The present invention relates to biostable gel comprising: (a) at least one silicon-containing polyol, polyamine, polyepoxy or polyisocyanate having 1 or more functional groups and a molecular weight of at least 20,000 which is cured in the presence of: (b) at least one diol, diamine or diisocyanate having a molecular weight of less than 10,000; and/or (c) an initiator, processes for their preparation and their use in the manufacture and repair of biomaterials and medical devices, articles or implants, in particular the manufacture of asoft tissue implant such as breast implants and the repair of orthopaedic joints such as spinal discs.