Non-degradable embolisation microsphere

Nonbiodegradable embolization microspheres comprising a crosslinked matrix, said matrix being based on at least:

• a) 45% to 95% of hydrophilic monomer selected from N-vinylpyrrolidone and poly(ethylene glycol) methyl ether methacrylate;
• b) 2% to 7% of a nonbiodegradable, linear or branched hydrophilic crosslinking monomer of the following formulas (IIa) or (IIb):
• (CH2═(CR7))CO—NH-A-HN—OC(CR8═CH2) (IIa),
• (CH2═(CR7))CO—O-A-O—OC(CR8═CH2) (IIb),
• in which R7 and R8 represent, independently of one another, H or a (C1—C)alkyl;
• and
• A represents, alone or with at least one of the atoms to which it is bound, a polyethylene glycol (PEG) having a length in the range from 200 to 10,000 g/mol;
• and
• c) 1.5% to 4.5% of transfer agent selected from alkyl halides and cycloaliphatic or aliphatic thiols, and optionally having another functional group selected from the amino, hydroxy and carboxy groups,
• the percentages of the monomers a) and b) being given in moles relative to the total number of moles of monomers and the percentages of compound c) being given in moles relative to the number of moles of the hydrophilic monomer a).