Modelling trilayer conducting polymer is still challenging as it exhibits interrelated coupled multiscale and non-linear characteristics. Therefore, this work proposes to review the underlying electro-chemo-mechanical principles in ultrathin PEDOT trilayer ionic conducting polymers based upon internal ion charge transport, conduction phenomena, redox process and elastic deformation. Microscale governing equations are first analyzed and the choice of appropriate assumptions depending of the used material is discussed. Since exact analytical solutions can be so far given only for some limited conditions, numerical solutions are developed to solve the problem. Then simulations in both sensing and actuating are successfully compared with experiments.