*Article* **Water Pores in Planar Lipid Bilayers at Fast and Slow Rise of Transmembrane Voltage**

**Alenka Maˇcek Lebar 1, \* ,† , Damijan Miklavˇciˇc 1 , Malgorzata Kotulska 2 , Peter Kramar 1, \* ,†**


**Abstract:** Basic understanding of the barrier properties of biological membranes can be obtained by studying model systems, such as planar lipid bilayers. Here, we study water pores in planar lipid bilayers in the presence of transmembrane voltage. Planar lipid bilayers were exposed to fast and slow linearly increasing voltage and current signals. We measured the capacitance, breakdown voltage, and rupture time of planar lipid bilayers composed of 1-pamitoyl 2-oleoyl phosphatidylcholine (POPC), 1-pamitoyl 2-oleoyl phosphatidylserine (POPS), and a mixture of both lipids in a 1:1 ratio. Based on the measurements, we evaluated the change in the capacitance of the planar lipid bilayer corresponding to water pores, the radius of water pores at membrane rupture, and the fraction of the area of the planar lipid bilayer occupied by water pores. The estimated pore radii determining the rupture of the planar lipid bilayer upon fast build-up of transmembrane voltage are 0.101 nm, 0.110 nm, and 0.106 nm for membranes composed of POPC, POPS, and POPC:POPS, respectively. The fraction of the surface occupied by water pores at the moment of rupture of the planar lipid bilayer is in the range of 0.1–1.8%.

**Keywords:** planar lipid bilayer; capacitance; voltage breakdown; water pores; hydrophobic pores
