Biological supramolecular assemblies, such as phospholipid bilayer membranes, have
been used to demonstrate signal processing via short-term synaptic plasticity (STP) in
the form of paired pulse facilitation and depression, emulating the brain’s efficiency and
flexible cognitive capabilities. However, STP memory in lipid bilayers is volatile and
cannot be stored or accessed over relevant periods of time, a key requirement for learning.
Using droplet interface bilayers (DIBs) composed of lipids, water and hexadecane, and
an electrical stimulation training protocol featuring repetitive sinusoidal voltage cycling,
we show that DIBs displaying memcapacitive properties can also exhibit persistent synaptic plasticity in the form of long-term potentiation (LTP) associated with capacitive
energy storage in the phospholipid bilayer. The time scales for the physical changes
associated with the LTP range between minutes and hours, and are substantially longer
than previous STP studies, where stored energy dissipated after only a few seconds. STP
behavior is the result of reversible changes in bilayer area and thickness. On the other
hand, LTP is the result of additional molecular and structural changes to the zwitterionic lipid headgroups and the dielectric properties of the lipid bilayer that result from
the buildup of an increasingly asymmetric charge distribution at the bilayer interfaces.
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