Ion channels have three conceptual elements which determine the nature of a channel, namely the gate, the selectivity filter and the unitary conductance.
The gate rules opening and closing of a channel. It's operation is regulated by a set of rate constants which are functions of membrane potential (voltage gated channel), agonist concentration (chemically gated channel) or some other physico-chemical elements (sensory transducing channel).
The selectivity filter determines the ionic species which can permeate through the channel. Because of this property, the channel can have a reversal potential which is strictly determined by the activity ratio of the intra-and the extra-cellular ions selected.
The ionic channel has relatively large unitary conductance in general, resulting in the mass-transportation of ions across the biological membrane.

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Dynamic characteristics of a model membrane constructed from a porous filter impregnated with synthesized lipids, dioleyl phosphate (DOPH), are studied. This model membrane shows a change in electric properties with the salt concentration. It is shown that a relaxation from the low-capacitance to high-capacitance state exhibits a sigmoid curve with time delay, while the transition from the high- to low-capacitance state is a simple exponential type. A hysteresis loop is time-dependent. A theoretical model of phase transition between oil droplets, spherical micelles and bilayers explains the experimental results. When the membrane is placed under the salt-concentration gradient and sometimes under the electric current and the pressure difference, the oscillation of electric potential across the membrane appears spontaneously. The burst as often found in neurons is also observed. The theoretical model proposed is able to describe the above oscillations, and shows that nonlinear coupling of phase transition of DOPH molecules with salt flow plays an essential role on the oscillations.

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Recent research progresses on membrane technologies to fermentation production processes are reviewed. First, a cross flow filtration of suspended yeast cells by a hollow fiber module was carried out to enhance the ethanol productivity in repeated fedbatch culture. The ethanol productivity obtained in this system was 11.3g ethanol/litre/h compared to 3 in a conventional Melle-Boinnot method. Second, in ethanol fermentation, an immobilized yeast cells system capable of forced permeation of the medium was developed in contrast to conventional immobilized yeast cell system where the mass transfer depends on “diffusion”. In this new system, the cell density on the surface of gel (1 mm thickness) became 1.7×10¹⁰ cells/ml gel compared to 4×10⁹ in the conventional system. Hence, the ethanol productivity in this system could increase 60g ethanol/ litre gel/h compared to 20 to 30 in conventional packed bed systems. Third, acetic acid fermentaion from ethanol was conducted in a hollow fiber module bioreactor where the bacterial cells absorbed on the fiber surface could consume oxygen running through the inside of hollow fiber and ethanol running through the outside the fiber. Thus, a mimic vineger production of conventional surface cluture could be established with a remarkable increase of the surface area per reactor volume. Finally, a sulfonated polysulfone membrane was used for in situ regeneration and retention of coenzymes NADP (H) using xylose reductase coupled with oxidoreductase system in the reduction of xylose to xylitol with hydrogen gas. The membrane could almost completely reject the permeation of NADP (H) and F420 and the required enzymes, but not reject for permeation of xylitol.

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Since the application fields of ultrafiltration (UF) are expanding very rapidly, an UF membrane must have various kinds of characteristics. In this article, the UF membranes developed recently are reviewed from the literatures published after 1980.
Recently, two kinds of heat-resistant UF membranes have been developed by using polyether-sulfone as membrane material. In particular, the commercially available DUS-40 membrane resists the sterilization by pressurized hot water at 128 °C.
Several kinds of solvent-resistant UF membranes have been developed by using various types of polyimide. One of these membranes has already been commercialized.
As the highly functional UF membrane, various kinds of negatively and positively charged UF membranes have been developed. Applications of the charged UF membranes are still under development.
Many types of UF membranes have also been developed by using various kinds of polymers.
Inorganic materials were also used in order to make UF membranes having large heat- and solvent-resistance.
Several kinds of porous ceramic membranes and porous glass membranes have, already been commercialized. Many of these membranes have pores whose diameters are 0.2-10, μm, and are mainly used as a microfiltration membrane.
Porous aluminium membranes are now under developing.
Dynamically formed inorganic UF membranes have been developed by using several kinds of inorganic colloids.

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The Millipore DOPH model membrane is composed of a Millipore filter whose pores are filled with a lipid analogue, dioleylphosphate (DOPH). The electric resistance of the Millipore DOPH membrane is increased in the mixed salt solution such as KCl and CaCl₂, and it oscillates rhythmically under which a current and a pressure stimulus are applied simultaneously. This paper showed electric current (I) -voltage (V) characteristics to discuss the plausible mechanism of the resistance oscillation, because I-V characteristics were one of important properties to described the mechanism of an electric oscillation, and these had not been examined in the Millipore DOPH membrane. The current and voltage responses were observed by using the current or voltage clamp circuits which were made by us. The current or the voltage increased linearly till 0.15 μA or 400mV, but above its oscillatory changes appeared. These meaned that it was difficult to discuss the oscillation mechanism with only I-V characteristics as compared with living cell membranes. The Millipore DOPH membrane consists of many small membranes such as oil droplets, spherical micelles and multilayers formed in filter pores. The oscillations observed under clamps come from this complex structure. From I-V characteristics, however, the structure of the Millipore DOPH membrane could be demonstrated by the simplest model connected with capillaries. Using the capillary model it could be easy illustrated that the sustained oscillation was caused by the conformational change of the small membrane in one capillary.

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Cellulose membranes for ultrafiltration were prepared by the casting method with dimethyl acetamide-LiCl system as the solvent. The unique point is that organic solvents were employed as the gelating solutions instead of aqueous solutions.
In this report, the characteristics of the membrane were described from the viewpoint of the relation between the fabrication process and the membrane structure. Moreover, water state in the membrane was discussed.
The properties of the membrane for hemodialysis were also investigated in comparison with those of Cuprophan membrane.
The membrane exhibited the excellent properties in that the solute permeabilities, especially in case of solutes with large molecular weight, were larger than those of Cuprophan.

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Separation of gaseous mixture of CO₂ and CH₄ was carried out using composite microporous glass membrane on ceramic tubing of which permeability are greater than that of the composite membrane by a factor of 10⁸.
The maximum separation. factor was obtained with composite membrane made, of casting solution of which ratio of B₂O₃ and SiO₂ was 1/4.
Composite membrane coated less than 5 times shows no ability of separation.
Carbon dioxide seems permeate through composite membrane obtained with decreased content of B₂O₃ (B₂O₃ : SiO₂= 1 : 14) by the surface diffusion mechanism on the pore.

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