Laser 3D-Printing of Organic Semiconductor-Carbon Nanotube Microstructures for Flexible Microelectronics and Circuitry

3D printing is taking the stage in the forefront of technological and industrial advancements, particularly in the emerging field of organic micro/nano electronics. Amongst various 3D printing techniques, Direct Laser Writing based on Two-Photon Polymerization (DLW-TPP) reigns supreme, owing to its unique capability to construct arbitrary-shaped 3D architectures in sub-micron resolution. Herein, we have directly incorporated 2 organic semiconductor fillers, i.e. poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) and multi-walled carbon nano tubes (MWCNTs), in a photosensitive ink which can be fabricated into highly conductive microstructures via DLW-TPP.<br/><br/>The photosensitive ink contained polymer crosslinker poly(ethylene glycol) diacrylate (PEGDA), two organic semiconductors (PEDOT:PSS and MWCNTs), photo-initiator (ethyl (2,4,6-trimethylbenzoyl) phenylphosphinate), and two miscible agents (dimethyl sulfoxide and pentaerythritol tetrakis (3-mercaptopropionate)). Formulation-wise, maximum content of PEDOT:PPS and MWCNTs in the conductive, homogeneous, and stable ink was found out to be 0.4 wt% and 0.15 wt%, respectively. Microstructures were constructed on flexible poly(dimethylsiloxane) substrates through 3D movement of XYZ stages and irradiation of 130 femtosecond pulses from two-photon laser, which solidified the resin at it’s focal point. <br/><br/>Current-voltage measurements revealed that conductivity of microstructures fabricated by the conductive ink was 140050 ± 29414 S m-1, almost ten orders of magnitude higher than their counterparts fabricated with inks without added conductive agent, i.e. 0.0002 ± 0.0003 S m-1 (n=5). Optical transparency of the conductive ink showed ink transmittance of 82% at 550 nm. Moreover, microstructures fabricated with the conductive ink presented average surface roughness of 258 ± 2 nm (n=4). <br/><br/>Fabrication and electrical/electrochemical characterization of various conductive microstructures were successfully demonstrated. In the space of flexible microelectronics and circuitry, printed circuit boards were fabricated via DLW-TPP based on the conductive ink. Functionality of sub-components such as resistors and capacitors were measured and confirmed. In another notable development, multi-site microelectrodes were constructed via DLW-TPP. Electrochemical impedance spectroscopy and cyclic voltammetry revealed that recording sites exhibited low impedance (18.28 ± 5.58 kΩ at 1 kHz) and high charge storage capacity (48.13 ± 4.67 mC cm-2), which promises their application in potential neural recording/stimulation. Development of these 3D-printed microstructures via DLW-TPP forges the path forward for development of advanced printed circuitry and wearable / implanted microelectronics.