SiO 2 Rich Surface Layer Formation
1. Hydrogel Properties
As previously stated, the primary focus of this research moving forward will be the development and characterization of hydrogels, which will be tailored to provide a structural scaffold and delivery system, upon which our Ga-containing bioactive glasses can be incorporated, that facilitates leaching of the beneficial ions from the glasses into the surrounding environment. In order to properly develop hydrogels for this application, we must first understand their inherent structures and properties. First developed by Wichterle and Lim for use as soft contact lenses155, hydrogels can be defined as cross- linked networks of hydrophilic polymers, which contain large amounts of water (usually in the range of 30-95 wt%).156 The presence of water drastically effects the properties of these materials and in the case of impregnating them with bioactive glasses and using them as implantable materials, higher water content can lead to increased rate of dissolution of the glass particles as the particles will be surrounded by fluid not only in the implanted environment, but within the material’s structure. Increased water content can also promote diffusion through the material of not only substances such as oxygen and proteins from the environment, but also diffusion of the ions being released from the glass particles into the environment.156 Increased water content also translates to a softer material, allowing for easier molding during implantation, and faster integration with tissues post-implantation.156
The cross-linking of these polymer networks can be achieved either chemically or physically, however, there are positive and negative consequences associated with both methods. In hydrogels where the polymer cross-linking is performed by a chemical process, as is performed in the current study, the resulting gel that forms is comprised of either an ionically or a covalently cross-linked network. These types of gels can be created through processes such as combining a polyelectrolyte with a multi-valent ion of opposite charge, combining polyelectrolytes of opposite charge, or by using a cross- linking agent to cross-link water-soluble polymers.157 Chemical cross-linking leads to a
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slightly more regular structure than physical cross-linking, which can result in increased mechanical strength, but also a decrease in biodegradability. In hydrogels where the polymer cross-linking is achieved through a physical process, the resulting gel that forms is held together by the entanglement of polymer chains, and/or secondary forces.158 Physically cross-linked polymers will form a more amorphous structure than their chemically cross-linked counterparts, which can result in increased biodegradability at the expense of mechanical strength. Illustrations of both forms of cross-linking can be seen below in Figure 12. Regardless of the method used to induce cross-linking, cross- linked hydrogels will reach an equilibrium swelling point upon submersion in an aqueous solution. This equilibrium swelling point is mainly dependent on the cross-link density present within the network, which can be estimated as the molecular weight between links.159
Figure 12. Illustrations of both chemical and physical cross-linking within polymer hydrogels.160
In addition to there being different methods for cross-linking polymer networks, there are also many different macro-molecular structures that can be formed. The three main structures observed in hydrogels are cross-linked networks of linear homopolymers (chains consisting of identical monomer units), networks of linear copolymers (chains consisting of multiple monomers), and graft copolymers (main chains and side chains are composed of different monomers).157 Since hydrogels can exhibit several different structures, they can also exhibit several different physical forms such as solid molds (e.g.
contact lenses), pressed powders (e.g. capsules for oral ingestion), coatings, membranes, and even liquids which can form gels upon the application or deprivation of heat. The
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desired cross-linking method, macro-molecular structure, and ultimate physical form of a hydrogel are all dictated by the intended application, which is why one must understand all three aspects of these materials in order to properly tailor their hydrogel to adequately perform in a specific setting.
As briefly discussed earlier, the water content of a hydrogel impregnated with glass particles can determine the permeability of nutrients into the gel, and ionic dissolution products out of the gel. When a dry hydrogel is created, and water is then added, the first water molecules to interact will become the “primary bound water”.
These water molecules enter the polymer matrix and hydrate the most polar, hydrophilic groups present.157 As hydration of these polar groups proceeds, the overall network begins to swell, resulting in the exposure of hydrophobic groups to the ingressing water molecules. These hydrophobic groups will lightly bind water molecules, resulting in what is known as “secondary bound water”.157 The combination of the “primary” and
“secondary” water molecules is called the “total bound water” and is an important characteristic when characterizing hydrogels. Beyond this point, the network can still absorb more water. The amount of additional water that can be endured by the gel is determined by the degree of opposition between the osmotic driving force of the polymer chains to achieve infinite dilution, and the inherent cross-link density of the polymer network. When these two forces are equal, the hydrogel will have reached its equilibrium swelling point. All of the water that is contained within the network that is not “bound”
water is called “free water”, and this “free water” is believed to occupy the spaces between chains and any pores that may be present throughout the structure. The total amount of water present in a hydrogel is what will determine the absorption and diffusion of solutes throughout the network.
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