Microchip-based Dialysis of Protein Samples Using Photopatterned Nanoporous Membranes

A novel method for flexibly implementing sample dialysis on microchips is introduced. Nanoporous polymer membranes (5-20 pm thickness) with up to 1OOO:l aspect ratio are fabricated in-situ on silica microchips via photopatterned phase-separation polymerization. These membranes enable selective diffusion of molecules smaller than a cutoff specified by the membrane pore size. Diffusion measurements on-chip indicate a molecular weight cutoff near 6 kD for typical membranes; further, this molecular weight cutoff can be engineered by varying the constitution of the solvent in the polymerized solution. The dialysis membranes reported here are hydrophilic and show essentially no protein adhesion. KEY’VVORDS phase-separated polymer, filtration, dialysis, microdialysis, sample preparation, polymer monolith INTRODUCTION Complex samples (e.g., cell extract) often require extensive cleanup before introduction to analysis channels in a miniaturized device. These processing steps are often performed off-chip using large volumes of sample and other reagents, and hence often add substantially to the total analysis time and cost. Dialysis, or size-based separation of species via selective diffusion through a semipermeable membrane, is a widely used technique for cleanup of biological samples [ 1,2]. We have developed a technique for fabricating thin (5-20 pm) polymer dialysis membranes within the channels of microchips. Controlled placement of the dialysis membrane(s) is accomplished using UV photoinitiated polymerization with a shaped laser beam. Laser-photopatteming of dialysis membranes is rapid and inexpensive, and the ability to place multiple membranes of varying molecular weight cuttoff will increase the functionality of integrated microfluidic devices. Here we demonstrate in-situ fabrication of nanoporous membranes with molecular weight cutoff near 6 kD, and demonstrate mass exchange across a lcm-long membrane in counterflow configuration. IN-SITU PHOTOPATTE~ING OF POLYMER MEMBRANES Thin (5-20 pm) porous polymer membranes are fabricated in-situ in glass microchannels by shaping and focusing the 355 nm output of a frequency-tripled Nd:YAG laser into a 515 pm sheet. We use this laser sheet to locally excite azo photoinitiators and generate 7th lnternat~onal Conference on Miniaturized Chemical and Blochemlcal Analysts Systems October 5-9, 2003, Squaw Valley, Callfornla USA O-974361 I-0.O/~TAS2003/$15.0002003TRF 421 phase-separation polymerization [3-51 in the irradiated region (Figure 1). The membrane consists of a zwitterionic methacrylate (2methacryloyloxyethyl dimethyl 3sulfopropyl ammonium) crosslinked with methylene bisacrylamide (Figure 2). The membrane is covalently attached to the silica surface by preceding polymerization with acid-catalyzed reaction of acrylate-mnctionalized organosilanes at the silica surface. The polymer membrane is precipitated from a water/2-methoxy-ethanol solution whose solvent properties dictate the membrane pore size. observation/alignment