In situ generation of pH gradients in microfluidic devices for biofabrication of freestanding, semi-permeable chitosan membranes

Abstract

We report the in situ generation of pH gradients in microfluidic devices for biofabrication of freestanding, semi-permeable chitosan membranes. The pH-stimuli-responsive polysaccharide chitosan was enlisted to form a freestanding hydrophilic membrane structure in microfluidic networks where pH gradients are generated at the converging interface between a slightly acidic chitosan solution and a slightly basic buffer solution. A simple and effective pumping strategy was devised to realize a stable flow interface thereby generating a stable, well-controlled and localized pH gradient. Chitosan molecules were deprotonated at the flow interface, causing gelation and solidification of a freestanding chitosan membrane from a nucleation point at the junction of two converging flow streams to an anchoring point where the two flow streams diverge to two output channels. The fabricated chitosan membranes were about 30-60 microm thick and uniform throughout the flow interface inside the microchannels. A T-shaped membrane formed by sequentially fabricating orthogonal membranes demonstrates flexibility of the assembly process. The membranes are permeable to aqueous solutions and are removed by mildly acidic solutions. Permeability tests suggested that the membrane pore size was a few nanometres, i.e., the size range of antibodies. Building on the widely reported use of chitosan as a soft interconnect for biological components and microfabricated devices and the broad applications of membrane functionalities in microsystems, we believe that the facile, rapid biofabrication of freestanding chitosan membranes can be applied to many biochemical, bioanalytical, biosensing applications and cellular studies.

On-Chip Fabrication of Carbon Nanoparticle–Chitosan Composite Membrane

“…In addition to the sandwich-type integration and the direct membrane preparation in the chip fabrication process, the in situ method is receiving greater interest. In this method, the membrane can be prepared by using emulsion photopolymerization [17,22,23] , laser-induced phase separation polymerization [24,25] , and laminar flow-based interfacial polymerization techniques [26][27][28][29] . Among these techniques, the first two allow researchers to form membranes with desired thicknesses and at a desired location.…”

“…For these devices, the first important issue that should be considered is the biocompatibility of the membranes, which refers to the choice of the membrane materials. In the literature, most membranes that are integrated in microdevices are made of non-biomaterials that could be toxic in subsequent biological applications [26] . Although the chemical modification of these membranes can improve the biocompatibility to some extent, it makes the microfabrication process more complicated [18] .…”

“…Although the chemical modification of these membranes can improve the biocompatibility to some extent, it makes the microfabrication process more complicated [18] . Recently, the use of a promising biomaterial, chitosan, was proposed for in situ microfabrication of porous membranes, which greatly expanded the function and application of membranes in microfluidic devices [26,32] . The second most important issue is the area of the membranes.…”

Fabrication of an optically driven pH gradient generator based on self-assembled proton pumps

“…Mass transport control for filtration, microdialysis, extraction, and gasliquid exchange in micro-systems can be achieved by integrated membranes (Luo et al 2010). In situ fabrication methods such as photo-polymerization and thermo-gelation have been developed for creating porous structures in microchannels; however, these techniques can leave residues that are toxic to biological processes.…”

“…In situ fabrication methods such as photo-polymerization and thermo-gelation have been developed for creating porous structures in microchannels; however, these techniques can leave residues that are toxic to biological processes. In this context, Luo et al (2010) proposed using in situgenerated pH gradients to fabricate semi-impermeable chitosan membranes. Rather than using high electric fields that can be problematic for the fabrication of chitosan membranes, the authors chose to exploit a freestanding hydrophilic membrane structure that enabled pH gradients to be generated at the converging interface between a slightly acidic chitosan solution and a slightly basic buffer solution.…”