Fabrication of thin and flexible PDMS membranes for biomechanical test applications

For continuous analysis of mechanical properties of soft biological tissues during mechanical stimulation we developed a bioreactor to apply controlled periodic deflection to a probe. The probe attached on an elastic membrane produces a counterpressure if a certain gas volume is applied to the pressure chamber under the membrane. The related counterpressure-volume relationship describes the mechanical properties of the membrane-probe combination. To improve the signal-to-noise ratio of measurements the use of highly flexible membranes is advantageous. We developed a specific adapter, attached securely to the central thread of a centrifuge, to fabricate silicone membranes on carrier rings with reproducible mechanical characteristics. As membrane material we used a two component elastomer that consists of base resin and curing agent. Components were mixed to a 10:1 (M1) and 15:1 (M2) weight ratio. Further test parameters were rotation time and rotation speed of the spin-coating procedure. The compliance of the solitary membranes was measured over 24 h to investigate long-time stability. With optical coherence tomography (OCT) the thickness of membranes was estimated. M2 (higher base resin to curing agent ratio) resulted in more flexible membranes compared to M1. Increasing rotation duration and speed led to higher compliance values. The membranes material characteristics did not change during 24 h mechanical stimulation. Analysis of thickness using OCT demonstrated that our method resulted in a homogenous profile and a thickness of softest membranes around 33-36 mu m. With our new technique highly pliant membranes with homogenous thickness and mechanical stability over a period of 24 h can be produced. The membranes in designated sizes are characterized by reproducible mechanical properties. For further applications membranes may variously be modified.