Polymers with two different aryloxy side groups have also been developed as elastomers for fire-resistance as well as thermal and sound insulation applications. Linear polyphosphazenes with oligo-ethyleneoxy side chains are gums that are good solvents for salts such as lithium triflate. These solutions function as electrolytes for lithium ion transport, and they were incorporated into fire-resistant rechargeable lithium-ion polymer battery. The same polymers are also of interest as the electrolyte in dye-sensitized solar cells. Other polyphosphazenes with sulfonated aryloxy side groups are proton conductors of interest for use in the membranes of proton exchange membrane fuel cells. Water-soluble poly(organophosphazenes) with oligo-ethyleneoxy side chains can be cross-linked by gamma-radiation. The cross-linked polymers absorb water to form hydrogels, which are responsive to temperature changes, expanding to a limit defined by the cross-link density below a critical solution temperature, but contracting above that temperature. This is the basis of controlled permeability membranes. Other polymers with both oligo-ethyleneoxy and carboxyphenoxy side groups expand in the presence of monovalent cations but contract in the presence of di- or tri-valent cations, which form ionic cross-links. Phosphazene hydrogels have been utilized for controlled drug release and other medical applications. The ease with which properties can be controlled and fine-tuned by the linkage of different side groups to polyphosphazene chains has prompted major efforts to address biomedical materials challenges using these polymers. Different polymers have been studied as macromolecular drug carriers, as membranes for the controlled delivery of drugs, as biostable elastomers, and especially as tailored bioerodible materials for the regeneration of living bone. An advantage for this last application is that poly(dichlorophosphazene) reacts with amino acid ethyl esters (such as ethyl glycinate or the corresponding ethyl esters of numerous other amino acids) through the amino terminus to form polyphosphazenes with amino acid ester side groups. These polymers hydrolyze slowly to a near-neutral, pH-buffered solution of the amino acid, ethanol, phosphate, and ammonium ion. The speed of hydrolysis depends on the amino acid ester, with half-lives that vary from weeks to months depending on the structure of the amino acid ester.