Micelle‐crosslinked hydrogels with stretchable, self‐healing, and selectively adhesive properties: Random copolymers work as dynamic yet self‐sorting domains

The design of crosslinking domains is a vital factor to create functional hydrogels with controlled physical, mechanical, and adhesive properties. This paper demonstrates versatile synthetic systems of micelle‐crosslinked hydrogels with highly stretchable, self‐healing, and selectively adhesive properties. For this, methacrylate‐bearing random copolymer micelles are designed as physical and covalent crosslink domains via the self‐assembly of amphiphilic random copolymers carrying hydrophilic poly(ethylene glycol) (PEG), hydrophobic butyl or dodecyl groups, and methacrylate‐terminal PEG in water. The size, aggregation number, and pendant methacrylate number of the micelles are controlled by the composition and degree of polymerization. Hydrogels are efficiently obtained from the free radical polymerization of hydrophilic monomers such as PEG acrylate and acrylamide in the presence of the micelle crosslinkers in water. Owing to the dynamic yet selective chain exchange properties of the micelle domains, the hydrogels are highly stretchable up to over 1000% and show self‐healing and selectively adhesive properties. The self‐healing of hydrogels is promoted upon heating due to the fast chain exchange of the micelle domains, whereas hydrogels consisting of micelles with different alkyl groups are never adhesive because of their self‐sorting properties.


INTRODUCTION
Hydrogels are soft materials consisting of three-dimensional networks swelled by water. Owing to the design versatility, tunable physical properties, and biocompatibility, hydrogels are utilized in various research fields and are typically effective as biomaterials for tissue engineering, drug delivery, and many others. [1][2][3][4][5] Many types of gelation systems have been developed to create network polymer materials and hydrogels with excellent mechanical properties and intriguing functions such as self-healing properties and shape memory effects. The physical properties and functions depend on the structure, density, and dynamic properties of crosslinking units, in addition to the design of network chains. In general, crosslinking of network chains involves covalent bond formation (chemical crosslinking) or physical association and entanglement via hydrophobic effects, hydrogen-bonding or ionic interactions, and complex formation. Importantly, dynamic crosslinking via physical association or mobile and exchangeable units serve as sacrificial bonding or resilient and reversible connection of network polymer chains when gel materials are strained by tensile or shear stress. [6][7][8] Therefore, such dynamic crosslinking systems often lead to highly stretchable and excellent mechanical properties and adhesive properties, as realized by double network gels, [9,10] interpenetrating gels, [11] slide-ring gels, [12,13] hydrogels with supramolecular recognition, [14,15] and micelle-crosslinked gels. [16][17][18][19][20][21][22][23][24][25][26][27][28][29] Among them, polymer micelles bearing multifunctional units such as post-reacting vinyl groups are promising S C H E M E 1 (A) Hydrogels prepared via free radical polymerization of hydrophilic monomers with methacrylate-bearing amphiphilic random copolymer micelles as physical and chemical crosslinking points. (B) Hydrogels via the self-assembly of ABA-random triblock copolymers. (C) Self-healing and adhesive properties of micelle-crosslinked hydrogels as physically and/or chemically crosslinked domains for hydrogels. [22][23][24][25][26][27][28][29] For example, pluronic surfactants [poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide), PEO-PPO-PEO], [22][23][24][25] macromolecular surfactants, [26,27] and amphiphilic block copolymers [28,29] have been employed for micelle crosslinking agents. Even after radical crosslinking reactions using hydrophilic monomers such as acrylamide (AAm), their polymer micelles bearing multiple vinyl groups reversibly and dynamically induce physical association by hydrophobic effects and thus offer energy dissipation in hydrogels, leading to highly stretchable and excellent mechanical properties.
Recently, we have developed controlled self-assembly systems of amphiphilic random copolymers with hydrophilic poly(ethylene glycol) (PEG) and hydrophobic alkyl groups as side chains to create size-controlled but small micelles in water. [30][31][32][33][34][35] Uniquely, the random copolymers induce chainfolding via the association of the hydrophobic alkyl groups in water and form folded micelles whose size (∼10 nm) is much smaller than that of corresponding block copolymer micelles.
The micelle size and aggregation number can precisely be controlled by their degree of polymerization (DP), copolymer composition, and side-chain structures. More importantly, the random copolymer micelles show dynamic self-sorting via chain exchange and self-recognition in water: The copolymer chains are exchanged between micelles with identical composition and hydrophobic groups, whereas copolymer chains consisting of different compositions and/or alkyl pendants are not associated each other and thus give discrete micelles of their respective copolymers.
Focusing on the dynamic self-assembly and self-sorting behavior, we previously designed amphiphilic ABA-triblock copolymers comprising the amphiphilic random copolymer A segments and a hydrophilic PEG chain B segment to produce hydrogels (Scheme 1B). [32] The random copolymer A segments induced self-assembly to form micelles as physical crosslink domains. The hydrogels showed self-healing properties via chain exchange between the crosslink domains but never adhered to hydrogels consisting of different micelles due to the self-sorting behavior. Thus, we found that random copolymer micelles were effective as crosslinked domains for hydrogels with self-healing but selectively adhesive properties.
Given these backgrounds, we herein report a versatile strategy to design micelle-crosslinked hydrogels with highly stretchable, self-healing, and selectively adhesive properties (Scheme 1A). The objectives of this paper are to establish a novel strategy to produce hydrogels with micelle-crosslinked domains and investigate their physical and adhesive properties. It should be noted that the synthetic approaches enable us to combine various hydrophilic polymer networks and micelle-crosslinkers via simple free radical polymerization (FRP) of hydrophilic vinyl monomers in the presence of multifunctional random copolymer micelles in water. Thus, this strategy is more efficient and easier than the previous gelation systems with the amphiphilic ABA-triblock copolymers in designing hydrogels: the previous system required the preparation of various macroinitiators (B segments) to modify network polymer chains for targeted physical properties and functions. Additionally, the micelles in this novel gelation system work as not only physical but also chemical crosslinking domains to afford the diverse tuning of the physical properties (mechanical, rheological, stretchable properties, etc.).
For this, we designed amphiphilic random copolymers bearing hydrophilic PEG, hydrophobic alkyl (butyl or dodecyl) groups, and methacrylate-capped PEG as side chains to produce folded multichain micelles as crosslinkers in water. The size and aggregation number of the micelles were controlled by the DP, alkyl groups, and composition, and thus the number of methacrylate units (for crosslinking) per a single micelle domain was also controlled. Micelle-crosslinked hydrogels were obtained from the FRP of hydrophilic monomers in the presence of the micelle crosslinkers. The rheological, stretching, self-healing, and adhesive properties of their hydrogels were investigated systematically by varying the micelle concentration and structures and hydrophilic monomers. Owing to the dynamic self-assembly and selfsorting properties of the micelle crosslinkers, the hydrogels showed highly stretchable, self-healing, but selectively adhesive properties (Scheme 1C).

Design and synthesis of methacrylate-bearing amphiphilic random copolymers
Methacrylate-carrying amphiphilic random copolymers (P1-M-P4-M) were designed as crosslinking domains for hydrogels. P1-M-P4-M were synthesized by living radical copolymerization of hydrophilic PEG methyl ether methacrylate (PEGMA: average number of oxyethylene units = 8.5), hydrophobic butyl or dodecyl methacrylate (BMA or DMA), and PEG methacrylate with a hydroxy group (PEGOHMA: average number of oxyethylene units = 6.2), followed by the post-functionalization of the resulting random copolymer precursors (P1-P4) with 2-methacryloyloxyethyl isocyanate (MOI) (Scheme 2). The methacrylate units introduced into the PEGOHMA terminals are expected to exist in the outer layers of their micelles and polymers in water and thus undergo efficient crosslinking. According to our previous investigations, [30,31] the content of BMA and DMA was set to 70 mol% and 50 mol%, respectively, to produce sizecontrolled micelles in water. P1-M and P2-M, both of which TA B L E 1 Characterization of amphiphilic random copolymers    Table 1 and Figure S1). PEGMA (PEGOHMA) and BMA or DMA were simultaneously consumed at the same speed during copolymerization, meaning that PEG and alkyl groups were randomly distributed along each polymer chain. All the products were analyzed by 1 H nuclear magnetic resonance ( 1 H NMR) spectroscopy to determine the composition, DP, and the number of monomer units (PEGMA/BMA or DMA/PEGOHMA = l/m/n) (Table 1, Figure 1, and Figure  S2). The l/m/n values, estimated from the area ratio of their monomer units to that of the initiator Ph group, were close to those calculated from the feed ratio of their respective monomers to the initiator and their monomer conversion. The number-average molecular weight of the copolymers (M n ) was determined to be 24,200-36,400 by 1 H NMR.
P1-M-P4-M were obtained by mixing P1-P4 and MOI with dibutyltin dilaurate as a catalyst in dichloromethane at 25 • C. As confirmed by 1 H NMR, the hydroxyl groups of P1-P4 completely disappeared, and the methacrylate units were introduced quantitatively ( Figure 1 and Figure S3). P1-M-P4-M had molecular weight close to their corresponding precursors (M n = 22,600-30,500, M w /M n = 1.17-1.32 by SEC in DMF with PMMA standard calibration, Figure 1 and Table 1). The number-average molecular weight of P1-M-P4-M was determined to be 25,500-36,900 by 1 H NMR. Additionally, P1-M-P4-M and their precursors were analyzed by SEC coupled with multi-angle laser light scattering (SEC-MALLS) in DMF to determine absolute weightaverage molecular weight (M w,DMF ) for the calculation of the aggregation number of their micelles in water (Table 1).

Self-Assembly of copolymers into micelles for crosslinking domains in water
Self-assembly of amphiphilic random copolymers (P1-M-P4-M, P1-P4) into micelles in water was examined by SEC-MALLS. All the copolymers were easily dissolved in water. The micelles were prepared via the following processes: The copolymers were mixed with pure water in vials at 25 • C. The aqueous mixtures ([polymer] = 1 mg/m) were sonicated for 15 min and then filtrated with poly(tetrafluoroethylene) membrane filters (pore size: 0.45 μm) before measurements. The aqueous mixtures were kept at 25 • C for about 24 h and then injected into the SEC-MALLS system using a 100 mM NaCl aqueous solution as an eluent. The apparent size of the copolymers and their micelles in water was evaluated with their SEC curves (by refractive index detector) on the basis of poly(ethylene oxide) (PEO) standard calibration; the peaktop molecular weight in water was compared with that in DMF (good solvent). The absolute weight-average molecular weight (M w,H 2 O ) of the copolymers and their micelles in water was also determined by the MALLS detector to estimate the aggregation number (N agg ).
As shown in Figure 2A and Figure S4, P1, P2, and P3 showed unimodal SEC curves in water. The SEC curves in water shifted to molecular weight higher than corresponding counterparts in DMF. This indicates that the random copolymers induce intermolecular self-assembly to form multichain micelles in water. Similarly, P1-M, P2-M, and P3-M showed SEC curves with a peak-top molecular weight larger than those in DMF ( Figure 2B-D). P1-M and P3-M exhibited relatively narrow molecular weight distribution in water.  Figure 2C), indi-cating that multichain micelles partly associated into larger aggregates. [34] This is because the methacrylate (MOI)capped PEGs are more hydrophobic than other methyl ether PEGs (PEGMA). Considering that the apparent molecular weight of the P2-M micelle by SEC is larger than that of P2 micelle with N agg of 9, the P2-M micelle should have over 10 N agg and thereby over 100 pendant methacrylate units. The P2-M micelle was further analyzed by 1 H NMR in D 2 O ( Figure 1E). The proton signals of the hydrophobic butyl groups and the polymethacrylate backbone turned broad, whereas those of the hydrophilic PEG chains and methacrylate units incorporated into the PEG terminals were clearly observed. This importantly supports that P2-M forms folded micelles via the association of hydrophobic butyl groups and In contrast, P4 and P4-M without hydrophobic alkyl groups showed unimodal SEC curves in water and the peak-top molecular weight was close to that in DMF ( Figure 2E and Figure S4). N agg of P4-M was estimated to be 1.2 (Table 1), indicating that P4-M unimolecularly dissolves in H 2 O. The small peak shift of P4-M to lower molecular weight means the formation of a more compact structure in water than that in DMF due to the weak association of their pendant methacrylate units.
PEGMA/BMA (70 mol%) or DMA (50 mol%) random copolymer micelles are known to show lower critical solution temperature-type solubility in water and the cloud point (Cp) temperature of PEGMA/BMA (70 mol%) random copolymers is typically around 45 • C. [31] This implies that radical crosslinking of their micelles should be conducted below the Cp temperature. Self-assembly and thermoresponsive properties of their copolymers into micelles may also be affected by hydrophilic monomers used for network polymer chains. Thus, we examined effects of temperature and hydrophilic monomers (used for network chains) on their random copolymer micelles in water by the Cp measurements of the micelle with about 10-20 nm diameter were mainly formed. Importantly, the copolymer still maintained small micelles even in the presence of their hydrophilic monomers in water up to 45 • C without decomposition of their self-assemblies into unimers. Therefore, we found that P1-M-P3-M micelles were available as crosslinking domains in water by applying appropriate temperatures between 25 • C and 45 • C in hydrogel preparation.   The internal structures of the AAm/P1-M micelle hydrogel in D 2 O were analyzed by small-angle neutron scattering (SANS) ( Figure 5A). The P1-M micelle showed a scattering profile originating from the globular structure in D 2 O, and the radius of gyration (R g ) was estimated to be 5 nm by the Guinier plot of the profile ( Figure S7). The scattering profile of the hydrogel almost overlapped that of the P1-M micelle in the high-q region (approximately 0.6-2 nm −1 ). This demon-strates that the hydrogel consists of polyAAm network chains crosslinked with P1-M micelles. In addition, the scattering intensity of hydrogel around q < 0.1 nm −1 increased with decreasing q, reflecting the spatial inhomogeneity of the network structure. [36] To clarify the structure of the micelles and hydrogel in detail, we carried out model fitting ( Figure 5B and Supporting Information). First, we utilized a fitting model representing the core-shell spherical structure of the micelles. In this model, called the separate micelle model, a micelle is composed of a spherical BMA core with radius R c and a shell with thickness t s formed by PEGMA (volume fraction: ϕ s ) and D 2 O (volume fraction: 1 − ϕ s ). Therefore, the overall radius of the micelle is R s = R c + t s . We assumed the Gaussian distribution of R c , taking the size distribution of micelles into account. The results of the fitting are given in Figure 5A (blue line) and Table 3. The scattering function well represented the entire scattering profile of the P1-M micelle solution and the high-q region of the hydrogel profile, indicating that the compact structure of micelles (R s ∼ 5 nm) was preserved in the hydrogel. In addition, the volume fraction of D 2 O in the micelle shells was large (1 − ϕ s = 0.77). This means that the PEG chains in the shells were highly hydrated. The experimental datapoints of the hydrogel in the middle-q and low-q regions, however, did not agree with the theoretical scattering function of the separate micelles. This is probably because multiple micelles were interconnected by polyAAm chains.

Preparation and structure analysis of micelle-crosslinked hydrogels
To address the effects of interconnected adjacent two micelles, which is thought to be the dominating factor in causing the abovementioned deviation, we used another model (micelle pair model). [37] In this model, we suppose a pair of micelles with the abovementioned core-shell structure, whose surface-to-surface distance is l pair and obeys Gaussian distribution. The best fit to the experimental data is shown in Figure 5A (red line) and Table 3. The experimental data points almost agreed with the theoretical curve, but the former slightly diverged from the latter, presumably due to the inhomogeneity of network structure and

Rheological properties of hydrogels
We investigated the effects of (1) micelle concentration, (2) crosslinking temperature, (3) crosslinking domain structures, and (4) hydrophilic monomers on viscoelastic properties of resulting hydrogels (Gel 1-Gel 12) by changing their synthetic conditions ( Table 2). The rheological properties of their hydrogels were evaluated by oscillatory strain or frequency sweep measurements.

Effects of micelle concentration on PEGA hydrogels
We prepared In the feed conditions, the concentration of their micelle domains is relatively close in both gels but the total concentration of the pendant methacrylate units in Gel 2 is larger than that in Gel 1. Gel 3 consists of a small amount of P2-M micelle (5.5 mg/mL) but the concentration of the pendant methacrylate is close to that in Gel 1. Figure 6A shows the shear storage modulus (G′) and loss modulus (G″) for Gel 1-Gel 3 from oscillatory strain sweep measurement at a frequency of 1 Hz at 25 • C. Typically, at 1% strain, G′ for Gel 1 and Gel 2 was almost identical (∼200 Pa) and over ten times larger than G″. The linear viscoelastic region for Gel 1 (∼100% strain) was larger than that for Gel 2 (∼20% strain). G′ for Gel 3 was much smaller than that for Gel 1 and Gel 2 and only about twice larger than G″. G′ and G″ for Gel 1-Gel 3 were evaluated by oscillatory frequency sweep measurements at 1% strain from 10 −1 to 10 2 rad/s at 25 • C ( Figure 6B). Gel 1 and Gel 2 showed almost constant G′ in a wide range of frequency (10 −1 -10 2 rad/s) and the G′ values were larger than G″ in the frequency region, indicative of the efficient gelation on Gel 1 and Gel 2.
Taking these results into consideration, we found the following trends: (1) G′ for their hydrogels depends on the concentration of micelle domains and is hardly affected by the local concentration of pendant methacrylates per a single micelle (n x N agg ) in the case of 30 mg/mL of their micelles. About 20 methacrylate units per micelle domain were enough to crosslink polyPEGA network chains if 30 mg/mL of micelles were used. (2) Gel 2 was more brittle than Gel 1. This is because P2-M micelles with a large number of methacrylate units (>100) contribute as chemical crosslinking in Gel 2 more greatly than P1-M micelles in Gel 1; the reversible and physical crosslinking by P2-M micelles would be more restricted than those by P1-M micelles. It is also confirmed that poly(PEGA) network chains were not only physically but also covalently crosslinked with a P1-M micelle in Gel 1 by the following experiment: Gel 1 mostly collapsed in methanol but the insoluble aggregates and microgels partly remained ( Figure S8).

Effects of crosslinking conditions and micelle domains on AAm hydrogels
To evaluate the effects of the crosslinking conditions on the rheological properties, we prepared AAm hydrogels using a P1-M micelle crosslinker in water by the following condi-   Figure S9). G′ and G″ for the three hydrogels were evaluated by oscillatory strain sweep measurement at a frequency of 1 Hz at 25 • C ( Figure 6C,E and Figure S9). All the hydrogels showed the linear viscoelastic region of G′ up to about 50% strain; G′ was 5-10 times larger than G″ at 1% strain. The G′ values depended on the crosslinking conditions: 1,010 Pa (with APS at 45 • C, Gel 4), 797 Pa (with APS/TMEDA at 25 • C, Gel 9), 267 Pa (with APS at 25 • C). The difference of G′ is probably related to the length of polyAAm network chains and/or crosslinking efficiency dependent on their reaction conditions. A

Design of hydrophilic polymer networks
We further prepared hydrogels (Gel 8-Gel 12) consisting of various hydrophilic polymer networks by FRP of HEA, AAm, HEAAm, NIPAM, and DMAAm as hydrophilic monomers with a P1-M micelle and APS/TMEDA at 25 • C. All the hydrogels showed the linear viscoelastic region of G′ up to around 100% strain in oscillatory strain sweep measurement at a frequency of 1 Hz at 25 • C ( Figure 6E). G′ and G″ for the hydrogels were also evaluated by oscillatory frequency sweep measurements at 1% strain from 10 −1 to 10 2 rad/s at 25 • C ( Figure 6F). For example, Gel 8 with polyHEA networks showed almost constant G′ in a wide range of frequency (10 −1 -10 2 rad/s) and the G′ values were larger than G″ in the frequency region. As demonstrated here, we successfully developed versatile synthetic systems of micelle-crosslinked hydrogels via FRP of hydrophilic monomers in the presence of olefin-bearing micelles in water.

Tensile and stress relaxation behavior of hydrogels
PEGMA-based random copolymer micelles induce chain exchange in the water around room temperature. [32,35] The exchange kinetics depend on the hydrophobic monomer units consisting of the micelle cores: The chain exchange of PEGMA/BMA random copolymer micelles is faster than that of PEGMA/DMA random copolymer micelles at 25 • C. Thus, crosslinking domains of random copolymer micelles are also expected to show chain exchange within hydrogels. [32] To investigate the effects of such chain exchange on the mechanical properties of hydrogels, tensile tests and stress relaxation measurements were conducted for Gel 4, Gel 5, and Gel 6. All the hydrogels showed highly stretchable properties ( Figure 7A). Gel 4 consisting of P1-M micelles was more stretchable up to over 1,000% than Gel 5 consisting of P3-M micelles. The stretchable properties of Gel 4 most likely arise from the efficient chain exchange of PEGMA/BMA-based P1-M micelles within the network structures: The micellecrosslinking domains can reversibly be regenerated within the hydrogel networks even though the random copolymer chains are once pulled out from the micelle domains during the elongation process of the hydrogel. We can also stretch Gel 4 to over 1,000% strain by hand without breaking ( Figure 7B). Gel 6 consisting of covalent crosslinking with P4-M was also well stretched. This is probably due to the concentration of P4-M (3 mg/mL) lower than the other micelle crosslinking domains (P1-M and P3-M, 30 mg/mL). The breaking strain of Gel 4 was close to that of Gel 6.
The chain exchange behavior of micelle-crosslinking domains was examined by shear stress relaxation measurements of Gel 4, Gel 5, and Gel 6 at 5, 25, and 40 • C, where the stain was kept at 10% ( Figure 7C-E). The shear stress of Gel 4 gradually decreased with time progress and the stress relaxation was promoted by increasing temperature. Similarly, Gel 5 also showed stress relaxation at 40 • C, whereas the decay of the stress was slower than that for Gel 4 at 40 • C. In contrast, Gel 6 only showed a small decrease in the stress and the trend was almost independent of the temperature. Gel 4 and Gel 5 showed quite slow decay of the stress at 5 • C at almost the same speed as that for Gel 6. These results indicate the followings: (1) The stress relaxation on Gel 4 and Gel 5 originates from the chain exchange of micelle-crosslinking domains. The chain exchange within their hydrogel networks, as well as that of corresponding random copolymer micelles in water, [32] is accelerated upon heating. (2) The chain exchange of PEGMA/BMA random copolymer micelles in Gel 4 is faster than that of PEGMA/DMA counterparts in Gel 5. PEGMA/DMA random copolymer chains were hardly exchanged at 25 • C. (3) Micelle-crosslinking domains within Gel 4 and Gel 5 are "frozen" at 5 • C without chain exchange like covalently crosslinked domains in Gel 6. The slow decay of the stress for Gel 4 and Gel 5 at 5 • C, as well as that for Gel 6, is probably due to the relaxation of their entangled polyAAm network chains.

Self-healing and adhesive properties of hydrogels
Focusing on the chain exchange of random copolymer micelles, we examined the self-healing and adhesive properties of hydrogels. First, Gel 4 crosslinked with P1-M micelles was cut into two pieces and the cut gels were again contacted at 5 • C, 25 • C, and 40 • C ( Figure 8A). The cut gels adhered at 25 • C for at least 2 h or at 40 • C for 15 min, whereas the gels were not adhesive at 5 • C at all. The self-healing properties are derived from the chain exchange between micelle domains on the gel surfaces; the fast adhesion at 40 • C is due to the fast chain exchange upon heating. Gel 5 crosslinked with P3-M micelles adhered at 40 • C in 2 h ( Figure 8D), whereas Gel 5 was hardly adhesive at 25 • C in 15 h ( Figure 8C). This is due to the quite slow exchange of PEGMA/DMA copolymer chains at 25 • C. The adhesion of cut Gel 5 at 40 • C was weaker than that of cut Gel 4 at 25 • C. This suggests that P3-M micelles contain relatively large amounts of pendant methacrylates to contribute more as covalent crosslinking units. Those healing and adhesive properties were consistent with the trend of the stress-relaxation of Gel 4 and Gel 5. Gel 6 crosslinked covalently with P4-M was never adhesive at all ( Figure 8F).
Interestingly, Gel 4 (AAm/P1-M) adhered to Gel 9 (DMAAm/P1-M) at 25 • C in 2 h although the network polymer chains of their hydrogels were different ( Figure 8B). This means that hydrogels consisting of identical random copolymer micelles as crosslinking domains are adhesive, independent of the structures of network polymers. In contrast, Gel 4 containing P1-M micelles with BMA units never adhered to Gel 5 containing P3-M micelles with DMA units at 40 • C in 2 h ( Figure 8E) although Gel 5 was adhesive each other at 40 • C in 2 h ( Figure 8D). These results indicate that their micelle-crosslinking domains exhibit self-sorting (selective self-assembly) behavior on their gel surfaces. Such selective adhesion properties are one of the characteristics of hydrogels consisting of random copolymer micelles as crosslinked domains.
Finally, a tensile test of the self-healed Gel 4 was conducted ( Figure S11). The sample was prepared by the adhesion of two cut gel pieces at 25 • C for 2 h. The connected gel stretched up to about 80% strain and then fractured at the adhesion point, although the original Gel 4 was stretched around 1,000%. This is because the broken covalent bonds of multiple polyAAm network chains are not repaired and the tensile stress is concentrated on micelle-crosslinking domains regenerated at the adhesion interface of the cut gels. [38] The cut and re-connecting method, examined herein, would be useful to produce hydrogels with controlled breaking points by introducing the relatively weak adhesion interface into the desired sites.

CONCLUSION
In summary, we established versatile synthetic systems of highly stretchable, self-healing, and selectively adhesive hydrogels via FRP of hydrophilic vinyl monomers coupled with methacrylate-bearing random copolymer micelles. The random copolymer micelles efficiently served as physical and covalent crosslinking domains for hydrogels. Thus, the physical properties of hydrogels were controlled by designing the micelles as crosslinked domains. The self-healing and adhesive properties and their kinetics depended on the chain exchange and selective association (self-sorting) of the micelles. This system further affords the versatile design of network polymer chains. Therefore, the synthetic strategy of micelle-crosslinked hydrogels, developed herein, opened a new possibility to produce hydrogels with stretchable, self-healing, and selectively adhesive properties as desired and would bring innovation in various research fields for applications such as selectively adhesive biomaterials.

C O N F L I C T O F I N T E R E S T
The authors declare no conflict of interest.

D ATA AVA I L A B I L I T Y S TAT E M E N T
The data that support the findings of this study are available from the corresponding author upon reasonable request.