Actuation of a Hyperelastic PDMS Membrane Suspended inside a Microfluidic Channel: From Computer Simulation to Microfabrication

Abstract

This Master-project was done in the Department of Physics at NTNU in the spring of 2012. The project develops micro- and nanofabrication processes useful for lab-on-a-chip systems. These systems aim to scale down and automate lab processes primarily in the fields of biomedicine and chemistry.A hyperelastic membrane suspended inside a microfluidic channel was the starting point of the project. The membrane was made from poly(dimethylsiloxane) (PDMS) and structured through physical and chemical processes developed largely by the semiconductor industry. The membrane can be stretched reversibly by applying air-pressure to the channel side-walls. This mechanism can be used to actuate functionality built into the membrane.Finite element method computer simulations in COMSOL MultiPhysics software were used to investigate the feasibility of the project. Experimental data obtained from literature for different mixing ratios (5:1, 10:1 and 15:1) of the commercially available Sylgard 184 PDMS from Dow Corning formed the basis for modeling the system using three different hyperelastic material models (neo-Hookean, Mooney-Rivlin and Ogden). By optimizing the design we made an elliptic pore in the center of the membrane that can triple in size when a pressure of 100kPa is applied to the side-walls.A microfluidic device based on the design was made from Sylgard 184 PDMS using multilayer SU-8 soft lithography. The SU-8 molds were patterned through standard photolithographic processes and used for cast molding. Hyperelastic membranes were made by spin-coating PDMS. An elliptic pore was etched in the center of the membrane using SF6/O2 reactive-ion etching. A novel method for hard masking PDMS using chromium was successfully developed, but will require further optimization. The membrane was irreversibly bonded inside the microfluidic channel and actuated by applying air-pressure to parallel channels using a syringe pump. Preliminary results indicate that the elastic response of the membrane seemingly mimics the computer model, but further investigation is required to acquire the necessary data for an accurate comparison. Potential applications of the mechanism developed here include particle sorting and mimicking of complex cellular micro-environments. The processes developed are more general and can become valuable assets in development of advanced microfluidic systems in the future.