(General Student Poster Award Winner, 3rd Place) Ion Transport across Bilayer Lipid Membranes Formed in the Porous Membrane Filter
Authors: Chuang Weipai , Sowa Keisei , Kitazumi Yuki , Shirai Osamu 
Published: December 2024

Bilayer lipid membranes (BLMs), comprising of lecithin and cholesterol, are useful to analyze ion transport across the BLM in the presence of hydrophobic ions, carrier compounds, and channel proteins. The application of BLMs can prevent the unpredictable influence on other coexisting biological substances such as various enzymes and transporters within biomembranes.

However, because the physically fragility of BLMs restricts their application, it is difficult to measure the ion transport phenomena over a wide range of membrane potentials (< 0.15 V) and/or for long time. In this study, a new method for enhancing physical stability of the BLMs was developed using a track-etched membrane (TM). TM is an effective porous sheet to construct the BLMs within the cylindrical pores. BLMs were formed by dropping the n-decane solution containing lecithin and cholesterol, and then the BLMs were automatically formed within the pores of TM. The electrochemical cell employed in this study was composed by two glass chambers. The two chambers (W1 and W2) were filled with aqueous solution containing 0.1 mol dm-
3 (M) of KCl and separated by TM. The formation of BLMs within TM was confirmed by cyclic voltammetry. The capacitance and thickness of the BLMs can be described by the Eq. (1).

Here, C
m, ε
0, ε
m, A, and d are the mean capacitance of membrane, the vacuum permittivity (8.854 pF m-1), the dielectric constant of the BLM (ε
m = 2.001), the total area of BLMs (0.057 cm2), and the BLM thickness (nm), respectively. Assuming the BLM as a flat capacitor, d can be expressed by the Eq. (2).

Here, v is a potential scanning rate and I
W1-W2 is a ion transport current between W1 and W2. The C
m and d of BLMs were estimated to be 0.51 ± 0.05 μF cm-2 and 3.5 ± 0.4 nm, respectively. Comparing with those reported by the former researches, the formation of BLMs within TM was confirmed.

BLMs formed within TM were stable enough to be applied the membrane potential up to 1.5 V, as shown in Figure 1. The potential range was about 10 times larger than that of the BLM formed by the conventional method. The wide potential range and long lifetime of BLMs within TM provide the ability to measure the transport phenomena happened under high voltage.

Ions transport across the BLMs between W1 and W2 containing 0.1 M KCl was discussed. A small amount of KCl was distributed from aqueous phases to the BLM. Applying the potential difference between W1 and W2, K+ and Cl− transported in the opposite direction simultaneously, as shown in Scheme 1. When the concentration of KCl in W1 was different from that in W2, the membrane potential at the zero current (E
W1‒W2, i=0) shifted in the positive or negative direction. In the case that the concentration of KCl in W2 was r times higher than that in W1, the relationship between E
W1‒W2, i=0 and r was able to be expressed by Eq. (3).

Here, α, R, T, and F are the ratio of diffusion coefficient in BLM (D) of K+ to Cl− (α = D
K+/D
Cl−), the gas coefficient, the absolute temperature, and the Faraday constant, respectively. Based on the high linear relationship between the ln(r) and the measured E
W1‒W2, i=0, the α was calculated to be 0.3 ± 0.3. Therefore, the D
Cl− was assumed to be 3.2 times higher than D
K+.

The authors believe that this BLM-forming method can be applied to the studies of various transport phenomena and coupling reactions between electron as well as ion transports.

Figure 1