, our group has focused on making planar devices because we felt that chip or wafer bonding would make devices that are too expensive for biomedical applications.
The top image is a check valve that operates using viscous forces (rather than inertia like a standard checkvalve). Designed and fabricated by Ajay Deshmuhk, PhD, M.E., 2002). The bottom device is an active valve made by Alex Papavisilou (PhD, M.E., 2001).
Mixing in Microdevices
This is the original project in microfluidics starting with Al Pisano and John Evans (PhD '98). Many people thought it was a dumb project based on the then-held belief that because microfluidic length scales are short, mixing must not be an issue.
In reality, mixing is tough in microenvironments because the Reynolds Numbers are so small that vortical or turbulent motions are impossible to obtain. Furthermore, MEMS devices generally have small aspect ratios (i.e. they are wider than they are deep) which substantially slows down diffusion. The goal of micromixers are to increase the interfacial area between two fluids so that diffusion becomes effective. The example on the right is a simulation of a mixer using Chaotic Advection after the work by Hassan Aref and Scott Jones in the '80's at UCSD.
A continuous micromixers designed by Ajay Deshmuhk (PhD 2003) is shown below. The approach uses Ajay's check valves with bubbles to make two positive displacement pumps. Mixing is achieved by exploiting the time-dependence of the pumps with Taylor dispersion as the flow moves down the center channel.
The Liepmann lab spent over seven years working on micro-pumps, starting with the work of John Evans. The latest implementation shown on the right was a collaboration with the Pisano Lab. It uses Jeremy Frank's 'hole-in-the-wall' valve designs that support very large pressures (pressure drop ratio ~ 1200). This latest micro-pump was designed and fabricated by Stefan Zimmerman and Jeremy Frank.