Recent results have demonstrated that lipid bilayers have the ability to "self-heal" after mechanical failure. In a previous
study the maximum pressure that could be withstood by a bilayer lipid membrane (BLM) formed over a porous substrate
was measured and reported. This paper expands on this subject by exploring the ability of a BLM to spontaneously "self-heal"
or reform after having been pressurized until failure. A 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine
(SOPC) BLM is reconstituted over a silicon substrate that contains a single square aperture (25 x 25 μm) and is
pressurized until failure. It is found that the BLM spontaneously reforms multiple times over the aperture after the initial
failure. For each experiment the BLM is subjected to several pressurization cycles with a 70 mV potential applied
across the BLM. The current is measured using an impedance analyzer and indicates the presence of a BLM formed
over the aperture. It is found that electrical current conducted across the BLM increases from approximately 100 pA to
650 nA during each BLM failure and returns to 100 pA after BLM reformation. These results demonstrate that the
bilayer is reforming because the electrical resistance across the aperture is increasing by several orders of magnitude.
A new methodology has been developed to measure the mechanical integrity of a bilayer lipid membrane (BLM)
formed over porous substrates. A custom test fixture was fabricated in which a stepper motor linear actuator
drives a piston in order to apply pressure to a BLM in very fine increments. The pressure, monitored with a
pressure transducer, is observed to increase until the BLM reaches its failure pressure, and then drop. This
experiment was performed on 1-Stearoyl-2-Oleoyl-sn-Glycero-3-Phosphocholine (SOPC) lipid bilayers formed
over porous polycarbonate substrates with various pore sizes ranging from 0.05 - 10 &mgr;m in diameter. A trend
of increasing failure pressure with decreasing pore size was observed. The same set of experiments was repeated
for BLMs that were formed from a mixture of SOPC and cholesterol (CHOL) at a cholesterol concentration of
50 mol%. The presence of cholesterol was found to increase the failure pressure of the BLMs by 1.5 times on
average. A model of the characteristic pressure curve from this experiment was developed based on an initially
closed fluid system in which pressure increases as it is loaded by a moving piston, and which upon reaching a
critical failure pressure allows pressure to decrease as fluid escapes through a porous medium. Since the BLM
is formed over many pores, this model assumes that the failure pressure for each micro-BLM follows a normal
distribution over all pores. The model is able to accurately predict the major trends in the pressurization curves
by curve-fitting a few statistical parameters.
The motion and growth of plants is the inspiration for a new biomimetic actuator that uses fluid transport across a bilayer lipid membrane (BLM) to create internal pressure and cause displacement in the actuator. In order for the actuator to be viable the BLM must be able to withstand this internal pressure without failing. In this study BLMs are formed over a porous polycarbonate substrate and a hydrostatic pressure is applied to the BLM and gradually increased until it fails. This test is performed over different pore sizes to measure the failure pressure of the BLM as a function of pore radius. A similar test is used for polymer films to compare the failure pressure trends of a BLM to conventional engineering materials. The polymer films and BLMs are modeled as a simply supported circular plate under uniform load, first with the assumption of small deflections and then with the assumption of large deflections. It was found that the large deflection model better represents the trend of failure pressure versus pore radius than the small deflection model.