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Single-macromolecule particles, each formed of a single macromolecule,

• the single macromolecule having a molecular weight distribution Mw/Mn of not more than 1.5,
• the single macromolecule being present in a form of single macromolecular chains in an aqueous solvent without inter-molecular secondary association,
• the single macromolecule being a structure including a hydrophilic macromolecule A, the structure being (i) a structure consisting of a single hydrophilic macromolecule A that is composed solely of a linear structure or (ii) a structure in which one or more side chains are bonded to one or more of units included in a main chain, the main chain consisting of a single hydrophilic macromolecule A that is a polymer molecule including a multiple repetition of the units, each of the one or more side chains being a hydrophilic macromolecule B,
• the hydrophilic macromolecule A being a polypeptide, a polysaccharide, a vinyl-based macromolecule, a polyether-based macromolecule, a polyester-based macromolecule, or a polyoxazoline, and
• the hydrophilic macromolecule B being polyalkylene glycol, polyvinyl alcohol, polyvinyl pyrrolidone, polyhydroxyethyl methacrylate, poly(2-methoxyethyl acrylate), a polyoxazoline, carboxymethyl cellulose, methyl cellulose, hydroxyethyl cellulose, or a carboxy vinyl polymer.