Conjugated-Polymer Micro- and Milliactuators for Biological Applications
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Conjugated-Polymer Micro- and Milliactuators for Biological Applications
C. Immerstrand, K. Holmgren-Peterson, K.-E. Magnusson, E. Jager, M. Krogh, M. Skoglund, A. Selbing, and O. Inganäs Abstract The development of new conjugated-polymer tools for the study of the biological realm, and for use in a clinical setting, is reviewed in this article. Conjugated-polymer actuators, based on the changes of volume of the active conjugated polymer during redox transformation, can be used in electrolytes employed in cell-culture media and in biological fluids such as blood, plasma, and urine. Actuators ranging in size from 10 m to 100 m suitable for building structures to manipulate single cells are produced with photolithographic techniques. Larger actuators may be used for the manipulation of blood vessels and biological tissue. Keywords: artificial muscles, electroactive organic materials, biomaterials, conjugated polymers.
Introduction Conjugated polymers have added a new dimension to the field of polymer materials; electronic and photonic processes typical of semiconductors and metals can be implemented in soft and fusible polymers. This electronic structure is crucial for inducing charge on the polymer chain, whether by chemical, electrochemical, optical, or electrical methods. By the appropriate combination of the electrochemical properties of conjugated polymers with the structural properties of polymers, new material hybrids appear that can be made stiff or soft by electrochemical oxidation and reduction processes; they may also be induced to swell or to shrink. Redox processes in conjugated polymers lead to geometrical changes in the polymer chain; the introduction of charge on the polymer chain leads to polaron (radical cation) formation with concomitant changes of bond lengths
MRS BULLETIN/JUNE 2002
and chain conformation. This realization led to the suggestion, in the early 1990s, that conjugated polymers may be used as mechanical actuators.1,2 Several groups have pursued this goal,3–13 leading to numerous studies of redox-induced volume change in conjugated polymers, to a large degree related to ion and solvent insertion/ deinsertion in the polymer.14 More recently, the development of carbon nanotube actuators has been pursued, where the higher elastic moduli of nanotubes is a feature that could lead to strong charge-induced actuation in the tubes.15 As ion transport from an ion-storage medium (an electrolyte) to a solid material is common for both types of electroactive actuators, whether based on nanotubes or polymers, the rate of ion transport is crucial to the scalability of the actuators. With solid polymers like polypyrrole, which
can easily be combined with aqueous electrolytes, ion diffusion is slow. This rewards thinner layers of polymer in which the mechanical changes may be more rapid due to shorter diffusion lengths. It is therefore attractive to use thin layers of polypyrrole in making artificial micromuscles, in order to reduce the length of ion transport. These thin actuators will, however,
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