The conducting polymer poly(3,4-ethylenedioxythiophene) doped with poly(4-styrenesulfonate) (PEDOT-PSS) was patterned by micromolding in capillaries (MIMIC), in the form of nanowires on a glass or a Si wafer. The periods of the molded nanowires were 833 or 278 nm. By applying force on top of the stamp during MIMIC, the height of these nanowires could be changed. An alternative method of preparing structured surfaces is the liquid embossing technique, used to pattern polymers deposited from dispersion. Nanowires (278 nm) and 2-D nanodots on semiconducting polymer (poly(3-(2'-methoxy-5'-octyphenyl) thiophene)) POMeOPT were also achieved by another soft lithography technique-soft-embossing. The possibility to pattern both semiconducting and metallic conjugated polymers from aqueous solutions or organic solvents on a submicron level makes it possible to use these materials in flexible optoelectronic devices where light propagation and electronic paths are defined by patterning.

We have explored new organic materials and fabrication methods to fabricate organic photodiodes and light emitting diodes. Grafting of a fullerene derivative to a polythiophene backbone yielded an integrated acceptor-donor polymer that we used as the active material in organic photodiodes. Using a method of soft lithography. soft embossing, we fabricated submicron structures to be used as organic light emitting diodes. Employing a silicone rubber replica (stamp) of an optical diffraction grating we transferred the grating pattern to an organic resist layer by placing the stamp in conformal contact with the resist. The transferred pattern was subsequently used as an etch mask for the processing of the device. The structures were successfully utilized as light emitting diodes and photodiodes, with device characteristics influenced by the imposed structure. (C) 2002 Published by Elsevier Science B.V.

This thesis describes fabrication methods for optoelectronic devices (light emitting diodes (LEDs) and photodiodes) and structures for neural interfacing (neural electrodes and nerve guidance structures) using semiconducting and conducting organic materials as the active elements. Special importance has been attached to the patterning and processing of these devices. Soft lithographic patterning methods constitute the key element for the fabrication of the optoelectronic devices while photolithography is the base for the fabrication of neural electrodes.

When fabricating organic optoelectronic devices material properties of the semiconducting polymers put demands on device geometry. Low mobility of the materials sets restriction on the active layer thicknesses, usually less than 100 nm. For a photodiode a thicker layer increases light absorption but decreases the possibility to extract the generated charge. An intriguing approach to solve this conflict is to impose a light collecting structure on the active layer. Active layer thickness can be kept small while light absorption is increased. These structures may be submicrometer sized and cover large areas ( ~cm²). A soft lithographic patterning method, soft imprint, for fabricating submicrometer features over large areas was developed and used to fabricate LEDs and photodiodes.

A tentative alternative route to increase light collection in thin layers is to include three-dimensional optical elements in the device. Using selfassembly of water to form microdomes, I devised a method of fabricating micrometer sized optical lenses in a polymer substrate. These structures were successfully used as substrates for building organic photodiodes with an "inverted" geometry.

Efficient function of "standard" organic optoelectric devices relies, among else, on a thin homogenous layer of an organic film sandwiched between two metal electrodes. The last processing step of the "standard"device is the deposition of the top metal electrode, by vacuum evaporation, on the organic layer. The heat from the evaporation and the momentum of metal atoms may be destructive to the thin organic layer, creating short circuits in the device. Breaking the standard planar geometry of a device with an imposed topography, up to two orders the magnitude of the layer thickness, increases the risk of defects. An "inverted" device geometry, where the last evaporation step was substituted with a method of spin coating a conducting polymer as the top electrode, was thus explored. This approach to apply the top electrode in an "inverted" structure was proven in successful fabrication of organicphotodiodes.

A polymer approach to a neural interface was devised by the use of a polymer hydrogel electrode. Metal electrodes used for neural excitation depend on electrochemical reactions at the metal surface to generate a stimulating, faradaic, current. As unwanted products from the electrochemical reactions may have deleterious consequences for the surrounding tissue, these currents should be minimized. The polymer hydrogel electrode acts as a reservoir of charge that can be expelled by the application of a potential to the electrode. By applying a small stimulating potential, faradaic current is kept at a minimum and substituted by a capacitive current, which avoids electrochemical reactions at the interface. Simulations of a polymer hydrogel electrode show that the electrode has the electrical requirements for exciting myelinated nerve fibers. To selectively electrically address a part of a nerve fiber population, it is of interest to use biology and sort nerve fibers into different compartments with a low signal crosstalk. Using chemical cues it was possible to sort regenerating motor and sensory axons into different branches of a silicone Y-tube.

A conducting polymer hydrogel electrode was electrochemically deposited in micromachined via holes and the charge delivery capacity (CDC) was studied. Polymer hydrogel microelectrodes, with a geometric area of 1000 mum(2), and a capacitance of up to 850 nF were fabricated. The impedance of a 1,000 mum(2) polymer hydrogel electrode deposited with 8 muC was measured as low as 8.5 kOmega. We studied neural cell growth on structures to be used as neural interfaces. Directed cell growth was achieved by imposing a topographical structure on the substrate. Due to the interesting mechanical and chemical adaptability of the polymer hydrogel material and its large charge delivery capacity and low impedance we think that it is an interesting material for neural communication.

We have used a method of soft lithography, soft imprinting, to fabricate submicrometre structures to be used as light emitting polymer diodes and photodiodes. Using a silicone rubber replica (stamp) of an optical diffraction grating we transferred the grating pattern to an organic resist layer by placing the stamp in conformal contact with the resist. The transferred pattern was subsequently used as an etch mask for the processing of the device. This cheap and fast process, not limited by optical diffraction, was used to fabricate submicrometre structures over large areas, square millimetres. The structures were successfully utilized as light emitting diodes and photodiodes, with device characteristics influenced by the imposed structure.

The functional outcome of microsurgical repair of divided nerves is disappointing since many regenerating axons fail to reach appropriate targets. Sorting of regenerating axons according to target tissue might be used to improve functional regeneration. The aim of the present study is to see if regenerating axons can be sorted into functionally different bundles with target-derived molecules. The proximal stump of the adult rat sciatic nerve was sutured into the inlet of a silicon Y-tube. The two branches of the Y-tube were filled with agarose primed with filtrates prepared from skin and muscle homogenates from the operated rat. The tibial and sural nerves were inserted in the two branches of the Y-tube. Six weeks later the sciatic nerve axons showed vigorous regeneration into both branches. Electron microscopic examination of regenerated nerve segments showed numerous myelinated and unmyelinated axons. The proportion of myelinated axons was significantly larger in the muscle-gel branch than in the skin-gel branch. Retrograde tracing from the nerve regenerates with Fast Blue and Fluoro-Ruby showed that ventral horn neurons at L4–L5 segmental levels were preferentially labeled from the muscle-gel branch. Neurons in corresponding dorsal root ganglia were labeled from both Y-tube branches (no significant numerical difference). A few neurons of both types contained both tracers. Measurements revealed that sensory neurons labeled from the muscle-gel branch were significantly larger (mean perikaryal area 870 μm²) than neurons labeled from the skin-gel branch (mean area 580 μm²). We conclude that regenerating motor and sensory axons can be sorted with target-derived molecules.

Increasing the conversion efficiency is very important in photovoltaic devices, as is cheap and simple technology. Here is demonstrated a soft embossing technique for printing a submicrometer grating with an elastomeric mold into an optically active polymer layer in a photovoltaic device (see Figure). The light trapping due to the grating pattern enhances the photoconversion efficiency by more than 25 % at normal light incidence (see also inside front cover).