Abstracts

Rationale: The principal challenge in achieving long-term, high fidelity neural recording is maintaining long-term viability of the electrode-tissue interface. One reason for this challenge is the need for rigid, strong, large electrodes - thin, flexible electrodes buckle from the force of insertion. Using rigid, large electrodes leads to trauma from insertion, long-term damage, and encapsulation at the electrode-tissue interface due to aggravation of the foreign-body response. This damage and encapsulation results in a loss of signal fidelity over time. Previous studies showed macrophage and astrocyte clusters at the electrode interface of 165μm diameter electrodes; while 25μm diameter electrodes showed little to no scarring. By reducing the mismatch of the mechanical properties between the electrode and tissue, trauma is minimized. However, the 25μm electrodes require numerous steps in fabrication and showed variability in accuracy and depth of magnetic insertion. Thus, a new electrode design is desirable. Methods: The procedure uses a magnetic insertion system where a microelectrode is loaded in an anti-static ejection tube inside a driver coil. A current pulse is discharged through the coil, generating a transient magnetic field in the ejection tube and accelerating the electrode through a craniotomy into brain tissue. Through a growing relationship with Fort Wayne Metals, we are in the process of testing their customized 25μm and 10μm microelectrodes. These electrodes are made from a ferromagnetic and biocompatible alloy, Fe60Pt. They are insulated with parylene C, and magnetically accelerated into rat deep brain structures. The microelectrode accelerates at high enough speeds to prevent buckling, overcoming the disadvantage of microelectrodes. The implanted end of the electrode interfaces with brain tissue while the other end protruding from the skull is connected to circuit pins, which can connect to external circuitry. Electrochemical impedance spectroscopy (EIS) is used on awake, behaving animals to assess the reactive tissue response and infer the degree of electrode encapsulation. Results: Long-term implantation and recording of 25µm electrodes in a rat using the magnetic insertion system is demonstrated. The new Fe60Pt microelectrodes offer advantages over stainless steel microelectrodes by being more biocompatible and therefore foregoing gold plating. This simplifies the electrode production process and may increase consistency with insertion and recording. Additionally, this new material exhibits more ferromagnetism, allowing for greater electrode accuracy and depth control. Conclusions: Chronic performance of flexible microelectrodes implanted using the magnetic insertion system and integrated with recording circuitry is supported, although additional investigation is required. This technology allows for the chronic monitoring of small neural populations in live rodent models, possibly improving implantable deep brain stimulation (DBS) devices for the treatment of many neurological diseases including epilepsy by reducing the foreign-body response at the electrode-tissue interface.