Viscoelastic polyacrylamide MR elastography phantoms with tunable damping ratio independent of shear stiffness.

Physiologically modeled test samples with known properties and characteristics, or phantoms, are essential for developing sensitive, repeatable, and accurate quantitative MRI techniques. Magnetic resonance elastography (MRE) is one such technique used to estimate tissue mechanical properties, and it is advantageous to use phantoms with independently tunable mechanical properties to benchmark the accuracy of MRE methods. Phantoms with tunable shear stiffness are commonly used for MRE, but tuning the viscosity or damping ratio has proven to be difficult. A promising candidate for MRE phantoms with tunable damping ratio is polyacrylamide (PAA). While pure PAA has very low attenuation, viscoelastic hydrogels have been made by entrapping linear polyacrylamide strands (LPAA) within the PAA network. In this study, we evaluate the use of LPAA/PAA gels as physiologically accurate phantoms with tunable damping ratio, independent of shear stiffness, via MRE. Phantoms were made with 15.3 wt% PAA while the LPAA concentration ranged from 4.5 wt% to 8.0 wt%. MRE was performed at 9.4 T with 400 Hz vibration on all phantoms revealing a strong, positive correlation between damping ratio and LPAA content (p < 0.001). There was no significant correlation between shear stiffness and LPAA content, confirming a constant PAA concentration yielded constant shear stiffness. Rheometry at 10 Hz was performed to verify the damping ratio of the phantoms. Nearly identical slopes for damping ratio versus LPAA content were found from both MRE and rheometry (0.0073 and 0.0075 respectively). Ultimately, this study validates the adaptation of polyacrylamide gels into physiologically-relevant MRE phantoms to enable testing of MRE estimates of damping ratio.

Harvesting Silk Fibers from Plodia interpunctella: Role of Environmental Rearing Conditions in Fiber Production and Properties.

Silk fibers are produced by a wide variety of insects. The silkworm Bombyx mori (Bombyx) was domesticated because the physical properties of its silk fibers were amenable to the production of fine textiles. Subsequently, engineers have regenerated silk fibroin to form biomaterials. The monocular focus on Bombyx silk has underutilized the expanse of diverse silk proteins produced by more than 100,000 other arthropods. This vast array of silk fibers could be utilized for biomedical engineering challenges if sufficient rearing and purification processes are developed. Herein, we show that the moth, Plodia interpunctella (Plodia), represents an alternative silk source that is easily reared in highly regulated culture environments allowing for greater consistency in the silk produced. We controlled the temperature, resource availability (larvae/gram diet), and population density (larvae/mL) with the goal of increasing silk fiber production and improving homogeneity in Plodia silk proteins. We determined that higher temperatures accelerated insect growth and reduced life cycle length. Furthermore, we established initial protocols for the production of Plodia silk with optimal silk production occurring at 24 °C, with a resource availability of 10 larvae/gram and a population density of 0.72 larvae/mL. Population density was shown to be the most prominent driving force of Plodia silk mat formation among the three parameters assessed. Future work will need to link gene expression, protein production and purification, and resulting mechanical properties as a function of environmental cues to further transition Plodia silk into regenerated silk fibroin biomaterials.

Biomimetic Substrate to Probe Dynamic Interplay of Topography and Stiffness on Cardiac Fibroblast Activation.

Materials with the ability to change properties can expand the capabilities of in vitro models of biological processes and diseases as it has become increasingly clear that static, stiff materials with smooth surfaces fall short in recapitulating the in vivo cellular microenvironment. Here, we introduce a patterned material that can be rapidly stiffened and softened in situ in response to an external magnetic field through the addition of magnetic inclusions into a soft silicone elastomer with topographic surface patterning. This substrate can be used for cell culture to investigate short-term cellular responses to dynamic stiffening or softening and the interaction with topography that encourages cells to assume a specific morphology. We investigated short-term cellular responses to dynamic stiffening or softening in human ventricular cardiac fibroblasts. Our results indicate that the combination of dynamic changes in stiffness with and without topographic cues induces different effects on the alignment and activation or deactivation of myofibroblasts. Cells cultured on patterned substrates exhibited a more aligned morphology than cells cultured on flat material; moreover, cell alignment was not dependent on substrate stiffness. On a patterned substrate, there was no significant change in the number of activated myofibroblasts when the material was temporally stiffened, but temporal softening caused a significant decrease in myofibroblast activation (50% to 38%), indicating a competing interaction of these characteristics on cell behavior. This material provides a unique in vitro platform to observe the time-dependent dynamics of cells by better mimicking more complex behaviors and realistic microenvironments for investigating biological processes, such as the development of fibrosis.