3D Hyaluronic Acid Hydrogels for Modeling Oligodendrocyte Progenitor Cell Behavior as a Function of Matrix Stiffness.

The lack of regenerative solutions for demyelination within the central nervous system motivates the development of strategies to expand and drive the bioactivity of the cells, including oligodendrocyte progenitor cells (OPCs), that ultimately give rise to myelination. In this work, we introduce a 3D hyaluronic acid (HA) hydrogel system to study the effects of microenvironmental mechanical properties on the behavior of OPCs. We tuned the stiffness of the hydrogels to match the brain tissue (storage modulus 200-2000 Pa) and studied the effects of stiffness on metabolic activity, proliferation, and cell morphology of OPCs over a 7 day period. Although hydrogel mesh size decreased with increasing stiffness, all hydrogel groups facilitated OPC proliferation and mitochondrial metabolic activity to similar degrees. However, OPCs in the two lower stiffness hydrogel groups (170 ± 42 and 794 ± 203 Pa) supported greater adenosine triphosphate levels per cell than the highest stiffness hydrogels (2179 ± 127 Pa). Lower stiffness hydrogels also supported higher levels of cell viability and larger cell spheroid formation compared to the highest stiffness hydrogels. Together, these data suggest that 3D HA hydrogels are a useful platform for studying OPC behavior and that OPC growth/metabolic health may be favored in lower stiffness microenvironments mimicking brain tissue mechanics.

Engineering biomaterial microenvironments to promote myelination in the central nervous system.

Promoting remyelination and/or minimizing demyelination are key therapeutic strategies under investigation for diseases and injuries like multiple sclerosis (MS), spinal cord injury, stroke, and virus-induced encephalopathy. Myelination is essential for efficacious neuronal signaling. This myelination process is originated by oligodendrocyte progenitor cells (OPCs) in the central nervous system (CNS). Resident OPCs are capable of both proliferation and differentiation, and also migration to demyelinated injury sites. OPCs can then engage with these unmyelinated or demyelinated axons and differentiate into myelin-forming oligodendrocytes (OLs). However this process is frequently incomplete and often does not occur at all. Biomaterial strategies can now be used to guide OPC and OL development with the goal of regenerating healthy myelin sheaths in formerly damaged CNS tissue. Growth and neurotrophic factors delivered from such materials can promote proliferation of OPCs or differentiation into OLs. While cell transplantation techniques have been used to replace damaged cells in wound sites, they have also resulted in poor transplant cell viability, uncontrollable differentiation, and poor integration into the host. Biomaterial scaffolds made from extracellular matrix (ECM) mimics that are naturally or synthetically derived can improve transplanted cell survival, support both transplanted and endogenous cell populations, and direct their fate. In particular, stiffness and degradability of these scaffolds are two parameters that can influence the fate of OPCs and OLs. The future outlook for biomaterials research includes 3D in vitro models of myelination / remyelination / demyelination to better mimic and study these processes. These models should provide simple relationships of myelination to microenvironmental biophysical and biochemical properties to inform improved therapeutic approaches.

Self-assembly of doublets from flattened polymer colloids.

Bottom-up fabrication methods are used to assemble strong yet flexible colloidal doublets. Part of a spherical particle is flattened, increasing the effective interaction area with another particle having a flat region. In the presence of a moderate ionic strength, the flat region on one particle will preferentially "bond" to a flat region on another particle in a deep (≥10 kT) secondary energy minimum. No external field is applied during the assembly process. Under the right conditions, the flat-flat bonding strength is ≥10× that of a sphere-sphere interaction. Not only can flat-flat bonds be quite strong, but they are expected to remain freely rotatable and flexible, with negligible energy barriers for rotation because particles reside in a deep secondary energy minimum with a ~20-30 nm layer of fluid between the ~1 µm radius particles. We present a controlled technique to flatten the particles at room temperature, the modeling of the interparticle forces for flattened spheres, and the experimental data for the self-assembly of flat-flat doublets.