**3. Chemical structures of dialysis membrane**

#### **3.1. Main material of the membrane**

Chemical structure of the dialysis membrane usually refers to the chemical structural formula of the main material(s) of the dialysis membrane. Most chemical structural formulae of the main material of the membrane are tabulated in Table 1, including both natural and synthetic polymers. Among them AN-69®, ethylenevinylalcohol (EVAL) co-polymer, polyester polymer alloy (PEPA, Nikkiso Co., Tokyo, Japan, Figure 2) include two main materials. Actually PMMA is also a stereo complex co-polymer of two kinds of PMMA, isotactic and syndiotactic. Isotactic PMMA has acetate groups on only one side of the main chain, resulting a curled structure, whereas syndiotactic PMMA has acetate groups alternately on either side of the main chain, resulting a fairly straight structure. Combining these two polymers, membranes with low to high hydraulic permeability has been brought to realization [4].

#### **3.2. Hydrophilic agent**

In general, cellulosic membranes are hydrophilic in nature, including original RC and its derivatives such as CA, CDA, and CTA in which hydroxyl group(s) are replaced by acetate group(s). On the contrary, since synthetic polymeric membranes are originated from petrole‐ um, generally speaking they are hydrophobic in nature. Blood coagulation usually occurs soon after blood interacts with hydrophobic materials. Most synthetic polymeric membranes, therefore, include so-called hydrophilic agent such as polyvinylpyrrolidone (PVP) to make membrane hydrophilic. Figure 2 includes the chemical structure of PVP together with two other polymers (polyarrylate and polyethersulfone). PEPA is composed of these two polymers with or without PVP, the former shows little hydrophobicity, while the latter has strong adsorptive characteristic to various proteins due to its hydrophobicity (see section 4).

U.S.A.). The basic structure of the hollow-fiber dialyzer is the same as the one of multi-tube heat exchanger that is compact and has large surface area. Because of these advantages, dialyzers with hollow fiber membrane have been become widely used. The first dialyzer with a synthetic polymeric hollow fiber membrane sterilized by gamma-ray was introduced by Toray Co. (Tokyo, Japan), in which polymethylmethacrylate (PMMA) was used as a main

In order to improve solute and hydraulic permeabilities as well as biocompatibilities, many synthetic polymeric membranes have been introduced to the market since early 1980's, and currently these membranes are the main stream. Among them, polysulfone (PSf) and the like (including polyethersulfone (PES), polyarylethersulfone (PAES), etc.) have the highest market share over the world. Since these membranes are made from petroleum, they are hydrophobic in nature. Then most of these membranes include so-called hydrophilic agent that also plays a role of pore-forming agent when cast. The role of the hydrophilic agent is discussed later from the chemical (section 3), mass transport (section 4), and biological (section 5) points of

Chemical structure of the dialysis membrane usually refers to the chemical structural formula of the main material(s) of the dialysis membrane. Most chemical structural formulae of the main material of the membrane are tabulated in Table 1, including both natural and synthetic polymers. Among them AN-69®, ethylenevinylalcohol (EVAL) co-polymer, polyester polymer alloy (PEPA, Nikkiso Co., Tokyo, Japan, Figure 2) include two main materials. Actually PMMA is also a stereo complex co-polymer of two kinds of PMMA, isotactic and syndiotactic. Isotactic PMMA has acetate groups on only one side of the main chain, resulting a curled structure, whereas syndiotactic PMMA has acetate groups alternately on either side of the main chain, resulting a fairly straight structure. Combining these two polymers, membranes with low to high hydraulic permeability has been brought to realization [4].

In general, cellulosic membranes are hydrophilic in nature, including original RC and its derivatives such as CA, CDA, and CTA in which hydroxyl group(s) are replaced by acetate group(s). On the contrary, since synthetic polymeric membranes are originated from petrole‐ um, generally speaking they are hydrophobic in nature. Blood coagulation usually occurs soon after blood interacts with hydrophobic materials. Most synthetic polymeric membranes, therefore, include so-called hydrophilic agent such as polyvinylpyrrolidone (PVP) to make membrane hydrophilic. Figure 2 includes the chemical structure of PVP together with two other polymers (polyarrylate and polyethersulfone). PEPA is composed of these two polymers with or without PVP, the former shows little hydrophobicity, while the latter has strong

adsorptive characteristic to various proteins due to its hydrophobicity (see section 4).

material of the membrane [4].

166 Updates in Hemodialysis

**3. Chemical structures of dialysis membrane**

**3.1. Main material of the membrane**

**3.2. Hydrophilic agent**

view.

Figure 2-1 Chemical structures of polyester polymer alloy (PEPA) composed of PES and PAR with polyvinylpyrroridone (PVP) **Figure 2.** Chemical structures of polyester polymer alloy (PEPA) composed of PES and PAR with polyvinylpyrrori‐ done (PVP). Table 2-1 Chemical structures of cellulosic and synthetic polymeric

membranes for blood purification

**Table 1.** Chemical structures of cellulosic and synthetic polymeric membranes for blood purification.

Since PVP is water-soluble, excess amount of PVP may be rinsed out from the membrane after cast that forms pores for solute and water transport. Therefore PVP is also known as a poreforming agent. Namely, it should be understood that PVP residues in or on the membrane after rinse behave as a hydrophilic agent. Both an average and a distribution of molecular weight of PVP are important as well as the amount of PVP used in the membrane. Moreover, PVP may be cross-linked together and to the main material of the membrane by irradiating gamma-ray in the final sterilization process. With this procedure, PVP should be tightly attached together and/or on the membrane that does not allow PVP to behave as a "cushion" (cushion effect) to the blood corpuscles [5].

Acrylic acid is specifically chosen for polyacrylonitrile (PAN, Asahi Kasei, different from AN-69®) as a hydrophilic agent, whereas no hydrophilic agent is used in PMMA, EVAL and the original PEPA in which micro-layer separation technology plays a significant role in casting procedure.
