The dispersion of
microgels with two interpenetrating
polymer networks of
poly( N-isopropylacrylamide) and
poly(acrylic acid) (
PNIPAM-IPN-PAAc) has been studied for its viscoelastic behavior, biocompatibility, and in vivo release properties. The IPN
microgels in water had an average hydrodynamic radius of about 85 nm at 21 degrees C, measured by dynamic light scattering method. The atomic force microscope image showed that the particles were much smaller after they were dried but remained spherical shape. The storage and loss moduli ( G' and G'') of dispersions of IPN
microgels were measured in the linear stress regime as functions of temperature and frequency at various
polymer concentrations using a stress-controlled rheometer. For dispersions with
polymer concentrations of 3.0 and 6.0 wt % above 33 degrees C, the samples behave as viscoelastic solids and the storage modulus was larger than the loss modulus over the entire frequency range. The loss tangent was measured at various frequencies as a function of temperature. The gelation temperature was determined to be 33 degrees C at the point where a frequency-independent value of the loss tangent was first observed. At pH 2.5, when heated above the gelation temperature, IPN
microgels flocculate by pumping a large amount of water from the gel. When the pH value was adjusted to neutral, deprotonation of -COOH groups on PAAc made the
microgel keep water even above the gelation temperature. Using an animal implantation model, the biocompatibility and drug release properties of the IPN
microgel dispersion were evaluated.
Fluorescein as a model
drug was mixed into an aqueous
microgel dispersion at ambient temperature. This
drug-loaded liquid was then injected subcutaneously in Balb/C mice from Taconic Farms. The test results have shown that the IPN
microgels did not adversely promote
foreign body reactions in this acute implantation model and the presence of gelled
microgel dispersion substantially slowed the release of
fluorescein.