Crosslinked mixed charge hydrogels

1. A crosslinked hydrogel, comprising:

a crosslinked polymer having repeating units comprising a plurality of positively charged repeating units and a plurality of negatively charged repeating units, wherein the crosslinked ratio of the number of positively charged repeating units to the number of the negatively charged repeating units is from about 1:1.1 to about 1:0.5; and

a plurality of crosslinks,

wherein each crosslink is represented by the formula:

wherein

R 1 and R 2 are independently selected from the group consisting of hydrogen, fluorine, trifluoromethyl, C1-C6 alkyl, and C6-C12 aryl groups;

R 3 is selected from the group consisting of C1-C6 alkyl, C6-C12 aryl, CH 2 ═C(R 1 )-L 1 -, CH 2 ═C(R 2 )-L 2 -, or R 3 is the residual portion of a third crosslink, -L 1 -CR 1 —CH 2 — or -L 2 -CR 2 —CH 2 —;

L 1 and L 2 are independently selected from the group consisting of —C(═O)O—(CH 2 ) n — and —C(═O)NH—(CH 2 ) n —, wherein n is an integer from 1 to 20;

L 3 is —(CH 2 ) n —, where n is an integer from 1 to 20;

A 1 is C, S, SO, or PO;

x is an integer from about 5 to about 10,000;

X − is a counter ion associated with the N + cationic center; and

M + is a counter ion associated with the (A=O)O − anionic center.

2. The crosslinked hydrogel of claim 1 , wherein each repeating unit is represented by the formula:

wherein

R 7 and R 8 are independently selected from hydrogen, fluorine, trifluoromethyl, C1-C6 alkyl, and C6-C12 aryl groups;

R 9 , R 10 , and R 11 are independently selected from alkyl and aryl, or taken together with the nitrogen to which they are attached form a cationic center;

A 3 (=O)—OM) is an anionic center, wherein A 3 is C, S, SO, or PO, and M is a metal or organic counterion;

L 6 is a linker that covalently couples the cationic center [N + (R 9 )(R 10 )(R 11 )] to the polymer backbone;

L 7 is a linker that covalently couples the anionic center [A 3 (=O)—OM] to the polymer backbone;

X − is the counter ion associated with the cationic center [N+(R9)(R10)(R11)];

M + is a counter ion associated with the (A=O)O − anionic center;

n is an integer from 5 to about 10,000; and

p is an integer from 5 to about 10,000.

3. The hydrogel of claim 2 , wherein R 1 , R 2 , R 3 , R 7 , R 8 , R 9 , R 10 , and R 11 are independently selected from the group consisting of C1-C3 alkyl.

4. The hydrogel of claim 2 , wherein R 7 , R 8 , R 9 , R 10 , and R 11 are methyl.

5. The hydrogel of claim 1 , wherein L 1 and L 2 are —C(═O)O—(CH 2 ) n —, wherein n is 1-6.

6. The hydrogel of claim 1 , wherein L 3 is —(CH 2 ) n —, where n is an integer from 1-6.

7. The hydrogel of claim 2 , wherein L 6 is selected from the group consisting of —C(═O)O—(CH 2 ) n — and —C(═O)NH—(CH 2 ) n —, wherein n is an integer from 1-20.

8. The hydrogel of claim 2 , wherein L 7 is a C1-C20 alkylene chain.

9. The hydrogel of claim 2 , wherein L 6 and L 7 are —C(═O)O—(CH 2 ) 2 —.

10. The hydrogel of claim 1 , wherein A 1 is C or SO.

11. The hydrogel of claim 2 , wherein M + at each occurrence is selected from the group consisting of a metal and organic ion.

12. The hydrogel of claim 2 , wherein X − at each occurrence is selected from the group consisting of halide, carboxylate, alkylsulfonate, sulfate, nitrate, perchlorate, tetrafluoroborate, hexafluorophosphate, trifluoromethylsulfonate, bis(trifluoromethylsulfonyl)amide, lactate, and salicylate.

13. The hydrogel of claim 1 , wherein x is an integer from about 10 to about 1,000.

14. The hydrogel of claim 2 , wherein n is an integer from about 10 to about 1,000.

15. The hydrogel of claim 2 , wherein p is an integer from about 10 to about 1,000.

16. The hydrogel of claim 1 , wherein the hydrogel is a gradient hydrogel.

17. The hydrogel of claim 1 , further comprising a particle encapsulated therein.

18. The hydrogel of claim 17 , wherein the particle is a nanoparticle.

19. The hydrogel of claim 18 , wherein the nanoparticle has a diameter of about 5 nm to about 200 nm.

20. The hydrogel of claim 18 , wherein the nanoparticle is a gold nanoparticle.

21. The hydrogel of claim 18 , wherein the nanoparticle is a polymer-coated nanoparticle.

22. The hydrogel of claim 21 , wherein the nanoparticle is coated with a polymer comprising carboxybetaine methacrylate (CBMA) or oligo(ethylene glycol) methyl ether methacrylate (OEGMA).

23. A surface of a substrate, wherein the surface comprises a hydrogel of claim 1 .

24. The surface of claim 23 , wherein the substrate is selected from the group consisting of a medical device, a glucose sensor, a particle, a drug carrier, a non-viral gene delivery system, a biosensor, a membrane, an implantable sensor, a subcutaneous sensor, an implant, and a contact lens.

25. The surface of claim 24 , wherein the particle is a nanoparticle.