Conformal and adhesive gel for stable electrophysiology on hairy animals without shaving

Domain: chemistry
Published: 2026-03-07
Editorial outcome: major_revision

Abstract
Non-invasive collection of weak electroneurographic signals is crucial for cognitive neuroscience. However, animal scalps are often covered by dense hair, which presents a substantial barrier in the scalp-electrode interface for accurate electrophysiological acquisition. Although scalp hair removal is common practice in clinical settings, it has detrimental effects on the test subjects and is infeasible in many non-clinical scenarios. Here we report a hair-adaptable and adhesion-tunable (HAAT) scalp-electrode interface that can accurately acquire electroneurographic signals from hairy skins of animal and human. By synthetically integrating dynamic covalent bonds and ionic conduction channels in the copolymer poly(sodium thioctate-co-sulfobetaine methacrylate), we implement the key functionalities of the HAAT interface including penetration of dense hair layers, strong conformal skin-electrode adhesion, on-demand painless detachment, and sensitive neuroelectric coupling. We achieve stable and precise electroneurographic signals acquisition from human, monkey and mouse’s scalps with various hair densities. The accurate event-related potential monitored during monkey vision-attention tasks facilitates the study of monkey brain activity. These results demonstrate the HAAT interface as an outstanding platform for conducting non-invasive research on animal cognition.

Introduction
The non-invasive and precise collection of electroneurographic signals from the brain, such as electroencephalogram (EEG) and event-related potential (ERP), is a crucial technology widely applied in cognitive neuroscience1–4. A primary challenge in developing non-invasive brain neural electrodes lies in establishing a robust and conformal interface with the scalp, a prerequisite for minimizing signal artifacts and ensuring reliable neuroelectric coupling5,6. This challenge becomes particularly pronounced in test subjects, e.g., rodents and primates, whose scalps are covered with amounts of hair. The hair layer creates gaps at the scalp-electrode interface and significantly hinders stable and intimate contact. While hair removal is an established pre-operative protocol in clinical settings such as craniotomy, where patient compliance is implicitly secured by therapeutic necessity, this approach fails to translate to many scenarios. For example, pediatric subjects have very delicate scalps, so hair removal can trigger psychological and physiological harm. In cognitive neuroscience research, hair removal from healthy volunteers encounters substantial cultural, comfort, and esthetic barriers. In animal behavior studies, fur and feathers serve vital biological functions, rendering their removal both impractical in field studies and against the ethical treatment of laboratory animals. Thus, there is a pressing need for non-invasive brain neural interfaces capable of overcoming the challenges posed by hair while ensuring stable and conformal contact with the scalp.

Current strategies involve two types of non-invasive neural interfaces to achieve hair adaptability for brain study (Supplementary Fig. 1a, b). Solid electrodes7–10 have been designed with specific structures, such as claw configurations, to address this issue. However, these solid electrodes were fixed by external pressure on the scalp, where the continuously strong force often leads to extreme discomfort. Fluid interfacial materials11,12 such as saline bypass hair and conform to skin wrinkles. Nevertheless, they require EEG caps to fix electrodes, and introduce risks of positional shifts as well as signal crosstalk in high-density arrays. The development of a non-fluid and imperceivable interface for non-invasive neural electrodes to comfortably and efficiently acquire electrophysiology at hairy regions of animal scalps remains challenging.

We envision a hair-adaptable and adhesion-tunable scalp-electrode interface, termed HAAT, with the following features: (1) the capability to penetrate dense hair layers and conform to skin wrinkles, for example, through liquid-solid-liquid phase transitions; (2) robust mechanical and electrical coupling to the scalp for reliable recording; and (3) tunable adhesion for on-demand painless detachment. Biogels including gelatin and poly(N-isopropylacrylamide)13–15 are capable of penetrating dense hairs through fluid-gel transition. However, they exhibit either weak adhesion, causing unexpected detachment, or excessively strong adhesion, inducing hair loss or skin irritation (Supplementary Fig. 1c, d, Supplementary Table 1). Polymers with dynamic covalent bonds in the backbone have emerged as promising bioadhesives with potential hair-adaptability. Dynamic dissociation and formation of the covalent bonds facilitate fluid-gel transitions, strong interfacial bonding, and tunable adhesion modulated by external stimuli. Notably, disulfide-based poly(lipoic acid) (PLA) has recently been reported as a recyclable surgical super glue due to its high-strength adhesion and excellent biocompatibility16–18. Leveraging these properties, a dynamic covalent polymer incorporated with a disulfide bond emerges as a suitable interface for non-invasive neural electrodes for hairy animals, one that not only adapts to hair but also offers robust and customizable adhesion. Despite their outstanding mechanical properties and biocompatibility, poly(lipoic acid) based gels have found limited use in electrophysiological recording, highlighting the critical need for material innovation to fully realize the HAAT design.

Results
Mechanical design principle of HAAT
Ensuring a mechanically robust scalp-electrode contact is crucial for the non-invasive acquisition of high-quality electroneurographic signals from the brain (Fig. 1a). The robustness of the electrode-skin adhesion can be characterized by the critical peeling force (Fc) that is needed to break the contact19,20. According to the Irwin-Kies relation:

Fc / Gc C A n+1 (1)

where Gc, C, A and n are the critical fracture energy, compliance of the electrode material, contact area, and a peeling mode-dependent constant, respectively. Gc is further broken down into the intrinsic fracture energy (G0) the dissipative term (GD) that depends on specific peeling conditions including peeling rate (a_), temperature (T), etc.

Gc = G0 + GD = G0 (1 + f(a_, T, etc.)) (2)

Therefore, large modulus (low C) and strong interfacial bonding (large G0) favor stable adhesion21–23. An ideal HAAT goes through three steps in its lifecycle: 1) low-modulus fluid state for hair penetration; 2) high-modulus and adhesive gel state for stable skin-electrode interface; 3) non-adhesive state for hair-friendly removal (Fig. 1b). Dynamic polymers undergo fluid-gel transitions triggered by temperature, ultraviolet irradiation or solvent24–26, facilitating hair penetration and conformal attachment to skin. The stimuli-triggered formation and dissociation of covalent bonds in the polymer backbone and on polymer-skin interfaces alter the modulus and interfacial fracture energy, allowing both strong adhesion and easy on-demand detachment of the electrode. Dynamic polymers augmented with ionic conductivity, therefore, represent an ideal and generalizable strategy to implement HAAT for non-invasive electroneurographic sensing on the skins of varying hair densities (Fig. 1c).

Fabrication of HAAT
We demonstrate the HAAT principle using the copolymer of sodium thioctate (ST) and sulfobetaine methacrylate (SBMA), hereafter denoted as poly(ST-co-SBMA) (Fig. 2a). The disulfide bonds in the backbone can be formed and cleaved by heating and reducing reagents, giving the copolymer its dynamic behavior. We use ST, instead of its conjugate α-lipoic acid, to enhance the water dispersibility of the monomer and hydrophilicity of the polymer16. The PSBMA subunit plays several critical roles. First, the zwitterions in the side chain create ionic conduction channels, promoting sensitivity to electrophysiological signals. Skin interfacial impedances of the HAAT electrodes are comparable or even lower over a broad range of frequencies than commercial electrodes, which is attributed to the improved bio-interface as well as sufficiently high ionic conductivity (Supplementary Figs. 2, 3). Second, it suppresses the spontaneous ring-closing depolymerization of PST initiated by terminal sulfur radicals16,27,28. Electron paramagnetic resonance (EPR) spectroscopy, Fourier transform infrared (FTIR), Raman, and X-ray diffraction (XRD) indicate that the disulfide bonds of ST undergo homolytic cleavage upon heating, generating thiyl radicals to initiate the polymerization of SBMA. Simultaneously, ST diradical coupling occurs, resulting in the formation of poly(ST-co-SBMA) (Supplementary Figs. 4–10). To further increase the modulus, ferric ions are added as crosslinkers through the formation of coordination bonds (Supplementary Fig. 11). This copolymer thus enlists multiple interactions, including coordination, hydrogen bonds, electrostatic interactions, and dynamic disulfide bonds, to synergistically improve its modulus, stretchability, adhesion, and ionic conductivity (Fig. 2a, Supplementary Figs. 12–14). By adjusting the temperature and concentration of disulfide and coordination bonds, the mechanical properties of poly(ST-co-SBMA) can be effectively controlled.

Hair adaptability
Poly(ST-co-SBMA) exhibits a reversible fluid-gel transition within the range of body-friendly temperature (Supplementary Fig. 15). Rheological analysis reveals that the storage modulus (G’) and the loss modulus (G”) decrease, crossing at 62 °C (Fig. 2b). Differential scanning calorimetry (DSC) indicates a phase transition from gel to fluid at the same temperature13 (Supplementary Figs. 16, 17). The transition temperature is promoted and suppressed by the addition of disulfide and Fe3+, respectively (Supplementary Figs. 18–21). By optimizing the disulfide and Fe3+ concentrations, we achieve a fluidic poly(ST-co-SBMA) at slightly above body temperature, which easily penetrates thick hairs (Fig. 2c, Supplementary Figs. 22, 23 and Supplementary Movie 1) and fills the microstructures of the skin, thereby achieving conformal contact (Fig. 2d).

Raman spectroscopy further reveals the temperature-driven dynamic behavior of the disulfide bonds. Temperature-dependent Raman spectroscopy is conducted over the range of 25 to 95 °C. Of note, the dissociation of disulfide bonds does not lead to substantial mass loss, as revealed by the thermogravimetric analysis (TGA, Supplementary Fig. 24). Intensities of the S–S stretching modes at 509 and 527 cm−1 decrease with rising temperature, while the intensity of the S-H stretching at approximately 2536 cm−1 slightly increases29 (Fig. 2e). Two-dimensional correlation spectroscopy (2DCOS) provides additional insight23,30, with synchronous and asynchronous spectra revealing the sequence of changes: 509, 527→2536 (→ denotes preceding) (Fig. 2f, g). According to Noda’s rule, this progression indicates the cleavage of disulfide bonds into thiol groups upon heating. The resulting reduction in polymer chain length drives the transition to a fluid state with lower modulus. Conversely, during cooling, the reformation of disulfide bonds lengthens the polymer chains, restoring the HAAT to a gel state with a higher modulus (Supplementary Fig. 25).

Tunable adhesion
Tunable adhesion and on-demand detachment are crucial features of HAAT (Fig. 3a). To this end, we first investigate the effect of Fe3+ and ST contents on the modulus and interfacial toughness of poly(ST-co-SBMA). When Fe3+ concentration is low, higher Fe3+ content increases modulus by forming more Fe3+-carboxylate coordination bonds; accordingly, interfacial toughness increases by the contribution of the dissipative term (GD)19. At high Fe3+ concentration, excessive crosslinking reduces polymer chain mobility31,32, limiting the movement of adhesive groups to the interface and reducing interfacial toughness through the reduction of G0 (Fig. 3b). Initial increase of ST content raises the proportion of dynamic disulfide bonds, leading to a less rigid polymer backbone and lower modulus. Concomitantly, the more abundant adhesive groups increase G0, dominating over decreasing GD and leading to enhanced interfacial toughness. With increasing ST content, increased rigidity limits the availability of adhesive groups on the interface, leading to reduced interfacial toughness (Fig. 3c). By tuning Fe3+ and ST content, we simultaneously optimize G0 and GD, achieving the largest interfacial toughness of 523 N/m.

Next, we demonstrate that chemically triggered cleavage of the dynamic bonds drastically reduces interfacial toughness, enabling painless detachment of the electrode (Fig. 3d). This is achieved by delivering a detachment solution consisting of glutathione (GSH) and NaCl to the skin-gel interface. GSH cleaves the disulfide bonds both at the interface and within the polymer backbone, resulting in reduced intrinsic adhesion and cohesion by breaking the polymer main chains33. NaCl disrupts the interfacial non-covalent dynamic interactions, such as electrostatic interactions and hydrogen bonding34. PBS solution (simulated sweat) alone cannot detach HAAT from the scalp, and large adhesion loss only occurs in the presence of reducing agents capable of cleaving disulfide bonds (Supplementary Movie 2). These observations suggest that interfacial disulfide bonds are the determining factor of interfacial toughness. Upon the delivery of the detachment solution, interfacial toughness between HAAT and porcine skin drastically decreases from 523 N/m to almost zero under a 90-degree peeling test (Fig. 3e, Supplementary Figs. 26, 27). The Raman intensities of υ(S–S) at 509 and 527 cm−1 decrease substantially upon addition of the detachment solution, confirming the cleavage of disulfide bonds (Fig. 3f). These results highlight the on-demand detachment of HAAT without skin irritation or hair loss (Figs. 3g top, 3h, Supplementary Movie 3), which is advantageous over commercial electrode (Fig. 3g bottom, Supplementary Fig. 28). Notably, the detachment solution modulates the interfacial toughness (Γ) by over 50 times, ranking the highest among both hair-in-adaptable and adaptable EEG gels in literature (Fig. 3i, Supplementary Fig. 29 and Supplementary Table 2). In the cytotoxicity assessments of HAAT (Supplementary Fig. 30), the fluorescent images exhibit no observable signs of apoptosis or necrosis. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay further demonstrates a higher proliferation rate of cells on HAAT compared to the control group after 3 days of culture, indicating excellent biocompatibility of HAAT.

Hairy scalp EEG recorded by HAAT electrode
Human and animals of different evolution levels are important research subjects in brain cognition2,35–37. Animals, particularly primates and rodents, possess structural and functional similarities with humans, making them excellent models for uncovering the physiological mechanisms and cognitive processes of the human brain. Electroneurographic signals, such as scalp electroencephalogram (EEG), offer prominent evidence for various brain activities38,39. To evaluate the acquisition ability of scalp EEG, HAAT electrode is first applied on human occipital region that is covered by thick hair (Fig. 4a). Compared to commercial EEG pastes, HAAT is able to penetrate hair without the need of additional pressure, tightly and stably adhere to scalp during prolonged usage in the presence of simulated sweat, and is readily removed by a detachment solution (inset of Fig. 4c, Supplementary Figs. 31–33). Characteristic brainwaves (α, β, γ, θ) are distinguished after filtering the raw signals40 (Fig. 4b, Supplementary Fig. 34). The power spectral density (PSD) of α and β waves obtained by HAAT is higher than that by commercial paste, highlighting its superior performance in scalp EEG acquisition on human41,42 (Fig. 4c, Supplementary Fig. 35). HAAT is also able to penetrate the denser and finer hair of monkey, and construct a stable bio-interface enabling accurate EEG acquisition without the need of pressing (Supplementary Movies 4, 5). Harnessing the positional stability of HAAT, we subsequently utilize it in a 16-channel EEG system (O1, O2, P3, and P4) to concurrently record monkey EEG from different brain regions without signal crosstalk (Fig. 4e, f, Supplementary Figs. 36, 38), demonstrating its potential for high-density EEG mapping. Accurate acquisition of scalp EEG from animals with smaller heads and denser hair, such as mice, remains an outstanding challenge. Commercial EEG pastes fail to penetrate mouse fur and acquire an EEG signal even in the presence of applied pressure (Supplementary Fig. 39, Supplementary Movie 6). In contrast, the HAAT electrode successfully detects auditory evoked potentials (AEP) from the mouse scalp, demonstrating their superior hair adaptability (Fig. 4h, Supplementary Figs. 40, 41). A side-by-side impedance comparison between commercial gel and HAAT on skin and hairy skin is summarized in Fig. 4i, Supplementary Figs. 42.

ERP monitoring in a monkey’s vision-attention task
To further demonstrate the capability of HAAT in hairy scalp EEG electrodes, we incorporate it into the monitoring of weak electroneurographic signals to study a monkey’s cognitive process. Vision is a primary mode of interaction with the external environment, and has been extensively studied in cognitive neuroscience36. A vision-attention task is designed to investigate attention’s influence on the visual processing of monkey (Fig. 5a). In this task, monkey is tasked with detecting a bright dot that emerged after a bright annulus over 4 h (Fig. 5b). Throughout this process, the event-related potential (ERP), which mirrors brain’s responses to distinct sensory, cognitive, or motor events, is meticulously monitored by HAAT electrode. ERP components are extracted and averaged over repeated trials to minimize noise and highlight signals associated with the events. Stimuli-evoked C1 (50–100 ms), P1 (100–130 ms), and N1 (120–200 ms) components are observed under eight different conditions43 (Supplementary Figs. 43–50, Supplementary Table 3), indicating the high quality of signals and reflecting stimuli-evoked working processes. Heatmaps of neighboring channels (P3, P4 by HAAT electrodes and PO7, PO8 by commercial electrodes) demonstrate comparable ERP patterns under these conditions (Fig. 5c–e). The signal-to-noise ratio (SNR) in the early stage of the task is slightly higher than that in the end period, indicating that HAAT achieves equivalent performance throughout the entire task (Supplementary Figs. 51, 52). These further confirm that HAAT can serve as an advanced alternative to commercial pastes for ERP collection.

To investigate the monkeys’ visual processing, ERP signals are compared across different stimulus (annulus and dot) and locations (left and right) in the task requiring the monkeys to detect the position of the target. Under the condition of Annulus Right (the annulus was presented on the right visual field), a pronounced P1 component is observed in the contralateral channel (on the left hemisphere of the brain) (Fig. 5f, orange), while the P1 component barely rises in the ipsilateral channel (on the right hemisphere brain) (Fig. 5f, orange). This aligns with the principle of contralateral processing of visual information44 (Fig. 5g). However, under the condition of Annulus Left, not only the contralateral channel (Fig. 5f, blue) but also the ipsilateral channel (Fig. 5f, blue) exhibits a significant P1 component, suggesting a stronger response to left-side stimuli. This indicates a lateralized attention bias toward the left visual field. The ERP amplitudes (P1/N1) for Annulus Left (Figs. 5f and 5f, blue) are larger than those for Dot Left (Supplementary Fig. 53, blue) conditions, because annulus stimuli are larger and brighter than dot stimuli. This task demonstrates the feasibility of using HAAT electrodes for recording ERP signals in hairy regions, underscoring their potential applications in cognitive neuroscience research. The HAAT electrode could be further used to explore exogenous attention effects on ERP by exploring more complex task conditions, such as setting the annulus as a cue and the dot as a target appearing in the same experimental trial.

Discussion
In this work, we propose a hair-adaptable and adhesion-tunable scalp-electrode interface achieved by incorporating dynamic covalent bonding into ionically conductive polymers. Poly(ST-co-SBMA) serves as a proof-of-concept of HAAT, with which we demonstrate non-invasive electroencephalographic recording on the hairy scalp for an animal cognition study. The fluid-gel phase transition facilitates facile hair penetration and adaptability to the scalp surface. Dynamic covalent bonds provide robust adhesion during bio-signal acquisition and allow for on-demand removal via a detachment solution. HAAT electrodes successfully acquired high-quality electroneurographic signals from hairy-skin regions of human, monkey, and mouse, highlighting its potential for studying brain cognition. We believe that the present design can be extended to other dynamic covalent bonding polymers for the development of hair-adaptable and adhesion-tunable scalp-electrode interfaces.

Methods
Materials
DL-α-Thioctic acid (LA), Sulfobetaine methacrylate (SBMA), Iron chloride hexahydrate (FeCl3·6H2O), Sodium chloride (NaCl) and Glycerol are obtained from Aladdin. Glutathione (GSH) and Sodium hydroxide (NaOH) were obtained from Innochem. All purchased chemicals are of analytical purity. The commercial electroneurographic electrode was purchased from (X-1, Hangzhou Xunda). The commercial EEG paste (for human, monkey and mouse) and conductive gel were purchased from (Ten20, Weaver and Company) and (GT10, GREENTEK), respectively.

Synthesis of HAAT
HAAT was prepared via the following steps. Firstly, LA (4 g) and NaOH (0.7755 g) were dispersed uniformly in deionized water (10 mL) to prepare the ST solution. The ST solution was stored at 4 °C in the dark. Secondly, FeCl3·6H2O (2 g) was dissolved in deionized water (5 mL) to form an FeCl3 aqueous solution. Thirdly, a certain amount of FeCl3 aqueous solution, ST (2 mL), SBMA (2 g), glycerol (1 mL) was added into deionized water (2 mL) and then stirring in the dark to form the precursor solution of HAAT. The specific contents of each component are listed in Supplementary Tables 4, 5. Finally, the precursor solution was polymerized at 90 °C for 210 min to form the HAAT. Additionally, GSH (0.5 g) and NaCl (0.1 g) were dispersed uniformly in deionized water (9.4 g) as the detachment solution (5 wt% GSH and 1 wt% NaCl).

Characterizations of HAAT
The cross-sectional morphology of the HAAT was observed by SEM (SU-8010, HITACHI). EPR is measured at 9.853 GHz on a Bruker A300 instrument. Raman spectroscopy is performed by LabRAM HR Evolution (Horiba) equipped with an objective (50×, Olympus). The wavelength of the excitation laser is 532 nm. FTIR spectra of the monomers and the freeze-dried HAAT are collected on a Fourier transform infrared spectrophotometer (IRAffinity-1, SHIMADZU) over the spectral range from 4000 cm−1 to 500 cm−1. XRD of LA monomer and the freeze-dried bulk HAAT is collected on a diffractometer (X’Pert Pro MPD, Panalytical). The thermal Imaging Camera (FLIR E54) is used for real-time observation of the temperature of HAAT. DSC is obtained on a METTLER TOLEDO DSC1 instrument at a ramp rate of 3 K/min. The rheological testing is conducted on a rheometer (MCR 302, Anton Paar). After frequency sweep (Supplementary Fig. 18a) and amplitude sweep (Supplementary Fig. 18b), the temperature ramp is conducted at the condition of 40% strain and 1 rad/s with the ramp rate of 5 K/min. Thermogravimetric analysis (TGA) is performed with a heating rate of 10 K/min from room temperature to 700 °C in N2 atmosphere (STA200, HITACHI).

Mechanical and adhesive performance measurements
The samples for mechanical performance were prepared in a Telfon mold (50 mm × 25 mm × 1.5 mm). The precursor solution was poured into the mold before heating. After heating and cooling, HAAT with a thickness of about 1 mm was transferred from the mold and cut into strips (approximately 20 mm × 10 mm × 1 mm) for the tensile test. The tensile tests were conducted on a tensile tester (F105-IM, MARK-10). The tensile test was performed at a speed of 60 mm/min.

Heated HAAT was brushed on dried porcine skin or other substrates. The width of the specimen was about 10 mm. The sample is used for the 90-degree peeling test after cooling to room temperature. A tape was attached to the top surface of HAAT with the help of clamps, gauze and super glue (Supplementary Fig. 26). The peeling test was performed at a speed of 20 mm/min.

Cytotoxicity test
The cytocompatibility of HAAT was evaluated using an extract-based cell culture method. Human skin fibroblasts (HSFs) were seeded in a 96-well plate at an initial density of 5000 cells per well and cultured overnight to allow cells to attach and adapt. Subsequently, sterilized HAAT extract (prepared by dissolving 2 g of HAAT in 1 mL of culture medium) was added to the wells. A control group consisting of HSFs cultured in standard medium without HAAT extract was also established. Cell viability and cytotoxicity were assessed after 24, 48, and 72 h. At each time point, the pristine medium was removed, and the cells were stained using a Calcein-AM/PI Cytotoxicity Assay Kit. Fluorescent images were captured using a Confocal Laser Scanning Microscope (Stellaris 5, Leica) to visualize cell morphology. Cell viability was further conducted using an MTT kit (Thermo Fisher). All parallel holes were set in five groups for each test.

Subjects
One male macaque monkey (Macaca rhesus DN, aged 8 to 9 years at the beginning of the experiments and weighing 10 to 11 kg) participated in the experimental tasks. To immobilize the animal’s head during behavioral training, a titanium post was surgically attached to its skull using bone screws. The procedure involved inducing general anesthesia with ketamine (10 mg/kg) and maintaining it under isoflurane (1.5%–2.0%). All procedures followed the guidelines outlined in the National Institutes of Health Guide for the Care and Use of Laboratory Animals. Approval for the experiments was obtained from the Institutional Animal Care and Use Committee of Beijing Normal University. C57BL/6J male mouse, aged 3–6 months and weighing 25–30 grams (Charles River Laboratories), was used in this experiment. The mouse was allowed to acclimate to the environment for at least one week prior to the experiment. It was housed under standard conditions of 22 ± 2 °C, 50 ± 10% humidity, and a 12-h light/dark cycle. For procedures requiring anesthesia, the mouse was anesthetized using 3% isoflurane delivered via an inhalation mask. Anesthesia was maintained throughout the procedure, and the mouse was monitored for signs of distress. Afterward, the mouse was placed in a recovery room at 22 ± 2 °C, with regular monitoring of their body temperature and overall health. All experimental procedures were approved by the Animal Ethics Committee of Beijing Normal University and were conducted in accordance with national animal welfare regulations.

Vision-attention task
In the vision-attention task, a monkey was required to detect a bright dot (as a target) that appeared following the presentation of a bright annulus (as a cue). To initiate a trial, the monkey fixated on a 0.3° fixation point (FP) for 300 ms (within a 3° window). The main part of the trial lasted 2.5 s, during which the monkey had to maintain fixation on the FP (within a 3° window). After a 350 ms delay following the main part of the trial, a bright annulus appeared on a gray background, either 5° to the left or right of the fixation point, and lasted for 20 ms. 580 ms after the disappearance of the annulus, a bright dot appeared, also 5° to the left or right of the fixation point, and lasted for 20 ms. The animal was required to maintain fixation until the end of the trial, at which point a response prompt display appeared, presenting two bright dots 5° to the left and right of the fixation point, indicating that the animal should make a saccade to one of the locations36. During this response phase, the animal was required to make a saccade to the location it believed the target had appeared, and maintain fixation for 150 ms. Within 0–2000 ms after the appearance of the response prompt display, trials in which the animal made a saccade to the correct location were counted as correct and rewarded with water; trials in which the animal broke fixation or responded incorrectly were not rewarded.

In most trials, both the cue (bright annulus) and the target (bright dot) stimulus appeared. In a smaller number of trials, only the target stimulus appeared, meaning that at the time the cue appeared, only the fixation point was presented. Whether a reward was given in these trials depended on the animal correctly detecting the target’s location. In some trials, only the cue was presented, meaning that at the time the target appeared, only the fixation point was present. In these cases, rewards were given randomly.

Stimulus details
Visual stimuli were generated using a stimulus generator (ViSaGe; Cambridge Research Systems) controlled by a custom C++ program developed in our lab. These stimuli were presented on a 22-inch CRT monitor (Dell, P1230, 1200 × 900 pixels, mean luminance 29.4 cd/m², 100 Hz refresh rate). The viewing distance was 114 cm. In the vision-attention task, all stimuli were presented on a gray background. A bright annulus appeared on either the left or right side of the screen, positioned 5° from the fixation point. The annulus had a contrast of 0.9, with an inner diameter of 2.2° and an outer diameter of 2.5°. Additionally, a bright dot was also displayed on either the left or right side of the screen, 5° from the fixation point. This dot had a contrast of 0.56 and a diameter of 0.35°. The positions of the annulus and the dot were independent of each other.

Data recording and analysis
To evaluate the electrical performance, we measured the skin contact impedance of the HAAT electrode and commercial electrode by an electrochemical workstation (CHI660E) in the range from 1 Hz to 100 kHz with the applied voltage of 20 mV.

The human and mouse EEGs in the present work were recorded by a neuroelectric signal recording box (Human Spike Box, Backyard Brains). For dual-channel EEG recording, two electrodes (Au cup electrodes with commercial EEG paste or HAAT) were placed on occipital region (Supplementary Fig. 32), and a commercial ECG electrode was placed behind the ear as a ground electrode. For mouse EEG recording, two HAAT electrodes were placed mouse scalp, one of which is a reference (Supplementary Fig. 40). The hairy scalp EEG was recorded by the box with a bandpass filter (1 Hz to 50 Hz).

The monkey EEG signal processing was performed on the collected data using MATLAB (v. 2023a) for fundamental signal analysis (independent component analysis/superposition average/fast-Fourier transform/filter). A 16-channel EEG cap was used to record monkey EEG, with one reference on the forehead (placed as Fig. 4e). Raw data were acquired with a 128-channel system (Blackrock Microsystems). The raw data were downsampled to 500 Hz and subsequently bandpass filtered between 0.3 and 50 Hz. Trials exhibiting excessive signal variability were excluded from the analysis. For each trial, the main segment of 2500 ms and the preceding 100 ms will be included in the analysis. Additionally, the results of all valid trials for each condition were averaged to obtain the ERP.

The SNR value is calculated according to the equation below.

SNR = 20 log10 (RMS_ERP / RMS_PRE)

SNR represents the signal noise ratio of the ERP, RMS_ERP and RMS_PRE represents the root mean square of the ERP signal and baseline, respectively. Following the Bootstrap Method, 5000 SNR values are obtained for comparison of signal quality by different electrodes.

Statistics and reproducibility
All experiments were repeated independently with similar results at least three times.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In this manuscript, the authors presented a novel adhesive gel that leverages dynamic covalent bonding and thermoresponsive polymer design to enable EEG recordings from hairy scalps in humans, monkeys and mice. The use of dynamic disulfide adhesive to enable reversible transitions between fluid and gel states can allow a good scalp contact thus increasing recording stability. This novel adhesive gel has the potential to replace existing EEG interfacing gels. Overall, this paper makes a significant contribution to material innovation for application in EEG recordings. Therefore, I recommend its publication in Nature Communications after addressing the comments below.\n1. In Fig.2, it seems that temperature needs to be increased to around 60 C to have the gel transitioning to liquid state, which seems a little high for clinical applications. Is it possible to reduce the transitioning temperature by slightly modifying the gel material? Alternatively, are there other ways to mitigate the heating effects on the scalp when applying?\n2. The main focus of the manuscript is to develop a bioahesive gel that has the potential to replace existing EEG gel. I would recommend change “Scalp-Electrode Interface” to “Scalp-Electrode Interfacing Gel” in the title to increase clarity.\n3. Reference to existing literatures on state-of-the-art bioadhesive EEG electrodes are missing and should be added, for example, https://www.nature.com/articles/s44385-025-00009-x\n4. It will be beneficial to add a supplementary table to compare the gel developed in this paper, commercial gels, and gels developed by other researchers to highlight the novelty of this paper.\n5. A minor one: In the abstract, event-related potential (EPR) should be (ERP).", "reviewer_2": "In general, the HAAT gels presented here is lack of novelty. It seems to me that any hydrogel systems that allow sol-gel transition at a rather physiologically-acceptable condition can be used for such a purpose. As for the capacity for sensing electroneurographic signals, most hydrogels capable of electron transduction can be used for this purpose. Therefore,the advantages and novelty of the current study is not clear. Moreover, it is plausible that the presence of hairs at the scalp-electrode interface seemed to be a significant challenge, but in practice, this problem can be simply addressed by cutting the hair of the patients. Why on earth take so much efforts to address such easy problem. Issues like anti-evaporation, tolerance to more complications such as sweat or body movements seem to be more valuable in the relevant fields. Therefore, I don’t think the manuscript reach the level of Nature Communications.\nSome other issues that should be addressed.\n1.The rheological behavior of the gel electrodes should be studied with more details.\nTypical requirement for hydrogel electrodes should be clarified based on literature, the HAAT gels presented herein seems to be very weak, with the G’ values less than 5 Pa. I doubt such weak gels can really be useful for realistic applications, since such weak gels can hardly withstand the shear forces or even gravity.\n2.The stability of gel gels is not clear. For instance, whether the gel electrodes can tolerate sweat, or long-term exposure to dry air, body movements, or other common conditions that patients can experience during their daily lives.\nMore importantly, these conditions might influence the accuracy of the readout of electroneurographic signals. This should also be evaluated.\n3.Adhesion behavior to hairly skin should also be tested with the presence of liquid condition such as simulated sweat or different humidity.\n4.The adhesion of the gels to the hairly skin can be released by adding GSH and NaCl. NaCl could also present in body sweat, which means the mechanical functions can be significantly compromised if body secret sweat, which seems to be unavoidable. How to solve this contradiction?\n5.For animal tests, comparisons of the sensitivity of HAAT gels versus currently available materials should be added.", "reviewer_3": "The present manuscript describes the preparation of hair-adaptable and adhesion-tunable (HAAT) scalp electrode using a copolymer with sodium thioctate, featuring dynamic disulfide bonds for temperature-triggered fluid–gel transitions. This allows easy penetration through hair, strong yet reversible adhesion, and gentle removal. The HAAT gel electrode reliably records EEG from hairy scalps of humans, monkeys, and mice. In monkey studies, it captured event-related potentials revealing neural lateralization, marking the first use of dynamic covalent polymers for non-invasive brain recordings in animals. However, there is unclear mechanism of gel electrode formation and how this gel improves conductivity. These are required detailed explanations and scientific evidence. I firmly suggest that the author must do the following comments. I still suggest the authors to add more insights into the studies carried out. To this end, the following list of papers can be considered and cited, where fitting within the existing narrative flow. The quality of the manuscript is satisfactory. Major revisions needed.\nFollowing have to be addressed,\n1. Many articles have been published on recording electroneurographic signals. In which specific aspects does this work differ from existing studies?\n2. Since the thermal and chemical properties of polymers are crucial for their design, the author is advised to include TGA (Thermogravimetric Analysis) and DMA (Dynamic Mechanical Analysis) in addition to DSC (Differential Scanning Calorimetry) to more accurately determine the glass transition temperature of the composite gel.\n3. Is the gel biocompatible? Given that the gel is in direct contact with skin, the author is encouraged to conduct basic cell viability tests, possibly using both Gram-positive and Gram-negative bacteria.\n4. Could the author specify the solvents used for removing the gel from skin, and describe the gel’s solubility in various solvents, including hot water? Additionally, was an adhesion test conducted on wet or oily skin?\n5. What is the gel’s reproducibility after applying the detachment solvent? The author is advised to include a cyclic peeling test to evaluate repeated use.\n6. The Materials and Methods section lacks clarity, making it difficult to understand key experimental details such as chemical compositions, mixing ratios, reaction conditions, and solvent usage. The author should revise this section with more comprehensive information.\n7. The role of Fe in coordination and its effect on mechanical properties is well-discussed. However, optimization of electrical performance is not clearly illustrated. For example, the addition of NaCl is not described in the experimental section. The author is therefore recommended to revise the experimental section to include missing information and to characterize the resistance change during the gel-to-liquid phase transition.\n8. Does the gel possess self-healing properties, considering the author’s claim about the presence of reversible S–S covalent bonds?\n9. The optimization of adhesive properties through Fe-ion coordination and ST disulfide ratios is well-described using Young’s modulus and interfacial toughness measurements. However, the author is also encouraged to discuss viscosity changes with appropriate characterization methods.\n10. In order to strengthen your introduction, please consider the following references. I suggest authors to cite some of the recent related literatures. Authors can cite recent references, which can deliver and connect the audience with the current study.\n(a) Nat Biomed Eng 3, 194–205 (2019). Doi.org/10.1038/s41551-019-0347-x.\n(b) Adv. Sci. 2021, 8, 21, 2102275. Doi.org/10.1002/advs.202102275\n(c) Adv. Mater. 2023, 35, 2303267. DOI: 10.1002/adma.202303267.\n(d) Neural Regeneration Research, (2024) 19 (3), pp. 680 – 686. DOI: 10.4103/1673-5374.380908.\n(e) Adv. Sci. 2024, 11, 2404019. Doi.org/10.1002/advs.202404019.\n(f) Bioactive Materials, (2024) 31, pp. 408 – 421. DOI: 10.1016/j.bioactmat.2023.08.021.\n(g) Nano Energy 133 (2025) 110416. Doi.org/10.1016/j.nanoen.2024.110416.\n11. The gel electrode’s conductivity is stated to be based on NaCl; however, the author also claims that NaCl disrupts interfacial non-covalent interactions, such as hydrogen bonding, during the peeling experiment. This apparent contradiction needs to be clarified. Additionally, the effects of varying NaCl concentration on the mechanical properties-specifically Young’s modulus and interfacial toughness-should be investigated and reported to better understand its role in balancing conductivity and adhesion performance.\n12. How does skin moisture, such as sweat, affect the adhesive properties of the gel electrode? Since real-world applications involve contact with sweaty skin, it is important to evaluate the adhesion performance under such conditions?", "reviewer_4": "This manuscript presents a highly innovative study on the development of a hair-adaptable and adhesion-tunable scalp-electrode interface (HAAT) based on dynamic disulfide bonding polymers for non-invasive electroneurographic recording in humans and animal models. The authors convincingly show how HAAT overcomes key challenges in EEG acquisition from hairy scalps, particularly in monkeys and mice. The temperature-triggered fluid-to-gel transition and glutathione-based detachment are elegantly designed and offer strong translational potential. The study is thoroughly validated through comprehensive material characterization, multi-species EEG/ERP recordings, and a well-designed cognitive task in non-human primates. The manuscript is clearly written, well organized, and supported by solid data and analysis. The integration of advanced materials science with cognitive neuroscience is both timely and impactful. Given its significant contribution to advancing non-invasive neurotechnology, I recommend accepting this manuscript for publication in Nature Communications, pending the authors’ resolution of the following issues.\n1. Some figures lack adequate description and require clarification. For example, in Figure 1a, the size and location of the mouse brain in the schematic are inconsistent with anatomical accuracy and should be revised. In Figure 1c, the subject shown is unclear and should be explicitly identified. Additionally, Figure 3h is missing a scale bar, which is recommended for proper interpretation.\n2. While fabricating HAAT polymer, EPR was conducted to prove that the disulfide bonds of ST undergo cleavage upon heating, generating radicals to initiate the polymerization of SBMA. The authors should explain in detail how EPR results substantiate this conclusion.\n3. The authors indicate that the ST/SBMA monomer ratio affects mechanical properties (Supplementary Figure S9). A detail discussion on how this ratio influences mechanical behavior would be helpful.\n4. Understanding the mechanism behind hair adaptability is central to the HAAT design. The authors attribute HAAT’s thermal responsiveness to dynamic disulfide bonds and present supporting rheological and DSC data. However, the results also suggest that Fe³⁺-carboxyl coordination bonds may contribute to this transition. The authors are encouraged to clarify whether additional factors, beyond disulfide bonds, contribute to thermal responsiveness and to explain their respective roles.\n5. The authors need to provide the preparation details of the detachment solution (GSH and NaCl) and explain the rationale for choosing these components.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": "We are grateful to the reviewer for the positive comments on our work. Our point-by-point responses are listed as below.\n1. “In Fig.2, it seems that temperature needs to be increased to around 60 °C to have the gel transitioning to liquid state, which seems a little high for clinical applications. Is it possible to reduce the transitioning temperature by slightly modifying the gel material? Alternatively, are there other ways to mitigate the heating effects on the scalp when applying?”\nResponse: We thank the reviewer for raising this concern. Indeed, the transition temperature can be modulated by adjusting the concentration of dynamic interactions, which depends primarily on the contents of sodium thioctate (ST) and Fe3+ ions. As shown in Supplementary Figs. 19 and 20, increasing the ST and Fe3⁺ concentration influences the transition temperature. As the amount of disulfide bond increase, polymer network becomes more susceptible to temperature, facilitating state transition. Conversely, an increase in crosslinking density enhances the robustness of the polymer network, requiring more energy to break Fe3+-carboxyl coordination bonds during heating. This results in a higher state transition threshold. A detailed explanation is provided in the \"Hair Adaptability\" section of the revised manuscript and SI.\nWe would like to notice that our volunteers did not experience any obvious heat sensation in the skins or hair during the application of the HAAT electrode. We believe that, although the gel was initially heated to 60 °C, it quickly cooled down as it penetrates the hair, approaching skin temperature as it approached the scalp. Considering the small amount of gel and short exposure time above skin temperature, application of HAAT has minimal risk of scalding on scalp.\n2. “The main focus of the manuscript is to develop a bioadhesive gel that has the potential to replace existing EEG gel. I would recommend change “Scalp-Electrode Interface” to “Scalp-Electrode Interfacing Gel” in the title to increase clarity.”\nResponse: We thank the reviewer for this suggestion. Following your suggestion, we have changed the title of the manuscript to be: “Hair-Adaptable and Adhesion-Tunable Scalp-Electrode Interfacing Gel for Animal Cognition Study”.\n3. “Reference to existing literatures on state-of-the-art bioadhesive EEG electrodes are missing and should be added, for example, https://www.nature.com/articles/s44385-025-00009-x ”\n4. “It will be beneficial to add a supplementary table to compare the gel developed in this paper, commercial gels, and gels developed by other researchers to highlight the novelty of this paper.”\nResponse: We thank the reviewer for the suggestions. We have cited 10 additional references in the revised manuscript (see below), and compared hair adaptability and adhesion tunability among HAAT, commercial interfacial materials and references in the revised Supplementary Information (Supplementary Table 2).\nIn the revised manuscript, we have added the following discussion.\n“……, which is attributed by the design of dynamic covalent bonding (Fig. 3i, Supplementary Fig. 29 and Table 2).”\nSupplementary Table 2. Comparison of the hair adaptability and adhesion performance between HAAT and state-of-art interfacial materials.\nHair adaptability Adhesion tunability\nNo need\nSpontaneous\nMaterials additional maximum Stable self-Minimum Gentle Ref.\nhair Location\nfixture on adhesion adhesion adhesion removal\npenetration\nhairy skin\nGel × × Hairy ×\nN/A -- -- --\n(Xunda, X-1) forearm\nPaste × × ×\nCommercial (Weaver and company, Ten20) Hairy scalp N/A -- -- --\ninterfacial\nmaterials Gel × √ Hairy scalp N/A × -- -- --\n(Greentek, GT10)\nGel × √ ×\nHairy scalp N/A -- -- --\n(Weaver and company, Nuprep)\nPGA/GelMA hydrogel N/A N/A Hairless skin 8.1 N/m × ~0 -- 1\nPEDOT:PSS/SBMA/PEGDA √ √ Hairy lower ×\n6.5 kPa -- -- 2\nhydrogel leg\nGel LA/MBAA/Fe3+ hydrogel √ × Hairless skin 180 N/m √ 80 N/m × 3\nPBAc/AA/AAc-NHS/polyol √ × √ √\nHairless skin 400 N/m 24 N/m 4\nhydrogel\nPVA/PAA/NHS hydrogel N/A N/A Hairless skin ＞400 N/m √ ~40 N/m √ 5\nPAA/PU/NHS hydrogel √ N/A Hairy scalp 70 N/m √ N/A √ 6\nPLA/N,N-diacryloyl-L-lysine/Au- √\nN/A N/A Hairless skin 6 N/A 7\nGA hydrogel\nSA/TA polymer N/A × Hairless skin 45.7 N/m √ 3.0 N/m √ 10\n√ √ Hairy\nforearm √ √ This\nHAAT 523 N/m ~0\nwork\n√ √\nHairy scalp\nPGA: polymerized gallic acid; GelMA: gelatin methacryloyl; SBMA: [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide; AA: acrylic acid; PEGDA: poly(ethylene glycol) diacrylate; LA: lipoic acid; MBAA: N,N’-methylenebis(acrylamide); PBAc: a copolymer prepared from a free radical copolymerization of acrylic acid (AA), 4-vinylphenylboronic acid (4-VPBA), and hydroxyethyl methacrylate (HEMA); AAc-NHS: acrylic acid N-hydroxysuccinimide ester; PVA: polyvinyl alcohol; NHS: N-hydroxysuccinimide; PU: polyurethane; SPBS: sorbitol-modified polyborosiloxane; CA: catechol; SA: stearyl acrylate; TA: tetradecyl acrylate.\nNote: “N/A” indicates “not available” in the references.\nReference:\n1. Jiang, Y. et al. Infant Skin Friendly Adhesive Hydrogel Patch Activated at Body Temperature for Bioelectronics Securing and Diabetic Wound Healing. ACS Nano 16, 8662-8676 (2022).\n2. Huang, X. et al. In Situ Forming Dual-Conductive Hydrogels Enable Conformal, Self-Adhesive and Antibacterial Epidermal Electrodes. Advanced Functional Materials 33, 2302846 (2023).\n3. Cheng, Y. et al. An Elastic and Damage-Tolerant Dry Epidermal Patch with Robust Skin Adhesion for Bioelectronic Interfacing. ACS Nano 16, 18608-18620 (2022).\n4. Xue, Y. et al. Trigger-Detachable Hydrogel Adhesives for Bioelectronic Interfaces. Advanced Functional Materials 31, 2106446 (2021).\n5. Chen, X., Yuk, H., Wu, J., Nabzdyk, C. S. & Zhao, X. Instant tough bioadhesive with triggerable benign detachment. Proceedings of the National Academy of Sciences 117, 15497-15503 (2020).\n6. Ahmed, S. et al. Stick-and-play bioadhesive hairlike electrodes for chronic EEG recording on human. npj Biomedical Innovations 2, 9 (2025).\n7. Zhang, Z. et al. Combination of biodegradable hydrogel and antioxidant bioadhesive for treatment of breast cancer recurrence and radiation skin injury. Bioactive Materials 31, 408-421 (2024).\n8. Tian, Q. et al. Hairy-Skin-Adaptive Viscoelastic Dry Electrodes for Long-Term Electrophysiological Monitoring. Advanced Materials 35, 2211236 (2023).\n9. Cai, C., Wu, S., Tan, Z., Li, F. & Dong, S. On-Site Supramolecular Adhesion to Wet and Soft Surfaces via Solvent Exchange. ACS Applied Materials & Interfaces 13, 53083-53090 (2021).\n10. Gao, M. et al. Skin temperature-triggered, debonding-on-demand sticker for a self-powered mechanosensitive communication system. Matter 4, 1962-1974 (2021).\n5. “In the abstract, event-related potential (EPR) should be (ERP).”\nResponse: We thank the reviewer for finding this typo, and have corrected it in the revised manuscript.\n“Moreover, the accurate event-related potential (ERP) monitored during monkey vision-attention tasks facilitates the study of monkey brain activity, implying that HAAT is an ideal bio-interfacing material for conducting non-invasive research on animal cognition.”\nReviewer #2:\n“In general, the HAAT gels presented here is lack of novelty. It seems to me that any hydrogel systems that allow sol-gel transition at a rather physiologically-acceptable condition can be used for such a purpose. As for the capacity for sensing electroneurographic signals, most hydrogels capable of electron transduction can be used for this purpose. Therefore, the advantages and novelty of the current study is not clear. Moreover, it is plausible that the presence of hairs at the scalp-electrode interface seemed to be a significant challenge, but in practice, this problem can be simply addressed by cutting the hair of the patients. Why on earth take so much efforts to address such easy problem. Issues like anti-evaporation, tolerance to more complications such as sweat or body movements seem to be more valuable in the relevant fields. Therefore, I don’t think the manuscript reach the level of Nature Communications.”\nWe welcome the opportunity to clarify the significance and novelty of this work. We strongly believe that developing hair-adaptable neural electrodes is crucial on both scientific and ethical grounds, and our work constitutes a significant contribution to this goal through material innovation.\nGeneral Comment 1: Significance of Hair Adaptability\nResponse: A primary concern of the reviewer is the significance for developing scalp-electrode interface with hair adaptability. The reviewer believes that cutting the hair of patients can simply address the hair barrier in scalp-electrode interface. However, hair removal is neither practical nor ethical in many scenarios:\n1. In daily life, healthy people are reluctant to shave hair for non-clinical purpose: In clinical settings, hair removal is an established pre-operative protocol for procedures such as craniotomy, where patient compliance is implicitly secured by therapeutic necessity. However, this paradigm fails to translate to non-clinical scenarios. In cognitive neuroscience research, healthy volunteers constitute the main group for EEG studies for understanding psychology or behaviors. As exemplified by the event-related potential (ERP) analyzed in vision-attention study, they are able to mirrors brain’s responses to distinct sensory, cognitive, or motor events. These volunteers are recruited as research collaborators rather than patients, rendering hair removal infeasible due to cultural, comfort, and aesthetics considerations. The development of hair-adaptable interface materials with tunable adhesion thus represents a great advancement, enabling the non-invasive EEG monitoring while respecting volunteers’ autonomy.\n2. Animal fur and feathers serve vital biological functions: EEG recordings in animal behavior studies require frequent and repeated removal of hairs due to their fast growth rate, which severely compromises the well-being of test subjects. It may affect their physiological states, alter mobility/thermoregulation, elevate stress, and introduce artifacts in neurophysiological measurements. For instance, plucking a bird's feathers would severely impair its flight, and shaving a bear’s fur would compromise its thermal insulation, essential for survival in winter.\n3. Repeated shaving animal hair is against the well-being of animals: The Guide for the Care and Use of Laboratory Animals clearly states that all scientific research procedures must practice “avoidance or minimization of discomfort, distress, and pain”. Repeated shaving can disturb the skin, possibly causing skin irritation, local inflammation, all of which are contrary to the principle of “Refinement”. (National Research Council. Committee for the Update of the Guide for the Care and Use of Laboratory Animals. Guide for the Care and Use of Laboratory Animals. 8th ed. Washington, DC: National Academies Press; 2011, Section 1, pp. 4.). Additionally, the \"Environment, housing and Management\" section of the Guide states that animal care providers should observe hair condition as one of the health indicators and avoid interventions that affect the natural behavior of the animals. Repeated shaving may affect an animal's body temperature regulation, social behavior and stress levels. (National Research Council. Committee for the Update of the Guide for the Care and Use of Laboratory Animals. Guide for the Care and Use of Laboratory Animals. 8th ed. Washington, DC: National Academies Press; 2011, Section 3, pp. 12.).\nTherefore, based on scientific and ethical grounds, we strongly believe that the present work addresses a key challenge in the broad field of behavioral neuroscience.\nWe have clarified the significance of hair adaptability in the revised manuscript as following:\n“Abstract\nNon-invasive collection of weak electroneurographic signals is crucial for cognitive neuroscience. However, animal scalps are often covered by dense hair, which presents a substantial barrier in the scalp-electrode interface for accurate electrophysiological acquisition. Although scalp hair removal is common practice in clinical settings, it has detrimental effects on the test subjects and is infeasible in many non-clinical scenarios. Here we report a hair-adaptable and adhesion-tunable (HAAT) scalp-electrode interface that can accurately acquire electroneurographic signals from hairy skins of animal and human. By synthetically integrating dynamic covalent bonds and ionic conduction channels in the copolymer poly(sodium thioctate-co-sulfobetaine methacrylate), we implement the key functionalities of the HAAT interface including penetration of dense hair layers, strong conformal skin-electrode adhesion, on-demand painless detachment, and sensitive neuroelectric coupling. We achieve stable and precise electroneurographic signals acquisition from human, monkey and mouse’s scalps with various hair densities. The accurate event-related potential monitored during monkey vision-attention tasks facilitates the study of monkey brain activity. These results demonstrate the HAAT interface as an outstanding platform for conducting non-invasive research on animal cognition.\nIntroduction\nThe non-invasive and precise collection of electroneurographic signals from the brain, such as electroencephalogram (EEG) and event-related potential (ERP), is a crucial technology widely applied in cognitive neuroscience1-4. A primary challenge in developing non-invasive brain neural electrodes lies in establishing a robust and conformal interface with the scalp, a prerequisite for minimizing signal artifacts and ensuring reliable neuroelectric coupling5,6. This challenge becomes particularly pronounced in test subjects, e.g., rodents and primates, whose scalps are covered with amounts of hair. The hair layer creates gaps at the scalp-electrode interface and significantly hinders stable and intimate contact. While hair removal is an established pre-operative protocol in clinical settings such as craniotomy, where patient compliance is implicitly secured by therapeutic necessity, this approach fails to translate to non-clinical scenarios. In cognitive neuroscience research, hair removal from healthy volunteers encounters substantial cultural, comfort, and aesthetics barriers. In animal behavior studies, fur and feathers serve vital biological functions, rendering their removal both impractical in field studies and against the ethical treatment of laboratory animals. Thus, there is a pressing need for non-invasive brain neural interfaces capable of overcoming the challenges posed by hair while ensuring stable and conformal contact with the scalp.”\nGeneral Comment 2: Novelty and advantages of the HAAT interface\nResponse: We respectfully disagree with Review 2’s assessment on the (lack of) novelty of the design presented in this work. Hair-adaptable electrophysiological recording requires a material that (1) penetrates hair layers, (2) stick stably to the skin and couples sensitively to weak (µV level) EEG signals, and (3) detach on demand without hair loss or skin irritation. The reviewer stated that “any hydrogel systems that allow sol-gel transition at a rather physiologically-acceptable condition can be used for such a purpose”. However, our thorough literature search shows that there is currently no material that satisfies all the three criteria (Supplementary Table 1). Specifically:\n(i) Conductive gels or other sol-gel transitional hydrogels (e.g., Gelatin and poly(N-isopropylacrylamide) PNIPAm), are extensively studied and used clinically as electrode materials. However, they exhibit either weak adhesion leading to unexpected detachment and positional shift, or excessively strong adhesion causing hair loss or skin irritation upon detachment (Supplementary Figure 1c, d, Supplementary Table 1).\n(ii) Dynamic polymers such as poly(α-lipoic acid) and their copolymers have recently been shown as surgical superglues and wound dressings with on-demand tunable adhesion. However, these materials have not been demonstrated with EEG recording capabilities.\nThe core novelty of this work is the design and implementation of a covalent bond based dynamic polymer that provides stable attachment, on-demand detachment and reliable EEG recording, thereby satisfying all the criteria for hair-adaptable electrodes. We achieve this by synthetically incorporating disulfide bonds and ionic conduction channels in the polymer network, thereby achieving strong, gapless skin-electrode interface in the hairy regions of test subjects that affords accurate, sensitive and long-term stable EEG acquisition. Harnessing the dynamic nature of the disulfide bonds, we achieve controlled bond cleavage using physiologically feasible external stimuli, which allows on-demand detachment of the gel electrode without hair loss. The superior performance of our design over existing EEG gels is clearly manifested in the comparison in Table S2 in the revised SI. Of note, disulfide-based adhesive poly(α-lipoic acid) (αLA polymer) was recently demonstrated as a surgical superglue due to its high-strength adhesion and excellent biocompatibility (Pal et al. Science 2024, 385, 877-883). Leveraging the similar merit of disulfide covalent bonding, our HAAT design adds new, active functionalities to the material, that is, a non-invasive neural electrode for hairy animals, which not only adapts to hair but also offers robust and customizable adhesion.\nFigure R1. Schematic of the hair-adaptable and adhesion-tunable (HAAT) bio-interfacing electrodes for animal cognition study. a, The hair challenge in electroneurographic signals acquisition for animal cognition study. b, Working mechanism of hair adaptability and tunable adhesion of HAAT. Temperature-triggered dynamic bonding dissociation and reassociation allow fluid-gel state transitions for hair-adaptability. Stimulus-triggered interfacial covalent and non-covalent bonding cleavage allows adhesion-tuning for hair-friendly removal.\nRobust and customizable adhesion is indispensable for constructing robust and conformal interface between scalp and electrode. Solid electrodes have been designed with claw structures, and are fixed by external pressure on scalp, where the continuously strong force leads to discomfort (Figure R2a). Saline, as a type of fluid interfacial materials, it introduces risks of positional shifts (Figure R2b). Conductive gels, or other sol-gel transitional hydrogels (e.g., Gelatin and PNIPAm) without dynamic covalent bonding, they exhibit either weak adhesion bringing unexpected detachment (Figure R2c), or strong adhesion inducing hair loss or skin irritation (Figure R2d).\nFigure R2. Current strategies of non-invasive brain electrodes designed for hairy skin.\nIn contrast, covalent bond based dynamic polymer is able to achieve robust and customizable adhesion as well as hair-adaptability, because it can undergo three steps (Figure R3): 1) low-modulus fluid state allows hair penetration; 2) high modulus and covalent bonding at the interface to construct stably adhesive bio-interface; 3) easily interfacial bonding cleavage for hair-friendly removal.\nFigure R3. Mechanism of dynamic covalent bonding polymer achieving hair-adaptable and adhesion-tunable properties.\nRegarding the comments that “It seems to me that any hydrogel systems that allow sol-gel transition at a rather physiologically-acceptable condition can be used for such a purpose”, many types of hydrogels were carefully compared in terms of state-transition ability, adhesion property and adhesion tunability. It is clearly demonstrated that polymers without dynamic covalent bonding features are not able to achieve hair-adaptable and adhesion tunable properties simultaneously.\nWe have clarified the novelty and advantages of HAAT design in the revised introduction as following:\n“Current strategies involve two types of non-invasive neural interfaces to achieve hair adaptability for brain study (Supplementary Fig. 1a, b). Solid electrodes7-10 have been designed with specific structures, such as claw configurations, to address this issue. However, these solid electrodes were fixed by external pressure on scalp, where the continuously strong force often leads to extreme discomfort. Fluid interfacial materials11,12 such as saltwater bypass hair and conform to skin wrinkles. Nevertheless, they require EEG caps to fix electrodes, and introduces risks of positional shifts as well as signal crosstalk in high-density arrays. The development of non-fluid and imperceivable interface for non-invasive neural electrodes to comfortably and efficiently acquire electrophysiology at hairy regions of animal scalps remains challenging.\nWe envision a hair-adaptable and adhesion-tunable scalp-electrode interface, termed HAAT, with the following features: (1) the capability to penetrate dense hair layers and conform to skin wrinkles, for example, through liquid-solid phase transitions; (2) robust mechanical and electrical coupling to the scalp for reliable recording; and (3) tunable adhesion for on-demand painless detachment. Biogels including gelatin and poly(N-isopropylacrylamide)13-15 are capable of penetrating dense hairs through fluid-gel transition. However, they exhibit either weak adhesion causing unexpected detachment, or excessively strong adhesion inducing hair loss or skin irritation (Supplementary Figure 1c, d and Supplementary Table 1). Polymers with dynamic covalent bonds in the backbone have emerged as promising bioadhesives with potential hair-adaptability. Dynamic dissociation and formation of the covalent bonds facilitate fluid-gel transitions, strong interfacial bonding, and tunable adhesion modulated by external stimuli. Notably, disulfide-based poly(lipoic acid) (PLA) is recently reported to be an ideal skin adhesive as a recyclable surgical superglue due to its high-strength adhesion and excellent biocompatibility16-18. Leveraging these properties, dynamic covalent polymer incorporated with disulfide bond emerges as suitable interface for non-invasive neural electrodes for hairy animals, one that not only adapts to hair but also offers robust and customizable adhesion. Despite their outstanding mechanical properties and biocompatibility, poly(lipoic acid) based gels have found limited use in electrophysiological recording, highlighting the critical need for material innovation to fully realize the HAAT design.”\nIn the revised Supplementary Information, we have added the following table.\nSupplementary Table 1. Comparisons of state transition property and adhesion performance between HAAT and hydrogels/polymers with potential of hair adaptability.\nAdhesion\nMaterials with potential of hair penetration State transition condition Adhesion property Ref.\ntunability\nLap shear test:\ngelatin/PVA/CNT hydrogel N/A No 1\n7.7 kPa (porcine skin)\nLap shear test:\nPAA/gelatin/tannic acid/AlCl hydrogel N/A No 2\n3\n26.8 kPa (porcine skin)\nLap shear test:\nGelatin-based gelatin-based (Tsg-THA&Fe) hydrogel N/A No 3\n4.97 - 18.23 kPa (porcine skin)\nhydrogel\nLap shear test:\nGelatinol®-based hydrogels N/A No 4\n3.8 ± 0.3 MPa (skin)\n90-degree peel test: 197.6 N/m\nChitosan−gelatin hydrogel N/A No 5\nLap-shear test: 51.2 N/m\ngelatin/sodium citrate/glycerol hydrogel Temperature (23.8 – 35.9 ℃) No 90-degree peel test: 6\n80 ± 7.6 N/m (skin)\nTemperature (33.2 ℃) and 180-degree peel test:\nGelMA/PEGDA/LAP/AgNWs ink No 7\nLED light 50 - 196 N/m (porcine skin)\n180-degree peel test:\nPDA-NPs/PNIPAM hydrogel Temperature (32 ℃) No 8\n25 - 90 N/m (porcine skin)\nNIPAM-based Lap-shear test:\nPNIPAM/SA/GO hydrogel Temperature (32 ℃) No 9\n~2.14 - ~8.61 kPa (porcine skin)\nhydrogel\n(PAA-g-PNIPAM) Temperature (32 ℃) and salt Underwater Adhesion:\nNo 10\n(PDMAPAA-g-PNIPAM). concentration 60.6 kPa (glass)\n90-degree peel test:\nALA/MBAA/Fe3+ copolymer Temperature (85 ℃) No 11\n80 - 180 N/m (skin)\nLap shear test:\nOther materials PEDOT:PSS/SBMA/PEGDA hydrogel Time (7 min) No 12\n6.5 kPa (porcine skin)\nLap shear test:\nTA/CA copolymer Temperature (-10 ℃) No 13\n62.5 - 84.8 kPa (porcine skin)\nTemperature (60 °C: high-\n90-degree peel test:\nThis\nHAAT poly(ST-co-SBMA) modulus gel → 25 °C: low-Yes\nwork\nmodulus fluid)\nPVA: polyvinyl alcohol; AA: acrylic acid; THA: hydroxybenzaldehyde; GelMA: gelatin methacryloyl; PEGDA: poly(ethylene glycol) diacrylate; LAP: lithium phenyl-2,4,6-trimethylbenzoylphosphonate; PDA-NPs: polydopamine nanoparticles; PNIPAM: poly(N-isopropylacrylamide); SA: sodium alginate; GO: graphene oxide; LA: lipoic acid; MBAA: N,N’-methylenebis(acrylamide); SBMA: [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide; TA: tetradecyl acrylate. CA: catechol;\nNote: “N/A” indicates “not available” in the references.\nReference:\n1. Liu, F., Liu, X. & Gu, H. Multi-Network Poly(β-cyclodextrin)/PVA/Gelatin/Carbon Nanotubes Composite Hydrogels Constructed by Multiple Dynamic Crosslinking as Flexible Electronic Devices. Macromolecular Materials and Engineering 307, 2100724 (2022).\n2. He, Z. & Yuan, W. Adhesive, Stretchable, and Transparent Organohydrogels for Antifreezing, Antidrying, and Sensitive Ionic Skins. ACS Applied Materials & Interfaces 13, 1474-1485 (2021).\n3. Lu, Y. et al. A physicochemical double-cross-linked gelatin hydrogel with enhanced antibacterial and anti-inflammatory capabilities for improving wound healing. Journal of Nanobiotechnology 20, 426 (2022).\n4. Osetrov, K., Uspenskaya, M., Zaripova, F. & Olekhnovich, R. Nanoarchitectonics of a Skin-Adhesive Hydrogel Based on the Gelatin Resuscitation Fluid Gelatinol®. Gels 9, 330 (2023).\n5. Xu, Z. et al. Tough and Biocompatible Hydrogel Tissue Adhesives Entirely Based on Naturally Derived Ingredients. ACS Applied Polymer Materials 6, 1141-1151 (2024).\n6. Wang, C. et al. On-skin paintable biogel for long-term high-fidelity electroencephalogram recording. Science Advances 8, eabo1396 (2022).\n7. Wan, C. et al. In Situ Formation of Conductive Epidermal Electrodes Using a Fully Integrated Flexible System and Injectable Photocurable Ink. ACS Nano 17, 10689-10700 (2023).\n8. Han, L. et al. Polydopamine Nanoparticles Modulating Stimuli-Responsive PNIPAM Hydrogels with Cell/Tissue Adhesiveness. ACS Applied Materials & Interfaces 8, 29088-29100 (2016).\n9. Cao, Y. et al. Double-crosslinked PNIPAM-based hydrogel dressings with adjustable adhesion and contractility. Regenerative Biomaterials 10, (2023).\n10. Dompé, M. et al. Enhancement of the Adhesive Properties by Optimizing the Water Content in PNIPAM-Functionalized Complex Coacervates. ACS Applied Polymer Materials 2, 1722-1730 (2020).\n11. Cai, C., Wu, S., Tan, Z., Li, F. & Dong, S. On-Site Supramolecular Adhesion to Wet and Soft Surfaces via Solvent Exchange. ACS Applied Materials & Interfaces 13, 53083-53090 (2021).\n12. Huang, X. et al. In Situ Forming Dual-Conductive Hydrogels Enable Conformal, Self-Adhesive and Antibacterial Epidermal Electrodes. Advanced Functional Materials 33, 2302846 (2023).3.\n13. Cheng, Y. et al. An Elastic and Damage-Tolerant Dry Epidermal Patch with Robust Skin Adhesion for Bioelectronic Interfacing. ACS Nano 16, 18608-18620 (2022).\n“Issues like anti-evaporation, tolerance to more complications such as sweat or body movements seem to be more valuable in the relevant fields.”\nResponse: We agree with the reviewer on the importance of sweat and motion tolerance, and have carried out meticulous studies on our HAAT system. These findings are summarized in Supplementary Figures. 31, 50, 51 and video 6. We find that HAAT is not affected by sweat and able to monitor EEG on hairy skin over extended periods. Please see Responses to point 3 and 4 for details.\nWe address the specific comments from Reviewer 2 point-by-point below:\n1. “The rheological behavior of the gel electrodes should be studied with more details. Typical requirement for hydrogel electrodes should be clarified based on literature, the HAAT gels presented herein seems to be very weak, with the G’ values less than 5 Pa. I doubt such weak gels can really be useful for realistic applications, since such weak gels can hardly withstand the shear forces or even gravity.”\nResponse: We thank the reviewer for raising this concern. After carefully checking the data of rheological test (Figure 2b and Supplementary Figs. 18-21), we realize that the unclear y-axis unit may mislead the reviewer. It is in logarithm form of storage and loss modulus (G’ and G’’ in Pa unit) in the title of vertical coordinate (lg G’ and lg G’’). The actual G’ and G” are on the order of 103 Pa, comparable to typical bioadhesive gels for EEG recording (see, for example, https://www.nature.com/articles/s44385-025-00009-x).\nIn the revised manuscript and Supplementary Information, we have clarified these figures and emphasizing logarithmic coordinates in the figure caption.\nFigure 2. Hair adaptability of HAAT induced by dynamic bonding. a, Chemical structure of poly(ST-co-SBMA). b, Logarithm of storage and loss modulus (lg G’ and lg G’’) and the loss factor (tan  = G”/G’) from 25 to 95°C. c, Scanning electron microscopy (SEM) image of the HAAT applied on hairy rat skin. d, SEM image of the HAAT applied on porcine skin. e, Temperature-variable in-situ Raman spectra of HAAT at 25, 35, 50, 65, 90 and 95 °C. f-g, 2DCOS synchronous and asynchronous spectra generated from the Raman spectra during heating. The warm color (red) represents positive intensities, while the cold color (blue) represents negative intensities.\nSupplementary Table 4. Specific contents of a series of HAAT with different ST content.\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n0.5 2 0.06 1 2 0.06 1.5 2 0.06 2 2 0.06 2.5 2 0.06\nST solution (mL) 0.5 1 1.5 2 2.5\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n2 2 0.01 2 2 0.02 2 2 0.04 2 2 0.06 2 2 0.08\nST solution (mL) 2 2 2 2 2\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\nIt could be demonstrated that the Linear Viscoelastic Region (LVER) is within the strain of 40%.\nHAAT is T A Fe . T A means that the precursor solution contains x mL ST and y g SBMA, while\n2 2 0.01 x y\ncontents of other ingredients remain unchanged. Similarly, Fe means the molar ratio of Fe3+ to ST is\nz\nz.\nSupplementary Fig. 19. Rheological characterization of storage and loss modulus (Pa) in logarithmic form (lg G’ and lg G’’) at temperature sweeps of HAAT.\nThey suggest that with the increase of disulfide bond, polymer network becomes more susceptible to temperature, facilitating state transition. HAAT are T A Fe and T A Fe . T A means that the\n2 2 0.01 2.5 2 0.01 x y\nprecursor solution contains x mL ST and y g SBMA, while contents of other ingredients remain\nunchanged. Similarly, Fez means the molar ratio of Fe3+ to ST is z.\nSupplementary Fig. 20. Rheological characterization of storage and loss modulus (Pa) in logarithmic form (lg G’ and lg G’’) at temperature sweeps of HAA.\nAt an increasing amount of Fe3+, the crosslinking density will be enhanced, leading to the robustness of polymer network. More energy will thus be required to break Fe³⁺-carboxyl coordination bonds during heating, resulting in a higher state transition threshold. HAAT are T A Fe and T A Fe .\n2 2 0.01 2 2 0.06\nT A means that the precursor solution contains x mL ST and y g SBMA, while contents of other\nx y\ningredients remain unchanged. Similarly, Fez means the molar ratio of Fe3+ to ST is z.\n2. “The stability of gel gels is not clear. For instance, whether the gel electrodes can tolerate sweat, or long-term exposure to dry air, body movements, or other common conditions that patients can experience during their daily lives. More importantly, these conditions might influence the accuracy of the readout of electroneurographic signals. This should also be evaluated.”\n3. “Adhesion behavior to hairy skin should also be tested with the presence of liquid condition such as simulated sweat or different humidity.”\nResponse: We thank the reviewer for raising these concerns. Subjects are usually provided with a relatively comfortable environment (appropriate temperature, humidity, brightness, etc.) when monitoring EEG for cognition study, thus we initially did not take sweat tolerance of HAAT as a primary design goal. Instead, we focus on hair-adaptability and adhesion-tunability in this work. In the revised manuscript, following the reviewer’s suggestion, we evaluate the environmental stability of HAAT by monitoring water retention over 10 days and taking videos while spraying simulated sweat solution on HAAT electrodes (newly added Supplementary Fig. 31 and Supplementary Movie 6). Additionally, we compared the ERP signal quality in the first and last stage of monkey vision-attention task (newly added Supplementary Fig. 50). All these tests show that our HAAT is sufficiently stable at high humidity for the use of electrode-scalp interface.\nIn the revised manuscript and Supplementary Information, we have added following discussion, figures and movie.\n“Compared to commercial EEG pastes, HAAT is able to penetrate hair without the need of additional pressure, tightly and stably adhere to scalp during prolonged usage in the presence of simulated sweat, and is readily remove by a detachment solution (inset of Fig. 4c and Supplementary Figs. 31-33).”\nSupplementary Fig. 31. Water retention of HAAT. m and m denote mass of HAAT before and\n0\nafter water retention experiments.\nThree samples of HAAT (approximately 5 mm x 15 mm x 1.5 mm) were respectively placed on three open Petri dishes at room temperature for 10 days, during which the masses of HAAT were monitored.\nLittle mass losses but slightly mass increases were observed. This is because in two of the components of HAAT, SBMA (sulfobetaine methacrylate) and glycerol, sulfonic acid groups and hydroxyl groups can form hydrogen bonds with water molecules, leading to better environmental stability.\nSupplementary Movie 6_Comparison of adhesion stability of HAAT and commercial materials by spraying simulated sweat solution.\n“The signal to noise ratio (SNR) in the early stage of the task is slightly higher than that in the end period, indicating that HAAT achieves equivalent performance throughout the entire task.\n(Supplementary Figs. 50, 51). These further confirm that HAAT can serve as an advancing alternative to commercial pastes for ERP collection.”\nSupplementary Fig. 50. Comparison of the SNR of ERP in early and late stage of the monkey’s vision-attention task.\nHist and box plot of SNRs are obtained from HAAT channels. Blue represents the 100 trials in the early stage of the task, and orange represents the 100 trials near the end of the task. The task lasts approximately 4 hours. SNRs of ERP are obtained under the condition of annulus occurred in the contralateral visual field. Specifically, channels O2 and P4 correspond to the annulus presented in the left visual field (Annulus Left).\n4. “The adhesion of the gels to the hairly skin can be released by adding GSH and NaCl. NaCl could also present in body sweat, which means the mechanical functions can be significantly compromised if body secret sweat, which seems to be unavoidable. How to solve this contradiction?”\nResponse: We thank the reviewer for raising this concern. We emphasize that the interfacial disulfide bonds, which are not affected by sweat, dominate over other weaker interactions (electrostatic, hydrogen bonding) by orders of magnitude and constitute the primary source of long-term adhesion stability. We add NaCl to the detachment solution to fully disrupt the interfacial interactions and ease the peeling, yet significant adhesion loss occurs only in the presence of reducing agents capable of cleaving disulfide bonds. Electrolytes in PBS solution (i.e., simulated sweat) may partially disrupt interfacial non-covalent interactions, such as electrostatic interactions and hydrogen bonding, yet only result in a slight and temporary reduction in adhesion. As shown in the newly added Supplementary Movie 6, HAAT exhibits excellent self-adhesive capability, functioning reliably even in the presence of simulated sweat, while the three commercial interface materials fail to maintain stable adhesion without external force for fixation. These results highlight the clear advantage of HAAT over conventional materials under normal sweating conditions.\nAdditionally, in practical EEG applications, scalp preparation and environmental control minimize sweat and oil. The insensible sweat volume is approximately 12 to 42 g m−2 h−1 (Adv. Sci. 2022, 9, 2103257). Unlike immersion in water, the insensible sweat only appears in the form of interfacial water between the HAAT-skin interface, which HAAT can readily absorb. Consequently, the mechanical and electrical impact of sweat on HAAT is minimal, and the temporary disruption of electrostatic interactions caused by salts in sweat has a negligible effect on adhesion.\nWe have added the following discussion in the revised manuscript.\n“NaCl disrupts the interfacial non-covalent dynamic interactions such as electrostatic interactions and hydrogen bonding37. PBS solution (simulated sweat) alone cannot detach HAAT from scalp, and large adhesion loss only occurs in the presence of reducing agents capable of cleaving disulfide bonds (Supplementary Movie 6). These observations suggest that interfacial disulfide bonds are the determining factor of interfacial toughness. Consequently, with above detachment solution, during 90-degree peeling tests, interfacial toughness between HAAT and porcine skin rapidly decreased from 523 N/m to almost zero (Fig. 3e and Supplementary Figs. 24-25).”\n5. “For animal tests, comparisons of the sensitivity of HAAT gels versus currently available materials should be added.”\nResponse: We thank the reviewer for the suggestion. We further compared and analyzed the amplitudes and SNR of ERP collected by HAAT electrode and commercial paste electrode with external forces on scalp during monkey’s vision-attention task. As shown in Supplementary Fig. 51, there is almost no signal difference between commercial paste and HAAT electrodes. Smaller animals, such as mice, possess smaller heads and denser hair, presenting a more significant barrier for accurate acquisition of scalp EEG. Despite applying pressure, commercial EEG pastes were unable to penetrate mouse fur, leading to the failure in EEG acquisition (Supplementary Fig. 39 and Supplementary Movie 2). In contrast, HAAT electrode successfully distinguished auditory evoked potentials (AEP) from mouse scalp, showcasing their superior hair adaptability (Fig. 4h and Supplementary Figs. 40-41).\nSupplementary Fig. 51. Amplitude and SNR of ERP change as a function of trials during monkey’s vision-attention task.\nSNR (a-b) and amplitude (c-d) of EEG under the condition of annulus occurred in the contralateral visual field. Specifically, channels PO7, O1, and P3 correspond to the condition where the annulus appeared in the right visual field (Annulus Right), while channels PO8, O2, and P4 correspond to the annulus presented in the left visual field (Annulus Left). After applying a 3 Hz low-pass filter and excluding trials that exceeded the defined artifact threshold, a total of 365 valid trials were obtained.\nThe signal quality (SNR and amplitude) observed in the HAAT channels (O1, O2, P3, P4) is comparable to that of the commercial paste channels (PO7, PO8), indicating that HAAT achieves equivalent performance throughout the entire task. Notably, both SNR and amplitude showed a marked decline around the 150th trial, which may be attributed to fluctuations in the monkeys’ attentiveness or external environmental disturbances during the experiment.\nSupplementary Fig. 41. Auditory evoked potentials (AEP) distinguished from mouse EEG captured by HAAT electrode from mouse scalp.\nReviewer #3:\n“The present manuscript describes the preparation of hair-adaptable and adhesion-tunable (HAAT) scalp electrode using a copolymer with sodium thioctate, featuring dynamic disulfide bonds for temperature-triggered fluid-gel transitions. This allows easy penetration through hair, strong yet reversible adhesion, and gentle removal. The HAAT gel electrode reliably records EEG from hairy scalps of humans, monkeys, and mice. In monkey studies, it captured event-related potentials revealing neural lateralization, marking the first use of dynamic covalent polymers for non-invasive brain recordings in animals. However, there is unclear mechanism of gel electrode formation and how this gel improves conductivity. These are required detailed explanations and scientific evidence. I firmly suggest that the author must do the following comments. I still suggest the authors to add more insights into the studies carried out. To this end, the following list of papers can be considered and cited, where fitting within the existing narrative flow. The quality of the manuscript is satisfactory. Major revisions needed”\nWe thank the reviewer for the positive and insightful comments. Our point-by-point responses are listed as below.\n1. “Many articles have been published on recording electroneurographic signals. In which specific aspects does this work differ from existing studies?”\n10. “In order to strengthen your introduction, please consider the following references. I suggest authors to cite some of the recent related literatures. Authors can cite recent references, which can deliver and connect the audience with the current study.\n(a) Nat Biomed Eng 3, 194–205 (2019). Doi.org/10.1038/s41551-019-0347-x.\n(b) Adv. Sci. 2021, 8, 21, 2102275. Doi.org/10.1002/advs.202102275\n(c) Adv. Mater. 2023, 35, 2303267. DOI: 10.1002/adma.202303267.\n(d) Neural Regeneration Research, (2024) 19 (3), pp. 680 – 686. DOI: 10.4103/1673-5374.380908.\n(e) Adv. Sci. 2024, 11, 2404019. Doi.org/10.1002/advs.202404019.\n(f) Bioactive Materials, (2024) 31, pp. 408 – 421. DOI: 10.1016/j.bioactmat.2023.08.021.\n(g) Nano Energy 133 (2025) 110416. Doi.org/10.1016/j.nanoen.2024.110416.”\nResponse: We thank the reviewer for raising this concern.\nThe core novelty of this work is the design and implementation of a covalent bond based dynamic polymer for working as a hair-adaptable and adhesion-tunable bio-interfacing electrode (namely HAAT electrode). We create a dynamic polymer with hair infusion and penetration as well as strong adhesion to the hairy skin by synthetically incorporating disulfide bonds in the polymer network, thereby achieving strong, gapless skin-electrode interface in the hairy regions of test subjects that affords accurate, sensitive and long-term stable EEG acquisition. Harnessing the dynamic nature of the disulfide bonds, we achieve controlled bond cleavage using physiologically feasible external stimuli, which allows on-demand detachment of the gel electrode without hair loss. The superior performance of our design over existing EEG gels is clearly manifested in the comparison in Table S2 in the revised SI. Of note, disulfide-based adhesive poly(α-lipoic acid) (αLA polymer) was recently demonstrated as a surgical superglue due to its high-strength adhesion and excellent biocompatibility (Pal et al. Science 2024, 385, 877-883). Leveraging the similar merit of disulfide covalent bonding, our HAAT design adds new, active functionalities to the material, that is, a non-invasive neural electrode for hairy animals, which not only adapts to hair but also offers robust and customizable adhesion.\nIn the revised manuscript, we have updated our introduction as following:\n“Current strategies involve two types of non-invasive neural interfaces to achieve hair adaptability for brain study (Supplementary Fig. 1a, b). Solid electrodes7-10 have been designed with specific structures, such as claw configurations, to address this issue. However, these solid electrodes were fixed by external pressure on scalp, where the continuously strong force often leads to extreme discomfort. Fluid interfacial materials11,12 such as saline bypass hair and conform to skin wrinkles. Nevertheless, they require EEG caps to fix electrodes, and introduces risks of positional shifts as well as signal crosstalk in high-density arrays. The development of non-fluid and imperceivable interface for non-invasive neural electrodes to comfortably and efficiently acquire electrophysiology at hairy regions of animal scalps remains challenging.\nWe envision a hair-adaptable and adhesion-tunable scalp-electrode interface, termed HAAT, with the following features: (1) the capability to penetrate dense hair layers and conform to skin wrinkles, for example, through liquid-solid phase transitions; (2) robust mechanical and electrical coupling to the scalp for reliable recording; and (3) tunable adhesion for on-demand painless detachment. Biogels including gelatin and poly(N-isopropylacrylamide)13-15 are capable of penetrating dense hairs through fluid-gel transition. However, they exhibit either weak adhesion causing unexpected detachment, or excessively strong adhesion inducing hair loss or skin irritation (Supplementary Figure 1c, d and Supplementary Table 1). Polymers with dynamic covalent bonds in the backbone have emerged as promising bioadhesives with potential hair-adaptability. Dynamic dissociation and formation of the covalent bonds facilitate fluid-gel transitions, strong interfacial bonding, and tunable adhesion modulated by external stimuli. Notably, disulfide-based poly(lipoic acid) (PLA) is recently reported as a recyclable surgical superglue due to its high-strength adhesion and excellent biocompatibility16-18. Leveraging these properties, dynamic covalent polymer incorporated with disulfide bond emerges as suitable interface for non-invasive neural electrodes for hairy animals, one that not only adapts to hair but also offers robust and customizable adhesion. Despite their outstanding mechanical properties and biocompatibility, poly(lipoic acid) based gels have found limited use in electrophysiological recording, highlighting the critical need for material innovation to fully realize the HAAT design.”\n“Notably, the detachment solution modulates the interfacial toughness () by over 50 times, ranking the highest among both hair-in-adaptable and adaptable EEG gels in literature (Fig. 3i, Supplementary Fig. 29 and Supplementary Table 2).”\nSupplementary Table 2. Comparison of the hair adaptability and adhesion performance between HAAT and state-of-art interfacial materials.\nHair adaptability Adhesion tunability\nNo need\nSpontaneous\nMaterials additional maximum Stable self-Minimum Gentle Ref.\nhair Location\nfixture on adhesion adhesion adhesion removal\npenetration\nhairy skin\nGel × × Hairy ×\nN/A -- -- --\n(Xunda, X-1) forearm\nPaste × × ×\nCommercial (Weaver and company, Ten20) Hairy scalp N/A -- -- --\ninterfacial\nmaterials Gel × √ Hairy scalp N/A × -- -- --\n(Greentek, GT10)\nGel × √ ×\nHairy scalp N/A -- -- --\n(Weaver and company, Nuprep)\nPGA/GelMA hydrogel N/A N/A Hairless skin 8.1 N/m × ~0 -- 1\nPEDOT:PSS/SBMA/PEGDA √ √ Hairy lower ×\n6.5 kPa -- -- 2\nhydrogel leg\nGel\nLA/MBAA/Fe3+ hydrogel √ × Hairless skin 180 N/m √ 80 N/m × 3\nPBAc/AA/AAc-NHS/polyol √ × √ √\nHairless skin 400 N/m 24 N/m 4\nhydrogel\nPVA/PAA/NHS hydrogel N/A N/A Hairless skin ＞400 N/m √ ~40 N/m √ 5\nPAA/PU/NHS hydrogel √ N/A Hairy scalp 70 N/m √ N/A √ 6\nPLA/N,N-diacryloyl-L-lysine/Au- √\nN/A N/A Hairless skin 6 N/A 7\nGA hydrogel\nSA/TA polymer N/A × Hairless skin 45.7 N/m √ 3.0 N/m √ 10\n√ √ Hairy\nforearm √ √ This\nHAAT 523 N/m ~0\nwork\n√ √\nHairy scalp\nPGA: polymerized gallic acid; GelMA: gelatin methacryloyl; SBMA: [2-(methacryloyloxy) ethyl]dimethyl-(3-sulfopropyl) ammonium hydroxide; AA: acrylic acid; PEGDA: poly(ethylene glycol) diacrylate; LA: lipoic acid; MBAA: N,N’-methylenebis(acrylamide); PBAc: a copolymer prepared from a free radical copolymerization of acrylic acid (AA), 4-vinylphenylboronic acid (4-VPBA), and hydroxyethyl methacrylate (HEMA); AAc-NHS: acrylic acid N-hydroxysuccinimide ester; PVA: polyvinyl alcohol; NHS: N-hydroxysuccinimide; PU: polyurethane; SPBS: sorbitol-modified polyborosiloxane; CA: catechol; SA: stearyl acrylate; TA: tetradecyl acrylate.\nNote: “N/A” indicates “not available” in the references.\nReference:\n1. Jiang, Y. et al. Infant Skin Friendly Adhesive Hydrogel Patch Activated at Body Temperature for Bioelectronics Securing and Diabetic Wound Healing. ACS Nano. 16, 8662-8676 (2022).\n2. Huang, X. et al. In Situ Forming Dual-Conductive Hydrogels Enable Conformal, Self-Adhesive and Antibacterial Epidermal Electrodes. Advanced Functional Materials. 33, 2302846 (2023).\n3. Cheng, Y. et al. An Elastic and Damage-Tolerant Dry Epidermal Patch with Robust Skin Adhesion for Bioelectronic Interfacing. ACS Nano. 16, 18608-18620 (2022).\n4. Xue, Y. et al. Trigger-Detachable Hydrogel Adhesives for Bioelectronic Interfaces. Advanced Functional Materials. 31, 2106446 (2021).\n5. Chen, X., Yuk, H., Wu, J., Nabzdyk, C. S. & Zhao, X. Instant tough bioadhesive with triggerable benign detachment. Proceedings of the National Academy of Sciences. 117, 15497-15503 (2020).\n6. Ahmed, S. et al. Stick-and-play bioadhesive hairlike electrodes for chronic EEG recording on human. npj Biomedical Innovations. 2, 9 (2025).\n7. Zhang, Z. et al. Combination of biodegradable hydrogel and antioxidant bioadhesive for treatment of breast cancer recurrence and radiation skin injury. Bioactive Materials. 31, 408-421 (2024).\n8. Tian, Q. et al. Hairy-Skin-Adaptive Viscoelastic Dry Electrodes for Long-Term Electrophysiological Monitoring. Advanced Materials. 35, 2211236 (2023).\n9. Cai, C., Wu, S., Tan, Z., Li, F. & Dong, S. On-Site Supramolecular Adhesion to Wet and Soft Surfaces via Solvent Exchange. ACS Applied Materials & Interfaces. 13, 53083-53090 (2021).\n10. Gao, M. et al. Skin temperature-triggered, debonding-on-demand sticker for a self-powered mechanosensitive communication system. Matter. 4, 1962-1974 (2021).\nWe have added the references suggested by the reviewer as following:\n“The non-invasive yet precise collection of electroneurographic signals from the brain, such as electroencephalogram (EEG) and event-related potential (ERP), is a crucial technology widely applied in cognitive neuroscience1-4.”\nReference:\n3. Tian, L. et al. Large-area MRI-compatible epidermal electronic interfaces for prosthetic control and cognitive monitoring. Nature Biomedical Engineering. 3, 194-205 (2019).\n4. Xu, S. et al. Illuminating the Brain: Advances and Perspectives in Optoelectronics for Neural Activity Monitoring and Modulation. Advanced Materials. 35, 2303267 (2023).\n2. “Since the thermal and chemical properties of polymers are crucial for their design, the author is advised to include TGA (Thermogravimetric Analysis) and DMA (Dynamic Mechanical Analysis) in addition to DSC (Differential Scanning Calorimetry) to more accurately determine the glass transition temperature of the composite gel.”\nResponse: We thank the reviewer for the suggestions. DMA is generally applied to study solid materials (e.g. plastics, rubber, fiber); obtaining moduli of our system in the fluidic state would be technically challenging with DMA. Instead, we use rheology to obtain temperature-dependent storage modulus (G’) and the loss modulus (G’’) across the gel-fluid transition (Figure 2b, Supplementary Figs. 18-21). Following your suggestion, we have added thermogravimetric data (TGA) of poly(ST-co-SBMA), which is shown in the newly added Supplementary Fig. 24. TGA shows a gradual and slight loss of mass in the sub-100 °C range, which is commonly observed in hydrogels and likely due to loss of water. Combined with DSC (Supplementary Figs. 16, 17) and rheological tests, the fluid-gel phase transition can be confirmed. Rheological analysis revealed that as the temperature increases, both G’and G’’ decrease, crossing at approximately 62 °C (Figure 2b).\nIn the revised manuscript and Supplementary Information, we have added the following details and data.\n“Thermogravimetric analysis (TGA) was performed with a heating rate of 10 K/min from room temperature to 700 °C at N atmosphere (STA200, HITACHI).”\n3. “Is the gel biocompatible? Given that the gel is in direct contact with skin, the author is encouraged to conduct basic cell viability tests, possibly using both Gram-positive and Gram-negative bacteria.”\nResponse: We thank the reviewer for the suggestions. To further validate the biocompatibility of HAAT, we have added In vitro cytotoxicity test. Gram-positive and Gram-negative bacteria are generally used in antibacterial studies. Thus, we prefer Human Skin Fibroblasts (HSF) to conducting cytotoxicity test.\nIn the revised manuscript and Supplementary Information, we have added the following description and data.\n“These results highlight the on-demand detachment of HAAT without skin irritation or hair loss (Figs. 3g top, 3h, and Supplementary Movie 1), which is advantageous over commercial electrode (Fig. 3g bottom and Supplementary Fig. 28). Notably, the detachment solution modulates the interfacial toughness () by over 50 times, ranking the highest among both hair-in-adaptable and adaptable EEG gels in literature (Fig. 3i, Supplementary Fig. 29 and Supplementary Table 2). In the cytotoxicity assessments of HAAT (Supplementary Fig. 30), the fluorescent images exhibit no observable signs of apoptosis or necrosis. The 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay further demonstrates a higher proliferation rate of cells on HAAT compared to control group after 3 days of culture, indicating excellent biocompatibility of HAAT.”\n“Method\n……\nCytotoxicity test\nThe cytocompatibility of HAAT was evaluated using an extract-based cell culture method. Human skin fibroblasts (HSFs) were seeded in a 96-well plate at an initial density of 5,000 cells per well and cultured overnight to allow cells to attach and adapt. Subsequently, sterilized HAAT extract (prepared by dissolving 2 g of HAAT in 1 mL of culture medium) was added to the wells. A control group consisting of HSFs cultured in standard medium without HAAT extract was also established. Cell viability and cytotoxicity were assessed after 24, 48, and 72 hours. At each time point, the pristine medium was removed and the cells were stained using a Calcein-AM/PI Cytotoxicity Assay Kit. Fluorescent images were captured using a Confocal Laser Scanning Microscope (Stellaris 5, Leica) to visualize cell morphology. Cell viability was further conducted using an MTT kit (Thermo Fisher). All parallel holes were set in five groups for each test.”\nSupplementary Fig. 30. Fluorescent images (a) and cell viability (b) of HAAT vs. control group (ctrl).\nScale bar is 100 μm. Fluorescent images of Human Skin Fibroblasts (HSF) cells with HAAT and control group (the environment in which cells grow under the best conditions) show the impact of HAAT on HSF death. Fluorescent images were stained with Calcein-AM (live cells, green) and propidium iodide (PI) (dead cells, red). Corresponding comparisons of HSF cell viability (n=5 independent samples) at various days are presented as means ± SD.\n4. “Could the author specify the solvents used for removing the gel from skin, and describe the gel’s solubility in various solvents, including hot water? Additionally, was an adhesion test conducted on wet or oily skin?”\nResponse: We thank the reviewer for raising the concerns.\nThe detachment solution consists of a mild reducing agent (glutathione, GSH) and an electrolyte (NaCl) in this work. GSH mainly undergoes REDOX reactions with disulfide bonds, cleaving both disulfide bonds within the polymer network and interfacial disulfide bonds. Simultaneously, the Na+ ions released from NaCl neutralize negatively charged carboxylate groups weakening interfacial electrostatic interaction. PBS solution, employed as simulated sweat and sprayed onto scalp, is not able to detach HAAT (newly added Supplementary Movie 6). The synergistic effect of GSH and NaCl leads to a rapid decrease in adhesion. Large adhesion loss occurs in the presence of reducing agents capable of cleaving disulfide bonds, as shown in a 90-degree peeling test (Supplementary Fig. 24). Therefore, the crucial factor in the detachment solution lies in the reducing agents’ ability to chemically cleave disulfide bonds through reduction.\nAs a gel, HAAT naturally swells in aqueous environments, allowing free ions to diffuse outward. The coordination bonds, disulfide bonds and other interaction that maintain the stability of polymer network would remain intact in room-temperature water. However, elevated water temperature might probably disrupt these interactions and destabilize the HAAT.\nAll adhesion tests in this work were conducted on dry porcine skin. The experiment setup was shown in Supplementary Fig. 26. The dry porcine skin inherently retains residual surface oils, which can reasonably simulate oily skin.\nWe have added the following discussion in the revised manuscript.\n“NaCl disrupts the interfacial non-covalent dynamic interactions such as electrostatic interactions and hydrogen bonding34. PBS solution (simulated sweat) alone cannot detach HAAT from scalp, and large adhesion loss only occurs in the presence of reducing agents capable of cleaving disulfide bonds (Supplementary Movie 6). These observations suggest that interfacial disulfide bonds are the determining factor of interfacial toughness. Upon the delivery of the detachment solution, interfacial toughness between HAAT and porcine skin drastically decreases from 523 N/m to almost zero under 90-degree peeling test (Fig. 3e and Supplementary Figs. 26-27).”\n5. “What is the gel’s reproducibility after applying the detachment solvent? The author is advised to include a cyclic peeling test to evaluate repeated use.”\nResponse: We thank the reviewer for raising the concerns. Due to the high interfacial toughness of HAAT, cohesive failure occurs upon peeling from porcine skin without the use of detachment solution, resulting in residual hydrogel on the surface. Upon treatment with the detachment solution, disulfide bonds responsible for strong adhesion undergo irreversible redox reactions. Consequently, HAAT loses its adhesive capability and cannot be reused repeatedly.\n6. “The Materials and Methods section lacks clarity, making it difficult to understand key experimental details such as chemical compositions, mixing ratios, reaction conditions, and solvent usage. The author should revise this section with more comprehensive information.”\nResponse: We thank the reviewer for the suggestions. We have rewritten the “Synthesis of HAAT” section in “Method”.\nIn the revised manuscript and Supplementary Information, we have updated the “Synthesis of HAAT” and following tables.\n“Synthesis of HAAT\nHAAT was prepared via following steps. Firstly, LA (4 g) and NaOH (0.7755 g) were dispersed uniformly in deionized water (10 mL) to prepare ST solution. The ST solution was stored at 4 °C and out of light. Secondly, FeCl ·6H O (2 g) was dissolved in deionized water (5 mL) to form FeCl\n3 2 3\naqueous solution. Thirdly, a certain amount of FeCl aqueous solution, ST (2 mL), SBMA (2 g),\n3\nglycerol (1 mL) was added into deionized water (2 mL) and then stirring in the dark to form the precursor solution of HAAT. The specific contents of each component are listed in Supplementary Table 4-5. Finally, the precursor solution was polymerized at 90 °C for 210 min to form the HAAT. Additionally, GSH (0.5 g) and NaCl (0.1 g) was dispersed uniformly in deionized water (9.4 g) as the detachment solution (5 wt% GSH and 1 wt% NaCl).”\nSupplementary Table 4. Specific contents of a series of HAAT with different ST content.\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n0.5 2 0.06 1 2 0.06 1.5 2 0.06 2 2 0.06 2.5 2 0.06\nST solution (mL) 0.5 1 1.5 2 2.5\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n2 2 0.01 2 2 0.02 2 2 0.04 2 2 0.06 2 2 0.08\nST solution (mL) 2 2 2 2 2\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\n3\n7. “The role of Fe in coordination and its effect on mechanical properties is well-discussed. However, optimization of electrical performance is not clearly illustrated. For example, the addition of NaCl is not described in the experimental section. The author is therefore recommended to revise the experimental section to include missing information and to characterize the resistance change during the gel-to-liquid phase transition.”\nResponse: We thank the reviewer for raising these concerns. We have conducted additional experiments. The results have been shown and discussed in Supplementary Fig. 3.\nThe conductivity arises from the presence of freely moving ions, primarily Na⁺ and Cl⁻. The Na⁺ ions originate from ST or poly(ST), while the Cl⁻ ions are introduced via the crosslinking agent FeCl .\nThe electrical properties of HAAT were all evaluated in its gel state. Besides, during the process of monitoring EEG, HAAT was also in a gel state. The fluid-to-gel transition occurs only during hair penetration before the test. Therefore, the transient resistance change during state transition is not critical to our work, as it does not affect the performance during actual signal acquisition.\nRelevant discussion and data have been added in the revised Supplementary Information as below.\nSupplementary Fig. 3. The skin contact impedance of a series of HAAT compared with commercial Ag/AgCl electrode.\n“The skin contact impedance of a series of HAAT with different ST (a-b) and Fe3+ (c-d) content compared with commercial Ag/AgCl electrode. Due to its excellent hair adaptability and strong adhesion, HAAT can easily penetrate hair and form seamlessly conformal contact with the skin, laying the foundation for constructing stable and efficient charge transfer interface on hairy skin. Furthermore, the presence of a large number of freely moving ions within the system as charge carriers is an important reason for achieving low skin contact impedance. Therefore, theoretically, adjusting the addition amounts of ST monomer and crosslinking agent FeCl can regulate the skin contact impedance\n3\nof HAAT. As shown in the figures above, the skin contact impedance of the HAAT electrode shows values comparable to or even smaller than those of the commercial Ag/AgCl electrode over a wide frequency range, but the variation pattern is not obvious.\nWith the increase of ST content, the number of freely moving ions (Na+) in the system increases, improving the conductivity. However, when the ST content exceeds the critical value, due to the more complex polymer network, ion mobilizing becomes more restricted, which is not conducive to conductivity.\nIncreasing of the content of crosslinking agent FeCl , although the number of freely ions (Cl-) in the\n3\nsystem increases, at the same time, the crosslinking of the polymer network becomes tighter. This leads to a narrowing of the ion diffusion channel and an increase in the diffusion path, which is detrimental to conductivity.”\n8. “Does the gel possess self-healing properties, considering the author’s claim about the presence of reversible S-S covalent bonds?”\nResponse: We thank the reviewer for raising this concern. According to references, self-healing behavior can be achieved through non-covalent bonding (e.g. hydrogen bonding, metal-ligand interaction and host-guest interaction) and covalent bonding through various mechanisms (e.g. Diels-Alder reactions, disulfides, and boronic ester). In this work, the presence of abundant reversible disulfide bonds within the system indeed endows HAAT with self-healing property. However, during EEG monitoring, minor scalp deformations do not induce rupture in the highly stretchable HAAT. Therefore, the self-healing property is not a primary focus of this work.\nReference:\n1. He, C. et al. Super Tough and Spontaneous Water-Assisted Autonomous Self-Healing Elastomer for Underwater Wearable Electronics. Advanced. Science. 8, 2102275 (2021).\n2. Sun, T. et al. MXene-reinforced water insensitive self-healing piezoelectric nanogenerator for ambient and aquatic mechano-pressure sensing. Nano Energy. 133, 11041 (2025).\n9. “The optimization of adhesive properties through Fe-ion coordination and ST disulfide ratios is well-described using Young’s modulus and interfacial toughness measurements. However, the author is also encouraged to discuss viscosity changes with appropriate characterization methods.”\nResponse: We thank the reviewer for this suggestion. We have analyzed the viscosity change of HAAT during the temperature ramp in rheological tests and studied the influences of ST and Fe3+ content on the viscosity.\nRelevant discussion and data have been added in the revised Supplementary Information as below.\nSupplementary Fig. 21. Complex viscosity changes during temperature ramp of HAAT.\nFe represents T A Fe and Fe represents T A Fe . ST represents T A Fe and ST\n0.01 2 2 0.01 0.06 2 2 0.06 2 2 2 0.01 2.5\nrepresents T A Fe (see Supplementary Table 4 and 5).\n2.5 2 0.01\nRising temperature progressively breaks disulfide bonds and coordination bonds, leading to a reduction in viscosity. Besides, higher Fe3+ concentration increases viscosity by forming more Fe3+-carboxylate coordination bonds. Increasing ST content raises the proportion of dynamic disulfide bonds, leading to the increase in dissipation energy component increase and the decrease in viscosity.\n11. “The gel electrode’s conductivity is stated to be based on NaCl; however, the author also claims that NaCl disrupts interfacial non-covalent interactions, such as hydrogen bonding, during the peeling experiment. This apparent contradiction needs to be clarified. Additionally, the effects of varying NaCl concentration on the mechanical properties specifically Young’s modulus and interfacial toughness-should be investigated and reported to better understand its role in balancing conductivity and adhesion performance.”\nResponse: We thank the reviewer for raising these concerns.\nThe reviewer is correct in stating that Na+ and Cl- are present in the gel and serve as charge carriers for ionic conduction. These two ions are introduced by the precursors (ST for Na+ and Cl- for FeCl )\n3\nduring synthesis; no additional NaCl is introduced. Our results indicate that even in the presence of these ions in the bulk of the gel, the gel-skin adhesion is exceptionally well (Video 6). We postulate that these ions may be loosely anchored to the polymer backbone by coordination bonds, and do not aggregate at the gel-skin interface. We note that, to detach the HAAT gel, (1) GSH is needed to cleave the disulfide bonds; and (2) a NaCl solution (1%wt) is directly delivered to the edge of the gel-skin interface for weakening interfacial electrostatic interaction and hydrogen bonding. This process creates a different scenario, where high ionic strength is localized on the interface to shield the electrostatic interaction and hydrogen bonding and facilitate detachment. To clarify the preparation process for readers, we have revised the “Synthesis of HAAT” section in “Method”.\nNo additional NaCl was introduced during HAAT preparation. The effects of Na⁺ and Cl⁻ concentrations on mechanical properties correspond to variations in ST and FeCl content, which had\n3\nbeen systematically investigated in Figure 3b-c.\nCorresponding discussion and details have been added in the revised manuscript and Supplementary Information as below.\n“Synthesis of HAAT\nHAAT was prepared via following steps. Firstly, LA (4 g) and NaOH (0.7755 g) were dispersed uniformly in deionized water (10 mL) to prepare ST solution. The ST solution was stored at 4 °C and out of light. Secondly, FeCl ·6H O (2 g) was dissolved in deionized water (5 mL) to form FeCl\n3 2 3\naqueous solution. Thirdly, a certain amount of FeCl aqueous solution, ST (2 mL), SBMA (2 g),\n3\nglycerol (1 mL) was added into deionized water (2 mL) and then stirring in the dark to form the precursor solution of HAAT. The specific contents of each component are listed in Supplementary Table 4-5. Finally, the precursor solution was polymerized at 90 °C for 210 min to form the HAAT. Additionally, GSH (0.5 g) and NaCl (0.1 g) was dispersed uniformly in deionized water (9.4 g) as the detachment solution (5 wt% GSH and 1 wt% NaCl).”\nSupplementary Table 4. Specific contents of a series of HAAT with different ST content.\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n0.5 2 0.06 1 2 0.06 1.5 2 0.06 2 2 0.06 2.5 2 0.06\nST solution (mL) 0.5 1 1.5 2 2.5\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\nSample T A Fe T A Fe T A Fe T A Fe T A Fe\n2 2 0.01 2 2 0.02 2 2 0.04 2 2 0.06 2 2 0.08\nST solution (mL) 2 2 2 2 2\nSBMA (g) 2 2 2 2 2\nGlycerol (mL) 1 1 1 1 1\nH O (mL) 2 2 2 2 2\n3\n12. “How does skin moisture, such as sweat, affect the adhesive properties of the gel electrode? Since real-world applications involve contact with sweaty skin, it is important to evaluate the adhesion performance under such conditions?”\nResponse: We thank the reviewer for raising these concerns. Subjects are usually provided with a relatively comfortable environment (appropriate temperature, humidity, brightness, etc.) when monitoring EEG for cognition study, thus we initially did not take sweat tolerance of HAAT as a primary design goal. Instead, we focus on hair-adaptability and adhesion-tunability in this work. In the revised manuscript, following the reviewer’s suggestion, we evaluate the environmental stability of HAAT by monitoring water retention over 10 days and taking videos while spraying simulated sweat solution on HAAT electrodes (newly added Supplementary Fig. 31 and Supplementary Movie 6). Additionally, we compared the ERP signal quality in the first and last stage of monkey vision-attention task (newly added Supplementary Fig. 50). All these tests show that our HAAT is sufficiently stable at high humidity for the use of electrode-scalp interface.\nIn the revised manuscript and Supplementary Information, we have added following discussion, figures and movie.\n“Compared to commercial EEG pastes, HAAT is able to penetrate hair without the need of additional pressure, tightly and stably adhere to scalp during prolonged usage in the presence of simulated sweat, and is readily remove by a detachment solution (inset of Fig. 4c and Supplementary Figs. 31-33).”\nSupplementary Fig. 31. Water retention of HAAT. m and m denote mass of HAAT before and\n0\nafter water retention experiments.\nThree samples of HAAT (approximately 5 mm x 15 mm x 1.5 mm) were respectively placed on three open Petri dishes at room temperature for 10 days, during which the masses of HAAT were monitored.\nLittle mass losses but slightly mass increases were observed. This is because in two of the components of HAAT, SBMA (sulfobetaine methacrylate) and glycerol, sulfonic acid groups and hydroxyl groups can form hydrogen bonds with water molecules, leading to better environmental stability.\nSupplementary Movie 6_Comparison of adhesion stability of HAAT and commercial materials by spraying simulated sweat solution.\n“The signal to noise ratio (SNR) in the early stage of the task is slightly higher than that in the end period, indicating that HAAT achieves equivalent performance throughout the entire task.\n(Supplementary Figs. 50, 51). These further confirm that HAAT can serve as an advancing alternative to commercial pastes for ERP collection.”\nSupplementary Fig. 50. Comparison of the SNR of ERP in early and late stage of the monkey’s vision-attention task.\nHist and box plot of SNRs are obtained from HAAT channels. Blue represents the 100 trials in the early stage of the task, and orange represents the 100 trials near the end of the task. The task lasts approximately 4 hours. SNRs of ERP are obtained under the condition of annulus occurred in the contralateral visual field. Specifically, channels O2 and P4 correspond to the annulus presented in the left visual field (Annulus Left).\nReviewer #4:\n“This manuscript presents a highly innovative study on the development of a hair-adaptable and adhesion-tunable scalp-electrode interface (HAAT) based on dynamic disulfide bonding polymers for non-invasive electroneurographic recording in humans and animal models. The authors convincingly show how HAAT overcomes key challenges in EEG acquisition from hairy scalps, particularly in monkeys and mice. The temperature-triggered fluid-to-gel transition and glutathione-based detachment are elegantly designed and offer strong translational potential. The study is thoroughly validated through comprehensive material characterization, multi-species EEG/ERP recordings, and a well-designed cognitive task in non-human primates. The manuscript is clearly written, well organized, and supported by solid data and analysis. The integration of advanced materials science with cognitive neuroscience is both timely and impactful. Given its significant contribution to advancing non-invasive neurotechnology, I recommend accepting this manuscript for publication in Nature Communications, pending the authors’ resolution of the following issues.”\nWe are grateful to the reviewer for the positive comments on our work. Our point-by-point responses are listed as below.\n1. “Some figures lack adequate description and require clarification. For example, in Figure 1a, the size and location of the mouse brain in the schematic are inconsistent with anatomical accuracy and should be revised. In Figure 1c, the subject shown is unclear and should be explicitly identified. Additionally, Figure 3h is missing a scale bar, which is recommended for proper interpretation.”\nResponse: We thank the reviewer for the reminder. In Figure 1a, the size of mouse brain has been adjusted and the location has been corrected (the olfactory bulb is closer to the nose). In Figure 1c, the legends of whose hair have been clearly labeled. In Figure 3h, scale bar has been added.\nCorresponding changes have been updated in the revised manuscript as below.\nFigure 1. Design of hair-adaptable and adhesion-tunable scalp-electrode interface (HAAT).\nFigure 3. Tunable adhesion of HAAT induced by dynamic disulfide bond. a, Schematic illustration …… h, Photographs of the scalp before and after HAAT removal by detachment solution. No hair loses by HAAT. Scale bar is 0.5 cm. i, Comparison …….\n2. “While fabricating HAAT polymer, ERP was conducted to prove that the disulfide bonds of ST undergo cleavage upon heating, generating radicals to initiate the polymerization of SBMA. The authors should explain in detail how ERP results substantiate this conclusion.”\nResponse: We thank the reviewer for the suggestion. We have supplemented the explanation in the revised Supplementary Information below Supplementary Fig. 6 as following.\n“The g value of ST monomer solution (100 g/L) was 2.0039, while the g value of the ST/SBMA mixed solution (ST: 100 g/L, SBMA: 250 g/L) was 2.0016. The change of g value can indicate that the single electron of the free radical has been transferred under light exposure. It is speculated that the ST free radical is transferred to the SBMA monomer through a chain initiation reaction, which is in line with the chain initiation process and indirectly proves the rationality of the predicted synthetic mechanism.”\n3.“The authors indicate that the ST/SBMA monomer ratio affects mechanical properties (Supplementary Figure S9). A detail discussion on how this ratio influences mechanical behavior would be helpful.”\nResponse: We thank the reviewer for the suggestion. We have added a detailed discussion below the Supplementary Fig. 11 (9 in 1st submission version) as following.\n“In the tensile test, as the ST content increases, the number of dynamic covalent bonds within the system increases, the energy dissipation during tensile testing increases, leading to tensile property enhances. Meanwhile, the increase in the proportion of dynamic disulfide bonds in the polymer chain enhances the flexibility of polymer chains, resulting in a decrease in Young's modulus. However, when the ST content exceeds the critical value, due to the increase in the number of monomers, the molecular weight of the polymer network increases, resulting in an increase in rigidity, which leads to a decrease in the elongation at break and a slight increase in Young's modulus.”\n4. “Understanding the mechanism behind hair adaptability is central to the HAAT design. The authors attribute HAAT’s thermal responsiveness to dynamic disulfide bonds and present supporting rheological and DSC data. However, the results also suggest that Fe³⁺-carboxyl coordination bonds may contribute to this transition. The authors are encouraged to clarify whether additional factors, beyond disulfide bonds, contribute to thermal responsiveness and to explain their respective roles.”\nResponse: We sincerely appreciate this valuable comment. In investigating the mechanism of the state transition, we have indeed considered the contributions of other interactions. As the reviewer correctly pointed out, in addition to disulfide bonds, Fe3+-carboxylate coordination bonds and various other dynamic interactions are also temperature-sensitive and collectively contributed to the thermal responsiveness of the system. However, our primary focus is on disulfide bonds, as they are more abundant and play a dominant role in the phase transition behavior. This is supported by our Raman spectroscopy results (Figure 2e) and rheological analyses (Supplementary Fig. 20), which highlight the significant contribution of disulfide bonds to thermal responsiveness.\nSupplementary Fig. 20. Rheological characterization of storage and loss modulus (Pa) in logarithmic form (lg G’ and lg G’’) at temperature sweeps of HAAT.\nAt an increasing amount of Fe3+, the crosslinking density will be enhanced, leading to the robustness of polymer network. More energy will thus be required to break Fe3+-carboxylate coordination bonds during heating, resulting in a higher state transition threshold. HAAT are T2A2Fe0.01 and T2A2Fe0.06. TxAy means that the precursor solution contains x mL ST and y g SBMA, while contents of other ingredients remain unchanged. Similarly, Fez means the molar ratio of Fe3+ to ST is z.\n5. “The authors need to provide the preparation details of the detachment solution (GSH and NaCl) and explain the rationale for choosing these components.”\nResponse: We thank for the reminder and suggestion. We are sorry for the oversights. We have now included the preparation process of detachment solution in “Synthesis of HAAT” section of “Method”.\nThe rationale for selecting glutathione (GSH) and sodium chloride (NaCl) is based on the principle of combining agents that can disrupt interfacial disulfide bonds and non-covalent interactions, while ensuring biocompatibility. Specifically, we selected GSH, a naturally occurring tripeptide with reducing properties, as a mild and non-toxic agent capable of cleaving disulfide bonds through redox reactions. Additionally, NaCl was chosen as a widely used, biocompatible electrolyte. The Na+ and Cl- ions ionized from NaCl serve to neutralize the charged groups and disrupt the hydrogen bonding at the interface. This synergistic effect of GSH and NaCl results in a rapid reduction in the adhesion of HAAT, enabling safe and gentle detachment without causing damage to skin or hair.\nRelated details have been added in the revised manuscript as following.\n“NaCl disrupts the interfacial non-covalent dynamic interactions such as electrostatic interactions and hydrogen bonding34. PBS solution (simulated sweat) alone cannot detach HAAT from scalp, and large adhesion loss only occurs in the presence of reducing agents capable of cleaving disulfide bonds (Supplementary Movie 6). These observations suggest that interfacial disulfide bonds are the determining factor of interfacial toughness. Upon the delivery of the detachment solution, interfacial toughness between HAAT and porcine skin drastically decreases from 523 N/m to almost zero under 90-degree peeling test (Fig. 3e and Supplementary Figs. 26-27).”\n“Synthesis of HAAT\nHAAT was prepared via following steps. Firstly, LA (4 g) and NaOH (0.7755 g) were dispersed uniformly in deionized water (10 mL) to prepare ST solution. The ST solution was stored at 4 °C and out of light. Secondly, FeCl ·6H O (2 g) was dissolved in deionized water (5 mL) to form FeCl\n3 2 3\naqueous solution. Thirdly, a certain amount of FeCl aqueous solution, ST (2 mL), SBMA (2 g),\n3\nglycerol (1 mL) was added into deionized water (2 mL) and then stirring in the dark to form the precursor solution of HAAT. The specific contents of each component are listed in Supplementary Table 4-5. Finally, the precursor solution was polymerized at 90 °C for 210 min to form the HAAT. Additionally, GSH (0.5 g) and NaCl (0.1 g) was dispersed uniformly in deionized water (9.4 g) as the detachment solution (5 wt% GSH and 1 wt% NaCl).”"}, {"round": 2, "reviewer_comments": {"reviewer_2": "no more comment.", "reviewer_3": "After careful evaluation, I find that the authors have adequately addressed the previously raised comments with appropriate material characterizations. Therefore, I recommend the revised manuscript for publication in Nature Communications. However, the figure and reference numbering, as well as the overall flow, should be revised before final acceptance.", "reviewer_4": "The authors has addressed all my concerns and I highly recommend the publication of this work as it is.", "reviewer_5": "I co-reviewed this manuscript with one of the reviewers who provided the listed reports. This is part of the Nature Communications initiative to facilitate training in peer review and to provide appropriate recognition for Early Career Researchers who co-review manuscripts."}, "author_response": ""}]

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