Intelligent polyelectrolytes and gels in oral drug delivery

The present review concerns smart, or intelligent polymers for oral administration that change conformation in aqueous solutions in response to external stimuli such as pH or temperature. We concentrate on charged polymers and gels with polyelectrolyte properties. Because of the ionization at a certain pH or in response to changes in the ionic composition of the solution, a polyelectrolyte has better chances of displaying smart properties than a neutral polymer. When such smart polyelectrolyte is cross-linked by covalent or hydrogen bonding and/or physical aggregation or is entangled, it forms an environmentally sensitive gel capable of swelling and collapse in an aqueous medium. Varying pH, temperature, and microbial flora are found in the gastrointestinal tract, and thus pH- sensitive polymers and gels that can be degraded by specific enzymes and/or inhibit proteolytic enzymes can be tailored for the efficient site-specific therapy. Smart polymers wield a lot of promise in the targeted, site-specific administration where they can provide advantages in loading of sensitive drugs such as proteins and peptides, while releasing the drug at a specific pH or in response to the presence of certain microbial flora.