**8. Conclusions**

Since hemodialysis experiments with canines were first reported, many membranes, either natural or synthetic polymeric ones, have been developed and the latter have been the main stream due to higher solute and hydraulic permeabilities as well as better biocompatibility. The mass transport mechanism across the membrane can be expressed by the Fick's 1-st law of diffusion; however, not only the membrane permeability but also the design specifications are important for assembling dialyzers with better performances.

The chemical structure of the dialysis membrane determines the hydrophilicity and hydro‐ phobicity of the membrane. Since all synthetic polymeric membranes are made from petrole‐ um, they are hydrophobic in nature. Most of these membranes include a hydrophilic agent such as PVP for anti-thrombosis purpose. According to the in vitro experiments and clinical observations, it was proved that PVP was closely related to the sieving coefficient for albumin and had big influence on the complement activation. Then we must pay much attention on additives in addition to the main material(s) of the membrane.

Physical structure of the dialysis membrane can be discussed in two ways, i.e., direct obser‐ vations by taking microscopic views (SEM) and the theoretical analysis by using a mathemat‐ ical model. There are two kind of dialysis membranes, "homogeneous" and "asymmetry", among which the latter is gaining popularity because of the much smaller thickness that contributes to the resistance of solute and water transport. The pore theory is a useful tool for analyzing mass and water transport across the membrane and for designing a physical structure of the membrane.

Since the number of on-line hemodiafiltration (HDF) is growing these days not only from the solute removal point of view but also from the improvement of dialysis-induced hypotension during the treatment, membranes specifically designed for performing HDF has to be more extensively studied both clinically and fundamentally. Importance of biocompatibility of the membrane should be more carefully taken into account for selecting a device, considering membrane characteristic such as adsorption, especially in the field of acute kidney injury (AKI).
