**2. Inkjet 3D printing of optical nanostructures**

The inkjet 3D printing has already been used for more than 20 years. Up to now, this method remains to be one of the most popular due to its low price both for home using, and for commercial applications. During this period, the techniques of the inkjet 3D printing made an incredible progress [8]. The step of drop resolution advanced from 500 μm (30 dpi) to several micrometers (9000 dpi), reaching the value boundary to the nanoscale range. The absence of high temperatures and a wide variety of inks made a revolution in the field of biosensor and print electronic devices. This method was used for printing first organic light-emitting diode (OLED) displays and panels.

The possibility of the point deposition of thin layer coatings is often used to create conductive layers [9]. Under an intensive study is the possibility of applying the inkjet 3D printing for the graphene electronics fabrication, production of ordered arrays with metallic conductivity and high transparency, for deposition of electrodes of a predetermined shape to a substrate, and also for generation of 2D microarrays in printing electronics. A well-known method is printing of metal precursors (the most popular are Au+ , Ag+ , Ni2+, Cu2+), followed by the deposition of a reducing agent from another cartridge. These approaches are widely used in industrial printing of electrochemical sensors and combined differential thermocouples, as well as in deposition of substrate electrodes on the conductive polymeric panels.

prevention during the 3D printing, which significantly levels the potential advantages of using thematerialsinthenano-orsubmicronstate.Oneofthewaystosolvethisproblemistheisolation of nanoparticles into the inert matrices where they do not undergo any aggregation or "ag‐ ing" and can be controllably released with the retained of chemical and phase composition [1–

Stabilization of nanoparticles in a polymeric matrix and additionally reinforced porous structure makes it possible to arrange a desired distribution of the nanoparticles in the polymer and thus to protect them from agglomeration, oxidation, and corrosion and even to design the FGS. The results indicate that nanoparticles mechanically reinforced the polymer matrix and elastic modulus and the maximum stress significantly increased. Finally, the correlations "prehistory of obtaining (i.e., "background") - chemical composition of the nanoparticles volume and surface condition - phase/structural composition - morphology - perspective

The present review will demonstrate how laser-assisted techniques of the 3D synthesis could be used to prepare a porous core-shell polymer structures containing different encapsulated nanoparticles distributed heterogeneously over the sintered polymer and dangerous for cancer tissue account of thermal hyperthermia or cytotoxic effect. We demonstrated a principal feasibility for fabrication of functionally graded 3D parts with the structural ordering of iron oxide particles and determined corresponding laser optimal regimes. The SLS-fabricated 3D samples of biocompatible iron oxide core/(polyetheretherketone (PEEK) or polycaprolactone (PCL)) shell magnetic nanocomposites have potential medical application for the tissue

Functionally graded 3D parts with alternating ferromagnetic Ni-PC and nonmagnetic Cu-PC layers [5] exhibited hysteresis phenomena that can probably be used in microelectromechan‐ ical systems (MEMS)-nanoelectromechanical systems (NEMS) applications [6] also, where the time response must depend on the relaxation rate. The synthesized nanocomposites with high porosity and large-specific surface could also find their application in catalysis, lab-on-chips, drug delivery systems, and 3D crystalline structures for hydrogen storage devices [7].

The inkjet 3D printing has already been used for more than 20 years. Up to now, this method remains to be one of the most popular due to its low price both for home using, and for commercial applications. During this period, the techniques of the inkjet 3D printing made an incredible progress [8]. The step of drop resolution advanced from 500 μm (30 dpi) to several micrometers (9000 dpi), reaching the value boundary to the nanoscale range. The absence of high temperatures and a wide variety of inks made a revolution in the field of biosensor and print electronic devices. This method was used for printing first organic light-emitting diode

The possibility of the point deposition of thin layer coatings is often used to create conductive layers [9]. Under an intensive study is the possibility of applying the inkjet 3D printing for the

properties" will determine the nanoparticles behavior in further applications.

engineering scaffolds and cell targeting systems [4].

**2. Inkjet 3D printing of optical nanostructures**

(OLED) displays and panels.

3].

238 New Trends in 3D Printing

No less revolutionary is the application of the inkjet 3D printing in the bioengineering field. For example, for the first time the selective bacterial test systems for the analysis of drinking water quality were researched and developed, mechanisms of diagnosing pancreatitis were studied with the aid of biosensors applied to the inkjet 3D printing substrate. Just that very method was used to obtain noninvasive high-sensitive biochips estimating the content of hydrogen peroxide and glucose, as well as to design colorimetric sensors aimed to detect neurotoxins and pesticides [5, 10]. However, the use of the inkjet 3D printing mechanism for bioengineering basically reduced to finding of new approaches to the biomolecules fixation into porous matrices that ensure a stable trapped substance for a long period of time after its deposition on the substrate and drying. Thus, the fabrication of highly inert colloids capable of biomolecules capturing is the most important task for the 3D bioprinting.

The optical nanostructures designed to control the photons flow are of a high practical and fundamental importance. Studies on the interaction of electromagnetic radiation (EMR) with low-dimensional semiconductor and dielectric media suggest the occurrence of a whole set of multifactor processes that jointly determine the probability of photon transport as a key mechanism for the quantum communication. Hence, the control of light flows in nanostructure constructions brings closer to the developing of new photonic devices. Thus, the development of widely available 3D inkjet printing technology for getting optical nanostructures and construction applicable in quantum communications is a pressing problem. This method proved to be successful in the field of electronics and biosensor engineering, including the generation of microchips and microintegrated circuits. The use of the nano-oxide-based system ensures a high stability and repeatability of 3D process. Earlier, we have shown the crystalline phase obtained from liquid solutions with the following condensation of boehmite, anatase, and magnetite under temperatures not exceeding 100°C and without annealing of the samples obtained [11].

Hence, it is possible to fabricate photon-induced panels with a uniform spreading of the light wave in the thin layer. These objects are unique both from the standpoints of the effect proper, and from the viewpoint of their fabrication by 3D methods of a soft chemistry. Such hetero‐ structures will allow to carry out the uniform photon transport (for the specified wavelength) by the whole area perimeter and irrespective of the excitation point. The substrate structure can serve the base for transfer and reading of information in the new PC generation operating by the photon-signal principle. The deposition of planar waveguides on flexible polymeric substrates by the inkjet methods from solutions will greatly simplify and accelerate machinereadable signals pickup on carriers. However, the deposition of transparent dielectric struc‐ tures with the accuracy of several nanometers by the 3D inkjet technology is still inaccessible. A complex "structure—property" correlation is required to be determined, ensuring the controllability of optical characteristics of the sample being formed at the stage of the "ink" preparation for the jet printing.

The question of the materials and 3D inkjet technology use for optics remains open. This is due to the fact that the droplet coating aimed to form a solid phase on the substrate surface can embrace a predetermined range of 10−3–10−6 m with a high accuracy, while the transition to nanoscale remains uncontrollable. But this nanorange is of a particular importance for the optical use, since nanostructures are commensurable with the wavelength of light in the visible range (400–700 nm).

On the other hand, printing of photonic materials and optical structure is still gaining in popularity, and is solely determined by the development of printing technologies and by increase in their accuracy. The size effect has a major impact on the photon flow control thus imposing many restrictions to the used ink. Still unsolved is the problem of a high heteroge‐ neity of morphology in the drying process (coffee-ring effect). The maximal achievements were obtained under the controlled application of photonic crystals. However, in this case, the light control does not depend on the 3D inkjet technology and is exclusively resulted by the dimensions of spherical particles composing the ink.

As the universal method to control the photon flow, the use of layers with a high refractive index (RI), encased into a core with a low RI can serve. Classically, these approaches are often used for the creation of antireflective coatings and planar waveguides. However, the formation of the high RI transparent layers (more than 2.0 in the visible range) it is not an easy task. To solve this task, attempts were made to modify the polymers by using various nanoscale crystalline additives improving the optical properties of the polymers. In addition to this, the adjustment of 3D printing parameters implies the evaluation of viscosity and surface tension, as well as fine-tuning of the printer for a specific composition. Generally, in order to change the rheology, the additives increasing viscosity, such as glycerin, and surfactants decreasing the surface tension are used [9, 12].

Now only a few of inorganic materials among a great variety of those adapted to the 3D inkjet printing and widely used, can be attributed to highly refractive ones, having high transparency and not expensive, they are for example, ZrO2, TiO2, ZnO, and several mixed oxides. TiO2 and magnetite are considered to be the most versatile for the obtaining of ink with high RI. This soft chemistry approach is well presented in publications, due to known values of the RI for TiO2 in the anatase phase (2.61), and for Fe3O4 (3.2). At the same time, TiO2 is completely transparent in the visible range, and its colloids are readily formed in an aqueous medium, and Fe3O4-based films are magnetically controlled. Printed aluminum and silicon oxides can be used in the polymer environment with a low RI. Now they are the most common matrixes for the storage and delivery of biomolecules. The alumina is used in microelectronics for deposition of dielectric layers, including the use of the 3D inkjet technology. Due to the tendency of these two systems to spontaneous and uncontrolled polycondensation it is necessary to study the influence of sonication on the particles stabilization. For planar waveguides obtaining by the inkjet technology, the multipass printing should be applied. For this, the ink combinations with different RI from cartridges of different types should be alternated. This approach concept consists in using of a layered heterostructure wherein the layer with the minimum RI is formed of spherical nanoparticles with a narrow distribution by size. If the sphere diameter is commensurable with the wavelength of the external light excitation, then the effect of the critical angle of total reflection will be minimized.

A complex "structure—property" correlation is required to be determined, ensuring the controllability of optical characteristics of the sample being formed at the stage of the "ink"

The question of the materials and 3D inkjet technology use for optics remains open. This is due to the fact that the droplet coating aimed to form a solid phase on the substrate surface can embrace a predetermined range of 10−3–10−6 m with a high accuracy, while the transition to nanoscale remains uncontrollable. But this nanorange is of a particular importance for the optical use, since nanostructures are commensurable with the wavelength of light in the visible

On the other hand, printing of photonic materials and optical structure is still gaining in popularity, and is solely determined by the development of printing technologies and by increase in their accuracy. The size effect has a major impact on the photon flow control thus imposing many restrictions to the used ink. Still unsolved is the problem of a high heteroge‐ neity of morphology in the drying process (coffee-ring effect). The maximal achievements were obtained under the controlled application of photonic crystals. However, in this case, the light control does not depend on the 3D inkjet technology and is exclusively resulted by the

As the universal method to control the photon flow, the use of layers with a high refractive index (RI), encased into a core with a low RI can serve. Classically, these approaches are often used for the creation of antireflective coatings and planar waveguides. However, the formation of the high RI transparent layers (more than 2.0 in the visible range) it is not an easy task. To solve this task, attempts were made to modify the polymers by using various nanoscale crystalline additives improving the optical properties of the polymers. In addition to this, the adjustment of 3D printing parameters implies the evaluation of viscosity and surface tension, as well as fine-tuning of the printer for a specific composition. Generally, in order to change the rheology, the additives increasing viscosity, such as glycerin, and surfactants decreasing

Now only a few of inorganic materials among a great variety of those adapted to the 3D inkjet printing and widely used, can be attributed to highly refractive ones, having high transparency and not expensive, they are for example, ZrO2, TiO2, ZnO, and several mixed oxides. TiO2 and magnetite are considered to be the most versatile for the obtaining of ink with high RI. This soft chemistry approach is well presented in publications, due to known values of the RI for TiO2 in the anatase phase (2.61), and for Fe3O4 (3.2). At the same time, TiO2 is completely transparent in the visible range, and its colloids are readily formed in an aqueous medium, and Fe3O4-based films are magnetically controlled. Printed aluminum and silicon oxides can be used in the polymer environment with a low RI. Now they are the most common matrixes for the storage and delivery of biomolecules. The alumina is used in microelectronics for deposition of dielectric layers, including the use of the 3D inkjet technology. Due to the tendency of these two systems to spontaneous and uncontrolled polycondensation it is necessary to study the influence of sonication on the particles stabilization. For planar waveguides obtaining by the inkjet technology, the multipass printing should be applied. For this, the ink combinations with different RI from cartridges of different types should be

preparation for the jet printing.

dimensions of spherical particles composing the ink.

the surface tension are used [9, 12].

range (400–700 nm).

240 New Trends in 3D Printing
