Elastic Wave Manipulations via 3D-printed Chains of Hollow Elliptical Cylinders

Thesis (Ph.D.)--University of Washington, 2018Lattice structures have been studied for a long time but have come into spotlight in recent years as a test bed for various wave manipulations. Researchers have started to pay attention not only to their interesting static/dynamic behaviors but also to their high tunability. In this work, we explore linear and nonlinear elastic wave dynamics in lattice chains. Specifically, the chains are composed of 3D-printed hollow elliptical cylinders (HECs). 3D-printing provides us the freedom of altering the design of the HECs, hence offering high tunability of the HEC lattice chains. This implies that we can assemble different systems for various wave manipulations. First, we investigate shock wave propagation in the homogeneous chain. We experimentally and numerically demonstrate the formation of dispersive rarefaction shocks in the 3D-printed soft HEC chain. We claim that the dispersion in the wave tails and the rarefaction in the leading pulse of the dispersive rarefaction shocks provide innate advantage in energy absorption. Next, we consider graded chains made of HECs with varying thicknesses, where asymmetric wave dynamics is invoked. In the decreasing thickness chain, we find out that elastic waves are trapped at a specific location of the chain, which is based on the principle of the Bloch oscillations. This trapping mechanism depends on the input frequency of the propagating elastic waves. In the increasing thickness chain, however, we observe waves are reflected back in the middle of the chain whose location depends on the input frequency. Finally, we show asymmetric nonlinear wave dynamics in the graded HEC chain. Under the same striker impact, the wave decelerates in the decreasing thickness chain whereas it accelerates in the increasing thickness chain. We discover that there is near an order-of-magnitude difference in transmitted force between these two directions. We extend our findings from the 1D systems to a 2D lattice, with a possibility of using it as a core material in sandwich structures. These results suggest that the 3D-printed HECs can be built into different structures to manipulate mechanical waves in various ways, such as attenuation, localization, and filtering. We can exploit the findings in this work for potential applications in impact mitigation, vibration isolation, and energy harvesting

Electrospun Silk Fibroin Nanofibrous Scaffolds with Two-Stage Hydroxyapatite Functionalization for Enhancing the Osteogenic Differentiation of Human Adipose-Derived Mesenchymal Stem Cells

The development of functional scaffolds with improved osteogenic potential is important for successful bone formation and mineralization in bone tissue engineering. In this study, we developed a functional electrospun silk fibroin (SF) nanofibrous scaffold functionalized with two-stage hydroxyapatite (HAp) particles, using mussel adhesive-inspired polydopamine (PDA) chemistry. HAp particles were first incorporated into SF scaffolds during the electrospinning process, and then immobilized onto the electrospun SF nanofibrous scaffolds containing HAp via PDA-mediated adhesive chemistry. We obtained two-stage HAp-functionalized SF nanofibrous scaffolds with improved mechanical properties and capable of providing a bone-specific physiological microenvironment. The developed scaffolds were tested for their ability to enhance the osteogenic differentiation of human adipose-derived mesenchymal stem cells (hADMSCs) in vitro and repair bone defect in vivo. To boost their ability for bone repair, we genetically modified hADMSCs with the transcriptional coactivator with PDZbinding motif (TAZ) via polymer nanoparticle-mediated gene delivery. TAZ is a well-known transcriptional modulator that activates the osteogenic differentiation of mesenchymal stem cells (MSCs). Two-stage HAp-functionalized SF scaffolds significantly promoted the osteogenic differentiation of TAZ-transfected hADMSCs in vitro and enhanced mineralized bone formation in a critical-sized calvarial bone defect model. Our study shows the potential utility of SF scaffolds with nanofibrous structures and enriched inorganic components in bone tissue engineering.© 2017 American Chemical Societ