Considering their specific structure, porous polymers have high adsorptive capacity, high flexibility, and high surface area compared to solid material. Highly flexible, deformable, and ultralightweight structures are required for advanced sensing applications such as wearable electronics and robotics. Hence, porous conductive polymer nanocomposites (CPNCs) have attracted significant attention for developing flexible piezoresistive sensors. In the first part of this dissertation, the application of solvent evaporation-induced phase separation (EIPS) as a promising technique to create porous polymer structures is investigated. The ternary polymer solution consisting of polymer/solvent/nonsolvent is explored. The ternary phase diagram is constructed, showing the thermodynamic equilibrium state for polymeric solutions consisting of Polydimethylsiloxane (PDMS)/Water/Tetrahydrofuran (THF). The possible composition path during the heat treatment and phase separation procedure is obtained. Moreover, the fabrication and characterization of porous PDMS structures developed by the EIPS technique are explored. The porous PDMS structures are formed by phase separation induced by removing the solvent, leading to water enriched droplets formation and removal during the stepping heat treatment procedure. The results show that the isolated pores with the adjustable pore size ranging from 330 µm to 1900 µm are obtained by tuning the water to the THF ratio. A wide range of elastic modulus ranging between 0.49-1.05 MPa was achieved without affecting the density of the porous sample by adjusting the solvent and non-solvent content in the solution. The second part of the dissertation proposes a two-step phase separation synthesis protocol based on a ternary polymer solution. THF and Toluene with various mixing ratios are utilized as the solvent phase. Two distinct pore size distributions were observed in the cast PDMS sheets. The large pores with an average of 509 µm are formed during the first step of the phase separation after THF is evaporated. The second phase separation occurs later at higher temperatures by the evaporation of Toluene, resulting in much smaller pores with an average size of 28 µm. The experiments reveal that raising the THF/solvent ratio increases the large pore concentration, and the small pore density is reduced. The elastic modulus is varied between 0.64-0.95 MPa, indicating that the proposed method can create porous structures with a wide range of flexibility while keeping the density constant. In the third part of the dissertation, a novel approach to synthesizing highly flexible and ultralightweight piezoresistive sensors is developed by combining the direct ink writing (DIW) and EIPS method. CPNC is prepared by dispersing carbon nanotubes (CNTs) at various concentrations in PDMS polymer, followed by mixing with solvent and nonsolvent phases to achieve a homogenous solution. Macroscale pores are established by designing structural printing patterns with adjustable infill densities, while the microscale pores are developed by EIPS of the deposited CPNC solution ink. Silica nanoparticles are utilized to modify the rheological properties of the DIW, evaluated by rheology experiments. A tunable porosity of up to 84% is achieved by controlling macroscale (infill density) and microscale porosity (polymer weight). The effect of macroscale/microscale porosity and printing nozzle sizes on the mechanical and piezoresistive behavior of the CPNC structures is explored. The electrical and mechanical testing demonstrate a durable, extremely deformable, and sensitive piezoresistive response without sacrificing mechanical performance. The flexibility and sensitivity of the CPNC structure are enhanced up to 900% and 67% with the development of dual-scale porosity. The application of the developed porous piezoresistive sensor for detecting human motion is also evaluated.