Biological material encapsulated in a modified alginate comprising one or more covalently modified monomers defined by Formula I Formula I wherein, X is oxygen, R 1 is substituted alkyl, wherein the substituted alkyl comprises substituted aryl, substituted heteroaryl, or Y 1 and Y 2 independently are hydrogen or -PO 2; or Y 2 is absent, and Y 1, together with the two oxygen atoms to which Y 1 and Y 2 are attached form a cyclic structure as shown below and R 2 and R 3 are, independently, hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, alkoxy, substituted alkoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, arylthio, substituted arylthio, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, polyaryl, substituted polyaryl, C 3 -C 20 cyclic, substituted C 3 -C 20 cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(ethylene glycol), peptide, or polypeptide group; or R 2 and R 3, together with the carbon atom to which they are attached, form a 3- to 8-membered unsubstituted or substituted carbocyclic or heterocyclic ring; and R is, independently for each occurrence, hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, alkoxy, substituted alkoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, arylthio, substituted arylthio, polyaryl, substituted polyaryl, C 3 -C 20 cyclic, substituted C 3 -C 20 cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(ethylene glycol), peptide, or polypeptide group. X is oxygen, R 1 is substituted alkyl, wherein the substituted alkyl comprises substituted aryl, substituted heteroaryl, or Y 1 and Y 2 independently are hydrogen or -PO 2; or Y 2 is absent, and Y 1, together with the two oxygen atoms to which Y 1 and Y 2 are attached form a cyclic structure as shown below and R 2 and R 3 are, independently, hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, alkoxy, substituted alkoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, arylthio, substituted arylthio, carbonyl, substituted carbonyl, carboxyl, substituted carboxyl, amino, substituted amino, amido, substituted amido, polyaryl, substituted polyaryl, C 3 -C 20 cyclic, substituted C 3 -C 20 cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(ethylene glycol), peptide, or polypeptide group; or R 2 and R 3, together with the carbon atom to which they are attached, form a 3- to 8-membered unsubstituted or substituted carbocyclic or heterocyclic ring; and R is, independently for each occurrence, hydrogen, alkyl, substituted alkyl, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, aryl, substituted aryl, heteroaryl, substituted heteroaryl, alkoxy, substituted alkoxy, aroxy, substituted aroxy, alkylthio, substituted alkylthio, arylthio, substituted arylthio, polyaryl, substituted polyaryl, C 3 -C 20 cyclic, substituted C 3 -C 20 cyclic, heterocyclic, substituted heterocyclic, amino acid, poly(ethylene glycol), peptide, or polypeptide group.