No abstract

Electrospun fiber mats (EFMs) are highly versatile biomaterials used in a myriad of biomedical applications. Whereas some facets of EFMs are well studied and can be highly tuned (e.g., pore size, fiber diameter, etc.), other features are under characterized. For example, although substrate mechanics have been explored by several groups, most studies rely on Young’s modulus alone as a characterization variable. The influence of fiber mat thickness and the effect of supports are variables that are often not considered when evaluating cell-mechanical response. To assay the role of these features in EFM scaffold design and to improve understanding of scaffold mechanical properties, we designed EFM scaffolds with varying thickness (50–200 µm) and supporting methodologies. EFM scaffolds were comprised of polycaprolactone and were either electrospun directly onto a support, suspended across an annulus (3 or 10 mm inner diameter), or “tension-released” and then suspended across an annulus. Then, single cell spreading (i.e., Feret diameter) was measured in the presence of these different features. Cells were sensitive to EFM thickness and suspended gap diameter. Overall, cell spreading was greatest for 50 µm thick EFMs suspended over a 3 mm gap, which was the smallest thickness and gap investigated. These results are counterintuitive to conventional understanding in mechanobiology, which suggests that stiffer materials, such as thicker, supported EFMs, should elicit greater cell polarization. Additional experiments with 50 µm thick EFMs on polystyrene and polydimethylsiloxane (PDMS) supports demonstrated that cells can “feel” the support underlying the EFM if it is rigid, similar to previous results in hydrogels. These results also suggest that EFM curvature may play a role in cell response, separate from Young’s modulus, possibly because of internal tension generated. These parameters are not often considered in EFM design and could improve scaffold performance and ultimately patient outcomes.

Polyimide-based azobenzene polymer networks have demonstrated superior photomechanical performance over more conventional azobenzene-doped pendent and cross-linked polyacrylate networks. These materials exhibit larger yield stress and glass transition temperatures and thus provide robustness for active control of adaptive structures directly with polarized, visible light. Whereas photochemical reactions clearly lead to deformation, as indicated by a rotation of a linear polarized light source, temperature and viscoelasticity can also influence deformation and complicate interpretation of the photostrictive and shape memory constitutive behavior. To better understand this behavior we develop a rate-dependent constitutive model and experimentally quantify the material behavior in these materials. The rate dependent deformation induced in these materials is quantified experimentally through photomechanical stress measurements and infrared camera measurements. Bayesian uncertainty analysis is used to assess the role of internal polymer network evolution and azobenzene excitation on both thermomechanical and photomechanical deformation in the presence polarized light of different orientations. A modified Arrhenius relation is proposed and validated using Bayesian statistics which provide connections between free volume, shape memory, and polarized light.

<div class="section abstract"><div class="htmlview paragraph">Understanding cylinder-kit tribology is pivotal to durability, emission management, reduced oil consumption, and efficiency of the internal combustion engine. This work addresses the understanding of the fundamental aspects of oil transport and combustion gas flow in the cylinder kit, using simulation tools and high-performance computing. A dynamic three-dimensional multi-phase, multi-component modeling methodology is demonstrated to study cylinder-kit assembly tribology during the four-stroke cycle of a piston engine. The percentage of oil and gas transported through different regions of the piston ring pack is predicted, and the mechanisms behind this transport are analyzed. The velocity field shows substantial circumferential flow in the piston ring pack, leading to blowback into the combustion chamber during the expansion stroke. Oil initialization and management of a continuous supply of oil throughout the cycle are observed to govern how much oil would be lost to the crankcase and combustion chamber. The calculated blow-by results agree with the results of a quasi-one-dimensional cylinder-kit analysis system of programs known as CASE (Cylinder-kit Analysis System for Engines). Implementing this three-dimensional methodology leads to a better understanding of cylinder-kit fluid flow physics. The findings presented in this work pave the way to further the ongoing development effort of optimum cylinder kit designs with controlled gas leakage, low oil consumption, and low cylinder kit friction.</div></div>