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A technique to fabricate electrically conductive all-polymer 3D microstructures is reported. Superior conductivity, high spatial resolution and three-dimensionality are achieved by successive application of two-photon polymerization and in situ oxidative polymerization to a bi-component formulation, containing a photosensitive host matrix and an intrinsically conductive polymer precursor. By using polyethylene glycol diacrylate (PEG-DA) and 3,4-ethylenedioxythiophene (EDOT), the conductivity of 0.04 S/cm is reached, which is the highest value for the two-photon polymerized all-polymer microstructures to date. The measured electrical conductivity dependency on the EDOT concentration indicates percolation phenomenon and a three-dimensional nature of the conductive pathways. Tunable conductivity, biocompatibility, and environmental stability are the characteristics offered by PEG-DA/EDOT blends which can be employed in biomedicine, MEMS, microfluidics, and sensorics.
© 2013 Optical Society of America
1. Introduction
Two-photon polymerization (2PP) is an additive 3D manufacturing technology based on highly localized absorption of femtosecond laser pulses in photoresist materials [1,2]. 2PP offers sub-100 nm resolution allowing the realization of photonic metamaterials, microfluidic and biomedical devices, MEMS and actuators [3–6]. Scaffold structures were also fabricated by 2PP and tested for regenerative medicine and tissue engineering [7]. Excellent results on structuring by 2PP were achieved with hybrid organic/inorganic sol-gels [8], epoxy [9], and acrylic based negative-tone resins [10]. So far, the greatest efforts were made to address 2PP processability, structuring resolution, and mechanical characteristics of photoresist materials. However, the increasing demand for smart materials requires the broadening of photoresist functionalities. Molecular devices [11], ion-exchange media [12], chemical sensors [13], and organic photovoltaic [14] will benefit from the technology that enables 3D microstructuring of electrically conductive polymer materials. Recent reports show, that 2PP can be applied for mechanical mixtures of photoresists with single-wall nanotubes (SWNT) [15,16], graphene and ionogels [17]. However, only low conductivities of up to 1x10−6 S/cm have been demonstrated with these materials.
Intrinsically Conductive Polymer (ICP) materials are attracting much interest since their first discovery in 1977 [18]. They offer high electrical conductivity owing to the conjugated electronic system that can be doped through oxidation. This doping allows to tune electrical conductivity, optical transparency, mechanical, and other physical properties [19]. However, ICP materials have limited solubility, and do not have melting and glass-liquid transitions. Therefore, extrusion, injection, thermoforming, or other conventional 3D polymer processing techniques are not applicable. Only planar technologies with ICP materials achieved an industrial success either by polymerizing ICPs in situ or solubilizing and applying casting, inkjet, or screen printing [20–22]. The undergoing research in this field is aimed to achieve high structuring resolution by stamping [23], photoinduced doping [24], UV lithography [25] and thin film nanoimprint lithography [26]. An attempt to pattern ICP materials in 3D directly via two-photon absorption was also reported [27]. However, only photopolymerization inside supporting media was demonstrated and no measurements of the electrical conductivity were made.
2. Experimental technique
The largest obstacle in obtaining free-standing 3D microstructures of ICPs directly by 2PP is their strong absorption in the visible and near infrared spectral ranges by polaron carriers [28]. In contrast to polymerized ICPs, several of their precursors, such as pyrrole, aniline or 3,4-ethylenedioxythiophene, are optically transparent. In this contribution, we propose a 3D processing technique that combines 2PP and in situ oxidative polymerization of an ICP precursor. A suitable formulation can be produced by dissolving a transparent precursor in a liquid photoresist. The processing workflow is illustrated in Fig. 1. The fabrication starts by scanning a focused femtosecond laser beam in the volume of the material formulation to create a desired microstructure by 2PP in the photoresist. The microstructure functions as a host matrix and encapsulates the ICP precursor. Next, the microstructure is developed by removing the uncured photoresist and non-encapsulated ICP precursor in a solvent bath. Subsequently, electrical conductivity of the microstructure is obtained by in situ oxidative polymerization of the precursor. We examine 3,4-ethylenedioxythiophene (EDOT) monomer as the ICP precursor. After in situ oxidative polymerization, EDOT is converted into poly(3,4-ethylenedioxythiophene) (PEDOT) which is characterized by high conductivity, environmental stability, and biocompatibility [29–31]. The accelerated oxidative polymerization of EDOT is achieved using an undiluted oxidizer melt. As a host matrix poly(ethylene glycol) diacrylate (PEG-DA) is selected. PEG-DA is a biocompatible and 2PP-curable compound that can be photosensitized by addition of a free-radical photoinitiator. Mechanical and physical characteristics of PEG-DA can be flexibly adjusted by selecting oligomers with an appropriate molecular weight [32]. The important aspect of the separation of 2PP and oxidative polymerization processes is the freedom to choose and optimize oxidizing agents and host matrix material independently.
Fig. 1 The workflow of 3D ICP processing based on 2PP. 1) Direct 2PP fabrication of the microstructures in photoresist/ICP precursor formulation, 2) Removal of uncured photoresist, 3) Oxidative polymerization of the encapsulated ICP precursor, 4) Removal of the remaining soluble byproducts.
2.1 Preparation of PEG-DA/EDOT blends
EDOT monomer Clevios M V2 (MW 142.18) (Heraeus Clevios GmbH, Leverkusen, Germany) and polyethylene glycol diacrylate SR610 (MW 742) (Sartomer, Paris, France) were degassed in a vacuum chamber under 1x10−2 mbar for one hour prior use. The blends were prepared by dissolving a required amount of EDOT in PEG-DA and stirring for 30 minutes. In total, a set of seven PEG-DA/EDOT blends, containing 5, 8, 12, 17, 23, 30 and 38 vol% EDOT, and PEG-DA reference, were prepared. A free-radical photoinitiator 2-hydroxy-4-(2-hydroxyethoxy)-2-methyl-propiophenone (Irgacure 2959) (Ciba, Basel, Switzerland) was added in the ratio of 5 wt% (blend total) and stirred further for 30 minutes. After filtering with a 0.2 µm syringe filter, the blends were additionally degassed under 1x10−2 mbar for 5 minutes.
2.2 Substrate treatment
For all PEG-DA/PEDOT microstructures, microscope cover slips were used as substrates. To enhance material adhesion, silanization from a liquid solution was applied [33]. Substrates were stored in a vacuum chamber under 1x10−2 mbar until to be used.
2.3 Microstructuring of PEG-DA/EDOT blends
A second harmonic (520 nm) of an ytterbium-based femtosecond oscillator femtoTRAIN (High Q Laser Production GmbH, Austria), running at 20 MHz repetition rate and pulse duration of 250 fs, was used for 2PP structuring. The average power available in second harmonic was up to 300 mW. 300 µm thick layers of liquid PEG-DA/EDOT blends were suspended between two parallel rigidly fixed #5 microscope cover slips (Menzel GmbH & Co KG, Braunschweig, Germany). A three-axis positioning system ABL10100 (Aerotech Inc., USA) was used to scan the samples and write a layer-by-layer rastered model. Laser beam was focused by 20x, NA = 0.5 microscope objective (Zeiss, Oberkochen, Germany). The obtained microstructures were developed by rinsing in ethanol for a duration of 1 minute.
As the prepared PEG-DA/EDOT blends contained different photoresist concentrations, 2PP structuring parameters were selected to achieve degree of polymerization sufficient for good substrate attachment and resistance to the development/oxidation processes. For this arrays of sample microstructures, covering available power and scan speed ranges, were fabricated and inspected under SEM (Scanning Electron Microscope). Parameters, resulting in highest throughput and yet high micrsotructure fidelity, were selected for each blend, see Table 1 for details.
Table 1. 2PP Structuring Parameters
2.4 Oxidative polymerization
The microstructures, produced using 2PP, were oxidatively polymerized in a bath of iron(III)chloride hexahydrate melt at 120 °C for 7 minutes. Samples were rinsed first using ethanol, 1 minute, and then water, 1 minute.
3. Results
All seven formulation blends containing 5 to 38 vol% EDOT and a PEG-DA reference without ICP precursor were used to fabricate microstructures and to characterize electrical properties. The design of these microstructures was based on 3D bars with two large area contacts and a 450 x 50 x 50 µm measurement segment, see Fig. 2(1) for details. SEM image of these structures directly after 2PP fabrication and the development step is shown in Fig. 2(2). SEM image of the oxidatively polymerized and prepared for the electrical characterization structures is shown in Fig. 2(3). 2PP fabrication and conductivity measurements were not possible with more than 17 vol% of EDOT content due to insufficient attachment to the substrate and deformations of the microstructures.
Fig. 2 Schematic representation of microstructures, used to characterize PEG-DA/EDOT electrically by two-probe method (1), SEM images of 2PP fabricated microstructures after development (2) and after oxidative polymerization of ICP precursor (3).
Fig. 3 Electrical conductivity of PEG-DA/PEDOT measured on microstructures, produced by 2PP. The red line represents a Belehradek power fit.
After the oxidative polymerization, the parallel bar sets, as produced by 2PP, were connected electrically using a conductive silver paste at the contact areas. Agilent 34405A multimeter (Agilent Technologies) was used in two-probe layout to measure electrical resistance of the composite PEG-DA/PEDOT microstructure sets. PEG-DA references were used as a negative control. The measurements were carried out in a vacuum chamber under 1x10−2 mbar after 1 hour stabilizing period. The conductivity was calculated based on the actual dimensions of the measurement segment, as evaluated using an optical microscope.
Conductivity measurement data are provided in Fig. 3. A threshold increase of the conductivity can be observed as EDOT concentration grows. This behavior can be explained by the percolation theory [34], used to describe rapid onset of conductivity in bi-component systems, containing an insulating matrix and a conducting filler. The conductive filler concentration, corresponding to the formation of conductive pathways is commonly referred to as the critical conductive filler content or percolation threshold. The conductivity dependence such systems is well approximated by the Belehradek function fit (1):
Here σ denotes electrical conductivity, p is the conductive filler content, pc is the critical conductive filler content, t is the topological parameter. The topological parameter characterizes the dimensionality of the conductive pathways. The fit to our conductivity measurement data is shown as a red line in Fig. 3. The estimated topological parameter and percolation threshold are t = 1.1 and 5 vol% of EDOT, correspondingly. The maximum conductivity of 0.04 S/cm is achieved at the maximum EDOT concentration of 17 vol%.A set of true-3D microstructures was fabricated by 2PP. The structures are designed as scaffolds for nerve tissue regeneration [35]. SEM images, shown in Fig. 4, correspond to PEG-DA (reference sample) and two PEG-DA/EDOT blends, with 8 and 17 vol% of EDOT. Initial 2PP structures and oxidized scaffolds are shown for comparison. In general, higher microstructure fidelity is achieved using lower EDOT concentrations. Little or no modifications are introduced during the rapid oxidative polymerization process. However, as listed in Table 1, 2PP fabrication process requires slower scan speed and larger average laser powers as higher EDOT concentrations are used. As in the case of conductivity measurement, scaffolds did not attach to a substrate well and were strongly deforming if any blend with more than 17 vol% of EDOT was used for the fabrication. Therefore for a particular application case the necessary microstructure resolution, electrical conductivity and physical stability should be all taken into account.
Fig. 4 3D micro-scaffolds fabricated using 2PP of PEG-DA and PEG-DA/EDOT with 8 and 17 vol% of EDOT content. Top row: microstructures after development, bottom row: oxidized microstructures.
4. Conclusion
In conclusion, a novel route to fabricate 3D conductive polymer microstructures has been demonstrated. PEG-DA/EDOT formulations were successfully structured using 2PP technology. Electrical conductivity of the composite material was induced by in situ oxidative polymerization of EDOT using iron(III)chloride hexahydrate melt. Conductivity of the 2PP processed materials is limited by the EDOT content and reached 0.04 S/cm at the maximum EDOT concentration of 17 vol%. The experimental conductivity data of PEG-DA/PEDOT composites correspond to the percolation process with a threshold value of 5 vol% and three dimensional internal topologies.
Acknowledgments
We gratefully acknowledge financial support from the Deutsche Forschungsgemeinschaft (DFG) grant “Development and fabrication of functional micromechanical and MicroOptoElectro-Mechanical Systems (MOEMS) by ultra-high resolution 3D multi-photon material processing of new polymer materials”. K. Kurselis was supported by ITN TOPBIO (PITN-GA-2010-264362). This work was partially supported by the Ministry of Education and Science of Russian Federation (Project 14.B25.31.0019).
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