(272a) Improving the Mechanical Performance Hydrogels by Controlling the Nanostructure

Hydrogels are water swellable polymer networks that have many technological applications including biochemical separations, consumer products, drug delivery systems and tissue engineering scaffolds. In many applications, high water content is a desirable feature, however, as the polymer content is diluted by hydration, the mechanical properties of the hydrogel usually deteriorate. Poor mechanical performance typically limits the capabilities of hydrogels in most applications. However, there are many biological hydrogels such as the lens of the eye, cartilage, certain insect cuticles and so on which have much better mechanical properties than those typically observed in synthetic hydrogels. The most notable difference between biological hydrogels and synthetic hydrogels is that biological hydrogels generally have two or more macromolecular components organized in well-defined nanostructures that are responsible for distributing load and inhibiting crack propagation, while typical synthetic gels typically lack such nanostructure.

Thus we are focusing on the development of biomimetic, nanostructured gels to improve their mechanical performance with a particular focus on their use as tissue engineering scaffolds. We have two basic approaches to this problem: the development of semi-interpenetrating networks based on the biocompatible polymers polyethylene glycol (PEG) and agarose, and gels in which the networks are formed sequentially with different structures at different submicron length scales. The mechanical performance of these gels are comprehensively evaluated to determine a number of different properties, including the Young's modulus (E), shear modulus (G), elastic (storage) modulus (E'), viscous (loss) modulus (E?), tan δ, fracture stress, fracture strain and toughness. The goal is to develop biocompatible, high strength networks by scalable methods and to correlate the mechanical properties with the underlying nanostructure.

For example, we have shown that PEGDA-agarose semi-IPNs offer superior mechanical performance to PEGDA (PEG diacrylate) and agarose single networks, being up to 9x stiffer than agarose and 5x tougher then PEGDA. Macroscopic properties have been correlated with microscopic mechanical properties as measured by atomic force microscopy (AFM). The improved fracture properties of the semi-IPN are suggested to be a consequence of a heterogeneous microstructure that retards crack propagation. Diverse mechanical properties were achieved with the composite PEGDA-agarose semi-IPN gels depending on its polymer composition and concentration, suggesting such networks can be easily tailored to a variety of biomedical applications. It has also been demonstrated that chondrocytes can be encapsulated in these networks during synthesis while maintaining their viability, demonstrating the potential of such networks as scaffolds for tissue engineering.

Ed Cussler is the progenitor of this line of research as Steve Gehrke earned his Ph.D with Ed in 1986 on the synthesis and characterization of hydrogels for use in bioseparations. Ed suggested to him that biomedical and pharmaceutical applications of hydrogels would be a promising basis upon which to build an academic career. Steve would like to use this forum to publicly thank Ed for providing him with the foundation for a very satisfying academic career in exactly this research area.