(262c) Swelling, Shrinking, and Mechanical Relaxations In Glucose-Sensitive Hydrogels

Hydrogels containing phenylboronic acid (PBA) sidechains bind glucose and other sugar molecules.  Such binding stabilizes the ionized Lewis base form of PBA, giving rise to an osmotic swelling force.  Binding occurs by formation of bisbidentate condensation complexes between cis-diols on PBA and tetragonal hydroxyls of the PBA Lewis base.  Unlike other sugars, glucose possesses two cis-diols, which provides the opportunity to form reversible crosslinks between polymer chains, leading to shrinkage of the hydrogel.  These properties enable PBA-hydrogels to be utilized in microfabricated implantable wireless glucose sensors.  In this presentation, data concerning swelling and mechanical stress relaxation of PBA-based hydrogels is reported as a function of pH and concentrations of fructose or glucose.  Hydrogels contained methacrylamidophenylboronic acid (MPBA), acrylamide (AAm), and dimethyaminopropylmethacrylamide (DMP).

Hydrogels containing MPBA and AAM swelled in the presence of fructose at all pH values, whereas glucose had a swelling effect at lower pH values but cause shrinking at higher pH values, reflecting the effect of pH on the fraction of PBA moeities that are in the Lewis base form.  Swelling data was well fit by a modified Flory-Rehner-Donnan-Langmuir model, with the interaction parameter depending linearly on the proportion of PBA units that are ionized.  Mechanical stress relaxation studies demonstrated that glucose-mediated crosslinks are reversible, with a wide spectrum or relaxation times observed, reflecting kinetics of crosslink breakage and reformation, network reorganization, and collective network diffision.

Hydrogels containing MPBA, DMP, and AAm deswelled with increasing glucose concentration near physiologic pH, reflecting the polyampholytic character of these hydrogels in this pH range, and the Lewis base action of DMP on MPBA.