HARDWARE
The objective of the hardware system in this project is to construct an accessible, reproducible, and highly operable EEG acquisition demonstration platform. This platform serves to support the development, validation, and exhibition of epilepsy monitoring and treatment systems. Given the extremely low amplitude of EEG signals and their susceptibility to external interference, the system design places signal quality, safety, and user-friendliness at the core. Targeted optimization measures have been implemented in multiple key links to ensure that the system performance reaches the best.
The system primarily consists of three components:
1.EEG Signal Acquisition: EEG activities are recorded using an Elastic EEG Cap and wet electrodes.
2.Signal Sampling and Processing: The OpenBCI Cyton Biosensing Board (8 channels) is employed to amplify and digitize the signals.
3.Data Transmission and Display: Data is transmitted via Bluetooth to the computer and visualized and analyzed in real-time using OpenBCI GUI.
At the EEG signal acquisition stage, the Elastic EEG Cap was selected. Composed of elastic webbing, this cap can adapt to various head shapes, enhancing comfort and facilitating electrode positioning. This design alleviates the pressure associated with traditional rigid electrode caps during extended use, making it more suitable for continuous or long-duration monitoring.
For electrodes, wet electrodes in conjunction with Ten20 conductive paste were chosen. Wet electrodes reduce impedance between the skin and electrodes through a conductive medium, significantly improving signal quality and stability, particularly for low-amplitude EEG signals. Ten20, a widely used conductive paste in the EEG field, offers low impedance, excellent stability, skin compatibility, and easy cleaning. Its viscous nature ensures consistent electrode contact during prolonged experiments, resulting in clearer and more stable EEG signals.
The OpenBCI platform was selected for signal acquisition. As a well-established open-source platform, OpenBCI has numerous research applications and a supportive user community, ensuring reliability and providing valuable references. Its comprehensive circuit board design and excellent compatibility enable seamless integration with electrodes and other hardware components. Moreover, its modularity and compatibility facilitate both current experimental requirements and future expansions, such as multi-channel acquisition or integration with other sensors.
Regarding power supply, a 6V battery box (utilizing four AA dry batteries) was chosen over an adapter or charger. Given the low amplitude of EEG signals, power supply stability is crucial for signal quality. Battery-based DC power supply exhibits favorable linear characteristics, eliminating minor voltage fluctuations and effectively preventing interference with EEG signals. In contrast, common AC power adapters, chargers, or computer USB power supplies often introduce power frequency interference, compromising signal purity. Thus, the battery box not only adheres to the recommended 4V - 6V voltage range of OpenBCI but also enhances the accuracy and reliability of experimental data.
In the hardware design of this project, a custom enclosure for the EEG acquisition circuit board was required. Due to the low amplitude of EEG signals and the sensitivity of circuit components to static electricity, anti-static properties were a top priority for the enclosure. Compared to commonly used insulating materials in 3D printing, such as PLA and ABS, ESD Resin has a surface resistivity of 10^5 - 10^9 Ω/sq, effectively discharging static electricity and preventing charge accumulation on the enclosure surface. This property minimizes interference and potential damage to the circuit board during the acquisition of weak EEG signals. Additionally, ESD Resin offers superior strength, heat resistance, and aesthetic appeal, meeting the long-term requirements of the experimental environment and maintaining a professional appearance during demonstrations.
In the structural design of the enclosure, the first option (left image) was selected. This design incorporates screw holes within the housing, securely fastening the circuit board and preventing issues such as poor electrical contact or mechanical damage caused by vibration or displacement. In contrast, the second option, a more common design in the market (including the official OpenBCI enclosure), relies primarily on housing encapsulation to secure the circuit board. While this design is aesthetically simple and widely adopted, the large contact area between the circuit board and the housing can generate unnecessary mechanical stress due to friction, resulting in relatively lower stability during long-term use. Therefore, despite the prevalence of the second design, the first option was chosen to ensure the stability of the circuit board during transportation, operation, and display, thereby meeting the stringent safety and reliability requirements of scientific research.
Material List:
OpenBCI Cyton 8-channel board
USB Dongle
6V 4 AA battery box
Ear clip electrodes
Gold-plated bowl-shaped electrodes
EEG cap
Ten20 conductive paste
Insulating pillars
3D printed enclosure
Hardware Assembly:
1.Insulating Pillar Installation: Plastic insulating pillars were installed in the four mounting holes surrounding the OpenBCI main board. This elevated the circuit board, preventing direct contact with the desktop and mitigating the risk of static electricity or short circuits.
2.Power Supply Configuration: Dry batteries or lithium batteries were recommended to maintain a supply voltage within the 4V - 6V range. Battery-based DC power supply offers linear characteristics, minimizing power frequency interference and ensuring signal integrity.
3.Wireless Receiver Connection: The USB Dongle was connected to the computer's USB port to enable wireless communication with the OpenBCI acquisition board.
4.Powering on the Acquisition Board: The power switch on the acquisition board was set to the "PC" position to establish a connection with the USB Dongle. A lit LED on the Dongle indicated a successful connection.
5.Electrode Attachment:
- The BIAS interface was connected to one earlobe as the reference electrode.
- The SRB2 interface was connected to the other earlobe as the ground electrode.
- The remaining 1N - 8N interfaces (closest to the circuit board) were connected to EEG electrodes for recording electrical signals from the target brain regions.
- Conductive paste was used to secure all electrodes, ensuring optimal skin contact and minimizing signal degradation.
After the initial setup and confirmation of system stability, the temporary insulating pillars were removed and replaced with a custom 3D printed enclosure to enhance protection and usability.
Software Installation:
1.Driver Installation: Inserting the USB Dongle triggered the automatic installation of the serial port driver. The newly assigned serial port was verified in the device manager.
2.OpenBCI GUI Installation: The appropriate OpenBCI_GUI version for the operating system was downloaded and extracted from the official OpenBCI website. First, OpenBCIHub (communication bridge) was launched, and upon confirmation of its normal operation, OpenBCI_GUI was executed as an administrator.
3.Connection Establishment: In the GUI main interface, the appropriate serial port was selected, and "START SYSTEM" was clicked. A successful connection granted access to the data acquisition interface, enabling real-time data streaming. Power frequency Notch filtering (50 Hz for China, 60 Hz for the US) was applied to suppress power line interference, and the band-pass range was adjusted according to experimental requirements (commonly 0.5 - 50 Hz for EEG) to isolate the relevant brainwave frequencies and minimize noise.
I. Design Philosophy
In the initial conception, we aimed to replace the traditional electrode cap with the cEEGrid, a flexible electrode array placed behind the ear. Unlike the bulky cap design, cEEGrid is fixed in the area behind the ear with ten Ag/AgCl flexible conductive patches arranged in a C shape, hardly affecting daily activities. Its comfort and low profile mean that users can conduct long-term EEG monitoring in various life scenarios. For patients who need continuous tracking of epileptic seizures, this design represents a significant shift from "laboratory equipment" to "daily wearable devices".
II. System Configuration
Our original plan was to integrate cEEGrid with the OpenBCI Cyton module based on the existing hardware platform. The Cyton module provides eight-channel input and a sampling rate ranging from 250 to 1000 Hz, which can fully cover the eight electrode contacts of cEEGrid. After the signals are acquired at the electrodes behind the ears, the Cyton board is responsible for signal amplification and sampling. Subsequently, the signals are stably transmitted to the host computer via wireless transmission. The OpenBCI GUI on the PC can display waveforms in real time, perform filtering operations, and record data, thereby establishing a complete chain from electrode signal acquisition to visualization. This design is not only highly compatible with our existing system in terms of hardware interface but also provides a high-quality data foundation for subsequent algorithm training and event annotation.
III. Application Advantages
Compared with traditional electrode caps, the cEEGrid has advantages in terms of comfort and concealment. The flexible electrodes behind the ears eliminate the pressure and restraint of electrode caps, making long-term monitoring possible. Additionally, the electrodes of the cEEGrid naturally fit the area behind the ears and are almost imperceptible in daily life, reducing the psychological burden of wearing them. Research on signal aspects shows that although the overall sensitivity of the electrodes behind the ears is lower than that of full-head high-density EEG, in adjacent areas such as the temporal lobe, it can obtain effective signals with amplitudes close to those of traditional EEG. This makes the cEEGrid particularly suitable for partial brain region recording related to epilepsy monitoring. At the same time, the multi-electrode design of the cEEGrid is more stable than a single bipolar electrode, can cover more signal sources, and reduce data loss caused by individual differences.
Due to budget constraints, we chose the more economical electrode cap as the actual acquisition solution in this project. However, the cEEGrid solution remains our ideal direction from the design stage. The comfort, portability, and practicality it embodies are precisely the trends in the development of future EEG monitoring devices. We hope that under the conditions of subsequent research and if circumstances permit, we can truly realize this vision, enabling our system not only to operate in the laboratory but also to truly enter the daily lives of patients.
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