We present photonic devices based on a nematic liquid crystals (NLC) infiltrated in polydimethylsiloxane (PDMS) channels, named LC:PDMS waveguides, for flexible photonic integrated circuits. A simulation study of the NLC ordering and possible defects under an electric field between coplanar gold electrodes has been carried out by a Monte Carlo approach. The minimization of the free energy in the NLC core waveguides computed by means of finite elements allows to derive the refractive index profile, which is implemented in a BPM algorithm to design a large variety of switchable and tuneable photonic devices. In particular we report the design of two photonic devices based on LC:PDMS waveguide technology: a 2x2 optical switch made of a zero-gap electro-optical controlled directional coupler and a multimodal interferometer (MMI) acting as an optical multi/demultiplexer. The electro-optical controlled directional coupler is able to switch light from one waveguide to a second one with an extinction ratio of 16 dB by applying a voltage of just 1.62 V to coplanar gold electrodes deposited on PDMS by electroplating technique. Light remains in the same waveguide with an extinction ratio of about 18 dB with a voltage of 1.76 V. An MMI has been also designed to demultiplex wavelengths at 980 nm and 1550 nm in two output waveguides with an extinction ratio better than 11 dB by applying about 7 V. Main advantage of these devices is the low driving voltage due to the combination of electro-optic effect of the NLC with an optimized design.