Candidate: Evan D. Richards

 Date: Tuesday, October 18, 2022

 Time: 3:00 PM – 5:00 PM

 In-person: 1145 Benedum Hall

 Title: Development of a Selective Cation Exchanger to Adsorb Ammonium by Coating with a Gas-permeable and Hydrophobic Membrane 

Chair:

Dr. Lei Li, Chemical and Petroleum Engineering Department, University of Pittsburgh

Committee:

Dr. Susan Fullerton, Chemical and Petroleum Engineering Department, University of Pittsburgh

 Dr. Tagbo Niepa, Chemical and Petroleum Engineering Department, University of Pittsburgh

 Dr. William Wagner, Department of Surgery, McGowan Institute for Regenerative Medicine, Department of Bioengineering and Department of Chemical and Petroleum Engineering University of Pittsburgh

Dr. Sang-Ho Ye, Department of Surgery, University of Pittsburgh

Dr. Stephen R. Ash, MD – Hemocleanse Technologies, Pittsburgh, PA

 Abstract: A sorbent with a high enough capacity for NH₄⁺ could serve as an oral binder to lower urea levels in end-stage kidney disease patients. A hydrogen-loaded cation exchanger such as zirconium phosphate Zr(HPO₄)₂·H₂O (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 developed on an amorphous form of ZrP using tetraethyl orthosilicate and methoxy-terminated polydimethylsiloxane. The hydrophobic coating served as a barrier to ions in water solution from reaching the ion-exchanger’s surface. Meanwhile, its gas-permeable nature allowed for gaseous ammonia transfer to the cation exchanger. In vitro studies replicated the small intestine’s expected ion concentrations and exposure time to the sorbent. The effectiveness of the coating was measured with NH₄⁺ and Ca²⁺ solutions and uncoated ZrP as the negative control. X-ray photoelectron spectroscopy and scanning electron microscopy measurements showed the coating successfully modified the surface of the cation exchanger. Water contact angle (WCA) studies indicated that coated ZrP was hydrophobic with an angle of (149.8° ± 2.5°). Simulated small intestine solution studies showed the coated ZrP removed 94% (± 11%) more NH₄⁺ than uncoated ZrP in the presence of Ca²⁺. Meanwhile, Ca²⁺ binding decreased by 64% (± 6%). The nearly four-fold increase in NH₄⁺ selectivity can be attributed to the gas-permeable and hydrophobic coating applied on ZrP’s surface. 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 made of a perfluorocarbon backbone was next investigated – 1H,1H,2H,2H-perfluorooctyltriethoxysilane. The coating was assembled on a polysiloxane-formed membrane on ZrP’s surface. Calculated Langmuir equilibrium and removal rate plots showed that decreasing %1H,1H,2H,2H-perfluorooctyltriethoxysilane in coating solution from 10% to 4% improved total NH₄⁺ removal by 27% and removal rate by nearly 675%. In vitro studies indicated the perfluorocarbon-based structure attached to ZrP offers complete selectivity for NH₄⁺ over Ca²⁺. The studies also showed the material removed as much NH₄⁺ from solution as the polydimethylsiloxane-based coating. But 1H,1H,2H,2H-perfluorooctyltriethoxysilane-coated ZrP maintained its selectivity and total NH₄⁺ removal capacity after acid exposure.