A device that is implantable in an animal to electrically stimulate tissue, the device comprising: (a) a can housing electronics capable of producing electrical signals; (b) electrical connectors external of the can and in electrical connection with the electronics; and (c) a polyurethane header in which the electrical connectors are embedded, the polyurethane header being formed by a method comprising the steps of: (i) mixing a prepolymer comprising a silicon-containing diol of formula HO − R 5 − R 1 | Si | R 3 − R 2 | R 7 − Si | R 4 n − R 6 − OH in which R 1, R 2, R 3 and R 4 are independently selected from C 1-6 alkyl; R 5 and R 6 are independently selected from C 1-6 alkylene; R 7 is C 1-6 alkylene, O, S or NR in which R is H or C 1-6 alkyl; and n is 1 to 3 and a diisocyanate; and a C 2-12 alkanediol (ii) injecting the mixture of the prepolymer and the C 2-12 alkanediol into a mould having a cavity shaped to form a header; (iii) allowing the mixture to cure and form a polyurethane header within the mould; and (iv) releasing the formed polyurethane header from the mould. (a) a can housing electronics capable of producing electrical signals; (b) electrical connectors external of the can and in electrical connection with the electronics; and (c) a polyurethane header in which the electrical connectors are embedded, the polyurethane header being formed by a method comprising the steps of: (i) mixing a prepolymer comprising a silicon-containing diol of formula HO − R 5 − R 1 | Si | R 3 − R 2 | R 7 − Si | R 4 n − R 6 − OH in which R 1, R 2, R 3 and R 4 are independently selected from C 1-6 alkyl; R 5 and R 6 are independently selected from C 1-6 alkylene; R 7 is C 1-6 alkylene, O, S or NR in which R is H or C 1-6 alkyl; and n is 1 to 3 and a diisocyanate; and a C 2-12 alkanediol (ii) injecting the mixture of the prepolymer and the C 2-12 alkanediol into a mould having a cavity shaped to form a header; (iii) allowing the mixture to cure and form a polyurethane header within the mould; and (iv) releasing the formed polyurethane header from the mould. (i) mixing a prepolymer comprising a silicon-containing diol of formula HO − R 5 − R 1 | Si | R 3 − R 2 | R 7 − Si | R 4 n − R 6 − OH in which R 1, R 2, R 3 and R 4 are independently selected from C 1-6 alkyl; R 5 and R 6 are independently selected from C 1-6 alkylene; R 7 is C 1-6 alkylene, O, S or NR in which R is H or C 1-6 alkyl; and n is 1 to 3 and a diisocyanate; and a C 2-12 alkanediol a prepolymer comprising a silicon-containing diol of formula HO − R 5 − R 1 | Si | R 3 − R 2 | R 7 − Si | R 4 n − R 6 − OH in which R 1, R 2, R 3 and R 4 are independently selected from C 1-6 alkyl; R 5 and R 6 are independently selected from C 1-6 alkylene; R 7 is C 1-6 alkylene, O, S or NR in which R is H or C 1-6 alkyl; and n is 1 to 3 and a diisocyanate; and in which R 1, R 2, R 3 and R 4 are independently selected from C 1-6 alkyl; R 5 and R 6 are independently selected from C 1-6 alkylene; R 7 is C 1-6 alkylene, O, S or NR in which R is H or C 1-6 alkyl; and n is 1 to 3 and a diisocyanate; and a C 2-12 alkanediol (ii) injecting the mixture of the prepolymer and the C 2-12 alkanediol into a mould having a cavity shaped to form a header; (iii) allowing the mixture to cure and form a polyurethane header within the mould; and (iv) releasing the formed polyurethane header from the mould.