ADDITIVE MANUFACTURING BY 3D AND 4D PRINTING METHODS: A REVIEW OF MATERIALS, METHODS AND APPLICATIONS.

3D printing, also known as additive manufacturing, is the process of creating a solid product by depositing layerby-layer a molten material in accordance with the parameters of the digitized model. The idea of 3D printing has come at the end of the 20th century. In the 1980s, it has started to compete with conventional manufacturing methods because of the extraordinary material efficiency, excellent surface finish, and one-step manufacturing provided. This technique has gradually been introduced to the fields of biomedicine, electronics, self-regeneration and biomimetics. However, it cannot control the dimensional changes caused by the deformation and the anisotropic behavior of the material. These difficulties are overcome by the 4D printing, which allows dynamic changes in the structures. The fourth parameter provides the products flexibility because, through an external stimulation, the smart materials used for the production can change the size, the properties and other parameters of the product. The smart or environmentally sensitive materials (metal alloys, polymers, ceramics, composites) can be activated by changes in the temperature, the water absorption, the electromagnetic and infrared radiation, the magnetic field, the current, the voltage, the changes of pH, etc. This smart behavior of the materials is important for the delivery of drugs, for sensors, in mobile electronics, for fashion products and other engineering objects. The unusual features of the 4D printing are based on the material shape memory effect and the ability of the materials to respond to external stimuli. [ABSTRACT FROM AUTHOR]