Rebecca B. Dupaix

Inherent Interfacial Mechanical Gradients in 3D Hydrogels Influence Tumor Cell Behaviors

Cells sense and respond to the rigidity of their microenvironment by altering their morphology and migration behavior. To examine this response, hydrogels with a range of moduli or mechanical gradients have been developed. Here, we show that edge effects inherent in hydrogels supported on rigid substrates also influence cell behavior. A Matrigel hydrogel was supported on a rigid glass substrate, an interface which computational techniques revealed to yield relative stiffening close to the rigid substrate support. To explore the influence of these gradients in 3D, hydrogels of varying Matrigel content were synthesized and the morphology, spreading, actin organization, and migration of glioblastoma multiforme (GBM) tumor cells were examined at the lowest (<50 µm) and highest (>500 µm) gel positions. GBMs adopted bipolar morphologies, displayed actin stress fiber formation, and evidenced fast, mesenchymal migration close to the substrate, whereas away from the interface, they adopted more rounded or ellipsoid morphologies, displayed poor actin architecture, and evidenced slow migration with some amoeboid characteristics. Mechanical gradients produced via edge effects could be observed with other hydrogels and substrates and permit observation of responses to multiple mechanical environments in a single hydrogel. Thus, hydrogel-support edge effects could be used to explore mechanosensitivity in a single 3D hydrogel system and should be considered in 3D hydrogel cell culture systems.

Large Strain Mechanical Behavior of Poly(methyl methacrylate) (PMMA) Near the Glass Transition Temperature

The mechanical behavior of amorphous thermoplastics, such as poly(methyl methacrylate) (PMMA), strongly depends on temperature and strain rate. Understanding these dependencies is critical/or many polymer processing applications and, in particular, for those occurring near the glass transition temperature, such as hot embossing. In this study, the large strain mechanical behavior of PMMA is investigated using uniaxial compression tests at varying temperatures and strain rates. In this study we capture the temperature and rate of deformation dependence of PMMA, and results correlate well to previous experimental work found in the literature for similar temperatures and strain rates. A three-dimensional constitutive model previously used to describe the mechanical behavior of another amorphous polymer, poly(ethylene terephthalate)-glycol (PETG), is applied to model the observed behavior of PMMA. A comparison with the experimental results reveals that the model is able to successfully capture the observed stress-strain behavior of PMMA, including the initial elastic modulus, flow stress, initial strain hardening, and final dramatic strain hardening behavior in uniaxial compression near the glass transition temperature.

Constitutive Modeling of Rate-Dependent Stress–Strain Behavior of Human Liver in Blunt Impact Loading

An understanding of the mechanical deformation behavior of the liver under high strain rate loading conditions could aid in the development of vehicle safety measures to reduce the occurrence of blunt liver injury. The purpose of this study was to develop a constitutive model of the stress–strain behavior of the human liver in blunt impact loading. Experimental stress and strain data was obtained from impact tests of 12 unembalmed human livers using a drop tower technique. A constitutive model previously developed for finite strain behavior of amorphous polymers was adapted to model the observed liver behavior. The elements of the model include a nonlinear spring in parallel with a linear spring and nonlinear dashpot. The model captures three features of liver stress–strain behavior in impact loading: (1) relatively stiff initial modulus, (2) rate-dependent yield or rollover to viscous “flow” behavior, and (3) strain hardening at large strains. Six material properties were used to define the constitutive model. This study represents a novel application of polymer mechanics concepts to understand the rate-dependent large strain behavior of human liver tissue under high strain rate loading. Applications of this research include finite element simulations of injury-producing liver or abdominal impact events.

Exploring the mechanical behavior of degrading swine neural tissue at low strain rates via the fractional Zener constitutive model.

The ability of the fractional Zener constitutive model to predict the behavior of postmortem swine brain tissue was examined in this work. Understanding tissue behavior attributed to degradation is invaluable in many fields such as the forensic sciences or cases where only cadaveric tissue is available. To understand how material properties change with postmortem age, the fractional Zener model was considered as it includes parameters to describe brain stiffness and also the parameter α, which quantifies the viscoelasticity of a material. The relationship between the viscoelasticity described by α and tissue degradation was examined by fitting the model to data collected in a previous study (Bentil, 2013). This previous study subjected swine neural tissue to in vitro unconfined compression tests using four postmortem age groups (<6h, 24h, 3 days, and 1 week). All samples were compressed to a strain level of 10% using two compressive rates: 1mm/min and 5mm/min. Statistical analysis was used as a tool to study the influence of the fractional Zener constants on factors such as tissue degradation and compressive rate. Application of the fractional Zener constitutive model to the experimental data showed that swine neural tissue becomes less stiff with increased postmortem age. The fractional Zener model was also able to capture the nonlinear viscoelastic features of the brain tissue at low strain rates. The results showed that the parameter α was better correlated with compressive rate than with postmortem age.

Stress-Relaxation Behavior of Poly(Methyl Methacrylate) (PMMA) Across the Glass Transition Temperature

Characterizing Poly(methyl methacrylate) (PMMA) across its glass transition temperature is essential for modeling warm deformation processes such as hot embossing. Its mechanical properties vary significantly across the glass transition as well as with strain rate. Several previous models have attempted to capture this behavior utilizing uniaxial compression experimental data with limited success. In this work, compression experiments including stress relaxation at large strains are conducted to aid researchers in developing better models. Multiple temperatures, final strains, and strain rates are examined to characterize the material across values found in typical hot-embossing processes. It was found that the amount of stress relaxed is highly dependent on the temperature and strain at which it is held. With this data, a model can be developed that will accurately capture stress relaxation with the final goal of being able to simulate hot embossing processes.

Characterizing the Temperature Dependent Spring-Back Behavior of Poly(Methyl Methacrylate) (PMMA) for Hot Embossing

Characterization of the temperature dependent spring-back behavior of poly(methyl methacrylate) (PMMA) is essential to model hot embossing. The constitutive model must capture several deformation modes including uniaxial compression and stress relaxation with cooling in order to predict spring-back. In this work, the thermo-mechanical coupling of spring-back is investigated through finite element simulations utilizing a constitutive model that captures stress relaxation. It was found that the material model successfully predicts spring-back trends under a variety of heat transfer conditions. At the cooling times used experimentally, spring-back decreased with held strain and increased with embossing temperature. Initial simulations utilizing the experimentally obtained platen temperature under predicted spring-back. After performing a simple heat transfer simulation, spring-back predictions were improved by altering the temperature profile according to the heat transfer simulation and matched experimentally obtained values. To investigate the effect of a thermal gradient, a fully coupled thermo-mechanical simulation was performed. From this, it was found that the thermal gradient had a minimal effect on spring-back. Rather, the rapid cooling upon release of PMMA was found to cease spring-back and can be modeled without a fully coupled simulation. These results indicate that temperature, along with strain level, and cooling time are important to the process of spring-back.

Mechanical Behavior of a Series of Copolyester Blends near the Glass Transition: Monotonic and Load-Hold Behavior in Compression

Monotonic loading tests were conducted on five commercial blends of poly(ethylene terephthalate) (PET) and poly(1,4-cyclohexylenedimethylene terephthalate) (PCT) at temperatures of 90°C and 100°C and strain rates of 0.1/s, 0.05/s, and 0.005/s in uniaxial and plane strain compression. On comparing the mechanical behavior of the five materials, it was found that the behavior of the low-PCT content materials was different from the high-PCT content materials only at conditions that favored strain-induced crystallization, particularly in plane strain compression. Load-hold tests were also conducted on three of the blends with similar results to the monotonic tests. Material differences were only pronounced at certain conditions, and in these cases the low-PCT content materials showed increased strain hardening after the hold period while the high-PCT content material did not. Therefore, it was found that the addition of a hold period was not exclusively required to observe differences in the crystallizable materials over the noncrystallizing blends. The increased strain hardening likely associated with crystallization in PET was only observed when the following conditions were met: (i) strain rates of 0.1/s and above, (ii) temperatures of 90°C–100°C, (iii) plane strain compression, and (iv) after a certain level of deformation.

Viscoelastic Properties of Macaque Neural Tissue at Low Strain Rates

Increased knowledge of the mechanical properties of soft tissue subjected to low strain rates is beneficial to biomedical applications, such as designing bio-compatible implants, developing minimally invasive surgical techniques and surgical simulation devices for training surgeons. Unconfined compression and indentation experiments were conducted to extract macro- and micro-level mechanical properties of Macaque neural tissue. The tissues were placed in physiological saline solution and tested at room temperature within one hour post-sacrifice and three weeks post sacrifice using unconfined compression and indentation experiments. For each test, the temporal lobe was sectioned into 26 mm diameter disks that were subjected to 1%, 2%, 5%, and 10% strain at a loading rate of 5 mm per minute. The impermeable platen used in the unconfined compression test had a diameter equivalent to the tissue sample. The diameter of the flat tip used in the indentation experiment was 5 mm. Both test configurations utilized a ramp-and-hold strain input to capture the features of the tissue attributed to the stress-strain relationship (ramp) and stress-relaxation (hold). Viscoelastic theory was applied to the experimental data for the calculation of the secant and relaxation modulus, which correspond to the ramp and hold portion of the strain input, respectively. The resulting viscoelastic behavior of the Macaque brain at the macro- and micro-scale were compared with (i) bovine and porcine neural tissue, commonly found in the literature and (ii) viscoelastic models for neural tissue, which in the literature are generally applicable only to large deformations.Copyright

Finite Element Modeling of Reheat Stretch Blow Molding of PET

Poly (ethylene terephthalate) or PET is a polymer used as a packaging material for consumer products such as beverages, food or other liquids, and in other applications including drawn fibers and stretched films. Key features that make it widely used are its transparency, dimensional stability, gas impermeability, impact resistance, and high stiffness and strength in certain preferential directions. These commercially useful properties arise from the fact that PET crystallizes upon deformation above the glass transition temperature. Additionally, this strain‐induced crystallization causes the deformation behavior of PET to be highly sensitive to processing conditions. It is thus crucial for engineers to be able to predict its performance at various process temperatures, strain rates and strain states so as to optimize the manufacturing process. In addressing these issues; a finite element analysis of the reheat blow molding process with PET has been carried out using ABAQUS. The simulation employed a cons...