A Control-Oriented Dynamical Model of Deposited Droplet Volume in Electrohydrodynamic Jet Printing

This paper discusses a run-to-run iterative learning control (ILC) algorithm for Electrohydrodynamic jet (E-jet) printing. E-jet printing is a nano-manufacturing process that uses electric field induced fluid jet printing through nano-scale nozzles for achieving better control and resolution than traditional jet-printing processes. The printing process is controlled by changing the voltage potential between the nozzle and the substrate. However, it is difficult to maintain constant operating conditions such as stand-off height during a run of the printing process. The change in operating conditions results in fluctuating jet frequency. For stabilizing the jetting frequency across a single run, we propose a proportional ILC algorithm. We determine the jetting frequency by recording the electric current pulses when ink droplets are released from the nozzle. The frequency profile obtained from current measurements is then used to shape the voltage profile across a run to compensate for changing operating conditions. Experimental results are presented to validate the proposed control method.

High-resolution flexible electronic devices are widely used in the fields of soft robotics, smart human-machine interaction, and intelligent e-healthcare monitoring due to their mechanical flexibility, ductility, and compactness. The electrohydrodynamic jet printing (e-jet printing) technique is used for constructing high-resolution and cross-scale flexible electronic devices such as field-effect transistors (FETs), flexible sensors, and flexible displays. As a result, researchers are paying close attention to e-jet printing flexible electronic devices. In this review, we focused on the latest advancements in high-resolution flexible electronics made by e-jet printing technology, including various materials used in e-jet printing inks, the process control of e-jet printing, and their applications. First, we summarized various functional ink materials available for e-jet printing, including organic, inorganic, and hybrid materials. Then, the interface controlling the progress of e-jet printing was discussed in detail, including the physical and chemical properties of the functional ink, the interfacial wettability between the ink and substrate, and the micro-droplet injection behavior in a high-voltage field. Additionally, various applications of e-jet printing in the fields of flexible electrodes, FETs, flexible sensors, and flexible displays were demonstrated. Finally, the future problems and potential associated with the development of next generation e-jet printing technology for flexible electronic devices were also presented.

Electrohydrodynamic jet (E-jet) printing has broad application prospects in the preparation of flexible electronics and optical devices. Ejection cycle time and droplet size are two key factors affecting E-jet-printing quality, but due to the complex process of E-jet printing, it remains a challenge to establish accurate relationships among ejection cycle time and droplet diameter and printing parameters. This paper develops a model based on random forest regression (RFR) for E-jet-printing prediction. Trained with 72 groups of experimental data obtained under four printing parameters (voltage, nozzle-to-substrate distance, liquid viscosity, and liquid conductivity), the RFR model achieved a MAPE (mean absolute percent error) of 4.35% and an RMSE (root mean square error) of 0.04 ms for eject cycle prediction, as well as a MAPE of 2.89% and an RMSE of 0.96 μm for droplet diameter prediction. With limited training data, the RFR model achieved the best prediction accuracy among several machine-learning models (RFR, CART, SVR, and ANN). The proposed prediction model provides an efficient and effective way to simultaneously predict the ejection cycle time and droplet diameter, advancing E-jet printing toward the goal of accurate, drop-on-demand printing.

Electrohydrodynamic jet (E-jet) printing is a recent technique for high resolution additive micromanufacturing. With high resolution comes sensitivity to small disturbances, which has kept this technique from reaching its industrial potential. Closed loop control of E-jet printing can overcome these disturbances, but it requires an improved understanding of ink droplet spreading on the substrate and a physical model to predict printed feature locations and geometries from process inputs and disturbances. This manuscript examines a model of ink droplet spreading that uses assumptions that are important to the e-jet process. Our model leverages previous energy balance models that were derived for larger length scale droplets. At the smaller length scale, we find that viscous losses are a significant portion of the energy budget and must be accounted for; this is in contrast to models at length scales two orders of magnitude larger. Our model predicts the droplet height, base radius and contact angle in time from an initial volume and E-jet printing control parameters. The model is validated with published droplet spreading data and new measurements of E-jet printed droplets of diameter 8 μm. The viscous friction calculated in the new model is found to be significant compared to surface energy.

High-resolution additive direct writing of metal micro/nanostructures by electrohydrodynamic jet printing

This paper reports the use of advanced forms of electrohydrodynamic jet (e-jet) printing for creating micro- and nanoscale patterns of proteins on various surfaces ranging from flat silica substrates to structured plasmonic crystals, suitable for micro/nanoarray analysis and other applications in both fluorescent and plasmonic detection modes. The approaches function well with diverse classes of proteins, including streptavidin, IgG, fibrinogen, and γ-globulin. Detailed study reveals that the printing process does not adversely alter the protein structure or function, as demonstrated in the specific case of streptavidin through measurements of its binding specificity to biotin-modified DNA. Multinozzle printing systems enable several types of proteins (up to four currently) to be patterned on a single substrate, in rapid fashion and with excellent control over spatial dimensions and registration. High-speed, pulsed operational modes allow large-area printing, with narrow statistical distributions of drop size and spacing in patterns that include millions of droplets. The process is also compatible with the structured surfaces of plasmonic crystal substrates to enable detection without fluorescence. These collective characteristics suggest potential utility of e-jet techniques in wide-ranging areas of biotechnology, where its compatibility with various biomaterials and substrates with different topographies and surface chemistries, and ability to form deposits that range from thick films to submonolayer coatings, derive from the remote, noncontacting physical material transfer mode of operation.

Design, Development and Experimental Investigation of E-jet Based Additive Manufacturing Process

Electrohydrodynamic jet printing of PZT thick film micro-scale structures

Bioelectronic interfaces and biohybrid technologies have emerged as powerful solutions for sensing and manipulation applications. A unique method of underwater chemical sensing via a combination of engineered microbes and bioelectronic interfaces is discussed here with the goal of reporting to autonomous robotics through an electronic nervous system. The fabrication of these interfaces warrants a deposition method capable of precisely implementing the electronic, interfacial, and biological layers necessary for reliable devices. Electrohydrodynamic jet (e-jet) printing has been shown capable of doing so at a biologically relevant scale of 240 ± 50 nm. An e-jet printing system was fabricated and modified to use additive manufacturing to fabricate heterogeneous interfaces for the biological reporters Nitric Oxide and luminescence. These tasks included the fabrication of electronics that typically have multiple materials and thus multiple layers in their architecture; a potential point of failure in this liquid phase deposition method. The implementation of code based ejection modalities and a heated vacuum chuck allow for controlled volume deposition and rapid solvent evaporation, alleviating dissolution events in the sublayer. By eliminating these events, controlling film thickness to deter pinhole formation, and generating inks with solvents that were incompatible with sublayers, multi-layer electrochemical nitric oxide sensors and photosensors were produced. The fabricated multi-layer nitric oxide sensor has a lower limit of detection range of 188.5 nM - 314.5 nM; levels at which RAW 264.7 macrophage depolarization can be monitored in real time. An alternative bioelectronic interface is being studied wherein a modified version of this sensor is interfaced with Shewanella oneidensis to detect aqueous toxins. The photosensor architecture is comprised of a transparent PET substrate, an ITO cathode (pre-deposited), a bulk heterojunction P3HT:PC₇₁BM active layer, and a silver anode, with a final maximum spectral response of 7 mA/W and 90 mA/W/mm² at 514 nm, with a broad visible light spectrum from 400 nm to 620 nm. In interfacing these sensors with Vibrio fischeri, emitting light at 490 nm, the concentration of auto inducers in solution has been monitored in real time. These studies have created a clear path toward the fruition of olfactory guided, underwater autonomous robotics.

Here, we suggest simple method for multi value slope In‐Ga‐Zn O (IGZO) thin‐film transistors (TFTs) using electrohydrodynamic (EHD) jet printing. To induce multi value slope, parasitic channel was intentionally formed on top of IGZO main channel using EHD jet printing. By precisely controlling parasitic channel regime with EHD jet printing, multi value slope could be realized without degradation in electrical performance of original IGZO TFTs.

Numerical simulation of stable electrohydrodynamic cone-jet formation and printing on flexible substrate

Fabrication of piezoelectric thick-film stator using electrohydrodynamic jet printing for micro rotary ultrasonic motors

<p indent="0mm">Microfluidic chips have attracted increasing research attention in biology, chemistry, and medicine; in particular, microfluidic pipes containing metal electrodes have broad prospects in capillary electrophoresis, electrochemical microdetection, biomedical engineering, and flexible electronics. In this study, a simple on-demand method for fabricating arrayed flexible microfluidic pipes with embedded metal electrodes was proposed. The method is based on the direct-writing technology of electrohydrodynamic jet printing (E-jet), which is combined with the conventional mold turning and wet etching processes to obtain flexible microfluidic pipe arrays with embedded metal electrodes. First, a linear convex microstructure with a smooth surface (line width of 10–100 μm and aspect ratio &gt;1:2) was prepared by stacking direct-writing polyvinyl alcohol fibers using the E-jet printing system with a linear rotation-receiving substrate to serve as a template. Using the template, the arrayed microfluidic channels were obtained. Second, a flexible metal patterned conductive electrode (line width &lt;5 μm) was printed on the flexible microfluidic pipe array substrate using the E-jet printing system with a linear translation-receiving substrate and the wet etching process, with a direct-writing photoresist as the protective layer. Finally, the microfluidic pipes were electrically tested with different salt solution concentrations to verify the conductivity of the pipes and the metal electrode. The results indicate that the integrated manufacturing process based on the E-jet printing could be used to simply and efficiently fabricate flexible microfluidic pipe arrays embedded with gold electrodes on-demand and at a low cost. These arrays can be broadly applied in biomedical engineering and flexible electronics.

In the dc voltage-applied electrohydrodynamic (EHD) jet printing of metal nanoparticles, the residual charge of droplets deposited on a substrate changes the electrostatic field distribution and interrupts the subsequent printing behaviour, especially for insulating substrates that have slow charge decay rates. In this paper, a sinusoidal ac voltage was used in the EHD jet printing process to switch the charge polarity of droplets containing Ag nanoparticles, thereby neutralizing the charge on a polyethylene terephthalate (PET) substrate. Printed Ag lines with a width of 10 µm were invisible to the naked eye. After sintering lines with 500 µm of line pitch at 180 °C, a grid-type transparent electrode (TE) with a sheet resistance of ∼7 Ω sq−1 and a dc to optical conductivity ratio of ∼300 at ∼84.2% optical transmittance was obtained, values that were superior to previously reported results. In order to evaluate the durability of the TE under bending stresses, the sheet resistance was measured as the number of bending cycles was increased. The sheet resistance of the Ag grid electrode increased only slightly, by less than 20% from its original value, even after 500 cycles. To the best of our knowledge, this is the first time that Ag (invisible) grid TEs have been fabricated on PET substrates by ac voltage applied EHD jet printing.

Invisible Ag mesh transparent electrodes (TEs), with a width of 7 μm, were prepared on a curved glass surface by electrohydrodynamic (EHD) jet printing. With a 100 μm pitch, the EHD jet printed the Ag mesh on the convex glass which had a sheet resistance of 1.49 Ω/□. The printing speed was 30 cm s−1 using Ag ink, which had a 10 000 cPs viscosity and a 70 wt% Ag nanoparticle concentration. We further showed the performance of a 3-D transparent heater using the Ag mesh transparent electrode. The EHD jet printed an invisible Ag grid transparent electrode with good electrical and optical properties with promising applications on printed optoelectronic devices.