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Typical durable TPUs will absorb between 1 and 3% by weight water under ASTM conditions depending on PU composition. In contrast, some biomedical applications may require that a polymer matrix contain between 20 and 40% by weight water. Such applications may include soft contact lenses, wound dressing adhesive components, implant coatings, and media for drug delivery [50]. Polymer systems that can swell with water to this extent are often termed "hydrogels." The design latitude that PU possess makes them effective hydrogel-forming polymers. A challenge for PU, and all polymers, is that the material must maintain substantial physical properties even when highly swollen. In addition, they should not present a fertile environment for microbial growth. Polyethylene glycol soft segments are commonly used in hydrogel design to provide hygroscopic base properties and a microbe resistant interface [51, 52]. PU chemistry also presents numerous design options for developing cross-links to buttress swollen polymer physical properties. Cross-linking agents may be relatively small like trimethylolpropane or diethanolamine. Alternatively, they can be higher-molecular-weight materials like ethoxylated glycerine. The hard segment for hydrogels is invariably aliphatic—usually hexane diisocyanate or isophorone diisocyanates (Chapter 2). The equilibrium water content and the physical properties are mutually dependent on chemistry and the degree of cross-linking.

Cross-link density in the swollen matrix is designed from the building blocks, but the actual molecular weight between cross-links must be determined empirically. A convenient method for calculation of cross-link density is the Flory-Rehner equation (11.1) based on measurements of equilibrium matrix swelling by a solvent [53]:

In Equation 11.1, Mc is the molecular weight between cross-links, vs is the molar volume of the solvent, dp is the density of the polymer, V is the volume fraction of the swollen polymer in the solvent, andZ12 is the polymer-solvent interaction parameter (see Chapter 4). As discussed in detail in Chapter 4, the interaction parameter can be calculated from Equation 11.2:

TABLE 11.2 Solubility parameters of representative solvents useful for determining cross-link density and molecular weight between cross-links for a polyurethane foam using the Flory-Rehner equation 11.1

Solubility parameters of representative solvents useful for determining cross-link density and molecular weight between cross-links for a polyurethane foam using the Flory-Rehner equation 11.1

In Equation 11.2, 8p and 8s are the solubility parameters of the polymer and solvent, respectively. A discussion and table of solubility parameters useful for PU polymers can be found in Section 4.1. Solubility parameters for standard solvents relevant to measuring cross-linking in PU are found in Table 11.2 [54].

The cross-link density is calculated from Equation 11.3 following calculation of Mc from Equation 11.1:

Since the volume of PU used for hydrogels is inconsequential, the significance of this technology is the fundamental science it offers, the investment in future therapies consequential to human well-being, and as a signpost for future high-technology research in PU science. Activity on medical PU hydrogels is approximately 52 patents from 1990 to 2014. Only one company has filed more than two patents in this time, while over 500 articles in the open literature have been published.

 
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