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 a soft tissue implant such as breast implants and the repair of orthopaedic joints such as spinal discs.