Photo-responsive spiropyran-based N-isopropylacrylamide (NIPAM) gels

1. A photo-responsive hydrogel comprising a copolymer formed from comonomers comprising:

(i) N-isopropylacrylamide (NIPAM),

(ii) a polymerisable unit of benzospiropyran having a polymerizable group capable of being polymerized into the copolymer backbone,

(iii) a cross-linking agent, and

(iv) an acid having a pKa of less than 6,

wherein the acid comonomer (iv) is copolymerized, together with comonomers (i)-(iii), into the copolymer backbone and retains acid functionality, such that the copolymerised acid in the copolymer backbone retains a pKa of less than 6 and provides an internal source of protons for the hydrogel to self-protonate without external acidification.

2. A photo-responsive hydrogel according to claim 1 wherein said hydrogel exhibits spontaneous swelling in the dark, and photo-induced shrinking upon white light irradiation.

3. A photo-responsive hydrogel according to claim 1 wherein said polymerisable unit of benzospiropyran comprises at least one acrylate group.

4. A photo-responsive hydrogel according to claim 3 wherein said polymerisable unit of benzospiropyran comprises benzospiropyran acrylate (BSP-A).

5. A photo-responsive hydrogel according to claim 1 wherein the copolymerisable acid is selected from the group consisting of acrylic acid (AA), methacrylic acid, 2-ethylacrylic acid, 2-propylacrylic acid, 2-(Trifluoromethyl)acrylic acid, 2-(Bromomethyl)acrylic acid, 2-Bromacrylic acid, vinylsulphonic acid, 4-styrenesulfonic acid, 2-Acrylamido-2-methyl-1-propanesulfonic acid and vinylphosphonic acid.

6. A photo-responsive hydrogel according to claim 1 wherein the cross-linking agent is selected from the group consisting of N,N′-methylenebisacrylamide (MBIS), N,N′-ethylenebisacrylamide, butanediol diacrylate, hexanedioldiacrylate, poly(ethyleneglycol) diacrylate and poly(propyleneglycol) diacrylate.

7. A photo-responsive hydrogel according to claim 6 comprising a copolymer comprising acrylic acid-co-benzospiropyran acrylate-co-N-isopropylacrylamide-co-N,N′-methylenebisacrylamide (AA-co-BSP-A-co-NIPAM-co-MBIS).

8. A photo-responsive hydrogel according to claim 7 comprising NIPAM in an amount of 100 mol %; BSP-A in an amount in the range 1 to 3 mol %, acrylic acid (AA) in an amount in the range 1 to 15 mol % and MBIS in an amount in the range 0.1 to 3 mol % with all mol % being determined relative to the amount of NIPAM in mol.

9. A photo-responsive hydrogel according to claim 7 wherein the copolymer comprises AA-co-BSP-A-co-NIPAM-co-MBIS having the structure

wherein X:Y:Z:C=5:1:100:3.

10. A photo-responsive hydrogel according to claim 7 wherein the gel shrinks by an amount in the range 1 to 90% upon white light irradiation for a period of 0.01 seconds to 30 minutes.

11. A photo-responsive hydrogel according to claim 8 wherein the gel comprises 1 mol % BSP-A and re-swells when in darkness to ≥50% of its initial diameter within a period of 1 to 30 minutes upon exposure to water, for a period of 15-60 minutes.

12. A photo-responsive hydrogel according to claim 1 , further comprising a pore-forming agent, wherein said pore-forming agent comprises poly(ethylene glycol) having a molecular weight in the range 2,000 g/mol to 20,000 g/mol.

13. A process for the preparation of a hydrogel comprising

(a) forming a mixture of comonomers comprising (i) N-isopropylacrylamide (NIPAM), (iii) a cross-linking agent, (ii) a polymerisable unit of benzospiropyran having a polymerizable group capable of being polymerized into the copolymer backbone, and (iv) a copolymerisable acid having a pKa less than 6;

(b) dissolving said mixture in an organic solvent;

(c) adding an initiator; and

(d) polymerising the comonomers (i)-(iv) with white or UV light, to form a copolymer, wherein the acid comonomer (iv) is copolymerized, together with comonomers (i)-(iii), into the copolymer backbone and retains acid functionality, such that the copolymerised acid in the copolymer backbone retains a pKa of less than 6 and provides an internal source of protons for the hydrogel to self-protonate without external acidification.

14. A process according to claim 13 wherein the initiator comprises a white light photo-initiator or a UV-initiator.

15. A process according to claim 13 wherein the polymerisable unit of benzospiropyran comprises benzospiropyran acrylate.

16. A process according to claim 13 wherein the copolymerisable acid is selected from the group consisting of acrylic acid, methacrylic acid, 2-ethylacrylic acid, 2-propylacrylic acid, 2-(Trifluoromethyl)acrylic acid, 2-(Bromomethyl)acrylic acid, 2-Bromacrylic acid, vinylsulphonic acid, 4-styrenesulfonic acid, 2-Acrylamido-2-methyl-1-propanesulfonic acid and vinylphosphonic acid.

17. A process according to claim 13 , further comprising the step of adding a pore-forming agent to said organic solvent prior to step (b), wherein said pore-forming agent comprises poly(ethylene glycol).

18. The process according to claim 13 wherein said organic solvent is 1,4-dioxane, ethanol, acetone, tetrahydrofuran (THF), dimethyl sulfoxide (DMSO), or mixtures thereof with water.

19. A method for actuating a hydrogel, comprising:

providing a photo-responsive hydrogel comprising a copolymer formed from comonomers comprising:

i) N-isopropylacrylamide (NIPAM),

ii) a polymerisable unit of benzospiropyran having a polymerizable group capable of being polymerized into the copolymer backbone,

iii) a cross-linking agent, and

iv) an acid having a pKa of less than 6,

wherein the acid comonomer (iv) is copolymerized, together with comonomers (i)-(iii), into the copolymer backbone and retains acid functionality, such that the copolymerized acid in the copolymer backbone retains a pKa of less than 6 and provides an internal source of protons; and

exposing the photo-responsive hydrogel to water such that the photo-responsive hydrogel spontaneously self-protonates, without external acidification, and expands from a first size to a second larger size.