  Zhelev and Needham have recently created large, quasi-stable pores in
artificial lipid bilayer vesicles [Biochim. Biophys. Acta 1147 (1993) 89].
Initially created by electroporation, the pores remain open for up to several
seconds before quickly snapping shut. This result is surprising in light of the
large line tension for holes in bilayer membranes and the rapid time scale for
closure of large pores. We show how pores can be dynamically stabilized via a
new feedback mechanism. We also explain quantitatively the observed sudden pore
closure as a tangent bifurcation. Finally we show how Zhelev and Needham's
experiment can be used to measure accurately the pore line tension, an
important material parameter. For their SOPC/CHOL mixture we obtain a line
tension of 2.6 10^{-6} erg/cm.
