Bio‐Inspired Adhesive with Reset‐On Demand, Reuse‐Many (RORM) Modes

Development of tough, reusable adhesives is important, but remains a major challenge, especially in water. A tough reusable adhesive that resets entirely to its virgin condition when needed is reported using caffeic acid. Here, caffeic acid is employed as adhesive moiety to achieve such the functions due to its dual characteristics: an adhesive moiety from mussel‐inspired catechol and a photo‐reversible crosslink from cinnamic acid. Adhesion involves a two‐step process. First, the caffeic acid‐functionalized polymer is applied to the adherend, followed by UV irradiation (peak wavelength of light‐emitting diode, λP: 365 nm) to form a durable pre‐applied adhesive (PAA) layer through crosslinking among the caffeic acid moieties. Second, thermal activation of the PAA layer ensures repeated adhesion to a variety of adherends (Reuse‐Many mode). The cyclic dimer of the caffeic acid moiety is de‐crosslinked by UV irradiation at λP: 254 nm. This allows the complete removal of the adhesive residues from the adherends when the adhesive is no longer needed (Reset‐On demand mode). Furthermore, using magnetic nanoparticles, the caffeic acid‐functionalized polymer can be activated remotely under water by magnetic induction heating. This study paves the way for the rational design of bio‐inspired adhesives that outperform nature using plant‐derived raw materials.


Introduction
During the British industrial revolution of the 19th century, the invention of postal stamps that comprised an adhesive applied to paper to form a non-sticky layer that could be activated on request as often as necessary made a significant contribution to the development of the postal system. The invention facilitated the distribution of goods and communication between distant peoples. Since then, various types of preapplied adhesive (PAA) have been developed that are repeatedly activated by external stimuli, such as electricity, heat, and light. However, the concept of a non-sticky PAA that transforms into an adhesive layer when incited by an external stimulus remains important in various fields, such as medicine, semiconductors, and structural materials. [1,2,3,4] PAAs could contribute to a circular economy by providing the means of separating and recycling each component of a structural material. However, as a structural material, a PAA has inherent limitations in terms of practical applications. That is, it cannot simultaneously satisfy the requirements of both toughness and reusability. These limitations are exacerbated under wet conditions because the interactions between the adhesive and the adherend are easily disrupted by water. [5] Moreover, to achieve a closed circular economy, the substrate should be returned to its virgin state by completely removing the adhesive when it is no longer needed, and the adhesive should also be recycled.
Marine mussels have perfected the art of adhering tenaciously to various surfaces, even under wet conditions, by secreting adhesive foot proteins that include the peculiar but abundant catechol group l-3,4-dihydroxyphenylalanine (l-DOPA). [6] l-DOPA is thought to be responsible for two functions: adhesion and cohesion. With regard to adhesion, which is the force of attraction between the adhesive and the adherend, the catechol unit in l-DOPA serves as a versatile adhesion moiety through intermolecular interactions such as hydrogen bonding, [7] metal coordination, [8] and even hydrophobic interactions. [9] Utilizing these unique adhesive properties, a variety of adhesives and coatings containing catechol-functionalized polymers have been developed for a wide range of applications. [10] More recently, on-demand activation/deactivation of the adhesiveness of the Figure 1. Design of the tough, mussel-mimetic, photo-induced, sustainable adhesive. A strategy combining mussel-glue chemistry and reversible photoreactive chemistry is proposed for designing an adhesive material, poly1. (M1: (E)-2-(acryloyloxy)ethyl-3-(3,4-dihydroxyphenyl) acrylate and EHMA = 2ethylhexylmethacrylate). A) The adhesive contains a catechol moiety that mimics mussel l-DOPA and a light-sensitive moiety that is present in cinnamates extracted from plant materials such as coffee beans. B) Preparation of a light-induced pre-applied adhesive layer (LPAA) and its closed-loop sustainability. As adhesives in viscous liquid form or solvent-dissolved form are coated onto the substrate (i), polymer chains in the interfaces anchor the substrates through strong H-bonds as well as complex interactions of the catechol with substrates to produce adhesive domains, while internal interactions among the polymer chains form non-covalent cohesion domains (e.g., H-bonds and entanglement). Long wavelength UV (UV L ) induces crosslinking of the cinnamoyl group in the polymer chain to create a covalent network that enhances cohesion and forms a stable, non-sticky LPAA layer (ii). Following thermal activation, the adhesive forms a tough bond with the substrate (iii). The adhesive can be used repeatedly owing to dynamic H-bonding and complex interactions (iv). If short wavelength UV (UV S ) is used, decrosslinking of the cyclobutene can reduce cohesion (v), which allows the adhesive to be easily removed by organic solvents, so that both the adhesive and the substrate can be recycled (vi).
catechol groups by external stimuli, such as pH, [11] electricity, [12] and enzymatic reactions, [13] has been proposed. Light is particularly attractive as a clean stimulus because it provides noncontact precise control of adhesive functionality. [14] The mechanism by which this strategy works is as follows. First, the catechol unit is transformed into its caged form by protecting the hydroxyl groups with a photo-dissociative o-nitrophenyl moiety, [15] or is protected by the formation of a catechol-Fe 3+ complex [16] or a catechol-acetonide. [17] Subsequently, the catechol unit is deprotected in the presence of a photoacid generator. However, light activation of the catechol-functionalized adhesive layer is not suitable for a reworkable tough PAA because the protection/deprotection of the catechol groups requires tedious chemical preparations.
In view of the considerations mentioned above, we focused our attention on another feature of the l-DOPA adhesive moiety, i.e., cohesion, which is the force of attraction among adhesive polymers. l-DOPA is known to reinforce adhesive foot proteins by forming crosslinks among the polymer chains of the proteins in seawater through diverse chemical reactions such as Michael additions, [18] metal coordination, [19] and autoxidation. [20] The extraordinary toughness of the mussel's byssus (a secreted bundle of filaments that attaches the mollusk to a solid surface) is attributed to the cohesive strength of the foot proteins resulting from l-DOPA-induced crosslinking, as well as the adhesive strength of the phenolic groups of l-DOPA. [21] Therefore, we expect that if the cohesive strength among the catechol-functionalized polymers could be reversibly controllable by an external stimulus, it would be possible to develop a PAA system that combines toughness, reusability, and resettability.
Herein, we report caffeic acid, as a novel photoreactive musselinspired adhesive motif ( Figure 1A). It enabled us to develop a tough PAA that was effective even under water. It is important to note that caffeic acid has dual functionality: adhesiveness from the catechol moiety [22] and photo-reactivity from the cinnamoyl unit. [23] Caffeic acid can be classified as a cinnamic acid derivative in which the phenyl ring of the cinnamic acid is substituted by hydroxy groups at positions 3 and 4. Therefore, it should be possible to utilize it as an underwater adhesive moiety, just as l-DOPAs are exploited by marine mussels, because it can also be regarded as a catechol derivative. However, the cinnamoyl group undergoes [2+2] photo-cyclo-addition and de-cyclization by UV irradiation at P : 365 nm (UV L ) and 254 nm (UV S ), respectively. [24] Therefore, we thought it would be possible to reversibly control the crosslinking of a caffeic acid-functionalized adhesive, and consequently its cohesive strength, by controlling the wavelength of the UV radiation stimulus ( Figure 1A). However, unlike l-DOPA in the adhesive proteins of mussels, caffeic acid is not used as an adhesive moiety, either naturally or synthetically.
In the present study, we demonstrated a viable approach to the fabrication of a light-tunable pre-applied adhesive (LPAA) with "Reset On demand, Reuse Many" (RORM) modes by using caffeic acid as a photo-crosslinkable/decrosslinkable adhesive moiety. This approach enabled us to develop a sustainable closed-loop adhesive system ( Figure 1B). The obtained LPAA is applicable to a variety of substrates including polyethylene (PE), polypropylene (PP), and polytetrafluoroethylene (PTFE). For the first time, we demonstrated the remote application of the underwater adhesive used in conjunction with induction heating.

Preparation of a Caffeic Acid-Functionalized Adhesive Polymer
Previously, we reported the rational design of a mussel-inspired tough adhesive copolymers, which are obtained by varying the alkyl chain length/structural isomer of alkylmethacrylate as well as the composition ratio of alkylmethacrylate to Dopaminefunctionalized methacrylamide (Figure 2, M2). [25] Eventually, we selected a 2-ethylhexylmethacrylate (EHMA) copolymerizing with 8 mol% of M2 as the most suitable combination in terms of adhesive strength and ductility (Poly2) (Figure 2). We predicted that the unique hydrophobic segments resulting from the relatively long and branched alkyl chain of EHMA [26] would prevent oxidation of the catechol unit through hydrophobic interactions. [27] In the present study, we designed a caffeic acid-functionalized adhesive polymer (Poly1) based on Poly2, because caffeic acid can be regarded as an analog of catechol. Therefore, we prepared a series of caffeic acid-functionalized copolymers of various molecular weights (Poly1a-1h) by the free radical polymerization of (E)-2-((3-(3,4dihydroxyphenyl)acryloyl)oxy)ethyl acrylate (M1) and EHMA in various ratios ( Table 1). The resulting caffeic acid-functionalized Poly1s were characterized by proton nuclear magnetic resonance ( 1 H NMR) spectroscopy, Fourier-transform infrared (FT-IR) spectroscopy, and gel permeation chromatography (GPC). The detailed procedures are described in the supporting information, and the characterizations are summarized in Table 1 and Figures S2-S4 and S41-S52 (Supporting Information). To determine the respective contributions of the catechol unit and the photo-crosslinking of Poly1 on adhesion and cohesion, we prepared Poly2, Poly3, and Poly4 (Table 1). Poly2 was used as an analog of Poly1 without the photo-crosslinkable vinylene unit. However, Poly3 and Poly4 were utilized as analogs of Poly1 without the hydroxyl groups (M3) and with protected catechol hydroxyl groups (M4), respectively ( Figure 2) .

Light-Induced Pre-Applied Adhesive
In general, PAA requires a two-step preparation process: first, a stable, non-sticky pre-applied adhesive layer is prepared, and is subsequently activated by an external stimulus ( Figure 1B and Figure S1, Supporting Information). In the present study, the photo-crosslinking reaction of Poly1 was exploited in the PAA layer. First, we used UV-vis and 1 H NMR spectroscopic analyses to confirm whether the caffeic acid moiety underwent photo-reversible [2+2] cycloaddition when introduced into the side chain of Poly1. Figure 3A shows the time course of the UV absorption spectra of Poly1f with low weight-averaged molecular weight (M w = 7000) cast on a quartz substrate following UV irradiation with a light-emitting diode (LED) at P : 365 nm (UV L ). The UV-vis absorption band with a maximal wavelength ( max ) at 325 nm decreased linearly, suggesting that the photo-crosslinking reaction preferentially occurred in the caffeic acid moiety through [2+2] cycloaddition of the vinylene units in the side chains of Poly1f without unexpected side reactions ( Figure 3A′). [24] Similar change of UV-vis absorption can also be found in other polymers such as Poly1a and Poly1g ( Figure S19, Supporting Information). To further understand this photo-crosslinking mechanism, we investigated the kinetics of the crosslinking reaction of caffeic acid in Poly1. The linear relationship between ln(A 0 /A t ) and the UV L irradiation time revealed that the crosslinking reaction followed first order kinetics with regard to the concentration of caffeic acid ( Figure 3B). Herein, A 0 and A t indicate the intensity of UV absorption at 325 nm before UV L irradiation and after t min of UV L irradiation,  respectively. This photo-cycloaddition reaction could be supported by 1 H-NMR spectroscopy of homopolymer Poly 1i ( Figure 3C and Figure S20, Supporting Information). New signal peaks appeared in the chemical shift range 4.66-4.72 ppm upon UV L irradiation, indicating that a cyclobutane group had formed. However, unlike the homopolymer Poly1i, the formation of cyclobutane by crosslinking of cinnamyl groups in Poly1f could not be observed by 1 H NMR spectroscopy. This is because after UV L irradiation, Poly1f turned to insoluble in the common organic solvents. Similarly, 13 C NMR of Poly1f showed broad peaks only. Solubility of Poly1f before/after UV L irradiation, after thermal activation, and after UV S irradiation were listed in Table S1 (Supporting Information). Especially, changes in solubility of Poly1f in CDCl 3 was monitored with UV L irradiation time ( Figure S22, Supporting Information). In addition, we realized that the formation of the photo-crosslinks affected the thermal properties of Poly1 ( Figure S17, Supporting Information). Figure 3D shows changes in the glass transition temperature (T g ) and the number of unreacted vinylene units in Poly1f upon UV L irradiation. The obtained plots could be curve-fitted by a Gaussian curve fitting and an exponential decay function with good correlation coefficients (r) more than 0.99. Considering the good fit to these regression functions, it was suggested that the remaining vinylene units were exponentially decreased upon UV L irradiation, along with exponentially increase in T g values. Furthermore, Poly1f gradually changed from a viscous liquid ( Figure 3D′) to a nonsticky solid ( Figure 3D′′). These results suggest that Poly1 can be used as the photo-curable PAA, where adhesives in a viscous liquid form or dissolved in organic solvents when applied, transform into a hard yet non-sticky layer by UV L irradiation.

Design of a Tough and Universal LPAA on Demand
We carried out a single-lap shear test to demonstrate a reusable tough PAA comprising Poly1. A specimen was prepared as follows. First, Poly1 was applied to a substrate. It was then irradiated with UV L for various times to stimulate the [2+2] cycloaddition of the vinylene units in Poly1. The resulting non-sticky PAA layer was chemically stable, and its adhesive capability remained unchanged over 2 years of storage under ambient conditions ( Figure  S26, Supporting Information). The PAA layer was thermally activated to prepare an adhesive joint, directly placed on another piece of the substrate, and cured at 120°C for various times. As a control experiment, a specimen was prepared that was not irradiated with UV light. Unless otherwise noted, a degreased aluminum alloy sample (A6061P-T6: 1 mm thick, 25 mm wide, and 100 mm long), polished with a sandpaper of 800 SiC grit, was used as the substrate. First, we evaluated the adhesive capability of Poly1 with respect to its composition ratio of M1. Figure  4A is a plot of adhesive strength as a function of M1 (mol%) by a single-lap shear joint of Poly1. The weight-averaged molecular weight (M w = 7000) was almost identical among Poly1s with different composition ratio of M1. The adhesive strength of the specimens before UV L irradiation gradually increased as M1 increased and reached a maximum of 3.2 MPa at ≈9 mol% of M1 ( Figure 4A, blue solid line). However, when M1 further increased over 11.3 mol%, the adhesive strength gradually decreased, although the T g increased linearly ( Figure 4B, and Figure S16, Sup-porting Information). This trend agreed well with that found in the Poly2 samples with M2 as a side chain, [25] suggesting that there is an optimal balance between T g and adhesive capability. To our surprise, after UV L irradiation the adhesive strength of Poly1 increased significantly by ≈twofold between 0 and 20 mol% of M1. In particular, Poly1 with M1 (9.4 mol%) reached a maximum adhesive strength of 7.2 MPa ( Figure 4A, red solid line).
To further determine how the unique dual nature and reliability of our designed LPAA adhesive Poly1, we performed similar adhesive control experiments on Poly2, Poly3, and Poly4( Figure 4C). We specifically chose samples with similar low molecular weights (M w :7000-8000) to eliminate the effects of molecular weight-dependent physical and mechanical properties, such as the topological entanglement of polymer chains. Figure 4C shows the changes in the adhesive strength of the corresponding polymers before/after UV L irradiation. Before UV L irradiation, the single-lap shear test revealed very low adhesive strength in the range of several kPa in all the specimens, owing to the viscous liquid nature of low-molecular-weight Poly1. After UV L irradiation, the adhesive strength of Poly1f was over 6 MPa, i.e., ≈95 times that of its adhesive strength before UV L irradiation. The topological and cohesive entanglements, which lead to a significant increase in the adhesive strength, can be attributed to photo-crosslinking. Similarly, the adhesive strengths of Poly3 and Poly4, which contained photo-crosslinkable vinylene units, increased by 7 times after UV L irradiation that is much smaller than in poly1f ( Figure 4C), implying that catechol plays a critical role for adhesiveness. In contrast, the adhesive strength of Poly2 did not change after the same treatment, which is probably attributed to stabile against UV exposure ( Figure S15, Supporting Information), due to no photosensitive cinnamoyl functional group. These results clearly indicate that cohesion by physical and chemical entanglement is essential for tough adhesion, as well as adhesion by non-covalent bonding between the catechol groups and the topmost surface of the adherend, as demonstrated by the mussel's bioadhesive. [28] To further clarify the effect of the entanglement of poly1 on adhesive behavior before/after UV L irradiation, we prepared Poly1a-h, which had M w values in the range of ≈5000-64 000. According to Figure 4A, the Poly1a-h samples with M1 of ≈9 mol% had the greatest adhesive strength. The apparent features are summarized in Table 1 along with M w and polydispersity (Ð) values. As a result, Poly1 changed from a viscous liquid to a brittle solid as the M w of Poly1h increased to that of Poly1a (Table 1). Next, to optimize the UV L irradiation time, we investigated its relationship with adhesive strength. We used Poly1a and Poly1f as samples with high and low M w values, respectively. Figure 4D shows the strengths of Poly1a and Poly1f versus UV L irradiation time. The adhesive strength of Poly1f could be fitted to a sigmoidal regression model, suggesting that a cooperative behavior among polymer chains occurred by photo-crosslinking ( Figure 4D, red solid line). Thus, the red trend line gradually increased with an induction period of several minutes as the UV L irradiation time increased, and reached a plateau after 50 min. On the other hand, adhesive strength of Poly1a increased immediately after the commencement of UV L irradiation, and reached a maximum value of 7.2 MPa after 5 min. However, as the UV L irradiation time increased, the adhesive strength abruptly decreased, although the T g gradually increased after this inflection point according to a saturation-growth rate model ( Figure 4D′). More interestingly, the failure mode changed from cohesive to partial adhesive across the inflection point ( Figure S13, Supporting Information), which event can also be found in other substrates, such as Cu, Fe, and Al substrate ( Figure S14, Supporting Information). It is likely that the lightinduced cyclization reaction increased the cohesive force among the polymer chains as the crosslink density increased, and the adhesive force at the interface decreased as compensation for the increased cohesion. [29] To further confirm the cohesive effect from UV irradiation, the change in moduli (G′) of Poly1a before and after UV L irradiation was conducted by rheological study ( Figure  S18, Supporting Information). The storage moduli (G′) increased by ≈one order of magnitude after UV L irradiation. To further support the hypothesized mechanism behind increased adhesive strength after UV exposure, molecular dynamics simulations were performed. [5] A model representing a random copolymer of M1 and EMHA with 10 mol% M1 content was created and optimized, and its cohesive energy densities without any crosslinks and with 30% crosslinking density were calculated ( Figure S36, Supporting Information). The cohesive energy density was found to increase from 1.549 × 10 8 to 1.646 × 10 8 J m −3 , representing an increase of ≈6.0%. This suggested that the improved adhesion properties upon UV radiation exposure could be partially explained by an increase in cohesion strength due to crosslinking. Combined with the effect of UV on the T g, and MD simulation, it is reasonable to infer that UV irradiation intensifies cross-linking density, resulting in stronger chain entanglements, which favors an increase in cohesion to resist bond failure. [28b] To further verify the dependency of adhesiveness on M w , we determined the UV L irradiation time when the adhesive strength of the photocrosslinked Poly1 reached its maximum, as summarized along with the corresponding values of adhesive strength in Figure 4E, and Figures S5-S11 (Supporting Information). As a comparison, we also determined the adhesive strength of the non-crosslinked Poly1 before UV L irradiation. As a result, the adhesive strength of the non-crosslinked Poly1 increased in a sigmoidal manner, and subsequently reached a maximum value of ≈3 MPa as the M w increased ( Figure 4E, blue line). Such nonlinear mechanical property relationships of non-crosslinked polymers are often attributed to either topologically or cohesively entangled polymer www.advancedsciencenews.com www.afm-journal.de networks. [30] As an example of a biomimetic adhesive, the synergistic effects of the topological/cohesive entanglements among polymer chains, and the non-covalent interactions among catechol moieties have been reported in a catechol-functionalized styrene copolymer. [29] Considering that the appearance of Poly1 changed from that of a viscous liquid to that of a solid at a critical region above a M w of ≈10 000 (Table 1), the topological/cohesive entanglement of the polymer chains in these M w regions is likely to contribute to the nonlinear increase in adhesive strength, although the involvement of entanglement in the crystallization process has always been controversial. [31] However, once Poly1 was subjected to UV L irradiation and successive thermal activation, its adhesive strength increased dramatically to a maximum over 7 MPa ( Figure 4E, red solid line). In particular, when the low-molecular-weight Poly1f was used, its adhesive strength increased by approaching two orders of magnitude compared with that before UV L irradiation. More interestingly, when the UV L irradiation time was optimized to maximize the adhesive strength of the Poly1 with various M w , an unexpected phenomenon was revealed: at the optimum UV L irradiation time, the plot of the adhesive strength versus the M w apparently decayed exponentially ( Figure 4E, green solid line). Thus, the optimum UV L irradiation time for maximum adhesive strength was shortened from 120 min for low M w Poly1h to 5 min for high M w Poly1a, a reduction of ≈1/24. This suggests that the optimization of the adhesive strength of LPAA during the photo-crosslinking reaction should take into account the complementary effects of physical entanglement and photochemical crosslinking resulting from the molecular weight effect and the photocyclization reaction, respectively. Furthermore, we also found the molecular weight affected both the optimal UV L irradiation time and the toughness of the resulting adhesives. Figure 4F shows the changes in the stress-strain curves of Poly1a and Poly1f. Before UV L irradiation (0 min), Poly1f had a weak and ductile nature, according to the gentle slope of the stress-strain curve. After UV L irradiation, the curve steepened but smoothly arched at top, suggesting that the Poly1f had turned into a tough adhesive that was strong and ductile. [32] In contrast, there was a sudden decrease in the gradient of the Poly1a stress-strain curve, even before UV L irradiation, suggesting that Poly1a had a strong but brittle adhesive nature. [28] This trend remained unchanged after the UV L irradiation time was increased, although the adhesive strength reached a maximum at 5 min of UV L irradiation ( Figure S5, Supporting Information). This result was expected from the apparent behaviors of Poly1a and 1f before/after UV L irradiation. Thus, Poly1a with high molecular weight was a brittle solid both before and after UV L irradiation, whereas Poly1f with low molecular weight changed from a viscous to a waxy solid upon UV L irradiation ( Figure 3D′,D′′). This was further supported by a rheology experiments, because of relatively high storage modulus of Poly1a with ≈1 MPa even before UV L -crosslinking ( Figure S18, Supporting Information). Notably, strong bio-inspired adhesives are often brittle, and need additional sacrificial weak interactions, such as hydrogen bonding, to enhance their toughness and bonding performance. [32] In contrast, it was possible to maximize the toughness of the LPAA system described herein by simply controlling its molecular weight and UV L curing time.
Poly1 demonstrated its potential as a light-induced on-demand PAA for various materials (Figure 5A), such as metals with na-tive oxide surfaces (e.g., copper (Cu), stainless steel (Fe), and aluminum alloy (Al)), wood, graphite, and synthetic polymers (e.g., polyethylene (PE), polypropylene (PP), polymethylmethacrylate (PMMA), and polyvinyl chloride (PVC)). Poly1f exhibited relatively weak adhesiveness on these substrates, whereas its adhesive strength increased dramatically (by more than 20-fold) on metal, wood, and graphite substrates after UV L irradiation. Furthermore, its adhesive strength with regard to PMMA, PVC, and graphite increased by ≈10 times, and its adhesive strength with regard to PP and PE, which are inert hydrophobic substrates, increased by 27 times to 1.5 MPa and by 18 times to 2.3 MPa, respectively, compared with the corresponding values before UV L irradiation. Moreover, the performance of Poly1 was superior to that of commercial structural adhesives with regard to a PE substrate, in which hydrophobic interactions are dominant, and an Al substrate, in which electrostatic interactions are dominant ( Figure 5B). Another outstanding advantage of our LPAA is that it takes only a few minutes of thermal curing to reach maximum adhesive strength, whereas common commercial adhesives require a curing time of tens of hours to reach maximum strength. Figure 5C shows the correlation between the curing time and adhesive strength with regard to a PP substrate. Despite the poor adhesion of commercial adhesives to PP substrates, Poly1 achieved maximum adhesive strength as fast as cyanoacrylate, which is considered an instant glue. However, from a practical standpoint, cyanoacrylate-based adhesives have the disadvantage of not being able to withstand long-term storage because they react aggressively with the moisture in air, whereas poly1 has favorable storage and instant curing properties, with no deterioration in adhesive performance over 2 years ( Figure S26, Supporting Information). More importantly, it was possible to reversibly control the non-covalent interactions between catechol and the chemical species on the substrate surface by triggering them with light; this has not been achieved in either commercial or biomimetic adhesives. Finally, Figure 6 shows practical demonstrations of the unique adhesive capabilities of Poly1. To demonstrate the toughness and superior adhesion of Poly1f with regard to inert hydrophobic substrates, we carried out repeated bending tests on single lab joints comprising PTFE ( Figure 6Ai,ii, and Movie S1, Supporting Information). Although catecholbased biomimetic adhesives have been reported to exhibit some adhesion to PTFE, [10d,e] to the best of our knowledge, there have been no demonstrations of such toughness in bonded specimens of PTFE. Similarly, we used Poly1 to adhesively connect cracked silicone tubes and recover their mechanical properties and demonstrated no water leakage when subjected to great water pressure (a velocity of 8.37 m s −1 ) ( Figure 6B-i-iii) and Movie S2, Supporting Information). Furthermore, it was easy to reverse this tough adhesion on demand by irradiating the adhesive with light. Figure 6C shows a UV L -toughened bonded piece consisting of quartz and PP under a load of 40 kilogram-force (kgf) units and there was no detachment even after 72 h ( Figure 6C-ii, and Movie S3, Supporting Information). However, it was possible to dismantle the quartz and PP by simply irradiating the quartz surface with UV S for ≈30 min ( Figure 6C-iii). Tough adhesive joints that can be readily dismantled by the application of an external stimulus are important from the perspective of industrial applications including automobiles, smart robots, and semiconductors, wherein accurate temporary fixation is a requirement. In addition, photo-induced cyclization/decyclization can be used as a new energy conversion method from solar energy to chemical reaction. We conducted a sunlight exposure test of the pre-applied Poly1f on aluminum (Al) and polypropylene (PP) substrates as adherends. One set of pre-applied specimens was exposed directly to sunlight for 16 days ( Figure S32a, Supporting Information), while the other set of specimens was exposed outdoors in a glass container of ≈10 mm thickness ( Figure S32b, Supporting Information). As a result, although the maximum adhesion strength to the respective substrates (2.17 MPa for PP and 6.05 MPa for Al) as shown in Figure 5A was not reached, the adhesion strength increased with the exposure time to sunlight for both Al and PP substrates and showed sufficient adhesion strength for practical use of ≈0.8 MPa for Al and ≈0.6 MPa for PP ( Figure S32c,d, Supporting Information). In addition, direct exposure to sunlight tended to increase the bond strength more than through the glass. This could be because the amount of solar energy was in the range of 2.46-3.45 mW cm −2 when the sunlight passed through the glass plate, whereas it was higher in the range of 3.94-4.75 mW cm −2 when the substrates were directly exposed to sunlight. The findings from the sunlight exposure test are expected to lead to the novel molecular design for energy conversion from sunlight to chemical reaction, such as self-healing and post-curing.

Adhesive with Reset-On Demand, Reuse-Many (RORM) Modes
In modern society, the development of sustainable materials for a circular economy is an emergent objective. [33] Therefore, reusable adhesives that enable repeated bonding and debonding have been developed. However, dismantlable adhesives that require thermal activation are mainly used for one-time debonding applications. Light-induced reusable adhesives tend to have low adhesive strength and are restricted to use with sufficiently transparent quartz glass adherends. [34] Based on our findings with regard to the light-triggered control of adhesive strength, we propose a new concept for sustainable adhesives: Reset-On demand, Reuse-Many (RORM) modes. Figure 7 shows the workflow of a sustainable adhesive system with RORM modes. In this system, Poly1 was applied to the substrate ( Figure 7A), and irradiated with UV L to obtain the LPAA layer ( Figure 7B). The specimen was prepared by thermal activation and was subjected to a lap shear test. In Reuse-Many mode, the tested specimen recovered its original adhesive performance following thermal activation of the same pair of substrates that was repeated more than 30 times without significant adhesion change ( Figure 7C and Figure S34, Supporting Information). After lap-shear tests, the cohesive failure was dominantly observed, called mixed failure, as typically shown in iii) However, the dumbbell was released when the adhesive on the surface of the quartz was irradiated with UVs. i) An enlarged photograph shows that there were no cracks in the bonded substrates.
Figures S13 and S14 (Supporting Information). It is most likely that the bonding strength between adhesive and adherend are higher than their individual strength. Therefore, basically, the Reuse-Many mode is designed to use the same combination of substrates repeatedly in order to keep the amount of adhesive applied to the adhesive side constant. However, if a new substrate is used for reworking, the adhesive should be pre-applied on the fresh substrate side. Here, it is important to confirm the solubility of Poly1f at each process step to realize the RORM concept. Poly1f showed good solubility in solvents with dielectric constants in the range of about = 47-4.7, except alcohols (Table  S1 and Figure S21, Supporting Information). When Poly1f was irradiated with UV L , the initially soluble solvents became insoluble as the crosslinking reaction of Poly1f progressed. This tendency remained unchanged after thermal activation. However, when Poly1f was irradiated with UV S , it became soluble in the polar solvents in which it was originally soluble ( Figure 7D and Figure S25, Supporting Information). These results suggest that Poly1f underwent a reversible crosslinking-decrosslinking reaction upon UV L and UV S irradiation whereas the thermal activation did not affect to both crosslinking-decrosslinking reaction. These chemical stability of Poly1f against heating and light irradiation during RORM process was confirmed by 1 H NMR ( Figures  S23 and S24, Supporting Information) and thermogravimetric analysis (TGA) (Figures S34 and S35, Supporting Information). Comparing with 1 H NMR of Poly1f before UV L irradiation in Figure 3, the peak position and number of Poly1f after successive UV L and UV S irradiation did not change, and no other peaks were appeared ( Figure S23, Supporting Information). Furthermore, it was possible to recover the eluted Poly1 by solvent removal and reuse it without loss of adhesive capability ( Figure 7D′). Here, poly1f is an example of a lab small-scale RORM pattern that supports our concept well. Through this RORM adhesive system, recycled valuable adhesive and substrates could be obtained, eventually contribute to materials development to realize the circular economy. [33]

Underwater Curing of a Tough Adhesive by Induction Heating
The RORM modes enabled us remotely to control the adhesiveness of a tough underwater adhesive system by integrating it with electromagnetic technology. [35] We used an electromagnetic induction system comprising magneto-sensitive ferrimagnetic magnetite (Fe 3 O 4 ) nanoparticle (MNP)s embedded in the adhesive for remote thermal activation (Figure 8, and Figures  S28, S40, and Movie S4, Supporting Information). First, the sticky Poly1f with MNPs was applied to a PP substrate submerged in water ( Figure 8A,A′). To ensure maximum heating efficiency through electromagnetic induction and strong adhesion in water, we determined the optimal amount of MNPs in advance using lap-shear testing under air conditions. Through these tests, we found that adding 30 wt% MNPs provided the best results as shown in Figure S27 (Supporting Information). The adhesive was then irradiated with UV L to prepare a PAA Figure 7. A) Reset-On demand, Reuse-Many (RORM) modes of the designed LPAA demonstrated using Poly1f. The low viscosity adhesive was easily applied to the substrate. B) UV L irradiation was used to generate the LPAA layer. C) Following thermal activation, the adhesive demonstrated superstrong adhesive strength when subjected to a lap shear test, and did not show any signs of fatigue after 30 cycles of thermal activation (Reuse-Many mode). Once the super-strong adhesive was no longer needed, the substrates were easily dismantled by irradiating the adhesive with UV s to cause a decrosslinking of covalent bonds in tough adhesive, which makes it soluble in common solvents such as acetone, tetrahydrofuran, or chloroform. D) The eluted adhesive could be recovered by removing the solvent, and it could be reused without loss of adhesive capability. Therefore, it was possible to recycle both the adhesive and the substrate (Reset-on demand mode). D′) A photograph of the recycled adhesive. It could be reused for the subsequent bonding of various substrates on demand. layer ( Figure 8B,B′). Subsequently, the specimen was placed in an alternating magnetic field inductor (coil) ( Figure 8C,C′). When the preset electric current was applied, the Fe 3 O 4 nanoparticles embedded in the PAA layer were locally heated up to around 80°C within 1 min. We thereby demonstrated the practical performance of the adhesive with RORM modes by using it to join a submerged PP substrate, delivering in situ non-contact bonding on demand. Subsequent bending tests on cantilevered beams with loads of at least 60 kPa demonstrated that the adhesive had sufficient strength ( Figure 8D). Specifically, the lap shear test showed that the bond strength was 1.2 MPa, demonstrating the enduring strength of the adhesive with respect to hydrophobic PP ( Figure S31, Supporting Information). For thermal activation, Poly1 was first subjected to adhesion tests under air with varying temperatures of thermal activation. The results showed that the adhesive strength gradually increased as the temperature increased and became a constant value at temperatures above about 120°C ( Figure S30, Supporting Information). Therefore, the optimum thermal activation temperature was determined to be 120°C. When a similar thermal activation was performed using electromagnetic induction in air, we confirmed that the temperature reached 120°C or higher within a few seconds. On the other hand, when remote thermal activation was performed in water using the electromagnetic induction, the local temperature of the adhesive layer of Poly1f with 30 wt% MNPs did not rise above 80°C. This may be due to the diffusion of the locally elevated heat from the adhesive layer of Poly1f with MNPs into the surrounding water. Therefore, it is required that the adhesives in this method thermally activate below 80°C. Furthermore, it is worth noting that electromagnetic induction is not limited to adhesives containing MNPs but can also be applied to metals and alloys such as iron, aluminum and copper and their alloys and conductive materials as the adherend. For example, when an Al substrate was used as an adherend, remote thermal activation could be performed with Poly1f without MNPs, and sufficient adhesive strength was obtained ( Figure S31, Supporting Information). The in situ non-contact underwater adhesion requires remote irradiation of UV L on the adherend surface with a pre-applied Poly1f. Therefore, when UV L is absorbed or scattered by contamination or turbidity before UV L reaches the adherend surface, it is necessary that UV L source places very close to the adherend surface through an optical fiber, or that a cross-linking reaction with UV L carries out in air beforehand, and then immersing the specimen in such the liquids, followed by thermal activation with electromagnetic induction. The latter method was demonstrated using porcine serum and natural seawater without purification as Figure 8. Tough underwater adhesive curing by combining UV with electromagnetic induction heating. A) A magnetic nanoparticle (MNP)-doped adhesive (Poly1f) was coated on a polypropylene (PP) substrate, and B) irradiated with UV L to produce a pre-applied adhesive layer. C) An electromagnetic field was then applied to heat the adhesive to 80°C within 1 min, thereby tightly bonding the two PP substrates. D) The bonded specimen could easily lift up to 0.5 kgf weight, demonstrating the strength of the bond without failure (see Movie S4, Supporting Information). models of biochemical conditions and the natural environment. Poly1f containing 30 wt% MNPs was applied to PP substrates and photo-crosslinked by UV L irradiation for 50 min ( Figure S29a,b, Supporting Information). The photo-crosslinked Poly1f-applied adherends were immersed in the respective solutions for 5 min and then heated by electromagnetic induction. As a result, the temperature of the Poly1f layer immediately increased, and then remained constant at ≈80°C, due to the thermal diffusion of the local heat to the bulk water. After induction heating, the specimens were removed from the glass vial and the adhesive strength was evaluated by the lap-shear test ( Figure S29c, Supporting Information). Consequently, the adhesive strength of thermally activated Poly1f in porcine serum and seawater was comparable to that of the adhesive specimen prepared in deionized water. On the other hand, when the induction heating was performed without photo-crosslinking under UV L irradiation, the adhesive strength was greatly reduced ( Figure S29c-1, Supporting Information).

Conclusions
In the present study, we developed a tough, mussel-inspired, light-triggered caffeic acid-functionalized pre-applied adhesive (LPAA) that can behave in Reset-On demand, Reuse-Many (RORM) modes under both wet and dry conditions. The adhesive was based on caffeic acid, which has dual characteristics: a mussel-inspired adhesive property and a photo-reversible dynamic crosslinking function. Even under water, the tough LPAA was effective with regard to various substrates such as PTFE, PP, and silicone rubber, which are difficult to bond with commer-cially available adhesives. The two-step process of LPAA adhesion by photo-crosslinking and curing by thermal activation enabled us to achieve both resettability and reusability. The tough LPAA had greater adhesive strength than previously reported biomimetic adhesives, and solved the problems of recyclability and resettability, which have recently emerged as challenging issues in the manufacture of materials in a circular economy. Furthermore, by embedding magnetic nanoparticles in the caffeic acid-functionalized PAA, we have developed a novel and facile adhesive that can be cured underwater by non-contact local induction heating. The novel bio-derived adhesive motif of caffeic acid opens the way to the development of multifunctional PAAs that are tough, reversible, recyclable, fast-curing, and capable of functioning underwater. Such qualities, which surpass those of the original bioadhesive, are required of sustainable materials for applications such as electronics, transportation, robotics, and infrastructure maintenance.

Experimental Section
All the experimental procedures are described in the Supporting Information.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.