3D reactive inkjet printing of aliphatic polyureas using in-air coalescence technique

An in-flight coalescence reactive inkjet printer has been developed to facilitate the in-air collision of two reactive microdroplets. This way precise volumes of reactive inks can be mixed and subsequently deposited on the substrate to produce the desired product by polymer synthesis and patterning in a single step. In this work, we validate the printer capabilities by fabrication of a series of 3D structures using an aliphatic polyurea system (isophorone diisocyanate IPDI and poly(propylene glycol) bis(2-aminopropyl ether) PEA-400). The influence of temperature and ink ratio on the material properties has been investigated. An increase in both IPDI and temperature facilitates the production of materials with higher Young's Modulus E and higher ultimate strength U. The possibility of printing different materials i.e. ductile (U = 2 MPa, ε B = 450%), quasi-brittle (U = 14 MPa, ε B = 350%), and brittle (U = 10 MPa, ε B = 11%) by varying the printing process parameters using one set of inks has been presented. The anisotropy of the material properties arising from different printing directions is at the 20% level.

High-Throughput and Combinatorial Approaches for the Development of Multifunctional Polymers

High-throughput (HT) development of new multifunctional polymers is accomplished by the combination of different HT tools established in polymer sciences in the last decade. Important advances are robotic/HT synthesis of polymer libraries, the HT characterization of polymers, and the application of spatially resolved polymer library formats, explicitly microarray and gradient libraries. HT polymer synthesis enables the generation of material libraries with combinatorial design motifs. Polymer composition, molecular weight, macromolecular architecture, etc. may be varied in a systematic, fine-graded manner to obtain libraries with high chemical diversity and sufficient compositional resolution as model systems for the screening of these materials for the functions aimed. HT characterization allows a fast assessment of complementary properties, which are employed to decipher quantitative structure-properties relationships. Moreover, these methods facilitate the HT determination of important surface parameters by spatially resolved characterization methods, including time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. Here current methods for the high-throughput robotic synthesis of multifunctional polymers as well as their characterization are presented and advantages as well as present limitations are discussed.

Miniaturized high-throughput synthesis and screening of responsive hydrogels using nanoliter compartments

The traditional pipeline of hydrogel development includes individual one-by-one synthesis and characterization of hydrogels. This approach is associated with the disadvantages of low-throughput and high cost. As an alternative approach to classical one-by-one synthesis, high-throughput development of hydrogels is still tremendously under-represented in the field of responsive material development, despite the urgent requirement for such techniques. Here, we report a platform that combines highly miniaturized hydrogel synthesis with screening for responsive properties in a high-throughput manner. The platform comprises a standard glass slide patterned with 1 ​× ​1 ​mm hydrophilic regions separated by superhydrophobic liquid-impermeable barriers, thus allowing deposition of various precursor solutions onto the hydrophilic spots without cross-contamination. The confinement of these solutions provided by the hydrophilic/superhydrophobic pattern allows encapsulation of cells within the hydrogel, and enables variation in hydrogel height and width. We have also proved the proper mixing of chemicals within the nanoliter-sized droplets. We have successfully implemented this platform for the synthesis of hydrogels, constructing 53 unique hydrogels, to demonstrate the versatility and utility of the platform. Photodegradation studies were performed on 20 hydrogels, revealing structure/function relationships between the hydrogel composition and photodegradability, and covering the range of degradability from non-degradable to rapidly degradable materials.

Printable Transparent Conductive Films for Flexible Electronics

Printed electronics are an important enabling technology for the development of low-cost, large-area, and flexible optoelectronic devices. Transparent conductive films (TCFs) made from solution-processable transparent conductive materials, such as metal nanoparticles/nanowires, carbon nanotubes, graphene, and conductive polymers, can simultaneously exhibit high mechanical flexibility, low cost, and better photoelectric properties compared to the commonly used sputtered indium-tin-oxide-based TCFs, and are thus receiving great attention. This Review summarizes recent advances of large-area flexible TCFs enabled by several roll-to-roll-compatible printed techniques including inkjet printing, screen printing, offset printing, and gravure printing using the emerging transparent conductive materials. The preparation of TCFs including ink formulation, substrate treatment, patterning, and postprocessing, and their potential applications in solar cells, organic light-emitting diodes, and touch panels are discussed in detail. The rational combination of a variety of printed techniques with emerging transparent conductive materials is believed to extend the opportunities for the development of printed electronics within the realm of flexible electronics and beyond.

A new photocrosslinkable polycaprolactone-based ink for three-dimensional inkjet printing

A new type of photocrosslinkable polycaprolactone (PCL) based ink that is suitable for three-dimensional (3D) inkjet printing has been developed. Photocrosslinkable Polycaprolactone dimethylacrylate (PCLDMA) was synthesized and mixed with poly(ethylene glycol) diacrylate (PEGDA) to prepare an ink with a suitable viscosity for inkjet printing. The ink performance under different printing environments, initiator concentrations, and post processes was studied. This showed that a nitrogen atmosphere during printing was beneficial for curing and material property optimization, as well as improving the quality of structures produced. A simple structure, built in the z-direction, demonstrated the potential for this material for the production of 3D printed objects. Cell tests were carried out to investigate the biocompatibility of the developed ink. © 2016 The Authors Journal of Biomedical Materials Research Part B: Applied Biomaterials Published by Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 105B: 1645-1657, 2017.

Creating Patterned Conjugated Polymer Images Using Water-Compatible Reactive Inkjet Printing

The fabrication of patterned conjugated polymer images on solid substrates has gained significant attention recently. Office inkjet printers can be used to generate flexible designs of functional materials on substrates on a large scale and in an inexpensive manner. Although creating patterns of conjugated polymers on paper using common office inkjet printers has been reported, only a few examples exist, such as polyaniline (PANI) and poly(3,4-ethylenedioxythiophene) (PEDOT), because only water-compatible inks can be utilized. Herein, we describe the production of poly(phenylenevinylene) (PPV) patterns on paper by employing a reactive inkjet printing (RIJ) method. In this process, printing of a hydrophilic terephthaldehyde, bis(triphenylphosphonium salt) and potassium t-butoxide using a common office inkjet printer leads to formation PPV patterns as a consequence of an in situ Wittig reaction. In addition, microarrayed PPV patterns are also readily generated on solid substrates, such as glass and PDMS, when a piezoelectric dispenser system is employed. The in situ prepared PPV was found to be insoluble in water and chloroform. As a result, unreacted excess reagents and byproducts can be efficiently removed by washing with these solvents.

Effect of Cross-Linking on the Structure and Growth of Polymer Films Prepared by Interfacial Polymerization

Interfacial polymerization of tri- and bifunctional monomers (A3B2 polymerization) is investigated by dissipative particle dynamics to reveal an effect of cross-linking on the reaction kinetics and structure of the growing polymer film. Regardless of the comonomer reactivity and miscibility, the kinetics in an initially bilayer melt passes from the reaction to diffusion control. Within the crossover period, branched macromolecules undergo gelation, which drastically changes the scenario of the polymerization process. Comparison with the previously studied linear interfacial polymerization (Berezkin, A. V.; Kudryavtsev, Y. V. Linear Interfacial Polymerization: Theory and Simulations with Dissipative Particle Dynamics J. Chem. Phys. 2014, 141, 194906) shows similar conversion rates but very different product characteristics. Cross-linked polymer films are markedly heterogeneous in density, their average polymerization degree grows with the comonomer miscibility, and end groups are mostly trapped deeply in the film core. Products of linear interfacial polymerization demonstrate opposite trends as they are spontaneously homogenized by a convective flow of macromolecules expelled from the reactive zone to the film periphery, which we call the reactive extrusion effect and which is hampered in branched polymerization. Influence of the comonomer architecture on the polymer film characteristics could be used in various practical applications of interfacial polymerization, such as fabrication of membranes, micro- and nanocapsules and 3D printing.

An inverse method for determining the spatially resolved properties of viscoelastic-viscoplastic three-dimensional printed materials

A method using experimental nanoindentation and inverse finite-element analysis (FEA) has been developed that enables the spatial variation of material constitutive properties to be accurately determined. The method was used to measure property variation in a three-dimensional printed (3DP) polymeric material. The accuracy of the method is dependent on the applicability of the constitutive model used in the inverse FEA, hence four potential material models: viscoelastic, viscoelastic–viscoplastic, nonlinear viscoelastic and nonlinear viscoelastic–viscoplastic were evaluated, with the latter enabling the best fit to experimental data. Significant changes in material properties were seen in the depth direction of the 3DP sample, which could be linked to the degree of cross-linking within the material, a feature inherent in a UV-cured layer-by-layer construction method. It is proposed that the method is a powerful tool in the analysis of manufacturing processes with potential spatial property variation that will also enable the accurate prediction of final manufactured part performance.

Patterning fluorescent quantum dot nanocomposites by reactive inkjet printing

Fluorescent quantum dot nanocomposites, including polymer and photonic crystal quantum dots, have been fabricated by reactive inkjet printing. This reactive inkjet printing method has the potential to be broadened to fabrication of other functional nanomaterials, which will find promising applications in optoelectronic devices.

Autonomic composite hydrogels by reactive printing: materials and oscillatory response

Autonomic materials are those that automatically respond to a change in environmental conditions, such as temperature or chemical composition. While such materials hold incredible potential for a wide range of uses, their implementation is limited by the small number of fully-developed material systems. To broaden the number of available systems, we have developed a post-functionalization technique where a reactive Ru catalyst ink is printed onto a non-responsive polymer substrate. Using a succinimide-amine coupling reaction, patterns are printed onto co-polymer or biomacromolecular films containing primary amine functionality, such as polyacrylamide (PAAm) or poly-N-isopropyl acrylamide (PNIPAAm) copolymerized with poly-N-(3-Aminopropyl)methacrylamide (PAPMAAm). When the films are placed in the Belousov-Zhabotinsky (BZ) solution medium, the reaction takes place only inside the printed nodes. In comparison to alternative BZ systems, where Ru-containing monomers are copolymerized with base monomers, reactive printing provides facile tuning of a range of hydrogel compositions, as well as enabling the formation of mechanically robust composite monoliths. The autonomic response of the printed nodes is similar for all matrices in the BZ solution concentrations examined, where the period of oscillation decreases in response to increasing sodium bromate or nitric acid concentration. A temperature increase reduces the period of oscillations and temperature gradients are shown to function as pace-makers, dictating the direction of the autonomic response (chemical waves).

Mixing and internal dynamics of droplets impacting and coalescing on a solid surface

The coalescence and mixing of a sessile and an impacting liquid droplet on a solid surface are studied experimentally and numerically in terms of lateral separation and droplet speed. Two droplet generators are used to produce differently colored droplets. Two high-speed imaging systems are used to investigate the impact and coalescence of the droplets in color from a side view with a simultaneous gray-scale view from below. Millimeter-sized droplets were used with dynamical conditions, based on the Reynolds and Weber numbers, relevant to microfluidics and commercial inkjet printing. Experimental measurements of advancing and receding static contact angles are used to calibrate a contact angle hysteresis model within a lattice Boltzmann framework, which is shown to capture the observed dynamics qualitatively and the final droplet configuration quantitatively. Our results show that no detectable mixing occurs during impact and coalescence of similar-sized droplets, but when the sessile droplet is sufficiently larger than the impacting droplet vortex ring generation can be observed. Finally we show how a gradient of wettability on the substrate can potentially enhance mixing.

Plasma and microwave flash sintering of a tailored silver nanoparticle ink, yielding 60% bulk conductivity on cost-effective polymer foils

A combination of plasma and microwave flash sintering is used to sinter an inkjet-printed and tailored silver nanoparticle formulation. By using two sintering techniques sequentially, the obtained conductivity is 60%, while keeping the processing temperature well below the glass transition temperature (T(g)) of the used polymer substrate. This approach leads to highly conductive features on cost-effective polymer substrates in relatively short times, which are compatible with roll-to-roll (R2R) production. An electroluminescence device is prepared as an example.

A substrate-independent lift-off approach for patterning functional surfaces

A lift-off method for creating multifunctional patterned surfaces has been devised. It entails consecutive pulsed plasmachemical deposition of a reactive bottom layer and a protective top release layer. By way of example, a bottom/top layer combination comprising pulsed plasma deposited poly(glycidyl methacrylate)/poly(pentafluorostyrene) has been shown to display selective adhesive lift-off of the latter. Application of a prepatterned adhesive template yields well-defined arrays of reactive epoxide functionality surrounded by a passive fluoropolymer background or vice versa.