Journal of Membrane Science 219 (2003) 15–25 Optimum dialysis membrane for endotoxin blocking Masayo Hayama a , Takehiro Miyasaka b , Seiichi Mochizuki c , Hiroko Asahara b , Ken-ichiro Yamamoto a , Fukashi Kohori a , Katsuhiko Tsujioka b , Kiyotaka Sakai a,* a Department of Chemical Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan b Department of Physiology, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama 701-0192, Japan c Department of Medical Engineering, Kawasaki Medical School, 577 Matsushima, Kurashiki, Okayama 701-0192, Japan Received 30 August 2002; received in revised form 23 January 2003; accepted 2 February 2003 Abstract We have reported a novel method of visualizing endotoxin (Et) distribution inside an Et-blocking filtration membrane using both fluorescence-labeled Et and a confocal laser scanning fluorescence microscope (CLSFM) in our previous paper [J. Membr. Sci. 210 (2002) 45]. The objective of the present study is to clarify Et-blocking mechanism of dialysis membranes. Six kinds of dialysis membranes with varying materials (hydrophilic and hydrophobic) and varying structures (pore diameter, skin layer location and thickness, and water content) were evaluated by CLSFM together with other techniques such as atomic force microscopy (AFM). Physicochemical property of a membrane material affects Et-adsorbing efficiency, and further membrane structure affects Et-plugging efficiency. Rejected Et distribution in the membranes with varying materials and structures is successfully visualized using fluorescence-labeled Et by CLSFM. Et adsorption on the membranes occurs first, followed by the narrowing of their pores, and afterward pore plugging is continued. Adsorption plays a vital role in Et-blocking. Double skin layer structure is valid for preventing of Et contamination than only inner skin layer structure because the double skin layer structure blocks Et more farther from blood-side surfaces than the only inner skin layer structure. © 2003 Elsevier Science B.V. All rights reserved. Keywords: Endotoxin; Hemodialysis; Highly permeable dialysis membranes; Confocal laser scanning fluorescence microscopy; Atomic force microscopy 1. Introduction Since the finding of  2 -microglobulin (M W : 11,800) as a causal substance in the carpal tunnel syndrome of chronic hemodialysis patients,  2 -microglobulin has been removed using highly permeable dialysis mem- branes having larger pores [2–4]. Such large-pore membranes allow endotoxin (Et), harmful substances contained in the dialysate, to enter the blood [5–7]. * Corresponding author. Tel.: +81-3-5286-3216; fax: +81-3-3209-7957. E-mail address: kisakai@waseda.jp (K. Sakai). Therefore, high functional dialysis membranes are strongly desired to be developed with contradictory performance, high Et-blocking efficiency and high diffusive permeability. Moreover, a greatly sterile dialysate is needed. Various kinds of Et-blocking fil- tration membranes are presently used to remove Et from the dialysate [8–13]. Et-blocking filtration mem- branes should be further improved so that they stand long use and have excellent Et-blocking efficiency on high flux conditions. In solutions, Et is present as a dimer, a configuration like a sandwich with hydrophilic polysaccharide part outside and hydrophobic lipid A part inside. They form 0376-7388/$ – see front matter © 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0376-7388(03)00131-5