Stick-and-play bioadhesive hairlike electrodes for persistent EEG recording on human

3D printed hairlike gadget

To manufacture the hairlike EEG recording gadget, we utilized direct ink writing (DIW) 3D printing¹⁴, a flexible method that enables exact patterning of assorted inks via managed nozzle motion in three-dimensional, constructing the gadget layer by layer¹⁵. This technique affords excessive spatial decision, decreased time in gadget fabrication, and glorious reproducibility, making it a perfect methodology for establishing intricate, multi-layered buildings with the precision required for our software. We employed conducting polymer hydrogel materials for the electrode layer which consists of poly(3,4-ethylenedioxythiophene): polystyrene sulfonate (PEDOT: PSS) and hydrophilic polyurethane (PU). This hydrogel materials displays excessive electrical conductivity (>11 Scm⁻¹)¹⁶ and excessive stretchability (~400%)¹⁶ making certain optimum properties for our designed hairlike gadgets. Structurally, the hairlike gadget consists of 4 layers (Fig 1a (ii)): the central conducting polymer hydrogel electrode layer is sandwiched between two PDMS encapsulation layers. The electrode layer has 3 parts: The interface, the interconnect and the output unit. On the interface web site, a single PDMS layer is used to reveal the electrode, permitting it to instantly purchase indicators from the scalp. Moreover, an adhesive layer is printed across the electrode layer on high of the underside PDMS layer to safe the gadget in place.

**Fig. 1: Bioadhesive hairlike electronics for secure persistent EEG monitoring.**

We designed the hairlike gadget with the intention that it may be worn constantly over prolonged intervals, permitting topics to take care of their day by day routines with out experiencing any interference or noticeable beauty variations. The interface a part of the hairlike gadget that adheres to the scalp is round with a diameter of 1.5 mm, optimizing contact with the pores and skin with minimal intrusiveness. The interconnect a part of the gadget is designed to be 300 µm in width, offering a skinny, hairlike construction that seamlessly integrates with the pure hair (Fig. 1b). This slender design minimizes any aesthetic affect and ensures the gadget’s light-weight and versatile nature, enhancing consumer consolation. Upon placement, the gadget meshes naturally with the hair, turning into just about invisible and preserving the consumer’s pure look (Fig. 1c). This design facet is important for consumer acceptance, because it permits the gadget to be worn discreetly in varied social {and professional} settings with out attracting discover. Along with beauty benefits, the hairlike design ensures that the gadget doesn’t intrude with day by day actions. Its sturdy adhesion and suppleness permit it to stay robustly in place throughout motion, train, and routine hair grooming (Fig. 1d). To cater for topics with totally different hair colours, we additionally designed the hairlike gadget for varied hair colours (Fig. 1e) by adopting biocompatible dyes within the printing materials (see Strategies for detailed process).

3D printable adhesive ink

The bioadhesive was synthesized utilizing a UV-assisted polymerization technique (see Strategies for detailed process). An answer containing PU, ethanol, deionized water, acrylic acid, α-ketoglutaric acid, and benzophenone was uncovered to UV gentle. On this course of, α-ketoglutaric acid initiated the polymerization of acrylic acid, whereas benzophenone generated radical websites within the PU, facilitating the covalent bonding of poly(acrylic) acid (PAA). The resultant PAA-integrated PU was then purified via dialysis in ethanol and water sequentially to take away any unreacted reagents, adopted by completely drying. To transform the synthesized PAA-integrated PU right into a printable ink, we dissolve it in an aqueous ethanol resolution. N-(3-dimethylethylaminopropyl)—N-ethylcarbodiimide hydrochloride (EDC) and N-ethyl-N′-(, N-hydroxysuccinimide) (NHS) had been launched within the resolution to introduce NHS ester purposeful teams on the PAA chains (Supplementary Fig. 1). The introduction of NHS esters considerably enhances the adhesive properties of the polymer by rising its reactivity with amine teams current in proteins and different biomolecules. This elevated reactivity facilitates the formation of sturdy and sturdy covalent bonds^17,18, making the adhesive extremely efficient for organic purposes. The covalent bonding ensures strong attachment of hairlike gadget to the pores and skin. Previous to 3D printing the answer was blended with PU to finely modify its viscosity to allow high-resolution 3D printing¹⁹.

The rheological properties of the as developed bioadhesive displays a shear thinning conduct (Supplementary Fig. 2), which is attribute of non-Newtonian fluids. For direct-ink-write (DIW) printing, the ink should circulate easily via the nozzle below stress and exhibit fast restoration to its authentic form post-extrusion. The quickly decreased viscosity at greater shear price allows the ink to circulate simply via the superb nozzle below stress, minimizing the chance of nozzle clogging and enabling a steady, uninterrupted circulate. Excessive viscosity at low shear price makes the fabric proof against spreading upon printing, permitting exact printing of the adhesive layer of hairlike gadget.

To measure the adhesion efficiency of the bioadhesive to the pores and skin floor, we measured the interfacial adhesive drive between the bioadhesive and pores and skin. The bioadhesive confirmed favorable adhesion to the pores and skin floor with a mean adhesion drive of 0.7 N/cm (Fig. second). Owing to a excessive density of charged carboxylic acid teams, PAA chains can rapidly consolidate with tissue surfaces to type intermolecular hydrogen bonds. Reactive NHS ester teams additional improve tissue adhesion by interacting with main amines on tissue surfaces to type covalent amide bonds¹⁷. Hydrophilic PAA chains improve fast interfacial water absorption¹⁸, permitting the bioadhesive to take care of sturdy adhesion to the pores and skin even throughout publicity to water, corresponding to throughout showers, or when sweat types on the scalp throughout train. We additionally evaluated the adhesion efficiency of the hairlike gadget utilizing bioadhesive compared to a business adhesive EEG paste (Fig. 2e). The hairlike gadget with the bioadhesive demonstrated considerably stronger and extra sturdy adhesion to the pores and skin floor in comparison with the business adhesive EEG paste (Fig. 2 f-g). The bioadhesive confirmed adhesion efficiency which is sort of double the power of economic EEG gel (n = 4) (Fig. 2g). The bioadhesive exhibited mechanical robustness with excessive flexibility and stretchability, permitting for seamless integration with nonplanar pores and skin surfaces. The bioadhesive might be stretched about eight instances of its authentic size with a Younger’s modulus of 42 kPa (Fig. 2h), which is akin to that of the native tissue of muscle and pores and skin²⁰. The low modulus of the bioadhesive ensures conformal contact with the curved floor of the pores and skin, enhancing consolation. Moreover, it may be eliminated with out inflicting any injury to the pores and skin. The AFM picture (Supplementary Fig. 3) of the bioadhesive post-attachment to human pores and skin reveals a clean floor with no residue left on the bioadhesive after removing.

**Fig. 2: Bioadhesive supplies for hairlike electrodes.**

Electrical and mechanical efficiency of hairlike gadget

An intimate interface between the electrode of the hairlike gadget and the pores and skin is critical for electrical communication. Leveraging the extremely strong nature of the bioadhesive, we had been in a position to set up a secure interface between the electrode and the pores and skin. To guage their interface, we performed electrochemical impedance spectroscopy (EIS) characterization of the hairlike gadget to review the interfacial impedances of the gadget electrode and pores and skin on each the wrist and the scalp. For comparative evaluation, we additionally measured the pores and skin impedance utilizing a gold electrode with business EEG paste. The bioadhesive’s strong adhesion and the hydrogel electrode’s flexibility facilitated conformal contact with the pores and skin tissue. Consequently, the hairlike gadget demonstrated a decrease electrode-skin electrical impedance in comparison with the business gold electrode. Particularly, the hairlike gadget exhibited pores and skin impedances of 26.02 kΩ at 100 Hz and 43.89 kΩ at 10 Hz on the wrist (Fig. 3b). On the scalp, the hairlike gadget confirmed impedances of 14.83 kΩ at 100 Hz and 16.72 kΩ at 10 Hz (Fig. 3c). The hairlike electrode additionally confirmed a excessive cost storage capability (Supplementary Fig. 4).

**Fig. 3: Printing of hairlike electrodes and characterization of their stability, electrical and mechanical properties.**

To make sure the hairlike gadget is unobtrusive and doesn’t intrude with day by day actions, we examined its long-term adhesion and electrical efficiency. The gadget was positioned on the scalp of a topic who then proceeded with their regular day by day actions. We examined the location and the adhesion efficiency of the gadget, which remained securely hooked up to the scalp for twenty-four hours (Fig. 3d). We additionally carried out electrochemical impedance spectroscopy (EIS) characterization to find out interfacial impedances of the gadget electrode and pores and skin on the scalp after 12 and 24 hours of steady put on. The outcomes confirmed that the impedance remained secure with no noticeable improve after 12 and 24 hours (Fig. 3e and Supplementary Fig. 5). These findings underscore the hairlike gadget’s potential for long-term high-fidelity electrophysiological recordings, as evidenced by its decrease impedance values and secure skin-electrode interface.

Earlier than 3D printing, we have now examined rheological conduct of all of the parts of hairlike electrode, Rheology characterizations (Supplementary Fig. 6) reveal that each the PDMS ink and the PEDOT:PSS electrode ink display shear-thinning conduct, the place viscosity decreases with rising shear price making certain clean materials circulate below mechanical stress whereas sustaining structural integrity after printing. As a wearable gadget, the hairlike gadget should endure varied forces encountered throughout regular actions, corresponding to combing or dealing with hair. Due to this fact, it is necessary for a hairlike gadget to have the ability to face up to varied forces. For dependable software, a tool should possess restoration properties from deformation. We examined the efficiency of the hairlike gadget below cyclic tensile loading and single tensile loading. The hairlike gadget exhibited mechanical robustness with excessive stretchability and tensile cyclability, which is helpful for real-world purposes. Successive cyclic tensile checks with a most pressure of 10% had been performed for the hairlike gadget. After cyclic loading, we examined the pores and skin impedance of the hairlike gadget on the wrist. The hairlike gadget confirmed secure electrical efficiency after 10, 50 and 100 cycles of loading (Fig. 3f). The hairlike gadget can also be extremely stretchable with an final pressure as much as 200% (Fig. 3g).

Excessive-fidelity long-term EEG recording with hairlike gadget

Excessive-quality EEG recording is essential for each medical and research-based neurological purposes, together with the prognosis of neurological issues and the event of brain-machine interfaces (BMIs)^21,22. EEG recording affords a noninvasive and environment friendly technique for monitoring mind electrophysiological actions. It offers good temporal decision that’s important for precisely capturing fast neural dynamics that’s important for diagnosing circumstances corresponding to epilepsy²³, sleep issues²⁴, mind accidents²⁵, in addition to for advancing BMIs, which depend on exact neural sign interpretation to regulate exterior gadgets. Attaining extremely efficient recording of EEG indicators is especially difficult as a result of inherently weak nature of the indicators, which have amplitudes within the microvolt vary²⁶. Moreover, dense hair on the scalp introduces vital interference, complicating the acquisition of clear and correct readings²⁷. For EEG, sign high quality critically relies on the gadget’s skill to evolve carefully to the pores and skin. This shut conformity ensures optimum contact between the electrodes and the scalp, which is important for capturing the faint electrical indicators generated by mind exercise. Any hole can considerably degrade sign high quality by rising impedance and rising interference from exterior noise²⁸. Owing to the superb adhesion efficiency of the bioadhesive layer and the pliability and stretchability of the encapsulation layers and hydrogel electrode layer, the hairlike gadget achieves glorious conformability and compliance with the scalp, even within the presence of dense hair. The hairlike gadget design ensures that there’s minimal interference from hair, permitting the gadget to take care of intimate contact with the scalp. The strong adhesion prevents any shifting or lifting of the electrodes, whereas the versatile supplies conform to the contours of the scalp, making certain constant sign acquisition. Because of this, the hairlike gadget can seize high-quality EEG indicators. To validate the efficacy of our hairlike EEG recording gadget, we performed a proof-of-concept examine specializing in EEG alpha exercise, recorded from the occipital area of a bushy scalp. This space is especially vital for producing alpha waves, that are sometimes noticed when people are in a relaxed state with their eyes closed²⁹. We recorded EEG indicators in each eyes-open and eyes-closed states, utilizing the hairlike gadget (Fig. 4). The recordings revealed clear variations between the indicators detected in these two paradigms. Within the eyes-closed states (Fig. 4a-c and Supplementary Fig. 7a), the ability spectral density evaluation (PSDA) (Fig. 4c) confirmed a pronounced alpha rhythm with a transparent central peak round 10 Hz, a attribute function of alpha rhythms. Alpha rhythms sometimes vary from 8 to 12 Hz and are related to a state of relaxed wakefulness³⁰ with decreased visible processing and a extra introspective mind-set. Alpha waves are most distinguished when the visible cortex is idle, corresponding to when the eyes are closed³⁰, permitting for a clearer detection of those mind waves. In distinction, in the course of the eyes-open state (Fig. 4d-f and Supplementary Fig. 7b), the PSDA (Fig. 4f) confirmed an absence of alpha rhythm, according to the identified suppression of alpha waves when the eyes are open, because the mind shifts its exercise in the direction of processing visible data and exterior stimuli. The clear central peak round 10 Hz noticed within the PSDA of eyes closed state highlights the hairlike gadget’s skill to precisely seize EEG alpha rhythms. For comparative evaluation, we recorded EEG indicators from the occipital area utilizing a business gold electrode with EEG gel (Fig. 4g-l). This comparability was performed to benchmark the efficiency of the hairlike gadget in opposition to a traditional EEG recording normal. The outcomes demonstrated that the hairlike gadget successfully captured high-quality EEG indicators, exhibiting constancy akin to, and in some situations surpassing, that of the business gold electrodes.

**Fig. 4: Excessive-fidelity persistent EEG sign monitoring from bushy scalp utilizing hairlike electrodes and standard gold electrodes.**

As well as, the hairlike gadget enabled high-fidelity recording of EEG indicators over an prolonged interval, sustaining dependable efficiency for over 24 hours of steady sporting (Fig. 5). This long-term stability is attributed to the gadget’s glorious electrical stability, superior mechanical properties, and strong contact with the scalp. EEG indicators had been recorded upon preliminary placement of the electrode on the scalp, following which the topics engaged of their regular day by day actions. Subsequent EEG recordings had been collected at 3-hour, 6-hour, and 12-hour intervals. The themes maintained their common sleep schedules and sleeping positions in a single day. On the next day, after 24 hours of steady put on, EEG indicators had been recorded once more. The EEG alpha waves recorded at varied time factors confirmed no notable degradation in high quality. The recordings at totally different time spans persistently demonstrated excessive constancy, with clear patterns characterised by a peak frequency round 10 Hz within the eyes-closed state (Fig. 5a) and no discernible peak within the eyes-open state (Fig. 5b). The consistency of recorded indicators throughout various time spans strongly underscores the robustness of the hairlike gadget for long-term EEG monitoring.