Universal Hydrogel Adhesives with Robust Chain Entanglement for Bridging Soft Electronic Materials

doi:10.21203/rs.3.rs-4169072/v1

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Universal Hydrogel Adhesives with Robust Chain Entanglement for Bridging Soft Electronic Materials

https://doi.org/10.21203/rs.3.rs-4169072/v1

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published 12 Jul, 2024

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Ensuring stable integration of diverse soft electronic components for reliable operation under dynamic conditions is crucial. However, integrating soft electronics, comprising various materials like polymers, metals, and hydrogels, poses challenges due to their different mechanical and chemical properties. This study introduces a dried-hydrogel adhesive made of poly(vinyl alcohol) and tannic acid multilayers (d-HAPT), which integrates soft electronic materials through moisture-derived chain entanglement. d-HAPT is a thin (~ 1µm) and highly transparent (over 85% transmittance in the visible light region) adhesive, showing robust bonding (up to 3.6 MPa) within a short time (< 1 min). d-HAPT demonstrates practical application in wearable devices, including a hydrogel touch panel and strain sensors. Additionally, the potential of d-HAPT for use in implantable electronics is demonstrated through in vivo neuromodulation and electrocardiographic recording experiments while confirming its biocompatibility both in vitro and in vivo. It is expected that d-HAPT will provide a reliable platform for integrating soft electronic applications.

Physical sciences/Engineering/Electrical and electronic engineering

Physical sciences/Materials science/Materials for devices

Soft electronics have received significant attention due to their potential to revolutionize human-machine interaction by bridging the mechanical disparity between traditional rigid electronics and soft tissues^1–3. These soft electronic devices are composed of soft and rigid building blocks. The soft building blocks usually comprise stretchable and flexible polymers^4,5, and hydrogels^6,7, serving as deformable substrates, interconnectors, and electrodes. These soft building blocks facilitate the conformal contact between the device and the human body, allowing reliable sensing of various biological signals or stimulation of body tissue. In particular, many studies have introduced hydrogel-based electronic devices by exploiting the distinctive properties of hydrogels, including their high water content, low Young’s modulus, and biocompatibility^7,8. However, the realization of soft electronics with only soft components is considered infeasible due to limitations in handling the tasks of data processing or transmission. Therefore, the use of rigid electronic components (e.g., IC chips and printed circuit boards (PCBs)) is essential⁹. These soft and rigid components should be seamlessly integrated and robustly combined for the reliable operation of the devices under dynamic movement conditions. However, achieving stable integration is challenging due to their inherent differences in chemical and mechanical properties. For instance, the strong adhesion of hydrogels (wet materials) to other building blocks (dry materials) is difficult owing to their high water content, which prevents intimate contact and adhesion between them. Therefore, universal adhesion strategies that achieve flexible but robust integration not only at dry/dry interfaces but also at wet/dry interfaces are required.

Stable integration of the building blocks using conventional adhesives has been inappropriate due to unstable bonding interface and biocompatibility. For example, cyanoacrylate-based superglues are frequently adopted for their immediate bonding capability. However, the bonded interface lacks flexibility, and resistance to mechanical stress, and temperature¹⁰. Besides, typical superglues could not be cured on silicone-based elastomers such as polydimethylsiloxane (PDMS) and Eco-flex, which are extensively used in soft electronic devices. Furthermore, the cytotoxicity of cyanoacetate and formaldehyde has been a concern as toxic components could be released during the degradation of polycyanoacrylate¹¹. Epoxies are also widely adopted as adhesives for soft electronics. Nonetheless, they have limited adhesion to certain plastics and require a long curing time. In addition, the difficulty of controlling the shape of the soldered form restricts their applications for minimized soft electronic devices¹². Silver pastes provide electrical connectivity and adhesiveness. However, cytotoxicity and poor bonding strength hinder bioelectronic applications^12,13. Most importantly, these conventional adhesives enable the bonding between dry and rigid materials, but stable bonding of soft or wet materials remains a challenge due to their differences in mechanical modulus and water content. To improve the bonding properties of the conventional adhesives, surface activation, and a modified cyanoacrylate-based adhesive were introduced as the adhesion strategies that establish conformal bonding interfaces to soft materials, including hydrogels. However, each strategy showed limited selectivity, and biocompatibility was not investigated. Therefore, there is a need to develop universal and reliable adhesion strategies that offer biocompatibility, flexibility, electrical adaptability, and the capability to facilitate robust bonding and integration of various soft/rigid and wet/dry electronic materials.

Hydrogel adhesives are emerging as potential breakthroughs for biocompatible and universal adhesive materials. Nature-derived hydrogel adhesives (e.g., chitosan-based^14,15, gelatin-based^16,17, and mussel-inspired^18,19) exhibit non-selective adhesion due to their abundant functional groups. The functional groups within the adhesive facilitate the physical and chemical interactions with adherents. However, these adhesives usually result in thick bonding interfaces and have relatively low bonding strength, which hinders their application in electronics. Recently, dry hydrogel adhesives that improve the bonding strength of typical hydrogel adhesives have been introduced, utilizing their maximized water-swellable property^20–22. Dry hydrogels rapidly absorb the surrounding moisture and undergo rapid hydration and swelling. This enhances the molecular chain mobility that facilitates the formation of tough bonding interfaces through diverse mechanical interactions and entanglement. This property of dry hydrogels holds immense potential as adhesive materials. Consequently, there have been reports on the development of dry hydrogel adhesives that seamlessly attach soft electronic devices and tissues^23,24. However, their effectiveness in integrating the building blocks that comprise soft electronic devices remains unexplored.

Here, we introduce a universal and biocompatible dried-hydrogel adhesive made of poly(vinyl alcohol) (PVA) and tannic acid (TA) multilayers (d-HAPT) that enable robust bonding and integration of diverse soft/rigid and wet/dry building blocks. d-HAPT exhibits softness, biocompatibility, and excellent adhesion properties by taking advantage of dry hydrogel adhesives. Especially, the abundant functional groups in TA and moisture-derived chain entanglement facilitate the robust integration of dry/dry materials (DD bonding) and wet/dry materials (WD bonding) regardless of their mechanical and surface properties. d-HAPT is successfully applied to substrates of various sizes, from small electrodes to large Si wafers by dip, spray, or brush coating. Moreover, d-HAPT has a thin thickness of less than 1µm and exhibits high transparency with a transmittance of over 85% in the visible light region while enabling strong integration within 1 min. The DD bonding strength of metals and polymers was 4 MPa and 600 kPa, respectively. Moreover, the tough hydrogel was firmly attached to the d-HAPT-coated Eco-flex substrate even at 630% elongation. To demonstrate the practical adaptability of d-HAPT in wearable soft electronics, we fabricated hydrogel-based wearable devices including a touch panel and strain sensors. Each device maintained stable adhesion and operation in bending and stretching situations. Furthermore, non-toxicity was demonstrated by in vitro and in vivo biocompatibility tests. Given the biocompatibility of d-HAPT, implantable soft electronic devices based on it have been developed to perform in vivo neuromodulation and electrocardiographic recording. Considering the biocompatibility, adhesiveness, and versatility, d-HAPT could be a promising candidate adhesive to replace conventional adhesives used for sophisticated soft electronics.

Integration of soft electronic devices facilitated by d-HAPT

Figure 1a shows a schematic of possible applications of d-HAPT in soft electronic devices composed of various components comprising a soft substrate, electrodes, rigid electronics, and a flexible interconnector. The integration of these materials is seamlessly facilitated by d-HAPT via two distinct adhesion mechanisms: 1) dry/dry materials bonding (DD bonding) and 2) wet/dry materials bonding (WD bonding). DD bonding enables adhesion among dry materials including elastomers, plastics, rigid electronics, metals, etc. As illustrated in Fig. 1a(i), d-HAPT coated on the substrates swells by absorbing a little interfacial water, which leads to chain entanglement of d-HAPT. The subsequent mild thermal treatment facilitates robust bonding between the substrates by evaporating moisture in d-HAPT instantly. WD bonding utilizes the water-rich property of hydrogels leading to the attachment of them to the dry substrates (Fig. 1a(ii)). When the hydrogel is attached to the d-HAPT-coated substrate, d-HAPT absorbs the water at the interface of the hydrogel matrix. The absorbed water leads to the diffusion of polymer chains of adhesive into the hydrogel matrix, thereby entangling the hydrogel network. These two bonding mechanisms accomplish robust adhesion of diverse materials with a broad spectrum of mechanical moduli, attributed to the swellable and soft nature of hydrogel constituents of d-HAPT. Figure 1b and c show representative examples showing the efficacy of DD bonding and WD bonding. To demonstrate the robustness of DD bonding, polymethyl siloxane (PDMS) (E ≈ 1 MPa) discs were attached to Eco-flex substrates (E ≈ 120 kPa) (Fig. 1b and Supplementary Movie 1)²⁵. The strong adhesion is maintained even when the sample is subjected to repeated areal strain of up to 350%. Furthermore, to confirm the strong adhesion of hydrogel bonding, we designed a light-emitting diode (LED) circuit system employing a conductive hydrogel (Fig. 1c and Supplementary Movie 2). The conductive hydrogel was adhered to a pair of stainless steel (SS) components coated with d-HAPT (E ≈ 200 GPa). The emitted light from the LED remained stable even when the hydrogel underwent a 200% extension of its initial length. These outcomes comprise substantial evidence that d-HAPT enables robust adhesion between various materials, enduring mechanical stresses including cyclic elongation and relaxation.

Fabrication and characterization of d-HAPT

Figure 2a shows the schematic illustration of the d-HAPT fabrication process through layer-by-layer (LbL) deposition onto substrates. TA layer was adopted as the first layer, because of its capacity to interact with various materials attributed to the plentiful galloyl groups in TA molecules. It is known that the hydroxyl groups in the TA molecule induce hydrogen bonds and metal coordinate bonds, whereas benzene rings bring about hydrophobic interaction and π–π interaction²⁶. These diverse physical and chemical interactions facilitate the stable TA-driven surface coating on a variety of materials, regardless of the substrate being organic/inorganic, and hydrophilic/hydrophobic. Sequentially, the PVA and TA layers are alternately stacked. PVA is a polymeric component that increases adhesion by forming multidentate bonds with TA through hydrogen bonds¹⁸. Furthermore, the strong hydrogen bond at the interface of the PVA and TA layer allows the uniform deposition of PVA/TA multilayers^18,27. ATR-FTIR spectroscopy analysis was conducted to affirm the successive layer stacking via the LbL assembly process and the presence of the strong hydrogen bonding interlinking each layer (Fig. 2b). Each step is denoted as T_nP_m, where n and m indicate the number of TA layers and PVA layers, respectively. After the coating of the primary TA layer, the broad and dull peak showed at 3100–3500 cm^− 1, indicating a hydroxyl group (–OH) stretching band²⁸. Upon the introduction of the subsequent PVA layer, the methylene group (–CH₂–) stretching peaks at 2908 and 2941 cm^− 1, characteristic of PVA, appear alongside the OH-stretching band²⁹. This result demonstrates the successful deposition of the PVA layer after the initial TA layer. Moreover, as successive layers are stacked, the OH-stretching peak gradually shifts to higher wavenumbers, from 3295 cm^− 1 for pure PVA to 3349 cm^− 1 for the T₃P₂. The obvious blue shift of the peak suggests the enhancement of strong hydrogen bonding between PVA and TA during LbL assembly^30,31. The stepwise thickness growth seen in Supplementary Fig. 1, provides additional confirmation of the continuous and stable multilayer coating achieved through the LbL assembly technique. According to the thickness data, the PVA layer is dominant in terms of thickness, and the overall thickness of the adhesive remains within the submicron scale (958.9 ± 25 nm). The SEM image visualized a sub-1 µm thick d-HAPT conformally coated on the glass substrate, similar to the thickness measurement data above (Fig. 2c and Supplementary Fig. 2). The areal surface roughness (S_a) data analyzed by the laser confocal microscope image (0.014 µm) and AFM image (0.0171 µm) further clearly demonstrated the uniformity of the dip-coated d-HAPT (Fig. 2d, e). Additionally, the spray-coated, and brush-coated d-HAPT are investigated (Fig. 2f and Supplementary Fig. 3). The thickness and roughness of spray-coated d-HAPT was 3.2 µm and 0.022 µm, and that of brush-coated d-HAPT was 4.5 µm and 0.163 µm. These results indicate that the d-HAPT-coated surfaces are uniform irrespective of the coating methods. The versatility of coating methods is a notable advantage in applicability across a wide spectrum ranging from wide wafer scale to meticulous electrodes (Supplementary Fig. 4). Additionally, d-HAPT can be fabricated into a free-standing film, enhancing its practical application (Supplementary Fig. 5). For d-HAPT to establish a robust bonding at the interface of two materials, it must remain securely affixed to the substrate, even under the influence of external forces. Therefore, to demonstrate the mechanical stability of d-HAPT, a crosscut test was conducted (Fig. 2g). A series of optical images demonstrate the results of the crosscut test onto two other substrates: glass (hydrophilic) and TPU (hydrophobic). For both substrates, d-HAPT is kept securely after 10 cycles of tape peeling. Moreover, the transparency of d-HAPT was assessed using ultraviolet-visible (UV–Vis) spectroscopy (Fig. 2h). The d-HAPT-coated glass substrates exhibit a transmittance of approximately 85% across the entire visible light spectrum (400–750 nm). This exceptional transparency is attributed to the ability of the d-HAPT to coat the substrate conformally and uniformly as confirmed above. In contrast, when solutions of TA and PVA were mixed and coated on glass (referred to as 'Mixed'), the transmittance dropped dramatically to below 40%. Visual observations of the samples with optical images corroborate these results. In the case of a mixed sample, it appears blurry behind the glass. Opacity is caused by the formation of yellowish precipitates in the case the PVA and TA solutions are mixed due to the strong hydrogen bond between PVA chains and TA molecules. Whereas the transparency of d-HAPT-coated glass closely resembles that of bare glass, enabling clear visibility of text and colors behind the glass. This is due to the controlled formation of hydrogen bonds at the interface of the PVA layer and TA layer. Consequently, the LbL approach yields a uniformly surfaced d-HAPT with minimal diffuse reflection and high transparency. This suggests that the d-HAPT could provide reliable alignment of the chips and electrodes demonstrating its potential application in optical soft electronic devices.

DD bonding mechanism and durability of the adhesion

Figure 3a illustrates the DD bonding process and its underlying mechanism. For DD bonding, interfacial water should be formed between d-HAPT-coated dry surfaces. The dry hydrogel constituents of d-HAPT allow it to swell by absorbing the interfacial water, thereby increasing chain mobility^20,32. The enhanced chain mobility leads to the entanglement of chains across the dry substrates. Subsequently, a mild heat is applied to evaporate the residual water of d-HAPT. This causes the entangled chains to aggregate and be fixed, which leads to robust bonding between the dry substrates.

To investigate the effect of heating temperature on the adhesion, SS substrates were bonded under various temperature conditions (70 to 160°C) with a fixed bonding time of 1 min (Supplementary Fig. 6). The results showed strong integration of substrates at all temperature conditions, with increasing bonding strength with the increasing bonding temperature (from 3 MPa at 70°C to 5.5 MPa at 160°C). This is attributed to the different interfacial water evaporation rates affecting the aggregation and fixation of chains. Moreover, the effect of heating time was investigated by bonding PDMS substrates at temperature conditions of 70 and 100°C for 1 and 5 min (Supplementary Table 1). These results indicate that substrates can be effectively bonded under various temperature and time conditions. Suitable bonding conditions can be selected according to the specific characteristics of the substrate. To ensure a simple and fast process, the standard bonding condition was determined to be 130°C for 1 min in this article, which can be easily achievable with a hair iron (Supplementary Fig. 7 and Supplementary Movie 3).

Figure 3b presents an optical image of d-HAPT bonded PDMS substrates. The d-HAPT showed high transparency in the visible light region (Supplementary Fig. 8). Additionally, it exhibits exceptional flexibility, attributed to the softness of hydrogel-based components within d-HAPT. The cross-sectional SEM image reveals seamless bonding between the PDMS substrates and the d-HAPT (Fig. 3c). Furthermore, the boundary of the d-HAPT was not visible, affirming that the two d-HAPT layers effectively entangled during the bonding process. To assess bonding strength, lap shear tests were conducted on various materials widely adopted for soft electronic devices, including metals and polymers (Fig. 3d). Initially, two identical metal substrates were bonded using a hair iron in standard condition. The bonding strength for these metals exceeded 2.5 MPa, with stainless steel demonstrating the highest bonding strength over 4 MPa (4.1 ± 1.2 MPa for SS, 2.6 ± 0.5 MPa for aluminum (Al), 2.5 ± 0.9 MPa for Copper (Cu)) (Fig. 3e). These values are 50 times higher than the bonding strength observed in other hydrogel-based adhesives capable of attaching metals with an intensity of up to 80 kPa^14,18. Figure 3f shows the strong adhesion of SS substrates sufficient to support a 20 kg water tank. This excellent adhesion in metals is attributed to the metal coordinate bonds, and hydrogen bonds between TA and PVA^18,27. Subsequently, the bonding strength of polymer substrates was investigated (Fig. 3g). Considering the glass transition temperature of polymers, the bonding condition was adjusted to 1 min in a drying oven at 100°C. The bonding strength exceeded 500 kPa for all polymer materials (637.6 ± 105 kPa for polyimide (PI), 556.7 ± 98 kPa for acryl, 537.911 ± 60 kPa for polypropylene (PP)). In the case of PDMS, they were stretched without debonding until fracture occurred due to the inherent stretchability of PDMS. The fractured section of the samples corresponds to a non-adhered region, signifying that d-HATP sufficiently withstands the stress that induces PDMS breakage (Fig. 3h). These overall results demonstrate that d-HAPT could form strong bonds between the same materials. For practical applications, adhesive compatibility between different materials is crucial. Bonding heterogeneous substrates presents more challenges compared to bonding the same materials, primarily due to the disparities in mechanical and chemical properties among materials. Nevertheless, d-HAPT with abundant functional groups enables robust bonding regardless of substrate properties. To evaluate this, lap shear tests were conducted across various substrate combinations. Samples were prepared by sandwiching a material with a lower modulus (substrate 2) between two substrates with higher moduli (substrate 1) to minimize the effect of deformation of elastomer substrates during measurements (Fig. 3i). The samples exhibited effective integration to a large extent that elastomer substrate 2 (Eco-flex, PDMS) fractured during testing (Fig. 3j, k). Consequently, d-HAPT facilitates robust adhesion among diverse materials and substrate combinations through a substrate bonding process, all accomplished within a short time. In conclusion, d-HAPT holds promise for integrating solid materials used in soft electronic devices regardless of material properties.

To demonstrate the remarkable performance of d-HAPT as an adhesive, a comparative analysis was conducted against the existing adhesives used in soft electronics (Fig. 3l and Supplementary Table 2)^33–36. d-HAPT is a thin adhesive that enables rapid and robust bonding (shear strength ≈ 200 kPa, T-peel strength ≈ 4 N/cm for soft materials) (Supplementary Fig. 9). A few polymer-based adhesives with comparable bonding strength and thickness were reported, but they required prolonged heat treatment for chain entanglement (2 days), or adhesive network stabilization (> 10 min) compared to d-HAPT. In the case of hydrogel-based adhesives, the polar groups within their network facilitate instant bonding between soft materials. However, d-HAPT exhibits stronger adhesion than the hydrogel-based adhesives, even at a thousandth of thickness.

WD bonding mechanism and durability of the adhesion

Hydrogel has been a promising candidate for advanced soft electronics by leveraging soft and tissue-like properties. Nonetheless, integrating hydrogels with diverse materials poses a significant challenge due to their innate water-rich composition and exceptionally low modulus^37,38. As the hydrogel is susceptible to heat applied during thermal bonding, a modified bonding process has been adopted for bonding between hydrogel and dry substrates (WD bonding). Figure 4a illustrates the WD bonding process and its underlying mechanism. To attach the hydrogels, they are simply placed onto the adhesive-coated substrates. Then, the d-HAPT absorbs the water at the interface with the hydrogel matrix, leading to the diffusion of its hydrogel polymer chains into the d-HAPT. These diffused hydrogel chains become entangled with the d-HAPT, ultimately resulting in the integration of hydrogel and substrates.

Figure 4b demonstrates the mechanical robustness of WD bonding. When the polyacrylamide (PAAm)-Alginate tough hydrogel is affixed to the d-HAPT-treated Eco-flex substrate, it exhibits remarkable stretchability. The bonded substrates endured strains over 600% of their original length without delamination. This stable adhesion between the hydrogel and Eco-flex remained intact even when the sample was fractured. In contrast, when the tough hydrogel was attached to an untreated Eco-flex substrate, the hydrogel was delaminated from the substrate not being stretched along with the elastomer (Fig. 4c). This outcome implies incomplete adhesion between the hydrogel and the elastomer substrate and demonstrates the crucial role of d-HAPT in establishing a robust hydrogel-elastomer interface. To evaluate the bonding strength of WD bonding, lap shear tests were conducted by sandwiching hydrogels between substrates with various combinations (Fig. 4d). Three types of hydrogels were selected: a tough hydrogel (PVA-TA)³⁹, a single-network hydrogel (PVA)³⁷, and a double network tough hydrogel (PAAm-Alginate)^37,40. These hydrogels adhere robustly on the d-HAPT coated substrates compared to the bare substrates (Fig. 4e). The differences in bonding strength between hydrogels were attributed to the inherent mechanical properties and degree of polymer chain diffusion. In the case of PVA-TA tough hydrogel, the PVA polymer chain easily diffuses into the d-HAPT leading to the strong adhesion strength. However, single-network PVA hydrogel shows comparatively low bonding strength due to its inherent low mechanical modulus and weak cohesion. For PAAm-Alginate hydrogel, long-chain and dissipative polymer networks are densely entangled, which hinders the active diffusion of the hydrogel chains into the d-HAPT. Nevertheless, the intact interface of PVA hydrogel and PAAm-Alginate hydrogel with the substrates during the tests evidenced their stable integration (Supplementary Fig. 10, 11). Remarkably, each hydrogel exhibited a similar bonding strength across all substrates (Fig. 4f and Supplementary Fig. 12). This uniformity could be attributed to the universal adhesion of d-HAPT on the substrate, which is derived from the abundant functional groups within TA molecules. These adhesion test results demonstrate that d-HAPT could offer a stable and universal WD bonding interface despite its relatively low bonding strength, showing intriguing potential for a dry hydrogel adhesion strategy. Existing adhesion strategies for hydrogels have employed hydrogel pre-gel solutions to induce adhesion to substrates^37,38. Although these approaches offer the advantage of a stronger interfacial toughness, they face practical limitations, requiring additional curing processes. Additionally, specific chemical modifications must be performed depending on the hydrogel. However, d-HAPT facilitates stable adhesion to soft substrates regardless of its mechanical and chemical properties owing to moisture-derived robust chain entanglement. This presents a significant advantage for practical application providing convenience in the process and versatile applicability.

To demonstrate the durability and applicability of the d-HAPT as an adhesive for hydrogel-based electronics, we fabricated a simple stretchable circuit system (Fig. 4g and Supplementary Movie 4) using conductive hydrogel. The hydrogel was 3D printed on d-HAPT-treated Eco-flex to serve as the interconnector. Then, LED chips coated with d-HAPT by brushing were adhered to the hydrogel interconnectors. The fabricated device exhibited stretchability, without any delamination of LED chips. Upon applying a voltage to both ends, the light of the LED was turned on and maintained even when the device was stretched to 150% of its initial length. The diminished light intensity was recovered as soon as the device was relaxed to its original length suggesting the stable adhesion of the soft and rigid components via d-HAPT.

Supplementary Table 3 demonstrates the comparison of various adhesion methods that attach various building blocks of soft electronics^{23,24,33–38,41,42}. While some studies exhibit the exceptional adhesive strength, none of them show multifunctional adhesive properties including soft/soft, rigid/rigid, soft/rigid, soft/hydrogel, and rigid/hydrogel and their electrical applications.

Wearable electronics applications

Figure 5a illustrates the wearable touch panel comprising three layers: an upper ionic hydrogel layer for touch sensing, a flexible flat cable (FFC) for signal transmission, and a lower PDMS layer serving as an insulator⁶. The conformal assembly of the layers was secured using d-HAPT, which substantially contributed to sustained sensing performance and reliable signal transmission. For example, maintaining stable interconnection between the ionic hydrogel and the FFC is challenging during dynamic body movement due to their relatively minor contact area and mechanical mismatch. However, d-HAPT exhibited strong adhesion at the interface of the FFC cable and touch panel enduring applied strains (Supplementary Fig. 13 and Supplementary Movie 5). The transparent ionic hydrogel layer was colored purple for visualization. As the FFC cable adhered to the hydrogel was pulled, the hydrogel was also stretched showing stable interfaces. Upon further pulling, a fracture was initiated on the hydrogel without debonding of the FFC cable, as seen in the inset. This durable adhesion enables the stable operation of wearable touch panels even in dynamic conditions. The operational principle of this wearable touch panel is based on a surface capacitive system, as elucidated in Fig. 5b. A uniform electrostatic field is established across the touch panel by applying an identical alternating current voltage to its four corners. When a finger contacts the touch panel, the touch point becomes grounded, allowing current to flow from the corner electrodes toward the touch point. To investigate the current at the four corners depending on the touch position, we evaluated the current variations as contacting with Point 1 to Point 4 on the panel surface sequentially. Visible difference in current was observed upon the contact of a finger (Fig. 5c). Notably, it was confirmed that the current was proportional to touch point proximity to the corner electrodes. Based on this outcome, the formula was deduced to investigate the correlation between the current measured at the corners and the specific touch point (Supplementary Fig. 14). A controller board was employed to convert the current data to the position on the touch panel. This wearable touch panel was attached to the forearm seamlessly and interfaced with a computer system through the controller board (Supplementary Fig. 15, 16). To test the reflection of the touch on the monitor, we wrote the word ‘BLISS’ on the touch panel (Fig. 5d and Supplementary Movie 6). As a result, high-resolution letters were successfully acquired, despite minor distortions along the edges. Additionally, a video game, avoiding obstacles by jumping while running, was performed using this touch panel. As shown in Fig. 5e, tactile interaction with the touch panel induced the character jumps (Supplementary Movie 7). These results suggest that the hydrogel touch panel integrated through d-HAPT successfully operated on the forearm.

The hydrogel strain sensors are promising bioelectronic applications exhibiting the capability of monitoring various human movements. However, their practical utility has been hampered by unstable connections between integrating components, such as a substrate, a cable, and a sensing layer. The difficulty causes problems in maintaining its shapes and securing consistent data acquisition during vigorous physical motion. In response to these issues, we have introduced a ring-shaped wearable strain sensor designed to mitigate the risk of detachment from the body (Fig. 5f). The foundational design comprises a stretchable Eco-flex substrate fashioned into a circular configuration. On this substrate, a carbon nanotube (CNT) hydrogel⁴³ responsible for sensing, and a conductive thread for transmitting data were affixed by utilizing d-HAPT. The fabricated strain sensors exhibited excellent softness due to the intrinsic stretchability of conductive hydrogel and Eco-flex. As a result, this ring-shaped wearable strain sensor offers stretchability, enabling repeated use while ensuring the user’s comfort. To comprehensively assess the versatility and robustness of this strain sensor, sensors were applied to distinct joints, such as a finger, a wrist, and an elbow (Fig. 5g-i). Notably, even during the bending of these joints, the measurement of the electrical resistance was seamless without any disruption to the hydrogel-substrate interface. Although the resistance modulation was inherently influenced by the range of motion within the joints, a resistance alteration rate across all cases exceeded 20%. Also, the gauge factor (GF) of this sensor was 1.22, surpassing that of the existing strain sensors. Leveraging the exceptional sensitivity of the sensor, precise angle-dependent sensing capabilities were demonstrated on the finger (Fig. 5j). Additionally, extensive stretching cycles of the strain sensor were systematically conducted to evaluate the durability of the sensor. Throughout 300 stretching cycles, the strain sensor consistently exhibited stable and reliable sensing characteristics, attesting to its robustness and durability (Fig. 5k). Consequently, d-HAPT enabled the stable operation of wearable soft electronics by providing robust adhesion between each building block. These devices were capable of sensing signals accurately even in bending and stretching situations that occur during the use of soft electronics.

Biocompatibility of d-HAPT

Soft electronics have gained prominence as a solution to mechanical challenges in implantable devices such as neural probes⁴⁴, artificial vascular sensors⁴⁵, and electronic sutures⁴⁶. For implantable devices, the materials should be non-toxic. To evaluate the biocompatibility of d-HAPT for implantable devices, a series of in vitro cell viability and morphological analyses were conducted. As shown in Fig. 6a, the d-HAPT-coated on the flexible PI film was incubated with NIH3T3 fibroblasts in cell culture media using a transwell system. The cell biocompatibility test using transwell insert investigates the effect of physiological environment change on the cells by allowing cells to be in indirect contact with the tested materials. The results of the live/dead assay and the CCK-8 assay showed no significant differences in cell viability after incubation for 1, 3, and 5 days (Fig. 6b and Supplementary Fig. 17, 18). Moreover, analyzation of cell morphological changes can serve as another biomarker to evaluate the material’s biocompatibility (Fig. 6c and Supplementary Fig. 19). When foreign materials are implanted, activated inflammation can cause fibroblasts to undergo morphological changes, transitioning from a healthy spindle shape to a pathological circular shape as a result of cytoskeletal remodeling. The morphology of the fibroblast was evaluated using cell aspect ratio after TRITC phalloidin staining. The results showed no significant differences in bare PI and d-HAPT-coated PI. Also, d-HAPT-bonded PDMS devices were implanted into the subcutaneous region of mice to investigate in vivo biocompatibility (Fig. 6d). To mimic actual application conditions and prevent the potential of enzymatic degradation of PVA and TA, the devices were encapsulated. After 7 days of implantation, the devices were retrieved and remained intact without any detachments, suggesting the durability of d-HAPT in the physiological environment (Fig. 6e). Histological staining with Hematoxylin & eosin (H&E), toluidine blue (TB), and Masson's trichrome (MT) staining indicated that the implanted device did not trigger significant inflammatory responses or necrosis of surrounding tissues (Fig. 6f). Furthermore, there was no observed increase in mast cells, as highlighted by organ arrows, or noticeable increase in collagen deposition. To evaluate any systemic response triggered by the implantation, serum levels of Aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) were measured before and after 3, 7, and 10 days of implantation (Fig. 6g). There were no significant alterations in AST and ALT serum levels between the Sham and implanted groups across all time points, indicating that implantation did not induce hepatotoxic effects.

Implantable bioelectronics applications

Given that the adhesive showed biocompatibility in vitro and in vivo experiments, we fabricated the implantable bioelectronic devices to evaluate the feasibility of d-HAPT as an adhesive for implantable applications (Fig. 6h). To fabricate the device, a PVA-liquid metal composite was printed on d-HAPT coated PDMS substrate as stretchable hydrogel electrodes. Then a FFC was integrated through the DD bonding process, similar to the structure of soft electronics illustrated in Fig. 1a. The device was firmly integrated despite the application of external forces, such as bending or stretching deformation (Fig. 6i). Using this device, we first performed in vivo neuromodulation on the rat sciatic nerve (Fig. 6j). As the stimulation was applied, leg of the rat responded showing different moving angle upon increase in the applied current (Fig. 6k). The electrocardiogram signal was also recorded using the identical device (Fig. 6l). P wave, QRS complex, and T wave, which play a vital role in diagnosing various cardiac disorders, were distinctly confirmed through the analyzed signal (Fig. 6m). In both sciatic nerve stimulation and electrocardiogram recording, the d-HAPT stably integrated the soft electronics during the intense movement of the leg and heart owing to the excellent adhesiveness. These overall results suggest the potential of d-HAPT for diverse implantable electronics.

The chemical and mechanical mismatches between materials used in soft electronics have hindered the instant and stable bonding of these materials using conventional adhesives. In this study, we have introduced a simple yet effective dry hydrogel adhesive with biocompatibility and softness that could bring a paradigm shift in adhesion strategy for soft electronics. As a promising strategy for instant adhesion to diverse soft electronic materials, d-HAPT robustly interfaces them regardless of the types and shapes of the materials either dry or wet ones. The sub-micron d-HAPT exhibits excellent uniformity, transparency, and mechanical stability. Its water-swellable property enables strong bonding of diverse materials through the adhesive’s polymer chain entanglement, upon absorbing moisture. Using d-HAPT, the hydrogel touch panel and strain sensors were fabricated, which demonstrated the applicability of wearable soft electronics. d-HAPT also showed excellent biocompatibility in both in vitro and in vivo experiments. Furthermore, the neuromodulation on the sciatic nerve, and recording of the electrocardiogram signal were successfully performed with implantable soft electronic devices integrated with d-HAPT. Overall, the excellent mechanical stability, softness, and biocompatibility of d-HAPT present a promising approach for utilizing hydrogel adhesives in a wide range of applications, from wearable to implantable biomedical engineering.

Materials

The chemicals were purchased and used without further purification. Poly(vinyl alcohol) (PVA, M_w 89,000–98,000), tannic acid (TA), acrylamide (AAm), acrylic acid (AA), N, N’-methylenebis(acrylamide) (MBAA), ammonium persulfate (APS), tetramethyl-ethylenediamine (TEMED), ammonium persulfate (APS), sodium alginate were purchased at Sigma Aldrich. A carbon nanotube (CNT) was purchased at Nanolab, Korea. Lithium chloride (LiCl) was purchased at Duksan, Korea. Gallium and Indium were purchased at AliExpress, China.

Preparation of d-HAPT

PVA (2.5% w/v) was dissolved in deionized (DI) water at 140°C by stirring at 300 rpm for 2 hours. TA solution was prepared by dissolving TA (2.5% w/v) in DI water using a vortex mixer. Substrates were coated layer by layer using the PVA and TA solution with a dip coater with a speed of 130 mm/min. The first and the last layer were designed to be the TA layer. Before depositing each layer, the samples were dried at 45°C for 10 min. To fabricating d-HAPT into a free-standing film, T₄P₃ layers were dip-coated with 7.5% w/v of TA and PVA solution at a speed of 130 mm/min.

Preparation of Hydrogels

The hydrogels were fabricated as described in the previous research. To synthesize PVA hydrogel, PVA (4,000 mg) was dissolved in DI water (20 mL) at 140°C while stirring for 20 min. The PVA solution was poured into the mold. The PVA solution was freeze-thawed at temperatures between − 20 and 25°C to yield the PVA hydrogel. To synthesize PVA-TA hydrogel³⁹, TA (4,000 mg) was dissolved in the PVA solution at 140°C. After 2 hours of stirring, the viscous PVA-TA solution was fabricated. The PVA/TA solution was spread on the mold to make its thickness 2 mm. Then, the PVA-TA solution was frozen and subsequently thawed to yield the PVA-TA hydrogel. The carbon nanotube (CNT) conductive hydrogel was fabricated following our previous research⁴³. To synthesize PAAm-LiCl hydrogel, a precursor solution was obtained by dissolving AAm monomer (146.35 mg/ml), MBAA (0.07% of AAm), APS (0.1% of AAm), 0.4 µL/mL of TEMED, and LiCl in DI water. The solution was poured into the mold with a thickness of 3mm. Then, the PAAm-LiCl hydrogel was cured for 4 hours at room temperature⁴⁷. To fabricate PAAm-Alginate hydrogel, alginate (29 mg/ml) was dissolved in DI water by stirring for an hour at room temperature. An AAm precursor solution (AAm monomer (666 mg/ml), MBAA (0.06% of AAm), APS (0.75% of AAm), and TEMED (2 µl/ml)) was added to the alginate solution. The PAAm-Alginate solution was poured into the mold with a thickness of 3mm. After 4 hours of the curing process, the PAAm-Alginate hydrogel was fabricated⁴⁸.

d-HAPT Characterization

The chemical structures were analyzed through ATR-FTIR (Vertex 70, Bruker, USA) with a range of 2800 to 3800 cm^-1. The thickness was analyzed using a surface profiler (DektakXT stylus Profiler, Bruker), and the surface morphology was acquired through AFM (NX-10, Park Systems), FE-SEM (IT-500HR, JEOL), and laser confocal microscope (VK-X3050, Keyenece). The mechanical stability was tested using a cross-cut test (ISO 2409). Cross-cut adhesion test (ISO 2409) was performed to test the durability of the coating. 100 blocks of size 10×10 mm² were formed on the d-HAPT coated glass and TPU surface by scratching the surface using a crosscutter. Then, the adhesive tape was attached to the surface and removed from it. This process was repeated 10 times, and the integrity of the coating was assessed. The transmittance was measured using a UV–Vis spectrophotometer (V-650, JASCO) with a wavelength range of 300 to 900 nm.

Bonding process

For DD bonding, the interfacial water is formed on d-HAPT-coated surfaces by dipping, brushing, or spraying. Then, the substrates were overlapped for a minute to induce moist absorption, thereby the polymer chain entanglement. The overlapped samples were heated for 1 min to evaporate residual interfacial water, leading to chain fixation. The heating temperature varied with the substrates from 100 to 130°C. For WD bonding, the hydrogel was placed on the coated surface of the substrate for 1 min.

Lap shear tests

The bonding strength was measured through lap shear tests using a tensile testing machine (Multitest-dV, Mecmesin). The samples were prepared with a bonding area of 15×20 mm². The tests were performed with a constant peeling speed of 5 mm/min. The bonding strength was detected with a 2500 N load cell.

T-peel tests

Peel strength was measured using the combination of two stretchable substrates of PDMS and Eco-flex. Two substrates with a size of 10×40 mm² were bonded with an adhered area of 10×10 mm² using d-HAPT. Then, the peel strength of the samples was tested using a tension tester (Multitest-dV, Mecmesin), with a strain rate of 50 mm/min.

Stretchable hydrogel circuit

The d-HAPT was formed on the Eco-flex and the electrodes of the LED chips (Adafruit LED, Adafruit). CNT·TA·PVA·PAA hydrogel was printed on the substrate using a 3D printer (EZROBO-5GX 3D printer, Iwashita) with a dispenser (AD3300C dispenser, Iwashita). Then, the coated LED chips were placed on the printed circuit. DC power supply (DP832, Rigol) was used to turn on LED chips.

Wearable touch panel

The wearable touch panel was fabricated by placing the PAAm/LiCl hydrogel with a thickness of 3mm on the d-HAPT-coated PDMS. Four vertexes of the ionic hydrogel were attached to the d-HAPT-coated FFCs (flat flexible cables) for connecting the touch panel to the devices that analyzed current changes caused by contact. To measure the degree of changes in current from touch, an AC voltage of 100 kHz from − 1 to 1V was applied to the vertexes using a function generator (AFG1022, Tektronix). Then the current was measured through the multimeter (34461A, Keysight) at each point. To confirm the operation of the wearable touch panel, FFCs attached to the touch panel were connected to a controller board (EXII-7720SC, 3M Touch Systems). Software MT7(3M, MicroTouch) was used as a calibration tool for the touch panel.

Strain sensor

d-HAPT-coated Eco-flex was used as a substrate, which is windable to body parts such as a finger, a wrist, and an elbow. CNT·TA·PVA·PAA hydrogel was placed on the substrate to measure the changes in resistance by bending the joint. For the measurement, the hydrogel was connected to the multimeter using the coated conductive thread (conductive stainless thread, DFRobot).

In vitro biocompatibility test

The 5×5 mm² bare and d-HAPT-coated PI film was prepared. Afterward, a two-chamber transwell system (8 µm pore size; Corning Inc.) was introduced for cell viability and analysis of morphological change. The prepared PI films were placed on the insert and incubated with cells thorough the experimental time. The NIH-3T3 fibroblast cells (0.5×10⁵ cells/mL) were cultured in 6 well cell culture plates with 1.5 mL of Dulbecco′s Modified Eagle′s Medium - high glucose (DMEM) supplemented with 10% bovine calf serum and 1% penicillin-streptomycin. Cell viability was evaluated using a Live/Dead kit (L3224, Invitrogen, USA) according to the manufacturer's instructions. The intensity of fluorescence cell images was measured using an inverted fluorescence microscope (IX81, Olympus, Japan) with a 10× magnification. Similarly, morphological change was measured following the aspect ratio calculation method which is the major cell axis divided by the minor one. For visualization, the cytoskeleton of NIH 3T3 cells was stained using an Alexa 594-conjugated phalloidin (Thermo Scientific, Pittsburgh, PA, USA). The cell morphology was evaluated using a confocal microscope (LSM 980, Carl Zeiss, Oberkochen, Germany) and calculated by the ImageJ/FIJI software program.

In vivo biocompatibility test

d-HAPT-bonded PDMS (10×10×1 mm³) was subcutaneously implanted into 6-week-old male CD-1 (ICR) mice for 3, 7, and 10 days. The animal experimental protocols were approved by the Institutional Animal Care and Use Committee of Yonsei University College of Medicine (IACUC No. 2023-0097), and all experiments adhered to the guidelines set by an Association for the Assessment and Accreditation of Laboratory Animal Care (AAALAC)-accredited facility. Within this facility, animals were housed under 12-hour alternating light/dark cycles and had unrestricted access to food and water. Before the mice were anesthetized using ketamine (100 mg/kg) and Rompum (10 mg/kg). Mice were procured from Orient Bio Incorporation, Seongnam, Korea. To assess any hepatotoxicity resulting from d-HAPT, blood samples were drawn through retro-orbital bleeding both before and after 3, 7, and 10 days of implantation (n = 4 for both the Sham and implanted groups). Blood was collected in heparinized capillary tubes (Micro-Hematocrit Capillary Tube Plain, Kimble Chase, Vineland, NJ, USA) and left to clot in a serum separation tube (BD Microtainer, BD, Franklin Lakes, NJ, USA) at room temperature for 2 hours. Subsequently, the samples underwent centrifugation at 300 g for 15 min, followed by 10 min at 600 g twice, after which the supernatant serum was extracted and stored at -80°C. Levels of AST and ALT in the serum were determined using a Dri-Chem 4000i biochemical analyzer (Fujifilm, Tokyo, Japan). For histological evaluations, mice were euthanized 3- and 7 days post-implantation, after which the implants and surrounding skin tissues were extracted and preserved in 10% formalin (Sigma-Aldrich, St. Louis, USA). The preserved tissues were then processed (ASP300S, Leica Biosystems, Nussloch, Germany), and embedded in paraffin blocks (Histo Core Arcadia, Leica Biosystems. Samples were then sectioned to a thickness of 4 µm and mounted onto slides (RM2255, Leica Biosystems) in preparation for histological staining with Hematoxylin and Eosin (H&E), Toluidine blue (TB), and Masson's Trichrome (MT).

Implantable bioelectronic device applications

All surgical procedures for mice were performed in accordance with protocols approved by the Institutional Animal Care and Use Committee (IACUC) at Korea Advanced Institute of Science and Technology (KAIST). 12-week-old female Sprague-Dawley rats (Koatech, South Korea) were anesthetized with isoflurane (5% induction, 2% maintenance). A heating pad was used to maintain the body temperature of the rat. To fabricate the implantable device, the composite of 3g of liquid metal (75% gallium and 25% indium) and 2 ml of PVA solution (5 wt%) is prepared as electrodes. 1 mm × 7 mm size of two electrodes were spray-printed on a 6 mm × 10 mm d-HAPT-coated PDMS substrate using a mask placed 2 mm apart. Subsequently, the d-HAPT-coated FFC was connected to the electrodes through DD bonding.

Sciatic nerve stimulation

Incision along the left thigh was made to expose the sciatic nerve. The electrodes are gently placed beneath the sciatic nerve in order to contact the epineurium. The biphasic current was administered at a frequency of 10 Hz, with a pulse duration of 25ms and current ranging from 0.03 to 0.09 mA, using an isolated pulse stimulator (2100, A-M systems). The leg movement was quantified using a protractor positioned beneath the leg.

Electrocardiogram recording

An acute tracheotomy was conducted, and the rat was intubated using a ventilator (VentElite Small Animal Ventilator, Harvard Apparatus) at a rate of 80 breaths per minute. Following a thoracic cavity incision, the ribs were carefully dissected to expose the heart. Subsequently, working and reference electrodes were positioned on the left ventricle. The electrocardiogram signals were recorded with an electrophysiology system (Lab Rat, Tucker-Davis Technologies). The recorded signals underwent bandpass filtering within the range of 0.3 Hz to 50 Hz.

Informed consent

The research participants in the images in Fig. 5 agree to the publication of these photographs. It was confirmed that a study using wearable devices simply touching the skin doesn’t need institutional review board approval.

Statistical analysis

The data of all experiments were statistically analyzed with a minimum sample size of 3 using Graphpad Prism 8 software (Graphpad Software Inc., USA). In the statistical analysis between groups, the unpaired t-test was used. ns was considered not significant. Statistical analyses for the in vivo study were performed using SPSS software (Version 26.0, IBM, Armonk, NY, USA). The Wilcoxon signed-rank test was employed to compare pre- and post-implantation data. A p-value of less than 0.05 was considered indicative of statistical significance.

Data availability

Data included in this manuscript is available and will be shared upon request.

Acknowledgements

This work was financially supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (Project numbers: NRF-2022R1A2C4001652).

Author information

These authors contributed equally: Yejin Jo and Yurim Lee

Authors and Affiliations

School of Electronic and Electronic Engineering, Yonsei University, Seoul 03722, Republic of Korea

Yejin Jo, Yurim Lee, Tae Young Kim, Kijun Park, Soye Kim & Jungmok Seo

Department of Physiology Yonsei University College of Medicine Seoul 03722, Republic of Korea

Jung Hyun Heo & Yoonhee Jin

Program of Brain and Cognitive Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Yeonzu Son

Department of Bio and Brain Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Seongjun Park

Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology (KAIST), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Seongjun Park

KAIST Institute for NanoCentury (KINC), 291 Daehak-ro, Yuseong-gu, Daejeon 34141, Republic of Korea

Seongjun Park

Contributions

Yejin J. and Y.L. contributed equally to this work. Yejin J. and Y.L. designed the experiment, analyzed the data, and wrote the manuscript. Yejin J. synthesized and characterized d-HAPT. Yejin J., Y.L., and S.K. performed mechanical tests. Yejin J., Y.L. conducted the experiment about the wearable electronics. Yejin J., J.H.H., and T.Y.K. performed the biocompatibility assay. Y.S. conducted the experiment regarding the implantable electronics. Yoonhee J., S.P., and J.S. were responsible for administrating this project. J.S. and K.P. revised and edited the manuscript. J.S. proposed the original concept and managed all aspects of this work. All authors discussed the results and commented on the manuscript.

Corresponding authors

Correspondence to Jungmok Seo

Ethics declarations

Competing interests

The authors declare no competing interests.

Rao, Z. et al. Soft electronics for the skin: from health monitors to human–machine interfaces. Advanced Materials Technologies 5, 2000233 (2020).
Byun, S.-H. et al. Mechanically transformative electronics, sensors, and implantable devices. Science advances 5, eaay0418 (2019).
Hong, Y. J., Jeong, H., Cho, K. W., Lu, N. & Kim, D. H. Wearable and implantable devices for cardiovascular healthcare: from monitoring to therapy based on flexible and stretchable electronics. Advanced Functional Materials 29, 1808247 (2019).
Gao, D., Lv, J. & Lee, P. S. Natural polymer in soft electronics: opportunities, challenges, and future prospects. Advanced Materials 34, 2105020 (2022).
Wang, T. et al. Nanoscale engineering of conducting polymers for emerging applications in soft electronics. Nano Research 14, 3112–3125 (2021).
Kim, C.-C., Lee, H.-H., Oh, K. H. & Sun, J.-Y. Highly stretchable, transparent ionic touch panel. Science 353, 682–687 (2016).
Yuk, H., Lu, B. & Zhao, X. Hydrogel bioelectronics. Chemical Society Reviews 48, 1642–1667 (2019).
Cong, Y. & Fu, J. Hydrogel–tissue interface interactions for implantable flexible bioelectronics. Langmuir 38, 11503–11513 (2022).
Liu, Y., Pharr, M. & Salvatore, G. A. Lab-on-skin: a review of flexible and stretchable electronics for wearable health monitoring. ACS nano 11, 9614–9635 (2017).
Yang, J., Steck, J., Bai, R. & Suo, Z. Topological adhesion II. Stretchable adhesion. Extreme Mechanics Letters 40, 100891 (2020).
Leggat, P. A., Smith, D. R. & Kedjarune, U. Surgical applications of cyanoacrylate adhesives: a review of toxicity. ANZ journal of surgery 77, 209–213 (2007).
Kim, M., Park, J. J., Cho, C. & Ko, S. H. Liquid Metal based Stretchable Room Temperature Soldering Sticker Patch for Stretchable Electronics Integration. Advanced Functional Materials, 2303286 (2023).
AshaRani, P., Low Kah Mun, G., Hande, M. P. & Valiyaveettil, S. Cytotoxicity and genotoxicity of silver nanoparticles in human cells. ACS nano 3, 279–290 (2009).
Shi, X. & Wu, P. A smart patch with on-demand detachable adhesion for bioelectronics. Small 17, 2101220 (2021).
Cintron-Cruz, J. A. et al. Rapid ultratough topological tissue adhesives. Advanced Materials 34, 2205567 (2022).
Liu, W. et al. A temperature responsive adhesive hydrogel for fabrication of flexible electronic sensors. npj Flexible Electronics 6, 68 (2022).
Zhang, Y., Dai, Y., Xia, F. & Zhang, X. Gelatin/polyacrylamide ionic conductive hydrogel with skin temperature-triggered adhesion for human motion sensing and body heat harvesting. Nano Energy 104, 107977 (2022).
Lee, D. et al. VATA: a poly (vinyl alcohol)-and tannic acid-based nontoxic underwater adhesive. ACS applied materials & interfaces 12, 20933–20941 (2020).
Xie, C., Wang, X., He, H., Ding, Y. & Lu, X. Mussel-inspired hydrogels for self‐adhesive bioelectronics. Advanced Functional Materials 30, 1909954 (2020).
Yuk, H. et al. Dry double-sided tape for adhesion of wet tissues and devices. Nature 575, 169–174 (2019).
Zhang, K. et al. Tough hydrogel bioadhesives for sutureless wound sealing, hemostasis and biointerfaces. Advanced Functional Materials 32, 2111465 (2022).
Peng, X. et al. Ultrafast self-gelling powder mediates robust wet adhesion to promote healing of gastrointestinal perforations. Science advances 7, eabe8739 (2021).
Deng, J. et al. Electrical bioadhesive interface for bioelectronics. Nature materials 20, 229–236 (2021).
Yang, Q. et al. Photocurable bioresorbable adhesives as functional interfaces between flexible bioelectronic devices and soft biological tissues. Nature materials 20, 1559–1570 (2021).
Qi, D., Zhang, K., Tian, G., Jiang, B. & Huang, Y. Stretchable electronics based on PDMS substrates. Advanced Materials 33, 2003155 (2021).
Sathishkumar, G. et al. Recent progress in tannic acid-driven antibacterial/antifouling surface coating strategies. Journal of Materials Chemistry B 10, 2296–2315 (2022).
Zhang, T. et al. All-organic multilayer coatings for advanced poly (lactic acid) films with high oxygen barrier and excellent antifogging properties. ACS applied polymer materials 1, 3470–3476 (2019).
Zhu, H. et al. Biobased plasticizers from tartaric acid: synthesis and effect of alkyl chain length on the properties of poly (vinyl chloride). ACS omega 6, 13161–13169 (2021).
Waly, A., Abdelghany, A. & Tarabiah, A. Study the structure of selenium modified polyethylene oxide/polyvinyl alcohol (PEO/PVA) polymer blend. Journal of Materials Research and Technology 14, 2962–2969 (2021).
Song, P. et al. Granular nanostructure: a facile biomimetic strategy for the design of supertough polymeric materials with high ductility and strength. Advanced Materials 29, 1704661 (2017).
Song, P. a., Xu, Z. & Guo, Q. Bioinspired strategy to reinforce PVA with improved toughness and thermal properties via hydrogen-bond self-assembly. ACS macro letters 2, 1100–1104 (2013).
Mao, X., Yuk, H. & Zhao, X. Hydration and swelling of dry polymers for wet adhesion. Journal of the Mechanics and Physics of Solids 137, 103863 (2020).
Gao, G. et al. Bioinspired self-healing human–machine interactive touch pad with pressure‐sensitive adhesiveness on targeted substrates. Advanced Materials 32, 2004290 (2020).
Jeong, K. et al. A Sub-Micron-Thick stretchable adhesive layer for the lamination of arbitrary elastomeric substrates with enhanced adhesion stability. Chemical Engineering Journal 429, 132250 (2022).
Dai, X. et al. Soft Robotic-Adapted Multimodal Sensors Derived from Entirely Intrinsic Self‐Healing and Stretchable Cross‐Linked Networks. Advanced Functional Materials 33, 2304415 (2023).
Zhang, Q. et al. Exploring a naturally tailored small molecule for stretchable, self-healing, and adhesive supramolecular polymers. Science advances 4, eaat8192 (2018).
Yuk, H., Zhang, T., Lin, S., Parada, G. A. & Zhao, X. Tough bonding of hydrogels to diverse non-porous surfaces. Nature materials 15, 190–196 (2016).
Yuk, H., Zhang, T., Parada, G. A., Liu, X. & Zhao, X. Skin-inspired hydrogel–elastomer hybrids with robust interfaces and functional microstructures. Nature communications 7, 12028 (2016).
Kim, S. A. et al. 3D printing of mechanically tough and self-healing hydrogels with carbon nanotube fillers. International Journal of Bioprinting 9 (2023).
Chen, Q., Chen, H., Zhu, L. & Zheng, J. Fundamentals of double network hydrogels. Journal of Materials Chemistry B 3, 3654–3676 (2015).
Wirthl, D. et al. Instant tough bonding of hydrogels for soft machines and electronics. Science advances 3, e1700053 (2017).
Cho, H. et al. Intrinsically reversible superglues via shape adaptation inspired by snail epiphragm. Proceedings of the National Academy of Sciences 116, 13774–13779 (2019).
Park, J. et al. Intrinsically Nonswellable Multifunctional Hydrogel with Dynamic Nanoconfinement Networks for Robust Tissue-Adaptable Bioelectronics. Advanced Science 10, 2207237 (2023).
Lee, Y. et al. A lubricated nonimmunogenic neural probe for acute insertion trauma minimization and long-term signal recording. Advanced Science 8, 2100231 (2021).
Park, K. et al. Resealable Antithrombotic Artificial Vascular Graft Integrated with a Self-Healing Blood Flow Sensor. ACS nano 17, 7296–7310 (2023).
Lee, Y. et al. A multifunctional electronic suture for continuous strain monitoring and on-demand drug release. Nanoscale 13, 18112–18124 (2021).
Yang, C. H., Zhou, S., Shian, S., Clarke, D. R. & Suo, Z. Organic liquid-crystal devices based on ionic conductors. Materials Horizons 4, 1102–1109 (2017).
Ohm, Y. et al. An electrically conductive silver–polyacrylamide–alginate hydrogel composite for soft electronics. Nature electronics 4, 185–192 (2021).

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Universal Hydrogel Adhesives with Robust Chain Entanglement for Bridging Soft Electronic Materials

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Journal Publication

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Abstract

Figures

Introduction

Results

Integration of soft electronic devices facilitated by d-HAPT

Fabrication and characterization of d-HAPT

DD bonding mechanism and durability of the adhesion

WD bonding mechanism and durability of the adhesion

Wearable electronics applications

Biocompatibility of d-HAPT

Implantable bioelectronics applications

Discussion

Methods

Materials

Preparation of d-HAPT

Preparation of Hydrogels

d-HAPT Characterization

Bonding process

Lap shear tests

T-peel tests

Stretchable hydrogel circuit

Wearable touch panel

Strain sensor

In vitro biocompatibility test

In vivo biocompatibility test

Implantable bioelectronic device applications

Sciatic nerve stimulation

Electrocardiogram recording

Declarations

References

Additional Declarations

Supplementary Files

Status:

Journal Publication

Version 1