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    Neural Engineering

    (K-429) Two-Photon Printed, 3-D Microelectrode Arrays for Intraneural Recording

    Friday, October 13, 2023
    9:30 AM – 10:30 AM PDT
    Location: Exhibit Hall - Row K - Poster # 429
    Introduction:: The ability to record neural activity from small nerves in freely moving animals has been a longstanding challenge in neuroscience, limiting our understanding of motor control signals underlying learned behaviors. Songbirds are a unique model for studying the brain and nervous system due to their highly stereotyped song, and their ability to modify and refine their song throughout their lives. This complex motor behavior involves precise control and signaling of the vocal organ which can provide insights into the neural mechanisms governing muscle control. Currently, devices for measuring action potentials in small nerves are limited by electromyographic noise, movement artifacts, and chronic tissue responses. Here, we present work that begins to meet these constraints by building on a previously published microscale implantable device (shown above) for chronic interfacing with fine peripheral nerves. The first generation of the device, now completed, involved only extraneural recording from flat electrodes that contact the surface of the nerve. The second-generation device aims to reduce noise through incorporation of intraneural spike electrodes to improve isolation of neural signals from external noise. 

    Materials and Methods::

    The Nanoclip and microneedle electrodes are written using our two-photon printing (TPP) method capable of creating high aspect ratio needles using customized a photocurable dental resins whose mechanical stiffness can be altered from 400 to 3000 MPa depending on degree of polymerization and post-print curing methods. In TPP, fast pulses of infrared light are used to initiate two-photon absorption within a photosensitive resist, causing the resist to polymerize. Complex arbitrary shapes can be written at micron resolution by controlling the laser focal point’s power and scan path through the photoresist. Our TPP system allows for a print speed of approximately 1 mm3/min while maintaining a print resolution of 1-3 µm. 




    Our flexible, polyimide-based device is fabricated using only FDA accepted bioinert materials. To fabricate the device, a silicon carrier wafer acts as a rigid backing for the polyimide substrate. Using TPP, we print a 4x1 array of hypodermic-inspired microelectrodes each with an electrically active surface area of approximately 0.02 mm2, a final tip width of ~2 μm, and a spacing of 100 μm. Platinum traces electrically connect the microneedle array after the TPP process. A second polyimide layer serves to insulate the interconnects from electromyographic noise and movement artifacts. The device is released from the carrier wafer via plasma etching and the Nanoclip is then printed through vias near the electrode sites. Finally, the device is bonded to an Omnetics connector for interfacing with Intan and OpenEPhys recording systems.




    Results, Conclusions, and Discussions::


    Results and Discussion: 




    Initial in vivo experiments have successfully demonstrated a prototype device’s capability to penetrate the zebra finch tracheosyringeal (TS) nerve prior to spike metallization, indicating the mechanical strength and robustness of our needles as printed. A dummy polyimide device was laser cut and released from a silicon carrier wafer, and a Nanoclip printed around our 4x1 needle array. The device was then placed around the TS nerve of an anesthetized male zebra finch, with the spikes clearly penetrating the nerve. The nerve body was held in place via the Nanoclip without the need for additional sutures. Following the surgical procedure, the spike-penetrated nerve was explanted and temporarily placed in phosphate buffered solution. Subsequent imaging required removal of the assembly from solution, which resulted in the dehydration of the nerve. Images taken through the Nanoclip body demonstrate good penetration of the uncoated spikes and secure attachment of the clip around the nerve. Experiments with electrically connected devices containing metallized recording surfaces are currently underway and involve ex vivo characterization of the device's electrochemical properties using cyclic voltammetry and electrochemical impedance spectroscopy. In vivo recordings of neural activity will soon be conducted in anesthetized and freely moving, freely singing zebra finches. In vivo recordings of spontaneous neural activity in anesthetized birds will allow for fine tuning of device parameters, while chronic recordings of spontaneous and stimulated neural activity in freely moving birds will capture neural activity during natural behavior. Correlating neural activity with song spectrograms will offer insights into the motor neural relationships underlying singing behavior. 




     




    Conclusions:  




    We have demonstrated progress towards a microfabricated, two-photon printed device for intraneural electrical recording and stimulation in small animals. This device has the potential to advance our understanding of neural function and develop novel therapies for neurological disorders. Future work will focus on improving the device performance, carrying out extensive recording bouts in singing birds, and evaluating its long-term biocompatibility. 





    Acknowledgements (Optional): : This material is based upon work supported by the National Science Foundation NRT program under Grant No DGE-2022168 and NIH grant 1RF1NS118424. We acknowledge the facilities and staff from the University of Oregon’s Technical Sciences Administration (TSA) and Center for Advanced Materials Characterization in Oregon (CAMCOR). 

    References (Optional): :