Supercritical CO2 foaming of radiation crosslinked polypropylene/high-density polyethylene blend: Cell structure and tensile property

Development of Advanced Biodevices Using Quantum Beam Microfabrication Technology

Biodevices with engineered micro- and nanostructures are strongly needed for advancements in medical technology such as regenerative medicine, drug discovery, diagnostic reagents, and drug delivery to secure high quality of life. The authors produced functional biocompatible plastics and hydrogels with physical and chemical properties and surface microscopic shapes that can be freely controlled in three dimensions during the production process using the superior properties of quantum beams. Nanostructures on a biocompatible poly(L-lactic acid) surface were fabricated using a focused ion beam. Soft hydrogels based on polysaccharides were micro-fabricated using a focused proton beam. Gelatin hydrogels were fabricated using γ-rays and electron beam, and their microstructures and stiffnesses were controlled for biological applications. HeLa cells proliferated three-dimensionally on the radiation-crosslinked gelatin hydrogels and, furthermore, their shapes can be controlled by the micro-fabricated surface of the hydrogel. Long-lasting hydrophilic concave structures were fabricated on the surface of silicone by radiation-induced crosslinking and oxidation. The demonstrated advanced biodevices have potential applications in three-dimensional cell culture, gene expression control, stem cell differentiation induction/suppression, cell aggregation into arbitrary shapes, tissue culture, and individual diagnosis in the medical field.

Improving the Supercritical CO₂ Foaming of Polypropylene by the Addition of Fluoroelastomer as a Nucleation Agent

In this study, a small amount of fluoroelastomer (FKM) was used as a nucleating agent to prepare well-defined microporous PP foam by supercritical CO₂. It was observed that solid FKM was present as the nanoscale independent phase in PP matrix and the FKM could induce a mass of CO₂ aggregation, which significantly enhanced the diffusion rate of CO₂ in PP. The resultant PP/FKM foams exhibited much smaller cell size (~24 μm), and more than 16 times cell density (3.2 × 10⁸ cells/cm³) as well as a much more uniform cell size distribution. PP/FKM foams possessed major concurrent enhancement in their tensile stress and compressive stress compared to neat PP foam. We believe that the added FKM played a key role in enhancing the heterogeneous nucleation, combined with the change of local strain in the multiple-phase system, which was responsible for the considerably improved cell morphology of PP foaming. This work provides a deep understanding of the scCO₂ foaming behavior of PP in the presence of FKM.

A new promising nucleating agent for polymer foaming: effects of hollow molecular-sieve particles on polypropylene supercritical CO2 microcellular foaming

Because polypropylene (PP) foam normally exhibits nonuniform cell size and cracked cellular structure, a narrow cell-size distribution and a well-defined morphology are always the focus of PP foaming technology. In this work, hollow molecular-sieve (MS) particles were applied as a potential nucleating agent in supercritical carbon dioxide (scCO2) foaming of PP. It was observed that the addition of MS particles largely narrowed the cell-size distribution. The resultant PP/MS foams exhibited significant concurrent enhancement in their cell density and mechanical properties: the cell density increased remarkably, by approximately 10 times, and the tensile strength increased from 6.1 MPa to 12.6 MPa. The hollow-structure MS particles resulted in a higher heterogeneous nucleation efficiency in the PP foaming process. We believe that the trapping of CO2 in the hollow holes of MS particles largely increased the solubility CO2 in PP and a number of gas cavities were formed. The existence of gas cavities reduced the energy barrier of heterogeneous nucleation, favoring the formation of a well-defined cellular structure. Additionally, the regular-hexagon shape of the cells might endow the PP foam with better mechanical properties compared with a circular cell shape.