Design of large-span stick-slip freely switchable hydrogels via dynamic multiscale contact synergy

Solid matter that can rapidly and reversibly switch between adhesive and non-adhesive states is desired in many technological domains including climbing robotics, actuators, wound dressings, and bioelectronics due to the ability for on-demand attachment and detachment. For most types of smart adhesive materials, however, reversible switching occurs only at narrow scales (nanoscale or microscale), which limits the realization of interchangeable surfaces with distinct adhesive states. Here, we report the design of a switchable adhesive hydrogel via dynamic multiscale contact synergy, termed as DMCS-hydrogel. The hydrogel rapidly switches between slippery (friction ~0.04 N/cm2) and sticky (adhesion ~3 N/cm2) states in the solid-solid contact process, exhibits large span, is switchable and dynamic, and features rapid adhesive switching. The design strategy of this material has wide applications ranging from programmable adhesive materials to intelligent devices.

The chemical characterization of poly (DMA-PFOMA) was tested by a FTIR spectrometer (Nicolet iS10, Thermo Scientific, USA). In the FTIR spectra, the characteristic peak at 2974.35 cm -1 , was attributed to the characteristic absorption of stretching vibration -CH3. The peak at 1750.68 cm −1 corresponded to the stretching vibration of C=O. Moreover, the peak at 1212.70 cm −1 and 1179.15 cm −1 were assigned to the stretching vibration of -CF3 and C-F bond. It is proved that the polymer has been successfully synthesized.

The Characterization of 4-(1-pyrenyl) butyl methacrylate Supplementary Fig. 3 | 1 H-NMR spectra and 13 C NMR spectra of 4-(4,8dihydropyren-1-yl)butyl methacrylate in DMSO-d6.
The chemical structure of it was characterized by 1 H NMR (left) and 13 C NMR (right) analysis using a Bruker AVANCE III HD NMR spectrometer. The above results proved the successful preparation of the monomer.

Effect of the covalent crosslink density of DMCS-hydrogels Supplementary Fig. 4 | Characterization of hydrogel microscopic morphologies.
To study the effect of crosslinking degree on modulus, we prepared hybrid gels with various concentrations of the crosslinker Bis. Scanning electron microscope images are hydrogels with (left) 0.0133 wt% Bis, (middle) 0.0200 wt% Bis, (right) 0.0266 wt% Bis. As the concentration of Bis increased, the crosslink density of the network increased. Hydrogels with higher crosslinking density had smaller pores. The corresponding elastic modulus of DMCS-hydrogels. The stress-strain curves of DMCShydrogels were measured using an electrical universal material testing machine with a 500 N load cell (EZ-Test, Shimadzu). The concentration of Bis did greatly affect the elastic modulus of DMCS-hydrogels. The trend is as follows. As the concentration of Bis increased, the elastic modulus of the hydrogel increased. The hydrogel with lower crosslinking density (0.0133 wt% Bis) has a higher fracture strain of 625% and elastic modulus of 37 kPa. This means that the elastic modulus can be adjusted by the degree of crosslinking. Error bars represent the standard deviation from at least three replicates. Data in (b) are presented as mean values ± SD.

Relationship between modulus and contact
Supplementary Fig. 9 | The contact process between hydrogels (different modulus) and quartz glass. (a) Schematic diagram of contact evolution. For soft adhesive hydrogels, the actual contact is a process in which the contact area gradually increases with the extension of contact time. At the beginning, only a small number of sites can be in contact with the substrate. With the increase of contact time, there are more and more contact sites, and the contact area increases gradually. In this process, the influence of modulus on contact process cannot be ignored and is very important. Supplementary figures demonstrate the contact process between hydrogel (different modulus) and quartz glass, showing the contact conditions (b) at 0s, 0.1s, 0.4s, and 4s, and the relationship between proportion of the contacted area (CA) and time. Dark blue is the part that has been touched, and light blue is the untouched part. Here, the contact area of 0.1s is defined as the initial contact area. (c) It can be seen that the low-modulus and high-modulus hydrogels have the same initial contact area (0.1 s contact), but the low-modulus hydrogel had a faster contact rate. The above experimental results confirm that the larger the modulus is, the slower the contact speed is. v: contact velocity; S0,1,2: contact area. Low M: Low Modulus; High M: High Modulus.

Relationship between roughness and contact
Supplementary Fig. 10 | The contact evolution between hydrogels (different roughness) and quartz glass. (a) Schematic diagram of contact evolution. For the high-roughness hydrogel, the average distance (L1) between the hydrogel surface and the substrate is large, resulting in insufficient contact (Z1, Z2). However, if the roughness is low, the distance (L2) is small, there are more contact sites at the beginning (Z3, Z4, Z5), and the contact area is large. Supplementary figures demonstrate the contact process between hydrogel (different roughness) and quartz glass, showing the contact conditions (b) at 0s, 0.1s, 0.4s, and 4s, and the relationship between proportion of the contacted area (CA) and time. Dark blue is the part that has been touched, and light blue is the untouched part. Here, the contact area of 0.1s is defined as the initial contact area. (c) At 0.1s, the sample with high roughness almost has no contact with the substrate (~0.15%). Conversely, low-roughness hydrogel has a sizable initial contact area (~10%), which is in sharp contrast to the high-roughness hydrogel., indicating that roughness determined initial contact area. L1,2: distance between the hydrogel surface and the substrate; Z1,2,3,4,5: initial contact area between the hydrogel surface and the substrate. Low R: Low Roughness; High R: High Roughness.

Dynamic wettability of DMCS-hydrogel
Supplementary Fig. 13 | Confocal laser scanning microscope images under low and high temperature conditions. At low temperature, the hydrogel showed strong fluorescence, but at high temperature, the fluorescence disappeared. Scale bar 100μm. The reversible cycling performance of DMCS-hydrogel does not weaken after 50 cycles. Among them, in order to prevent the hydrogel from losing water, the hydrogel was placed in water for 5 s to restore each 10 cycles, and then kept at 10 o C temperature and 60% humidity for 6 h. The contact angle is smaller at high temperature than at high temperature. Water droplet spreads faster on the high-temperature DMCS-hydrogel than on the low-temperature. Iron sheet and hydrogel were heated by silicone rubber heating sheet. The hydrogel was cut into strips and adhered to iron sheets, and the adhesive area is about 1cm 2 . The crosshead velocity was maintained at 100 mm/min. At low temperature, the sample was fully stretched, and DMCS-hydrogel was still firmly attached to the iron sheet. However, after heating the iron sheet with silicone rubber sheet, the adhesion force became very weak. The sample slips off the surface of the iron sheet with little stretch. F: force.

Demonstration of the adhesion strength between DMCS-hydrogel and iron sheet
Supplementary Fig. 28 | Adhesion of DMCS-hydrogel to iron sheet. The photos show the process of lifting the iron sheet at low temperature by DMCS hydrogel and the process of drop due to insufficient adhesion at high temperature. At low temperature, hydrogel can adhere to the iron sheet after being stretched. When heating the iron piece with the heating piece, the hydrogel underwent a phase change and the adhesion gradually decreased, so the iron piece fell straight down.

Switchable between sticky and slippery state
Supplementary Fig. 29 | Switching between adhesion and lubrication at high and low temperatures. (a) At low temperature, DMCS-hydrogel can be stretched to a very long length without de-bonding, which indicates that its adhesion is very strong. (b) However, at high temperature, the strategy of dynamic multiscale contact results in the hydrogel sliding on its surface. Once the low-temperature gel came into contact with the high-temperature iron sheet, the transition occurred in less than 1s. F: force.

Comparison of DMCS-hydrogel and other switchable adhesive materials Supplementary Fig. 30 | Literature research on the switchable adhesive materials.
In order to prove the superiority of the sample we designed and prepared, DMCShydrogel (yellow area) was compared with a verity of other switchable adhesive materials. This figure demonstrates the high and low adhesion strength of them, which are divided into interface adhesive materials (orange area) and non-interface adhesive materials (blue area).

Supplementary Table 2. Comparison of self-prepared hydrogel materials with other switchable adhesive materials.
Please note that the "--" symbol means "not mentioned in the article".