*2.3. The Effect of The Crosslinking Degree of PVA-SbQ Hydrogels on Hydrogel Adhesion, and Mechanical and Swelling Properties*

In addition to the F-T cycle and thermal-crosslinking, photo-crosslinking is another common method for preparing hydrogels. To prove the generality of the effect of crosslinking degree on hydrogel adhesion, the PVA-SbQ hydrogels with different degrees were prepared by adjusting the photo-crosslinking duration. Instead of the conventional F-T cycle or using chemical crosslinkers, PVA-SbQ hydrogels could be formed by a fast and facile photo-crosslinking method via the photodimerization of carbon–carbon double bonds (C=C bonds) on SbQ functional groups (Figure 3a,b) [33,34]. As shown in Figure 3c, the adhesion of a PVA-SbQ hydrogel sharply decreased with the increase in photo-crosslinking time. The increased photo-crosslinking time also reduced the swelling ratio of PVA-SbQ hydrogels, which suggested an increase in the crosslinking degree (Figure 3d) [30–32]. Conversely, the compressive strength and tensile strength steadily increased as the photo-crosslinking duration was increased (Figure 3e,f). As with the PAM hydrogels, the decrease in hydrogel adhesion primarily resulted from the limitation of segment mobility, and the enhanced mechanical properties were caused by the increase in the crosslinking degree. This proved that photo-crosslinked hydrogels also followed the aforementioned relationship between the degree of crosslinking and hydrogel adhesiveness.

### *2.4. The Strategies for Constructing Strong Hydrogel Adhesion*

In the study, three kinds of hydrogels were prepared by the freezing–thawing cycle, thermal-crosslinking and photo-crosslinking, respectively. The hydrogel adhesion was primarily generated by hydrogen bonding because of the amino and hydroxyl groups. In addition to the amino and hydroxyl groups shown in these hydrogels, carboxyl groups can also contribute towards hydrogel adhesion. Therefore, amino, hydroxyl or carboxyl groups are required to construct hydrogel adhesion generated by hydrogen bonding (Figure 4a). However, these functional groups should be free. For instance, the maximum adhesive strength of PVA-SbQ hydrogels is much higher than that of PVA hydrogels (Figure 4b). Although there were many hydroxyl groups in PVA hydrogels, they formed hydrogen bonding used for crosslinking with themselves rather than interacting with the functional groups on the substrate. As a result, these hydroxyl groups are not free and will contribute towards hydrogel adhesion. On the contrary, the fabricated PVA-SbQ hydrogel holds massive free hydroxyl groups as the crosslinking was mainly driven by photodimerization of SbQ groups instead of hydrogen bonds forming between hydroxyl groups. The advantage of hydrogen bond-triggered adhesion is that it is recyclable since the hydrogen bonding formation utilizes dynamic and noncovalent interactions. The cyclic stripping test demonstrated that the adhesive strength of the PVA-SbQ hydrogel did not reduce

significantly with the increase in the number of stripping cycles (Figure 4c). In addition to the free functional groups, it was found that the crosslinking degree also affected the hydrogel adhesion. The increase in the crosslinking degree of the hydrogel restricted the mobility of the polymer chains (Figure 4d). Hence, the polymer chains with functional groups were not quickly diffused towards the hydrogel surface to form intimate contact with the substrate. As a result, excessive crosslinking of hydrogels led to a reduced adhesive strength (Figure 4e). creased (Figure 2g). Conversely, while the increase in the crosslinker content improved the crosslinking degree, the compressive strength of PAM hydrogels did not increase significantly (Figure 2h). This is because the higher crosslinker content resulted in the crosslinking of PAM without complete polymerization. Accordingly, the length of PAM polymer chains that make up the hydrogel network were short, which resulted in the slight decrease in compressive strength. Therefore, it is challenging to enhance the mechanical and adhesive properties of hydrogels simultaneously.

time and crosslinker content, the hydrogel adhesion decreased. This observation implied that a higher crosslinking degree was not helpful for hydrogel adhesion. Unlike the PVA hydrogels, the free functional groups used for adhesion did not decrease in PAM hydrogels [1,2]. The decrease in hydrogel adhesion mainly resulted from the limitation of segment mobility. Although there were a lot of free amino groups on the PAM hydrogels, the amino group could not move to the hydrogel surface to interact with some functional groups on substrates as the crosslinking degree of PAM hydrogels increased. In addition, with the increase in crosslinking time and crosslinker content, the swelling ratio of PAM hydrogels simultaneously decreased (Figure 2e,f). The decreased swelling ratio further corroborated that the crosslinking degree of the hydrogel increased and a denser hydrogel structure was formed [30–32]. With the increase in crosslinking degree caused by increased crosslinking time, the compressive strength of PAM hydrogels gradually in-

*Gels* **2022**, *8*, x FOR PEER REVIEW 4 of 10

**Figure 2.** (**a**) The formation mechanism of PAM hydrogels. (**b**) The preparation process of PAM hydrogels. (**c**) The adhesive strength of PAM hydrogels prepared by different thermal-crosslinking times. (**d**) The adhesive strength of PAM hydrogels prepared with different crosslinker contents. (**e**) **Figure 2.** (**a**) The formation mechanism of PAM hydrogels. (**b**) The preparation process of PAM hydrogels. (**c**) The adhesive strength of PAM hydrogels prepared by different thermal-crosslinking times. (**d**) The adhesive strength of PAM hydrogels prepared with different crosslinker contents. (**e**) The swelling ratio of PAM hydrogels prepared by different thermal-crosslinking times. (**f**) The swelling ratio of PAM hydrogels prepared with different crosslinker contents. (**g**) The compressive strength of PAM hydrogels prepared by different thermal-crosslinking times. (**h**) The compressive strength of PAM hydrogels prepared with different crosslinker contents.

**Figure 3.** (**a**) The formation mechanism of PVA-SbQ hydrogels. (**b**) The preparation process of PVA-SbQ hydrogels. (**c**) The adhesive strength of PVA-SbQ hydrogels prepared by different photo-crosslinking times. (**d**) The swelling ratio of PVA-SbQ hydrogels prepared by different photo-crosslinking times. (**e**) The compressive strength of PVA-SbQ hydrogels prepared by different photo-crosslinking times. (**f**) The tensile strength of PVA-SbQ hydrogels prepared by different photo-crosslinking times. **Figure 3.** (**a**) The formation mechanism of PVA-SbQ hydrogels. (**b**) The preparation process of PVA-SbQ hydrogels. (**c**) The adhesive strength of PVA-SbQ hydrogels prepared by different photocrosslinking times. (**d**) The swelling ratio of PVA-SbQ hydrogels prepared by different photocrosslinking times. (**e**) The compressive strength of PVA-SbQ hydrogels prepared by different photo-crosslinking times. (**f**) The tensile strength of PVA-SbQ hydrogels prepared by different photo-crosslinking times. reduce significantly with the increase in the number of stripping cycles (Figure 4c). In addition to the free functional groups, it was found that the crosslinking degree also affected the hydrogel adhesion. The increase in the crosslinking degree of the hydrogel restricted the mobility of the polymer chains (Figure 4d). Hence, the polymer chains with functional groups were not quickly diffused towards the hydrogel surface to form intimate contact with the substrate. As a result, excessive crosslinking of hydrogels led to a reduced adhesive strength (Figure 4e).

stripping test demonstrated that the adhesive strength of the PVA-SbQ hydrogel did not

The swelling ratio of PAM hydrogels prepared by different thermal-crosslinking times. (**f**) The swelling ratio of PAM hydrogels prepared with different crosslinker contents. (**g**) The compressive strength of PAM hydrogels prepared by different thermal-crosslinking times. (**h**) The compressive

*2.3. The Effect of The Crosslinking Degree of PVA-SbQ Hydrogels on Hydrogel Adhesion, and*

In addition to the F-T cycle and thermal-crosslinking, photo-crosslinking is another common method for preparing hydrogels. To prove the generality of the effect of crosslinking degree on hydrogel adhesion, the PVA-SbQ hydrogels with different degrees were prepared by adjusting the photo-crosslinking duration. Instead of the conventional F-T cycle or using chemical crosslinkers, PVA-SbQ hydrogels could be formed by a fast and facile photo-crosslinking method via the photodimerization of carbon–carbon double bonds (C=C bonds) on SbQ functional groups (Figure 3a,b) [33,34]. As shown in Figure 3c, the adhesion of a PVA-SbQ hydrogel sharply decreased with the increase in photo-crosslinking time. The increased photo-crosslinking time also reduced the swelling ratio of PVA-SbQ hydrogels, which suggested an increase in the crosslinking degree (Figure 3d) [30–32]. Conversely, the compressive strength and tensile strength steadily increased as the photo-crosslinking duration was increased (Figure 3e,f). As with the PAM hydrogels, the decrease in hydrogel adhesion primarily resulted from the limitation of segment mobility, and the enhanced mechanical properties were caused by the increase in the crosslinking degree. This proved that photo-crosslinked hydrogels also followed the aforementioned relationship between the degree of crosslinking and hydrogel adhesiveness.

strength of PAM hydrogels prepared with different crosslinker contents.

*Mechanical and Swelling Properties*

**Figure 4.** (**a**) The functional groups required for hydrogel adhesion generated by hydrogen bonds. (**b**) The maximum adhesive strength of PVA and PVA-SbQ hydrogels. (**c**) The adhesive strength of PVA-SbQ hydrogels under different stripping cycles. (**d**) The effect of crosslinking degree of hydrogels on chain movement. (**e**) The effect of crosslinking degree on hydrogel adhesion. **Figure 4.** (**a**) The functional groups required for hydrogel adhesion generated by hydrogen bonds. (**b**) The maximum adhesive strength of PVA and PVA-SbQ hydrogels. (**c**) The adhesive strength of PVA-SbQ hydrogels under different stripping cycles. (**d**) The effect of crosslinking degree of hydrogels on chain movement. (**e**) The effect of crosslinking degree on hydrogel adhesion.

### **3. Conclusions**

In this study, we investigated PVA, PAM and PVA-SbQ hydrogels fabricated by the freezing–thawing cycle, thermal-crosslinking and photo-crosslinking, respectively. These three types of hydrogels present representative models of hydrogels formed by different crosslinking mechanisms and extend the observations and derived conclusion to a wide class of materials. The hydrogel adhesion capability decreased with the increase in the crosslinking degree of these hydrogels. This is because a higher crosslinking degree would limit segment mobility. As a result, functional groups on polymer chains could not move to the hydrogel surface to interact with the substrate to generate hydrogel adhesion. Therefore, in addition to free functional groups used to interact with substrates, the flexibility of polymer chains that make up hydrogels is vital for hydrogel adhesion. However, the decrease in crosslinking degree in order to increase the availability of free functional groups and higher segment mobility would result in poor mechanical strength of the hydrogels and limit their functionality. Therefore, it is essential as well as challenging to maintain the balance between hydrogel adhesion and mechanical strength. However, a rational design of a hydrogel crosslinked network and free functional groups can allow better control over these hydrogel characteristics.
