In this paper, series-parallel (SP) current-division will be employed for the design of very low transconductance OTAs. From the theory and measurements, it will be shown that SP mirrors allow building current copiers with copy factors of thousands, without reducing matching or noise performance. SP mirrors will be applied to the design of OTAs ranging from 33pS to a few nS, with up to 1V linear range, consuming in the order of 100nW, and with a reduced area. An integrated 3.3s time-constant integrator will also be presented. Several design concerns will be revised: linearity, offset, noise, leakages; as well as layout techniques. A final comparative analysis concludes that SP-association of transistors allows the design of very efficient transconductors, for demanding applications in the field of implantable electronics among others.