Cortical interface electronics with wide coverage can effectively improve the accuracy and resolution of real-time synchronous monitoring of brain parts. This can help to localize specific and resolved areas of the lesion or to accommodate larger, more complex BCI systems. However, the current clinical implantation process still relies on large-scale craniotomy, which obviously carries risks such as neurogenic hypertension and chronic postcraniotomy headache. These complications clamp the clinical application of brain-computer interfaces. In addition, a secondary surgical removal of the electronic implant is often required after treatment or monitoring to prevent unwanted immune response, biofilm formation, and migration-induced damage.

Academician Seung-Kyun Kang (Academician of the Korea Academy of Science and Technology and Academician of the Academy of Engineering) from the Department of Materials Science and Engineering, Seoul National University, Korea, and Professor Ju-Young Kim from the Department of Materials Science and Engineering, Ulsan National Institute of Science and Technology, reported a biodegradable and self-deployable "tent" electrode. For the cerebral cortex interface. The system can be integrated with multiplexed arrays and wireless modules for near-field communication and data transmission. Programmable packaging and self-deployment using syringes for minimally invasive deployment through small holes. Once deployed, it can scale to cover areas that are approximately 200 times their original size. The electrodes also naturally decompose in the body after use, minimizing the impact of subsequent removal procedures. The cortical interface platform has been demonstrated in large animals to effectively stimulate cortical activity.

Biodegradable, self-deployable electronic "tents":

The image below shows a detailed schematic of a biodegradable and self-deployable electronic "tent". The top and bottom substrates are composed of PLCL-PLGA, which provide programmable deployment due to their shape memory properties. Integrates a variety of electronics, including electrodes, semiconductor field-effect transistors, thermal-resistive temperature sensors, piezoresistive strain sensors, protonated and deprotonated pH sensors, and inductively coupled coils, all constructed from degradable components. The device's radial mesh design allows for axial transformation, resulting in uniform and flexible deformation during packaging and deployment.

Finite element analysis (FEA) and physical drawings show how an electronic "tent" works. The procedure begins with sequential loading of electronic "tents" into syringes with an inner diameter of 5 mm. It folds at the center node, and each node bends as it pushes inward. The pressure at the time of injection provides the primary driver of recovery. When fully deployed, the coverage area of the electronic "tent" is increased by about 200 times. The wireless light-emitting diode successfully operates on a circular support geometry through a power receiver coil, proving that the electronics are not damaged in packaging and deployment. The device was immersed in PBS, and after 150 days, all components were almost completely dissolved into various by-products due to the hydrolytic properties of the material. These by-products hardly affect the tissue. During use, the electrical functionality of the device decreases due to hydrolysis of metal components or expansion due to polymer degradation.

Figure: Fully biodegradable and self-deployable electronic "tent" for minimally invasive monitoring throughout the intervention

Mechanical Modeling of Device Packaging and Deployment:

The diagram below illustrates the packaging of a simplified circular tent-shaped unit. This presentation provides insights into mechanical modeling and design rules during shaft transformations. The fill ratio (F) represents the ratio of the filled area (AF) of the chassis and support sections to the coverage area (AC) of the equipment unit design. This is especially critical for devices with a constant thickness, as it facilitates efficient folding and recovery during packaging and unfolding. The relationship between the maximum local strain and the filling ratio is determined by the width and number of chassis and supports. During unfolding, the bent and twisted supports are restored by temperature-dependent shape memory, which causes the device to diffuse in the radial direction. The unfolding direction is determined by the initial orientation of the tip, so the final unfolding position can be predicted. The interaction between the device and the underside affects the sliding of the cortex. Controlling the contact angle is critical as it affects the forward sliding behavior. The threshold is an angle of 60° during deployment, and the initial backward sliding of the interface often results in an unsuccessful recovery.

Figure: Mechanical modeling of a programmable, packaged, and self-deployable electronic "tent".

in vivo self-deployment and cortical activity recording:

A minimally invasive procedure was used to successfully validate the interface deployment of the canine brain. The procedure involves simply exposing the surface of the brain through an opening in the skull and injecting a module and lubricating fluid (concentrated glycerin) through the opening. To be able to visualize the entire process in situ, the skull was replaced with a transparent replica of the skull. The figure below shows the location of a 16-channel molybdenum electrode array, each with an area of 300 × 300μm2, a thickness of ~500 nm, and an impedance of ~15 kΩ at 1 kHz. The input/output (I/O) interfaces fixed to the central polycaprolactone node ensure stable data transmission and surgical difficulty. In neurocritical care, monitoring various physiological signals in the brain is essential to recognize signs of secondary brain injury. The authors' team further monitored the temperature changes caused by ECoG, infrared light irradiation, strain changes caused by extrusion, and pH changes after injection of brine through boreholes through electronic "tents" on the canine model.

Figure Demonstration of a self-deploying electronic "tent" in a canine model

Active multiplexed arrays for space-time mapping:

MOSFET-based active electrodes are composed of Si NM and have the advantage of high-density recording. This feature enables accurate localization of lesions while minimizing the complexity of wire interfaces. These arrays integrate seamlessly with eight separate synapses in an electronic "tent". The average gain, electron mobility, on/off ratio, and signal-to-noise ratio of the active electrode are ~0.91, ~350 cm2s−1V−1, ≳104, and ~9.76 dB, respectively. The signal-to-noise ratio depends on the thickness of the top and bottom substrates. The switchable behavior of the MOSFET is achieved by controlling the applied gate voltage, enabling data to be selected individually from each MOSFET for efficient multiplexing. In vivo measurements obtained from an array of active electrodes integrated into an electronic tent are shown in the figure. Figures e,f represent the signal distribution at a specific instance (t = 55, 83, 150, and 195 ms) based on the amplitude of each array. An array of active electrodes is located in the area of the right motor cortex and effectively detects an ascending signal during electrical stimulation of the left sciatic nerve.

Figure Integration of an active multiplexed electrode array for high-density recording

Wireless systems for neurophysiological signal monitoring:

NFC-based systems provide a wireless way to transmit or receive physiological sensing data. This approach eliminates the need for external power supplies and I/O lines, reducing potential complications. The wireless version of the NFC-based e-tent includes a custom NFC board on the central node, a sensor layer on the synapse, and a coil layer on a radial circular bracket for wireless communication based on inductive coupling. The data measured from the wireless platform is transmitted to an external reader board at a frequency of approximately 13.56 MHz via inductive coupling. In addition, the NFC module integrates various sensors (temperature, thermal conductivity, and strain) to detect physiological signals. For in vivo wireless physiological monitoring, a wireless sensing platform is deployed on the canine brain. Temperature changes due to infrared light irradiation, thermal conductivity changes due to changes in the surrounding environment, and strain due to extrusion were effectively measured. This experiment demonstrates the feasible application of wireless monitoring on the brain surface using electronic "tents" with integrated custom circuitry.

Figure NFC system integration for wireless surveillance

Biocompatibility and biodegradability:

Micro-CT images show the continuous biodegradation of an electronic tent implanted in a rat brain over 460 days. Over time, the geometry of the implantable electronic tent gradually decreases. Extraction of the implanted device at different time points. Observations have shown that the device dissolves over time and some tissue adsorbs to the device. At 2 weeks after implantation of the device, the fluorescence signal exhibited by GFAP and Iba1 in the implantation group was higher than in the sham group. However, as the treatment period was extended from 2 to 6 weeks, the expression levels of GFAP and Iba1 decreased. This result suggests that the procedure to implant the device into the brain led to changes in the brain environment, a response that may temporarily induce astrocyte versus microglia activation. H&E and TUNEL staining showed no significant toxicity or cell death. Magnetic resonance images taken at specific time intervals did not reveal significant damage to deep brain tissue due to the degradation products of the electronic "tent". There was little difference in chemical, electrolyte, or cellular levels of blood between implanted rats and control animals

Figure Biodegradability and biocompatibility in vivo of electronic "tents".

Brief summary:

For brain-computer interfaces, reducing the size of the interface device as much as possible and improving its expansion ability after deployment can minimize the contradiction between the cortical binding area and the size of the surgical wound. The ideal brain-computer interface should provide predictable and gentle deployment to ensure reliable localization while preventing tissue damage. Conformal contact is important to maintain position and obtain a high-quality signal throughout its functional lifecycle. Integrating fully biodegradable components or graftable wireless electronic devices can further enhance the overall patient experience by minimizing complications. Ideally, all components should be dissolved by hydrolysis or metabolic activity, eliminating the risk of potential biofilm formation or the need for a secondary removal process.

Here, the authors' team describes a biodegradable and self-deployable electronic "tent" electrode designed for cortical interfaces. This method provides a minimally invasive platform for monitoring and removal from insertion to post-implantation. The e-tent uses a programmable packaging method that provides repeatable, automated, and gentle deployment, thereby reducing the risk of surgical complications. Its initial small size and biodegradability can minimize complications associated with implantation and surgical removal. The platform also has the potential to accommodate advanced functional modalities, such as microelectromechanical systems and fluid and optical channels, which can expand therapeutic approaches, including chemotherapy or phototherapy.

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