Yifeng Hong

Compression induced chondrogenic differentiation of embryonic stem cells in 3-D PDMS scaffolds.

Embryonic stem cells (ESCs) are an ideal source for chondrogenic progenitors for the repair of damaged cartilage tissue. It is currently difficult to induce uniform and scalable ESC differentiation in vitro, a process required for stem cell therapy. This is partly because stem cell fate is determined by complex interactions with the native microenvironment and mechanical properties of the extracellular matrix. Mechanical signaling is considered to be one of the major factors regulating the proliferation and differentiation of chondrogenic cells both in vitro and in vivo. We used biocompatible and elastic polydimethylsiloxane (PDMS) scaffolds, capable of transducing mechanical signals, including compressive stress in vitro. ESCs seeded into the PDMS scaffolds and subjected to mechanical loading resulted in induction of differentiation. Differentiated ESC derivatives in three-dimensional (3-D) PDMS scaffolds exhibited elongated single cell rather than round clonal ESC morphology. They expressed chondrogenic marker, Col2, with concomitant reduction in the expression of pluripotent marker, Oct4. Immunocytochemical analysis also showed that the expression of COL2 protein was significantly higher in ESCs in 3-D scaffolds subjected to compressive stress. Further analysis showed that compressive stress also resulted in expression of early chondrogenic makers, Sox9 and Acan, but not hypertrophic chondrogenic markers, Runx2, Col10, and Mmp13. Compressive stress induced differentiation caused a reduction in the expression of β-Catenin and an increase in the expression of genes, Rhoa, Yap, and Taz, which are known to be affected by mechanosignaling. The chondroinductive role of RhoA was confirmed by its downregulation with simultaneous decrease in the transcriptional and translational expression of early chondrogenic markers, SOX9, COL2, and ACAN, when ESCs in PDMS scaffolds were subjected to compressive stress and treated with RhoA inhibitor, CCG-1432. Based on these observations, a model for compression induced chondrogenic differentiation of ESCs in 3-D scaffolds was proposed.

Rapid Vacuum Infusion and Curing of Epoxy Composites with a Rubber-Cushioned Mold Design

ABSTRACT Vacuum resin infusion has been developed as a low-cost process for manufacturing fiber-reinforced composites with high fiber loading. However, it is an extremely slow process due to the long time required to prepare a vacuum-tight mold and further to cure the resin at room temperature. In this work, a new mold with a rubber cushion suitable for rapid infusion and rapid curing under complete negative pressure was designed and fabricated. Experimental results demonstrated that the new mold is suitable for rapid heating and withstand dynamic loading under complete vacuum force. Composite plaques can be produced with a short cycle time of several minutes while desired mechanical properties are still achieved. Thermal analysis and rheological measurements were performed to study epoxy curing at elevated temperatures and define an optimal process window for rapid vacuum infusion. This study leads to the development of a completely vacuum-based liquid molding process that uses simple tooling, yet is suitable for rapid production. GRAPHICAL ABSTRACT

Vacuum Infusion for Processing Thermosetting Composites Containing High Loading Solid Fillers

A new processing route for making thermosetting materials and products containing high loading solid fillers including waste solid scraps and natural mineral fillers was investigated. Traditionally, solid fillers are incorporated into thermosetting polymers by liquid-state mixing. However, due to the high viscosity in mixing, high loading above 50% by weight is difficult to achieve. The new process overcomes this hurdle by a two-step processing strategy. First, solid fillers are placed in the mold and compacted to conform to the mold shape. Next, with the mold closed, a thermosetting resin is infused by vacuum into the particulate preform and after curing to form the desired product. In the exploratory study, epoxy and mineral particles including sand were chosen as material systems for feasibility demonstration. Thermal analysis and rheology were performed to assist in understanding of the fluid flow in heated porous media to achieve an optimized process window for rapid infusion. The results indicated that epoxy matrix composites with extremely high solid filler loading exceeding 70% by weight, as well as with complex external geometry, can be successfully produced. The resulting material is structurally uniform and its mechanical properties can be adjusted by changing the composition and makeup of the filler materials.Copyright

Formation and Characterization of Co-Continuous Shear Thickening Fluid/Polymer Blends

By synergistically combining distinct physical and chemical properties of different components, co-continuous polymer blending has become an important route to improve the performance of polymeric materials. Shear thickening fluid is a type of non-Newtonian fluid which has unique shear rate dependence and good damping properties. In this work, the authors combined the shear thickening fluid and a commodity polymer into a single system by forming a co-continuous blend via a melt processing technique. The processing window of such co-continuous blend was determined by referring to the thermal and rheological properties of raw materials and experimentally exploring various blending conditions. An increase of tanδ under dynamic mechanical analyzing testing was observed in the co-continuous blend compared with neat polymer as control, which indicated the enhancement of damping capabilities.Copyright