Jennifer A. Lee

Despite the ubiquity of poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) as a transparent conducting electrode in flexible organic electronic devices, its potential as a stretchable conductor has not been fully explored. This paper describes the electronic and morphological characteristics of PEDOT:PSS on stretchable poly(dimethylsiloxane) (PDMS) substrates. The evolution of resistance with strain depends dramatically on the methods used to coat the hydrophobic surface of PDMS with PEDOT:PSS, which is cast from an aqueous suspension. Treatment of the PDMS with an oxygen plasma produces a brittle skin that causes the PEDOT:PSS film to fracture and an increase in resistivity by four orders of magnitude at only 10% strain. In contrast, a mild treatment of the PDMS surface with ultraviolet/ozone (UV/O3) and the addition of 1% Zonyl fluorosurfactant to the PEDOT:PSS solution produces a mechanically resilient film whose resistance increases by a factor of only two at 50% strain and retains significant conductivity up to 188% strain. Examination of the strained surfaces of these resilient PEDOT:PSS films suggests alignment of the grains in the direction of strain. Wave-like buckles that form after the first stretch >10% render the film reversibly stretchable. Significant cracking (∼2 cracks mm–1) occurs at 30% uniaxial strain, beyond which the films are not reversibly stretchable. Cyclic loading (up to 1000 stretches) produces an increase in resistivity whose net increase in resistance increases with the value of the peak strain. As an application, these stretchable, conductive films are used as electrodes in transparent, capacitive pressure sensors for mechanically compliant optoelectronic devices.

The phylogenetic relationships of amoebae are poorly resolved. To address this difficult question, we have sequenced 1,280 expressed sequence tags from Mastigamoeba balamuthi and assembled a large data set containing 123 genes for representatives of three phenotypically highly divergent major amoeboid lineages: Pelobionta, Entamoebidae, and Mycetozoa. Phylogenetic reconstruction was performed on approximately 25,000 aa positions for 30 species by using maximum-likelihood approaches. All well-established eukaryotic groups were recovered with high statistical support, validating our approach. Interestingly, the three amoeboid lineages strongly clustered together in agreement with the Conosa hypothesis [as defined by T. Cavalier-Smith (1998) Biol. Rev. Cambridge Philos. Soc. 73, 203-266]. Two amitochondriate amoebae, the free-living Mastigamoeba and the human parasite Entamoeba, formed a significant sister group to the exclusion of the mycetozoan Dictyostelium. This result suggested that a part of the reductive process in the evolution of Entamoeba (e.g., loss of typical mitochondria) occurred in its free-living ancestors. Applying this inexpensive expressed sequence tag approach to many other lineages will surely improve our understanding of eukaryotic evolution.