Characterizing the Evolution of Gel Scaffolds Using Passive Microrheology

Kelly Schultz from Lehigh University

The evolution of gel scaffolds has implications in potential applications. Due to gel versatility,
these materials are used in applications that range from home care products to synthetic
materials that enhance wound healing. To meet the need of this broad range of applications, we
must first understand the change in material properties and scaffold structure during gelation and
degradation. We will discuss the characterization of two gel systems: a hydrogenated castor oil
(HCO) colloidal gel and an enzymatically degradable human mesenchymal stem cell (hMSC)-
laden poly(ethylene glycol) (PEG)-peptide scaffold. In this work, we use multiple particle tracking
microrheology (MPT) to measure dynamic material properties during phase transitions. MPT
measures the thermal motion of embedded probe particles to measure rheological properties.
HCO is a rheological modifier used in commercial products, including fabric and home care
products. Of concern in the design is whether the gradient that induces phase change can
overcome any processing history, particularly due to shear stress. In this work, we characterize
HCO evolution with MPT, μ2rheology, MPT in a microfluidic device, and bulk rheology. MPT
measures that the scaffold structure varies when a single sample is gelled or degraded. To
determine whether this is due to processing history we develop a new microfluidic device for
μ2rheology measurements. μ2rheology measurements of consecutive phase changes of HCO are
taken starting with both a sheared solution of HCO and an un-sheared HCO gel and measure
materials return to the same equilibrium properties. We conclude that equilibrium structures
depend on the shear history of the starting material, which can have implications in end use
products made with this colloidal gel scaffolds.
hMSCs are critical players in wound healing. During wound healing, hMSCs are called to the
wound by chemical cues in the environment. In response, they migrate out of their niche and
traverse mechanically distinct microenvironments to reach the wound and regulate inflammation.
To enhance wound healing, implantable synthetic hydrogels are designed to mimic in vivo
microenvironments to deliver hMSCs to the surrounding tissue. It is not understood how cells reengineer
their microenvironments and how the microenvironment influences cellular degradation
strategies. Our approach uses MPT to characterize the pericellular region during cellular
remodeling in a synthetic hydrogel scaffold. The material hMSCs are encapsulated in is degraded
by cell-secreted matrix metalloproteinases (MMPs). We measure that hMSCs degrade a gradient
into the material with the largest degradation furthest from the cell and are not motile. The hMSC
then degrades the entire scaffold and move at twice the speed generally reported for hMSC
motility. From these measurements, we hypothesize that hMSCs are secreting tissue inhibitors of
metalloproteinases (TIMPs), which inhibit MMP activity and enzymatic degradation directly
around the cell. Inhibition of TIMPs results in the opposite degradation profile and enhanced
hMSC motility. This work provides a strategy to enhance hMSC motility in an implantable wound
healing scaffold to more effectively deliver these cells to wounds to begin the healing process.