A selective ultrasonic foaming (SUF) process was developed to fabricate porous polymer for biomedical applications. The method employs a high intensity focused ultrasound (HIFU) transducer to selectively heat and implode gas-impregnated polymers. This acoustic method is solvent-free and capable of creating interconnected pores that have various topographical features at different length scales. In this paper, we investigate the effects of major process parameters of the SUF process, including the ultrasound power, scanning speed, and the specimen gas concentration. The pore size and interconnectivity of the porous structure were analyzed. The microstructures were characterized using the scanning electron microscopy (SEM) and a dye penetration test. It was found that the scanning speed of the ultrasound had a significant effect on the pore size control, and that low gas concentration was a necessary condition for interconnected porous structures.

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-CO 2 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.

The fabrication of 3-dimensional tissue scaffolds is an essential prerequisite to engineered tissues. A polymer processing technique has been developed in this study to cast the 3D pore architecture of alginate tissue scaffolds. The alginate/calcium gluconate hydrogel was quenched in a mold and freeze dried to form a spongelike architecture whose tiny pores retain the shape of the ice crystals during quenching. Knowing that the water in the alginate hydrogel would form ice crystals if frozen and that different drying might dramatically influence the sublimation of the ice crystals, and therefore the pore architecture, we examined the speed and direction of the heat transfer out of the hydrogel as it freeze dries with regard to pore size and orientation. The pore architecture at the different locations of the fabricated scaffolds was characterized using scanning electron microscopy. It has been found that the pore size, orientation, and uniformity are significantly affected by the condition of heat transfer during freeze drying. Tailoring the pore architecture of the scaffolds is feasible by controlling the heat transfer rate. This study provides an insight on pore architecture formation and control by altered process parameters to improve the mechanical integrity.

Cautery is a process to coagulate tissues and seal blood vessels using the heat. In this study, finite element modeling (FEM) was performed to analyze temperature distribution in biological tissue subject to cautery electrosurgical technique. FEM can provide detailed insight into the heat transfer in biological tissue to reduce the collateral thermal damage and improve the safety of cautery surgical procedure. A coupled thermal-electric FEM module was applied with temperature-dependent electrical and thermal properties for the tissue. Tissue temperature was measured at different locations during the electrosurgical experiments and compared to FEM results with good agreement. The temperature-dependent electrical conductivity has demonstrated to be critical. In comparison, the temperature-dependent thermal conductivity does not impact heat transfer as much as the electrical conductivity. FEM results show that the thermal effects can be varied with the electrode geometry that focuses the current density at the midline of the instrument profile.

The electrosurgical sealing method has risen to prominence in recent years as more vessel sealing procedures are being conducted in minimally invasive surgery. Electrosurgical sealing works by applying electrical current to coagulate and denature proteins in the vessel, thereby creating a bond. In this study, experiments were conducted to seal 3 and 8 mm diameter vessels in a porcine model with a bipolar electrosurgical device and measure the electrical voltage and current, and temperature distribution in the vessel near the area of the seal. The vessel seal was modeled with finite element analysis (FEA) and compared to the experimental data. FEA allow for insight into the correlation of temperature to quality of the seal in the vessel. The maximum temperatures were 83°C and 63°C in the 3 and 8 mm vessels, respectively. Validation of the experimental results was attempted through the use of a simplified FEA model. The model showed similar thermal profiles near the electrode, but further downstream temperatures did not rise as fast as the experimental results. Further refinement to overcome limitations in the model is identified.