A process for manufacturing a flexible cellulosic substrate comprises at least one functional circuit and/or at least one functional board. The flexible cellulosic substrates are made functional by printing with a functional ink, which provides good performance (signal speed/dielectric properties of the substrate), is economical, thermally and dimensionally stable, and is able to be produced simply and reproducibly at an industrial rate. The process starts with an aqueous fibrous suspension comprising paper pulp and/or a pulp of (micro/macro) cellulose fibrils and produces a wet fibrous mat from this suspension. One of the faces of the wet fibrous mat is printed by means of at least one functional ink capable of transmitting, emitting, and/or processing at least one signal in order to produce at least one topography comprising at least one track for circulation of the signal. Printed circuits and functional boards are obtained by the manufacturing process.