published_or_final_version / Civil Engineering / Doctoral / Doctor of Philosophy

Cellular structures are present from the atomic level all the way up to patterns found in human skeleton. They are prevailing structures in the nature and known for their excellent mechanical, thermal, and acoustic properties. Two typical types of cellular structures, lightweight structures and compliant mechanisms, are investigated. Lightweight structures are rigid and designed to reduce weight, while increasing strength and stiffness. Compliant mechanisms are designed to transform motions and forces. Most available artificial lightweight structures are patterns of primitives. However, the performance of lightweight structures can be enhanced by using adaptive cellular structures with conformal strut orientations and sizes, like the trabeculae in femoral bone. Bending, torsion, and nonlinear behaviors of compliant mechanisms have not been sufficiently studied. In order to design adaptive cellular structures, a new unit cell, the unit truss is proposed. The unit truss approach facilitates the design of adaptive cellular structures for enhanced mechanical properties via geometric modeling, finite element analysis, shape optimization, and additive fabrication. Four research questions, which address representation, structural analysis, design synthesis, and manufacturing respectively, are raised and answered. Unit truss enables representation and mechanics analysis for adaptive cellular structures. A synthesis method using engineering optimization algorithms is developed to systematically design adaptive cellular structure. Two examples, graded cellular structure for prosthesis and compliant mechanism for morphing wings, are studied to test the unit truss approach.

Indiana University-Purdue University Indianapolis (IUPUI) / Mechanical force environment is a major factor that influences cellular homeostasis and remodeling. The prevailing wisdom in this field demonstrated that a threshold of mechanical forces or deformation was required to affect cell signaling. However, by using a fluorescence resonance energy transfer (FRET)-based approach, we found that C28/I2 chondrocytes exhibited an increase in RhoA activities in response to high shear stress (10 or 20 dyn/cm2), while they showed a decrease in their RhoA activities to intermediate shear stress at 5 dyn/cm2. No changes were observed under low shear stress (2 dyn/ cm2). The observed two-level switch of RhoA activities was closely linked to the shear stress-induced alterations in actin cytoskeleton and traction forces. In the presence of constitutively active RhoA (RhoA-V14), intermediate shear stress suppressed RhoA activities, while high shear stress failed to activate them. Collectively, these results herein suggest that intensities of shear stress are critical in differential activation and inhibition of RhoA activities in chondrocytes.