METHODS OF USE

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority from U.S. Provisional Application No. 63/170,213, filed on April 2, 2021, which is herein incorporated by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR

DEVELOPMENT

[0002] This invention was made with government support under Department of Navy award N00014-20-1-2731 issued by the Office of Naval Research. The government has certain rights in the invention.

FIELD OF THE INVENTION

[0003] The present invention generally relates to zwitterionic polymeric hydrogel materials, particularly those having a non-fouling or foul-resistant property. The present invention more specifically relates to such hydrogel materials containing a poly(carboxybetaine) (pCB), poly(sulfobetaine) (pSB), poly(trimethylamine V-oxide) (pTMAO), or poly(phosphorylcholine) (pPC) component.

BACKGROUND OF THE INVENTION

[0004] Polymeric hydrogels as soft materials are generally composed of polymer networks and a significant amount of water (typically about 50-90%). Among the numerous synthetic materials available to make hydrogels, zwitterionic materials, such as poly(carboxybetaine) (pCB), poly(sulfobetaine) (pSB), poly(trimethylamine N-oxide) (pTMAO), and poly(phosphorylcholine) (pPC), have gained particular attention since they can effectively resist non-specific adsorption and fouling from biomolecules and microorganisms. However, conventional zwitterionic hydrogels often suffer from unacceptably low mechanical strength and toughness.

[0005] Another problem often encountered in zwitterionic hydrogels is the “anti polyelectrolyte” effect, which is the disruption of interactions between zwitterionic moieties by ions in saline environments. The break in interaction results in the swelling of the hydrogel and further decrease of mechanical performance. Efforts have been made to improve the mechanical properties of zwitterionic hydrogels by introducing hydrophobic or non-zwitterionic polyelectrolyte components or using a metal-coordination bond for crosslinking. However, these methods often compromise the non-fouling properties of the zwitterionic hydrogels. Thus, there is a yet unmet need for zwitterionic hydrogel materials having both outstanding non-fouling ability and mechanical properties. There is a further need for such materials having a significantly greater resistance to disruption of interactions between zwitterionic moieties in saline environments. There is yet a further need for such materials also having a better fouling-release ability.

SUMMARY OF THE INVENTION

[0006] The present disclosure is foremost directed to novel zwitterionic hydrogel compositions having outstanding non-fouling ability and mechanical properties, along with exceptional resistance to disruption of interactions between zwitterionic moieties in saline environments. The hydrogel compositions described herein also surprisingly exhibit exceptional fouling-release ability. As discussed in further detail below, the zwitterionic hydrogel compositions having these combination of properties are zwitterionic triple network (ZTN) hydrogel compositions containing a network entanglement of first, second, and third zwitterionic polymers. Typically, at least the second and third zwitterionic polymers contain at least 50 mol% zwitterionic moieties. The first zwitterionic polymer may contain at least, for example, 1, 2, 3, 4, 5, 10, 20, or 30 mol% zwitterionic moieties. In some embodiments, each of the first, second, and third zwitterionic polymers (or at least the second and third zwitterionic polymers) contains at least 50, 60, or 70 mol% zwitterionic moieties.

[0007] ZTN hydrogels with both high mechanical strength and excellent non-fouling and fouling-release properties in saline environments are provided. The saline environment is typically ocean water, but may be another type of brackish water, either from a natural or industrial source. The first, the second, and the third network polymer networks comprise more than 50 mol% of zwitterionic (Z) moieties. The as-prepared first network is swellable. The precursor solution of the second network then forms the second network, which introduces the “lock effect” to the first network in the as-prepared DN hydrogel. The as- prepared DN hydrogel is immersed in the precursor solution of the third network, forming the third network, which protects the “lock effect” and strengthens the hydrogel in saline environments. The ZTN hydrogel shows macroscopic transparency, high stability, and remarkable mechanical strength and excellent non-fouling performance in water or saline environments and can be used in such applications as devices and coatings in the medical and marine fields.

[0008] In some embodiments, at least one, two, or all (each) of the first, second, and third zwitterionic polymers has the following structure: wherein A is a backbone of the polymer, L is a linking portion, Z is a zwitterionic moiety, and n is typically an integer of at least 2, 5, or 10.

[0009] In some embodiments, at least one, two, or all (each) of the first, second, and third zwitterionic polymers has the following structure: wherein: R ^a is H or an alkyl group containing 1-3 carbon atoms; X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms; Z is a zwitterionic moiety; n is typically an integer of at least 2, 5, or 10; and m is an integer of at least 1. In some embodiments, at least two or all of the first, second, and third zwitterionic polymers are within the scope of Formula (1). The group Z may be selected from, for example, carboxybetaine, sulfobetaine, phosphobetaine, and trialkylamine-N-oxide zwitterionic moieties.

[0010] In a first particular set of embodiments, Z contains a positively charged group directly bound to a negatively charged group in at least one, two, or all (each) of the first, second, and third zwitterionic polymers. The at least one zwitterionic polymer containing a positively charged group directly bound to a negatively charged group may have the following structure: wherein R ^a , X, n, and m are as defined above; and Ci and C ₂ are independently selected as positively charged and negatively charged groups to form a zwitterionic moiety C ₁ -C _2. The C ₁ -C ₂ moiety may be, for example, a tri alkyl a i ne-A'-oxide zwitterionic moiety. In some embodiments, at least two or all of the first, second, and third zwitterionic polymers are within the scope of Formula (la).

[0011] When the C ₁ -C ₂ moiety is a tri alkyl ami ne-A-oxide zwitterionic moiety, Formula (la) can be further specified as the following formula: wherein R ^a , X, n, and m are as defined above; and R ¹ and R ² are independently selected from R ^a , as defined above. A particular example of a zwitterionic polymer of Formula (la- 1) is poly(trimethylamine N-oxide) polymer, i.e., pTMAO polymer, which may more specifically refer to poly(methacrylamidopropyl-trimethylamine N-oxide) (when X = N) or poly(methacryloylpropyl-trimethylamine N-oxide) (when X = O). In some embodiments, at least two or all of the first, second, and third zwitterionic polymers are within the scope of Formula (la-1).

[0012] In a second particular set of embodiments, Z contains a positively charged group indirectly bound to a negatively charged group via a linker in at least one, two, or all (each) of the first, second, and third zwitterionic polymers. The at least one zwitterionic polymer containing a positively charged group indirectly bound to a negatively charged group may have the following structure: wherein R ^a , X, n, and m are as defined above; Ci and C2 are independently selected as positively charged and negatively charged groups to form a zwitterionic moiety; and p is an integer of at least 1.

[0013] In some embodiments of Formula (lb), Ci is an ammonium group, in which case Formula (lb) can be further specified as the following formula: (lb-1) wherein R ^a , X, n, m, and p are as defined above; C2 is a negatively charged group; and R ¹ and R ² are independently selected from R ^a , as defined above. The group C2 may be, for example, a sulfonate or carboxylate group. Some examples of zwitterionic polymers of Formula (lb-1) when C2 is sulfonate include poly(sulfobetaine)acrylamide, poly(sulfobetaine)acrylate, and poly(sulfobetaine)methacrylate. Some examples of zwitterionic polymers of Formula (lb-1) when C2 is carboxylate include poly(carboxybetaine)acrylamide, poly(carboxybetaine)acrylate, and poly(carboxybetaine)methacrylate.

[0014] When C2 in Formula (lb-1) is a sulfonate group, Formula (lb-1) can be further specified as the following formula: [0015] When C2 in Formula (lb-1) is a carboxylate group, Formula (lb-1) can be further specified as the following formula:

[0016] In other embodiments of Formula (lb), Ci is a phosphonium group, in which case Formula (lb) can be further specified as the following formula: wherein R ^a , X, n, m, and p are as defined above; C2 is a negatively charged group; and R ¹ and R ² are independently selected from R ^a , as defined above. The group C2 may be, for example, a sulfonate or carboxylate group. Some examples of zwitterionic polymers of Formula (lb-4) when C2 is sulfonate include poly(sulfophosphobetaine)acrylamide, poly(sulfophosphobetaine)acrylate, and poly(sulfophosphobetaine)methacrylate. Some examples of zwitterionic polymers of Formula (lb-1) when C2 is carboxylate include poly(carboxyphosphobetaine)acrylamide, poly(carboxyphosphobetaine)acrylate, and poly(carboxyphosphobetaine)methacrylate.

[0017] When C2 in Formula (lb-4) is a sulfonate group, Formula (lb-4) can be further specified as the following formula:

[0018] When C2 in Formula (lb-4) is a carboxylate group, Formula (lb-4) can be further specified as the following formula: [0019] In other embodiments of Formula (lb), Ci is a phosphate group, in which case Formula (lb) can be further specified as the following formula: wherein R ^a , X, n, m, and p are as defined in claim 4; and C2 ⁺ is a positively charged group.

[0020] In some embodiments of Formula (lb-7), C2 ⁺ is an ammonium group, in which case Formula (lb-7) can be further specified as the following formula: wherein R ^a , X, n, m, and p are as defined above; and R ³ , R ⁴ , and R ⁵ are independently selected from R ^a , as defined above.

[0021] In some embodiments, precisely or at least one of the first, second, and/or third zwitterionic polymers, such as any of those depicted above, is crosslinked. In some embodiments, precisely or at least two (or all) of the first, second, and/or third zwitterionic polymers, such as any of those depicted above, are crosslinked. The crosslinking may be achieved by including a crosslinker (e.g., A) A ^f ’-methyl enebis(acryl amide) or MBAA) in the polymerization process during preparation of the zwitterionic polymer.

[0022] In some embodiments, two of the zwitterionic polymers selected from the first, second, and third zwitterionic polymers have the same structure. In other embodiments, two of the first, second, and third zwitterionic polymers have different structures. [0023] In another aspect, the present disclosure is directed to a method of producing the ZTN hydrogel described above. The method includes at least the following steps: (i) providing the first zwitterionic polymer; (ii) absorbing a first precursor solution into said first zwitterionic polymer, wherein said first precursor solution comprises a first zwitterionic monomer species dissolved in a solvent; (iii) polymerizing said first zwitterionic monomer species to form a second zwitterionic polymer while absorbed in said first zwitterionic polymer to form a zwitterionic double-network (ZDN) hydrogel composition containing said second zwitterionic polymer entangled in said first zwitterionic polymer; (iv) absorbing a second precursor solution into said ZDN hydrogel composition, wherein said second precursor solution comprises a second zwitterionic monomer species dissolved in a solvent; and (v) polymerizing said second zwitterionic monomer species to form a third zwitterionic polymer while absorbed in said ZDN hydrogel composition to form said zwitterionic triple-network (ZTN) hydrogel composition containing said third zwitterionic polymer entangled in said ZDN hydrogel composition; wherein at least the second and third zwitterionic polymers typically contain at least 50 mol% zwitterionic moieties.

[0024] In yet another aspect, the present disclosure is directed to a method of preventing or reducing the rate of fouling of a surface. The method includes at least the following steps: (i) coating the surface with the first zwitterionic polymer; (ii) absorbing a first precursor solution into said first zwitterionic polymer, wherein said first precursor solution comprises a first zwitterionic monomer species dissolved in a solvent; (iii) polymerizing said first zwitterionic monomer species to form a second zwitterionic polymer while absorbed in said first zwitterionic polymer to form a zwitterionic double-network (ZDN) hydrogel composition containing said second zwitterionic polymer entangled in said first zwitterionic polymer; (iv) absorbing a second precursor solution into said ZDN hydrogel composition, wherein said second precursor solution comprises a second zwitterionic monomer species dissolved in a solvent; and (v) polymerizing said second zwitterionic monomer species to form a third zwitterionic polymer while absorbed in said ZDN hydrogel composition to form said zwitterionic triple-network (ZTN) hydrogel composition containing said third zwitterionic polymer entangled in said ZDN hydrogel composition; wherein at least the second and third zwitterionic polymers typically contain at least 50 mol% zwitterionic moieties. [0025] In different embodiments, at least the second and third zwitterionic polymers in the Zwitterionic Triple-Network (ZTN) hydrogels comprise 100, 90, 80, 70, 60, or 50 mol% zwitterionic moieties. In other embodiments, the three polymer networks independently comprise 100, 90, 80, 70, 60, or 50 mol% zwitterionic moieties.

[0026] In further embodiments, the hydrogels have one or more of the following properties after the hydrogels are soaked in water or saline environment (e.g., phosphate-buffered saline and seawater): compressive fracture stress (> 0.5, 1, 3, 5, 10, 12, and 15 MPa) or a compressive fracture strain (> 50%, 80%, and 99%) along with Young’s modulus (> 0.01, 0.1, 0.5MPa, 1 MPa).

[0027] In further embodiments from any of the above embodiments, zwitterionic poly(sulfobetaine) (pSB) is used in the second and third zwitterionic polymer networks.

[0028] In further embodiments from any of the above embodiments, zwitterionic moieties can be, but not limited to, poly(carboxybetaine) (pCB), pSB, poly(sulfabetaine) (pSAB), poly(phosphobetaine) (pPB), poly(phosphorylcholine) (pPC), poly(choline phosphate) (pCP), poly(trimethylamine-N-oxide) (pTMAO) or their latent derivatives.

[0029] In further embodiments from any of the above embodiments, the first network is chemically or physically crosslinked.

[0030] In further embodiments from any of the above embodiments, the second or third network is chemically or physically crosslinked.

[0031] In further embodiments from any of the above embodiments, the zwitterionic moieties can be polymerized by, but not limited to, addition, condensation, ring-opening, and free radical polymerization.

[0032] In further embodiments from any of the above embodiments, the physical crosslinking may include ionic bonding, hydrogen bonding, or dipole-dipole crosslinking.

[0033] In further embodiments from any of the above embodiments, the hydrogel has a fibrinogen binding level of less than 20%, 15% or 10% relative to that of tissue culture polystyrene (TCPS) tested via a fibrinogen binding assay (polymer surface is incubated at 37°C for 90 minutes with a 1.0 mg/mL fibrinogen solution in 0.15 M phosphate-buffered saline at pH 7.4). [0034] In further embodiments from any of the above embodiments, the hydrogel has a fibrinogen binding level of less than 20%, 15% or 10% relative to that of tissue culture polystyrene (TCPS) tested via a fibrinogen binding assay (hydrogel is incubated at 37°C for 1.5 h with a 1.0 mg/mL fibrinogen solution in 0.15 M phosphate-buffered saline at pH 7.4).

[0035] In further embodiments from any of the above embodiments, the hydrogels have an undiluted human serum binding level of less than 20%, 15% or 10% relative to that of tissue culture polystyrene (TCPS) tested via the BCA method (hydrogel can be incubated at 37°C for 2 h in solution in undiluted human serum, then sonicated in phosphate-buffered saline +

1 wt% sodium n-dodecyl sulfate (SDS) solution for 5 minutes to desorb proteins. This solution can be analyzed with Micro-BCA assay for quantifying the amount of adsorbed proteins).

[0036] In further embodiments from any of the above embodiments, the hydrogel has a water content (>50%, 70%, 80%, and 90%) and low swelling (the swelling ratio, (Mw-Md)/ Md, is less than 300%, where Mw is the wet weight of the hydrogel and Md is the dry weight of the hydrogel soaked in DI water or 0.15 M phosphate-buffered saline at pH 7.4 until equilibrium or 36 g/L seawater).

[0037] In further embodiments from any of the above embodiments, the hydrogels can be prepared in a three-step method. The three-step method can be practiced as follows: the first network hydrogels comprising a zwitterionic polymer are formed first, the as-prepared first network hydrogels are soaked until equilibrium in the precursor solution of a second network comprising zwitterionic monomers, which are further polymerized to form the zwitterionic double-network (ZDN) hydrogels. After that, the as-prepared ZDN hydrogels are soaked until equilibrium in the precursor solution of a third network comprising zwitterionic monomers. The equilibrated ZDN hydrogels containing the precursor solution of the third network is further polymerized to form the zwitterionic triple-network (ZTN) hydrogels.

[0038] In further embodiments from any of the above embodiments, the first network hydrogels comprise a zwitterionic polymer that is highly swollen in the precursor solution of the second network. The swelling ratios ((Mw-Md)/ Md) of the first network hydrogels in water are from 200% to 6000%, where Mw is the wet weight of the hydrogel and Md is the dry weight of the hydrogel soaked in DI water. [0039] In further embodiments from any of the above embodiments, the ZTN hydrogels can be prepared in a three-step method, wherein three networks containing zwitterionic moieties are reacted independently and sequentially via chemical or physical crosslinking.

[0040] In further embodiments from any of the above embodiments, the zwitterionic double-network (ZDN) hydrogels can absorb a large number of the third network monomers. The third and second (3rd/2nd) network molar ratios in the zwitterionic triple network (ZTN) hydrogels may be from 0.2 to 5.

[0041] The ZTN hydrogels according to any of the above embodiments can be made of various shapes according to the customized design, including implantable sensing devices, nose and ear cartilages, and blood vessels.

[0042] The ZTN hydrogels according to any of the above embodiments can have particular properties, which include, but are not limited to, the ability to support other molecules, biomolecules, small molecule drug nanoparticles, microparticles, cells, tissues, or organs as a carrier, scaffold, or matrix. Moreover, applications for bulk material ZTN hydrogels according to any of the above embodiments can be made into a consumer product.

[0043] The ZTN hydrogels according to any of the above embodiments can be made into a marine product. The ZTN hydrogels according to any of the above embodiments can be made into marine products selected from, for example, marine vessel hulls, marine structures, bridges, propellers, heat exchangers, periscopes, sensors, fishnets, cables, tubes/pipes, containers, membranes, and oil booms.

[0044] The ZTN hydrogels according to any of the above embodiments can be made into a biomedical product. The ZTN hydrogels according to any of the above embodiments can be made into a biomedical product selected from, for example, catheters, ear drainage tubes, feeding tubes, glaucoma drainage tubes, hydrocephalous shunts, keratoprosthesis, nerve guidance tubes, tissue adhesives, x-ray guides, artificial joints, artificial heart valves, artificial blood vessels, pacemakers, left ventricular assist devices (LVAD), artery grafts, vascular grafts, stents, intravascular stents, cardiac valves, joint replacements, blood vessel prostheses, skin repair devices, cochlear replacements, contact lenses, artificial ligaments and tendons, dental implants, and tissue scaffolds for regenerative tissue engineering.

[0045] The present disclosure is also directed to a surface coating for a substrate, wherein the surface coating contains the ZTN hydrogels according to any of the above embodiments. In some embodiments, the substrate is a consumer product. In other embodiments, the substrate is a marine product. In particular embodiments, the substrate is a marine product selected from marine vessel hulls, marine structures, bridges, propellers, heat exchangers, periscopes, sensors, fishnets, cables, tubes/pipes, containers, membranes, and oil booms. In other embodiments, the substrate is a biomedical product. In particular embodiments, the substrate is a biomedical product selected from catheters, ear drainage tubes, feeding tubes, glaucoma drainage tubes, hydrocephalous shunts, keratoprosthesis, nerve guidance tubes, tissue adhesives, x-ray guides, artificial joints, artificial heart valves, artificial blood vessels, pacemakers, left ventricular assist devices (LVAD), artery grafts, vascular grafts, stents, intravascular stents, cardiac valves, joint replacements, blood vessel prostheses, skin repair devices, cochlear replacements, contact lenses, artificial ligaments and tendons, dental implants, and tissue scaffolds for regenerative tissue engineering. In some embodiments, the substrate has one or more particular properties, which include, but are not limited to, ability to support other molecules, biomolecules, small molecule drugs nanoparticles, microparticles, cells, tissues, or organs as a carrier, scaffold, or matrix.

BRIEF DESCRIPTION OF THE FIGURES

[0046] FIG. 1. Synthesis scheme of the ZTN hydrogels. The ZSN hydrogel is swellable in the precursor solution of the second network to prepare ZDN hydrogel. The prepared ZDN hydrogels are immersed in the precursor solution of the third network to obtain the ZTN hydrogels.

[0047] FIG. 2. Optical photos in schematic of the preparation process of the pTMAO/pSB/pSB ZTN hydrogels.

[0048] FIG. 3. Graph showing compressive fracture stress of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in water, PBS, and seawater, respectively. The ZTN hydrogel exhibited high compressive fracture stress (18.7±1.2 MPa in water, 18.2±1.4 MPa in PBS, and 18.2±1.4 MPa in seawater) both in water and saline environments due to the high swelling property of pTMAO and strong electrostatic interaction and network entanglement of pSB.

[0049] FIG. 4. Graph showing compressive fracture strain of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in water, PBS, and seawater, respectively. The compressive strain of ZTN hydrogel could be up to 99%. [0050] FIG. 5. Graph showing compressive toughness of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in water, PBS, and seawater, respectively. The ZTN hydrogel exhibited high toughness (1.76±0.05 MJ/m ³ in water, 1.70±0.03 MJ/m ³ in PBS, and 1.62±0.03 MJ/m ³ in seawater) both in water and saline environments.

[0051] FIG. 6. Graph showing the compressive modulus of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in water, PBS, and seawater, respectively. The ZTN hydrogel is stiffer than the ZSN and ZDN hydrogels, with a modulus of 0.37±0.02 MPa in water, 0.59±0.01 MPa in PBS, and 0.66±0.03 MPa in seawater.

[0052] FIG. 7. Graph showing the stress-strain curves of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in PBS under uniaxial compression.

[0053] FIG. 8. Graph showing the stress-strain curves of the pTMAO ZSN, pTMAO/pSB ZDN, and pTMAO/pSB/pSB ZTN hydrogels after immersing in seawater under uniaxial compression.

[0054] FIG. 9. Images of compression tests of the the pTMAO/pSB ZDN (left) and pTMAO/pSB/pSB ZTN (right) hydrogels after immersing in PBS. Photos demonstrate how the ZTN hydrogel sustains a high compression after immersion in PBS.

[0055] FIG. 10. Images of compression tests of the pTMAO/pSB ZDN (left) and pTMAO/pSB/pSB ZTN (right) hydrogels after immersing in seawater. Photos demonstrate how the ZTN hydrogel sustains a high compression after immersion in seawater.

[0056] FIG. 11. Graph showing stability of the pTMAO/pSB/pSB ZTN hydrogel after immersion in seawater for 30 days. No obvious change was observed after immersing for 30 days, which confirmed the long-term stability of the ZTN hydrogel.

[0057] FIG. 12. Graph showing the composition ratios of the pTMAO ZSN, pTMAO/pSB ZDN and pTMAO/pSB/pSB ZTN hydrogels. It can be seen that pSB is the major component of the ZDN and ZTN hydrogels. A large amount of the pSB component in the ZDN hydrogel is due to the superhydrophilicity and high swellability of the pTMAO component, while the significant absorption of SB monomers for the ZDN hydrogel is due to the strong electrostatic interaction of SB moieties. [0058] FIG. 13. Graph showing equilibrium water contents (EWCs) of pTMAO ZSN, pTMAO/pSB ZDN and pTMAO/pSB/pSB ZTN hydrogels. The EWC of the ZTN hydrogel was lower than that of the ZSN and ZDN hydrogels.

[0059] FIG. 14. Graph showing swelling ratios (SRs) of the pTMAO ZSN, pTMAO/pSB ZDN and pTMAO/pSB/pSB ZTN hydrogels. The SR of the ZTN hydrogel was lower than that of the ZSN and ZDN hydrogels. The difference of SR between the ZDN and ZTN hydrogels was enlarged in saline solutions. The strong interpenetrating chain entanglement of pSB network could inhibit the swelling of the hydrogel.

[0060] FIG. 15. Graph showing fibrinogen adhesion test of the pTMAO/pSB ZDN and pTMAO/pSB/pSB ZTN hydrogels. Both the ZDN and ZTN hydrogels exhibited low fibrinogen adsorption as compared to that of TCPS.

[0061] FIG. 16. Graph showing undiluted human serum proteins adhesion test of the pTMAO/pSB ZDN and pTMAO/pSB/pSB ZTN hydrogels. Both the ZDN and ZTN hydrogels exhibited low undiluted human serum proteins adhesion as compared to that of TCPS.

[0062] FIG. 17. Graph showing algal biofilm growth test of the pTMAO/pSB/pSB ZTN hydrogel coating. Coating #1 represents the ZTN hydrogel coating. Biofilm growth was reported as fluorescence intensity. The growth amount of algal biofilm on the surface of pTMAO/pSB/pSB ZTN hydrogel coating was much lower than that of other commercial marine coatings.

[0063] FIG. 18. Graph showing biomass remaining of algal on the suface of pTMAO/pSB/pSB ZTN hydrogel coating after treatment with water jet. Coating #1 represents the ZTN hydrogel coating. The coating was jetted for 10 seconds at a pressure of 10 psi or 20 psi. It can be seen that the biomass remaining of algal cell on the surface of pTMAO/pSB/pSB ZTN hydrogel coating was much lower than that of other commercial marine coatings.

[0064] FIG. 19. Graph showing percent removal of algal on the surface of pTMAO/pSB/pSB ZTN hydrogel coating after treatment with water jet. Coating #1 represents the ZTN hydrogel coating. The coating was jetted for 10 seconds at a pressure of 10 psi or 20 psi. The percent removal of algal on the pTMAO/pSB/pSB ZTN hydrogel coating surface was almost 100% after treatment with water jet at a pressure of 10 psi. [0065] FIG. 20. Graph showing the compressive fracture stress of the pTMAO/pSB/pCBl, pTMAO/pSB/pCB2 and pTMAO/pSB/pSB ZTN hydrogels after immersing in water, PBS, and seawater, respectively. The pTMAO/pSB/pSB ZTN hydrogel exhibited higher compressive fracture stress than those of the pTMAO/pSB/pCBl and pTMAO/pSB/pCB2 ZTN hydrogels.

DETAILED DESCRIPTION OF THE INVENTION

[0066] In one aspect, the present disclosure is directed to a zwitterionic triple-network (ZTN) hydrogel composition containing a network entanglement of first, second, and third zwitterionic polymers. As well known in the art, a network entanglement results from the interpenetration of polymer chains and/or pendant groups and resulting loss of degree of freedom of the polymers without any formal bonding between the polymer chains and/or pendant groups engaged in the network entanglement. A zwitterionic double-network (ZDN) hydrogel composition contains two zwitterionic polymers engaged in a network entanglement. A zwitterionic triple-network (ZTN) hydrogel composition contains three zwitterionic polymers engaged in a network entanglement. As also well known, the term “hydrogel” refers to polymers that are swellable by absorption of a liquid, typically water, while maintaining their structure and not dissolving in the liquid. The zwitterionic polymer typically contains at least or greater than 10, 50, 100, 200, 300, 400, 500, 1000, 5000, 10,000, 50,000, or 100,000 units, or a number of units within a range bounded by any two of the foregoing values.

[0067] As used herein, the term “zwitterionic polymer” refers to a polymer containing zwitterionic moieties, wherein a zwitterionic moiety contains both negative and positively charged groups covalently attached to the polymer. In one set of embodiments, the zwitterionic polymer is produced by polymerization of a polymerizable zwitterionic monomer or a monomer containing a precursor for a zwitterionic group. Zwitterionic monomers are electronically neutral monomers that include equal numbers of positive and negative charges per monomer. In another set of embodiments, the zwitterionic polymer is produced by polymerization of equal numbers of monomers containing negatively charged groups and monomers containing positively charged groups. In some cases, the polymer is produced solely from zwitterionic monomers or equal numbers of positively and negatively charged monomers, in which case the zwitterionic polymer has 100 mole percent (100 mol%) zwitterionic moieties. In other cases, the polymerization process may include uncharged monomers, which provides a zwitterionic copolymer having less than 100 mole percent zwitterionic moieties. For example, when a polymerizable zwitterionic monomer and polymerizable comonomer are present in equal proportions in the polymerization mixture, the product is a zwitterionic polymer having 50 mol% zwitterionic moieties. The uncharged monomer may be, for example, ethylene, propylene, styrene, methyl acrylate, methyl methacrylate, 2-hydroxyethyl acrylate, 2-ethoxyethyl acrylate, 2-(2- ethoxyethoxy)ethyl acrylate, acrylamide, vinyl alcohol, or acrylonitrile.

[0068] At least the second and third zwitterionic polymers contain at least 50 mol% zwitterionic moieties. In different embodiments, the second and/or third zwitterionic polymers independently contain precisely or at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mol% zwitterionic moieties, or a mol% within a range bounded by any two of the foregoing values (e.g., 50-100 mol%, 60-100 mol%, 70-100 mol%, 80-100 mol%, or 90- 100 mol%). In different embodiments, at least the second and/or third zwitterionic polymers, and optionally the first zwitterionic polymer, independently contain precisely or at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mol% zwitterionic moieties, or a mol% within a range bounded by any two of the foregoing values (e.g., 50-100 mol%, 60-100 mol%, 70-100 mol%, 80-100 mol%, or 90-100 mol%).

[0069] The first zwitterionic polymer may contain precisely or at least, for example, 1, 2,

3, 4, 5, 10, 20, 30, 40, 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mol% zwitterionic moieties, or a mol% within a range bounded by any two of the foregoing values (e.g., 1-5 mol%, 1-10 mol%, 2-10 mol%, 3-10 mol%, 1-20 mol%, 1-30 mol%, 1-40 mol%, 1-50 mol%, 50-100 mol%, 60-100 mol%, 70-100 mol%, 80-100 mol%, or 90-100 mol%).

[0070] In some embodiments, the first, second, and third zwitterionic polymers contain at least 50 mol% zwitterionic moieties. In different embodiments, one, two, or all (or each) of the first, second, and third zwitterionic polymers independently contain precisely or at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mol% zwitterionic moieties, or a mol% within a range bounded by any two of the foregoing values (e.g., 50-100 mol%, 60-100 mol%, 70- 100 mol%, 80-100 mol%, or 90-100 mol%).

[0071] In some embodiments, the zwitterionic polymer is a betaine polymer. In other embodiments, the zwitterionic polymer is a poly(phosphatidylcholine) polymer, poly(trimethylamine N-oxide) polymer, poly(zwitterionic phosphatidylserine) polymer, or glutamic acid-lysine (EK)-containing polypeptide. In some embodiments, zwitterionic phosphatidyl serine comprises one neighboring positive charged moiety to balance the negative charge of the phosphoserine. In some embodiments, zwitterionic phosphatidyl serine comprises a compound as described in “De novo design of functional zwitterionic biomimetic material for immunomodulation” Science Advances, 29 May 2020, Vol. 6, Issue 22, (DOI: 10.1126/sciadv.aba0754) which is hereby incorporated by reference in its entirety.

[0072] Some examples of betaine polymers include poly(carboxybetaine), poly(sulfobetaine), and poly(phosphobetaine) polymers. Suitable poly(carboxybetaine)s can be prepared from one or more monomers selected from, for example, carboxybetaine acrylates, carboxybetaine acrylamides, carboxybetaine vinyl compounds, carboxybetaine epoxides, and mixtures thereof. In one embodiment, the monomer is carboxybetaine methacrylate. Representative monomers for making carboxybetaine polymers useful in the invention include carboxybetaine methacrylates, such as 2-carboxy-N,N-dimethyl-N-(2’- methacryloyloxyethyl) ethanaminium inner salt; carboxybetaine acrylates; carboxybetaine acrylamides; carboxybetaine vinyl compounds; carboxybetaine epoxides; and other carboxybetaine compounds with hydroxyl, isocyanates, amino, or carboxylic acid groups. In a particular embodiment, the polymer is a poly(carboxybetaine methacrylate) (poly(CBMA)). In some embodiments, the second and third zwitterionic polymers both have a poly(sulfobetaine) composition.

[0073] In some embodiments, at least one, or at least two, or all (each) of the first, second, and third zwitterionic polymers independently has the following structure: wherein A is a backbone of the polymer, L is a linking portion, Z is a zwitterionic moiety, and n is typically an integer of at least 2, 5, or 10. The backbone (A) can be any polymeric bones known in the art, including linear, branched, and cyclic backbone structures. The subscript n is typically an integer of at least 2 (e.g., at least or greater than 2, 5, 10, 50, 100, 200, 300, 400, 500, 1000, 5000, 10,000, 50,000, or 100,000 units, or a number of units within a range bounded by any two of the foregoing values). The linker (L) can be any of the linkers commonly included in pendant groups of polymers, e.g., linear or branched alkyl linkers or cyclic linkers, any of which may contain precisely or at least 1, 2, 3, 4, 5, or 6 carbon atoms and optionally containing one or more heteroatoms (typically selected from oxygen and nitrogen atoms).

[0074] In one set of embodiments, at least one, or at least two, or all (each) of the first, second, and third zwitterionic polymers independently has the following structure:

[0075] In Formula (1) above, the variable R ^a is H or an alkyl group containing 1-3 carbon atoms. Some examples of alkyl groups containing 1-3 carbon atoms include methyl, ethyl, n-propyl, and isopropyl. In some embodiments, R ^a is H or methyl. The variable X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms. In some embodiments, R ^b is H or methyl. The variable Z is a zwitterionic moiety. The subscript n is typically an integer of at least 2, 5, or 10 (e.g., at least or greater than 10, 50, 100, 200, 300, 400, 500, 1000, 5000, 10,000, 50,000, or 100,000 units, or a number of units within a range bounded by any two of the foregoing values). The subscript m is an integer of at least 1, such as a value of precisely or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, or a value within a range bounded by any two of the foregoing values (e.g., 1-12, 1-10, 1-6, 1-4, 1-3, 2-4, or 2-3).

[0076] In some embodiments, the group Z represents a single zwitterionic group containing a positive and negative charge in the same Z group. The group Z may be selected from, for example, carboxybetaine, sulfobetaine, phosphobetaine, and tri alkyl a i ne-A'-oxide zwitterionic moieties. The resulting polymer according to Formula (1) is poly(carboxybetaine), poly(sulfobetaine), poly(phosphobetaine), and poly(trialkylamine-N-oxide), as well known in the art. In other embodiments, a certain number of the Z groups are positively charged and an equal number of Z groups are negatively charged to result in Z zwitterionic pairs (i.e., Z ⁺ Z zwitterionic pairs). [0077] Notably, although Formulas (I), (1), and sub-formulas provided throughout this disclosure may appear to depict zwitterionic homopolymers (100 mol% zwitterionic moieties), Formulas (I), (1), and sub-formulas thereof include the possibility that one or more non-zwitterionic or uncharged monomer units is situated (inserted) between zwitterionic monomeric units depicted in any of these formulas, thereby resulting in a copolymer. If zwitterionic monomeric units (such as any of those described above, i.e., where n = 1) are labeled as A units, and non-zwitterionic or uncharged monomeric units are labeled as B units, the copolymer can have any of the known copolymer arrangements, including alternating (e.g., A-B-A-B), block (e.g., A-A-A-A-B-B-B-B), or random (e.g., A- B-B-A-B-A-A-B-A-B-B). The zwitterionic polymer may also include more than one type of non-zwitterionic or uncharged monomer unit, such as in the structures A-B-C-A-B-C (repeating), A-A-A-B-B-B-C-C-C (block), or A-C-B-B-C-A-B-C-A-C-A-B-C-A-B-C-B (random), wherein B and C represent non-zwitterionic or uncharged monomeric units.

[0078] In some embodiments of Formula (1), Z contains a positively charged group directly bound to a negatively charged group in at least one of the first, second, and third zwitterionic polymers. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure:

[0079] In Formula (la), R ^a , X, n, and m are as defined above under Formula (1). The variables Ci and C2 are independently selected as positively charged and negatively charged moieties to form a zwitterionic moiety C1-C2. Some examples of positively charged moieties include ammonium (-NRV-) and phosphonium (-PRV-) moieties. Some examples of negatively charged moieties include terminal oxide (-O ), carboxylate (-C(O)O- ), phosphate (-OPO3 ), phosphonate (-PO3 ), sulfate (-OSO3 ), and sulfonate (-SO3 ). In some embodiments, Ci is positively charged and C2 is negatively charged. For example, Ci may be an ammonium moiety and C2 may be oxide, which together results in an ammonium A-oxide (-NR.V-0 ) zwitterionic group. As the ammonium moiety is also attached to a carbon atom of the polymer, the ammonium oxide zwitterionic group is also herein referred to as a trialkylamine-iV-oxide group. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In other separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0080] In specific embodiments of Formula (la), Ci is an ammonium moiety and C2 is oxide, which together results in an ammonium A'-oxide (-NRV-0 ) zwitterionic group. The resulting polymer is a poly(trialkylammonium oxide), i.e., pTMAO, and may have the following structure for at least one, two, or all (each) of the first, second, and third zwitterionic polymers:

[0081] In Formula (la-1), R ^a , X, n, and m are as defined above under Formula (1). The variables R ¹ and R ² are independently selected from R ^a . In some embodiments, R ¹ and R ² are both alkyl, or more particularly, both methyl. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0082] In other embodiments of Formula (1), Z contains a positively charged group indirectly bound to a negatively charged group via a linker in at least one of the first, second, and third zwitterionic polymers. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure: [0083] In Formula (lb), R ^a , X, n, and m are as defined above under Formula (1). The subscript p is an integer of at least 1, such as a value of precisely or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, or a value within a range bounded by any two of the foregoing values (e.g., 1-12, 1-10, 1-6, 1-4, 1-3, 2-4, or 2-3). The variables Ci and C ₂ are independently selected as positively charged and negatively charged moieties to form a zwitterionic spaced pair. Some examples of positively charged moieties include ammonium (-NRV-) and phosphonium (-PR.V-) moieties. Some examples of negatively charged moieties include terminal oxide (-O ), carboxylate (-C(O)O-), phosphate (-OPO3 ), phosphonate (-PO3 ), sulfate (-OSO ₃ ), and sulfonate (-SO ₃ ). In some embodiments, Ci is positively charged and C ₂ is negatively charged. For example, Ci may be an ammonium or phosphonium moiety and C ₂ may be carboxylate, sulfate, sulfonate, phosphate, or phosphonate moiety. In other embodiments, Ci is negatively charged and C ₂ is positively charged. For example, Ci may be a phosphate, phosphonate, sulfate, or sulfonate moiety and C ₂ may be an ammonium or phosphonium moiety. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In other separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0084] In specific embodiments of Formula (lb), Ci is an ammonium moiety and C ₂ is a negatively charged moiety, such as a carboxylate, sulfate, sulfonate, phosphate, or phosphonate moiety, which together results in a spaced zwitterionic group. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure:

[0085] In Formula (lb-1), R ^a , X, n, m, and p are as defined above under Formulas (1) and (lb). The variables R ¹ and R ² are independently selected from R ^a . In some embodiments, R ¹ and R ² are both alkyl, or more particularly, both methyl. C2 is a negatively charged moiety, such as a carboxylate, phosphate, phosphonate, sulfate, or sulfonate moiety. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl. In embodiments where C2 is a carboxylate moiety, the polymer of Formula (lb-1) can generally be referred to as a poly(carboxybetaine). In embodiments where C2 is a sulfonate moiety, the polymer of Formula (lb-1) can generally be referred to as a poly(sulfobetaine). In embodiments where C2 is a phosphate moiety, the polymer of Formula (lb-1) can generally be referred to as a poly(phosphobetaine).

[0086] In specific embodiments of Formula (lb-1), C2 is a sulfonate or carboxylate moiety. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have any of the following structures:

[0087] In particular embodiments, both the second and third zwitterionic polymers independently have the sulfobetaine structure of Formula (lb-2), or more particularly, the structure of pSB. The second and third zwitterionic polymers may have the same or different sulfobetaine structure, and the second and third zwitterionic polymers are independently crosslinked or uncrosslinked. When the second and third zwitterionic polymers have the sulfobetaine structure of Formula (lb-2), the first zwitterionic polymer typically has a non-sulfobetaine structure, such as any of those shown above, such as a trialkylamine oxide structure of Formula (la-1) or carboxybetaine structure of Formula (lb-

3). [0088] In Formulas (lb-2) and (lb-3), R ^a , X, n, m, and p are as defined above under Formulas (1) and (lb). The variables R ¹ and R ² are independently selected from R ^a . In some embodiments, R ¹ and R ² are both alkyl, or more particularly, both methyl. In separate or further specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0089] In other specific embodiments of Formula (lb), Ci is a phosphonium moiety and C2 is a negatively charged moiety, such as a carboxylate, sulfate, sulfonate, phosphate, or phosphonate moiety, which together results in a spaced zwitterionic group. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure:

[0090] In Formula (lb-4), R ^a , X, n, m, and p are as defined above under Formulas (1) and (lb). The variables R ¹ and R ² are independently selected from R ^a . In some embodiments, R ¹ and R ² are both alkyl, or more particularly, both methyl. C2 is a negatively charged moiety, such as a carboxylate, phosphate, phosphonate, sulfate, or sulfonate moiety. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0091] In specific embodiments of Formula (lb-4), C2 is a sulfonate or carboxylate moiety. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have any of the following structures: [0092] In Formulas (lb-5) and (lb-6), R ^a , X, n, m, and p are as defined above under Formulas (1) and (lb). The variables R ¹ and R ² are independently selected from R ^a . In some embodiments, R ¹ and R ² are both alkyl, or more particularly, both methyl. In separate or further specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0093] In other specific embodiments of Formula (lb), Ci is a phosphate moiety and C2 is a positively charged moiety, such as an ammonium or phosphonium moiety, which together results in a spaced zwitterionic group. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure:

[0094] In Formula (lb-7), R ^a , X, n, and m are as defined above under Formula (1). The subscript p is an integer of at least 1, such as a value of precisely or at least 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, or 12, or a value within a range bounded by any two of the foregoing values (e.g., 1-12, 1-10, 1-6, 1-4, 1-3, 2-4, or 2-3). The variable Ci ⁺ is a positively charged moiety that forms a zwitterionic spaced pair with the phosphate moiety in Formula (lb-7). Some examples of positively charged moieties include ammonium (-NRV-) and phosphonium (- PRV-) moieties. In specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In other separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0095] In specific embodiments of Formula (lb-7), Ci ⁺ is an ammonium moiety. The resulting polymer in at least one, two, or all (each) of the first, second, and third zwitterionic polymers may have the following structure: (lb-8)

[0096] In Formula (lb-8), R ^a , X, n, m, and p are as defined above under Formulas (1) and (lb). The variables R ³ , R ⁴ , and R ⁵ are independently selected from R ^a . In some embodiments, at least one or two of R ³ , R ⁴ , and R ⁵ are alkyl (or more specifically, methyl and/or ethyl) or all of R ³ , R ⁴ , and R ⁵ are alkyl (or more specifically, methyl and/or ethyl). In separate or further specific embodiments, m has a value of 1, 2, 3, or 4. In separate or further specific embodiments, p has a value of 1, 2, 3, or 4, or a value of 2, 3, or 4. In separate or further specific embodiments, R ^a is H or methyl. In separate or further specific embodiments, X is O or NR ^b , wherein R ^b is H or an alkyl group containing 1-3 carbon atoms, or R ^b is H or methyl.

[0097] Some specific examples of zwitterionic polymers within the above formulas include: [0098] Any one or more of the first, second, and third zwitterionic polymers may be selected from zwitterionic polymers within the scope of Formula (I) but not within the scope of Formula (1) or sub-formulas thereof. Some examples of zwitterionic polymers not within the scope of Formula (1) or sub-formulas thereof but which may be included in the ZTN hydrogel composition as a first, second, and/or third zwitterionic polymer include the following, wherein q can have any of the values given above for m (e.g., at least 1): v xi) [0099] Typically, at least one zwitterionic polymer out of the first, second, and third zwitterionic polymers is different than the other two (i.e., typically, the first, second, and third zwitterionic polymers are not all the same). In some embodiments, two of the zwitterionic polymers selected from the first, second, and third zwitterionic polymers are of the same type (i.e., within the same sub-formula) or have the same structure. In other embodiments, the first, second, and third zwitterionic polymers are not the same type or have different structures.

[00100] In some embodiments, precisely or at least one, two, or all (each) of the first, second, and third zwitterionic polymers is/are crosslinked. The crosslinking can be between the same polymer type or between different polymer types. In a first set of embodiments, only or at least the first zwitterionic polymer is crosslinked, which may be between chains of the first zwitterionic polymer, or between chains of the first and second zwitterionic polymers, or both. In a second set of embodiments, only or at least the second zwitterionic polymer is crosslinked, which may be between chains of the second zwitterionic polymer, or between chains of the second and third zwitterionic polymers, or both. In a third set of embodiments, only or at least the third zwitterionic polymer is crosslinked, which may be between chains of the third zwitterionic polymer, or between chains of the second and third zwitterionic polymers, or both. In some embodiments, all of the zwitterionic polymers are crosslinked.

[00101] Crosslinking of polymer chains is typically achieved by including a crosslinking monomer, such as a bis(acrylamide), bis(acrylate), or bis(methacrylate), in the polymerization reaction. In some embodiments, the crosslinking monomer is not charged or zwitterionic. In other embodiments, the crosslinking monomer is charged or zwitterionic, such as in carboxybetaine dimethacrylate. Some specific examples of crosslinking monomers include the following: y y y bis[(2-methacryloyloxy)ethyl]phosphate di (methacry 1 oy 1 amino)azob enzene bis(2-methacryloyl)oxyethyldisulfide carboxybetaine dimethacrylate

[00102] In a first set of particular embodiments, the ZTN hydrogel contains a first zwitterionic polymer having the structure of Formula (la) or (la-1), or more particularly, pTMAO, which may be crosslinked or uncrosslinked. In other particular embodiments, the ZTN hydrogel contains a first zwitterionic polymer having any of the structures of Formulas (lb), (lb-1), (lb-2), (lb-3), (lb-4), (lb-5), (lb-6), (lb-7), or (lb-8), or more particularly, pSB or pCB, any of which may be crosslinked or uncrosslinked.

[00103] In a second set of particular embodiments, the ZTN hydrogel contains a second zwitterionic polymer having the structure of Formula (la) or (la- 1), or more particularly, pTMAO, which may be crosslinked or uncrosslinked. In other particular embodiments, the ZTN hydrogel contains a second zwitterionic polymer having any of the structures of Formulas (lb), (lb-1), (lb-2), (lb-3), (lb-4), (lb-5), (lb-6), (lb-7), or (lb-8), or more particularly, pSB or pCB, any of which may be crosslinked or uncrosslinked.

[00104] In a third set of particular embodiments, the ZTN hydrogel contains a third zwitterionic polymer having the structure of Formula (la) or (la-1), or more particularly, pTMAO, which may be crosslinked or uncrosslinked. In other particular embodiments, the ZTN hydrogel contains a third zwitterionic polymer having any of the structures of Formulas (lb), (lb-1), (lb-2), (lb-3), (lb-4), (lb-5), (lb-6), (lb-7), or (lb-8), or more particularly, pSB or pCB, any of which may be crosslinked or uncrosslinked.

[00105] In some embodiments, any one of the first, second, or third particular embodiments provided above are combined. In a first particular example, the first zwitterionic polymer may be pTMAO and the second zwitterionic polymer may be pSB, pCB, or pTMAO. In a second particular example, the first zwitterionic polymer may be pSB and the second zwitterionic polymer may be pSB, pCB, or pTMAO. In a third particular example, the first zwitterionic polymer may be pCB and the second zwitterionic polymer may be pSB, pCB, or pTMAO. In any of the foregoing first, second, and third particular examples, the third zwitterionic polymer may be selected from, for example, pSB, pCB, or pTMAO. The resulting ZTN hydrogel may be, for example, pTMAO/pSB/pSB, pTMAO/pSB/pCB, pTMAO/pCB/pCB, pTMAO/pCB/pSB, pTMAO/pSB/pTMAO, pTMAO/pCB/pTMAO, pTMAO/pTMAO/pSB, pTMAO/pTMAO/pCB, pCB/pSB/pSB, pCB/pSB/pCB, pCB/pCB/pSB, pCB/pSB/pTMAO, pCB/pCB/TMAO, pCB/pSB/pTMAO, pCB/pTMAO/pCB, pCB/pTMAO/pSB, pSB/pCB/pCB, pSB/pCB/pSB, pSB/pSB/pCB, pSB/pCB/pTMAO, pSB/pSB/TMAO, pSB/pCB/pTMAO, pSB/pTMAO/pSB, or pSB/pTMAO/pCB, wherein each of the foregoing examples has the nomenclature (FZP/SZP/TZP), wherein FZP = first zwitterionic polymer, SZP = second zwitterionic polymer, and TZP = third zwitterionic polymer.

[00106] The zwitterionic polymer can be prepared by any suitable polymerization method, such as vinyl-addition (free radical) polymerization, atom transfer radical polymerization (ATRP), and reversible addition fragmentation chain transfer (RAFT) polymerization. Any of the known radical initiators, including photoinitiators, for polymerizing such monomers, may be used. The initiator (or photoinitiator) may be, for example, azobisisobutyronitrile (AIBN), 2-hydroxy-2-methylpropiophenone, or 2,2- dimethoxy-2-phenylacetophenone. For example, sulfobetaine methacrylate monomer may be polymerized, typically by a vinyl-addition process, to form poly(sulfobetaine methacrylate). Alternatively, a non-zwitterionic precursor polymer may first be produced, followed by conversion of the non-zwitterionic precursor polymer to a zwitterionic polymer. For example, an amino-containing polymer, such as poly(2- aminoethylmethacrylate), may be reacted with a haloalkyl molecule containing an anionic group (e.g., carboxylate or sulfonate) to result in conversion of the amino groups to ammonium groups attached indirectly to the anionic group.

[00107] In another aspect, the present disclosure is directed to a method for producing the ZTN hydrogel compositions described above. In the method, a first zwitterionic polymer is obtained (provided), either commercially or by synthesis. If by synthesis, the first zwitterionic polymer can be produced by polymerization of suitable monomers, in the absence or presence of a crosslinking monomer, typically in a mold or on the surface of a substrate. Alternatively, the first zwitterionic polymer may be affixed to (or coated onto) the surface of a substrate. In some embodiments, the first zwitterionic polymer is coated onto the surface of an object for which fouling is to be inhibited or prevented. The object may be, for example, a metallic fin, propeller, or structural material designed for use in an underwater (typically, seawater) or other saline environment. The saline environment is typically ocean water, but may be another type of brackish water, either from a natural or industrial source.

[00108] In a second step (step ii), a first precursor solution is absorbed into the first zwitterionic polymer, wherein the first precursor solution includes or exclusively contains a first zwitterionic monomer species dissolved in a solvent. The solvent may be, for example, water or an aqueous based solvent mixture (e.g., alcohol-water mixture).

[00109] In a third step (step iii), the first zwitterionic monomer species is polymerized to form a second zwitterionic polymer while absorbed in the first zwitterionic polymer, which results in formation of a zwitterionic double-network (ZDN) hydrogel composition containing the second zwitterionic polymer entangled in the first zwitterionic polymer. In some embodiments, after this two-step synthesis, the ZDN hydrogels are soaked in water for precisely or at least 1, 2, 3, 4, or 5 days to reach equilibrium, before proceeding with the fourth step (below).

[00110] In a fourth step (step iv), a second precursor solution is absorbed into the ZDN hydrogel composition, wherein the second precursor solution includes or exclusively contains a second zwitterionic monomer species dissolved in a solvent. The solvent may be, for example, water or an aqueous based solvent mixture (e.g., alcohol-water mixture), and may be the same or different as the solvent used in step (ii).

[00111] In a fifth step (step v), the second zwitterionic monomer species is polymerized to form a third zwitterionic polymer while absorbed in the ZDN hydrogel composition, which results in formation of the zwitterionic triple-network (ZTN) hydrogel composition containing the third zwitterionic polymer entangled in the ZDN hydrogel composition. The third zwitterionic polymer is typically entangled with both the first and second zwitterionic polymers in the ZTN hydrogel composition.

[00112] The first, second, and third zwitterionic polymers used in the method typically contain at least 50 mol% zwitterionic moieties. In different embodiments, one, two, or all (or each) of the first, second, and third zwitterionic polymers independently contain precisely or at least 50, 55, 60, 65, 70, 75, 80, 85, 90, 95, or 100 mol% zwitterionic moieties, or a mol% within a range bounded by any two of the foregoing values (e.g., 50- 100 mol%, 60-100 mol%, 70-100 mol%, 80-100 mol%, or 90-100 mol%).

[00113] Typically, in the method, at least one zwitterionic polymer out of the first, second, and third zwitterionic polymers is different than the other two (i.e., typically, the first, second, and third zwitterionic polymers are not all the same). In some embodiments, two of the zwitterionic polymers selected from the first, second, and third zwitterionic polymers have the same structure. In other embodiments, the first, second, and third zwitterionic polymers have different structures.

[00114] Notably, the entire process for producing the ZTN hydrogel composition, as described above, may be conducted on the surface of an object for which fouling is to be inhibited or prevented, starting with producing a coating of the first zwitterionic polymer on the surface. In some embodiments, before placing the coating of the first zwitterionic polymer on the surface, the surface is cleaned or pretreated to result in a stronger bond between the polymer and the surface.

[00115] As indicated earlier above, aside from possessing excellent resistance to fouling and mechanical properties, the ZTN hydrogel compositions described herein also surprisingly exhibit exceptional (high) fouling-release ability. The term “high fouling- release" is used herein to mean that bio-foulants can be removed from the ZTN hydrogel compositions at low waterjet pressure, e.g., no more than or below 10 or 20 Psia. The foregoing property is a significant advance since zwitterionic polymer networks of the art are typically not capable of such low-pressure release of bio-foulants.

[00116] Examples have been set forth below for the purpose of illustration and to describe the best mode of the invention at the present time. However, the scope of this invention is not to be in any way limited by the examples set forth herein.

Examples

[00117] Overview

[00118] The present disclosure offers an effective strategy to overcome the challenge for the design and synthesis of pure zwitterionic hydrogels with high mechanical and excellent non-fouling properties in saline environments. In particular embodiments, the design is based on poly(trimethylamine A-oxide) (pTMAO) with high swelling property and poly(sulfobetaine) (pSB) with strong electrostatic interaction and network entanglement as the main components to construct the Zwitterionic Triple-Network (ZTN) hydrogels. The triple-network structure can effectively inhibit the swelling of the hydrogel and protect the "locking" effect of pSB in saline environments. Example features for the new mechanism in this disclosure include the following:

[00119] (i) All networks can be made from zwitterionic moieties, which provide reliable non-fouling properties;

[00120] (ii) The first network of zwitterionic moieties is swellable in the precursor solution of the second network to prepare Zwitterionic Double-Network (ZDN) hydrogels. The prepared ZDN hydrogels are immersed in the precursor solution of the third network to obtain the ZTN hydrogels. The second network of the zwitterionic moiety provides the “lock effect”, while the incorporation of the third network of the zwitterionic moiety can protect the “lock effect” in saline environments; and

[00121] (iii) The mechanical properties or swelling ratio can be determined or designed for any zwitterionic moiety via adjusting the intermolecular and intramolecular forces. The intermolecular and intramolecular forces can be adjusted by changing the types of positive or negative groups in zwitterionic moiety, or by changing the distance between the positive and negative groups.

[00122] Example 1. Preparation of pTMAO/pSB/pSB ZTN hydrogels

[00123] The pTMAO/pSB/pSB ZTN hydrogels were synthesized by a three-step sequential free-radical polymerization. As shown in FIG. 1, the prepared Zwitterionic Single-Network (ZSN) hydrogel was immersed in the second network solution and photo-polymerized by UV irradiation to prepare the ZDN hydrogel. Subsequently, the prepared ZDN hydrogel was immersed in the third network solution and photo-polymerized by UV irradiation to obtain the ZTN hydrogel. Specifically, in the first step, pTMAO zwitterionic single network (ZSN) hydrogels were synthesized by a photopolymerization method using 1 M (molality) of TMAO monomer, 4 mol% of cross-linker A ^f ,A ^f -methylenebis(acryl amide) (MBAA), and 0.1 mol% of initiator 2-hydroxy-2-methylpropiophenone (1173) (molar percentages both were relative to the TMAO monomer) in a transparent sheet mold under ultraviolet (UV) irradiation having a wavelength of 302 nm and a power of 50 watts for 1 hour under a nitrogen atmosphere. In the second step, the ZSN hydrogels were immersed in an aqueous solution containing [2-(methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)- ammonium hydroxide (SB) at 4M concentration, 0.1 mol% of MBAA, and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The fully swollen hydrogel was further polymerized by UV irradiation having a wavelength of 302 nm and 50-watt power for 10 hours under a nitrogen atmosphere to obtain the zwitterionic double-network (ZDN) hydrogel. After this two-step synthesis, the ZDN hydrogels were soaked in water for at least 3 days to reach equilibrium. In the third step, the as-prepared ZDN hydrogels were immersed in SB solution containing 4M SB, 0.1 mol% of MBAA, and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The polymerization was performed by 302 nm UV irradiation with 50-watt power for 10 hours under a nitrogen atmosphere. After polymerization, the as-prepared pTMAO/pSB/pSB ZTN hydrogels were immersed in water for 1 week until they reached swelling equilibrium. FIG. 2 shows the optical photos of the preparation process of the pTMAO/pSB/pSB ZTN hydrogels.

[00124] Example 2. Compression testing of the pTMAO/pSB/pSB ZTN hydrogels

[00125] The compressive test of the hydrogels was conducted at room temperature by using a universal testing machine with a 10 kN load cell. Before the experiment, the tested hydrogels were equilibrated in water, phosphate-buffered saline (PBS, 10 mM phosphate, 138 mM sodium chloride, 2.7 mM potassium chloride, pH 7.4), and seawater (36 g of sea salt in 1L deionized water), respectively, and then cut into cylinders (5 mm in diameter and 2 mm in thickness) with the biopsy punch. The crosshead speed was set at 1 mm/min. Average data were acquired by testing five specimens for each sample. Modulus was calculated from the initial slope of the stress-strain curve with a strain 5-10%. The compressive toughness was calculated as the area under stress-strain curves.

[00126] As shown in FIG. 3, the equilibrated pTMAO/pSB/pSB ZTN hydrogel exhibited high compressive fracture stress (18.7 ± 1.2 MPa in water, 18.2+ 1.4 MPa in PBS, and 18.2 + 1.4 MPa in seawater) both in water and saline environments. The compressive strain of ZTN hydrogel could be up to 99%, as shown in FIG. 4. As shown in FIG. 5, the ZTN hydrogel exhibited high toughness (1.76±0.05 MJ/m ³ in water, 1.70±0.03 MJ/m ³ in PBS, and 1.62±0.03 MJ/m ³ in seawater) both in water and saline environments. However, the ZDN hydrogel became weak with toughness decreased to -0.5 MJ/m ³ in saline solutions due to the role of the “anti-polyelectrolyte” effect. Moreover, the ZTN hydrogel is stiffer than the ZSN and ZDN hydrogels, with a modulus of 0.37 + 0.02 MPa in water, 0.59 + 0.01 MPa in PBS, and 0.66 + 0.03 MPa in seawater, as shown in FIG. 6. FIG. 7 and FIG. 8 show the stress-strain curves of the ZSN, ZDN, and ZTN hydrogels after their immersion in PBS and seawater, respectively, indicating the high stress of ZTN hydrogel. FIG. 9 and FIG. 10 demonstrate how the ZTN hydrogel sustained a high compression (compressive strain up to 99%) while the ZDN hydrogel broke down easily after immersing in PBS and seawater. Based on the triple-network structure, strong interpenetrating chain entanglement of pSB network could form in the ZTN hydrogel, which will provide an effective energy dissipation for improving the mechanical strength of hydrogel in saline environments. Furthermore, the stability of the ZTN hydrogel in saline environments was tested. The ZTN hydrogel was soaked in seawater for 30 days. After that, their mechanical properties, including compressive fracture stress and fracture strain were tested. As shown in FIG. 11, no obvious change was observed after immersing for 30 days, which confirmed the long-term stability of the ZTN hydrogel.

[00127] Example 3. Composition, equilibrium water content (EWC), and swelling ratio (SR) test of the pTMAO/pSB/pSB ZTN hydrogels

[00128] The composition, EWCs, and SRs of the ZSN, ZDN, and ZTN hydrogels were measured by gravimetric analysis. The hydrogels were cut into circular samples (diameter 10 mm, thickness 1 mm) and soaked in water, PBS, and seawater, respectively, at room temperature until swelling equilibrium was reached. The composition ratio was defined by the molar ratio of the pSB and pTMAO components (pSB/pTMAO). As shown in FIG. 12, the molar ratios of pSB/pTMAO for the ZDN and ZTN hydrogels were 22.0±0.4 and 46.1±0.7, respectively, indicating that pSB is the major component of the ZDN and ZTN hydrogels and the ZDN hydrogel can absorb a large amount of SB monomers when immersing in SB solution. A large amount of the pSB component in the ZDN hydrogel is due to the superhydrophilicity and high swellability of the pTMAO component, while the significant absorption of SB monomers for the ZDN hydrogel is due to the strong electrostatic interaction of SB moieties. The EWC refers to the water content of hydrogel after soaking in water. The SR is defined as the fractional increase in the weight of the hydrogel due to water absorption. As shown in FIG. 13 and FIG. 14, the EWC and SR of the ZTN hydrogel (EWC:58.3 ±0.8%, SR: 152.5 ± 1.1%) were lower than those of the ZSN (EWC:97.8±0.2%, SR: 3803.2 + 73.9%) and ZDN hydrogel (EWC:62.8 + 0.6%, SR: 178.7 + 3.1%) in water. This suggests that the triple-network structure can effectively inhibit the swelling of the hydrogel. The difference of SR between the ZDN and ZTN hydrogels was enlarged in saline solutions that the SR of the ZDN hydrogel (PB S : 293.1 + 1.9%, seawater: 369.6+ 12.5%) became significantly higher than that of the ZTN hydrogel (PBS: 202.2 + 1.5%, seawater: 272.1 + 1.4%). The results indicate that the ZDN hydrogel swelled dramatically in saline environment. Based on the triple-network structure, strong interpenetrating chain entanglement of pSB network could form in the ZTN hydrogel and protect the "locking" effect of pSB in saline environments.

[00129] Example 4. Fibrinogen adhesion test of the pTMAO/pSB/pSB ZTN hydrogels

[00130] The human fibrinogen adhesion test of hydrogels was performed in 24-well tissue culture polystyrene (TCPS) plates utilizing an enzyme-linked immunosorbent assay (ELISA) method following our previous reports. Before the experiment, the tested hydrogels were equilibrated in PBS and cut into uniform disks (5 mm in diameter and 1 mm in thickness) with the biopsy punch. Each sample disk was first incubated in 1 mL of fibrinogen solution (1 mg/mL, freshly prepared in PBS) at 37°C for 1.5 hours. Before being transferred into new wells, the samples were rinsed with 5 x 2 mL PBS to remove dissociative fibrinogen. 1 mL of horseradish peroxidase (HRP) conjugated anti-fibrinogen solution (1 pg/mL, in PBS) was then added in each well followed by incubation at room temperature for 1.5 hours. All the samples were then transferred to new wells after another five washes with PBS. 1 mL of o-phenylenediamine (OPD) solution (1 mg/mL, with 0.1 M citrate phosphate, pH 5.0) containing 0.03% hydrogen peroxide was added. After 15 minutes of incubation at room temperature, the enzymatic reaction was stopped by adding 1 mL of IN HC1. The absorbance values at 492 nm of all the samples were recorded by a plate reader and were normalized to that of TCPS (96-well, control). Each measurement was performed in triplicate (independent replicates). As shown in FIG. 15, both the ZDN and ZTN hydrogels exhibited excellent non-fouling abilities by decreasing 99.1% and 99.5% of fibrinogen adsorption as compared to that of TCPS.

[00131] Example 5. Serum fouling test of the pTMAO/pSB/pSB ZTN hydrogels

[00132] Human serum fouling of the hydrogels was evaluated via the bicinchoninic acid assay (BCA) method. All the tested hydrogels were equilibrated in PBS and then cut into uniform disks (5 mm in diameter and 1 mm in thickness) with the biopsy punch. The hydrogels were immersed into 200 pL of undiluted human pooled serum in 96-well TCPS plates with one disk per well and allowed proteins to adsorb at 37°C for 2 hours. After 2 hours, the samples were removed from the undiluted human pooled serum and washed with 200 pL of PBS five times. Samples were then sonicated in 200 pL PBS + 1 wt% sodium n- dodecyl sulfate (SDS) solution for 5 minutes to desorb proteins. This solution was analyzed using the Micro-BCA assay for quantifying the amount of adsorbed proteins and the absorbance values at 562 nm of all the samples were recorded by a plate reader and were normalized to that of TCPS (96-well, control). Each measurement was performed in triplicate (independent replicates). As shown in FIG. 16, the ZTN hydrogel exhibited excellent non-fouling abilities by decreasing 98.4% of undiluted human serum as compared to that of TCPS due to the strong hydration capability of the zwitterionic polymers.

[00133] Example 6. Algae biofilm growth test of the pTMAO/pSB/pSB ZTN hydrogels

[00134] Before the experiment, the pTMAO/pSB/pSB ZTN hydrogel was pasted into a 24 well plate to prepare the hydrogel coating (Coating #1). Algae ( N . incertd) were diluted to and OD of 0.03 at 660 nm in artificial sea water (ASW) supplemented with nutrients. 1 mL of diluted algae solution was added to each well of the plate and allowed to incubate statically for 48 hours for biofilm growth. Algal biofilm growth was quantified by fluorescence measurement of DMSO extracts of chlorophyll. Biofilm growth was reported as fluorescence intensity, i.e., relative fluorescence units, as shown in FIG. 17. Error bars represent one standard deviation of the mean. The coatings prepared by intersleek 700 (700), intersleek 900 (900), intersleek 1100SR (1100SR), polyurethane (PU) and silastic-T2 (T2) were used as control. As shown in FIG. 17, the growth amount of algal biofilm on the surface of pTMAO/pSB/pSB ZTN hydrogel coating was much lower than that of other commercial marine coatings, exhibiting excellent non-fouling property.

[00135] Example 7. Algal cell adhesion test of the pTMAO/pSB/pSB ZTN hydrogels

[00136] Before the experiment, the pTMAO/pSB/pSB ZTN hydrogel was pasted into a 24 well plate to prepare the hydrogel coating (Coating #1). Initial cell attachment of algae (N. incertd) was assessed before water jet adhesion analysis. Algal cell attachment was quantified by fluorescence measurement of DMSO extracts of chlorophyll. Water jet adhesion was carried out after 2 hours of initial cell attachment. The first column of each plate was not treated and served as the measure of cell attachment after 2 hours. The second and third column of each coating was jetted for 10 seconds at a pressure of 10 psi and 20 psi, respectively. Algal adhesion was reported as a function of biomass remaining (FIG. 18) and percent removal (FIG. 19) on the material surface after treatment with each pressure indicated above. Error bars represent one standard deviation of the mean. The coatings prepared by intersleek 700 (700), intersleek 900 (900), intersleek 1100SR (1100SR), polyurethane (PU) and silastic-T2 (T2) were used as control. As shown in FIG. 18, the biomass remaining of algal cell on the surface of pTMAO/pSB/pSB ZTN hydrogel coating (Coating #1) was much lower than that of other commercial marine coatings, demonstrating excellent nonfouling property. The percent removal of algal on the pTMAO/pSB/pSB ZTN hydrogel coating surface was almost 100% after treatment with water jet at a pressure of 10 psi (FIG. 19).

[00137] Example 8. Preparation of pTMAO/pSB/pCB ZTN hydrogels

[00138] pTMAO/pSB/pCB ZTN hydrogels were synthesized by a three-step sequential free- radical polymerization. In the first step, pTMAO zwitterionic single network (ZSN) hydrogels were synthesized by photopolymerization using 1 M (molality) of TMAO monomer, 4 mol% of cross-linker N, /V-methylenebis(acrylamide) (MBAA), and 0.1 mol% of initiator 2-hydroxy-2-methylpropiophenone (1173) (molar percentages both were relative to the TMAO monomer) in a transparent sheet mold under ultraviolet (UV) irradiation having a wavelength of 302 nm and a power of 50 watts for 1 hour under a nitrogen atmosphere. In the second step, the ZSN hydrogel was immersed in an aqueous solution containing [2- (Methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SB) at 4M concentration, 0.1 mol% of MBAA, and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The fully swollen hydrogels were further polymerized by UV irradiation having a wavelength of 302 nm and 50-watt power for 10 hours under a nitrogen atmosphere to obtain zwitterionic double-network (ZDN) hydrogels. After this two-step synthesis, the ZDN hydrogels were soaked in water for at least 3 days to reach equilibrium. In the third step, the as-prepared ZDN hydrogels were immersed in CB solution containing 4M 1 -carboxy-V, V-di m ethyl -N-( 3 -aery 1 am i dopropy 1 ) ethanaminium inner salt (CB1) or 2- carboxy-V, V-di m ethyl -N-( 3 -aery 1 am i dopropy 1 ) ethanaminium inner salt (CB2), 0.1 mol% of MBAA and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The polymerization was performed by 302 nm UV irradiation with 50-watt power for 10 hours under a nitrogen atmosphere. After polymerization, the as-prepared pTMAO/pSB/pCB ZTN hydrogels were immersed in water for 1 week until they reached swelling equilibrium. The swelling ratios (mass increase) of the pTMAO/pSB/pCB 1 and pTMAO/pSB/pCB2 hydrogels in water were 250% and 270%, respectively. [00139] Example 9. Compression testing of the pTMAO/pSB/pCB ZTN hydrogels

[00140] The compressive test of the hydrogels was conducted at room temperature by using a universal testing machine with a 10 kN load cell. Before the experiment, the tested hydrogels were equilibrated in water, PBS, and seawater, respectively, and then cut into cylinders (5 mm in diameter and 2 mm in thickness) with the biopsy punch. The crosshead speed was set at 1 mm/min. Average data were acquired by testing five specimens for each sample. As shown in FIG. 20, the compressive fracture stress of pTMAO/pSB/pCB 1 and pTMAO/pSB/pCB2 ZTN hydrogels in water was 12 MPa and 14 MPa, respectively, and about 1.5 MPa in saline solutions (PBS and seawater), which was lower than that of pTMAO/pSB/pSB ZTN hydrogel. This is because the associations of SB moieties are stronger than those of CB moieties.

[00141] Example 10. Preparation of the pCB/pSB/pSB ZTN hydrogels

[00142] pCB/pSB/pSB ZTN hydrogels were synthesized by a three-step sequential free- radical polymerization. In the first step, the pTMAO zwitterionic signal network (ZSN) hydrogels were synthesized by photopolymerization using 1 M (molality) of 1-carboxy- A( A ^f -di methyl -N-(3’ -aery 1 ami dopropyl) ethanaminium inner salt (CB1) or 2-carboxy-A', A ^f - dimethyl-N-(3’-acrylamidopropyl) ethanaminium inner salt (CB2) monomer, 4 mol% of cross-linkerN, A-methylenebis(acrylamide) (MBAA), and 0.1 mol% of initiator 2-hydroxy - 2-methylpropiophenone (1173) (molar percentages both were relative to the CB monomer) in the transparent sheet mold under the ultraviolet (UV) irradiation with a wavelength of 302 nm and a power of 50 watts for 1 hour under a nitrogen atmosphere. In the second step, the ZSN hydrogels were then immersed in an aqueous solution containing [2- (methacryloyloxy)ethyl]dimethyl-(3-sulfopropyl)ammonium hydroxide (SB) at 4M concentration, 0.1 mol% of MBAA, and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The fully swollen hydrogels were further polymerized by UV irradiation with a wavelength of 302 nm and 50-watt power for 10 hours under a nitrogen atmosphere to obtain the zwitterionic double-network (ZDN) hydrogels. After this two-step synthesis, the ZDN hydrogels were soaked in water for at least 3 days to reach equilibrium. In the third step, the as-prepared ZDN hydrogels were immersed in SB solution containing 4M SB, 0.1 mol% of MBAA, and 0.01 mol% of photoinitiator 1173 for one day until equilibrium was reached. The polymerization was performed by 302 nm UV irradiation with 50-watt power for 10 hours under a nitrogen atmosphere. After polymerization, the as- prepared pCB/pSB/pSB ZTN hydrogels were immersed in water for 1 week until they reached swelling equilibrium.

[00143] While there have been shown and described what are at present considered the preferred embodiments of the invention, those skilled in the art may make various changes and modifications which remain within the scope of the invention defined by the appended claims.