In the Fused Filament Fabrication (FFF/FDM) technology, the multi-material manufacturing additive method is achieved by a single nozzle or multiple nozzles working simultaneously with different materials. However, the adhesion between different materials at the boundary interface in FDM multi-material printing is a limiting factor. These studies are concerned with improving and study the adhesion between two polymers.Due to the numerous applications and possibilities of 3D printed objects, combining different materials has become a subject of interest. PLA is an alternative to the use of petrochemical-based polymers. Thermoplastic Polyurethane is a flexible material that can achieve different characteristics when combined with a rigid filament, such as PLA. To improve the adhesion between PLA and TPU in multi-material FFF/FDM, we propose the comparison of different processes: post-processing with acetone immersion, surface activation during printing with Acetone, surface activation during printing with tetrahydrofuran, post-processing annealing, and connection of printed parts with tetrahydrofuran.Modifying the 3D printing process improved the quality of the adhesive bond between the two different polymers. Activation of the surface with THF is the treatment method recommended by the authors due to the low impact on the deformation/degradation of the object.In the study, adhesion was considered in relation to the circular pattern of surface development. Further analysis should include other surface development patterns and changes in printing parameters, e.g. process temperatures and layer application speed.3D printing with multi-materials, such as PLA biopolymer and thermoplastic polyurethane, allows for the creation of flexible connections. The strengthening of the biopolymer broadens the possibilities of using polylactide. Examples of applications include: automotive (elements, where flexible TPU absorbs vibrations and protects PLA from cracking), medicine (prostheses with flexible elements ensuring mobility in the joints).Multi-material printing is a new trend in 3D printing research, and this research is aimed at promoting the use and expanding the possibilities of using PLA biopolymer.