Microtubules that were made from a phospholipid and coated with copper to become electrically conductive were incorporated into a polyurethane-polydimethylsiloxane (PU-PDMS) interpenetrating polymer network. The tubule concentration ranged from 8 to 16 vol %, whereas the PDMS concentration ranged from 0 to 75 wt %. The composites' dielectric properties and morphology were characterized by using a vector network analyzer and an optical microscope. In the composites the tubules became segregated into the polyurethane phase and were excluded from the PDMS phase. This forced the tubules closer together and resulted in the sample being closer to percolation. Consequently, for a specific tubule concentration, the real part of the permittivity increased for higher PDMS concentrations. In addition, adding tubules changed the morphology of the matrix. The dispersed PDMS globules were smaller in the composites than in the neat interpenetrating polymer network. The reason was that the tubules increased the viscosity of the system, which prevented the smaller PDMS globules from coalescing to form larger globules. (C) 2003 Wiley Periodicals, Inc.