Walker, Travis

• Ph.D., Chemical Engineering, Stanford University

• M.S., Chemical Engineering, Stanford University

• B.S., Applied and Computational Mathematics, South Dakota School of Mines & Technology

• B.S., Chemical Engineering, South Dakota School of Mines & Technology

Project 1: Engineering Vascular Microphysiological Systems and Quantifying the Endothelial Cell Response
The field of biomedical engineering has seen an incredible surge over the last ten years to develop microphysiological systems, which are generally small microfluidic devices containing soft hydrogel materials on or within which cells are cultured. These small systems are engineered to be biomimetic in vitro models of specific tissues and their physiological systems. In vitro modeling of physiological systems provides the distinct capacity to expose specific human cell lines, cultured under physiological conditions, to a specific stimulus and to analyze the associated response. Data that is acquired from in vitro microphysiological systems has a significant benefit when compared to in vivo evaluations because the in vitro system allows us to identify specific cause and effect phenomena where the variable of multisystem crosstalk, unavoidable in vivo, is eliminated. This project encompasses fundamentals in computer aided drafting (CAD), 3D digital light processing (DLP) printing, and polydimethylsiloxane (PDMS) molding to engineer specialized microfluidic devices. Then, the devices will be used to construct a microphysiological environment that emulates the in vivo vasculature. After sustaining healthy mammalian cells in the device, a series of experiments will be conducted, including local versus bulk sampling, marker quantification, angiogenesis induction, and microscopic imaging.

Project 2: Examine the Dynamic Mechanical Properties of Hydrogel Matrix During Cell Migration
During angiogenesis that is commonly associated with inflammation or hypoxia, tissue sites exist where cytokines are being released, which results in new growth of blood vessels and in recruitment of cells. VEGF, a cytokine that is released in these scenarios, is associated with stimulating endothelial cell sprouting and migration in a chemotactic manner to generate growth of blood vessels to a tissue site. A concentration dependence of VEGF has been shown to correlate to the chemotactic responsiveness of endothelial cells, indicating a minimum threshold requirement to stimulate sprouting angiogenesis, which is related to filopodial development in endothelial cells. However, to navigate the ECM in response to cytokines, endothelial cells secrete matrix metalloproteinases (MMPs) which exhibit proteolytic activity that is targeted at ECM constituents like collagen. Degradation of the ECM is key to the progression of angiogenesis, and blocking MMP secretion has been shown to block the process. During angiogenesis, tip cells secrete MMPs to break down the ECM and to allow navigation of the surrounding tissue. MMP secretion by tip cells is distinct from the trailing stalk endothelial cells. We will use our parallel-channel microphysiological model to induce directed angiogenesis through a collagen hydrogel while probing the ECM environment during migration to evaluate the micro-scale changes in gel stiffness. Numerous studies have surfaced that investigate stiffness gradients and subsequent cell response; however, cell-mediated matrix stiffening during a critical process such as angiogenesis is under-represented in the literature. By employing microrheological techniques such as multi-particle tracking, we can investigate transient changes in gel stiffness during cell migration. This project encompasses fundamentals in cell biology, microscopy, and rheology.

Project 3: Investigation of Shear and Extensional Rheology of Silk Fibroin in Applications of Tissue Engineering
Many tissues of the body rely on innate elastic properties to convey the proper function of the tissue. Loss of elasticity, especially in blood vessels, often leads to the onset of pathology. The tissue composition of blood vessels contain a high percentage of a protein that is called elastin. Elastin is largely responsible for the expansion and recoil of the vessel wall throughout the cardiac cycle. Elastin is notoriously challenging to process ex vivo while still retaining the same level of mechanical properties. Silk fibroin is an elastic protein, similar to elastin, that has impressive mechanical properties and biocompatibility, making it an attractive material in the development of tissue engineered blood vessels. Silk fibroin can be easily isolated from Bombyx mori silkworm cocoons in a relatively well-defined process. Its versatility allows the material to be used in a wide array of applications such as electrospinning, centrifugal spinning, and dip coating. However, rheological evaluation of silk fibroin at different concentrations and under different methods of processing that vary upon the application has been under-explored. The purpose of this work is to correlate the material characteristics prior to its respective application and relate the rheological data to its either successful or unsuccessful application. Successfully defining processing-structure-property-performance relationships through strategic characterization will enable leveraging to regenerated silk fibroin for applications in tissue engineering.

Project 4: Rhealogical Properties that Determine the Processability of Sodium Alginate and the Corresponding Mechanical Characterization
Alginate is a naturally derived polysaccharide that is extracted from algae, which is commonly used for biomaterial applications such as hydrogels, sponges, and microcapsules. Rheological characteristics of the solutions of sodium alginate can determine how successful a particular method of processing will be. To better provide a description of the molecular level, we utilize a suite of measurements to identify the critical characteristics of each alginate from different sources. Measurements via small-amplitude oscillatory shear provide relevant rheological characteristics, such as viscosity, viscous modulus, and elastic modulus. Processing of sodium alginate to form a tube is completed by dip-coating or co-extrusion. Dip-coating is completed by dipping a mandrel into a solution of sodium alginate and then crosslinking the alginate by dipping the coated mandrel into a solution of calcium chloride. Co-extrusion proceeds via a novel device that utilizes co-flow of solutions of calcium chloride to extrude thin-walled polymer tubes. All tubes are collected for mechanical characterization, including measurements of compliance and burst pressure. We found that molecular variability exists among different sources of alginate, resulting in significant variations in rheology and processability of the polymer solutions. Thus, we aim to leverage process-structure-property-performance relationships for alginate through advanced characterization.