Advanced Microfabrication Strategies for Neuro-Inspired 3D Conductive Polymers

Neuroelectronic platforms aim at recapitulating neuronal communication and functions to monitor and eventually restore lost functionalities. Indeed, engineering neuroelectronic devices that can be smoothly interfaced with brain and neurons can provide a step forward in a better understanding of hampered electrical communication in e.g., neurodegenerative diseases. In this scenario, conductive polymers, such as PEDOT:PSS, have gained a prominent position in the realization of seamlessly integrated bioelectronic devices and soft flexible probes.¹ However, existing polymer-based devices are not yet able to effectively mimic and reproduce the complex neuronal environment, being flat and bidimensional. Neurons indeed interact with and respond to a great variety of mechanical and topographical stimuli; thus it is imperative to take inspiration from the extremely complex architecture of the brain.²<br/>Moreover, the cell-device interface plays a crucial role for an effective electrical coupling between the cell and the device.³Indeed, a cleft is typically formed between the cell and the electrode. For this reason, recent approaches demonstrated that non-planar substrates, featuring 3D and 2.5D structures,⁴ result in the reduction of the cleft and in an improvement of cell-device electrical coupling.⁵<br/>In the light of the above, here we introduce innovative microfabrication strategies for the realization of neuroelectronic platforms based on PEDOT that can recapitulate distinctive morphological neuronal features such as dendrites and dendritic spines.<br/>Dendrites were reproduced by the formation of PEDOT:PF₆ fibers by using AC electro polymerization⁶ on a multi electrode arrays (MEAs). By changing the parameters of the applied signal and the geometric relationship between the electrodes, the growth and morphology of the fibers was altered. Additionally, the fibers geometry and branching was tuned by changing the shape of the applied AC-signal and the manipulation of the electrical field. The so obtained dendrite-like fibers were morphologically characterized by means of scanning electron microscopy (SEM) and atomic force microscopy (AFM) and the neurite elongation and branching was compared to real neurons. Additionally, electrochemical properties as well as long term stability in aqueous media was investigated via electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV).<br/>On the other hand, dendritic spines, biological structures essential in synaptic communication, were recreated by means of 2-photon polymerization (2PP) lithography, given its high resolution, versatility, and design freedom² both achieving non-conductive and conductive material patterning via electropolymerization. The resulting 3D microstructures were characterized by SEM and electrochemical measurements (CV and EIS) .<br/>Finally, biocompatibility assays were carried out for both dendrites- and dendritic spines-like structures with neuronal cells, and the local adhesion processes to the 3D structures was characterized by means of optical and electron microscopy.<br/>Overall, the proposed strategies allow for the fabrication of 3D/2.5D electrodes with different levels of complexity that can effectively mimic neuronal features and enhance the cell-chip interface. Such systems will open the way to the possibility of sensing and/or stimulating cells and tissues in a more realistic environment.<br/><br/>1. A. Mariano, et al., <i>Chem. Rev.</i>, <b>2021</b>, 122, 4, 4552–4580.<br/>2. A. Mariano, et al., <i>Nanotechnology</i>,<b> 2022</b>, 33, 492501<br/>3. G. Wrobel, et al., <i>J. R. Soc. Interface</i>, <b>2008</b>, 5, 213–222<br/>4. M. E. Spira, et al., <i>Front. </i><i>Neurosci.</i> <b>2018</b>, 12.<br/>5. F. A. Pennacchio, et al., <i>J. Mater. Chem. B</i>, <b>2018</b>, 6, 7096.<br/>6. M. Cucchi, et al., <i>Adv. Electron. Mater., </i><b>2021</b>, 7, 2100586.