Hydrogels for Acute Spinal Cord Injury: Physical/Chemical Material Characterization and Assessment of Astrocytic Response

Spinal cord injury (SCI) is a physically, emotionally and financially debilitating condition. Hundreds of thousands of people in the United States are currently living with some degree of SCI. Traumatic insult to the spinal column causes large-scale tissue damage, resulting in the disconnection of electrical connectivity between the central and peripheral nervous systems and potentially a substantial loss of motor function. Current clinical treatments for SCI are limited to decompression of the spinal cord as well as acute treatment with the anti-inflammatory steroid methylprednisilone. However, the efficacy of these treatments is under debate and the majority of people who are hospitalized for SCI never regain full function. Thus, there is a significant need for an efficacious SCI treatment capable of promoting regeneration of lost neuronal connections and restoration of functional motor behavior. A number of biomaterial-based, tissue engineering approaches have been developed for different time periods (acute vs. chronic) and types of SCI (contusion vs. transection) including implantable polymer scaffolds with guidance tubes to direct cellular growth and injectable hydrogel materials that act a matrix for cellular growth and a vehicle for drug or cell delivery. Contusive injuries with irregular lesion geometries are the most common type of SCI, making injectable hydrogels an ideal biomaterials treatment for contusive SCI. Due to the formation of a dense, astrocytic glial scar surrounding the lesion site, treatment of chronic SCI is difficult and complete removal of the glial scar often promotes further harm. Furthermore, regenerating neurons tend to follow astrocyte processes into biomaterial scaffolds within the lesion site, possibly acting as a guide for neuronal growth. Promoting positive astrocyte interaction is crucial for any biomaterial treatment of SCI. Even if a material promotes significantly neuronal growth, significant formation of reactive astrocyte or glial scar formation would remove any potential benefit.;My doctoral research was focused on the development of a Ca2+ -responsive hydrogel material capable of interacting with detrimental levels of Ca2+ within the spinal cord lesion and exploring the astrocytic response to these hydrogel materials. Alginate was chosen as the base polymer for our hydrogel material as it forms hydrogels by crosslinking with Ca2+. Ca2+ concentration increases extracellularly following SCI and is implicated in progression of secondary neuronal damage. It was hypothesized that hydrogels composed of alginate may be able to utilize this Ca2+ for in situ gelation while simultaneously inhibiting secondary neuronal death. Results demonstrate that alginate hydrogels respond to physiologically relevant Ca2+ concentrations by initiating further crosslinking and exhibiting stronger hydrogel-like behavior in an in situ gelation model, while exhibiting similar mechanical behavior to native spinal cord tissue. By controlling crosslinking through incorporation of different amounts of chitosan and genipin, degradation rate and positive charge are tunable. Increased astrocyte activation is observed on hydrogels that exhibited the highest amount of astrocyte attachment. However, further analysis demonstrates that increased astrocyte attachment is not a product of increased astrocyte activation or proliferation and is instead a result of the relative amount of different crosslinking patterns within hydrogels. By investigating the mechanisms responsible for positive astrocyte interaction with hydrogels, the composition of hydrogels might be refined to enhance astrocyte behavior on composite alginate/chitosan hydrogels.