Flexible Neural Electrode: When Probes Learn to 'Soft Get Along' with the Brain - Interface Engineering from Material Selection to Precision Coating

1、 A "signal attenuation" problem that has plagued the neuroscience community for forty years

In the 1980s, the invention of Utah and Michigan electrode arrays allowed neuroscientists to record the firing activity of hundreds of neurons simultaneously for the first time. These silicon-based microelectrode arrays perform well in acute experiments with high signal-to-noise ratio and precise spatial resolution.

But when electrodes need to be implanted for a long time, a problem gradually emerges: the signal continues to decay for weeks to months and eventually disappears completely. The root of the problem lies not in the electronic design of the electrodes, but in the 'mechanical war' between the electrodes and the brain. The Young's modulus of silicon is approximately 130-185 GPa, while the modulus of brain tissue is only 1-10 kPa. This mechanical mismatch of over six orders of magnitude causes micro motion damage to every tiny brain movement - breathing, heartbeat, head rotation - at the electrode tissue interface. Injury triggers an immune response, and astrocytes and microglia form a glial scar wrap around the electrode, physically isolating the electrode from neurons. After 13 months of implantation of traditional stainless steel wire electrodes, the thickness of the coating exceeded 451 microns and was accompanied by significant cell apoptosis reactions.

The core engineering question that flexible neural electrodes need to answer is: how to make an "electronic probe" learn to "coexist peacefully" with the soft brain while maintaining high-density neural recording ability?

2、 From rigidity to flexibility: the engineering logic of material selection

The core technology of flexible neural electrodes lies in the selection of the substrate. The substrate determines the mechanical properties, biocompatibility, and processing feasibility of the electrode. The current mainstream flexible substrates include PEEK, LCP, and PI.

Polyether ether ketone (PEEK)Has excellent biocompatibility, chemical stability, and mechanical properties. The microstructure of PEEK film can achieve directional and ordered arrangement in topological structure, which has unique value in the design of composite neural electrodes. The wireless cortical EEG recording system based on PEEK packaging has demonstrated a low impedance of 12.8 ± 2.1 k Ω @ 1kHz and a high charge injection capacity of 1.2 mC/cm ² in preclinical studies.

Liquid Crystal Polymer (LCP)It is becoming an ideal base material for implantable neural electrodes. LCP has an extremely low water absorption rate (<0.1%) and can maintain stable electrical performance in humid environments and long-term use. The cytotoxicity test showed that the cell survival rate was>95%. Research has shown that thin film electrode arrays based on LCP substrates can achieve 25 times higher electrode density and 20 times higher channel count, while the stiffness is only 1/11 of traditional clinical electrodes. Accelerated life testing shows that the LCP based 25 channel electrode array can survive for 158 days in 87 ° C saline, with an estimated equivalent life of about 14 years under body temperature conditions.

Polyimide (PI)It is one of the most widely used substrates in flexible electrodes.Advanced Institute of TechnologyThe thickness of the PI gold-plated film substrate can be as thin as 5 μ m. The PI based flexible microelectrode array exhibits more stable signal-to-noise ratio, single unit output, and impedance characteristics in comparative experiments with rigid silicon-based electrodes, while the performance of rigid probes continues to deteriorate over time.

3、 Gold plating: Transforming flexible polymers from "insulating films" to "functional electrodes"

Flexible substrates provide a "skeleton" for mechanical adaptation, but the collection of neural signals ultimately depends on the conductive layer - the coating process is the key link in transforming flexible polymers from "insulating films" to "functional electrodes". Gold (Au) has become the core interface material for flexible neural electrodes due to its unique physical and chemical properties.

Gold does not corrode or release toxic ions in physiological environments, and its chemical inertness endows gold-plated electrodes with excellent corrosion resistance. The high conductivity of gold ensures low loss transmission of microvolt level neural electrical signals in the brain. The ductility of gold enables it to perfectly bond with flexible substrates such as PI, PEEK, LCP, etc.

Advanced Institute of TechnologyWe have independently built a magnetron sputtering and vacuum evaporation production line, using roll to roll continuous production technology, which can achieve precise deposition of metal layers on various flexible film surfaces such as PEEK, PI, PET, LCP, FEP, PPS, etc. In response to the core process challenges of flexible neural electrodes, the company has achieved triple breakthroughs:

• Breakthrough in interface integration - plasma pretreatment.In response to the industry problem of insufficient bonding strength between metal and polymer substrates, the plasma pretreatment process developed by Advanced Institute Technology effectively improves the bonding strength between the substrate and the metal layer by bombarding the substrate surface with plasma. In conjunction with gradient coating design, stress transition from substrate to metal layer is achieved, effectively avoiding interface delamination caused by modulus differences. The coating has a hundred grid adhesion of 5B level and a delamination strength exceeding 0.8 N/mm.
• Breakthrough in Coating Quality - Pulse Electroplating.Introducing pulse electroplating technology, by precisely controlling the switching time and frequency of the current, the porosity of the coating is effectively reduced, and the density and uniformity of the coating are improved.
• Breakthrough in Environmental Protection and Safety - Cyanide free Gold Plating.Adopting cyanide free gold plating technology in the gold plating process not only reduces environmental pollution but also enhances operational safety.

4、 Key Performance: What Engineers Should Pay Attention to

Data Source:Advanced Institute Technology Official WebsiteAnd industry public information

interface impedanceIt is the primary parameter that determines signal quality. The lower the impedance, the higher the signal transmission efficiency and the better the signal-to-noise ratio. The impedance at the electrode tissue interface is too high, and weak signals will be drowned out by noise. Low impedance means higher signal-to-noise ratio and more accurate signal acquisition, which can significantly reduce common mode interference and motion artifacts.

Signal to Noise Ratio (SNR)It is a direct indicator for measuring signal quality. The ultra-thin gold μ ECoG array maintained low baseline noise and high signal-to-noise ratio in acute and two-week recordings, with an SNR of up to 43.28 dB. The flexible probe modified with anti fouling coating increased the signal-to-noise ratio from 18.0 to 20.7 in 13 week recordings.

long-term stabilityThis is the core requirement for implantable electrodes. The accelerated life test of LCP based electrodes shows that their equivalent life under body temperature conditions can reach 14 years, which is sufficient to meet the clinical needs of long-term implantation.

5、 From Laboratory to Clinical: Application Layout of Flexible Neural Electrodes

Brain computer interface and neural signal recording.This is the most popular application area for flexible neural electrodes. In September 2025, the team of Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, published NeuroWorm results in the journal Nature - a flexible and driveable nerve fiber electrode with a diameter of only 196 microns, which can "wander" in the brain and actively change the monitoring target. After 13 months of electrode implantation, the average thickness of the fibrous coating was less than 23 microns, and the apoptosis rate of surrounding tissues was comparable to that of normal tissues. On May 18, 2026, China's first multi center clinical trial of a 128 channel fully implantable brain computer interface system was officially launched. The flexible electrodes are made of ultra-thin biocompatible materials, which can significantly reduce the immune response after implantation and accurately capture single neuron action potentials with high spatiotemporal resolution.

Neuroregulation and closed-loop therapy.The team of the Ninth People's Hospital affiliated to Shanghai Jiaotong University School of Medicine built a closed-loop brain computer interface system for the first time relying on theta band EEG markers and new hydrogel electrodes to achieve objective diagnosis of neuropathic pain, on-demand regulation and simultaneous intervention of emotional comorbidity.

Chronic neural recording and cortical electroencephalography.In a control experiment with rigid silicon-based electrodes, flexible microelectrode arrays showed lower microglial activation and IgG contamination within a 50 micron radius around the electrodes, and the expression of Piezo1 mechanosensitive ion channels was significantly lower than that of the rigid group. The wireless micro cortical EEG system based on PEEK packaging achieved a signal-to-noise ratio of 24.5 dB and an input reference noise of 2.3 μ V RMS in a 36 channel configuration.

6、 Selection considerations: Engineering decisions from substrate to process

• The choice of substrate depends on the application scenario.LCP is preferred for scenarios that require long-term implantation (>10 years), as its extremely low water absorption and chemical inertness provide the longest acceleration life data support; PEEK can be chosen for scenarios that require certain mechanical stress and chemical corrosion; PI can be chosen for applications that are more sensitive to processing maturity and cost.
• The plating scheme is based on gold.Gold provides a comprehensive advantage of chemical inertness, low impedance, and biocompatibility. For electrodes that require electrical stimulation function, platinum or iridium oxide modification can be considered; For high-density recording arrays that require lower impedance, gold nanostructures or PEDOT: PSS coating modifications can be considered.
• The adhesion and porosity of the coating determine long-term reliability.The 5B level adhesion (ASTM D3359 highest grade) and low porosity are the process thresholds to ensure that the coating does not peel off during repeated bending, implantation, and long-term service of the electrode. The pulse electroplating technology of advanced institute technology can significantly reduce the porosity of the coating.
• The processing accuracy meets the requirements of high-density arrays.High channel count neural probes require micrometer level patterning accuracy. Advanced Institute Technology supports a full chain process from surface activation, vacuum coating, to metal coating, and can customize the coating system and microstructure of electrode materials according to different application needs.

7、 Conclusion

The essence of flexible neural electrodes is to establish a stable, low-noise, and biologically friendly signal channel between rigid electronic devices and soft neural tissue. It matches the mechanical properties of brain tissue with flexible substrates (PEEK, LCP, PI), ensures signal quality with gold plating, and converts the chemical inertness of gold into reliable long-term performance through plasma pretreatment and pulse electroplating processes - all three work together to answer a core engineering question: how to make electronic devices as soft as brain tissue while also understanding the language of neurons.

From PEEK to LCP, from 5 μ m PI film to 196 μ m drivable fiber electrode, from static implantation to dynamic "migration" - the technological form of flexible neural electrodes is constantly evolving, but the position of coating as the core interface engineering has remained unchanged. Understanding the mechanical and chemical boundaries of different substrates, as well as the decisive impact of gold plating processes on electrode performance - these are the key to upgrading flexible neural electrodes from "cutting-edge materials" to "clinical tools".