Adjusting the Chemical and Physical Properties of Hydrogels Leads to Improved Stem Cell Survival and Tissue Ingrowth in Spinal Cord Injury Reconstruction: A Comparative Study of Four Methacrylate Hydrogels

Currently, there is no effective strategy for the treatment of spinal cord injury (SCI). A suitable combination of modern hydrogel materials, modified to effectively bridge the lesion cavity, combined with appropriate stem cell therapy seems to be a promising approach to repair spinal cord damage. We demonstrate the synergic effect of porosity and surface modification of hydrogels on mesenchymal stem cell (MSC) adhesiveness in vitro and their in vivo survival in an experimental model of SCI. MSCs were seeded on four different hydrogels: hydroxypropylmethacrylate-RGD prepared by heterophase separation (HPMA-HS-RGD) and three other hydrogels polymerized in the presence of a solid porogen: HPMA-SP, HPMA-SP-RGD, and hydroxy ethyl methacrylate [2-(methacryloyloxy)ethyl] trimethylammonium chloride (HEMA-MOETACl). Their adhesion capability and cell survival were evaluated at 1, 7, and 14 days after the seeding of MSCs on the hydrogel scaffolds. The cell-polymer scaffolds were then implanted into hemisected rat spinal cord, and MSC survival in vivo and the ingrowth of endogenous tissue elements were evaluated 1 month after implantation. In vitro data demonstrated that HEMA-MOETACl and HPMA-SP-RGD hydrogels were superior in the number of cells attached. In vivo, the highest cell survival was found in the HEMA-MOETACl hydrogels; however, only a small ingrowth of blood vessels and axons was observed. Both HPMA-SP and HPMA-SP-RGD hydrogels showed better survival of MSCs compared with the HPMA-HS-RGD hydrogel. The RGD sequence attached to both types of HPMA hydrogels significantly influenced the number of blood vessels inside the implanted hydrogels. Further, the porous structure of HPMA-SP hydrogels promoted a statistically significant greater ingrowth of axons and less connective tissue elements into the implant. Our results demonstrate that the physical and chemical properties of the HPMA-SP-RGD hydrogel show the best combination for bridging a spinal cord lesion, while the HEMA-MOETACl hydrogel serves as the best carrier of MSCs.