Priscilla G. Taylor

Detection of Transmitter Release from Single Living Cells Using Conducting Polymer Microelectrodes

The advent of organic electronics has made available a host of materials and devices with unique properties for interfacing with biology.1–2 One example is the use of conducting polymer coatings on metal electrodes that are implanted in the central nervous system and interface electrically with neurons, providing stimulation and recording the neurons electrical activity.3–5 Coating a metal electrode with a conducting polymer has been shown to lower the electrical impedance and decrease the mechanical properties mismatch at the interface with tissue, with beneficial effects on the lifetime of the implant.3, 6 Conducting polymers can also be functionalized with biomolecules that stimulate neural growth and minimize the immune response to the implant.3–5, 7 Other examples are organic electronic ion pumps,8 and ion transistors,9 which are recently invented devices capable of precise delivery of neurotransmitters to neurons. These devices were recently implanted in the ear of a guinea pig and were shown to control its hearing.10 Conducting polymers such as poly(3,4-ethylenedioxythiophene) doped with poly(styrene sulfonate) (PEDOT:PSS), a material that has been shown to be biocompatible with a variety of different cells,1 have been used for these applications. These examples highlight the main advantages that organic electronic materials bring to the interface with biology, including their “soft” nature, which offers better mechanical compatibility with tissue than traditional electronic materials, and natural compatibility with mechanically flexible substrates, which paves the way for the development of implants that better conform to the non-planar shape of organs. Finally, the ability of organics to transport ionic in addition to electronic charge creates the opportunity to interface with electrically active cells in novel ways, as the work on ion pumps indicates.

Acid-sensitive semiperfluoroalkyl resorcinarene: an imaging material for organic electronics.

An acid-sensitive semiperfluoroalkyl resorcinarene was synthesized, and its lithographic properties were evaluated. Its solubility in segregated hydrofluoroether solvents enables the patterning of delicate organic electronic materials.

Orthogonal processing: A new strategy for organic electronics

The concept of chemical orthogonality has long been practiced in the field of inorganic semiconductor fabrication, where it is necessary to deposit and remove a layer of photoresist without damaging the underlying layers. However, these processes involving light sensitive polymers often damage organic materials, preventing the use of photolithography to pattern organic electronic devices. In this article we show that new photoresist materials that are orthogonal to organics allow the fabrication of complex devices, such as hybrid organic/inorganic circuitry and full-colour organic displays. The examples demonstrate that properly designed photoresists enable the fabrication of organic electronic devices using existing infrastructure.

Fabrication of polymer-based electronic circuits using photolithography

We exploited the concept of solvent orthogonality to enable photolithography for high-resolution, high-throughput fabrication of electronic circuits based on a polymeric semiconductor. An array of ring oscillators utilizing top contact polymer thin film transistors with 1 μm channel length has been fabricated on a 100 mm wafer scale. We used high performance, air stable poly(2,5-bis(thiophene-2-yl)-(3,7-ditri-decanyltetrathienoacene) as our active semiconducting material. Owing to the small channel length and small overlap length, these devices have a signal propagation delay as low as 7 μs/stage.

Cross-linkable molecular glasses: low dielectric constant materials patternable in hydrofluoroethers.

We report a new approach to solution-processable low-dielectric-constant (low-k) materials including photolithographic patterning of these materials in chemically benign and environmentally friendly solvents. A series of semiperfluorinated molecular glasses with styrenic substituents were successfully synthesized. These small molecular materials were thermally stable up to 400 degrees C and also exhibited an amorphous nature, which is essential to forming uniform films. Differential scanning calorimetry studies revealed that a cross-linking reaction occurred in the presence of acid, resulting in the formation of robust polymeric films. Atomic force microscopy images of the cross-linked films showed uniform and pinhole-free surface properties. Dielectric constants determined by a capacitance measurement were 2.6-2.8 (100 kHz) at ambient conditions, which are comparable to other polymeric low-k materials. The incorporation of semiperfluorinated substituents was effective in decreasing the dielectric constant; in particular, the fluorinated alkyl ether structure proved best. In addition, the fluorinated substituents contributed to good solubility in hydrofluoroether (HFE) solvents, which enabled the successful photolithographic patterning of those materials in HFEs down to a submicrometer scale.

Orthogonal patterning of multiple biomolecules using an organic fluorinated resist and imprint lithography.

The ability to spatially deposit multiple biomolecules onto a single surface with high-resolution while retaining biomolecule stability and integrity is critical to the development of micro- and nanoscale biodevices. While conventional lithographic patterning methods are attractive for this application, they typically require the use of UV exposure and/or harsh solvents and imaging materials, which may be damaging to fragile biomolecules. Here, we report the development of a new patterning process based on a fluorinated patterning material that is soluble in hydrofluoroether solvents, which we show to be benign to biomolecules, including proteins and DNA. We demonstrate the implementation of these materials into an orthogonal processing system for patterning multibiomolecule arrays by imprint lithography at room temperature. We further showcase this methods capacity for fabricating patterns of receptor-specific ligands for fundamental cell studies.

High voltage polymer solar cell patterned with photolithography

An acid-sensitive semiperfluoroalkyl resorcinarene has recently been demonstrated as an effective photoresist for the photolithographic patterning of organic semiconductor materials. In this work, we show that it can be used to pattern polymer solar cells fabricated from a blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61 butyric acid methyl ester (PCBM), so as to obtain high open circuit voltages (Voc). An array of 300 solar cells in series, with a period of 50 µm, achieved a Voc of 90 V and a power conversion efficiency (PCE) of 0.3%.

Acid-diffusion behaviour in organic thin films and its effect on patterning

Acid and its formation and placement is one of the most important aspects in the chemically amplified photolithographic process. The choice of photoacid generator (PAG) in the photolithographic patterning of acidic substrates, such as PEDOT:PSS, has consequences for the resolution and overall quality of the patterned image. In this study, an acid exchange and diffusion mechanism is proposed for the undesired decomposition of the unexposed photoresist layer containing ionic PAGs. The use of non-ionic PAGs has been shown to be a solution to this decomposition problem. In addition, the acidic nature of the PEDOT:PSS substrate is employed to produce patterned images of a cross-linkable light-emitting polymer. With further optimization and development, this is potentially a fast and simple method to introduce patterns in various organic electronic devices.

Orthogonal lithography for organic electronics

Organic electronics has recently gained attention as a new field promising cheaper, flexible, and large-scale devices. Although photolithography has proven to be a high-resolution and high-throughput patterning method with excellent registration capabilities, the emerging field of organic electronics has been largely unsuccessful in adapting this well-established method as a viable approach to patterning. Chemical compatibility issues between organic materials and the processing solvents and chemicals required by photolithography have been the main problem. This challenge has led us to identify a set of non-damaging processing solvents and to develop alternative imaging materials in order to extend photolithographic patterning methods to organic electronics. We have identified supercritical carbon dioxide and hydrofluoroether (HFE) solvents as chemically benign to organic electronic materials and which are also suitable as processing solvents. We refer to these solvents as orthogonal in that they do not substantially interact with traditional aqueous and organic solvents. Multi-layered devices are easily realized by exploiting this orthogonality property; subsequent layers are deposited and patterned without damaging or otherwise adversely affecting previously deposited underlying layers. We have designed and synthesized novel photoresists, which are processible in these benign solvents.