DISEÑO DE LA RED CORPORATIVA

A novel star poly(ethylene oxide-co-lactide-glycolide acrylate) (SPELGA) macromonomer was synthesized and characterized with respect to gelation, sol fraction, degradation, and swelling in aqueous solution. We hypothesized that a multi-arm star

amphiphilic poly(ethylene oxide-co-lactide-glycolide) based macromonomer with each arm terminated with an acrylate group would significantly increase the rate of crosslinking, thus reducing the minimum required concentration of initiator/monomer to produce robust networks. Addition of only 0.4 mol% NVP to the polymerization mixture increased modulus by 2.2-fold but modulus did not change appreciably for higher NVP concentrations. The higher modulus can be explained by the dilution effect of polymer chains in the sol, to delay the onset of diffusion-controlled reaction, and by propagation of growing polymer chains with network-bound SPELGA acrylates to facilitate network formation. It is interesting to note that the minimum NVP concentration of 0.4% was at least an order of magnitude less than that used in previous studies for gelation of PLEOF macromonomers. Depending on macromonomer concentration, sol fraction ranged between 8-18%. The higher reactivity and lower steric hindrance of the acrylates in SPELGA compared to fumarates in PLEOF significantly reduced sol fraction and minimum NVP concentration to produce robust networks. SPELGA gels with highest LA/GL content (SPELGA800-L50) had the highest mass loss but lowest water uptake with incubation time which was attributed to the high fraction of degradable units and the autocatalytic effect of acidic degradation products. SPELGA gels with highest LA/GL content (SPELGA800-L75 and SPELGA800-L50) showed delayed swelling characteristics.

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Seyedsina Moeinzadeh, Danial Barati, Xuezhong He, Esmaiel Jabbari, Gelation Characteristics and Osteogenic Differentiation of Stromal Cells in Inert Hydrolytically Degradable Micellar Polyethylene Glycol Hydrogels. Biomacromolecules, 2012. 13(7): p. 2073-2086. Reprinted here with permission of publisher.

4.1. INTRODUCTION

The overall goal of this work was to synthesize inert and non-fouling degradable hydrogels as a matrix for cell encapsulation. Hydrogels are hydrophilic polymeric networks that retain a significant fraction of water in their structure in physiological solution without dissolving. Nutrient molecules and proteins diffusive readily through hydrogels and cells immobilized in hydrogels display higher biological activity.[21-25] Due to these unique properties, hydrogels are very attractive as a matrix for cell encapsulation and delivery to the regeneration site in regenerative medicine.[26-30]

Polyethylene glycol (PEG) hydrogels are used extensively as a matrix for cell encapsulation to elucidate the effect of physiochemical, mechanical, and biological factors in the microenvironment on cell fate in vitro.[36-39] PEG hydrogels, due to their inert, hydrophilic and immunogenic nature, provide enormous flexibility in designing and controlling the cell microenvironment.[40, 41] Unlike small-molecule monomers that cross the cell membrane, the flexible PEG macromonomers crosslink to produce hydrogels with high compressive modulus without adversely affecting the viability of the encapsulated cells.[42-46]

An exciting approach to in vivo tissue engineering is to deliver progenitor cells to the regeneration site in an inert matrix, such as the PEG hydrogel, and allow the encapsulated cells to secrete the desired extracellular matrix (ECM). In this approach, the encapsulated progenitor cells, guided by cell-cell interactions and soluble factors, create and reorganize their ECM as they go through lineage commitment, differentiation, and maturation. Although inert PEG hydrogels provide flexibility in controlling the cell microenvironment, their use for in vivo applications in tissue regeneration is limited by

their persistence (non-degradability) in the site of regeneration to provide free volume for tissue formation and remodeling. In that regard, design and synthesis of PEG hydrogels with hydrolytically degradable links would substantially increase their use as a cell delivery matrix in tissue regeneration.[47-49]

Copolymerization of PEG with poly(lactide) (PLA) has been used to impart degradability to PEG macromers. However, due to the hydrophobicity of lactide, these copolymers form thermo-responsive physical gels in aqueous solution with orders of magnitude lower modulus than the covalently crosslinked PEG hydrogels.[150, 235] The degradation and water content of the copolymers could be adjusted by the fraction of hydrophobic lactide segments,[88, 178] but solubility of the copolymers in aqueous solution decreased with increasing lactide content.[56] Furthermore, the covalently crosslinked copolymer gels had significantly lower modulus compared to PEG hydrogels due to entrapment of reactive groups in micellar domains.[56, 59, 236, 237]

We hypothesized that degradation and crosslink density of PEG-PLA gels and viability of encapsulated cells is strongly dependent on the number of lactides per macromonomer. We further hypothesized that PEG macromers with short lactide segments could produce mechanically robust hydrogels with tunable degradation rate. Previous molecular dynamic simulations demonstrated that the micelle size and reactivity of PEG-acrylates with short lactide segments depended strongly on the lactide segment length.[59]

The objective of this work was to synthesize star 4-arm poly(ethylene oxide-co- lactide) acrylate (SPELA) macromonomers and investigate the effect of number of lactides per macromonomer on gelation time, modulus, sol fraction, water content, and

degradation and to compare the results with the linear poly(ethylene oxide-co-lactide) acrylate (LPELA). Star PEG macromonomers have lower radius of gyration and shear viscosity, and higher density of functional groups than the linear PEGs,[25, 53] leading to the formation of gels with higher crosslink density and modulus.[54-57] Dissipative Particle Dynamics method[58, 59] was used to simulate aggregation and nanostructure formation and the distribution of reactive groups in SPELA and LPELA hydrogel precursor solutions. Bone marrow derived stromal cells (MSCs) were used to measure viability and differentiation of encapsulated cells to the osteogenic lineage.

4.2. MATERIALS AND METHODS