Alginate–polyester comacromer based hydrogels as physiochemically and biologically favorable entities for cardiac tissue engineering

Abstract

The physiochemical and biological responses of tissue engineering hydrogels are crucial in determining their desired performance. A hybrid comacromer was synthesized by copolymerizing alginate and poly(mannitol fumarate-co-sebacate) (pFMSA). Three bimodal hydrogels pFMSA-AA, pFMSA-MA and pFMSA-NMBA were synthesized by crosslinking with Ca²⁺ and vinyl monomers acrylic acid (AA), methacrylic acid (MA) and N,N′-methylene bisacrylamide (NMBA), respectively. Though all the hydrogels were cytocompatible and exhibited a normal cell cycle profile, pFMSA-AA exhibited superior physiochemical properties viz non-freezable water content (58.34%) and water absorption per unit mass (0.97 g water/g gel) and pore length (19.92 ± 3.91 μm) in comparing with other two hydrogels. The increased non-freezable water content and water absorption of pFMSA-AA hydrogels greatly influenced its biological performance, which was evident from long-term viability assay and cell cycle proliferation. The physiochemical and biological favorability of pFMSA-AA hydrogels signifies its suitability for cardiac tissue engineering.

Introduction

The cell based therapeutic strategies for regeneration of infracted heart that inject cells directly to the infarct zone are not being fully successful. Because more than 90% of the injected cells were reported to be lost by extrusion and 90% of the sustained cells died within 1 week of injection. The use of scaffold systems can improve the cell based cardiac therapies by providing a suitable micro niche that facilitate the integration with host [1], [2]. Such scaffolds should have the potential to orchestrate the tissue architecture by facilitating the diffusion based mass transfer to nullify the ischemic injury. The mechanical properties and the porosity of the scaffolds play key roles in the in the proper functioning of cardiac tissue engineering scaffolds [3], [4].

An ideal tissue engineering scaffold should provide both physiochemical and mechanical support and facilitate a suitable 3D micro niche for the genesis of living tissue mimic by promoting cell attachment and infiltration so as to repair and regenerate the lost function [5]. Even though the natural hydrogels like alginate have given various appreciable outcomes, few have met all the desired requirements [6]. Among natural hydrogels alginate gained importance owing to its potent viscoelastic nature, hydrophilicity to induce entrapment of the cells resembling the real ECM, hydration capacity to facilitate the management of highly exudation of lesions, low rate of degradation and toxicity. However, it was reported that the efficacy of alginate gels to promote mammalian cell attachment and proliferation was limited especially in its unmodified form [7]. Moreover the alginate hydrogels lose more than 60% of their mechanical properties within 15 h of exposure with the biological fluids [8].

The water content and status of a hydrogels strongly influences the growth, response and function of the cells seeded on to it. Apart from the bulk water, the freezable and non-freezable water also influence the long-term cell growth, penetration and survival. The freezable water enhances the availability of nutrients to the cells by facilitating its easy traffic by diffusion [9]. While the non-freezing bound water stabilize the hydrogels networks by forming extensive hydrogen bonds with the functional groups like COOH and OH [10]. Moreover, this type of water remove the solvation layer covering the solutes and facilitate their easy entry to the cell [11]. Therefore the status of water present in the hydrogel has greater influence on the long-term tissue response.

With this background, we aimed to improve the physiochemical, mechanical and biological properties of alginate hydrogels by co-polymerizing with novel unsaturated polyester made from the biomolecules fumaric acid, mannitol and sebacic acid. Three bimodal hydrogel scaffolds were made from this comacromer by crosslinking the alginate segments with calcium ions and the unsaturation of the esters with different vinyl monomers. These hybrid alginate hydrogel scaffolds were chemically identical, except for vinyl cross linkers. The hydrogels were evaluated for the difference in their water content/status and its implication on the biological responses to be applicable for cardiac tissue engineering.