Advancing Neural Stimulation: The Impact of Microporous Graphene Electrodes

Introduction: The development of effective therapies for chronic pain, neural disorders, and post-injury recovery has made high-performance neural electrodes essential. These electrodes, crucial for modulating neural activity, have traditionally used materials like platinum (Pt), which offers limited charge storage capacity (CSC) and charge injection capacity (CIC). To enhance these capacities, advancements have been made using composite materials like Pt-TiN and Pt-PEDOT:CNT, which increase the electrode's surface area and conductivity, thereby improving charge transfer efficiency. Moreover, nanostructuring techniques have significantly boosted CSC and CIC in materials like nanoporous Pt and carbon-based alternatives like carbon nanotubes and graphene. Graphene, known for its high electrical conductivity and biocompatibility, has been particularly promising. However, standard graphene's interfacial capacitance remains low, which affects its effectiveness in neural stimulation. To address this, nanostructured graphene has been developed, enhancing its surface area and electrochemical activity. Recent innovations include the development of porous graphene-based neural electrodes using laser technology on polyimide films, achieving CSCs of 50 mC cm^2 and CICs of 2 mC cm^2. This significant improvement is attributed to the material’s porous structure and increased surface area, facilitating more effective neural stimulation and charge transfer. These advancements are pivotal for applications in deep brain stimulation, spinal cord stimulation, and cochlear implants, highlighting the potential of engineered graphene structures in medical applications.

Materials and

Methods: In this study, we introduce a novel neural electrode made from 3D micro-/nano-scale porous graphene (3DPG), which demonstrates superior charge storage capacity (CSC) and enhanced charge injection capacity (CIC), thereby increasing neural stimulation efficiency. The 3DPG electrodes are fabricated using a scalable, cost-effective direct laser scribing technique on a fluorinated polyimide (fPI) precursor. This method produces a highly microporous graphene structure featuring micro-/nano-scale pores, which form as fluorine-rich gases are released during laser photothermal irradiation.

Microscopic imaging and photographs of these fPI-derived 3DPG neural electrodes (fPI-3DPG) show their integration into a printed circuit board (PCB) for electrochemical analysis. The enhanced CSC of these electrodes is attributed to their increased electrochemically active surface area, provided by the distinctive microporous architecture, which allows for greater charge storage at the interface and reduces impedance, thereby boosting electrode performance. Comparative analysis reveals that fPI-3DPG electrodes have an eightfold increase in CSC and a twofold improvement in CIC over their polyimide-derived counterparts (PI-3DPG), as illustrated in our figures. These electrodes leverage their enlarged surface area and unique electrical properties, such as high electron mobility, to deliver outstanding electrochemical performance with high CIC and low impedance. The structural design of fPI-3DPG, featuring numerous active sites and minimized ion travel distance within interconnected graphene pores, significantly enhances charge transfer efficiency and CSC, making these electrodes highly effective for neural interface applications and more efficient neural tissue stimulation.

Results, Conclusions, and Discussions: In this study, we present the development of innovative graphene-based neural electrodes with a 3D microporous structure, engineered using a novel one-step laser photothermal technique. This approach leverages a fluorinated polyimide precursor that is crucial for creating micro- and nano-scale pores. These pores emerge through the release of fluorine-rich gases during laser irradiation, contributing to an exceptionally large electrochemically active surface area and superior electrical properties, including high electron mobility. Our electrodes, characterized as fPI-3DPG, exhibit groundbreaking electrochemical metrics, achieving a charge storage capacity (CSC) of 362.4 mC cm−2 and a charge injection capacity (CIC) of 10.32 mC cm−2. These values represent an eightfold increase in CSC and a twofold increase in CIC compared to previous PI-3DPG electrodes. Notably, the fPI-3DPG's CSC surpasses that of conventional monolayer graphene and metal-based electrodes by over two orders of magnitude, a testament to its intricate porous structure and large surface area which enhance charge transfer and accumulation. Furthermore, the fPI-3DPG electrodes demonstrate remarkable mechanical flexibility, reliability, and durability, enduring up to 10,000 bending cycles, 106 electrical stimulation cycles, and 1000 continuous cyclic voltammetry tests without performance degradation. Their efficacy was validated ex vivo on lobster tail nerves, which responded with visible muscle contractions, showcasing the practical application of our electrodes.
The significant improvements in CSC and CIC, coupled with the transparency of the fPI films, open new avenues for these electrodes in advanced applications like optogenetics and neuroimaging. This allows for high-efficiency neural stimulation alongside imaging-based identification of peripheral nerves, demonstrating the potential to significantly advance both medical research and clinical applications.

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