  Animal cells are both encapsulated and subdivided by lipid bilayer membranes.
Beyond just acting as boundaries, these membranes' shapes influence the
function of cells and their compartments. Physically, membranes are
two-dimensional fluids with complex elastic behavior, which makes it
impossible, for all but a few simple cases, to predict membrane shapes
analytically. Instead, the shape and behavior of biological membranes can be
determined by simulations. However, the setup and use of such simulations
require a significant programming background. The availability of open-source
and user-friendly packages for simulating biological membranes needs
improvement.Here, we present flippy, an open-source package for simulating
lipid membrane shapes, their interaction with proteins or external particles,
and the effect of external forces. Our goal is to provide a tool that is easy
to use without sacrificing performance or versatility. flippy is an
implementation of a dynamically triangulated membrane. We use a precise yet
fast algorithm for calculating the geometric properties of membranes and can
also account for local spontaneous curvature, a feature not all discretizations
allow. Finally, in flippy we can also include regions of purely elastic
(non-fluid) membranes and thus explore various shapes encountered in living
systems.
