Cross-linked polymeric material

A cross-linked polymeric material formed by polymerizing a polyethylene glycol di(meth)acrylate and a sulfonate salt containing a double bond that facilitates covalent bonding of the sulfonate salt to the polyethylene glycol di(meth)acrylate, wherein, in said polymeric material, cations from the salt are replaced with metal ions selected from sodium, lithium, aluminum, magnesium, and zinc, and combinations thereof, wherein:

• (a) the sulfonate salt is vinylsulfonic acid sodium salt;
• and/or
• (b) the metal ions are lithium ions;
• and/or
• (c) the cross-linked polymeric material comprises a repeating unit of formula (I):
• wherein:
• R1 and R1′ are individually selected from hydrogen and methyl;
• R2 and R2′ are individually selected from H, Me, H,H and H,Me;
• A and A′ are individually selected from a bond, Ph, C1-6alkyl, —C(O)OC1-6alkyl, and —C(O)NHC1-6alkyl;
• M and M′ are individually selected from sodium, lithium, aluminum, magnesium, and zinc;
• m and m′ are each an integer selected from 1 to 50;
• n is an integer selected from 1 to 30;
• n′ is an integer selected from 0 to 30;
• p is an integer selected from 2 to 20;
• each * indicates a point of attachment to an adjacent polymer repeating unit;
• and
• *′ and *″ indicate a point of attachment to an adjacent polymer repeating unit, or, when R2 or R2′ is H,H or H,Me, *′ or *″, respectively, is absent.