Hollow metal cylinders produced from diacetylenic lipid

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A method was presented for the formation and metallization of cylindrical tubules from a diacetylenic lipid. This improved technique allowed for the production of metal microcylinders without the need for preliminary lipid purification and in large quantities. The physical and electrical properties of the material were investigated, and composites were used to form parallel plate capacitors. A comparison of the conductivity of the bulk material with the derived conductivity of a composite showing electromagnetic percolation showed the proportionality of the specific packing density and the critical volume fraction characteristic of percolating systems.

I. INTRODUCTION

Organic tubules formed from a diacetylenic lipid were first described by Yager and Schoen1 and achieved by a process of liposomal cooling. The lipid 1,2-bis(10,12tricosadiynoyl)-sn-glycero-3-phosphocholine (DC-8,9PC) was heated in water above 50 °C and cooled. Upon being cooled below the melting temperature (42 °C), the liposomes were observed to convert to tubules of submicron diameter. Georger et al.2 demonstrated how tubules could be generated from the same lipid by the addition of water as a nonsolvent to a solution of lipid in alcohol or propylene glycol. Tubules were allowed to grow for periods up to 6 months and resulted in average lengths of up to 100 microns. Ratna et al.3 investigated more closely the role of alcohols in tubule formation. DC-8,9-PC in alcohol/water solutions was heated to 60 °C, and tubules were formed by cooling to room temperature. The solvent 85% methanol was found to yield tubules with the greatest lengths with an average of 65 microns. Schnur et al.4 reported the preparation of lipid tubules with a metal coating. Permalloy-coated tubules have been used in composites to produce high-dielectric, lowloss materials by aligning the tubules with a magnetic field.5,6 A similar technique was used to produce composites with significant ferromagnetic properties.7 A successful application of this technology was demonstrated by Chow et al.8 Tubules were aligned in a magnetic field and cast in epoxy. Subsequently the epoxy was etched away and the surface sputter-coated with gold. This structure was used to show vacuum field emission of >10 ␮A. Progress in the application of tubule technology has been held back by the nonuniformity and incompleteness of the metal coating.8 Further, it was shown that the coating consisted in a large part of tin and nickel oxides rather than a metallic nickel alloy. Reported here is the 2368

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development of techniques for the metallization of lipid tubes to form cylinders with the properties of bulk metal, while retaining the characteristic high aspect ratio and light weight. This material has been used to create artificial dielectric composites for use in the investigation of electromagnetic percolation.9 II. EXPERIMENTAL PROCEDURES

The lipid 1,2-bis(10,12-tricosadiynoyl)-sn-glycero-3phosphocholine was purchased from J.P. Laboratories, Middlesex, NJ. To increase the t