Creating Open-celled Solid-state Foams Using Ultrasound

Open-celled solid-state foams are expected to have many applications, such as filtration, biochemical sensing, and tissue engineering scaffolding. For bio-related applications, the solid-state foaming process has a unique advantage, i.e., the process does not involve any organic solvent that could leave residues harmful to biological cells. The disadvantage, however, is that solid-state foams are mostly close-celled and do not allow biological cells or fluids to permeate through. This article presents a parametric study on a novel fabrication method to create open-celled solid-state foams using ultrasound. Biodegradable polymer was foamed in a solid-state foaming process. Ultrasound was then applied to break the pore walls. The parameters examined in this study included the pore size, ultrasound power, ultrasound frequency, and water temperature. The inter-pore connectivity was verified using permeability measurements, dye diffusion, and a degradability test. The permeability values were further used to analyze the effects of the experimental parameters. In the analysis a logistic regression model was first used to ensure sample integrity after the ultrasound treatment. A linear regression was then conducted to determine the significance of process variables. It was found that the pore size was the most significant factor affecting the ultrasound effectiveness. The bigger the pores are, the higher the permeability could be obtained with ultrasound. In order to increase the permeability of the ultrasound treated samples, high ultrasound power, high water temperature, and low ultrasound frequency should be used.