Review: understanding sorbent dialysis systems.

Although maintenance haemodialysis once had the benefit of two distinctly different dialysate preparation and delivery systems - (1) a pre-filtration and reverse osmosis water preparation plant linked to a single pass proportioning system and (2) a sorbent column dependent dialysate regeneration and recirculation system known as the REDY system - the first came to dominate the market and the second waned. By the early 1990s, the REDY had disappeared from clinical use. The REDY system had strengths. It was a small, mobile, portable and water-efficient, only 6 L of untreated water being required for each dialysis. In comparison, single pass systems are bulky, immobile and water (and power) voracious, typically needing 400-600 L/treatment of expensively pretreated water. A resurgence of interest in home haemodialysis - short and long, intermittent and daily - has provided impetus to redirect technological research into cost-competitive systems. Miniaturization, portability, flexibility, water-use efficiency and 'wearability' are ultimate goals. Sorbent systems are proving an integral component of this effort. In sorbent dialysate regeneration, rather than draining solute-rich dialyser effluent to waste - as do current systems - the effluent repetitively recirculates across a sorbent column capable of adsorption, ion exchange or catalytic conversion of all solute such that, at exit from the column, an ultra-pure water solution emerges. This then remixes with a known electrolyte concentrate for representation to the dialyser. As the same small water volume can recirculate, at least until column exhaustion, water source independence is assured. Many current technological developments in dialysis equipment are now focusing on sorbent-based dialysate circuitry. Although possibly déjà vu for some, it is timely for a brief review of sorbent chemistry and its application to dialysis systems.