Xi-Ya Fang

Periodic Segregation of Solute Atoms in Fully Coherent Twin Boundaries

The Strength of Impurities The practical strength of a material (rather than its theoretical strength) is influenced by the presence of defects between crystalline domains and the inclusion of impurities. In some cases, synergistic effects may arise where the impurity atoms segregate to the domain boundaries, although kinetic barriers may limit the extent to which the impurity atoms can order. Nie et al. (p. 957) show the segregation of oversized and undersized solute atoms at coherent twin boundaries in a magnesium alloy. The minimization of strain energy drives the differently sized impurities to different twin boundaries, strengthening the material. Thermally driven ordering of solute atoms can lead to an unexpected strengthening of a metal alloy. The formability and mechanical properties of many engineering alloys are intimately related to the formation and growth of twins. Understanding the structure and chemistry of twin boundaries at the atomic scale is crucial if we are to properly tailor twins to achieve a new range of desired properties. We report an unusual phenomenon in magnesium alloys that until now was thought unlikely: the equilibrium segregation of solute atoms into patterns within fully coherent terraces of deformation twin boundaries. This ordered segregation provides a pinning effect for twin boundaries, leading to a concomitant but unusual situation in which annealing strengthens rather than weakens these alloys. The findings point to a platform for engineering nano-twinned structures through solute atoms. This may lead to new alloy compositions and thermomechanical processes.

In Vitro enzymatic degradation of poly (glycerol sebacate)-based materials

Enzymatic degradation is a major feature of polyester implants in vivo. An in vitro experimental protocol that can simulate and predict the in vivo enzymatic degradation kinetics of implants is of importance not only to our understanding of the scientific issue, but also to the well-being of animals. In this study, we explored the enzymatic degradation of PGS-based materials in vitro, in tissue culture medium or a buffer solution at the pH optima and under static or cyclic mechanical-loading conditions, in the presence of defined concentrations of an esterase. Surprisingly, it was found that the in vitro enzymatic degradation rates of the PGS-based materials were higher in the tissue culture medium than in the buffered solution at the optimum pH 8. The in vitro enzymatic degradation rate of PGS-based biomaterials crosslinked at 125°C for 2 days was approximately 0.6-0.9 mm/month in tissue culture medium, which falls within the range of in vivo degradation rates (0.2-1.5mm/month) of PGS crosslinked at similar conditions. Enzymatic degradation was also further enhanced in relation to mechanical deformation. Hence, in vitro enzymatic degradation of PGS materials conducted in tissue culture medium under appropriate enzymatic conditions can quantitatively capture the features of in vivo degradation of PGS-based materials and can be used to indicate effective strategies for tuning the degradation rates of this material system prior to animal model testing.

Mechanically tissue-like elastomeric polymers and their potential as a vehicle to deliver functional cardiomyocytes.

One of the major challenges in the field of biomaterials engineering is the replication of the non-linear elasticity observed in soft tissues. In the present study, non-linearly elastic biomaterials were successfully fabricated from a chemically cross-linked elastomeric poly(glycerol sebacate) (PGS) and thermoplastic poly(L-lactic acid) (PLLA) using the core/shell electrospinning technique. The spun fibrous materials, containing a PGS core and PLLA shell, demonstrated J-shaped stress-strain curves, and having ultimate tensile strength, rupture elongation, and stiffness constants respectively comparable to muscle tissue properties. In vitro evaluations also showed that PGS/PLLA fibrous biomaterials possess excellent biocompatibility, capable of supporting human stem-cell-derived cardiomyocytes over several weeks in culture. Therefore, the core/shell electrospun elastomeric materials provide a new potential scaffold to support cells in the therapy of a wide range of soft tissues exposed to cyclic deformation, such as tendon, ligament, cardiac or smooth muscle and lung epithelium.

Non-linear elasticity of core/shell spun PGS/PLLA fibres and their effect on cell proliferation

An efficient delivery system is critical for the success of cell therapy. To deliver cells to a dynamic organ, the biomaterial vehicle should mechanically match with the non-linearly elastic host tissue. In this study, non-linearly elastic biomaterials have been fabricated from a chemically crosslinked elastomeric poly(glycerol sebacate) (PGS) and thermoplastic poly(l-lactic acid) (PLLA) using the core/shell electrospinning technique. The spun fibrous materials containing a PGS core and PLLA shell demonstrate J-shaped stress-strain curves, having ultimate tensile strength (UTS), rupture elongation and stiffness constants of 1 ± 0.2 MPa, 25 ± 3% and 12 ± 2, respectively, which are comparable to skin tissue properties reported previously. Our ex vivo and in vivo trials have shown that the elastomeric mesh supports and fosters the growth of enteric neural crest (ENC) progenitor cells, and that the cell-seeded elastomeric fibrous sheet physically remains in intimate contact with guts after grafting, providing the effective delivery of the progenitor cells to an embryonic and post-natal gut environment.

Tuning the oxygen functional groups in reduced graphene oxide papers to enhance the electromechanical actuation

The superior mechanical flexibility, mechanical strength, electrical conductivity, high specific surface area, and a special two-dimensional crystalline structure make graphene a very promising building block material for flexible electromechanical actuators. However, graphene papers have exhibited limited electromechanical actuation strain in aqueous electrolyte solution. In this paper, we show an easy approach to significantly improve the electromechanical actuation of reduced graphene oxide (rGO) papers via fine tuning the oxygen functional groups in rGO sheets, which was achieved by careful control of quantity of the reduction agent used in the chemical reduction process of graphene oxide. The actuation strains are enhanced up to 0.2% at an applied voltage of −1 V, which is more than a 2 fold increase compared to the regular pristine rGO paper. Further theoretical and experimental analyses reveal that the change of the capacitance and the stiffness of the rGO papers are two key factors responsible for the observed improvement.