Acknowledgments We are indebted to the Citomics unit

Acknowledgments We are indebted to the Citomics unit of the IDIBAPS for technical help. This work was supported by grants from the Spanish Ministry of Economia y CompetitividadBIO2011-30299-C02-02 and from CIDEM-Generalitat de CatalunyaVALTEC09-2-0047. and was developed at the Centro Esther Koplowitz, Barcelona, Spain. CIBER de Enfermedades Raras is an initiative of the ISCIII. CF group is partially financed by the Instituto de Salud Carlos III (IIS10/00014) and co-financed by Fondo Europeo de Desarrollo Regional (FEDER) and receives partial support from the Generalitat de CatalunyaSGR091527. AM is recipient of a FPU fellowship from the Spanish Ministry of Education.
Introduction The high water content and highly customizable nature of hydrogels make these materials well suited for tissue engineering, especially in the brain and spinal cord. Unfortunately, too many studies use hydrogels without reporting the tunable properties. As a consequence, they may not recognize how these properties can alter their final results. Hydrogel chemistry, including the molecular weight (mw) of the components, the type of polymerizing modifications, and the total polymer content, all contribute to the customization of hydrogel behaviors (e.g. the physical properties, such as polymerization and degradation, and the mechanical properties). These behaviors will determine the success of a hydrogel used for tissue engineering. For example, two studies treating spinal cord injury used chemically similar hydrogels but reported substantially different results: Park et al. (2010) found significant repair after spinal cord injury using a hyaluronic d-cycloserine (HA; mw 170kDa) hydrogel with an identified shear storage modulus (G′) of 0.3kPa, but an undefined final weight percent (wt.%) of HA and an unspecific HA modification to allow for polymerization. In contrast, Horn et al. (2007) found no repair of the spinal cord using a thiol-modified HA-based hydrogel at a 0.5 or 1.0wt.%, but failed to report the molecular weight of the HA or the mechanical properties of the hydrogel. While the hydrogels used in these two studies are very similar in chemistry, neither study provides enough detail about the tunable properties to be independently replicated. Hydrogel chemistry and subsequent physical and mechanical properties all have unique contributions to successful tissue engineering, specifically with regard to the reaction of the host tissue to the hydrogel and how replacement cells respond to hydrogel encapsulation (Aurand et al., 2012a,b). A comprehensive exploration of hydrogels well suited for neural tissue engineering, composed of commercially available materials with defined tunable properties may help standardize the use of hydrogels for neural tissue repair. Hydrogels comprised of HA and poly(ethylene glycol) (PEG) provide both the natural element (HA) for neural cell interaction and the synthetic element (PEG) for customization and functionalization. HA is the main polymer backbone of the extracellular matrix (ECM) of the brain and is degraded by hyaluronidases produced by both neurons and glia in vivo (Al\'Qteishat et al., 2006; Lindwall et al., 2013). Biologically inert PEG provides additional control over hydrogel physical properties and helps to enhance functionality through prefabrication of more complex polymers, allowing for the attachment of cells or incorporation of growth factors or drugs (Aurand et al., 2012a,b; Burdick et al., 2006; Lampe et al., 2011; Lin and Anseth, 2009; Lin et al., 2009, 2011; Sawhney et al., 1993; Young and Engler, 2011). Our study employs only these two components, without further modifications, to assess baseline biocompatibility and explore how changes in hydrogel polymer ratio and subsequent physical and mechanical properties affect the fate of encapsulated neural progenitor cells (NPC). Many studies have explored the use of neural cells and tissue for their use in treating neurological disorders (Goldman, 2011; Barker et al., 2003; Svendsen et al., 1999). Clinical trials have been undertaken implanting neural stem cells and NPC to treat a number of neurological diseases, including Parkinson\'s, Batten disease, and cerebral palsy (Gupta et al., 2012; Olanow et al., 2003; Trounson et al., 2011; Selden et al., 2013; Luan et al., 2012). Because of the ethical controversies surrounding the destruction of a fetus, research has also begun to explore the potential use of NPC derived from adult brain (Lo and Parham, 2009; Su et al., 2011). Currently published studies often treat NPC derived from fetal or adult brains as the same type of cells. Indeed, Pollard et al. (2006) found NPC from adult and fetal sources express the same neural progenitor markers (e.g., nestin, sox2, blbp and olig2) and respond similarly to basic fibroblast growth factor (bFGF) and epidermal growth factor (EGF). While the major molecular NPC qualities are common to both cell types, few studies have compared the fates of these two cell types in vitro and even fewer studies have addressed the fate of hydrogel-encapsulated NPC using cells from either source.