Publication date: 14th January 2022
Development of soft and conductive microstructures is of paramount importance in organic bioelectronics. Among numerous 3D micro/nano fabrication techniques, two-photon polymerization (TPP) based on direct laser writing stands out due to its unique capability to construct complex architectures in sub-micron resolution, using a wide range of resins. Herein, we introduce a new conductive resin which consists of poly(ethylene glycol) diacrylate (PEGDA) and the organic semiconductor poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT:PSS) that is compatible with TPP process. We have also proposed a novel method for fabrication of hybrid microelectrodes via TPP.
Conductive resin consisted of 4 components: PEGDA (72.5 wt.%), PEDOT:PSS 1.0 wt.% in H2O (0.5 wt.%), dimethyl sulfoxide (25 wt.%), and ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate (2 wt.%). Non-conductive resin did not contain PEDOT:PSS. Microstructures were constructed on a glass coverslip through 3D movement of XYZ stages and irradiation of 130 femtosecond pulse laser, which crosslinked the resin at the laser focal point. Michigan-style neural microelectrodes were fabricated in two steps via TPP: 1) construction of conductive microstructures including cylindrical electrode sites with diameters of 1 µm (site 1), 5 µm (site 2), 10 µm (site 3), 20 µm (site 4), 40 µm (site 5), and 80 µm (site 6) with height of 7 µm, cubical contact pads with dimensions of 20 µm (width) × 20 µm (length) × 7 µm (height), and interconnects with height of 2 µm using conductive resin (PEGDA/PEDOT:PSS) (red color); and 2) fabrication of electrode shank with height of 5 µm using non-conductive resin (PEGDA) (green color).
Current-Voltage (I-V) measurements revealed that electrical conductivity of microstructures fabricated from nonconductive resin (PEGDA) remarkably increased from 2×10-6 ± 6.5 2×10-7 S m-1 to 2.4×104 ± 9.3×102 S m-1 by incorporation of 0.5 wt.% PEDOT:PSS (n=5). The electrical conductance significantly increased from 61.91 ± 2.35 µS in site 1 to 127.60 ± 9.00 µS in site 6 (n=5) in the microelectrode. Electrochemical characterization of the microelectrode indicated that the impedance at 1 kHz decreased from 63.13 ± 4.56 kΩ in site 1 to 19.28 ± 3.08 kΩ in site 6, and charge storage capacity increased from 2.38 ± 0.18 nC µm-2 in site 1 to 89.73 ± 15.14 nC µm-2 in site 6 (n=3).
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