Featured Application The L-FDM technique enables the direct introduction of chemicals, dyes, radioactive substances, pesticides, antibiotics, nanoparticles, trace elements, fertilizers, phosphors, monomers for polymerization, proteins, peptides, and active ingredients in the direct printing process from a polymer material with a typical FDM printer. With the proposed technology, it is now possible to introduce chemical substances into polymer filaments that were previously impossible to apply due to undergoing physical or chemical transformations during previous processing processes. This article discusses methods that eliminate the need for costly and energy-consuming processing equipment. These methods can be utilized in any laboratory by users without access to specialized devices. A novel L-FDM technique that builds upon the fundamentals of the FDM additive manufacturing process has been developed. It includes a mechanism that directly incorporates a chemical substance and alters polymer fibers throughout the fine process. This method eliminates the need for additional extrusion operations and compounding equipment to introduce chemical additives and solvents. This advancement opens up new opportunities for printers to be used in chemical labs to test new or known chemical substances. The paper outlines the technological assumptions, potential applications, and practical examples of direct filament modification using the L-FDM technique. The modifications made to the mechanical properties of the printed objects were confirmed through thermal analysis techniques (DSC), water contact angle measurements, electron microscopy (SEM-EDS), and mechanical analysis.