(3bq) Microrheological Characterization Techniques for Biological Applications and Soft Material Design

Synthetic scaffolds are designed to be remodeled and degraded by migrating cells for applications such as tissue engineering, wound healing and stem cell culture. Although there are distinct advantages of these materials, little is known about the interactions cells have with these synthetic microenvironments, especially in the region immediately surrounding the cell, the pericellular region. Development of characterization methods that will measure the rheological properties of the hydrogel material, both with and without the presence of encapsulated cells will enable further advances within the field. Specifically, microrheological characterization techniques of synthetic hydrogel scaffolds have been developed to quantify the rheological properties of equilibrated scaffold materials and through transient conditions, such as those occurring during gelation and degradation.

My doctoral research focused on developing high-throughput characterization techniques using multiple particle tracking microrheology (MPT) that would rapidly measure material rheological properties while conserving both material and time. MPT is a passive microrheological technique that measures the Brownian motion of embedded probe particles. The trajectories of these particles are directly related to rheological properties, such as viscosity and creep compliance, using the Generalized Stokes-Einstein Relation (GSER). This technique requires small sample volumes (4-40 μL) while enabling rheological measurements of both the hydrogelation reaction and the final material properties. During the hydrogelation reaction, the material is monitored with time and the sol-gel transition is measured, yielding the critical gelation time and critical relaxation exponents. High-throughput microrheological measurements in a microfluidic device, μ²rheology, are then used to screen the equilibrated materials, significantly reducing the time of sample preparation and increasing the amount of samples evaluated. From the data, gelation state diagrams identify compositions where hydrogels form. Overall, our methods can be used to thoroughly screen the material rheological properties, yielding a high information density, but requiring only small volumetric amounts.

Building on my expertise in microrheological characterization, my postdoctoral research focuses on the characterization of hydrogelators during enzymatic and cellular mediated degradation. Our approach has been to measure the enzymatic degradation of a synthetic scaffold as the amount of matrix metalloproteinases (MMP) degradable peptide cross-links (KCGPQG*IWGQCK) is varied. The ability to tailor the amount of degradation in the material enables fundamental studies of the material properties with degradable cross-links close to the critical fraction of cross-links required to form a gel, p_c,A, calculated using Flory-Stockmayer theory. The material degrades at slightly higher concentrations of degradable cross-links than predicted with Flory-Stockmayer theory, due to nonidealities present in the matrix and not accounted for in the theory. With these investigations we are able to identify the gel-sol transition and spatially characterize material properties during degradation. These materials and results can then be used to study the pericellular region of encapsulated cells during remodeling and degradation of the material. These experiments will enable the identification of the rheological properties and spatial variations that are required to enhance cell motility.