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3D printing and additive manufacturing technologies are currently introducing new design rules and novel device architectures in optoelectronics and photonics. This is opening new opportunities for the 3D integration of the elementary electronic and optoelectronic devices, such as the possibility of out-of-plane positioning and interconnection of the individual components of a functional device. Despite such enormous opportunities, there are some challenges to be overcome by current 3D printing technologies in order to become a reliable fabrication platform for optical and optoelectronic systems, especially for those based on transparent and stimuli-responsive materials.
The xPRINT project is developing advanced additive manufacturing technologies specifically designed for photoresponsive materials, which will be exploited for the manufacturing of 4-dimensional optical components. These novel optical components, with time-changing optical properties, are expected to enable a novel class of optical devices, which can be reconfigured and reprogrammed in real-time by suitable external light fields. The technologies that are currently developed in the laboratories of xPRINT include:
Stereolithography (STL). In this 3D printing approach objects are fabricated by scanning a focused laser beam in bath of a liquid photosensitive pre-polymer 
(monomer/oligomer & photoinitiator), which is locally solidified due to the absorption of light. Composite systems can be produced by adding guest molecules and nanoparticles to the photocurable pre-polymer. Laser scanning allows arbitrarily shaped objects to be realized, with good spatial resolution (typically 10-100 µm for commercially available systems). In a similar approach, 3D objects are realized by exposing the liquid photocurable pre-polymer to UV flash images projected by means of a digital projector screen, a micro-mirror array devices or a dynamic liquid-crystal mask. This is the so-called digital light processing method.
Fused Deposition Modeling (FDM). FDM exploits filaments of thermoplastic polymers which are directed through a hot extruder. The fused polymer is deposited 
in a continuous way, solidifying as soon as the filament temperature falls below the glass transition temperature. The movement of the nozzle is precisely controlled by three-axis translation stages, in order to build the 3D object. The substrate can be heated up to 150 °C, in order to promote better adhesion between adjacent layers. The typical spatial resolution of fused deposition modelling is in the interval 100-500 µm.
Direct Ink Writing (DIW). This 3D printing technique is based on the use of a concentrated and viscous ink that is extruded through a nozzle 
by applying pressure or by heating. The motion of the nozzle is controlled by 3D translation micro-mechanical stages with a micrometric spatial precision. The minimum feature size of the printed structures can be < 1 µm, depending on the diameter of the nozzle and on the physico-chemical properties of the used ink. The composition of the ink has to be accurately adjusted in order to avoid clogging of the nozzle.
