To control fluid transport, microfluidic systems often make use of pressure driven flow and pneumatically actuated valves. However these require bulky external instrumentation. An integrated fluid control mechanism would make microfluidic systems more portable, closed and automated. The aim of this project was to create a pH-responsive membrane for integrated fluid control in microfluidic systems. pH-responsiveness will allow the membrane to interact directly with analytes in a microfluidic system.
First pH-responsive materials and fluid control mechanisms to create a pH-responsive membrane were reviewed. Cross-linked polymer hydrogels with pH-dependent volumetric swelling were identified as the most suitable material for the membrane. These materials respond to changes in pH by large volumetric transitions, which creates potential for control over a wide range of flow rates. Furthermore they have an inherent interaction with water and are permeable to small molecules such as H+ ions. The material is synthesized from a liquid precursor that can be cured through photo-initiated free radical polymerization.
We tested a system for measuring the pH response of the material that consisted of a hydrogel disk that was vertically constrained inside a fluidic channel. Due to this constraint, expansion only occurred in the lateral direction and could be measured using an optical microscope. Furthermore, we developed a method for manufacturing macro-porous hydrogel membranes that gives control over pore size, shape and position. A photo-lithography approach was used to pattern the membranes and thereby create pores, using a photo-mask that was manufactured on an office printer in a very fast and low-cost process.
The resulting membranes had a thickness of 140-190 um and pore diameters of 100-400 um. The pore size was measured for environmental pH of 1.6 and 7.1, within this range the pores doubled in diameter. Furthermore the pH-responsive deformation ratio of the pores increased significantly with increasing curing time and decreasing pore diameter. The results suggest that there is a difference in material properties around the pores that develops due to a local difference in received exposure dose during curing.
The fluidic properties and pH-response of the membrane can be adjusted to suit a specific application by changing the design, the curing time or the chemical composition of the membrane. The constrained disk system can then be used to measure and compare the pH-response of different potential materials.