A viscoelastic biostable foam comprising a triblock copolymer of formula residue I: wherein: each represents a point of attachment to a urethane or urea linkage; each of X and Y is independently an ether, an ester, a carbonate, or a fluorohydrocarbon, or mixture thereof; each of R 1, R 2, R 3, R 4, R 5 and R 6 is independently selected from one or more of R, OR, -CO 2 R, a fluorinated hydrocarbon, a polyether, a polyester or a fluoropolymer; each R is independently hydrogen, an optionally substituted C 1-20 aliphatic group, or an optionally substituted group selected from phenyl, 8-10 memberedbicyclic aryl, a 4-8 membered monocyclic saturated or partially unsaturated heterocyclic ring having 1-2 heteroatoms independently selected from nitrogen, oxygen, or sulphur, or 5-6 membered monocyclic or 8-10 membered bicyclic heteroaryl group having 1-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur; each of m, n and p is independently 2 to 100; and each of L 1 and L 2 is independently a bivalent C 1-20 hydrocarbon chain wherein 1-4 methylene units of the hydrocarbon chain are optionally and independently replaced by -O-, -S-, -N -, -C -, - C N -, -N C -, -SO 2 -, -SO 2 N -, -N SO 2 -, -OC -, -C O-, or a bivalent cycloalkylene, arylene, heterocyclene, or heteroarylene, provided that neither of L 1 nor L 2 comprises a urea or urethane moiety, wherein the viscoelastic biostable foam: has a density of 0.1 to 1.5 g/cm 3; takes up less than 30% by weight of water at equilibrium; has an elongation capacity of 200% to 1200%; and has a tensile strength of 0.1 to 1.0 MPa. each represents a point of attachment to a urethane or urea linkage; each of X and Y is independently an ether, an ester, a carbonate, or a fluorohydrocarbon, or mixture thereof; each of R 1, R 2, R 3, R 4, R 5 and R 6 is independently selected from one or more of R, OR, -CO 2 R, a fluorinated hydrocarbon, a polyether, a polyester or a fluoropolymer; each R is independently hydrogen, an optionally substituted C 1-20 aliphatic group, or an optionally substituted group selected from phenyl, 8-10 memberedbicyclic aryl, a 4-8 membered monocyclic saturated or partially unsaturated heterocyclic ring having 1-2 heteroatoms independently selected from nitrogen, oxygen, or sulphur, or 5-6 membered monocyclic or 8-10 membered bicyclic heteroaryl group having 1-4 heteroatoms independently selected from nitrogen, oxygen, or sulfur; each of m, n and p is independently 2 to 100; and each of L 1 and L 2 is independently a bivalent C 1-20 hydrocarbon chain wherein 1-4 methylene units of the hydrocarbon chain are optionally and independently replaced by -O-, -S-, -N -, -C -, - C N -, -N C -, -SO 2 -, -SO 2 N -, -N SO 2 -, -OC -, -C O-, or a bivalent cycloalkylene, arylene, heterocyclene, or heteroarylene, provided that neither of L 1 nor L 2 comprises a urea or urethane moiety, wherein the viscoelastic biostable foam: has a density of 0.1 to 1.5 g/cm 3; takes up less than 30% by weight of water at equilibrium; has an elongation capacity of 200% to 1200%; and has a tensile strength of 0.1 to 1.0 MPa. has a density of 0.1 to 1.5 g/cm 3; takes up less than 30% by weight of water at equilibrium; has an elongation capacity of 200% to 1200%; and has a tensile strength of 0.1 to 1.0 MPa.