The influence of poly(allylamine hydrochloride) hydrogel crosslinking density on its thermal and phosphate binding properties

Sevelamer hydrochloride (SH) or Renagel® is an effective phosphate binder prescribed to prevent the absorption of phosphate in end stage renal disease (ESRD) patients. The relationship between SH structure and binding capacity and affinity is very important and can be used in characterising the sensitivity of the hydrogel to binding conditions. Thus, a series of hydrogels were prepared by varying the amount of crosslinker, whilst the other hydrogel components were kept constant. Variation of this parameter influenced the hydrogel structure as shown by swelling data, differential scanning calorimetry and solid state nuclear magnetic resonance spectroscopy. The hydrogels' physical characteristics were found to correlate with the number of phosphate binding sites and affinity obtained from the Langmuir-Freundlich Isotherm (L-FI) and affinity distribution spectra (AD). The hydrogels formed using lower amounts of crosslinker showed a slight increase in binding capacity but with lower affinity. However, the influence of the pH of the binding media on the binding parameters of sevelamer hydrochloride was significant. This is the first report on the use of AD spectra generated from L-FI binding parameters in hydrogels, which demonstrates the sensitivity of the affinity and binding site numbers to changes in hydrogel physical properties and the pH of the binding media.