Porous 85/15 poly (DL-lactide-co-glycolide) or PLGA foams were produced by the pressure quench method using supercritical CO 2 as the blowing agent. The effects of saturation pressure and temperature on average cell size and relative density of the resulting foams were studied. Porous PLGA foams were generated with relative densities ranging from 0.107 to 0.232. Foams showed evidence of interconnected cells with porosities as high as 89%. The cell size ranged from 30 to 70 microns.

Polystyrene disks were saturated with nitrogen at 2000 psi pressure (13.8 Mpa) and foamed at 115 C in a pressurized cavity. It was found that the cavity pressure had no influence on the cell nucleation density. This was a surprising result, since based on classical nucleation models, we expected fewer and fewer cells to nucleate as we increased the cavity pressure.

This article considers the problem of achieving a desired supersaturated gas concentration profile inside a polymer part by controlling the gas concentration on the part boundaries at elevated pressure. Such controlled gas concentration can be used to achieve a desired cell-structure inside the polymer when foaming the gas inside as the part is rapidly heat-treated. The goal is to achieve the interior gas concentration at the end of the controllable pressurization, i.e., right before the heat treatment. When the gas diffusion dynamics is Fickian, the main contribution of this article is to show that any specified concentration profile can be achieved provided a sufficiently large relatively-flat offset of the concentration profile is acceptable. This is demonstrated by showing that the interior gas concentration is controllable by changing the gas concentration at the boundary, and the use of an offset ensures that the control input (the outside concentration) remains positive. The controllability, in turn, allows the use of standard optimal control to achieve the state transition to the desired interior gas concentration before the heat treatment. A model-based simulation is used to illustrate the proposed approach.

Polymer nanocomposites with carbon nano tubes (CNTs) added in the polymer matrices have been shown to have improved material properties comparing to their neat forms. Foaming of polymer nanocomposites is even more interesting because these foams promise higher strength-to-weigh ratio and multifunctionality. However, many foaming results on polymer nanocomposites are inconclusive, for example, the effect of nano-additives on the bubble nucleation. In this study, solid-state foaming of pure PMMA and PMMA/CNT composite were performed. CNTs were dispersed in PMMA solution with ultrasonication. Microcellular foams were achieved in both cases. The resulted relative density and microstructures were investigated. It was found that nano carbon tubes aggregated on the cell walls of the microcellular foams. In terms of the cell size and relative density, there is no significant difference between pure PMMA and PMMA/CNT foams. Better dispersion of CNTs is needed in order to further study the CNT effect on the solid-state foaming process.

Polylactic acid (PLA) is a biodegradable semi-crystalline thermoplastic polymer that can be used in many applications such as tissue engineering scaffolds and packaging. The crystallinity of PLA is an important factor that affects its process-ability, mechanical strength, and biodegradability. The solid-state foaming of semi-crystalline PLA has been a subject of recent investigations. In this paper, crystallization through out the solid state foaming process was studied. It was found that the crystallization reaches the equilibrium once the gas sorption reaches the equilibrium. There are two main factors that will affect the PLA crystallization: gas sorption during the saturation stage and the heating and stretching during the foaming stage. Within the range of 2 to 5 MPa saturation pressures and 60 to 100 °C foaming temperatures, a maximum crystallinity of approx. 25% was observed in the foamed PLA. Effects of stretching and foaming temperature on crystallinity of foamed specimens were also investigated.

Polycaprolacton (PCL) is a synthetic biodegradable polymer that is widely used in tissue engineering related studies. It is a semi-crystalline polymer, and has a glass transition temperature ( T g ) of −60°C and a melting temperature of 60°C. In this paper, we report on the progress in creating porous PCL foams using the solid-state foaming process. The objective of this study is to examine the foam-ability of PCL using room temperature saturation. PCL specimens were made using compression molding. A “quenching” process was introduced to manipulate the crystallinity of PCL samples. CO 2 was used for gas saturation. The effects of saturation pressure and foaming temperature were studied. The created microstructures were characterized using scanning electron microscopy (SEM). The preliminary results have shown that microstructures with pores on the scale of hundreds of nanometers were generated.

Biodegradable porous polymers with interconnected pores of sub-micrometers to a few hundred micrometers find many applications in emerging technology areas such as tissue engineering, controlled drug delivery, and biochemical sensors. However, most of the current fabrication processes involve organic solvents and chemical blowing agents that may cause environmental concerns and leave residues harmful to biological cells. This paper presents a solvent free fabrication approach for biodegradable porous polymers. Ultrasound cavitation is introduced after the solid state foaming process to produce open cell structures. The material used in this study is polylactic acid (PLA). It belongs to a family of biodegradable polymers that can be used for tissue engineering scaffolds. In order to identify suitable conditions to apply ultrasound, a saturation and foaming study is conducted for the PLA-CO 2 gas polymer system. The effects of various process variables are discussed.

One challenge for engineering educators is to design appropriate educational curricula that address both students’ needs and educators’ expectations. It is therefore important for the engineering education community to think about how to include students and faculty into the assignment design process. In this paper we discuss our use of user-centered design (UCD) principles, including early focus on users, empirical measurements, and iteration design, to design course-based portfolio (CBP) assignments for mechanical engineering students. To support the assignment design decisions, we collected early information about mechanical engineering students’ learning needs and educators’ expectations. Based on the design considerations that we identified for portfolio design, the CBP assignment is designed to ask students build a persuasive document and identify the connections between the course and mechanical engineering discipline. We measured our design by piloting the assignment with students and getting feedback from faculty in the Mechanical Engineering Department; this feedback reflects the effectiveness of the CBP and leads to design iterations and tradeoffs. We also identified several students’ learning opportunities and formed these opportunities into hypotheses to evaluate the impacts of course-based portfolios on students learning in the next formal study. This research demonstrates a successful practice of using user-centered design principles to design a course-based portfolio assignment for mechanical engineering students.

Most of the existing fabrication techniques for tissue engineering scaffolds require the use of organic solvents that may never be fully removed even after long leaching hours. The residues of these organic solvents may reduce the ability of biological cells to form new tissue. In this study, interconnected porous structures were created using solid-state foaming and ultrasound. The interconnectivity was verified with dye test. The pore sizes of the foams ranged from 300 to 500 μm. Permeability measurements were also performed to quantify the interconnectivity using a system developed in house. The achieved average permeability of PLA foams with 300 μm pore size is 3.1×10 -12 m 2 . The bigger the closed pores are, the easier ultrasound can make them open. There is an optimum temperature range in which ultrasound will enhance the interconnectivity of the solid-state foam the most efficiently. An ultrasound cavitation model was proposed as the dominant pore opening mechanism and was used to explain the temperature effects and pore size effects. The combined solidstate foaming and ultrasound processing provides a way to fabricate porous polymer for potential tissue engineering applications.