P8: A Coated ZrP Cation Exchanger with Excellent Ammonium Selectivity and Chemical Stability: An Oral Sorbent for End-Stage Kidney Disease (ESKD)

Abstract

A sorbent with high enough capacity for NH4+ could serve as an oral binder to lower urea levels in end-stage kidney disease patients (Figure 1). A hydrogen-loaded cation exchanger such as zirconium phosphate (ZrP) is a promising candidate for this application. However, the NH4+ binding selectivity versus other ions must be improved. A gas-permeable and hydrophobic surface coating was first attached to an amorphous form of ZrP using tetraethyl orthosilicate and methoxy-terminated polydimethylsiloxane (PDMS). The coating prevents ions in solution from reaching the ion-exchanger but allows NH3 transfer to the ZrP. The selectivity of the coating was measured with NH4+ and Ca2+ solutions versus uncoated ZrP. X-ray photoelectron spectroscopy and scanning electron microscopy measurements showed the coating successfully modified the surface of ZrP. Water contact angle (WCA) studies indicated coated ZrP was hydrophobic (149.8° ± 2.5°). The in vitro studies showed coated ZrP removed 94% (± 11%) more NH4+ than uncoated ZrP in the presence of Ca2+. And Ca2+ binding decreased by 64% (± 6%). The increase in NH4+ selectivity was due to the applied coating. But the material’s selectivity decreased by 72% after exposure to stomach acid conditions. An alternative hydrophobic (WCA = 145.0° ± 3.2°) and gas permeable coating was investigated – perfluorooctyltriethoxysilane (FOTS). The coating was attached to a polysiloxane membrane on ZrP’s surface. In vitro studies in Figure 2b indicated the FOTS structure attached to ZrP offers complete selectivity for NH4+ over Ca2+ with similar NH4+ capacity as the PDMS coating. FOTS-coated ZrP maintained its selectivity and NH4+ removal capacity after stomach acid exposure. The results indicate FOTS-coated ZrP could work as an oral sorbent for ESKD patients.Figure 1. The recirculation of urea and oral sorbent’s methodology for NH4+ adsorption. Figure has been adapted from Barret’s original artwork.1Figure 2. (a): SEM images of uncoated, PDMS-coated and FOTS-coated ZrP at 10,000x magnification. (b): Binding study results of uncoated, PDMS-coated and FOTS-coated ZrP.