Eric M. Burgeson

Parallel superposition flows have been examined in the rheology literature since 1965; they consist of simultaneously applied steady shear and oscillatory flows. One interesting result found from the study of parallel superposition flows has been reportedly negative values of the dynamic moduli. This apparently unphysical result has led to some speculation regarding its origin. In this work, we provide mathematical and experimental evidence showing that negative moduli are potentially due to the oscillatory signal being distorted by inclusion of the steady shear component in the Fourier analysis. However, the moduli can be easily corrected to restore physical meaning.

In soft materials, a clear relationship between material properties and human sensory perception has long been desired for design of consumer products, but the link has remained evasive. Favorable perception indicates that customers enjoy a product and are likely to continue using it or purchase it again. Perception is frequently measured subjectively by consumer test panels in terms of descriptive sensory words such as softness, smoothness, thickness, etc. that lack established scientific definitions. In this work, we move beyond ambiguous definitions and detail a method to objectively measure and quantify human-material interactions using a representative series of viscoelastic putties. We show that human behaviors have direct rheological meaning with features that are characterized using transient recovery rheology. The rheology scales logarithmically at perception-relevant timescales, akin to Fechner's law. Our work explains variability in user-reported perception and demonstrates a way to construct direct relationships between user behavior and measurable rheology.

The topic of thixotropy has historically received much attention due to its importance in a wide range of complex fluids and their applications. However, a thorough understanding of the phenomenon and how to model it remain outstanding challenges. In this work, we examine two materials that exhibit phenomenology often referred to as thixotropic through the lens of stress-controlled recovery rheology. When subjected to an oscillatory shear stress, the materials, an aqueous surfactant system that structurally forms multilamellar vesicles as well as a frequently studied fumed silica suspension, show a transient increase in the resulting strain amplitude. We use both creep and oscillatory tests in conjunction with recovery rheology to measure the elastic and viscous contributions to flow and deformation and find that the elastic contributions remain constant, even at larger amplitudes where nonlinear responses are induced. We conclude that the observed behavior is, therefore, strictly a viscous phenomenon, in contrast with common modeling efforts that describe both the viscous and elastic behaviors as being transient. We additionally examine how typical use of the dynamic moduli can give a misleading description of the material’s behavior, whereas examination of the energetic contributions provides a description consistent with the recovery measurements.

Decellularized extracellular matrices (dECM) have strong regenerative potential as tissue engineering scaffolds; however, current clinical options for dECM scaffolds are limited to freeze-drying its native form into sheets. Electrospinning is a versatile scaffold fabrication technique that allows control of macro- and microarchitecture. It remains challenging to electrospin dECM, which has led researchers to either blend it with synthetic materials or use enzymatic digestion to fully solubilize the dECM. Both strategies reduce the innate bioactivity of dECM and limit its regenerative potential. Herein, we developed a new suspension electrospinning method to fabricate a pure dECM fibrous mesh that retains its innate bioactivity. Systematic investigation of suspension parameters was used to identify critical rheological properties required to instill "spinnability," including homogenization, concentration, and particle size. Homogenization enhanced particle interaction to impart the requisite elastic behavior to withstand electrostatic drawing without breaking. A direct correlation between concentration and viscosity was observed that altered fiber morphology; whereas, particle size had minimal impact on suspension properties and fiber morphology. The versatility of this new method was demonstrated by electrospinning dECM with three common decellularization techniques (Abraham, Badylak, Luo) and tissue sources (intestinal submucosa, heart, skin). Bioactivity retention after electrospinning was confirmed using cell proliferation, angiogenesis, and macrophage polarization assays. Collectively, these findings provide a framework for researchers to electrospin dECM for diverse tissue engineering applications.

Decellularized extracellular matrices (dECM) have strong regenerative potential as tissue engineering scaffolds; however, current clinical options for dECM are limited to freeze-drying its native form into sheets. Electrospinning is a versatile scaffold fabrication technique that allows control of macro- and microarchitecture. It remains challenging to electrospin dECM; which has led researchers to either blend it with synthetic materials or use enzymatic digestion to fully solubilize the dECM. Both strategies reduce the innate bioactivity of dECM and limit its regenerative potential. Herein, we developed a new suspension electrospinning method to fabricate a pure dECM scaffold that retains its innate bioactivity. Systematic investigation of suspension parameters was used to identify critical rheological properties required to instill “spinnability,” including homogenization, concentration, and particle size. Homogenization enhanced particle interaction to impart the requisite elastic behavior to withstand electrostatic drawing without breaking. A direct correlation between concentration and viscosity was observed that altered fiber morphology; whereas, particle size had minimal impact on suspension properties and fiber morphology. The versatility of this new method was demonstrated by electrospinning dECM with three common decellularization techniques (Abraham, Badylak, Luo) and tissue origins (intestinal submucosa, heart, skin). Bioactivity retention after electrospinning was confirmed using cell proliferation, angiogenesis, and macrophage assays. Collectively, these findings provide a framework for researchers to electrospin dECM for diverse tissue engineering applications. Abstract Figure: