CHAPTER 4. Unwanted Reactions of Polymers During the Inkjet Printing Process

Collagen, fibrinogen, and thrombin proteins in aqueous buffer solutions are widely used as precursors of natural biopolymers for three-dimensional (3D) bioprinting applications. The proteins are sourced from animals and their quality may vary from batch to batch, inducing differences in the rheological properties of such solutions. In this work, we investigate the rheological response of collagen, fibrinogen, and thrombin protein solutions in bulk and at the solution/air interface. Interfacial rheological measurements show that fibrous collagen, fibrinogen and globular thrombin proteins adsorb and aggregate at the solution/air interface, forming a viscoelastic solid film at the interface. The viscoelastic film corrupts the bulk rheological measurements in rotational rheometers by contributing to an apparent yield stress, which increases the apparent bulk viscosity up to shear rates as high as 1000 s-1. The addition of a non-ionic surfactant, such as polysorbate 80 (PS80) in small amounts between 0.001 and 0.1 v/v%, prevents the formation of the interfacial layer, allowing the estimation of true bulk viscosity and viscoelastic properties of the solutions. The estimation of viscosity not only helps in identifying those protein solutions that are potentially printable with drop-on-demand (DOD) inkjet printing but also detects inconsistencies in flow behavior among the batches.

In the search for efficient methodologies focused on up scaling production of low cost functional ceramic oxides, the implementation of Chemical Solution Deposition (CSD) in combination with drop on demand (DoD) inkjet printing technology is a challenge in materials science. When simple additive processing is implemented in continuous reel to reel systems with novel low-cost materials, the potential to further decrease the production costs of the whole manufacturing process is immense.Sets of parallel YBa2Cu3O7−x (YBCO) stripes, 250 nm thick and 150 μm wide, with critical current densities (Jc) of about 1.1 MA/cm2 at 77K and self field are obtained.On the other hand, different designs of magnetoresistive La0.7Sr0.3MnO3 (LSMO) devices with electrical resistances in the range of 10 KΩ including the contact pads, are drawn onto LaAlO3 (LAO) and SrTiO3 (STO) single crystals. We particularly investigate different strategies to increase pattern resolution and accuracy. Preliminary morphological and electrical characterizations of the device on cheaper amorphous, polycrystalline and textured substrates are presented.Therefore, this paper gives a summary of functional ceramic oxides patterning by combining CSD and inkjet printing methodologies onto different single crystal, polycrystalline and textured substrates. The present work demonstrates a powerful method of fabricating multifilamentary YBCO patterned coatings for low losses coated conductors (CC's) and LSMO magnetoresistive analog encoders for position sensing.

Drop-on-demand (DoD) ink-jet printing is an efficient technology for depositing picoliter drops on various printing surfaces. DoD technology is compatible with various liquids and does not require contacting the printing media. DoD inkjet printing combines several advantages including high speed, quiet operation, and compatibility with a variety of printing surfaces. Moreover, DoD printing can make patterns without any additional lithographic process. Ink-jet printing can reduce the number of processing steps compared to conventional patterning processes, which results in a lower production cost. Therefore, DoD ink-jet technology is applied in many engineering and scientific applications (see Figure 1), such as the formation of the conductive tracks for printed circuit boards, color filters in flat panel displays and plasma displays, polymer light-emitting diode displays, organic transistors, and the construction of DNA microarrays [1]-[5].

Submicron sized silver powder was prepared from AgNO3 using a chemical-reduction method. A spherical silver power exhibiting an average particle size distribution of 0.2–0.4 μm and an excellent dispersibility was achieved and applied to the inkjet printing process. A drop-on-demand (DOD) inkjetting system was used to print the silver particles suspended in a terpineol solvent. Through sintering at 300 °C, the size of the particles adjacent to the borderline of droplets were gradually increased and necking was observed between the droplets. Alternatively, the substrate for the particles could be heated to a lower temperature, and the sintering process of the conducting line was completed by the application of a laser beam. Increase in the laser power reduces the resistivity of the line. Through microstructure analysis, the necks between droplets were sintered at a specific energy density (ψ = 0.0398 J mm−3). The conducting lines were soldered and of a larger aggregation, between which a discontinuous micro-crack was observed. This was attributed to the surface tension effect and shrinkage during solidification. Influence of the densification parameters on resistivity was significant.

Three-dimensional (3D) bioprinting techniques, developed over the past two decades, have the potential to fabricate living, patient-specific tissues and organs for use in regenerative medicine (Kesti et al. 2016). Inkjet-based bioprinting is one of the oldest yet promising technologies, which combines the solid freeform fabrication process and precise cell placement in 2D and 3D. Typically, inkjet printing takes digital data of an image or character from a computer and reproduces it on to a material or substrate using ink drops in a noncontact mode (Mohebi and Evans 2002). In addition to its wide application in automation office tool, it has also been widely used in microengineering for printing electronic materials (Sirringhaus et al. 2000). Inkjet printer deposits controlled volume of ink in a layer-by-layer fashion to predefined locations on successive layers to form a 3D construct (Xu et al. 2008). Drop-on-demand (DOD) printers, a commonly used inkjet printers, are used for biological and nonbiological applications. The idea of printing of biological substances was introduced by Klebe after using HP thermal DOD inkjet printer to deposit collagen and fibronectin in 1988 (Klebe 1988). Later, Thomas Boland customized the thermal inkjet printer for printing living cells in a viable state (Wilson and Boland 2003). After this, scientists have made great progress in patterning molecules such as DNA, depositing proteins (Eric et al. 2009, Ilkhanizadeh et al. 2007, Miller et al. 2006), and printing tissues and organs by inkjet printing (Okamoto et al. 2000). The current focus of inkjet bioprinting is fabrication of 3D structures with clinically significant heights to extend its effectiveness in the direction of clinical applications. In this chapter, we discuss about the principle of inkjet bioprinting technology, types of inkjet printers in use, materials used for printing, and the critical parameters involved in this technique. Then, challenges associated with techniques and the future perspectives are also discussed.

In this article, we discuss the relationship between a high-speed continuous ink jet system and the printing substrate. Nonabsorbent materials, with different surface topographies were used to evaluate the importance of the droplet impact and spreading without capillary penetration. Commercial papers were also used to evaluate the potential of the existing grades, and to find their essential performance parameters. A laboratory scale testing environment was used for the high-speed imaging of ink jet drops. The impact, spreading, absorption, and drying of the ink droplets on the samples can be observed and analyzed in this testing environment on the time scale of microseconds up to several minutes. The image technical performance of the samples was measured with an image analysis system which was specially designed for the analysis of print quality with non-impact printing techniques.

There are many different methods by which to apply marks and codes either directly on product packages (primary packaging) or in cartons or cases of that product (secondary packaging). The following is an overview of different types of digital printing technologies used for industrial marking applications. The paper will explain the use of piezo drop-on-demand (DOD) ink jet with both phase change and liquid inks. Other ink containing systems that will be reviewed include continuous ink jet, valve jet, and thermal transfer. In addition, one non-ink containing digital marking method – a laser printing system- will also be discussed. The end user's choice of digital printing technology is determined by the application requirements of the product being marked, including: the size and shape of the mark or code, print speed, adhesion requirements, substrate, environmental requirements, and cost per print. The benefits and disadvantages of each technology will be discussed in relationship to these requirements.

A process combining conventional photolithography and a novel inkjet printing method for the manufacture of high sensitivity three-dimensional-shape (3DS) sensing patches was proposed and demonstrated. The supporting curvature ranges from 1.41 to 6.24 × 10−2 mm−1 and the sensing patch has a thickness of less than 130 μm and 20 × 20 mm2 dimensions. A complete finite element method (FEM) model with simulation results was calculated and performed based on the buckling of columns and the deflection equation. The results show high compatibility of the drop-on-demand (DOD) inkjet printing with photolithography and the interferometer design also supports bi-directional detection of deformation. The 3DS sensing patch can be operated remotely without any power consumption. It provides a novel and alternative option compared with other optical curvature sensors.

Fabrication of metal oxide thin film transistor (MOTFT) arrays using the inkjet printing process has caused tremendous interest for low-cost and large-area flexible electronic devices. However, the inkjet-printed MOTFT arrays usually exhibited a non-uniform geometry due to the coffee ring effect, which restricted their commercial application. Therefore, in this work, a strategy is reported to control the geometry and enhance device performance of inkjet-printed MOTFT arrays by the addition of an insulating polymer to the precursor solution prior to film deposition. Moreover, the impact of the polymer molecular weight (MW) on the geometry, chemical constitution, crystallization, and MOTFT properties of inkjet-printed metal oxide depositions was investigated. The results demonstrated that with an increase of MW of polystyrene (PS) from 2000 to 200 000, the coffee ring was gradually faded and the coffee ring effect was completely eliminated when MW reached 200 000, which is associated with the enhanced viscosity with the insulating polymer, providing a high resistance to the outward capillary flow, which facilitated the depinning of the contact line, leading to the elimination of the coffee ring. More importantly, the carrier mobility increased significantly from 4.2 cm2 V−1 s−1 up to 13.7 cm2 V−1 s−1 as PS MW increased from 2000 to 200 000, which was about 3 times that of the pristine In2O3 TFTs. Grazing incidence X-ray diffraction and X-ray photoelectron spectroscopy results indicated that PS doping of In2O3 films not only frustrated crystallization but also altered chemical constitution by enhancing the formation of the M-O structure, both of which facilitated the carrier transport. These results demonstrated that the simple polymer additive process provides a promising method that can efficiently control the geometry of MO arrays during inkjet printing and maximize the device performance of MOTFT arrays, which showed great potential for the application in next generation printed displays and integrated circuits.

Performance Improvement of Professional Printing Systems: from theory to practice Markets demand continuously for higher quality, higher speed, and more energy-efficient professional printers. In this thesis, control strategies have been developed to improve the performance of both professional inkjet and laser printers. Drop-on-Demand (DoD) inkjet printing is considered as one of the most promising printing technologies. It offers several advantages including high speed, quiet operation, and compatibility with a variety of printing media. Nowadays, it has been used as low-cost and efficient manufacturing technology in a wide variety of markets. Although the performance requirements, which are imposed by the current applications, are tight, the future performance requirements are expected to be even more challenging. Several requirements are related to the jetted drop properties, namely, drop velocity, drop volume, drop velocity consistency, productivity, and reliability. Meeting the performance requirements is restricted by several operational issues that are associated with the design and operation of inkjet printheads. Major issues that are usually encountered are residual vibrations in and crosstalk among ink channels. This results in a poor printing quality for high-speed printing. Given any arbitrary bitmap, the main objective is to design actuation pulses such that variations in the velocity and volume of the jetted drops are minimized. Several model-based feedfoward control techniques using an existing model are implemented to generate appropriate input pulses for the printhead. Although the implementation of the model-based techniques shows a considerable improvement of the printhead performance compared with the current performance, further improvements are still necessary. We observe that besides the pulse shape the state of the ink surface at the nozzle plate (speed, position) at the start of the pulse influences the drop velocity considerably. This state at firing depends also on previous pixels in the bitmap of the image. Consequently, any pulse design has to guarantee almost the same initial state when firing a drop. Based on these facts, a model-free optimization scheme is developed to minimize the drop velocity variations taking into account the bitmap information. Experimental results show the effectiveness of the optimized pulses. Laser printing systems are highly depending on the appropriate combination of several design factors so as to become functional in a desired working range. The physical printing process involves multiple temperature set points at different places, precise electro-magnetic conditions, transfer of toner through certain pressures and layouts, and many other technical considerations. In the laser printing system there are several challenging issues and unknown disturbances. They originate from different sources, such as the printer itself (unknown phenomena appear, disturbances that are not foreseen, wear, contamination, failures, bugs), the environment of the system (power supply variations, temperature, humidity, vibrations), and the printing media (weight, coating, thermal properties, humidity characteristics, and initial temperature). These issues have a negative effect on the stability and performance of the laser printing system. The objective is to design a control scheme to achieve printing quality requirements and a high productivity. Good printing quality means that the fusing temperature should track a certain reference signal at different operating conditions. Based on the printing system behavior, we propose two different control schemes to cope with the large parameter variations and disturbances, namely, a Model Reference Adaptive Controller (MRAC) and a nonlinear (scheduled) observer-based output feedback control scheme. Both control techniques yield considerable performance improvements compared with the present industrial controller.

In this paper, silver powders with a uniform particle size of 0.2-0.4 μm and an excellent dispersibility were applied to produce a conductive ink for the inkjet printing process. The thermal behaviors of silver particles sintered at different temperature were investigated through X-ray diffraction (XRD) patterns. It was found out that both the particle size and crystal grains increase during the surface diffusion and sintering process. A drop-on-demand (DOD) inkjet printing system was employed to print the conductor by using as-mentioned silver particles suspended in terpineol/polyketone (PK) system as conductive materials. The optimized value of W PK/W silver was 5%, corresponding to a resistivity of 2.0 μΩ · cm. It was revealed that an increase in the W PK/W silver ratio resulted in the increase in both the resisitivity and adhesion strength of the conductor. The scanning electron microscopy (SEM) analysis based on the microstructures of silver conductor further illustrates that the densification of conductor and long-range interparticle connectivity ensure the silver conductor a low resistivity. The adhesiveness effect from PK resin enables the conductor a high adhesion strength.

Drop-on-demand (DOD) microdroplet formation and deposition play an important role in additive manufacturing, particularly in printing of three-dimensional (3D) in vitro biological models for pharmacological and pathological studies, for tissue engineering and regenerative medicine applications, and for building of cell-integrated microfluidic devices. In development of a DOD based microdroplet deposition process for 3D cell printing, the droplet formation, controlled on-demand deposition and at the single-cell level, and most importantly, maintaining the viability and functionality of the cells during and after the printing are all remaining to be challenged. This report presents our recent study on developing a novel DOD based microdroplet deposition process for 3D printing by utilization of an alternating viscous and inertial force jetting (AVIFJ) mechanism. The results include an analysis of droplet formation mechanism, the system configuration, and experimental study of the effects of process parameters on microdroplet formation. Sodium alginate solutions are used for microdroplet formation and deposition. Key process parameters include actuation signal waveforms, nozzle dimensional features, and solution viscosity. Sizes of formed microdroplets are examined by measuring the droplet diameter and velocity. Results show that by utilizing a nozzle at a 45 μm diameter, the size of the formed microdroplets is in the range of 52–72 μm in diameter and 0.4–2.0 m/s in jetting speed, respectively. Reproducibility of the system is also examined and the results show that the deviation of the formed microdroplet diameter and the droplet deposition accuracy is within 6% and 6.2 μm range, respectively. Experimental results demonstrate a high controllability and precision for the developed DOD microdroplet deposition system with a potential for precise cell printing.

One of the challenges in inkjet technology is the reduction of residual vibration and crosstalk that can result in the appearance of satellites and ligaments in the produced droplets, which affects the overall printing quality. This study evaluated the print quality produced by comparing basic waveform with several actuating waveforms, particularly in the application of the grayscale method or the multi-pulse-ejection method. Print quality was compared using a microscope to evaluate the appearance of ejected droplet on a printing surface. This study measured aspect ratio to illustrate the circularity of the droplets. Based on the findings in this study, the waveform design with adjustments made to the rear wave from the preliminary vibration is proved effective in improving print quality. The average of aspect ratio obtained approached 1, means almost perfect circular, which was 0.997, with the lowest standard deviation of 0.0151.

Piezo-acoustic drop-on-demand (DOD) inkjet printing is widely applied in high-end digital printing due to its unprecedented precision and reproducibility. However, the stability of piezo-DOD inkjet printing can sometimes be compromised through the stochastic entrainment of bubbles within the ink channel. The acoustically driven air bubble modifies the ink channel acoustics, and conversely, the modified ink channel acoustics influences the bubble dynamics. Here, we measure the acoustic eigenfrequency of a MEMS based silicon ink channel as a function of the bubble size. The eigenfrequency was measured using a pulse-echo system and the bubble size using a short-wave infrared imaging setup. We show that the measured eigenfrequency increases when an air bubble is entrained. Surprisingly, the ink channel resonance frequency plateaus at total bubble volumes larger than 10 pl. Moreover, it was found that at a constant total bubble volume, the resonance frequency increases with the number of entrained bubbles. We show that both experimental observations can be quantitatively explained from a simple lumped element model (LEM) comprising the ink channel Helmholtz resonator coupled to the bubble mass-spring system. The results of the LEM model were validated using a full numerical model of the coupled ink channel—bubble system.

The drop-on-demand (DOD) inkjet printing technology has been widely used in many fields and several types of droplet generators are developed. This paper presents the design, fabrication and tests of a piezoelectric bend mode drop-on-demand inkjet printhead with interchangeable nozzle. A disk-type PZT is actuated to push the liquid out of inkjet printhead by a function generator, and a droplet is formed because of surface tension. The interchangeable nozzle design enables the same printhead to be fitted with nozzles of different orifice size, thus a clogged nozzle can be easily removed for cleaning or replacement. An experimental platform for micro-droplet jetting is built in this paper. The droplet formation is recorded by a CCD camera as pictures, which can be used to measure the droplet dimension. The experiments are carried out by using the self-developed bend mode piezoelectric inkjet printing system. The influence of the drive parameters on the droplet quality is also studied by dispensing water.