Development of a 3D in vitro spinal cord injury model to investigate how the mechanical properties of the matrix affect CNS cell behaviour

Injury to the spinal cord can have devastating consequences. Advances in neuroregeneration have placed emphasis on the importance of biomaterials to deliver cellular therapies. However, little is known about how the mechanical properties of the matrix affect cellular behaviour.
The study aims to investigate how the mechanical properties of the matrix affects CNS cell behaviour at rest and following injury. A synthetic-biologic hydrogel was developed, which was capable of achieving a range of different Young’s moduli by altering the bloom of gelatin, the molecular weight of PEGDA or the polymer concentration. Astrocytes when cultured on the surface of the hydrogels had a significantly higher cell metabolism, viability and increased expression of GFAP when cultured on a stiff (700 Pa) matrix compared to a soft (20 Pa) or mid-range (250 Pa) stiffness hydrogel matrix. Microglia exhibited a significantly higher cell metabolism when cultured on a soft matrix opposed to the stiffer or mid-range stiffness matrix. Interestingly mixed glial cells exhibited similar stiffness preference to microglia and showed significantly higher cell metabolism and cell viability on the softer matrix.
In order to investigate how the mechanical properties of the matrix affects CNS cell behaviour during injury, the BOSE Electroforce BioDynamic Impaction system was utilised to simulate SCI within 3D collagen gels and resulted in a 3D in vitro model of SCI, that produced consistent contusion injuries that mimicked in vivo SCI. A significant increase in astrocyte and microglial metabolism was observed between a mild SCI (receiving an impaction displacement of 50%) and a severe SCI (receiving an impaction displacement of 100%) within four hours of injury. At twenty-four hours post impaction of the gel, astrocyte and microglial metabolism was significantly higher at 80% and 100% displacement depth, than all other displacement conditions. This indicated that the greater the impact severity, the higher the cellular reactivity by measurement of cell metabolism, within twenty-four hours of the injury event. Astrocytic expression of GFAP also increased with increasing displacement depth.
In summary a 2D synthetic-biologic hybrid hydrogel system was developed to investigate how the mechanical properties of the matrix affect CNS cell behaviour and a 3D in vitro collagen hydrogel model was used to investigate how CNS cells respond to SCI.