This invention relates to electrical devices that are implantable inside an animal, including humans. The invention particularly relates to polyurethane headers mounted on the electrical devices to house electrical contacts. The polyurethane headers are formed by (a) mixing (i) a prepolymer comprising a silicon-containing diol of formula (I) in which R1, R2, R3 and R4 are independently selected from C1-6alkyl; R5 and R6 are independently selected from C1-6alkylene; R7 is C1-6alkylene, O, S or NR in which R is H or C1-6alkyl; and n is 1 to 3 and a diisocyanate; and (ii) a C2-12alkanediol; (b) injecting the mixture of the prepolymer and the C2-12alkanediol into a mold having a cavity shaped to form a header; (c) allowing the mixture to cure and form a polyurethane header within the mold; and (d) releasing the formed polyurethane header from the mold.