Book Description

Hydrogels have long been considered ideal candidates for biomaterial and tissue engineering applications due to their many desirable properties, such as high water content and tunable gelation conditions. Although these materials have undergone extensive research and development, some mechanical and physical properties are still difficult to achieve. The reason for this is often related to the structure of the hydrogel network. Understanding how network structures are influenced by changes in formulation parameters (i.e. polymer molecular weight, initial polymer concentration, ratio of hydrophilic to hydrophobic polymer), and correlating these results to known mechanical and physical properties would yield well characterized systems that are more easily tuned for specific applications. The work presented in this thesis focuses on the characterization of the micro- to nano-scale network structures of three distinct hydrogel systems: tetra-functional poly(ethylene glycol) (PEG)-based hydrogels, tetra-functional PEG/polydimethylsiloxane (PDMS)-based hydrogels, and commercial contact lenses. The tetra-functional PEG and PEG/PDMS hydrogel systems were synthesized with a novel cross-linking technique that was developed by the Tew Group in the Polymer Science and Engineering department at the University of Massachusetts Amherst. This technique was designed to reduce the formation of network defects. The resulting hydrogels are optically clear, and display highly resilient mechanical properties which suggest relatively defect free (or ideal) network structures. In collaboration with the Tew group, we performed a series of small-angle neutron scattering (SANS) studies on these systems. The results from the tetra-functional PEG hydrogels confirmed the presence of nearly ideal network structures. Additionally, those from the tetra-functional PEG/PDMS hydrogels revealed the presence of a two-phase network structure with a local, lamellar-like order. For both systems, the resulting structures were found to be dependent upon polymer molecular weight, initial polymer concentration, and the ratio of hydrophilic to hydrophobic polymer. These results confirm the effectiveness of the novel cross-linking technique used to synthesize the PEG and PEG/PDMS tetra-functional hydrogels. Their unique and predictable network structures provide an excellent starting point for the development of these systems for specific applications, such as tendon tissue engineering scaffolds.