Double Network Hybrid Hydrogels with Nanocomposite Structures for Cartilage Tissue Engineering Applications
Authors: Mohammed Ali , Jones Julian R , Georgiou Theoni 
Published: July 2019

Hydrogels have become a popular source of research for cartilage tissue engineering but have been limited by their brittle nature at high water contents. Double network hydrogels (DNHG) are innovative materials that possess the ability to hold high water content whilst maintaining high mechanical strength, but require more accurate control over these properties. This study aims to achieve this goal by introducing a new concept by incorporating functionalized sol-gel nanoparticles (xSNP) as macro cross linkers rather than the conventional chemical cross linkers. DNHG are formed by a 1st network (1NW) polyelectrolyte and 2nd network (2NW) neutral polymer. This study investigates two separate DNHGs; polyacrylic acid (PAAc) and poly 2-acrylamido-2-methylpropane sulfonic acid (PAMPS) were the 1NW, chosen for their biocompatibility and hydrophilic nature. They were cross linked with amino-SNP (ASNP) and vinyl-SNP respectively. Polyacrylamide was chosen as the 2NW for both gels for its intrinsic strong mechanical properties. The aim of this study is to understand the effects of size and concentration of xSNP as a novel cross linking agent in DNHG for precisely controlling the properties of the gels. SNPs of 20, 50 and 100 nm were synthesized by the Stöber process, and functionalized in situ with 3-aminopropyl triethoxysilane and vinyl TEOS. The xSNP concentrations in the DNHGs were 0-50 wt. ? of the 1NW. SNPs were studied under TEM, SEM, FTIR, DLS, zeta potential and confocal microscopy to confirm size and functionalization. 1NW polymers were polymerized and cross linked in situ with xSNP under UV light; ASNP used carbodiimide chemistry to cross link with PAAc and VSNP was cross linked using a UV initiator. 1NW were soaked in 2NW solution and UV polymerized to form the DNHG. FTIR, swelling and water uptake studies were performed on heat/vacuum dried DNHGs. Compressive and dynamic mechanical properties were studied for fracture cyclic loading. DNHG cross sections were used for SEM and TEM imaging. Increasing size and concentration of xSNP caused a reduction in both water up take and swelling properties, providing evidence for higher cross linking in the DNHG. Water uptake ranged from 1230 ? for the control (0 wt. ? xSNP) to 750? for 50 wt. ? with 100 nm VSNP. Water content reduced from 93? for the control to 76? for 50 wt. ? with 100 nm VSNP, in the range of natural cartilage water content. Compressive strengths of the DNHGs increased with increased ASNP conc. and size up to a fracture stress of 15 MPawith 75? water content, providing evidence that the SNPs are acting as cross linkers in the 1NW rather than fillers. Cross sections of the DNHGs under SEM and TEM show homogenous dispersion of xSNP within the structure, indicating successful incorporation. FTIR data of the DNHG after 3 drying and saturation cycles show Si-O-Si bands supporting the evidence of xSNP incorporation into the DNHG. These results show potential for further research and application of sol-gel nanoparticles in hydrogel applications. The best hydrogels from this reserach were chosen to be optimised. As the photopolymerisations were done under open atmosphere it is understood that atmopsheric O2 will interact with the monomer solution and inhibit the polymerisation from completing. This leads to shorter chain polymers and a lower degree of monomer to polymer conversion; hence leading to less polymer entanglement and lower mechanical integrity. Oxygen can be depleted from the monomer system by introducing glucose oxidase (GOX). Oxygen is eaten up by this enzyme in the presence of glucose to produce hydrogen peroxide. Full oxygen depletion is reached at 200 nM GOX, and 100 nm glucose. The enzyme works best at pH 5-6 therefore it was not possible to optimise the first network polymer AMPS.