A hydrogel, comprising a first polymer covalently coupled to a second polymer, wherein the first polymer comprises a first core having two or more polymeric branches covalently coupled to and extending from the first core, wherein the polymeric branches covalently coupled to and extending from the first core comprise first constitutional units selected from the group consisting of zwitterionic constitutional units and mixed charge constitutional units, and wherein the two or more polymeric branches covalently coupled to and extending from the first core each comprise one or more first functional groups effective for covalently coupling the first polymer to the second polymer, wherein the second polymer comprises a second core having two or more polymeric branches covalently coupled to and extending from the second core, wherein the polymeric branches covalently coupled to and extending from the second core comprise second constitutional units selected from the group consisting of zwitterionic constitutional units and mixed charge constitutional units, and wherein the two or more polymeric branches covalently coupled to and extending from the second core each comprise one or more second functional groups effective for covalently coupling the second polymer to the first polymer; wherein the hydrogel comprises covalent bonds linking the first polymer and the second polymer formed by reaction of the one or more first functional groups and the one or more second functional groups; and wherein the one or more first functional groups and the one or more second functional groups are a click chemistry reactive pair selected from the pairs of an azide and an alkyne, an azide and an alkene, a thiol and a maleimide, or a thiol and a disulfide. wherein the first polymer comprises a first core having two or more polymeric branches covalently coupled to and extending from the first core, wherein the polymeric branches covalently coupled to and extending from the first core comprise first constitutional units selected from the group consisting of zwitterionic constitutional units and mixed charge constitutional units, and wherein the two or more polymeric branches covalently coupled to and extending from the first core each comprise one or more first functional groups effective for covalently coupling the first polymer to the second polymer, wherein the second polymer comprises a second core having two or more polymeric branches covalently coupled to and extending from the second core, wherein the polymeric branches covalently coupled to and extending from the second core comprise second constitutional units selected from the group consisting of zwitterionic constitutional units and mixed charge constitutional units, and wherein the two or more polymeric branches covalently coupled to and extending from the second core each comprise one or more second functional groups effective for covalently coupling the second polymer to the first polymer; wherein the hydrogel comprises covalent bonds linking the first polymer and the second polymer formed by reaction of the one or more first functional groups and the one or more second functional groups; and wherein the one or more first functional groups and the one or more second functional groups are a click chemistry reactive pair selected from the pairs of an azide and an alkyne, an azide and an alkene, a thiol and a maleimide, or a thiol and a disulfide.