FULL PAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 802 wileyonlinelibrary.com A Hydrogel Bridge Incorporating Immobilized Growth Factors and Neural Stem/Progenitor Cells to Treat Spinal Cord Injury Hang Li, Trevor R. Ham, Nicholas Neill, Mahmoud Farrag, Ashley E. Mohrman, Andrew M. Koenig, and Nic D. Leipzig* DOI: 10.1002/adhm.201500810 Spinal cord injury (SCI) causes permanent, often complete disruption of central nervous system (CNS) function below the damaged region, leaving patients without the ability to regenerate lost tissue. To engineer new CNS tissue, a unique spinal cord bridge is created to deliver stem cells and guide their organization and development with site-specifically immobilized growth factors. In this study, this bridge is tested, consisting of adult neural stem/progenitor cells contained within a methacrylamide chitosan (MAC) hydrogel and protected by a chitosan conduit. Interferon-γ (IFN-γ ) and platelet-derived growth factor-AA (PDGF-AA) are recombinantly produced and tagged with an N-terminal biotin. They are immobilized to streptavidin- functionalized MAC to induce either neuronal or oligodendrocytic line- ages, respectively. These bridges are tested in a rat hemisection model of SCI between T8 and T9. After eight weeks treatments including chitosan conduits result in a significant reduction in lesion area and macrophage infiltration around the lesion site ( p < 0.0001). Importantly, neither immo- bilized IFN-γ nor PDGF-AA increased macrophage infiltration. Retrograde tracing demonstrates improved neuronal regeneration through the use of immobilized growth factors. Immunohistochemistry staining demonstrates that immobilized growth factors are effective in differentiating encapsulated cells into their anticipated lineages within the hydrogel, while qualitatively reducing glial fibrillary acid protein expression. Dr. H. Li, N. Neill, A. E. Mohrman, A. M. Koenig, Prof. N. D. Leipzig Department of Chemical and Biomolecular Engineering The University of Akron Whitby Hall 211, Akron, OH 44325, USA E-mail: nl21@uakron.edu T. R. Ham, Prof. N. D. Leipzig Department of Biomedical Engineering The University of Akron Auburn Science and Engineering Center 275 West Tower, Akron, OH 44325, USA Dr. M. Farrag Department of Biology The University of Akron Auburn Science and Engineering Center D401 Akron, OH 44325, USA 1. Introduction Despite serving as the main control and sensory system of the body, the adult cen- tral nervous system (CNS) does not func- tionally regenerate following injury. Nearly half of all traumatic spinal cord injuries (SCIs) result in complete and permanent loss of function below the injury, with significant effects on a patient's quality of life and life expectancy. [1,2] In traumatic SCI, typically secondary damage and the chronic inflammatory response greatly intensifies injury severity and thus func- tional losses. [3] The most concerning part of SCI is the disconnection of axonal tracks, via either direct or secondary damage, which severely restrict the information exchange between brain and spinal cord. [4] During the secondary SCI cascade, a glial scar develops around the injury, which chemically inhibits axon regeneration while forming a physical barrier, pre- venting axons from crossing the gap. [4–6] Numerous strategies have been attempted with varying results to improve sparing and regeneration, including cell transfusions, stimulating and guiding axonal growth, or blocking inhibitory molecules. [7] There is evidence that spinal cord neurons retain their ability to regenerate following SCI, [8] suggesting that these neurons could be coaxed and guided into full-scale regeneration through the strategic administration of growth factors and a sufficient population of stem cells. Simply delivering undifferentiated NSPCs has been shown to be insuf- ficient in clinical trials; [9] their behavior (e.g., lineage commit- ment) must be guided towards encouraging neurogenesis. In the case of trans/hemisection or cut injuries it has been shown that a nerve guidance conduit is beneficial to reconnect the sev- ered cord, however, function is difficult to restore since we do not yet know how to achieve proper reconnection of the post- injury axonal connections. [10] It has become apparent that, due to the complexity of SCI pathophysiology, an effective SCI treat- ment strategy must combine multiple factors that address each challenge in a synergistic way (e.g., stem cells, a neuroinductive material, and growth factors). Adv. Healthcare Mater. 2016, 5, 802–812 www.advhealthmat.de www.MaterialsViews.com