Multicomponent Polymer Networks Enhance Mechanical Properties and Cellular Response of Hydrogel Scaffolds for Cartilage Tissue Engineering

A wide variety of hydrogel networks are under investigation as scaffolds to support the repair and regeneration of a variety of tissues because they provide a three-dimensional environment for cells with water contents comparable to the native extracelullar matrix.  However, hydrogels made of single components typically do not have either the mechanical properties or cellular interaction that adequately mimic the native ECM.  The approach taken in this work is to use multiple polymeric components to more closely capture the key features of the ECM necessary to provide the mechanical response and cytocompatibility needed to improve cellular response for cartilage tissue engineering.

In this work, synthetic and biological polymers of established utility in tissue engineering are combined in different fashions to obtain desirable combinations of mechanical and biological responses.  The primary approach taken is to use interpenetrating networks (IPNs) of agarose and poly(ethylene glycol) diacrylate (PEGDA)  to provide mechanical strength while supplementing the network with methacrylated chondroitin sulfate (MCS) or other glycosaminoglycans to improve the biological response.  In a related approach, PEGDA is used as a crosslinker for MCS to create a strong yet biodegradable scaffold in a single synthesis step.

IPNs are created by encapsulating porcine chondrocytes in thermally gelling agarose, followed by soaking in a solution of PEGDA and MCS which is subsequently photopolymerized to form a copolymer network which interpenetrates the agarose constructs. Crosslinked MCS gels are made in a similar fashion but by photopolymerizing a solution of MCS with PEGDA along with chondrocytes in a single step. 

For unconfined compression of hydrogel disks, the IPN displayed a 4-fold increase in shear modulus relative to a pure PEG-DA network (39.9 vs. 9.9 kPa) and a 4.9-fold increase relative to a pure agarose network (8.2 kPa). PEGDA and IPN compressive failure strains were found to be 71±17 and 74±17 percent, respectively, while pure agarose gels failed around 15 percent strain. Similar mechanical property improvements were seen in IPN gels with encapsulated chondrocytes.  Live/Dead assays demonstrated that the viability of IPN-encapsulated chondrocytes was over 90% initially, but viability while in culture drops to 45% 3 weeks after encapsulation. However, inclusion of MCS with the PEGDA solution links it to the PEGDA network, and 3 week viability was then raised to 70%.  A continuous increase in GAG content over this time was also observed in these networks. In principle, these IPNs could be made biodegradable by inclusion of degradable linkers.  However, another route to degradable with synthesized in a single step was to make a gel from a solution of  10 – 15% MCS and 1 – 3% PEGDA  (w/v%).  The combination of MCS and PEGDA substantially increased the Young's modulus and fracture stress of MCS gel to levels comparable to the IPNs.  The MCS network would also be biodegradable over time while releasing lower molecular PEG fragments that can ultimately cleared from the body.

This work was supported by the NIH (1 R21 EB008783-01, 5 P20 RR 16475-08), and the NSF (IOS 0726425 and DMR 0805264).