SUSPENDED, POROUS CELLULOSE ACETATE MEMBRANES FOR MICRODIALYSIS USE

George C. López1

Gary K. Fedder1,2

1Robotics Institute, 2Electrical 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 solutionProbe 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

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

1 2 3

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 7

5 67

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---

---Variable---

---Variable---

100% deionized water

Room temperature

Factorial experiment designed for three variables.

Visual confirmation of results using SEM, AFM, and light microscopy.

Polymer Concentration

(min)

(max)

(max)

(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 FilmSEM 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)

A porous cellulose acetate film was suspended over a silicon channel using a standard fabrication process of spin coating.

Although the polymer undergoes considerable stress during drying, a 75 micron wide cavity was spanned.

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. 625-663, 2003[2] Mulder, M., Basic Principles of Membrane Technology, Kluwer Academic Publishers: The Netherlands, 1996.[3] Vaessen, D., McCormick, A., Francis, L., Polymer 43(8) 2267-2277 (2002)

Conclusion