A novel technique was developed for the fabrication of porous polymer for biomedical applications using selective ultrasonic foaming. It was found that the gas concentration of saturated polymer samples plays an important role in the creation of open celled, interconnected porous structures. In this paper, we present a concentration-dependent diffusion model for PMMA-CO2 gas polymer system to understand the open-cell mechanism in the selective ultrasonic foaming process. An explicit finite difference scheme was employed to solve the nonlinear gas diffusion equation. By comparing with the polymer gas saturation data, a necessary condition for open celled structure is established in terms of the gas concentration levels.

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