Electrical Impedance Analysis of Phospholipid Bilayer Membranes for Enabling Engineering Design of Bio-based Devices
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Electrical Impedance Analysis of Phospholipid Bilayer Membranes for Enabling Engineering Design of Bio-based Devices Stephen A. Sarles, Vishnu B. Sundaresan, and Donald J. Leo Mechanical Engineering, Virginia Tech, 310 Durham Hall, Blacksburg, VA, 24061 ABSTRACT Recent research at Virginia Tech have shown that active transporter proteins reconstituted into suspended bilayer lipid membranes (BLMs) formed across an array of pores in synthetic substrates can convert chemical energy available in adenosine triphosphate (ATP) into electricity. Experimental results from this work show that this system—called BioCell—is capable of 1.7µW of electrical power per square centimeter of BLM area and per 15µL of ATPase enzyme. In support of such a system, the lipid membrane, as host to active biological proteins and channels, must be formed evenly across a porous substrate, remain stable and yet fluid-like for protein folding and activation, and provide sufficient electrical insulation. We report on the formation and characterization using electrical impedance spectroscopy (EIS) of BLMs formed across two types of porous substrates: polycarbonate filters and single-aperture silicon substrates. Equivalent electrical circuits describing the lipid membranes and their supporting substrates are approximated to fit the measured responses. The results show that BLMs formed in some but not all of the 400nm pores of the filters, while the formation of BLMs on the single-aperture silicon substrates was much more consistent. INTRODUCTION The role of the supporting substrate is critically linked to the stability of the lipid membrane(s) as well as the focus of the experiment. The Biocell, developed at Virginia Tech [1,2], features H+ATPase transmembrane proteins extracted from red beets and reconstituted into suspended planar BLMs that hydrolyze available adenosine triphosphate (ATP) in order to pump protons (H+) through the membrane. The established pH gradient across the membrane can then be sourced as electrical current (40µA/cm2/15µL ATPase) to external circuits. Initial studies on the Biocell verified the feasibility of a chemo-electric energy-conversion system containing millions of BLMs. Having a large number of BLMs provides increased membrane area for protein insertion, which may be ideal in an energy-conversion device, and offers higher stability and rupture strength than a single large BLM [3]. Single BLM systems, on the other hand, offer increased resolution of protein insertion and reduce the sensitivity of electrical measurements dominated by open pores (those lacking a BLM) [4]. In this paper, we investigate the formation of suspended BLMs on two different types of synthetic substrates: multi-pore polycarbonate filters and single aperture, silicon-based chips. Electrical impedance measurements are used to derive equivalent electrical circuits that describe the physical structure of planar BLMs. MATERIALS AND METHODS Suspended phospholipid membranes were formed across the pores of synthetic substrates using a 20mg/m
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