Characterization of injectable hydrogels based on poly(N-isopropylacrylamide)-g-chondroitin sulfate with adhesive properties for nucleus pulposus tissue engineering

Abstract

The goal of this work is to develop an injectable nucleus pulposus (NP) tissue engineering scaffold with the ability to form an adhesive interface with surrounding disc tissue. A family of in situ forming hydrogels based on poly(N-isopropylacrylamide)-graft-chondroitin sulfate (PNIPAAm-g-CS) were evaluated for their mechanical properties, bioadhesive strength, and cytocompatibility. It was shown experimentally and computationally with the Neo-hookean hyperelastic model that increasing the crosslink density and decreasing the CS concentration increased mechanical properties at 37 °C, generating several hydrogel formulations with unconfined compressive modulus values similar to what has been reported for the native NP. The adhesive tensile strength of PNIPAAm increased significantly with CS incorporation (p < 0.05), ranging from 0.4 to 1 kPa. Live/Dead and XTT assay results indicate that the copolymer is not cytotoxic to human embryonic kidney (HEK) 293 cells. Taken together, these data indicate the potential of PNIPAAm-g-CS to function as a scaffold for NP regeneration.

Synthesis of Thermogelling Poly(N-isopropylacrylamide)-graft-chondroitin Sulfate Composites with Alginate Microparticles for Tissue Engineering

“…Free radical polymerization of PNIPAAm-g-CS requires successful methacrylation of chondroitin sulfate, complete dissolution of monomer components, and oxygen-free reaction conditions. The ratio of NIPAAm monomer to methacrylated chondroitin sulfate in the reaction mixture was chosen because it has been demonstrated, in our previous work, to generate copolymers with mechanical properties similar to native intervertebral disc tissue 29 . Increasing the amount of chondroitin sulfate in the hydrogel will reduce water loss during gelation and thus decrease the compressive stiffness of the gel.…”

“…Increasing the amount of chondroitin sulfate in the hydrogel will reduce water loss during gelation and thus decrease the compressive stiffness of the gel. The selected degree of methacrylation has also been demonstrated to produce hydrogels with favorable handling properties for injectability through an 18 G needle when dissolved in aqueous solution 29 . An increased degree of methacrylate substitution of the CS will increase the crosslink density of the polymer network, causing increased solution viscosity below the LCST and reducing gel swelling capability.…”

“…However, PNIPAAm homopolymers exhibit poor elastic properties and hold little water at physiological temperature due to hydrophobicity 28 . In this work, we choose to incorporate chondroitin sulfate covalently into the PNIPAAm network, which offers the potential for enzymatic degradability 29 , antiinflammatory activity 30,31 , and increased water and nutrient absorption 32 . PNIPAAm copolymers with CS were prepared in our laboratory by polymerizing the monomer NIPAAm in the presence of methacrylate-functionalized CS to form grafted copolymer (PNIPAAm-g-CS).…”

“…PNIPAAm copolymers with CS were prepared in our laboratory by polymerizing the monomer NIPAAm in the presence of methacrylate-functionalized CS to form grafted copolymer (PNIPAAm-g-CS). Because of the low crosslinking density of the copolymer, PNIPAAm-g-CS forms a viscous solution in water at RT and an elastic gel at physiological temperature due to the LCST 29 . The polymer solutions become flowable again upon cooling below the LCST due to the reversibility of the transition.…”

“…We have demonstrated that PNIPAAm-g-CS has the potential to function as a tissue engineering scaffold, due to mechanical properties that can be tailored, degradability, and cytocompatibility with human embryonic kidney (HEK) 293 cells 29 . However, in certain load bearing areas, such as the intervertebral disc, tissue engineering scaffolds should have the ability to form a substantial interface with surrounding disc tissue to eliminate the risk of dislocation 3.…”