Nozzle-based precision patterning with micro-/nano fluidics integrated cantilevers

As the requirements of micro-/nano printing technologies are continuously increasing, direct writing technologies on a submicron scale are drawing attention. One of the promising methods is a nozzle-based precision patterning with atomic force microscopy (AFM), which has the advantages of high position controllability including nozzle-to-substrate distance feedback under a nanometer scale. It uses a fluidic channel and a dispensing nozzle integrated with a cantilever structure for deflection monitoring. In this paper, we introduce micro/nanofluidics integrated cantilevers for nozzle-based precision patterning in several considerations: 1) numerous fabrication strategies for nozzle-integrated fluidic cantilevers; 2) methods for liquid transport; 3) methods for pattern formation; and 4) applications with various printing materials.


Introduction
Direct writing, a patterning technique using nozzles, had been widely used in micro and nano printing because it allowed rapid prototyping without a pre-defined mask [1][2][3][4][5]. Although direct writing was a somewhat mature method, there is still an ongoing effort for researching highresolution patterning with various materials for novel functionalities. The two categories of direct writing are "contact" methods, in which the nozzle touches the printing substrate, and "noncontact" methods, in which the sample is dispensed from the nozzle at a moderate distance from the substrate.
The representative method of "non-contact" consisted of spraying the printing solution using an electric field, which was well known as electro-hydrodynamic dispensing (EHD) [6][7][8][9]. By controlling the cone-shaped liquid meniscus at the nozzle apex with an electric field, printing resolution could reach below the nozzle's opening size by 0.1 times. Although EHD has been utilized for printing various liquids (e.g., functional inks [10,11], molten metals [12,13]) for submicron-sized patterns [14][15][16], the high voltage requirements and low selectivity of the substrate still presented limitations. In contrast, the "contact" method had a wide-selection availability concerning the printing substrate. Indeed, for this type of printing, high-resolution patterning in sub-micron scale was achieved by miniaturizing the dispensing nozzle, a straightforward approach, which was achieved through various methods, including through pulled glass capillary and MEMS-fabricated hollow cantilever.
To fabricate a glass capillary with a narrow and sub-micron aperture, it was heated up to its glass transition temperature with a laser, and then a pulling force was applied to the partially melted capillary [17,18] (also commercially available in Sutter Instrument). Printed patterns under a two-micron resolution of perovskite for optoelectronics [19] and liquid metals for flexible devices [20] were implemented with this type of pulled capillary, having an aperture of 600 nm and 5 μm, respectively. Furthermore, nanoscale electrodeposition using pulled theta-capillary enabled multi-material printing of copper and silver with a printing resolution of about 500 nm [21]. As the printing scale went down, precision position control was introduced by integrating pulled capillary with quartz tuning forks (QTFs), a well-known platform for atomic force microscopy (AFM) [22,23].
Although the glass capillary showed a nice proof-of-concept for precise direct printing, it had a large variation in the size of the fabricated dispensing aperture due to the manual fabrication process. On the other hand, MEMS-fabricated hollow cantilevers, with a horizontal fluidic channel inside the cantilever and an outlet at its end, may serve as a reliable fabrication platform, replacing proof-of-concept experiments with pulled capillaries. However, because of the complex fabrication of integration of small channels, dispensing nozzles, and cantilevers for precision control, there had been many studies focused on developing the patterning nozzle as well as the system.
In this paper, studies of direct writings were reviewed, especially for the hollow cantilever printing nozzle types. Various approaches to fabricating the devices, operation methods with their working principles, and patterning results including the potential applications of nanoscale dispensing were discussed (Fig. 1). The paper concluded by discussing future directions for high-resolution 3D printing with hollow cantilever printing nozzles, referring to applications in other devices. 3D printing using an atomic microscope cantilever began with dip-pen nanolithography (DPN) [24]. In DPN, a small volume of patterning solution was transferred to the solid tip by dipping the tip in a sample reservoir and then to the patterning substrate by a water meniscus, resulting in a sub-micron patterning resolution. However, supplying the patterning solution by moving the tip back to the reservoir and partially dipping it into the solution during each operation time increased the complexity of the patterning process.

Fabrication history
Due to the complexities of the process, as well as the time needed for DPN, various fluidic integrations had been continuously developed toward reliable liquid supply and highresolution patterning. The nanoscale dispensing tip (NADIS) [25], which included reservoirs and dispensing nozzles to hold the solution in an atomic microscope cantilever, was presented as an alternative. Afterward, the imaging tip and the dispensing nozzles were separately integrated, thus nanoscale patterning by following the obtained topography was implemented [26]. On the other hand, a core solid tip inside the channel named the volcano tip was introduced, writing supplied solution on the substrate with the apex of the solid tip [27][28][29]. Induced by water meniscus or electric field, sub-100 nm molecular writing was presented with this device. As the size of the nozzle becomes smaller, the possibility of the nozzle being blocked by fine particles in the printing solution (called clogging) increased. To prevent this issue, a paper [30] reported on the efficiency of a micro-filter, which was made with a focused ion beam (FIB) in the back of the nozzle.
Up to this point, the solution was placed in a reservoir inside the cantilever body and patterned with the pressure difference at the tip-surface interface [31]. For precise control of the pressure and continuous supply of the patterning fluid, the design of fluidic channel connection with the outside reservoir was presented (also now commercially available as "FluidFM" in Cytosurge). By connecting the NADIS-type cantilever to an external channel, the fluorescent label in 3 µm size was injected into the cell through the tip [32,33]. Negative pressure control such as re-aspiration of the patterned solution was also presented [34], which was not possible only with the internal reservoir.  [28,32], operation [32,57,91], and applications [40,49,86]. All figures are reprinted with the permissions.
Applications of these types are now divided into those using tipless cantilevers and those using tip-integrated cantilevers [3,36]. For the tipless design, a liquid metal (mercury) was printed in an aqueous solution by using a syringe pump and a pressure sensor to regulate pressure, during the potential difference monitoring [37]. This tipless design was further used in the manipulation of a single cell with a SU-8 cantilever to place the cell individually at a desired point or to aspirate the cell for subtractive patterning [38,39]. In the tip-integrated design, nanoparticles were printed through the small aperture with constant pressure control, and a small volume of solution was injected into the cell while applying pressure pulses into the probe [40,41]. For the cell injection, a large aspect ratio of the tip was advantageous, thus wet oxidation with a deep-reactiveion-etching (DRIE) hole was adapted and resulted in an aspect ratio of ~15 [42,43]. This kind of injection nozzle was also applied to a neural probe with a microchannel for drug delivery [44], where a polydimethylsiloxane (PDMS) mold was used to connect the fluidic interface and the chip. In some studies, a polymer tip was attached to the end of a channel cantilever as a diffusion nozzle rather than directly dispensing the liquid as mass flow [45][46]. There was also an attempt to integrate the heating electrodes into the dispensing nozzle to make a temperature gradient for the patterning solution, which induced thermo-capillary transport for easy operation [47]. This patterning technology, by incorporating a solution delivery and a precise positioning of AFM, had the advantage of simultaneously supplying a tiny amount of fluid alongside a high printing resolution. With this advance, numerous kinds of research in the bio-field [35,48] had been actively conducted, such as drug or bio-reactive agents' delivery inside the cell, spatial manipulation, or adhesion force measurements. In addition, various research fields had been introduced, such as enabling local electroplating [49] by dispensing precursors in an electric field-applied solution and using the patterning devices in scanning ion-conductance microscopy (SICM) meas-urements [50]. From that point on, a variety of fabrication methods or operation ideas had been reported such as SU-8 nano-nozzle crack lithography [51], simultaneous dosing of two different solutions [52], or direct connection of fabricated cantilevers with fluidic tubing [53]. Overall, precision manufacturing with micro/nano-fluidics integrated cantilevers continuously widened research scopes, without being limited to a few specific fabrication skills or applications. This article's next subsection will cover several mainstream fabrication processes.

Channel fabrication
The following section focused on how the channels for solution delivery were integrated with the AFM cantilever. The fabrication methods were divided into two types, one was the case where the channel is exposed on the top of the cantilever (Sec. 2.2.1), and the other one was the case where the channel was buried inside the cantilever (Sec. 2.2.2) as shown in Fig. 2.

Exposed channel
In the paper of Saya et al. [54], an etch mask was made of silicon nitride (Si x N y ) and tetramethylammonium hydroxide (TMAH) was used for self-limited etching of the <111> direction silicon. The triangular cantilever head was similarly etched with a silicon dioxide (SiO 2 ) etch mask and anisotropically etched with TMAH. This was the simplest way to fabricate the fluidic integrated cantilever to our best knowledge, but there are several disadvantages from the exposed channel, such as evaporation of the printing solution and low controllability in the direction of solution transport. The solution was delivered to the tip of the cantilever by the flow of gravity while the cantilever was inclined.

Buried channel
Si x N y and SiO 2 , which have high hydrophilicity and chemical resistance, had been mostly used for the structural material of  [25,33]; (d) nozzle punctuation at the end of the fluidic channel using a scanning electron microscope (SEM)/FIB. Reprinted with permission from Ref. [36]. the embedded channel. Fabrication of a channel for transporting fluid inside the structure could be generally divided into two methods. The first method removed the sacrificial layer ( Fig.  2(a)). This was a method of forming a flow channel by etching of sacrificial layer with polysilicon [37] or SiO 2 [27,30] with a potassium hydroxide (KOH), followed by deposition of external structure on the sacrificial layer. The etching time, which was the critical parameter for channel formation, was determined according to the length or geometry of the channel.
The other method relied on two wafers bonded by fusion bonding [33,55], or anodic bonding [26]. Commonly, a groove that connected the dispensing aperture and the channel inside was formed by wet etching, and then another wafer with a fluid inlet port was bonded to the channel system. Afterward, a structural layer such as SiO 2 was generated by thermal oxidation and the cantilever was released by patterning and etching. Similarly, a method of bonding SiO 2 nano-needles to a silicon wafer had been reported, having a fluidic channel with its upper part and an inlet/outlet port at the lower part with a silicon-oninsulator (SOI) wafer [42,43]. In the Ref. [51], the SU-8 cover was thermally bonded after the SU-8 layer was placed on top of the structural layer for the cantilever to form a path for the solution to flow. In this other Ref. [44], after the anodic bonding of the glass wafer and silicon wafer, a chemical-mechanical polishing (CMP) process was performed to form a neural probe electrode on the glass wafer side. In addition to silicon, organic channels made in the form of photolithography could be easily fabricated by the bonding method [38,39].

Nozzle fabrication
Dispensing nozzle is the passage through which the solution inside the microfabricated channel is delivered to the printing substrate. In this part, various nozzle fabrication methods were covered. The main differences between the methods were that the mask etching method (Sec. 2.3.1) enabled the batch fabrication process while the focused ion-beam milling method (Sec. 2.3.2) was a one-by-one manual process. However, it certainly had advantages such as the high resolution of nozzle fabrication. Unconventional fabrication methods (Sec. 2.3.3) such as the nanoscale crack method were also included.

Mask etching
In the nano-fountain pen [26], a nozzle was fabricated simultaneously with the process by RIE etching of silicon nitride as a cantilever structure. In the tip-less FluidFM, a nozzle pattern was created with RIE on the structure by the deposition of Si x N y low-pressure chemical vapor deposition (LPCVD) after being filled with a sacrificial layer. It was designed for a fluid reservoir to be connected to the dispensing nozzle when fabricating the channel. In the FluidFM with a pyramid-shaped tip [35], when etching the sacrificial layer from the V-groove tip, the time for etching the sacrificial material was slower near the tip aperture so that remained residue was used as an inversion mask for the tip opening. In the volcano tips [27,28], etching the Si x N y layer at the tip ends revealed the sacrificial layer of the channel surrounding the core tip. If the sacrificial layer was removed during the channel formation process, the inside solid tip appeared. Variance in Si x N y etches rates came as a result of the difference in height between the inner core tip and the outer channel. In the Ref. [44], by using a delivery nozzle as a neural probe, a circular aperture of 60 μm was made by DRIE on a layer with a channel.

Focused ion-beam (FIB) milling
FIB was a process for material removal at the small desired part by injecting the ion into the selective area with a constant intensity. It uses a high acceleration voltage to the source to generate ions [56]. To prevent the charging of Si x N y during FIB, a thin metal reflective layer (ex. Cr 15 nm or Au 10 nm) was sometimes deposited. It was advantageous that the position, material, and nozzle fabrication size were relatively free compared to the chemical etching with the photolithography method, although the device must be individually operated, takes a relatively long time than photolithography, and requires expensive analytical equipment (SEM/FIB). In the Ref. [34], a 2-μm nozzle was created with FIB on the side of the pyramid next to the tip (Fig. 2(d)). This allowed dispensing from a small hole while maintaining the high sharpness of the imaging tip of 20 nm radius. The dispensing orifices in FluidFM were also fabricated depending on the applications [32] such that nozzles were placed at the tip apex or next to the pyramidal tip structure to preserve the sharp imaging tip radius. This method had been currently used by many researchers because it could easily form sub-micron holes at desired locations [36], even with a hole size of a few tens of nanometers, which was hard to reach by photolithography.

Extra fabrication
Besides these two representative methods, unconventional fabrication methods could be used to create the nanoscale nozzle on the SU-8 after the channel was created by photolithography [51]. When developing SU-8 cantilevers, nanoscale cracks were formed due to internal stress by heat generation and volume shrinkage. These cracks could be used as a dispensing nozzle, having a line width of 250 nm and a thickness of 20 nm with the process optimized by varying the material, SU-8 thickness, and UV exposure time.

Reservoir to the holder
From an engineering perspective, one of the important things in implementing a cantilever printing setup is a watertight fixation between the fluidic reservoir and the cantilever holder, and also between the holder and the channel cantilever for printing. As a convenient and fast method, glue had been used to connect both sides [57], which is known to cover up to 2-3 bars.
However, it was an almost irreversible method, so microfluidic fittings [53,58,59], and O-rings with robust fixations [60] had been introduced as reversible and modular interconnection.
Kramer et al. [53] formed 3D printed structure as the connector between the tube and the holder assisted with SLA technology where the tube was connected to the external reservoir. It was reported that the connector was semi-transparent, and it took only 2 hours and 10 minutes to build 30 connectors, which were deemed suitable for the mass production process, although it required post-processing steps, such as residue removal with IPA.
Previous research also demonstrated a PDMS-based connector in attaching the tube from the external reservoir to the holder [34]. The tube was punched to the PDMS glued on the top of the holder. PDMS fitting gave a good seal and flexibility to the tube during the fuse process. PDMS was also demonstrated in assisting the thermally fused process [31]. A thermal pump was tested by placing a wire across the PDMS to the onchip reservoir forming a free loop. Over-pressure could be made by passing a current to the wire and heating it. Bypassing the current, the wire heated and formed an over-pressure pushing the liquid from the reservoir to the nozzle tip. Glue was also used in connecting the tube directly to the holder. Oorschot et al. used acetone to remove the glue to separate the chip and interface making the devices reusable [57]. Song et al. proposed an easy and fast approach to connect the capillary tube to the micro-hole of the reservoir [58]. The micro-hole was designed to have a smaller inner diameter than the outer diameter of the tube (Fig. 3(a)). Furthermore, the micro-hole was also expected to have the shape of a countersink to prevent bending during the connection process. The force of the micro-hole would distort the capillary tube due to the smaller rigidity of the tube than the micro-hole. Measured that it took less than 13 N of force for manual insertion, Maillard et al. used a commercial thread luer as a microfluidic fitting to connect the syringe tube to the holder [59] (Fig. 3(b)). The holder was custom-made to match the female fitting thread. The thread formed a tight connection to the holder and prevent leakage.

Holder to the chip
For the connection between the holder to the microfabricated chip, directly bonding the cantilever with the epoxy applied to the on-chip reservoir was the most common method in previous studies [34,57,61] (Fig. 3(d)). This approach needed a handful of treatments to determine the printing position to produce a good seal between the holder to the cantilever, otherwise the channel might be affected or closed. Maillard et al. demonstrated a connection between the holder to the device assisted with O-ring (Fig. 3(e)) [59]. This enabled reversible connection but if the clamping force by metal clip was not enough, the fluid may be leaked. So, it is important to design in consideration of chip break (too much or undistributed clamping force) and fluid leakage (not enough clamping force). Recently, to uniform clamping force, instead of O-ring, a Viton rubber pad with O-ring material was CNC machined to place chips [62,63]. In addition, Kramer et al. directly printed the tip to the designed place on the surface of the holder where the cantilever was made by 2-photon-polymerization (2PP) technology [53].

Precision dispensing method
The process of filling the channel with fluid varied depending on the device type. In the case of in-reservoir, the sample was loaded in advance and the experiment was performed using the dispensing nozzle filled by the capillary force. As mentioned in Sec. 2.1, devices with out-reservoir were directly connected to the dispensing nozzle and the sample was continuously supplied through an externally connected reservoir. This part covered methods for accurately dispensing samples from micro-scale channels which are connected with the out-reservoir to printing substrates.

Flow rate control
Flow rate control was the technique that was most commonly used in prior studies on microfluidic devices [64][65][66] or patterning with nozzles of several tens of micrometers or more [67][68][69]. It used a syringe connected to the stepper motor, causing the Ref. [59]. Utilization of epoxy for connection from (c) reservoir to holder or (d) holder to chip; (e) utilization of O-ring to connect chip and holder without chip break and liquid leakage during the press from solid material such as a metal clip or plastic element. Reprinted with permission from Refs. [57,60]. flow rate control depending on the volume and moving speed of the syringe (Fig. 4(a)). For simple use of a cantilever leakage test, a syringe pump could be an appropriate method. In the Ref. [53], the flow rate in the cantilever was precisely adjusted using the flow rate of 10 μL/min of DI water. It was intuitive and easy to use, but difficult to make a uniform pattern because the vibration of the flow due to the stepping action of the motor greatly affected the resolution of several micrometers or more.
Because of its instinctual concept, this technique was frequently used in several fluidic platforms works but was not limited to the nozzle-integrated microfluidic devices, such as fluid delivery [53], inner surface modification of the channel [70], and hydrogel patterning inside bio platform channels [71]. Chen et al. [70] performed chemical modifications on the surface of channels with antibody immobilization. Flow rate techniques made it easier for controlling the modification and immobilization since it needed time and flow control for the antibody to be securely tethered to the inner surface of channels. They also used this method to deliver the artificial blood sample to validate the efficiency of the device.

Pressure control
Pressure control was the most common method for transporting samples using cantilever-type printing nozzles. By making the inside of the channel at a higher pressure than the outside, the sample was extruded [40,41,57,72] (Fig. 4(b)). It had the advantage of a fast response time and uniform flow rate without vibration, but it was necessary to calibrate the flow rate according to flow resistance, which was affected by the size of the printing nozzle and the dimension of the channel leading to the nozzle. The most important parameters which affected the pressure condition were the viscosity of the fluid and the surface energy. For example, in the case of mercury with high surface energy [37], 6 bars were applied to extrude mercury from the channel, but not distributed evenly inside the hydrophilic nitride channel.
Besides the method relying on external pressure, studies had been conducted to load samples with pressure differences generated by inducing evaporation inside the channel [57]. When the printing solution reached the opening reservoir along the hydrophilic fluid channel, it started to evaporate, while the capillary force simultaneously supplied the solution to the printing nozzle in the path [31,73].
Guillaume-Gentil et al. [40] performed biomolecule microinjection into the cell with pressure-based control for sample transport. As a defined pressure pulse was applied through the micro-channeled probe, a biomolecule solution loaded onto the probe was controlled to be released and injected through the cell. Pressure control was also possible to assist the deposition process of nanoparticles [41]. The work mentioned overpressure in the microchannel filled with nanoparticles leads to deposition in dots and lines under the tip of the microchannel. This pressure also affected the nanoparticle adsorption along with the contact time.

Electric field control
The electric field between the nozzle and the substrate could be used as a dispensing method with an electrically charged printing solution (Fig. 4(c)). The idea came from the limitation of pressure-driven printing in the case of high viscosity solutions [43,74,75]. For electrospray to begin, the minimum condition for electro-spraying with voltage was called the onset voltage [76]. As shown in Fig. 4, a high-voltage power amplifier charged the liquid as it passed through the nozzle. Under the nozzle, conductive support placed as a substrate also worked as a ground electrode. The positively charged liquid would move toward the opposite charged substrate or ground electrode and be attached to them [77]. The method was also known as electrohydrodynamic dispensing (EHD), which had been developed mainly by stainless steel or pulled-glass capil-

lary.
After an EHD with a cantilever-based approach was proposed by dipping the AFM probe slightly into the target patterning liquid [78], a customized probe with a liquid reservoir was utilized for uniform and high-resolution up to tens of nanometers dispensing [79]. Li et al. [74] studied various deformations of the viscous ink flows toward the substrate by controlling the applied voltage, such as the velocity of the liquid approaching the substrate, the diameter of the liquid jet, and the vertical angle of the liquid cone. Different filament structures were derived from this work, including straight lines, meanders, alternating loops, and translated coils. Other than poly(lactic acid) (PLA), poly(lactic-co-glycolic acid) (PLGA), and polycaprolactone (PCL) [74], this method could be applied to materials such as graphene, carbon nanotubes, and metal [76]. Moreover, Shibata et al. [43] demonstrated electrokinetically driven flow for the intracellular delivery method. It was suitable for highresolution pattern fabrication with a resolution of sub-50 nm. However, it was difficult to optimize shape control and patterning conditions. It also required high voltage which was difficult to apply on electrically vulnerable or affectable printing substrates such as biomaterials.

Extra control
In the Refs. [80,81], a design was proposed in which a polymer heating tube was mounted inside a microcantilever to transfer the molten polymer from the tip using a thermocapillary force. The temperature gradient between the cantilever and the substrate generated thermal capillary stress, causing the fluid to flow from high temperature to low temperature. As with a similar concept of thermal dip-pen nanolithography (tDPN) [82], the dispensing speed here also depended on the temperature balance between the heating tip and the substrate.

Patterning method
The printing solutions delivered to the nozzle were solidified (additive patterning) or used in material partial removal (subtractive patterning) in various ways. In this part, each method was used for solidifying liquid-phased samples on the substrate while maintaining their features (Fig. 5).

Evaporation
The first method relied on evaporation. As the sample was dispensed out of the nozzle, evaporation occurred at the same time, and the shape was crystallized. The patterning solution was a mixture of a volatile sample (guide ink) that evaporated during the patterning of the solution and a sample (precursor ink) that composed the main structure of patterning results. The influencing factors included the surface energy of the patterning material with the substrate and the ratio of solution dispensing speed to the evaporation speed, which was affected by temperature and humidity in the printing environment.
As shown in Fig. 5(a), the surface energy affected the print-ing result. When a liquid was dropped on a hydrophilic surface, the structure formed by natural evaporation maintained a constant contact area and formed a thin layer [83]. In the case of a hydrophobic surface, it maintained a constant contact angle and resulted in a more protruded structure. The ratio of the dispensing rate to the evaporation rate of the material was mainly the function of printing speed (contact time with the substrate) and dispensing pressure. In the Ref. [72], dots were patterned by changing the contact time of the patterning nozzle with the substrate using a fluorescence solution, and fluorescence intensity was analyzed with the variation in the size of the ejected droplets. It is clear that the shorter the contact time with the substrate, the smaller the pattern could be formed. However, if the speed of solution dispensing did not follow the cantilever moving speed, which is called a threshold speed and determined experimentally by the surface energy at the tip end and the applied pressure, a continuous pattern cannot be created [41]. The fluorescence imaging method was also used for analyzing patterning result dependence in applied pressure and AFM setpoint, which means the distance between the dispensing and the patterning substrate. Since the temperature and humidity in the patterning environment determined the rate at which the material evaporates, it was necessary to adjust the other control parameters after maintaining the temperature and humidity during printing by evaporation. Accordingly, a patterning chamber having desired temperature and humidity for the control environment had been reported [57].

Polymerization
The polymerization method consisted of first mixing a curing agent with a specific stimulus with the patterning solution, and then transferring it to the subtracted before post-curing it. Fig.  5(c) shows a typical photo-polymerization process. There was a monomer, the base material, and a cross-linker that created a linkage to the base material. For example, when a material that releases free radicals in a specific wavelength band of UV was mixed with an initiator and irradiated with UV, molecular structures could be connected by replacing the part of the monomer with a cross-linker of the generated free radicals, and a sample formed a structure. Using this method, in the paper [57], a layer of Loctite (UV curable epoxy) was printed with an AFM dispensing nozzle and UV-cured to maintain their structures. AFM dispensing nozzle was then located again with analyzing topography to print the multiple layers with the controlled position.
The principle of releasing a solution out of the nozzle was similar to the preceding method, so the factors affecting the patterning result were similar to the aforementioned conditions (surface energy, dispensing rate). Compared to the evaporation method where the ratio of the volatile sample to the patterning sample determined the volume difference between the amount of dispensing solution and the amount of final patterned residue, in the polymerization method, the change in volume during curing affected the final product.
Although this photo-polymerization needed to separate the printing nozzle and the substrate when the printed layer was cured to prevent the nozzle from clogging due to UV illumination, it showed the possibility of layer-by-layer structuring of UV curable materials with precision positioning control of the AFM. If adequate curing technology is developed (e.g., integrating fiber for UV light illumination at the end of the dispensing nozzle), the application and impact on the industry would be widened significantly. Fig. 5(b) shows a schematic illustration of the electroplating method, in which the metal solution was extruded from the dispensing nozzle to produce a local electroplating process [49,84]. The patterning substrate was placed in an electrolyte bath that served as a conducting medium and a potential difference compared to the patterning tip was applied to the substrate. Then, the locally extruded metal-ion solution from the tip is solidified. Since the surface of the substrate had a lower potential than the dispensing nozzle, the dispensed solution solidifies in the form of a layer accumulated on the surface. Even when creating a 3D structure with some distance from the surface, the solidified pattern acted as a medium for charge transportation, which made simultaneous layer-by-layer deposition available.

Electroplating
Similarly, the factors influencing the patterning result in this method included the amount of metal solution delivered. The most important parameter was the localized potential difference, assuming that the amount of the metal solution transferred was well controlled while maintaining a constant contact force of dispensing nozzle with the substrate in the AFM system. In the above papers, the potential difference across the electrolyte was fed back and adjusted with a potentiometer to form a uniform pattern under the same printing conditions [85].

Dissolution
The dissolution method on soft material allowed the selective and local disintegration of a substrate with the use of stimuli (Fig. 5(d)). The Ref. [86] focused on a 1, 3, 5-benzene tricarboxamide (BTA) derived gelator's hydrogel films and the use of alkaline solutions dispensed through an apparatus combining an AFM and nanofluidic pressure controller, the alkaline solution disassembling the hydrogel film. The velocity was limited to 6 μm/s since above this velocity the dissolution of the BTA film is incomplete due to an insufficient supply of dispensing solution. The size of the patterned line was limited in width considering the incomplete dissolution.

Application
The different methods and techniques surrounding nozzleassisted precision manufacturing yielded a diverse range of resulting patterns with an even wider scope of applications. These patterns could be divided into three categories depending on the dimensions they belong to. First, the material can be printed to create 1D patterns (such as the microdroplets) as shown in Fig. 6(a), which illustrates a crisscross made out of a UV-curing solution deposited using electrohydrodynamic tips. 1D patterns such as that in this figure could be used for flexible devices, defect repair, and tissue bioengineering, so on. Recently, as quantum dot (QD)-LEDs were getting much interest [87], there have been efforts to pattern QDs in high resolution. Such application could be utilized if AFM-based precise dispensing can provide wide-range and high throughput patterning.
2D patterns could be also created from various materials and methods as presented in Fig. 6(b). Since the line patterning of conductive materials was already well known in numerous research areas, there were many studies also in micro/nanoscale patterning with metal. In the Ref. [49], a copper solution was used to generate a figure composed of copper lines. The solution was deposited using an electrochemical balance between the printed solution and the substrate and then solidified by electroplating. While this method required the solvents to be dried and rinsed, a liquid metal pattern due to direct printing was also introduced [88,89]. The gallium alloys used to print the patterns that were stretchable and highly conductive, making them ideal for printing lines on non-flat surfaces and flexible supports found in stretchable electronics, soft robotics, and wearable electronics. In addition, elastomer printing was optimized with the use of an electric field and bipolar voltage wave- Fig. 6. Patterning results varying from 1D to 3D structures verified by AFM, scanning electron microscopy (SEM), or optical observation: (a) 1D microdroplet deposition with EHD. Reprinted with permission from Ref. [15]; (b) pressure-driven 1D and 2D electrochemical deposition. Reprinted with permission from Ref. [49]; (c) 2D to 3D structures with liquid metal (EGaIn) using the pressure-driven capillary tip. Reprinted with permission from Ref. [20]; (d) 3D structures of perovskite (CH 3 NH 3 PbI 3 ). Reprinted with permission from Ref. [19]; (e) 3D structures (scaffolds) of electrodeposited metal. Reprinted with permission from Ref. [84]; (f) 3D architecture metals. Reprinted with permission from Ref. [12]. forms to create complex surface patterns including the interconnection of micro LEDs [20]. Patterning 2D could also be applied to plane structures, such as a dielectric elastomer actuator (DEA) [5]. A multi-nozzled device was used to print several lines of elastomer encased by electrodes at once to create the flexible actuator.
Lastly, numerous 3D patterns were created using tipassisted technologies, such as meniscus guiding freeform perovskite 3D printing and manufacturing vertically stable arclike structures (Fig. 6(d)). The control of the meniscus of the printed solution allowed the control of the thickness, orientation, and placement of the printed patterns. Freestanding 3D structures were also created by electrodepositing a metal solution layer by layer (Fig. 6(e)). The manufacturing of this structure relied on the force-control feature of the modified printing hollow AFM probe, which allowed great flexibility of design concerning the printed shape: it could print dots, walls, hollow interior structures, overhanging structures, and intertwined shapes. Freeform micro/nano elements were printed and used in the production of finely integrated circuits. Fig. 6(f) shows the SEM image of a complex 3D structure. The structure was obtained by using two-photon lithography layer-by-layer on a metal-rich photoresist, then removing the organic component by pyrolyzing and evaporation to obtain a final metallic structure. 3D printing for making this kind of scaffold structure was widely used in biomedical applications, especially tissue engineering or cell growth [90]. Here, nozzle-based precision patterning, which could scale down the existing structures, might provide new opportunities in biomimetic or medical applications.

Conclusions
This review introduced various approaches for precision patterning with hollow cantilever dispensing nozzles integrated with AFM. In the first section, the history of the fabrication development and the detailed method was discussed. Most of them had used conventional silicon MEMS for device fabrication including sacrificial etching, mask etching, or wafer bonding. In the second section, operation methodologies such as device interconnection, fluid delivery, and pattern solidification were introduced. In the final section, micron-to nano-scale patterning results from 1D to 3D structures were discussed with their practical applications and the recent interests of numerous researchers. The patterning results were investigated by optical microscope, SEM, or AFM technologies.
Utilizing the AFM measurement mechanism in precise distance feedback of dispensing nozzle had certain advantagesnanoscale pattern generation or pattern generation with the nanoscale alignment onto the predefined surface. However, there had a limitation to the low selectivity of materials and lowfabrication throughput. To overcome this problem, a brand-new type of dispensing method using hollow cantilever nozzles could be considered for future challenges (Fig. 7). Instantaneous solidification of patterning solution with photoactive components or low-temperature melting metals in an atmospheric environment was one alternative. Although drop-on-demand inkjet dispensers had been developed and commercialized by many different researchers to increase the fabrication throughput, ultra-high resolution under a micron scale was still limited. With the aid of several conceptualizations to date, the non-contact dispensing methods are expected to be realized. They would use near-field patterning distance only for the high-resolution aiming rather than obtaining surface topography with AFM.
These types of patterning nozzles were also advantageous for the mass-producibility of the nozzles. In addition, the cantilever-type printing device could be monitored with multiple sensing parameters such as resonance frequency or phase change, which made full of information about the amount of force applied to the substrate, the mass of the injected liquid, reaction force applied to the cantilever, and so on. These features could be seen as a great advantage of cantilever technology over existing nozzle-based printing methods. With these advantages, if nozzle-based precision patterning is further developed, it would provide a new direction for printing technology.