(302d) Biopolymer-Based Ultrathin Film Double-Network Hydrogels with High Mechanical Strength

A tough and ductile ultrathin film double-network (UTDN) biopolymer-based hydrogel was developed and characterized. The UTDN hydrogel was made using a biopolymer, methacrylated chondroitin sulfate (MCS), and polyacrylamide (PAAm). The MCS was synthesized by modifying chondroitin sulfate (CS), a key structural component of cartilage. It was previously shown that the toughening mechanism of UTDN gels are the same as bulk DN gels¹, but UT hydrogels allow rapid equilibration of the gels with solutions, require minimal material usage and are amenable to study of the fracturing mechanisms.

The MCS/PAAm UTDN hydrogel was prepared in a multistep process. Using two glass plates with a 100 μm polyethylene film spacer, the MCS hydrogel was synthesized through free radical polymerization by photoinitiation.  The MCS hydrogel was taken out of the mold and allowed to swell to equilibrium in an acrylamide/bisacrylamide solution. The swollen gel was irradiated to form the second PAAm network. The MCS/PAAm UTDN were ~100-250 μm fully swollen in water. Tension and tearing tests were both performed; from these tests the stress–strain data, Young’s modulus, yield point, fracture stress, fracture strain, and tearing energy were determined.

The major conclusion is that MCS/PAAm UTDN hydrogels have proved to provide excellent toughness and strength for a biomaterial and are comparable to previously developed tough double-network gels of poly(2-acrylamido-2-methylpropane-sulfonic acid) (PAMPS)/PAAm. The MCS/PAAm UTDN hydrogels showed two of the three characteristic regions in the stress-strain curves: pre-yielding and yielding. The MCS/PAAm UTDN hydrogels failed in the yielding region thus having ductile rather than brittle failure. In comparison, the double-network (DN) of MCS/PAAm had a failure stress more than 20 times greater than the single-network (SN) of either MCS or PAAm. The Young’s modulus was also more than 10 times greater in the DN than the SN of PAAm and was also slightly greater than the SN of MCS. Single network of MCS had a failure strain of ~13% while the fracture strain was increased to ~250% by adding the second network of PAAm. The yielding stresses of the MCS/PAAm DN gels were over 1500 kPa, at least double that of PAMPS/PAAm hydrogels. The tearing energies were comparable to PAMPS/PAAm DNs, in the range of 10²-10³ J/m², and are not significantly dependent of the formulation. This is the first demonstration of a tough, ductile biopolymer-based network displaying a substantial necking region, demonstrating the generality of this failure mechanism in such DN gel formulations.  

1. Liang, S.; Wu, Z. L.; Hu, J.; Kurokawa, T.; Yu, Q. M.; Gong, J. P. Macromolecules.