Microstructure and Electrical Conductivity of CNTs/PMMA Nanocomposite Foams Foaming by Supercritical Carbon Dioxide

The carbon nanotubes（CNTs）/polymethylmethacrylate（PMMA）nanocomposite foams were prepared by the anti-solvent precipitation and supercriticalfoaming method.The morphology and the electricalconductivity of the foams with different kinds of CNTs were investigated.The experimentalresults showed that allthe foams had uniform cellstructure,and the cellsize changed from 1.9 to 10 μm when the foaming temperature ranged from 50 ℃ to 95 ℃.With smallcellsize（1.9-4.0 μm）,the conductivities of the foams were 3.34×10^（-6）-4.16×10^（-6） S/cm compared with the solid matrix since the introduction of micro cells did not destroy the conductive network.However,when the cellsize was biger（4.5-10 μm）,the aspect ratio of the CNTs played the dominant role of the conductivity.The foams with short CNTs had higher conductivity,since the short CNTs were hard to stretch and snap by the cells and can well-dispersed in the cellwalland celledges.The results of this work provided a novelmaterialdesign method for conductive foams based on the rule of both microstructure and aspect ratio of the CNTs.

Bi-/multi-modal pore formation of PLGA/hydroxyapatite composite scaffolds by heterogeneous nucleation in supercritical CO_2 foaming

Scaffolds with multimodal pore structure are essential to cells differentiation and proliferation in bone tissue engineering. Bi-/multi-modal porous PLGA/hydroxyapatite composite scaffolds were prepared by supercritical C02 foaming in which hydroxyapatite acted as heterogeneous nucleation agent. Bimodal porous scaffolds were prepared under certain conditions, i.e. hydroxyapatite addition of 5%, depressurization rate of 0.3 MPa. min-1, soaking temperature of 55 ℃, and pressure of 9 MPa. And scaffolds presented specific structure of small pores （122 μM ± 66 μm） in the cellular walls of large pores （552 μm ±127 μm）. Furthermore, multimodal porous PLGA scaffolds with micro-pores （37 μM ± 11μM） were obtained at low soaking pressure of 7.5 MPa. The interconnected porosity of scaffolds ranged from （52.53 ± 2.69）% to （83.08±2.42）% by adjusting depressurization rate, while compression modulus satisfied the requirement of bone tissue engineering. Solvent-free CO2 foaming method is promising to fabricate bi-/multi-modal porous scaffolds in one step, and bioactive particles for osteogenesis could serve as nucleation agents.

Layered Structural PBAT Composite Foams for Efficient Electromagnetic Interference Shielding

The utilization of eco-friendly,lightweight,high-efficiency and high-absorbing electromagnetic interference(EMI)shielding composites is imperative in light of the worldwide promotion of sustainable manufacturing.In this work,magnetic poly(butyleneadipate-coterephthalate)(PBAT)microspheres were firstly synthesized via phase separation method,then PBAT composite foams with layered structure was constructed through the supercritical carbon dioxide foaming and scraping techniques.The merits of integrating ferroferric oxideloaded multi-walled carbon nanotubes(Fe3O4@MWCNTs)nanoparticles,a microcellular framework,and a highly conductive silver layer have been judiciously orchestrated within this distinctive layered configuration.Microwaves are consumed throughout the process of“absorption-reflection-reabsorption”as much as possible,which greatly declines the secondary radiation pollution.The biodegradable PBAT composite foams achieved an EMI shielding effectiveness of up to 68 dB and an absorptivity of 77%,and authenticated favorable stabilization after the tape adhesion experiment.

Poly（L-lactide）-grafted Bioglass/Poly（lactide-co-glycolide） Scaffolds with Supercritical CO2 Foaming Reprocessing for Bone Tissue Engineering

The bioglass particles/poly（lactide-co-glycolide）（BG/PLGA） scaffold has been extensively explored for biomedical applications due to its excellent advantages of mechanical property and controllable degradation rate. In ottr previous studies, the BG nanoparticle sttrface-grafted with poly（L-lactide）（PLLA） could substantially improve the phase compatibility between the polymer matrix and the inorganic phase and the biocompatibility of the scaffolds. However, using the traditional preparation methods to prepare the composite scaffold can barely achieve a high po- rosity and porous connectivity. In this work, the PLLA-grafted bioglass/PLGA（g-BG/PLGA） scaffolds were prepared by supercritical carbon dioxide foaming（Sc-CO2） with before or after particulate leaching（PL） method（Sc-CO2-PL or PL-Sc-CO2 method, PL/Sc-CO2 methods） and their applications in bone replacement and tissue engineering were investigated. The porosities of the g-BG/PLGA scaffolds prepared by the PL/Sc-CO2 methods were higher than 90%, and their mechanical properties had similar values with human cancellous bone. The proliferations of osteoblasts on the scaffolds were dependent on different preparation methods. The PL/Sc-CO2 methods significantly increased the proliferations of the cells. Computed tomography（CT） three-dimensional（3D） reconstruction tomographies of the implantation study for repairing calvarium defects of rabbits demonstrated that the calvarium defects were almost completely filled by the osteotylus in PL/Sc-CO2 method group at 12 week post-surgery, while there was little callus formation in PL method group and untreated control group. These results indicate that the g-BG/PLGA scaffolds prepared by the PL/Sc-CO2 methods exhibit rapid mineralization and osteoconductivity and are the optimal composites for bone repair.

Supercritical fluid-assisted controllable fabrication of open and highly interconnected porous scaffolds for bone tissue engineering

Recently tremendous progress has been evidenced by the advancements in developing innovative three-dimensional(3 D)scaffolds using various techniques for addressing the autogenous grafting of bone. In this work, we demonstrated the fabrication of porous polycaprolactone(PCL) scaffolds for osteogenic differentiation based on supercritical fluid-assisted hybrid processes of phase inversion and foaming. This eco-friendly process resulted in the highly porous biomimetic scaffolds with open and interconnected architectures. Initially, a 2^3 factorial experiment was designed for investigating the relative significance of various processing parameters and achieving better control over the porosity as well as the compressive mechanical properties of the scaffold. Then, single factor experiment was carried out to understand the effects of various processing parameters on the morphology of scaffolds. On the other hand, we encapsulated a growth factor, i.e., bone morphogenic protein-2(BMP-2), as a model protein in these porous scaffolds for evaluating their osteogenic differentiation. In vitro investigations of growth factor loaded PCL scaffolds using bone marrow stromal cells(BMSCs) have shown that these growth factor-encumbered scaffolds were capable of differentiating the cells over the control experiments. Furthermore, the osteogenic differentiation was confirmed by measuring the cell proliferation, and alkaline phosphatase(ALP) activity, which were significantly higher demonstrating the active bone growth. Together, these results have suggested that the fabrication of growth factor-loaded porous scaffolds prepared by the eco-friendly hybrid processing efficiently promoted the osteogenic differentiation and may have a significant potential in bone tissue engineering.

Experimental Investigation of Immiscible Supercritical Carbon Dioxide Foam Rheology for Improved Oil Recovery

This paper presents the rheological behaviour of supercritical CO2 （sCO2） foam at reser- voir conditions of I 500 psi and 80 ℃. Different commercial surfactants were screened and utilized in or- der to generate a fairly stable CO2 foam. Mixed surfactant system was also introduced to generate strong foam. Foam rheology was studied for some specific foam qualities using a high pressure high tempera- ture （HPHT） foam loop rheometer. A typical shear thinning behaviour of the foam was observed and a significant increase in the foam viscosity was noticed with the increase of foam quality until 85%. A de- sired high apparent viscosity with coarse texture was found at 85% foam quality. Foam visualization above 85% showed an unstable foam due to extremely thin lamella which collapsed and totally disap- peared in the loop rheometer. Below 52%, a non-homogenous and unstable foam was found having low viscosity with some liquid accumulation at the bottom of the circulation loop. This research has demon- strated rheology of sCO2 foams at different qualities at HPHT to obtain optimal foam quality region for immiscible CO2 foam co-injection process.

Microcellular foamed polyamide 6/carbon nanotube composites withsuperior electromagnetic wave absorption

Electromagnetic (EM) wave pollution causing damage to precision equipment and threatening thehealth of living organisms has attracted considerable attention. Herein, promising microcellular foamedpolyamide 6 (PA6)/carbon nanotube (CNT) composites for highly efficient EM wave absorption were successfully fabricated using supercritical CO_(2) foaming. Nanocomposites foams with a void fraction rangingfrom 38.7% to 85.1% were achieved, providing a platform to assess the correlation of the electrical conductivity, the dielectric permittivity and the EM wave absorption properties with porosity. Notably, theFoam-257.5C sample with a void fraction of 38.7% exhibited outstanding EM wave absorption characteristics at a thickness of only 1.59 mm and an ultra-low reflection loss value of -55.3 dB (99.9997% wave absorption). Most importantly, the effective absorption bandwidth (EAB) of the Foam-257.5C sample couldcover the entire Ku band (12.4–18 GHz) by slightly adjusting the thickness from 1.20 to 1.60 mm. Thesuperior EM wave absorption performance of the Foam-257.5C sample was attributed to multiple reflections and scattering at the solid-gas interfaces, favorable impedance matching due to the existence ofa large polymer-air interface area, conductive loss near the interfaces and interfacial polarization. Thus,this study offers an eco-friendly, simple and versatile methodology to develop high-efficiency, flexiblepolymer-based EM wave absorbents.