SUSPENDED, POROUS CELLULOSE ACETATE MEMBRANES FOR MICRODIALYSIS USE George C. López 1 Gary K. Fedder 1,2 1 Robotics Institute, 2 Electrical and Computer Engineering Dept. Carnegie Mellon University Pittsburgh, Pennsylvania, USA Micro Total Analysis System (mTAS) 2004 Malmö, Sweden Patient Monitoring Microdialysis is a real-time technique to monitor the chemistry of the extracellular space in living tissue. Doctors usually monitor physiology on a real-time basis such as heart rate, blood pressure, etc. Assessing chemistry in a tissue’s vicinity should provide more accurate data on biochemical and pharmacological events occurring in a patient. A more direct indicator of a patient’s status is using microdialysis Microdialysis setup Microdialysis Semi-Permeable Membrane Sampled solution Probe Interior Glass beaker In vitro microdialysis setup with close-up of semi-permeable membrane interface Collected dialysate will contain a representation of tissue’s chemistry Physiological salt solution is pumped through the microdialysis probe. Concentration gradient develops across semi-permeable membrane interface. Sampled molecules diffuse into probe. Conventional Microdialysis Probe Disadvantages  Probes are hand assembled  Large fluid dead volume  Affects temporal resolution  Limited lifetime Microdialysis Membrane Silicon Substrate Advantages of silicon-based MEMS-based Device Concept  Semiconductor fabrication technology  Miniaturization, Multiplicity, and Microelectronics  Standard processes, no assembly required  Integrated approach  Acquisition, fractionated collection, and sensing on-chip Output fractionation and sensing Buffer Inlet (c) Porous polymer spin-on Mask Silicon Porous Polymer (a) Film patterning (b) Silicon etch Fabrication Process Flow  Deposition of polymer is final process step.  Able to span up to 75 micron wide silicon trench.  Polymer’s wetting characteristics are largely responsible for its ability to fill or span across a cavity. Cellulose acetate film Silicon substrate Microchannel Porous Polymer Fabrication Desired polymer is chosen, in my case it was cellulose acetate powder. Polymer is dissolved in a miscible solvent at a desired concentration. N,N-dimethylacetamide (DMAc) solvent used. Polymer-solvent mixture is created (Rotating arm, mixing takes overnight). Viscous solution is created Phase Inversion Process Step 1 Step 2 Step 3 Solution is spin cast onto a silicon substrate at a specific rotational speed. Substrate is then immersed into a room temperature water bath overnight. Solvent and non-solvent are immiscible. Cellulose acetate precipitates from solution with void regions. Substrate allowed to air dry. A thin, porous cellulose acetate film is formed. Step 4 Step 5 4 Step 6 Step Porous Polymer Fabrication Phase Separation Process Variables in the procedure  Choice of polymer  Choice of solvent  Polymer concentration  Spin speed  Evaporation time  Composition of coagulation bath  Temperature of coagulation bath Cellulose Acetate N,N-Dimethylacetamide ---Variable % deionized water Room temperature  Factorial experiment designed for three variables.  Visual confirmation of results using SEM, AFM, and light microscopy. Polymer Concentration (min) (max) Design of Experiments Evaporation Time Spin Speed Set of 15 different runs performed to determine optimum processing conditions Polymer Concentration (w/v)5%, 10%, 15% Spin Speed 1 kRPM, 2.5 kRPM, 4 kRPM Evaporation Time <5 sec., 30 sec., 1 min. Characterization of Polymer Film Low speeds caused considerable peeling of polymer At high polymer concentrations, rough surface texture and small density of pores At low polymer concentrations, film is too thin, coverage issues Optimal Settings for Porous Film SEM imaging of cellulose acetate Optimum conditions: Polymer Conc. 10% (w/v) Spin Speed 2.5 kRPM Evap. Time < 5 seconds Close-up magnification Polymer Spanning Long Channels  Stress in these phase separation films are from constrained in- plane shrinkage of the film during drying. Considerable amount of film stress causes delamination.  Long channels provide less support, therefore polymer tears and delaminates. Series of spaced bridges offer structural support as shown on right. Longer channels 50 um wide channel. Bridges: 5 um width, 50 um spacing Microbridges Microdialysis Chip: MWCO Determination Polymer Film Inlet/Outlet tubing SEM micrograph showing microchannels Photograph of Chip Fluidic interconnect was attached to the inlet/outlet ports to interface with the microchannels. The cellulose acetate showed permeability to myoglobin (MW=17 kDa) and soybean trypsin inhibitor (20 kDa) n A porous cellulose acetate film was suspended over a silicon channel using a standard fabrication process of spin coating. n Although the polymer undergoes considerable stress during drying, a 75 micron wide cavity was spanned. n Permeability tests indicate the ability of low molecular weight molecules to pass through the 20 micron thick cellulose acetate. References [1] Bergveld, P., Olthuis, W., Sprenkels, A.J., Pijanowska, D., Linden, H.J. van der, Bohm, S. Integrated Analytical Systems, Comprehensive Analytical Chemistry Series, 39, pp , 2003 [2] Mulder, M., Basic Principles of Membrane Technology, Kluwer Academic Publishers: The Netherlands, [3] Vaessen, D., McCormick, A., Francis, L., Polymer 43(8) (2002) Conclusion