A polymer nanocapsule comprising a polymer shell and a microRNA (miRNA) molecule, wherein the polymer shell comprises a) at least one positively charged monomer, b) at least one degradable cross-linker, and c) at least one neutral monomer for use in a method of treating cerebral or brain ischemia/reperfusion injury, comprising administering said polymer nanocapsule systemically, wherein the miRNA knocks down or decreases expression of an overexpressed gene and/or increases expression of a gene in which expression is otherwise reduced or inhibited, thereby treating the cerebral or brain ischemia/reperfusion injury; and wherein the positively charged monomer is selected from N-(3-aminopropyl)methacrylamide, N-(3-Aminopropyl) methacrylamide hydrochloride, acryl-spermine, dimethylamino ethyl methacrylate, (3- Acrylamidopropyl)trimethylammonium hydrochloride, (3- Acrylamidopropyl)trimethylammonium hydrochloride, N-(3-((4-((3-aminopropyl)amino)butyl)amino)propyl)methacrylamide, N-(3-((4-aminobutyl)amino)propyl)acrylamide, N-(3-((4-aminobutyl)amino)propyl)methacrylamide, N-(2-((2-aminoethyl)(methyl)amino)ethyl)methacrylamide, N-(piperazin-1-ylmethyl)acrylamide, N-(piperazin-1-ylmethyl)methacrylamide, N-(2-(bis(2-aminoethyl)amino)ethyl)acrylamide, and N-(2-(bis(2-aminoethyl)amino)ethyl)methacrylamide; and preferably wherein the positively charged monomer is N-(3-aminopropyl)methacrylamide; wherein the degradable cross-linker comprises a crosslinker selected from glycerol dimethacrylate (GDMA), 1,3-glycerol dimethacrylate, glycerol 1,3-diglycerolate diacrylate, N,N'-bis(acryloyl)cysteamine, bis[2-(methacryloyloxy)ethyl]phosphate, and bisacryloylated polypeptide; and preferably wherein the degradable cross- linker comprises glycerol dimethacrylate (GDMA); and wherein the neutral monomer is selected from acrylamide, poly(ethylene glycol) methyl ether acrylate (mPEG), or a combination thereof. wherein the positively charged monomer is selected from N-(3-aminopropyl)methacrylamide, N-(3-Aminopropyl) methacrylamide hydrochloride, acryl-spermine, dimethylamino ethyl methacrylate, (3- Acrylamidopropyl)trimethylammonium hydrochloride, (3- Acrylamidopropyl)trimethylammonium hydrochloride, N-(3-((4-((3-aminopropyl)amino)butyl)amino)propyl)methacrylamide, N-(3-((4-aminobutyl)amino)propyl)acrylamide, N-(3-((4-aminobutyl)amino)propyl)methacrylamide, N-(2-((2-aminoethyl)(methyl)amino)ethyl)methacrylamide, N-(piperazin-1-ylmethyl)acrylamide, N-(piperazin-1-ylmethyl)methacrylamide, N-(2-(bis(2-aminoethyl)amino)ethyl)acrylamide, and N-(2-(bis(2-aminoethyl)amino)ethyl)methacrylamide; and preferably wherein the positively charged monomer is N-(3-aminopropyl)methacrylamide; wherein the degradable cross-linker comprises a crosslinker selected from glycerol dimethacrylate (GDMA), 1,3-glycerol dimethacrylate, glycerol 1,3-diglycerolate diacrylate, N,N'-bis(acryloyl)cysteamine, bis[2-(methacryloyloxy)ethyl]phosphate, and bisacryloylated polypeptide; and preferably wherein the degradable cross- linker comprises glycerol dimethacrylate (GDMA); and wherein the neutral monomer is selected from acrylamide, poly(ethylene glycol) methyl ether acrylate (mPEG), or a combination thereof.