Classifications and Applications of Inkjet Printing Technology: A Review

Inkjet printing technology uses the low-cost direct deposition manufacturing technique for printing and is applicable in various fields including optics, ceramics, three-dimensional printing in biomedicine, and conductive circuitry. This study reviews the classifications and applications of inkjet printing technologies, with a focus on recent publications. The different design approaches, applications, and research progress of several inkjet printing techniques are reviewed. Among them, the piezoelectric inkjet printing technology is the main focus owing to its reliability and handling of a diverse range of inks. A piezo-driven inkjet printhead is activated by applying a voltage waveform to a piezoelectric membrane. The waveform ensures the formation of the designed droplet and a stable jet. A survey of various driving-voltage waveforms is conducted, which can serve as a reference to the research community that uses piezo-driven inkjet printheads. The challenges of printing quality, stability, and speed and their solutions as published in recent studies are reviewed. Technologies for producing high-viscosity inkjets are explored, and the applications of inkjet printing technology in textile, displays, and wearable devices are discussed.


I. INTRODUCTION
Inkjet printing technology has applications in the fields of bioengineering [1], three-dimensional (3D) printing of microstructures [2], flexible and textile electronics [3], and micromechanical and microfluidic devices [4]. In 1858, William Thomson and Abbe Nollet invented an inkjet-like recording device with a continuous inkjet head [5], which paved the way for inkjet printheads. This technology was further matured by the introduction of the equations of fluid motion, drop-on-demand (DoD) inkjet heads with the squeeze, bend, push, and shear deformation modes using a piezoelectric actuator, and DoD thermal inkjet (TIJ) printheads [6].
Inkjet printing technology can be classified into two groups, continuous ink jet (CIJ) and DOD ink jet [6]. In a CIJ system, a stream of droplets is ejected continuously under an applied electric field and a charging electrode. The uncharged droplets are received by a catcher. In a DOD printing system, the droplet can be ejected by a voltage waveform. Referenced from [6] and further modified with information on the latest printing technologies. Fig. 1 shows the classification of inkjet printing technology.
Several methods including needle-based printing [ All these methods have their merits and demerits that make it suitable for printing resolution, various materials and applications. For example piezoelectric, thermal, needlebased and acoustophoretic printing are nozzle-based methods that are prone to clogging when attempting to extend their use for printing the inks which can cause nozzle blockage. However, piezoelectric inkjet printing is the most mature and reliable technology [16]. The mechanical structure of a piezoelectric inkjet printhead (PIP) has an ink chamber connected to the ink cartridge through a narrow path called the restrictor or throttle. On top of the ink chamber is a membrane composed of a piezoelectric material sandwiched between two electrodes. A nozzle supplies ink droplets to the outer substrate by applying an electrical pulse to the piezodriven membrane on top of the ink chamber. This membrane pressurizes the chamber, increasing the fluid velocity at the nozzle. The high velocity of the fluid leads to droplet formation at the nozzle exit [16]. The piezoelectric membrane, ink chamber, and nozzle are manufactured on wafers using microelectromechanical systems (MEMS). These structures are combined using an MEMS-based bonding technique to fabricate a printhead device. The TIJ printhead consists of a resistor, a chamber and a nozzle. An electrical pulse applied to the resistor heats the fluid, forming a vapor bubble that pushes the fluid through the nozzle, thus producing a droplet or a series of droplets [16].
Several studies have reviewed inkjet printing technologies. Reference [17] reviewed various materials and the application of inkjet technology to print them. These technologies, ranging from PIP to EHD printing, have various applications in printing 2D or 3D materials at the micrometer and nanometer scales [18] [19]. Reference [6] reviewed the history of PIP dynamics, covering the topics of actuation, ink chamber and nozzle acoustics, droplet formation, wetting, and air bubbles [6]. Li et al. [8] reviewed various piezo-driven inkjet printhead designs, along with their applications and challenges. Kwon et al. [20] reviewed commercially available printheads, ink supply systems, and inkjet printing technologies. Reference [21] explored microdroplet generation methods [21].
In this study, we focused on the classifications and applications of printing technologies. Focus is laid on the piezoelectric inkjet printing technology, especially the various voltage waveforms, the influence of voltage waveform on droplet formation, and methods to optimize these waveforms. Research articles that discussed the challenges related to the printing quality, stability, and speed of piezo-driven printheads and their solutions were articulated. Furthermore, high-viscosity jet ink printing technologies were discussed. Finally, various applications of different inkjet printing technologies in the fields of digital textile printing (DTP), display pixel printing, MEMS and wearable, stretchable, flexible devices were reviewed.

A. PIEZOELECTRIC INKJET PRINTING
In a PIP, the shape of the piezoelectric transducer changes under an applied voltage. This generates a pressure pulse in the ink chamber, resulting in the ejection of ink droplet from the nozzle connected with the chamber. A PIP can be in the squeeze, push, shear, or bend mode (Fig. 2). Most studies have demonstrated the bend mode.

1) ACTUATION MECHANISMS AND EFFECT OF VOLTAGE WAVEFORM
The PIP can be actuated by using either the push-pull ( Fig.  3-a) or pull-push mode ( Fig. 3-b). Generally, for these modes, a single trapezoidal pulse called the standard voltage waveform is used, which is composed of the rise, dwell, and fall times. At the rising edge of the pull-push mode, the piezoelectric membrane bends upward (z-axis), causing the ink chamber to expand and generate a negative pressure. Meanwhile, the ink is sucked from the nozzle and restrictor toward the center of the ink chamber. The membrane is still bent upward along the z-axis during the dwell time. At the falling edge, the membrane bends toward the ink chamber, causing the chamber to contract and generate a positive pressure. Thus, the droplet is ejected from the nozzle exit. The push-pull mode works opposite of the pull-push mode, i.e., the membrane is pushed toward the ink chamber at the rising edge and bent upward in the zdirection at the falling edge of the voltage waveform. References [22] [23] have studies the push-pull actuation mechanism [22] [23], in which the jet pressure and ink droplet velocity are not affected by the change in the dwell time of the voltage waveform [23]. However, this mode has a low velocity. In the case of the pull-push mode, although the jet pressure and droplet velocity fluctuate with the dwell time, the velocity is high. Because printing applications require high jet velocities, researchers have focused on the pull-push actuation mechanism.
The ink chamber still undergoes pressure oscillations even after the first ink droplet has been ejected under an applied standard voltage waveform using the pull-push actuation mechanism [24]. These undesired pressure oscillations decay after several microseconds, cause variations in the properties of the subsequent ink drops, and form satellites, which degrades the inkjet printhead performance. A voltage waveform with two trapezoidal pulses to suppress the undesired pressure oscillations was formulated. Based on the polarity, the voltage waveform was divided into two categories by pulse type, unipolar [25] [26] and bipolar [27] [28]. In both the cases, the first pulse was used for jetting an ink drop, whereas the second one, with the same or opposite polarity, was used to damp the undesired pressure oscillations.
Kwon and Kim [25] used the waveform with unipolar pulses. They extracted the pressure wave information from the ink chamber using piezoelectric self-sensing measurement by measuring the piezo-current. The second pulse was used to suppress the undesired pressure oscillations. They set the magnitude of the second waveform to half that of the first one, which cancelled out the undesired pressure oscillations. The start time of the second waveform is highly sensitive to oscillations in the unipolar voltage waveform. The undesired pressure oscillations were suppressed with the optimal start time of the second pulse of 18.8 µs; however, if the start time were more or less than this threshold, then the jetting performance would be poor, or even worse than the single voltage waveform in the case of a decreased second pulse start time. In addition to the suppression of undesired pressure oscillations, the suppression of satellite droplet formation has also been demonstrated by using a voltage waveform with unipolar pulses [26]. The disadvantage of using this type of waveform is that it takes longer to dampen the undesired pressure oscillations than with the voltage waveform with bipolar pulses [29]. Therefore, mostly voltage waveforms with bipolar pulses have been used in the literature [27]- [30].
To obtain a better jet, the optimal parameters of the jet pulse must be determined. The optimal parameter can be defined as the highest possible velocity and sufficient volume of an ink droplet for a specific voltage waveform parameter at given amplitude. Generally, a voltage waveform with a rise and fall time of less than 3 µs is sufficient for good droplet ejection [31], however the dwell time must be considered. As the dwell time increases, the velocity and volume of the ink droplet increases. Above the optimal dwell time, it decreases drastically [32] [33]. This trend also applies to the M-shaped, W-shaped, or other complex waveforms when tuning their time parameters. Different approaches were presented for estimating the optimal dwell time of a voltage waveform for achieving a higher droplet velocity and required volume. Reference [31] used the wave propagation theory and recommended an optimal dwell time of l/c [31], whereas [34] recommended 2l/c, where l is the length of the tube and c is the speed of sound in inks. However, no accurate optimal dwell time exists in practice because l and c are likely to be unknown and the fluid viscosity effect cannot be considered without complicating the equation. Therefore, other methodologies, for e.g., numerical simulation [35], model based [23][29] [36][37][38], automatic tuning [39], and experimental approaches [25] [40] have also been recommended. Table II lists the studies that used these approaches along with their optimal waveforms parameters and optimization methods. Table I presents the different voltage waveforms and their effects on the printhead's performance.
The efficient deposition of different types of inks on a substrate is necessary in the inkjet industry for all applications. Therefore, it is necessary to have a mechanism to control droplet formation. The simplest method is to tune the voltage waveform. In the case of a fluid with low viscosity, the ejection of the first droplet from the nozzle outlet is followed by the formation of satellite droplets. High-velocity satellite droplets can combine to form a main droplet during flight to the desired spot on the substrate. However, low-velocity satellite droplets have poor directionality and may land on an arbitrary spot on the substrate, which reduces the print quality. Multipulse waveforms are needed to prevent satellite droplets and maintain the single primary droplet [41][42][43][44][45][46]. Shin et al. [41] applied a double waveform with two square pulses (unipolar M-shaped) to the actuator, which effectively ejected the single main droplet without forming satellites. The trajectory of the satellite droplets can be improved by tuning the separation time (t 4 ) between two pulses [42] [43]. Reference [43] suppressed satellite droplets by tuning the time interval (t 2 ) of a unipolar W-shaped waveform [44].
Droplet with ligament is another challenge during printing, especially in applications that use a moving substrate. In this case, the ligaments will not merge with the main droplet. This will in turn misshape the printed dot on the moving substrate [47]. Moreover, the length of the ligament increases as the applied voltage is increased. In the multidrop ejection method, a multipulse input voltage waveform is applied to the piezoelectric actuator. Applying different voltages to the pulses and tuning the pulse voltage can eliminate ligaments and satellites [47].
The quality of printing by inkjet technology has improved. Long-term productivity would require small droplets ejected at higher velocities. A dominant parameter, the droplet size can be reduced by reducing the nozzle orifice diameter and decreasing the pulse amplitude. However, a small orifice can be clogged and requires expensive manufacturing techniques. Moreover, decreasing the amplitude will decrease the droplet velocity. Therefore, alternative methods to reduce the droplet size must be researched. Reference [48] proposed three square-wave bipolar W-shaped voltage pulses with amplitudes of −46, 56, and −46 V, respectively. The method successfully reduced the droplet volume. The droplet volume can also be reduced by the M-shaped and bipolar waveforms; however, the W-shaped waveform has a higher percentage volume reduction effect than the two [42]. Fig. 3(c) illustrates the classification of the actuation mechanisms of a piezo-driven inkjet printhead and waveform optimization methods.   [43].  This waveform can be used to reduce the droplet volume [39].  The time interval t4 must be optimal as it is sensitive to both residual oscillations and satellite droplets.
This waveform can be used to reduce the droplet volume [48].  In a unipolar W-shaped waveform, the satellites can be suppressed by tuning time interval t2 [44].
The 2 nd pulse of this waveform is used to suppress the residual vibrations.  This waveform can be used to eject highly viscous inks as it allows for a high voltage difference [43].  The droplet velocity and volume decrease and subsequently increase as t4 increases [49].
The 2 nd pulse of this waveform is used to suppress residual vibrations.  Time interval t4 is designed to allow the chamber pressure to reach its minimum.

2) MISCELLANEOUS STUDIES AND RECENT RESEARCH IN PIEZOELECTRIC INKJET PRINTHEADS
Although piezoelectric inkjet printheads are commercial, the printing quality and jet stability can be improved to solve by solving the nozzle clogging, the nonaxisymmetric effect, and the entrainment of air bubbles in the ink channel.
The clogging of a nozzle strongly influences the inkjet printhead performance in various applications. For example, sieving, bridging, and aggregation of particles can block the flow of fluid in microfluidic devices [51]. The unconventional MEMS printhead process causes dirt particles composed of Si or glass to remain in the ink channel. The interaction of these remaining dirt particles with the oscillating meniscus, or cavitation in the bulk ink, cause the entrainment of air bubbles in the tube [52]. For example, the collision of silica with the internal surfaces of a printhead can clog the nozzle [53]. Nozzle blockage can be minimized by tuning the pH of silica sol [53]. Reference [53] visualized the growth, translation, and interaction of the entrained air bubbles using shortwave infrared imaging [54]. Nozzle blockage can also be solved by ink recirculation. A recirculation system can enhance the reliability of the jet and remove the entrapped air bubbles [20].
Another issue with piezoelectric inkjet printheads is the nonaxisymmetric effects, which degrade the stability and performance of inkjet printhead. A nonaxisymmetric droplet can prevent the capturing of satellite droplets as they diverge from the trajectory of the main droplet. Highfrequency jets can reduce the nonaxisymmetry effects as it forces the successive droplet to follow the first jetted asymmetrical droplet. The causes and their solutions must be researched to improve the printhead's performance and stability. Nonaxisymmetry can be caused by the misalignment of the printhead channel with the nozzle plate, dust particles at the nozzle surface, wetted nozzle plate, bubble trapped inside the nozzle, and unstable asymmetrical surface modes at the meniscus interface by a Rayleigh-Taylor-like mechanism [55]. Fig. 4 illustrates various axisymmetric droplets [55]. Furthermore, jet stability can be improved by cooling the ink [56].
Satellite droplets degrade the print quality and reproducibility. These droplets are formed by the breakage of the long ligament behind the main droplet. As mentioned earlier, a method to suppress satellite droplets is to tune the voltage waveform. Another method is to add viscoelasticity to the ink. Sen et al. [57] suppressed satellite droplets in water-based ink by infusing polymers. The addition of polymers stabilizes the ligament against the Rayleigh-Plateau instability. The ligament was pushed toward the main droplet, producing a single droplet without satellites [57]. Reference [56] suppressed satellite droplets by cooling the ink [56].

B. THERMAL INKJET PRINTING
The TIJ can be in the form of a roof-shooter, sideshooter, or suspended heater [59]. In the first configuration, the heater is placed behind the nozzle. In the second configuration, it is placed adjacent to the nozzle. In the third configuration, the heater is suspended within the ink chamber [59]. Most industries manufacture and utilize the roof-shooter TIJ [60]. Fig. 5 illustrates the three forms of TIJ.
The major problem with the TIJ printhead is its short lifetime because of the electromigration of the heater, damage by bubble cavitation, and thermal stress-induced cracks [9] [61]. The lifetime can be increased by increasing the thickness and shape of the heater [9][61] [62]. Another problem is kogation [63], a phenomenon where ink particles are deposited on the heater surface during the operation of the TIJ, affecting the formation of bubble and droplet ejection. The addition of anions to ink can prevent this phenomenon [64].

C. ELECTROHYDRODYNAMIC JET PRINTING
In EHD jet printing, ink is ejected from the nozzle exit by a high applied electrical field between the nozzle and the substrate. Depending on the electric field, the EHD printing can be performed in the CIJ mode or DOD mode. The CIJ mode requires a constant DC supply between the nozzle and the substrate, whereas the DOD mode requires pulsed DC voltage. The DOD mode has been a focus area because its jet emissions can be controlled. Three different methodologies, AC [65], pulsed DC [66], and single potential AC [67], shown in Fig. 6, were demonstrated by studies related the DOD mode under an applied electric field. In the case of AC or pulsed DC, the electric potential is applied to the nozzle and the substrate is grounded, whereas the AC voltage is applied to the substrate and the nozzle is kept electrically floating in the case of single potential AC.
Various materials have been successfully printed on different substrates using EHD [68][69][70][71][72][73]. However, issues such as liquid wetting, particle-substrate interaction, and low throughput persist [74] [75]. These challenges can be solved by changing the electrode shape, hydrophobic coating of the nozzle, and tuning the applied voltage and fluid flow rate [74]. Wu et al. [76] demonstrated an EHD printing system driven by a triboelectric nanogenerator (TENG). They claimed that the TENG can protect the substrate against the conventional high-voltage supply system. An array of nozzles can be used to solve the problem of low throughput [77]- [79]; however, the electrostatic crosstalk between the neighboring nozzles degrades the printhead's performance [80]- [82]. Several studies were conducted to suppress crosstalk. For example, researchers attempted to increase the space between nozzles. However, this can cause jet offset, especially in electrospinning [83]- [85]. Zhang et al. [86] confirmed that the linear arrangement of nozzles produced better uniformity in the jetted materials than the toothed nozzle arrangement.

D. NEEDLE-BASED PRINTING
The key components of the needle-based printing system are needle, seat, and nozzle (Fig. 7). Air pressure [87] or a piezoelectric-stack actuator [88] can be used to move the needle. The actuator exhibits rapid response and high efficiency [89]. However, the piezo-stack actuator does not produce adequate output displacement for fluid jetting; Shin HurAuthor Name: Preparation of Papers for IEEE Access (February 2017) 2 VOLUME XX, 2017 therefore, a displacement amplification mechanism, i.e., a mechanical amplifier (e.g., a lever) was adopted for the needle and actuator [90]. The two types of actuators are single [91] and dual [92] piezo-stack actuators. The dual piezo-stack actuator is used for high-viscosity and highfrequency jetting. The quick response of the actuator accelerates the needle movement [88] [93]. Droplets are ejected from the nozzle exit by the motion of the needle toward the inside of the seat. The output parameters associated with the motion of the needle can be predicted through fluid flow simulations [94] [95] or modeling [96]- [100]. Phung and Kwon [87] used the accelerometer to sense the needle motion. They also investigated the effects of various parameters on the motion and jetting behavior.
Various displacement amplification mechanisms were demonstrated to enhance the printing performance. For example, for the stress relaxation of a dual piezo-stack actuator, [101] presented the jetting dispenser based on a corner-filleted hinge attached to the actuator. The design was further improved in terms of the stress reduction in the actuator by introducing a cylindrical pivot and changing the shape of the amplifier block [102]. Zhou et al. [103] demonstrated a rhombic mechanical amplifier that exhibited a higher needle stroke than traditional mechanical amplifiers. Furthermore, the jetting performance can be enhanced by changing the needle shape. For example, the jetting velocity was increased by adding a side cap [104] and pin joint [105] to the needle.

E. AEROSOL JET PRINTING
AJP is a high-resolution direct-writing droplet-based technique that can print a variety of materials with viscosities ranging approximately from 1-2500 cP [106]. This method was successfully applied in the fabrication of interconnects [107], sensors [108]- [110], organic light emitting diodes [111], supercapacitors [111], transistors [112], and medical imaging devices [113]. AJP works on the principle of the atomization of ink by ultrasonification or pneumatic mechanism, which results in aerosol formation (suspension of liquid particles in a gas carrier). The aerosol is then transferred to the deposition head by an inert gas stream (e.g., N 2 ), after which it is ejected from the nozzle exit. Sheath gas is added to the deposition head to further improve the performance. In an ultrasonic atomizer, the atomization of ink occurs from the generation of highfrequency pressure waves by the piezoelectric ultrasonic transducer. In the case of a pneumatic atomizer, an atomizer nozzle is placed in an ink reservoir where a carrier gas with a high velocity is passed through the tip of the atomizer nozzle, atomizing the ink. Fig. 8 illustrates an aerosol jet with both the atomization methods. Both methods have their merits and demerits. Compared with the pneumatic atomizer, the ultrasonic atomizer produces uniform aerosol; however, it can only print very high-viscous inks [114].
To achieve optimal aerosol-jet printing with improved performance, the relevant parameters must be tuned including sheath gas flow rate (SHGFR) and carrier gas flow rate (CGFR), which are the two main parameters of AJP that influence the quality of the printed line. The printed line width decreases by increasing the focusing ratio (SHGFR/CGFR) [115]. The ideal operating window for deriving the optimal parameters has not yet been defined owing to the complex structure of AJP and a lack of research in this area. Trial-and-error strategies have been adopted to achieve enhanced printing quality [115] [116]. However, this approach is time consuming and inefficient. Modeling approaches that are faster, more efficient than trial-and-error methods, e.g., computational fluid dynamics model [106] and knowledge transfer framework [117], were proposed to predict these parameters for printing performance enhancement.

F. LASER-ASSISTED PRINTING
Laser-assisted printing system is composed of a laser beam and a special type of substrate called the donor substrate, the top of which is coated with an absorbing layer and the bottom side with an adhesive layer of ink, as shown in Fig.  9 [118]. The laser beam energy cavitates the ink layer, propelling the droplet to the collecting substrate [119]. Laser-assisted printing is a nozzle-less technique, and therefore, does not suffer from clogging. This technology is used for direct writing [120] and DOD printing [121]. Various materials, e.g., conductive inks [122], adhesives [123], and biomaterials [124], were printed using this technology.

G. ACOUSTIC PRINTING
The application of acoustic waves to propel an ink droplet is referred to as acoustic printing. The pioneering work of Elrod et al. [125][126] has led major research work in the field of acoustic printing. Scientists are developing various acoustic droplet-ejection methods [127] [128]. Acoustic printing is further categorized into surface acoustic wave (SAW) and acoustophoretic printing.

1) SURFACE ACOUSTIC WAVE PRINTING
In SAW printing, the acoustic waves are generated at the surface of the liquid to propel the droplet. Generally, in a SAW-driven jet, interdigital transducers (IDTs) are patterned on a piezoelectric substrate. The substrate contracts and expands by applying radio frequency (RF) power to the IDTs, generating pressure waves of higher frequencies. The droplet is ejected by the acoustic radiation force generated from the SAW [129]. The IDTs can be connected on either side, which will drive the liquid along the Rayleigh angle (θ R ), or on opposite sides, which causes the droplet to be ejected on a point perpendicular to the printing surface, if the opposite sides have the same energy [130] [131]. Furthermore, a pair of aligned IDTs can enhance the maximum jet speed and minimum jet time [132]. A schematic of the SAW-propelled jet with a pair of IDT electrodes is illustrated in Fig. 10. Because the SAW-driven printing technique is nozzleless, it does not have the clogging-related demerits of nozzle-based printing techniques. This is especially advantageous for printing with bio-inks. However, most SAW-driven printing devices do not allow flexible the tuning of the droplet size using the same device. To solve this problem, [133] demonstrated a pulsed SAW device to control the droplet size by changing the pulse width. The droplet size and velocity can also be controlled by changing the input RF power. Strong capillary waves at the droplet surface overcome the capillary stress and result in the atomization of the droplet as the RF power is further increased [134].

2) ACOUSTOPHORETIC WAVE PRINTING
Reference [14] devised an acoustic nozzle-based printing mechanism, especially to print with highly viscous liquids. The mechanism uses three devices, actuator, acoustic cavity, and nozzle. The actuator was connected at one end of the acoustic cavity and activated by a driving ultrasonic frequency of approximately 25 kHz. The ink was injected at a constant flow rate into the nozzle inlet . The nozzle outlet was adjusted inside a Fabry-Pérot (FP) resonator in the location with the maximum acoustic pressure, as depicted in Fig. 11-a. The acoustophoretic force is generated in the FP resonator to detach the droplet flowing through a nozzle. Jetting occurs when the acoustic (F a ) and gravitational forces (F g ) exceed the capillary force (F c ), as shown in Fig.  11-b. For high acoustophoretic fields, the accuracy of the droplet trajectory decreases as the acoustic force increases. The droplet size can be decreased by increasing the acoustic force. The acoustic force can be calculated by integrating the radiation pressure p rad over the surface S of the sample, as follows [14], where n is the normal component inward to S. The radiation pressure rad p has a direct relation with the rootmean-square acoustic pressure and acoustic particle velocity, which can be increased by applying a voltage to the actuator. In other words, the droplet volume can be decreased by increasing the voltage applied to the actuator.
Various materials with a wide range of Z number (the inverse of the Ohnesorge number), including water, honey, bio-inks, and liquid metals have been printed successfully. Honey (viscosity: 25 000 cP) was printed on white chocolate. A low viscosity (2 cP) metal ink composed of eutectic gallium-indium (eGaIn) was also printed. These printed materials are depicted in Fig. 11-c and 11-

H. DROP IMPACT PRINTING
Reference [15] proposed a DOD printing technique in which they replaced the nozzle with a sieve and dispensed a single satellite-free micrometer-sized droplet. The size of the jetted droplet was proportional to the pore size. The droplet size can be decreased by decreasing the pore opening. This technique can print for Z values ranging from 3-200. The size can be further improved in the case of printing highly viscous inks (Z < 3). Materials for biological and electronic applications were printed using this technique. The printing mechanism is illustrated in Fig.  12.

III. PRINTING TECHNOLOGIES FOR HIGH-VISCOSITY INKS
Viscosity governs the droplet formation in the inkjet printing process. The substrate's surface condition has a weak effect on the printed patterns of high-viscosity inks. A large force is required to eject droplets of these inks from the nozzle exit. In a piezo-driven inkjet printhead, highviscosity inks can be ejected by actuating the piezomembrane using a double bipolar voltage waveform instead of a single voltage waveform [43]. Jackson et al. [135] demonstrated a technique that used the XAAR 1003 recirculating inkjet printhead with a shear-mode actuator to eject high-viscosity inks. Due to the high fluid-recirculation rate, the printhead could eject ink with a viscosity of up to 98 cP. The piezo-driven inkjet printheads cannot jet the ink with high viscosity; therefore, other approaches must be used, e.g., EHD [136], which was used for direct writing techniques or droplet jetting [137]- [139]. In this technique, the ink inside the printhead must be charged for better electrostatic deflection, which requires it to be conductive. However, high-viscosity inks have a lower conductivity, which leads to poor electrostatic deflection [140]. Laserbased printing can also be used to jet high-viscosity inks [11] [122][123] [141]. Zhang et al. [141] performed direct writing of alginate solutions of a viscosity 8279 cP. Therodorakos et al. [122] performed droplet-based jetting of Ag nanoparticles with an effective viscosity of 590 cP in a diethylene glycol monobutyl ether solvent. Needle-based dispensers are another device to produce high-viscosity ink jets. The motion of the needle toward the nozzle in the ink chamber causes the droplets to form a jet at the nozzle exit.
The literature contains various studies related to needlebased dispensers for the jetting of high-viscosity inks [142]- [145]. The jetting of high-viscosity ink can be improved by increasing the radius of the needle. Lu et al. [144] achieved the jetting of a 58 000-cP adhesive with a droplet volume of 0.6 µl, a needle radius of 1.5 mm, and a nozzle orifice diameter of 0.1 mm. Aerosol jet printing is a highly preferred method of direct writing. AJP can print inks with a viscosity of 1-2500 cP [146]. Recently, Forestri et al. [14] presented an acoustophoretic printing method for forming jets of droplets with very-high-viscosity inks at the nozzle exit. The acoustophoretic printer consisted of a Fabry-Pérot resonator and an acoustic source. Most acoustic waves were reflected because of the mismatch in the acoustic impedance at the fluid-air interface. Standing waves are generated in the resonator. A spherical droplet is ejected from the nozzle exit by the acoustophoretic force. Various inks with viscosities ranging from 0.5-25 000 cP were successfully printed using this approach. Table III summarizes the methods to high-viscosity ink jets by the technology (piezoelectric, EHD, needle-based, laser, aerosol jet, and acoustophoretic printing). The table also lists the high-viscosity jetted materials and their printing mechanisms. Among the methods, the acoustophoretic printing mechanism was found to be the simplest, most accurate for forming very-high-viscosity ink droplet jets. The use of MEMS to manufacture acoustophoretic printers will enable printing with highfrequency ink jets.

IV. APPLICATIONS OF INKJET PRINTING
The conventional semiconductor fabrication process consists of several steps (from oxidation to photoresist removal) for manufacturing a single device, whereas the inkjet printing technique offers maskless lithography and involves fewer steps [147]. The conventional process and inkjet printing method are illustrated in Fig. 13. Various industries have been researching the application and translation of inkjet printing in manufacturing owing to its low cost, fewer steps involved, and low material loss. Studies have explored its application in printed electronics [148], chemical sensors [149], supercapacitors [150], carbon nanotubes [151], pharmaceuticals [152] [153], and conductive materials [154]. The focus of this section is the application of inkjet printing technology in DTP, display pixel printing, MEMS, and wearable, flexible, and stretchable devices.

A. DIGITAL TEXTILE PRINTING
Inkjet printing technology, also referred to as digital printing, is widely used in various applications, including textile and graphic arts, that relied on conventional printing devices including rollers and screen printers until recently. Textile printing has grown rapidly with inkjet printing technology. This technology produces less waste, and consumes 45% less electricity and 35% less water compared with conventional printing technologies [155]. As inkjet printing is a non-contact process, the jetted droplets spread on contacting the substrate surface. The spreading of droplets on the substrate will affect the quality of the printed product [156]. The impact of droplets on various substrate surfaces with phenomena such as spreading, splashing, receding, and bouncing has been demonstrated [157]- [162]. The interaction of droplets on textiles in DTP has drawn significant research interest. The spreading and coalescing of droplets can affect the image quality printed on fabrics [163]. Zhang et al. [164] demonstrated the impact of aqueous glycerol droplets on hydrophobic and hydrophilic nylon textiles. The results demonstrated that the droplet penetrated and formed liquid filaments beneath the textile surface. They also concluded that the spherical shape of the droplet on the textile does not change when it interacts with porous substrates with a pore size from 100-300 µm in short time periods. Reference [165] analyzed the penetration of droplets in polyester fabric pores with and without an underlying substrate. The spreading ratio of water-glycerol droplets on the substrate is different with and without the underlying substrate because of the volume loss of the liquid [165].
Environment, pretreatment, and posttreatment affect the color and its performance. The process must be conducted in a controlled environment to improve the efficiencies of the printer. For example, reactive dyes require a humid environment to develop a stronger bond with the fabric [166]. Acid and reactive dyes require streaming as posttreatment for bonding with fibers [166]. Pretreatment is often required to improve the DTP's performance. Various pretreatment agents and methods were demonstrated [167]- [170]. Reference [167] used commercially available pretreatment agents (DP-300 & DP-302 from Lubrizol Corporation) and demonstrated improved color intensity and gamut for pigment-based ink by pretreating cotton and polyethylene terephthalate fabrics. They used chemical padding as a pretreatment method. Kim et al. [168] used acrylic polymers as a pretreatment agent to improve the color of nylon fabrics. Li et al. [169] improved the color quality of a cotton/polyamide fabric substrate by pretreating it with alpha olefin sulfonate and sodium alginate. An et al. [171] improved the color performance by a combined process that used the protease enzyme and sodium alginate for wool fabric inkjet printing using reactive dyes.
Various studies have researched and developed materials for inkjet printing for printing on fabrics. Song et al. [172] synthesized RR218@PSBV by the absorption of RR218 (red dye 218) onto PSBV (poly (styrene-butyl acrylatevinylbenzyl trimethylammonium chloride) nanospheres and inkjet-printed it on cotton fabric, as shown in Fig. 14-a. The dye@copolymer nanosphere improved the color performance, image quality, and material conservation [172]. In [173], two reactive red dyes, RR218 and reactive red 24:1 were inkjet-printed and the resulting droplets were compared. RR218 had more stable droplets and a smaller spread area (high resolution) than reactive red 24:1 [173]. Gao et al. [174] inkjet-printed dye-based inks on polyester fabric and investigated the effect of viscosity, surface tension, and fluidity on the sharpness of the printing pattern. Fig. 13-b displays a sample polyester fabric printed with dye-based inks. Various other studies were conducted to improve the performance of DTP by printing dye or pigment-based inks on different fabrics [175]- [179].

B. DISPLAY PIXEL PRINTING
Display technology has a ubiquitous effect in daily life. There are various types of displays [180]. For example, organic light emitting diode (OLED) has become a mainstream display technology as it consumes less power, is flexible, has a high contrast ratio, and is ultra-thin [181]. OLED is made of an emissive layer (EML) sandwiched between two conductors. A hole transport layer (HTL), hole injection layer (HIL), electron transport (ETL), and electron injection layer (EIL) can be added to enhance its efficiency [182]. Fig. 15-a displays the cross-sectional view of all the layers along with the electrodes. A cross-sectional schematic of red, green, and blue (RGB) pixel inkjet printing is illustrated in Fig. 15-b. The applications of OLED include computer, laptop, automobile, TV, and mobile phone displays. Several methods including spin coating, transfer printing, lithography, evaporation, and inkjet printing are used to deposit multilayer thin films of OLED.
Inkjet printing offers a significant advantage in the field of display electronics because it offers maskless lithography, consumes less material, uses a simple fabrication technique, and has larger substrate scalability. Therefore, it has been adopted for the fabrication of OLED displays. We focus on the latest studies on OLED fabrication using inkjet printing. In [183], a 3-in blue OLED array was inkjet-printed, resulting in improved uniformity of the surface [183]. Yoon et al. [184] used an ink with various solvents to print the emissive layer of the OLED. Thus, they achieved a uniform film. Zhaobing et al. [185] inkjet-printed an EML, HTL, and HIL for an OLED and achieved uniformity. However, the device efficiency requires further improvement. An image of the inkjetprinted green EML layer is displayed in Fig. 15-c. Reference [186] inkjet-printed a poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) HIL, with polyethylene glycol (PEG) for improved efficiency and bending resistance of the OLED device.
Another study printed a PEDOT:PSS (dissolved with isopropanol and ethylene glycol) HIL using a multi-nozzle inkjet printing system [187]. They achieved uniform droplet ejection by changing the solvent and tuning the printing parameters. Amruth et al. [188] printed an EIL made of a cesium carbonate film (with an alcohol-based solvent) for a polymer OLED. The current efficiency and luminance were enhanced compared to an OLED without the film. Researchers printed micro-lenses for pixelated OLEDs to enhance the out-coupling efficiency [189] [190]. Reference [192] printed 31-in [191] and 55-in [192] active matrix OLED (AMOLED) displays, improving the performance of the display technology.
Quantum dot light-emitting diode (QLED) is another display that can be printed by inkjet technology. These LEDs have long lifetime, low power consumption, high contrast, wide color gamut, wide viewing angles, and high refresh rate [193] [194]. Kim et al. [195] inkjet-printed an octane-based QD ink for a green OLED. They tested various mixtures of solvent with the octane-based QD ink and found that the octane-cyclohexane mixture was the most stable. Chen et al. [196] printed highly efficient red QLEDs using a QD ink based on a mixture of n-tridecane and decalin solvents. Quantum dots can also be used as color-conversion layers in display technologies. Lin et al. [197] effectively suppressed the blue residual light by inkjet-printing QD inks. Hu et al. [198] fabricated a 6.6-in QLED display with a QD layer printed to eliminate the blue residual emissions. They demonstrated a reduction in the transmittance of the residual blue light by increasing the thickness of the QD pixel layer. Yang et al. proposed the application of a Bragg reflector with an inkjet-printed QD pixel layer to further increase the color conversion efficiency. The reflector was composed of a multilayer of silicon oxide / titanium oxide.
Perovskite quantum dots (PQDs) have become a significant area in display technology research owing to their distinct optical properties, including a tunable wavelength and narrow emission wavelength [199]. Shi et al. [200] inkjet-printed PQD inks with water as a solvent. Yoo et al. [201] printed with a perovskite ink on an ethyl cellulose film and concluded that a (3-aminopropyl) trimethoxysilane (APTES)-coated red perovskite ink had improved drying stability. Other studies reported improvement in color [202] and brightness [203] by inkjetprinting with PQD inks with various solvents.

C. MEMS DEVICES PRINTING
Inkjet printing is also considered a versatile technique in the manufacturing of MEMS devices because it is environmentally friendly, produces less waste, is maskless, and offers rapid and multimaterials deposition [204].

1) PHOTOLITHOGRAPHY
In photolithography, a photoresist material can be applied using various coating techniques. These coating techniques still has flaws, among which photoresist material wastage is one [205]. Inkjet printing can be used to deposit the photoresist materials to overcome this problem [206]. Bietsch et al. [207] deposited alkanethiolate monolayers and DNA oligonucleotides on Au films using inkjet printing. Fukushima et al. [208] inkjet-printed acrylic resin on Al. Qu et al. [209] used the EHD technique to print photoresist lines; the line width was controlled by the voltage supplied to the substrate and nozzle. Bernasconi et al. [210] inkjet-printed SU-8-2005, an epoxy-based photoresist material. This material cannot be printed in it pristine form by commercially available piezoelectric inkjet printheads owing to its higher viscosity. To decrease the viscosity, the material was diluted with cyclopentanone (CP), tetrahydrofuran (THF), and N-methyl-1-pyrrolidone (NMP). The jetting of SU-8-CP and SU-8-NMP was found to be stable, whereas that of SU-8-THF was not because of the partial blockage of the nozzle [210]. The authors claimed that the partial blockage was due to the lower boiling point of THF, i.e., it evaporates, leaving partially solidified SU-8 at the nozzle's meniscus. This can cause a misdirected jet.
Micro-lenses and micro-lens arrays play a vital role in various applications, including optical communications, optical storage devices, wavefront sensing, and biomedical instruments. Inkjet printing is widely used in micro-lens and micro-lens array fabrication. Compared with photolithography, inkjet printing can deposit various materials at precise locations on any substrate for the fabrication of micro-lenses [212]- [218].

2) ETCHING
Inkjet printing can be used to remove the small areas with predeposited films. The advantage of inkjet etching is minimal material wastage. The dielectrics of SiO and Si 3 N 4 were inkjet-etched by ejecting an NH 4 F solution onto a polyacrylic acid film, which produces HF [219]. Another interesting application of inkjet printing is polymer etching. Microstructures with varying shapes (concave to convex) can be prepared on top of polymer surfaces [220] [221]. Microstructures with microgrooves, microwells, and hexagonal holes were also fabricated using inkjet etching. The dimensions of these microstructures can be controlled by tuning the jetted droplet volume, modifying the polymer-solvent interaction, substrate temperature, and processing parameters [222].
In addition, inkjet printing has been used to fabricate thin-film transistors (TFTs) [223]. For example, Kim et al. [224] fabricated a TFT by inkjet-etching Ag. Etchant (a mixture of ferric nitride and deionized water) was impinged on the surface of Ag by etching Ag films with a thickness of 20-80 nm, which generated a source-drain electrode. Li et al. [225] fabricated a TFT array wholly by inkjet printing pure Cytop solvent on a Cytop layer.

3) DEPOSITION
Direct material deposition is the most common application of inkjet printing in the fabrication of MEMS devices. A variety of different materials, including conductive, insulator, sacrificial, piezoelectric, and two-dimensional (2D) materials, can be deposited [226]. Piezoelectric materials play a vital role in the manufacturing of MEMS sensors, actuators, energy storage devices, transformers, and transducers. Various studies were published on the direct deposition of piezoelectric materials by inkjet printing. Kuscer et al. [227] deposited lead zirconate titanate (PZT) thick films onto a platinized alumina substrate by inkjet printing. Ink was formed by dispersing PZT particles in a water-glycerol mixture. However, they observed a few defects in the deposited film [227]. Subsequently, they synthesized defect-free structures by adding polybenzoxazole (PbO) to the aqueous ink [228]. Godard et al. [229][230] deposited a thin-film PZT onto a platinized silicon substrate. Pabst et al. [231] demonstrated all-inkjet-printed of micropump actuator. Zheng et al. [232][233] fabricated 3D microstructures of ice by impinging water droplets onto an existing ice structure, after which the droplets immediately froze.
The application of signal path to the MEMS devices is performed by electrical conductors. The various inkjetprinted conductive inks, their electrical characterization and applications, and historical background were reviewed by [154]. Godard et al. [234] printed Ag as the top electrode on a thin-film PZT layer. In [231], both the top and bottom electrodes of Ag in a piezoelectric micropump actuator were deposited by inkjet printing.
Reference [235] fabricated entire MEMS electrostatic drive motor was fabricated by inkjet printing (Fig. 16). They used Au and Ag nanoparticle inks to fabricate the motor structure. The insulator (polyketone resin) was inkjet-printed to electrically isolate the Au and Ag electrode wires. Delekta et al. [236] presented a fully inkjet-printed graphene-based microsupercapacitor, in which the electrodes were made of graphene, and the electrolyte nanographene oxide. Kaneto et al. [237] demonstrated an MEMS capacitive sensor for humidity. The sensor was fabricated by combining transfer and inkjet printing. Graphene oxide (GO) nanoparticles were deposited by printing them on Au electrodes, which was prefabricated on a PET substrate by screen printing. This bilayered GO/Au thin film exhibited higher Young's modulus than the Au thin film.

4) MEMS PACKAGING
Optical and electrical interconnects and adhesives for sealing and bonding in MEMS packaging can be printed by inkjet printing [238]. In the 3D packaging of MEMS devices, through-silicon via (TSV) is mostly used as an interconnect structure for signal transmission. Studies were published regarding the conductive plating and filling of vias by inkjet printing. Khorramdel and Mantysalo [239][240] demonstrated the inkjet-printing of an Agnanoparticle ink for the partial filling of a TSV. Quack et al. [241] developed ink-jetted Au-filled TSV arrays. Yang et al. [242] deposited Ag and filled a via at a lower processing temperature and electrical resistivity, and further enhanced its electrical performance [243].
Khorramdel et al. [244] stated that the partial metallization of a TSV by inkjet printing was not sufficiently mature for volume production. Therefore, they used an alternate approach that involved filling a hollow metallized TSV with a dielectric polymer and depositing an Ag ink as an under-bump metallization (UBM) pad, as shown in Fig. 17. Both the dielectric polymer and metallic ink were inkjet-printed. Solder balls of SnAgCu-based inks were also impinged on top of the UBM pads. Two commercialized printheads, i.e., piezoelectric and EHD, were used for printing metals and polymers [244]. Roshanghias et al. [245] inkjet-printed redistribution layers (RDLs) and a metal route used to connect the MEMS microphone pads to application-specific integrated circuits (ASICs) and fan-out the signals via solder balls. In another study, they inkjet-printed the RDLs of Ag-ink nanoparticles for the fan-out packaging of capacitive micromachined ultrasound transducers [246]. Other studies have demonstrated the synthesis of adhesives by inkjet printing [247] [248].

D. WEARABLE, FLEXIBLE, AND STRETCHABLE DEVICES
Wearable, flexible, and stretchable (WFS) devices have achieved remarkable progress in smart devices and healthcare modality [249] [250]. These devices can be directly mounted on the human skin or attached to fabrics. Novel device fabrication approaches are required to achieve, and retain, high performance. Inkjet printing has various applications in fabricating WFS devices and circuits due to its low cost, material wastage, fewer steps, maskless working, and precise deposition of small droplets on the substrate surface [251] [252]. Inks consisting of dielectric, conductors, and semiconductors can be printed on fabrics and skin by inkjet printing. For example, Ag nanowires and nanoparticles were printed for flexible and stretchable electronics [253]- [257]. Mikkonen et al. [258] demonstrated an all-inkjet-printed electrical circuit device (Fig. 18). Both the conductive (Ag nanoparticles) and dielectric (polydimethylsiloxane (PDMS)) materials were jetted by inkjet printing. PDMS was used as a dielectric between the conductive tracks [258]. Wang et al. [259] developed an all-inkjet-printed flexible proximity sensor ( Fig. 19-a). Both ZnO and web-shaped top electrodes (Ag nanoparticles) were jetted on a flexible Al sheet used as a bottom electrode. Another all-inkjet-printed wearable device for electronic textiles (e-textiles) applications was proposed in [260]. They printed a graphene-Ag composite ink on a piece of cotton fabric.
The human body temperature provides information about health status. Wearable temperature sensors are used to measure this temperature. Inkjet printing can be used to devise wearable temperature sensors. Kuzubasoglu et al. [261] jetted an aqueous carbon nanotube (CNT) conductive ink to fabricate a wearable temperature sensor. Vuorinen et al. [262] demonstrated a temperature sensor by printing graphene/Poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS) ink on a skin-conformable polyurethane substrate. Fig. 19-b displays the photograph of the sensor printed on a subject's finger. Reference [264] used CNT/PEDOT:PSS composite ink to fabricate a temperature sensor [263]. Wang et al. [264] presented an all-inkjet-printed PEDOT-PSS-based temperature sensor with PEDOT:PSS as the sensing layer, fluorinated polymer (CYTOP) as the passivation layer, and Ag nanoparticles as the electrode. High stability in humidity was achieved for the sensor by introducing CYTOP as the passivation layer ( Fig. 19-c).