Implantable neural interface electrodes are crucial for neuroscience research and clinical applications in neurological disorders. Electrocorticography (ECOG) records brain electrical activity using electrodes placed on the brain's surface. While minimally invasive ECOG electrodes have advanced, long-term implants face challenges like micromotion causing electrode displacement and foreign body responses degrading signal quality. Current macro-electrodes lack sufficient conformability, while thin-film microelectrodes, though offering higher channel counts, suffer from insufficient mechanical strength. Research focuses on reducing electrode stiffness through structural designs (segmented, mesh, strip, kirigami) or using intrinsically soft materials like PDMS or hydrogels. However, PDMS patterning and bonding remain challenging, while hydrogels lack processing accuracy and long-term reliability. Another often-overlooked issue is electrode moisturizing. The brain's moist environment is crucial; drying can severely degrade signal quality. To address these challenges, this study utilizes bacterial cellulose (BC) film, a highly biocompatible, ultrasoft, and highly moisture-retentive material, as a substrate for micro-ECOG electrodes.

The literature review extensively discusses the limitations of existing ECOG electrodes, highlighting the trade-offs between conformability, mechanical strength, and long-term stability. It covers various approaches to improve electrode softness, including structural modifications and the use of different substrate materials like PDMS and hydrogels. The review also emphasizes the importance of maintaining a moist interface between the electrode and brain tissue and the challenges associated with achieving this during implantation and long-term use. Existing solutions are analyzed, revealing their shortcomings in terms of either biocompatibility, mechanical properties, processing accuracy, or long-term stability.

The researchers designed and fabricated the Brainmask micro-ECOG electrode, using a bacterial cellulose (BC) film as the substrate and serpentine parylene-C microelectrodes bonded to its surface. The assembly process involved transferring the microelectrodes from a silicon wafer using PVA tape, applying a thin layer of elastic silica gel as an adhesive, dissolving the PVA tape with water, and then drying and hot-pressing the electrode pads to a flexible flat cable (FFC) via an anisotropic conductive film (ACF). The device was then sealed with silicone sealant. The mechanical properties and conformability of the BC film were compared to PDMS and Ecoflex using experiments involving wrapping the materials around cylinders and spheres of different curvatures. Finite element analysis (FEA) was used to simulate the stress and strain distribution in the Brainmask device under different deformation conditions, such as twisting and attachment to a curved surface. In vivo experiments were conducted to evaluate the electrode's performance in acute and long-term recordings.

The Brainmask device demonstrated superior accurate positioning of microelectrode sites due to the assembly process which leverages the water absorption and swelling characteristics of the BC film. The ultrasoft BC film ensured conformal contact with curved brain surfaces, significantly outperforming PDMS and Ecoflex in adhesion tests on both developable and non-developable surfaces. FEA confirmed that the Brainmask device could withstand significant deformation without exceeding the yield strain of the gold metal layer. Acute in vivo experiments showed that the Brainmask device maintained high-quality signal recordings for at least one hour, while longer-term in vivo recordings (one week) further validated its performance in maintaining consistent signal quality. The superior conformability and moisture retention of the BC film were key to these results.

The Brainmask device successfully addresses several critical limitations of existing ECOG electrodes. Its ultrasoft and moist nature improves conformability, reducing micromotion and foreign body responses. The precise electrode positioning, confirmed by both experimental and FEA results, ensures stable long-term recording. The successful in vivo tests demonstrate the practicality and potential of Brainmask for long-term neural recordings in various applications, such as epilepsy monitoring and brain-computer interfaces. The use of BC film provides a biocompatible and highly adaptable solution, opening up new avenues for the development of advanced neural interfaces.

The Brainmask device represents a significant advancement in micro-ECOG electrode technology. Its superior conformability, moisture retention, and long-term recording capabilities are attributed to the use of a biocompatible BC film substrate. This study demonstrates the potential of Brainmask for various neuroscience and clinical applications, paving the way for improved neural interfaces with enhanced stability and longevity. Future research could explore higher-density electrode arrays and investigate the long-term biocompatibility of the device in chronic implantation studies.

While the study demonstrates promising results, further investigation is needed to assess the long-term biocompatibility and stability of the Brainmask device in chronic implantation settings. The current design uses a limited number of channels; future work should focus on scaling up the device to include a higher density of electrodes. The long-term effects of the BC film and associated materials on brain tissue need to be more thoroughly explored.