Enhanced Strength for Double Network Hydrogel Adhesive Through Cohesion‐Adhesion Balance

Hydrogel adhesives exhibit great potential in various biomedical fields such as tissue sealing and soft robotics. However, the high‐water content and defective network structures of these hydrogel adhesives result in low intrinsic mechanical strength, severely impeding their application. In this study, it is reported that the strong hydrogel adhesive strength can be achieved when a balance is established between adhesive forces and cohesive forces. Based on this principle, a new double network (DN) design is created to combine an adhesive polyvinyl alcohol (PVA) as the first network with a flexible sodium alginate (SA) as the second network. A delicate balance is achieved between cohesion and adhesion by adjusting the ratio between the first rigid adhesive network and the second flexible network. As a result, this balanced DN hydrogel adhesive exhibits a strong tissue adhesion strength, approximately three times higher than that in the non‐balance situation.


Introduction
[3] In recent years, hydrogel-based adhesives have garnered significant attention due to their excellent adhesion, tissue-like viscoelasticity, remarkable biocompatibility, etc. [4][5][6][7] The flexible DOI: 10.1002/adfm.202313322network structure of hydrogel adhesives is conducive to its exchange of substances with organisms, and it provides a range of capabilities for loading, releasing, and adsorption. [8,9]These hydrogel adhesives can not only seal wounds but also create a favorable moist microenvironment for injured tissue, thereby promoting wound healing. [10,11]14] However, the hydrogel adhesives' inherent high water content and defective network structures normally lead to low intrinsic mechanical strength.As a result, their ability to effectively seal larger tissue defect areas or maintain strong adhesion in load-bearing regions is severely limited. [15,16]t is widely accepted that the adhesive strength of adhesives is influenced by two distinct factors: cohesive force and adhesive force. [17,18]Cohesive force refers to the internal force that holds the hydrogel network together, while adhesive force refers to the bonding strength between the hydrogel adhesive and the surface of the adhered tissue. [18]The adhesive failure of hydrogel can Scheme 1. Illustration of the preparation of PSTC adhesive DN hydrogels.
sometimes be attributed to either the rupture of interfacial adhesion or cohesive failure caused by poor mechanical stability. [19]22][23][24] Nonetheless, such endeavors of solely improving either of these aspects while neglecting the other have been shown to result in limited improvements in hydrogel adhesive performance. [19,25,26]ence, we are speculating whether the optimal adhesive strength could be achieved when simultaneously considering and balancing both the cohesive force within the hydrogel adhesive and its surface adhesive force.
To achieve the simultaneous regulation of the cohesive and the adhesive force, the double network (DN) hydrogel design would be a desired candidate. [27,28]Previous research on DN hydrogels has mainly focused on how to adjust the proportion and crosslinking density between the two networks to control mechanical properties. [29]However, we believe that the inherent flexibility in the composition of DN hydrogels allows us to selectively match the two networks with distinct properties.This, in turn, enables us to strike a balance between cohesion and adhesion, leading to optimized adhesive strength.Following this design principle, we have fabricated a novel DN hydrogel adhesive by a two-step technique (Scheme 1).In the first step, a hydrogen bond crosslinked polyvinyl alcohol (PVA) rigid network hy-drogel was formed through three freeze-thaw cycles.In the second step, natural polyphenolic tannic acid (TA) was introduced to confer excellent adhesive properties onto the 1st PVA network.Simultaneously, the 2nd flexible sodium alginate (SA) network was crosslinked by calcium ions (Ca 2+ ), which imparts excellent cohesion properties to the DN hydrogel adhesives.By adjusting the PVA and SA ratio, a cohesion-adhesion balance of DN hydrogel adhesive was achieved, resulting in much enhanced hydrogel adhesive strength.The adhesion strength of this newly constructed DN hydrogel achieved a threefold enhancement despite the reduced content of the PVA adhesion network.Moreover, the presence of TA bestows antibacterial properties with this novel hydrogel adhesive, a feature of paramount importance in clinical applications.

Preparation of PSTC Hydrogels
As illustrated in Figure 1a, the cohesion-adhesion balanced DN hydrogel adhesives based on PVA/SA/TA/Ca 2+ (PSTC) system were fabricated through a two-step technique, encompassing two heterogeneous networks formed by hydrogen and ion bonding, respectively.In the first step, the mixture solution of PVA and SA undergoes three repeated cycles of freezing and thawing, resulting in the formation of a rigid single network (SN) hydrogel crosslinked by dense hydrogen bonds.The PVA chains tend to aggregate during temperature variations due to their high molecular weight and densely ordered hydroxyl structure. [30]During the freezing process, the crystallization of water induces a phase separation between the polymer and the solvent. [31]In addition, the expansion caused by the formation of ice crystals further compresses the PVA molecular chains, bringing them closer together, and facilitating hydrogen bonding. [32]During the thawing process, the molecular movement of the PVA chains accelerates in accordance with the laws of thermodynamics. [33]This molecular movement causes the previously compressed and aggregated PVA chains to re-expand, ultimately creating a network structure dominated by hydrogen bonding. [34]In the second step, hydrogels were immersed in a treatment solution containing TA and Ca 2+ .Here, a single Ca 2+ ion can crosslink four L-guluronic (G) units from the SA chains, forming an "egg-box" ion-binding structure, which produces the second interpenetrated hydrogel network. [35]Simultaneously, TA acts as a crosslinking agent and fosters additional bonding, leading to the development of a denser network structure. [36]he abbreviation PS is employed to denote the PVA/SA SN hydrogel, while PSTC signifies the hydrogel formed postimmersion in the second step.The mass content of PVA and SA in all hydrogels was maintained at 10%.Specifically, PS-0/PSTC-0, PS-1/PSTC-1, PS-2/PSTC-2, and PS-3/PSTC-3 correspond to compositions comprising PVA (10%) + SA (0%), PVA (9%) + SA (1%), PVA (8.5%) + SA (1.5%), and PVA (8%) + SA (2%) respectively.In addition, the PST and PSC represent PS hydrogels that were post-immersed in TA or Ca 2+ treatment solution only.As shown in Figure 1b, with the increase in freeze-thaw cycles of PS hydrogels, storage modulus (G′) gradually rises, while loss mudulus (G″) decreases which indicates that more freezethaw cycles create denser hydrogen bonding among the hydroxyl groups of the PVA side chains. [37]For PS-0 hydrogel, which has the highest PVA content, it leads to the formation of the most hydrogen bonds.However, during the initial three freeze-thaw cycles, physical entanglement between SA polysaccharide chains with the PVA framework and the high viscosity of SA itself contribute more to the increase in the G' of PS-1, PS-2, and PS-3.Therefore, after the first and second freeze-thaw cycles, the G′ of PS-0 hydrogel is lower than that of the other PS groups.However, when the number of freeze-thaw cycles exceeds three, the hydrogen bonds crosslinked by the higher quantity of PVA in PS-0 contribute more to the increase in G′ compared to the uncrosslinked SA in other groups.Consequently, after the 4th and 5th freeze-thaw cycles, the PS-0 hydrogel exhibits a higher G′.However, this implies that more hydroxyl groups are employed for internal crosslinking rather than binding with TA.As shown in Figure 1c, with the increase in freeze-thaw cycles or SA content, the amount of TA that can be immobilized by PSTC hydrogels diminishes progressively.This change potentially impairs the adhesive strength at the interface.To immobilize as much TA as possible while ensuring the complete molding and demolding of the hydrogel, a three-cycle freeze-thaw process was chosen as the standardized procedure for the preparation of PS and PSTC hydrogels.As shown in Figure 1d, the G′ gradually decreased from 1572 Pa in PS-0 to 927 Pa in PS-3 with the increase in SA content of PS hydrogels.After the second immersion step, the G′ of all PSTC hydrogel groups increased to different degrees.This is due to the additional crosslinking density provided by TA and the formation of the second network through Ca 2+ crosslinking.As shown in Figure 1e, the equilibrium swelling ratio of PSTC-0 to PSTC-3 hydrogels increased with the SA content.This is because the total crosslinking density in the interpenetrating polymer network is lower if the PVA content is lower.Although PSTC hydrogels act as a DN system, this slightly flexible network allows the polymer chains to be freer, which reduces the elastic restraint.As a result, the hydrogel can swell further and absorb more water, leading to a higher swelling capacity.Scanning electron microscope (SEM) images show that the freeze-dried PS-1 and PSTC-1 hydrogels have a comparatively uniform and dense porous structure (Figure 1f,g).The PS-1 hydrogel exhibits a microporous structure (Figure 1f).However, the PSTC-1 DN hydrogel has a considerably denser porous structure, attributed to the second network, and higher crosslinking density resulting from the second immersion step (Figure 1g).

Mechanical Properties of PS and PSTC Hydrogels
The inherent mechanical properties of hydrogel adhesives play a crucial role in determining their sealing performance. [20,38]Variations in mechanical properties have a significant impact on the balance between cohesion and adhesion, further determining the strength and durability of the adhesive bond. [18,19]Specifically, maximizing the ability of hydrogel adhesives to withstand mechanical stresses before adhesive failure is a key principle in the design of effective wound tissue sealing materials. [5,39]Moreover, hydrogels with higher compression and tensile strength are better equipped to withstand shear and peel forces, enabling them to maintain their adhesive properties, and provide durable and effective sealing. [15,21]In the PSTC DN hydrogel system, the excellent mechanical properties are attributed to the interpenetrated physical and ionic bonding DN structure formed by the two-step technique.First, the DN structure of PSTC hydrogels provides them with excellent energy dissipation and shape maintaining capabilities under large deformation conditions.Second, the cohesion of the interpenetrating DN can be further tuned by adjusting the ratio of PVA/SA to achieve a balance with the adhesion force.Currently, there is no readily available instrument to measure cohesion for hydrogel adhesives.Normally, cohesion is considered as the internal strength of the hydrogel for holding the network together. [19]The tensile test applies axial tension to a sample body until it fractures, and cohesion is evaluated based on its tensile strength and toughness.Therefore, we have chosen Tensile strength as the indicator for cohesion assessment.
As shown in Figure 2a,d, the tensile stress, strain, and toughness of the PSTC hydrogel were significantly improved by increasing the percentage of SA components.The tensile strength of PSTC-1 increased almost threefold from 73.79 to 210.61 kPa compared to PSTC-0.As the proportion of SA components increased from PSTC-1 to PSTC-3, the tensile limit stress, strain, and toughness of the PSTC DN hydrogels were further enhanced.The tensile toughness increased progressively from 39.86 kJ m −3 in PSTC-0 to 435.97 kJ m −3 in PSTC-1, 527.22 kJ m −3 in PSTC-2, and 665.46 kJ m −3 in PSTC-3, respectively.This mechanical enhancement of PSTC hydrogel can be attributed to the formation of an interpenetrating DN structure.In this case, a dense network predominantly composed of PVA/TA provides the hydrogel with an effective energy-dissipating structure.This structure provides sacrificial bonds during deformation, thereby increasing the energy required for crack extension.Meanwhile, the flexible network primarily composed of SA/Ca 2+ helps alleviate stress concentration while preserving the hydrogels' integrity.This, in turn, raises the threshold for crack extension.The synergistic interaction between these two networks provides both an energy-dissipating and shape-maintaining structure, enhancing the mechanical properties of the PSTC DN hydrogels.Figure 2b,e demonstrates that the alteration in hydrogel mechanical properties with the inclusion of SA components in the compression test follows a similar trend to that observed in the tensile test.The compression strength increased from 6.18 MPa in PSTC−0 to 19.01 MPa in PSTC-1, 22.42 MPa in PSTC-2, and 2.43 MPa in PSTC-3, respectively.In addition, the compressive toughness increased from 0.49 MJ m −3 in PSTC-0 to 1.77 MJ m −3 in PSTC-3.At the same time, the compressive strain increased gradually from 73.63% in PSTC-0 to 93.60% in PSTC-3.It is evident that the flexible and interpenetrating SA network significantly contributes to the deformation capacity of the PSTC hydrogel.This DN structure enhances the mechanical properties with less chain breakage.Furthermore, this advantage also proves beneficial for the anti-fatigue performance of the hydrogel.To assess the antifatigue properties of PSTC hydrogels, a compression limit of 80% of the limiting strain was applied to both the PSTC-0 and PSTC-1 groups respectively.Following 20 compression cycles, the compression stress of PSTC-0 decreased from 0.97 to 0.62 MPa (a reduction of 36%) (Figure 2c).Conversely, as shown in Figure 2h, the compression stress for PSTC-1 only slightly decreased from 1.36 to 1.22 MPa (a reduction of 10%).This substantial disparity highlights the exceptional fatigue resistance of the PSTC DN hydrogel.Figure 2g showcases the impressive mechanical properties of the PSTC-1 DN hydrogel.A strip-shaped hydrogel sample, with dimensions of 6 cm in length and 2 mm in diameter, exhibited a stretch of over 18 cm in length.Even after being knotted, it retained the ability to be stretched to nearly three times its original length without fracturing.Furthermore, the knotted hydrogel exhibited exceptional strength, capable of lifting 0.5 kg water without breaking.Through the construction of a DN structure, the PSTC hydrogels are able to achieve a signif-icant enhancement in toughness and anti-fatigue strength, leading to an improvement in cohesion.

Adhesive Properties of PSTC Hydrogels
Optimal adhesion strength is achieved when the hydrogel adhesive reaches a balance between cohesion and adhesion near the interface.Moreover, the rheological properties of hydrogels do influence their adhesive capabilities.Typically, hydrogels with higher G′ tend to display weaker adhesion.This occurs because high elasticity restricts the flexibility of polymer chains, leading to adhesive functional groups on polymer chains having fewer opportunities to connect with the substrate.However, in this study, the G′ for PSTC exhibited only minor variations among the groups, suggesting that they do not play a dominant role in adhesive performance changing (Figure 1d).Much like dopamineadhesives inspired by mussels, the adhesive capability of this PSTC hydrogel primarily stems from the inherent polyphenolic structure of TA.This structure facilitates the formation of strong hydrogen bonds with the tissue surface.Moreover, the quinonelike structure that emerges from the oxidation of TA forms covalent bonds with nucleophilic groups, such as -NH 2 or -SH, found on the tissue surface. [36,40]As evident from Figure 1c, the adjustment of the PVA/SA ratio alone effectively regulates the amount of immobilized TA, subsequently allowing for the tuning of adhesion strength in the PSTC DN hydrogel.As shown in Figure 3a, the PSTC hydrogels exhibit robust adhesion to a variety of tissues and materials, including bone, tendon, heart, muscle, glass, and plastic.Particularly noteworthy is their potent adhesive bond to skin, rendering them exceptionally suitable for application as tissue adhesives.To evaluate the adhesive performance of the PSTC hydrogels, lap shear tests were performed using porcine skin as the adherents.This method has been widely adopted in the literature to assess the adhesion properties of bio-materials and is particularly suitable for soft tissue adhesives.The use of porcine skin as a model substrate also provides a relevant and realistic simulation of in vivo conditions, as it closely mimics the mechanical and biochemical properties of human skin.The hydrogel samples were prepared in a square shape with a side length of 1.2 cm and were placed between two porcine skin tissues.As shown in Figure 3b, the lap shear strength of the PSTC-0, 1, 2, and 3 hydrogels was measured as 14.963, 16.916, 3.627, and 2.292 kPa, respectively, following 10 min of contact time.Furthermore, compared to PSTC-1, 2, and 3 hydrogels, the lap shear stress of PSTC-0 hydrogel reaches its limit earlier and subsequently drops rapidly.This is due to the higher TA content imparts a good tissue adhesion ability to PSTC-0.However, the excessive stiffness of the pure PVA hydrogel network leads to inferior mechanical properties, particularly a lower limit strain (Figure 2b,c).In the Lap shear test, the excessively high adhesion at the rigid interface of PSTC-0 cannot match the brittle cohesion, inevitably leading to adhesive failure at low strains.Moreover, this result also can be attributed to a greater proportion of SA components facilitating the construction of a more flexible and tough DN structure, consequently improving the limit adhesive strain.In addition, the lap shear strength of all PSTC hydrogels increased as the contact time extended from 10 min to 1 h, and then to 24 h (Figure 3b,c,e).Especially, the PSTC-1 DN hydrogel achieved the highest lap shear strength of 75.08 kPa through the balanced cohesion-adhesion structure after 24 h of contact time (Figure 3f).However, for the PSTC-0 hydrogel, the adhesive force on the surface surpasses the cohesive force.As a result, the interior of the adhesive is prone to fracture during the lap shear process, leading to a reduction in overall adhesion strength.For PSTC-2 and PSTC-3 DN hydrogels, too much SA results in excessive cohesive force.At the same time, too little PVA content results in insufficient adhesive force, leading to a significant reduction in macroscopic adhesion strength.Compared to the other PSTC hydrogels, PSTC-3 exhibited the lowest adhesive stress after 24 h of contact, measuring only 14.24 kPa (Figure 3e).
As shown in Figure S1 (Supporting Information), variations in testing methods result in a lack of complete alignment in the measured values of tensile strength and adhesive strength for PSTC hydrogels.However, the changing trend of PSTC, PST, and PSC hydrogels reveal the cohesion-adhesion balance of PSTC-1 hydrogel.Similar to PSC hydrogels, the presence of the DN structure in PSTC hydrogels leads to an increase in tensile strength (Figure S1a,c, Supporting Information).In contrast to PST hydrogels, the lap shear strength of PSTC hydrogels exhibits an initial increase followed by a decrease (Figure S1a,b, Supporting Information).This phenomenon can be attributed to the balance between gradually increasing cohesive forces and decreasing adhesive forces within the PSTC hydrogel.In the PSTC-1 DN hydrogel adhesive, the adhesion network, consisting of PVA, and TA, delivers a robust interfacial adhesive force.At the same time, the DN structure, comprising a flexible crosslinked SA network, and a tightly crosslinked PVA network, significantly enhances the cohesive force of the hydrogel.The synergistic balance between these two forces plays a pivotal role, resulting in an overall enhancement of adhesive strength and the manifestation of a hysteretic adhesive failure behavior.Interestingly, the adhesive of PSTC-1 hydrogel demonstrates a unique "filamentary" phenomenon during the lap shear test, as shown in Figure 3d.This macroscopic observation confirms the cohesion-adhesion balance within PSTC-1 hydrogel.

Cytocompatibility of PSTC Hydrogels
In addition to its excellent adhesive properties, PSTC DN hydrogel also demonstrates high cytocompatibility, derived from the use of intrinsic biosafe raw materials.To assess the cytotoxicity, AlamarBlue measurements, and Live/Dead staining assays were performed on HaCaT cells.As shown in Figure 4a, the PSTC-1 DN hydrogel did not exhibit any toxicity toward the HaCaT cells at 25% and 50% concentrations, and only a slight inhibition of cell proliferation was observed at 100% concentration at day 1.However, the PSTC-0 hydrogel showed slightly poorer cytocompatibility at 50% concentration on days 1 and 3, with a marked restriction of cell proliferation at the 100% concentration.In general, the cytocompatibility analysis revealed that both PSTC-0 and PSTC-1 hydrogels exhibited greater biocompatibility compared to the TA-only control.Fluorescence images of HaCaT cells cultivated with the control, PSTC-1, and PSTC-0 hydrogel extraction solutions were acquired (Figure 4b), and these images were further analyzed to examine the vitality and morphology of the cells.The findings demonstrated that the co-cultured HaCaT cells after 1 and 3 days exhibited desirable cell shape and displayed no signs of cell death.In addition, the rate of cell growth in the control group was significantly slower than the growth rate of cells in the PSTC-0 and PSTC-1 groups, which aligns with the results of the AlamarBlue test.In consideration of practical skin adhesion scenarios extending beyond keratinocytes alone, we conducted an experiment employing NIH-3T3 fibroblast cells to evaluate the cytocompatibility of PSTC hydrogels.As shown in Figure S2a (Supporting Information), the PSTC hydrogel demonstrates a cytocompatibility pattern with NIH-3T3 cells similar to what was observed with HaCaT cells.Despite the slower proliferation rate of NIH-3T3 cells, PSTC-1 hydrogel consistently sustains cell viability above 80% after 1 and 3 days of co-culture.Additionally, NIH-3T3 cells exhibit characteristic spindle or star-shaped flattened structures (Figure S2b, Supporting Information).The results reveal that PSTC hydrogels exhibit strong cytocompatibility with both HaCaT and NIH-3T3 cells, without causing a significant impact on cell viability and proliferation.

Antibacterial Properties of PSTC Hydrogels
In addition to exhibiting remarkable adhesion, mechanical toughness, and cytocompatibility, PSTC hydrogels also demonstrate excellent antibacterial properties.These inherent properties of PSTC hydrogels enable them to hinder the growth of germs near the wound surface.This, in turn, alleviates the proinflammatory state, and renders them valuable in preventing wounds infections. [41]To assess the antibacterial efficacy of the PSTC hydrogels, a spread plate experiment was conducted to evaluate the inhibitory effect of PSTC-0 and PSTC-1 hydrogels on the growth of Escherichia coli (E.Coli), a common gram-negative bacterium.The images presented in Figure 5a were obtained by co-culturing PSTC-0 and PSTC-1 with E. coli for 1 and 24 h, respectively.After 1 h of incubation, the blank plate showed a significantly higher number of colonies than the PSTC-0 and PSTC-1 groups.However, the PSTC-1 group demonstrated notable acute antibacterial properties, completely eliminating all bacteria on the plate surface after only 1 h of co-culture.In contrast, a small number of colonies remained on the PSTC-0 plate surface.These results were further confirmed by the subsequent colony forming units (CFU) experiment.As shown in Figure 5b, the PSTC-0 hydrogel exhibited notable antibacterial effects after a 24 h coculture period, resulting in an ≈88.5% reduction in the E. coli population.In contrast, the PSTC-1 hydrogel displayed superior efficacy by completely eradicating E. coli.When the co-culture pe-riod was extended to 48 h, both PSTC-0 and PSTC-1 hydrogels demonstrated highly potent antibacterial activity, resulting in the complete elimination of E. coli.These results highlight the potential of PSTC hydrogels as promising antimicrobial bioadhesives.

Conclusion
In summary, we have developed a new strategy for constructing a DN hydrogel adhesive.By adjusting the ratio between the first PVA rigid adhesive network and the second SA flexible network, a balance between cohesion and adhesion was achieved.This balance resulted in an approximately threefold enhancement in the adhesive strength of the hydrogel, accompanied by a hysteretic adhesive failure behavior observed during lap shear tests.Moreover, it achieved robust antibacterial performance and excellent biocompatibility, providing a favorable healing environment for sealed wounds.This cohesion-adhesion balance strategy has successfully guided the construction of a new double network hydrogel with high adhesion strength.

Figure 1 .
Figure 1.Characterization of PSTC hydrogel preparation process.a) general observation of PS pre-solution, PS SN hydrogel, and PSTC DN hydrogel; b) the G′ and G″ of PS hydrogels with different freeze/thaw cycles; c) TA content of PSTC hydrogels immobilized under different freeze-thaw cycles; d) the G′ of PS and PSTC hydrogels; e) swelling ratio of PSTC hydrogels; SEM images of f) PS-1 and g) PSTC-1.Scale bar: 50 μm.

Figure 2 .
Figure 2. Mechanical properties of PS and PSTC hydrogels.a) tensile stress-strain curves of PSTC hydrogels; b) compression strain-stress curves of PSTC hydrogels; c) repeated compression strain-stress curve of PSTC-0 hydrogel; d) tensile toughness of PSTC hydrogels; e) compression toughness of PSTC hydrogels; f) repeat compression chart of PSTC-1 DN hydrogel; g) general observation of stretching and hanging of 0.5 kg water by PSTC-1 DN hydrogel.

Figure 3 .
Figure 3. Adhesive properties of PSTC hydrogels.a) PSTC hydrogel adhesion to a variety of objects including tissue, glass, and plastic; b) lap shear stress-strain curve of PSTC hydrogels by porcine skin adhesive test with 10 min contact time; c) lap shear stress-strain curve of PSTC hydrogels with 1 h contact time; d) representative images of lap shear test of PSTC-1 and PSTC-0 hydrogel adhesives; e) lap shear stress-strain curve of PSTC hydrogels with 24 h contact time; f) summary image of lap shear strength of the porcine skin adhesive tests with a minimum of three samples per group.*, p <0.05.

Figure 4 .
Figure 4. Cytocompatibility of PSTC hydrogels.a) the cell viability of HaCaT cells was assessed after treatment with varying concentrations of the extraction solution from PSTC-0 and PSTC-1 hydrogels; b) live/dead staining of HaCaT cells cultured with the pure extraction solution from PSTC-0 and PSTC-1 hydrogels for 1 and 3 days.Scale bar: 200 μm.

Figure 5 .
Figure 5. Antibacterial properties of PSTC hydrogels.a) photographs of the growth of bacterial colonies of E. coli on agar plates after 1 and 24 h of co-culture with PSTC-0 or PSTC-1 hydrogel; b) killing rate of E.coli with treatment of PSTC hydrogels with 24 and 48 h.