Highly Efficient Switchable Underwater Adhesion in Channeled Hydrogel Networks

The ability to switch adhesion strength is a highly desirable property for adhesives applied in a wet environment. The major challenges involve the presence of a water layer between the substrate and adhesive, and the incorporation of efficient switching mechanisms. Despite the recent progresses in devising such systems, there exist several intrinsic limitations in the current strategies, such as high residual adhesion, the use of solid–liquid transition, or thin film configurations. Herein, a channeled poly( N -isopropylacrylamide) (PNIPAm) hydrogel containing bio-inspired dopamine-comonomers is reported, which undergoes temperature-controlled reversible switching of underwater adhesion on both hydrophilic and hydrophobic surfaces. The introduction of microscopic channels inside the hydrogel, achieved by removing a sacrificial agarose network, greatly facilitates water removal from the interface and thus promotes underwater adhesive strength. On glass, the maximum adhesive stress of the channeled hydrogel can reach six times that of hydrogels without channels. Additionally, high switching efficiency and low residual adhesion can be achieved by the thermal phase transition of the PNIPAm network, also demonstrated by the capture and release of lightweight, irregular, fragile, and biological objects using the hydrogel. The channeling strategy provides implications for designing future underwater adhesive systems for, e.g., soft robotics or biomedical applications.


Introduction
[3] Natural organisms show a plethora of chemical and physical mechanisms for underwater adhesion to various surfaces, such as the well-studied catechol chemistry in mussels, [1,[4][5][6][7] complex coacervation in sandcastle worms, [2,8] hexagonal surface structures and suckers in clingfish, [9,10] and suckers in octopuses. [3,11,12]][22][23][24] However, the adhesion and surface structures to enhance dissipation [34] or water removal, [36,46] and to induce suction. [35,37]erein, we report a channeled poly(N-isopropylacrylamide) hydrogel (ch-dopa-PNIPAm) containing bio-inspired dopaminecomonomers as adhesion promoters, [1] which can undergo temperature-controlled reversible switching of adhesion in an aqueous environment.The microscopic channels introduced by removing a sacrificial agarose network facilitate water removal from the interface, [49] while high switching efficiency is ensured by the thermoresponsive phase transition of PNIPAm.The underwater adhesion is studied by probe-tack measurement on both hydrophilic and hydrophobic substrates, while the drystate adhesion is also characterized.The application potential of the hydrogel system as a bulk and reversible adhesive is demonstrated by the implementation of adhesive grippers for soft, lightweight, irregular, fragile, and biological objects.

Highly Efficient Switchable Underwater Adhesion in a Channeled Hydrogel
The chemical structures of the constituents and the preparation procedures of the hydrogels are illustrated in Figure 1a.Briefly, N,N′-methylenebisacrylamide (BIS) was used as a chemical cross-linker in the PNIPAm network, and 1 mol.% of dopamine methacrylamide (DPMA) relative to the NIPAm was introduced as the adhesion promotor. [1]Radical polymerization of the N-isopropylacrylamide (NIPAm) and DPMA was carried out in the presence of an agarose network using a photoinitiator Irgacure 2959, with the catechol groups being protected by sodium tetraborate decahydrate (Borax). [50]he as-prepared double network gel is denoted as DN-dopa-PNIPAm.][53] The agarose was removed by heat-induced melting and repeated washing by shrinking and swelling cycles to form the channels. [49]The thus-formed channeled PNIPAm hydrogel is denoted as ch-dopa-PNIPAm.The hydrogels prepared with the same composition and procedure in the absence of agarose are referred to as standard dopamine-containing PNIPAm (dopa-PNIPAm).The detailed polymerization procedure and protection of catechol groups using Borax [50] are described in the Experimental Section and Figure S1 (Supporting Information), while the removal of Borax and agarose network [49] are confirmed by FTIR shown in Figure S2 (Supporting Information).In principle, the washing steps for agarose removal did not change the chemical composition of the PNIPAm or the catechol groups as shown in Figure S2 (Supporting Information).The effect of Borax and agarose removal on the porosity of the hydrogel is summarized in Figure S3 (Supporting Information), which confirms the formation of microscopic pores/channels in the range between 4-8 µm (Feret diameter) in the ch-dopa-PNIPAm compared to DN-dopa-PNIPAm gel.
To study the switching of the adhesive state caused by the phase transition of PNIPAm (lower critical solution temperature (LCST) between 33.1 °C measured by transmittance, Figure S4, Supporting Information), the adhesion strength was measured at room temperature (RT, 20 °C) and high temperature (HT, 45 °C) well above LCST using the probe-tack method [54] as illustrated in Figure 1b.The swelling curve of the ch-dopa-PNIPAm shows a similar phase transition at ≈34 °C in Figure S5 (Supporting Information) in good agreement with the transmittance measurement, where the linear size of the gel decreased by almost 70% as the temperature changed from RT to 40 °C.For potential applications, lower high-temperature values closer to LCST, such as 40 °C, are also explored later in the application demonstrations.Hydrogels with cylindrical shapes (outer diameter 6 mm) were synthesized and immobilized in a glass tube via covalent bonds. [49]One end of the hydrogel protrudes from the glass tube by a length of 4 mm, which was used as the probe.The hydrogel probe was brought into contact with a wet substrate at room temperature, followed by a prestress period of 10 s, where a compressive stress equal to the tensile modulus of the hydrogel was applied. [17]The tensile measurements of dopa-PNIPAm and ch-dopa-PNIPAm are shown in Figure S6 (Supporting Information).In the HT measurements, a large amount of water at 45 °C was added during the prestress period to heat up the hydrogel.Afterward, the hydrogel probe was retracted, and the force was measured until the hydrogel was fully detached from the substrate.Sufficient amount of water was used to ensure that the hydrogels remained under the water surface to prevent the influence of surface tension on the measurement.
Representative stress-strain curves from the adhesion measurement at HT and RT of dopamine-containing hydrogels are shown in Figure 1c,d.The ch-dopa-PNIPAm hydrogels contained 1 mol.%DPMA and 0.1 mol.%BIS, where 0.5 wt.% agarose was used to create the channels.The positive values represent tensile stress (adhesion), and the negative values represent compressive prestress.Average maximum adhesive stresses from five measurements at HT and RT are presented in Figure 1e.The switching efficiency r S is calculated according to Equation 1 where σ L refers to the maximum adhesive stress at the low adhesion state and σ H refers to maximum adhesive stress at the high adhesion state.In this way, r S = 1 refers to a 100% switching where the residual adhesion at low adhesion state is practically zero (σ L = 0), while r S = 0 indicates no switching in the adhesion, i.e., σ L = σ H .This definition has the advantage that values between 0 and 1 will be obtained for better comparison of high switching ratios, while the simple ratio of high adhesion to low adhesion [55] will lead to less comparable values if the low adhesion is close to 0 (division by zero).
For the representative example in Figure 1c,d, the maximum adhesive stress of the dopa-PNIPAm is σ H = 0.33 kPa at HT and σ L = 0.19 kPa at RT, while the ch-dopa-PNIPAm possesses significantly enhanced adhesive stress at HT. Figure 1e shows that σ H = 2.06 kPa at HT, i.e., six times higher compared to

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© 2023 Wiley-VCH GmbH dopa-PNIPAm.The RT adhesive stress of ch-dopa-PNIPAm is reasonably low at 0.34 kPa, resulting in a switching efficiency of 0.83 compared to 0.43 of the dopa-PNIPAm.The difference between the HT and RT adhesion for dopa-PNIPAm in Figure 1e is not statistically significant due to the small difference between the adhesion strength and the relatively large variations in measurement results.Therefore, both the maximum adhesive stress and the switching efficiency are improved in

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© 2023 Wiley-VCH GmbH the ch-dopa-PNIPAm.Since the ch-dopa-PNIPAm and dopa-PNIPAm hydrogels underwent same washing treatment, the improvement can be attributed to the formation of channels.The effect of dopamine groups is investigated by control experiments using channeled and standard PNIPAm without DPMA, denoted as ch-PNIPAm and PNIPAm, respectively, as shown in Figure S7 (Supporting Information).The ch-PNIPAm has a maximum adhesive stress of 0.2 kPa and switching efficiency of only 0.2.This indicates that the addition of catechol groups dramatically improved the maximum adhesive stress in ch-PNIPAm at HT and the switching efficiency.On the other hand, the catechol groups have only minor influence on the HT adhesive stress but also improved the switching efficiency from 0.06 to 0.43 of standard PNIPAm without channels.
Based on the experimental results presented above, we postulate the mechanism of improved adhesive stress in channeled PNIPAm as illustrated in Figure 1f.Due to the presence of channels, the water can be transported into the hydrogel or away from the contact area at the interface more efficiently [49] compared to the reference dopa-PNIPAm.This is reminiscent of previously reported surface structures, [36,46] but has the advantage that the channels are formed throughout the hydrogels, making the adhesion properties less susceptible to surface damage.This greatly reduces the interfacial water layer and improves the hydrogel adhesion during attachment.On the other hand, such channels may also assist in the back-diffusion of water to the interface, thus assisting in reducing the residue adhesive stress at RT.The presence of microscopic channels is confirmed by scanning electron microscope (SEM) using freeze-dried hydrogels (detailed preparation in Experimental Section) as shown in Figure S8 (Supporting Information).There exist abundant porous structures in the size range between 4 and 8 µm in the ch-dopa-PNIPAm hydrogel at both RT and HT compared to the sub-micron pores in the dopa-PNIPAm, especially considering the fraction of surface areas covered by these pores.This is different from our previous ch-PNIPAm system, where more pores in the submicron range are now observed in the channeled hydrogel, [49] presumably due to the addition of catechol groups in the hydrogel.The switching of adhesion is achieved by the hydrophilic-hydrophobic transition of PNIPAm upon temperature change, [56] which also resulted in the change in the porosity shown in Figure S8 (Supporting Information).Above the LCST, the PNIPAm is in the hydrophobic state, which facilitates water removal from the interface [48] and thus provides higher adhesion to the glass surface via the hydrogen bonds formed by the catechol groups. [1,57]Similar effect has been detected in mussel adhesive proteins. [42]The essential contribution of catechol to adhesion in ch-dopa-PNIPAm is corroborated by control experiments in Figure S7 (Supporting Information), where the ch-PNIPAm without catechol groups shows only 0.2 kPa adhesive stress above the LCST.It is noted that the microscopic channels are only formed in the presence of catechol groups in ch-dopa-PNIPAm, presumably due to interactions between the catechol groups and the agarose via hydrogen bonds. [1]The ch-PNIPAm lacks such microscopic channels (Figure S9, Supporting Information) and thus has even lower adhesion to hydrophilic substrates compared to conventional PNIPAm as shown in Figure S7 (Supporting Information).At room temperature, the hydrated hydrophilic PNIPAm chains repel the substrate surface, which reduces the adhesion strength significantly.In this way, the catechol groups and channeled structures notably improve the switching efficiency and adhesion strength.

Effect of Hydrogel Composition on Underwater Adhesion
The effect of hydrogel composition on the switching of underwater adhesion has been studied by varying the agarose and crosslinker (BIS) concentrations, as summarized in Figure 2. Figure 2a demonstrates representative stress-strain curves of probe-tack measurement of ch-dopa-PNIPAm prepared using 0.5 wt.% agarose as the template and 0.1 mol.%BIS as the crosslinker.][60] At RT, the maximum adhesion is much lower at σ L = 0.45 kPa, thus resulting in a switching efficiency of 0.84. Figure 2b summarizes the adhesive stress and switching efficiency of ch-dopa-PNIPAm prepared using different concentrations of agarose averaged from 5 measurements.The agarose concentration of 0.5 wt.% results in an optimum switching efficiency (0.83) and an adhesive strength close to the highest value achieved in 1 wt.% sample (σ H = 2.2 kPa).Since the ch-dopa-PNIPAm gels prepared with different agarose concentrations have similar mechanical properties (Figure S10, Supporting Information), we attribute this observation to the optimal channel structures in the gel achieved by 0.5 wt.% agarose, as shown in Figure S11 (Supporting Information).Agarose (0.5 wt% and 1 wt%) samples possess higher density of pores in the range between 4 and 8 µm at HT, which explains the high adhesive stress in these two samples.On the other hand, the 1 wt.% agarose sample has less pores between 4 and 8 µm at RT compared to 0.5 wt.% samples, which contributes to the higher residue adhesion at RT and thus lower switching efficiency for the 1 wt.% sample.The adhesion energies of the hydrogels are summarized in Figure S12a (Supporting Information) according to the definition by Creton and Ciccotti. [59]he highest adhesion energy (3.1-4.6 J m −2 ) is achieved for the hydrogel prepared using 0.5 wt.% agarose, while RT and HT measurements show no qualitative difference in terms of adhesion energy due to the high strain of RT measurement before detachment.To be noted is that the ch-dopa-PNIPAm hydrogels have higher tensile strength than the interfacial adhesion as shown in Figure S13 (Supporting Information), so that only adhesive failure occurs during the measurement, thus ensuring the reusability of the hydrogels.Figure 2c shows the cyclic tests of a ch-dopa-PNIPAm hydrogel probe being repeatedly attached to and detached from the glass surface at changing temperatures.Within the 10 cycles, the maximum adhesive stress and the switching efficiency showed no obvious decay within the error of measurement, thus demonstrating the switchability and reusability of the adhesive hydrogel in the aqueous environment.
The effect of the cross-linking density on the underwater adhesion is presented in Figure 2d-f.The highest adhesive stress is achieved using 0.2 mol.%BIS with an HT adhesive stress of σ H = 5.3 kPa and RT adhesive stress of σ L = 0.92, Adv.Funct.Mater.2023, 2214091

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© 2023 Wiley-VCH GmbH leading to a switching efficiency of 0.83.It has been shown that the adhesive strength depends on the interplay between chemistry, topology, and mechanics of the adhesive hydrogel. [29]ince the gels have almost identical chemistry and topology, we attribute the difference in adhesive stress to the mechanics of the gels, where the gel containing 0.2 mol.%BIS has an intermediate modulus of 3.45 kPa (Figure S13, Supporting Information).The adhesive stress is limited by the softness of gels softer than the 0.2 mol.% sample, while the higher modulus of 11 kPa in the 0.5 mol.%BIS sample limits conformal contact with the substrate, resulting in lower adhesive stress.The highest switching efficiency, 0.91, is reached using 0.5 mol.%BIS with an HT adhesive stress of σ H = 3.0 kPa.The tensile test results of ch-dopa-PNIPAm containing different BIS concentrations are summarized in Figure S13 (Supporting Information), which shows a positive correlation between the concentration of BIS and the Young's modulus in the range between 0.05 to 0.5 mol.% of BIS.For ch-dopa-PNIPAm the highest adhesion stress is achieved at Young's modulus of σ H = 3.45 kPa, and the highest switching efficiency is achieved at a Young's modulus of 11 kPa.
The underwater adhesion tests on hydrophobic substrates, illustrated by polystyrene (PS), are presented in Figure 3.In this case, efficient switching has been observed for different agarose and BIS concentrations.For samples containing 0.1 wt.% BIS, the maximum adhesive stress is close to σ H = 3 kPa at HT, and below σ L = 0.8 kPa at RT for agarose concentrations between 0 and 1 wt.%, resulting in switching efficiencies between 0.6 and 0.8.The overall higher adhesive stress achieved on polystyrene compared to that on glass (c.f. Figure 2b) can be attributed to improved water removal from the hydrophobic interface, [42] hydrophobic interaction between the substrate and the hydrogel above the LCST, [48] as well as π-π interaction between the aromatic groups in catechol and polystyrene. [3]In contrast to the hydrophilic glass substrate, the maximum adhesive stress and switching efficiency of dopa-PNIPAm (0 wt. % agarose) on PS surface are comparable to that of ch-dopa-PNIPAm prepared with different agarose concentrations (Figure 3c).This can be attributed to the strong hydrophobic interactions between hydrophobic PNIPAm chains and the PS substrate at HT.The water layer can be more effectively expelled between two hydrophobic surfaces so that the effect of channels plays a minor role in improving the adhesion.
The increase in adhesion on PS compared to glass can also be observed when comparing the effect of cross-linker concentration in Figures 2d-f
Figure 4 compares the current system with previously reported switchable underwater adhesive systems measured by probe-tack method.For clarity, each system is presented in a separate panel, while the combined chart is shown in Figure S14 (Supporting Information).Four parameters are chosen, representing switching efficiency, maximum adhesive stress at high adhesion state, material state in solid (gel) or liquid (sol) during switching, and material geometry in bulk or film.More detailed descriptions and references are summarized in Table S1 (Supporting Information).Notably, the current system is the first one to achieve high switching efficiency (0.91, glass surface) and adhesive strength (6.2 kPa, PS surface) in a bulk and solid material that does not undergo solid-liquid (sol-gel) transition.As discussed in the introduction, the deformability of adhesives in the form of films strongly depends on the backing materials, such as reported by Zhao et al. [39] and Ma et al., [32] which limits their use to regular and non-fragile surfaces despite having excellent switching efficiencies.Another strategy is to utilize sol-gel transition of the material, as reported by Xu et al. [21] and Dompé et al., [28] which can also deliver a switching efficiency of almost 1.However, the material's reusability and continuous operation in an aqueous environment is limited due to the dissolution of the adhesive in the sol state.Wang et al. reported an adhesive system with extraordinary adhesive stress (130-340 kPa in terms of adhesive failure) based on anthracene dimerization controlled by light and temperature, [25] but such system has considerable residual adhesion in the low adhesion state (80-130 kPa) and thus much lower switching efficiency (0.35-0.6).This limits its use for release of lightweight and fragile objects.The present system using the channeling strategy thus has the advantage of 1) having high switching efficiency and adhesive strength; 2) remaining as a solid (but soft) material during switching that does not dissolve in water, ensuring its reusability and continuous operation; and 3) being a soft bulk material that can accommodate large deformations and irregular surface for the adhesion and release of fragile objects.
Compared to reported strategies to repel water from the interface, in particular by using surface microstructures, [36,46] the current strategy has the advantage that the channels are formed in the bulk of the material and are thus less prone to surface wear.Besides, the shape of the material is not limited to flat surface that is typically required for the replication of microstructures.On the other hand, the channeling strategy has certain limitations such as pre-stress to ensure the removal of water, relatively weak mechanical properties of the hydrogels (Figure S6

Adhesion in Dry State
In addition to underwater adhesion, the ch-dopa-PNIPAm also demonstrates efficient switchable adhesion to dry surfaces with switching efficiencies between 0.5 and 0.8 as exemplified in Figure 5.A similar protocol of probe-tack measurement was used compared to the underwater test, except that the substrate is pre-heated for HT measurement, as shown in Figure 5a.This protocol is used to unify the pre-stress duration, since heating up the substrate from RT will take several minutes, and to minimize the influence of hydrogel shrinking during heating that will influence the accuracy of force measurement.The hydrogel's maximum adhesive stress at RT is overall improved in the dry-state measurement compared to the underwater tests, reaching ≈1 kPa as shown in Figure 5b.This increase also corroborates the effect of water layer in decreasing the interfacial adhesion, as no excess water exist at the interface in dry state.On the other hand, the HT adhesion has been dramatically decreased compared to underwater test, with a reduction to below 0.5 kPa.This can be attributed to the fact that 1) water is expelled from the hydrogel probe's surface upon contact with a hot surface due to phase transition (shrinking), [49] which decreases the adhesion, [41] and 2) only a thin layer of the hydrogel at the contact point undergoes phase transition due to heating from the substrate, which leads to non-uniform deformation in the hydrogel and thus tension at the interface.Therefore, the maximum adhesive stress at RT is higher than that at HT, and switching is achieved in the reverse way compared to underwater conditions.
In addition, the dopa-PNIPAm also demonstrates comparable maximum adhesive stress, switching efficiency, and adhesion energy as ch-dopa-PNIPAm, as shown in Figure 5d,e.This can be explained by the lack of aqueous environment, where the effect of channels for water transport is strongly undermined.On hydrophobic PS surfaces in dry state (Figure S15, Supporting Information), the ch-dopa-PNIPAm shows much lower switching efficiency (0.2), where the maximum adhesive stress for HT and RT is 1.65 and 1.3 kPa, respectively.This can be again attributed to increased hydrophobic interactions   [28] c) Chitosan-catechol-PNIPAm conjugate based on sol-gel transitions. [21]d) Photo-and thermo-responsive adhesive based on anthracene dimerization. [25]e) Gecko-inspired structures coated with thermoresponsive catechol-containing copolymers. [32]f) Thermoresponsive thin films adsorbed via host-guest interactions. [39]214091 (8 of 12) © 2023 Wiley-VCH GmbH between the PS surface and PNIPAm at HT, increased the adhesion at HT compared to dry glass surface.

Soft Adhesive Gripper
The reasonably strong adhesion, highly efficient switching, and low residual adhesion make the ch-dopa-PNIPAm an ideal candidate for applications involving capture and release of objects underwater or in the dry state, particularly for soft and fragile surfaces.Figure 6 and Video S1 (Supporting Information) demonstrate the ability of ch-dopa-PNIPAm to capture objects at high adhesive state, transport, and release the objects upon switching to low adhesive state.Here, a similar sample geometry is used as in the case of probe-tack measurement, where a cylindrical piece of hydrogel is fixed inside a glass tube with a free end of 4 mm extruding from the tube that acts as the adhesive gripper.The hydrogel contained 10 wt.% NIPAm, 0.1 mol.%BIS, and 1 mol.%DPMA, and 0.5 wt.% agarose was used as the sacrificial template.Notably, a lightweight and fragile cargo with an uneven surface, i.e., a flower petal weighing 4.3 mg, can be captured by the hydrogel gripper in an aqueous environment at 40 °C.This is facilitated by the bulk material that allows large deformations at low stress to form conformal contact with the object.The petal can then be removed out of the water while being attached to the hydrogel, which is then transported and released in a water bath at RT upon gentle shaking (Figure 6a).Note that the same shaking does not cause detachment of the petal at 40 °C (Video S1, Supporting Information).The facile release of the petal at RT is achieved by the high switching ratio, which allows low residual adhesion and thus release of an intact petal.Furthermore, we demonstrate the switchable adhesion on biological tissues exemplified by the capture and release of a piece of chicken meat (110 mg) shown in Figure 6b.The meat could be adhered to the hydrogel gripper at 40 °C and readily released at RT in an aqueous environment without damage to it.The video of the capture and release process is shown in Video S2 (Supporting Information).
In addition to the light-weight objects, the adhesion of the gripper at 40 °C is sufficient to lift much heavier metal objects in a water bath, as demonstrated in Figure 6c.The hydrogel can be used to attach and lift different shapes of metal objects, up Adv.Funct.Mater.2023, 2214091 to a 2.1 g metal nut.objects can also be easily released in an RT water bath due to their heavy weight.The duration of the hydrogel gripper holding different objects before adhesive failure has been tested and summarized in Figure S16 (Supporting Information).In principle, the hydrogel can hold hydrophilic glass or hydrophobic PS samples for at least 18 h without adhesive failure, while the adhesion on metal surface lasted from few seconds to a few hours depending on the weight and the shape of the objects.
The gripper can also function in dry state due to the thermoresponsive switchability, as shown in Figure 6d.In this case, the hydrogel adhesion is higher at room temperature, allowing quick gripping of a fragile and curved eggshell weighing 1.7 g as demonstrated in Figure 6d and Video S3 (Supporting Information).When the hydrogel is heated using a heat gun, the eggshell detaches by its own weight without leaving residue of hydrogel on the shell.The switching of adhesion can be even applied solely at room temperature, by taking advantage of the difference in dry-state and underwater adhesion (Figures 2  and 5).For instance, a Teflon septum weighing 62 mg can be grabbed in dry state at room temperature and then released by placing it in a room-temperature water bath as shown in Figure 6e.
As demonstrated above, the ch-dopa-PNIPAm can be used as a versatile switchable adhesive for the capture and release of irregularly shaped and fragile objects, and in particular lightweight objects due to the high switching ratio.The switching of adhesive states can be achieved by temperature or the change from a dry to a wet state.The robust and repeatable and release thus promise great potential in, e.g., soft robotic and biomedical applications, [31] since the biocompatibility of PNIPAm and other types of hydrogels containing catechol chemistry has been well demonstrated in literature. [1,21,62,63]Furthermore, such system can be potentially controlled remotely by incorporating gold or Fe 3 O 4 nanoparticles in the hydrogel, which will allow light-induced photothermal heating. [32,37,49]

Conclusion
This work presents a new strategy to enhance the underwater adhesion properties in a PNIPAm hydrogel (ch-dopa-PNIPAm) copolymerized with bio-inspired dopamine monomers, by introducing channel structures formed upon removal of a sacrificial agarose network.The channel structures facilitate water transport away from the interface, which promotes close contact between the polymer network of the hydrogel and the substrate surface.In this way, the maximum adhesive stress on hydrophilic glass surfaces is enhanced six-fold compared to dopa-PNIPAm without channels.Highly efficient switching of the adhesion can be achieved by changing the temperature between 45 °C and RT, taking advantage of the LCST phase transition of the PNIPAm.The maximum adhesive stress at 45 °C can reach 5.3 kPa and the highest switching efficiency of 0.91 is achieved.On hydrophobic polystyrene surfaces, similar efficient switching and high adhesion are observed for ch-dopa-PNIPAm with a maximum adhesive stress of 6.2 kPa and switching efficiency of 0.83.Here the dopa-PNIPAm shows similar adhesive stress and switching efficiency compared to ch-dopa-PNIPAm due to the enhanced hydrophobic interaction between the hydrogel and substrate.In addition, the adhesion switching behavior can also be achieved on dry substrates, though the high adhesive state is achieved at RT instead of at 45 °C.The present system shows high adhesion strength and switching efficiency on par with the state-of-the-art underwater adhesive systems, yet possessing the unique properties of being a bulk and solid material that enables its application as a reversible and reusable adhesive on soft, fragile, irregular, and biological surfaces.A gripper made of the ch-dopa-PNIPAm is demonstrated, where capture and release of fragile and lightweight objects with repeatable switching can be controlled by temperature or wet-dry changes.The highly efficient switchable adhesion in the channeled hydrogel network promises great potential in, e.g., soft robotics, surgical, and biomedical applications.
Glass Silanization: Disposable culture tubes (6 × 50 mm lime glass, Fisher Scientific) were cut into 3 cm pieces and cleaned by sonication in deionized water for 10 min.After 5 min plasma treatment (Pico, Diener Electronic GmbH, Germany), the tubes were placed into a desiccator containing 3-(trimethoxysilyl)propyl acrylate (100 µl), and the desiccator was evacuated to a vacuum of 8 × 10 −2 Pa.The tubes were left under vacuum overnight for the gas phase deposition to take place, and the remaining silane on a watch glass was removed afterward.Finally, the desiccator was fully evacuated for 2 h at 1 × 10 −3 Pa, after which the silanized tubes were taken out and stored in a fridge.
PDMS Cap Preparation: Caps for the glass tubes were prepared using Sylgard 184 by thoroughly mixing a 10:1 ratio of base and curing agent.The mixture was placed in vacuum of 60 mbar for 1 h to remove bubbles and then poured into polystyrene cuvettes, where glass tubes were placed inside as molds.The cuvettes were placed into 70 °C oven for 2 h, after which the cured PDMS caps were taken out of the cuvettes.The PDMS caps were used to seal the glass tubes during polymerization of hydrogel.
Preparation of Dopamine-Containing Channeled Hydrogel: The PDMS caps were cleaned by sonication in deionized water for 10 min and dried using compressed air.One end of the glass tubes was closed by the caps, leaving room for the 4 mm probe end of the hydrogel.The assembly was stored under N 2 protection in a vial overnight, to remove the oxygen inside the PDMS.DPMA was protected using Borax following Yang et al. [50] Briefly, Borax (110.93 mg) was dissolved in water (2985 µl) and bubbled with N 2 .DPMA (29.85 mg) was dissolved into a mixture of ethanol (100 µl) and water (200 µl), which was added to the Borax solution.The solution was mixed under nitrogen for 2 h, and lyophilized.The product was used to prepare the monomer solution.
The monomer solution was prepared similarly to the previous report, [49] by dissolving agarose into water using heating and vortexing, which was used to dissolve NIPAm, Borax-protected DPMA, and the photoinitiator in a vial.The BIS was dissolved in water to form a 100 mm stock solution, which was added to achieve the desired concentration.To prepare 1 ml of monomer solution with 10 wt.% NIPAm, 0.1 mol.%BIS, 1 mol.%DPMA, 0.5 wt.% agarose, and 1 mol.%initiator, NIPAm (112.04 mg), borax-protected DPMA (9.84 mg), and Irgacure 2959 (2.44 mg) were weighed in a vial.Agarose (8.21 mg) was dissolved by heating and vortexing in water (1.454 ml), and the solution (990 µl) was used to dissolve the monomer and photoinitiator.The stock solution of BIS (9.90 µl) was added to this monomer solution.After dissolution, the monomer solution was bubbled with N 2 for 10 min.The solutions with a higher concentration of agarose were bubbled with N 2 in a water bath at 40 °C to prevent premature gelation of agarose.The degassed monomer solution was injected into the glass tubes under N 2 protection.The vials were then placed into a fridge at 4 °C for 30 min for gelation of agarose, and the monomer solution was then polymerized in a UV reactor (8 × 14 W lamps, 350 nm, Rayonet, USA) for 30 min.The caps were removed to expose the 4 mm free end of the gel.
The acid treatment of the gels to deprotect catechol groups was done by storing the samples in an HCl solution at pH 1 overnight, followed by washing at 40 °C for 10 min, and then storage at a pH 4 solution of HCl for 30 min.Finally, the hydrogels were washed at pH 4 by heating to 70 °C for 10 min and swelling at RT for 30 min twice to remove the agarose.The hydrogels were stored in a pH 4 solution overnight, and the adhesion was measured the following day.The hydrogels without catechol groups were prepared in the same way without addition of DPMA.The dopa-PNIPAm and standard PNIPAm were prepared using water instead of agarose solution.
FTIR Measurement: Hydrogels were first dried in air and then under vacuum overnight.The IR spectra were measured using Nicolet 380 (ThermoFisher Scientific, USA) with Smart Orbit Diamond ATR.The spectra were normalized for comparison.

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© 2023 Wiley-VCH GmbH Adv.Funct.Mater.2023, 2214091 UV-vis Measurement: The transmittance of a 1 thick hydrogel film was measured using a Cary 5000 (Agilent, USA) UV-vis spectrophotometer.Transmittance was recorded at 500 nm and the temperature was controlled by a dual cell Peltier sample holder.The transition temperature was determined as the temperature at which the transmittance reached 50%.
Tensile Testing: The hydrogel films for tensile tests were prepared between glass slides separated by Parafilm spacers with a thickness of 1 mm.The polymerization was carried under N 2 protection, and the hydrogels were treated as described above.The samples with a rectangular shape were heated to 40 °C before measurement to recover the as-prepared size (7 × 25 mm).The tensile testing was conducted at room temperature using Instron 5567 (Instron, USA) mechanical tester with a 100 N load cell and at a speed of 10 mm min −1 .
SEM Characterization: Hydrogels equilibrated at room temperature or 40 °C were frozen in liquid nitrogen and then freeze fractured.The frozen samples were dried in a freeze-dryer (FreeZone −105 °C 4.5 Liter Cascade Benchtop Freeze Dry System, Labconco, USA) overnight.The samples were sputter coated with 5 nm Pt/Pd using EM ACE600 (Leica, Germany).SEM Sigma VP (Zeiss, Germany) was used for imaging.
Porosity Analysis: The SEM images were converted to binary ones using the Threshold function of ImageJ (V 1.53t), and the resulting images were analyzed using the Analyze Particles function to calculate the Feret diameter.Feret diameter refers to the maximum caliper or the maximum distance between two points on the edge of the pore.Pores smaller than 200 nm in diameter were excluded from the analysis for clarity.More than 1000 pores were analyzed for each of the images.The fraction of surface area was defined as the fraction of the area occupied by pores of a certain size relative to the total area of the SEM image.
Adhesion test: Adhesion was measured using an Instron 5567 (Instron, USA) mechanical tester with a load cell of 100 N at a speed of 10 mm min −1 .pH 4 HCl solution was used in all measurements to prevent catechol oxidation and the formation of irreversible covalent bonding.The hydrogels were heated to 40 °C to recover the as-prepared size before measurement.The hydrogel was brought to contact with the substrate at RT to a compressive prestress equal to the Young's modulus of the gel.The contact time was 10 s.When measuring the underwater adhesion at HT, a small amount of water was added to the substrate surface before the experiment, and a large amount of 45 °C water was added to heat up the gel after contact.For room temperature measurements a similar amount of water was added before applying prestress.For measuring dry adhesion, a TMS94 temperature controller (Linkam, UK) was used to control the substrate temperature, which was kept constant throughout the measurement.Only the measurements of adhesive failure were included in the presented data.
Swelling Test: Hydrogel films (0.5 mm thick) were prepared according to the same protocol detailed above, and an 8 mm disk-shape piece of hydrogel was cut out and placed inside a capillary with pH 4 HCl solution.The capillary was heated at a rate of 0.2 °C min −1 using a TMS94 temperature controller (Linkam, UK), and images were captured using a digital camera.ImageJ (V 1.53t) was used to measure the size of the hydrogel.
Video and Photo Capture: Canon 80D with a Sigma 105 mm 1:2.8 DG macro HSM lens was used to capture the videos.VCL media player (3.0.16Vetinari) was used to extract images from the video.
and 3d-f.Similar to glass surface, the 0.2 wt.% BIS concentration delivers the highest adhesive stress on PS surface at HT, reaching σ H = 6.2 kPa.The RT adhesion Adv.Funct.Mater.2023, 2214091

Figure 2 .
Figure 2. Switchable underwater adhesion on glass surface.a) Representative stress-strain curves of probe-tack adhesion measurements of ch-dopa-PNIPAm.b) Maximum adhesive stress and corresponding switching efficiency of ch-dopa-PNIPAm prepared with different agarose concentrations.The line is to guide the eye.Data averaged from 5 measurements.c) Cyclic adhesion tests of ch-dopa-PNIPAm by temperature switching with average adhesive stress marked by the red (HT) and blue (RT) lines.d) HT and e) RT adhesion measurements of ch-dopa-PNIPAm with different cross-linker (BIS) concentrations.f) Maximum adhesive stress and switching efficiency of ch-dopa-PNIPAm with different cross-linker concentrations.The line is to guide the eye.Unless otherwise stated, the hydrogels were prepared using 10 wt.% of NIPAm, 1 mol.% of DPMA, 0.1 mol.% of BIS, and 0.5 wt.% of agarose.Error bars indicate standard deviations.* indicates statistical significance.n = 5, α = 0.05.*p < 0.05, **p < 0.01, ***p < 0.001, not significant (NS) p > 0.05.

Figure 3 .
Figure 3. Switchable underwater adhesion on hydrophobic polystyrene surface.a) HT and b) RT stress-strain curves of underwater probe-tack measurements using ch-dopa-PNIPAm hydrogels prepared with different agarose concentrations.c) Summary of the maximum adhesive stress and switching efficiency depending on agarose concentration.The line is to guide the eye.d) HT and e) RT stress-strain curves of underwater probe-tack measurements using ch-dopa-PNIPAm hydrogels with different cross-linker BIS concentrations.f) Summary of the maximum adhesive stress and switching efficiency depending on BIS concentration.The line is to guide the eye.Unless specified, the hydrogels were prepared using 10 wt.% of NIPAm, 1 mol.% of DPMA, 0.1 mol.% of BIS, and 0.5 wt.% of agarose.Error bars indicate standard deviations.* indicates statistical significance.n = 5, α = 0.05.*p < 0.05, **p < 0.01, ***p < 0.001, not significant (NS) p > 0.05.

Figure 6 .
Figure 6.Capture and release of various objects using the ch-dopa-PNIPAm gripper.a) Underwater capture and release of a flower petal (4.3 mg) at 40 °C and RT, respectively.b) Underwater capture and release of a piece of chicken meat (110 mg) at 40 °C and RT, respectively.c) Underwater capture of metal items with different weights at 40 °C.d) Capture at RT and release upon heating of an eggshell (1.67 g) in dry state.e) Capture of a Teflon septum (61.8 mg) in dry state and release in water, both at RT.All scale bars: 10 mm.