Optimization of Piezo-Driven Jet Valve Dispensing Process for the Geometrical Control of Printed Sensors Based on Silver and MXene Inks

Printed sensors offer unique benefits—highly customizable designs, low-cost prototyping, and a wide range of materials and properties—which can be exploited. Piezo-driven jet valve dispensing is a printing technique suitable for the fabrication of sensors and electronics directly on the surfaces (both 2-D and 3-D) of smart objects and devices but needs additional analysis and study. In this work, we studied the influence of printing parameters on the performance of the Nordson PICO Jetting system mounted on the Neotech PJ15X machine by depositing a silver ink, typically used for the fabrication of sensors and interconnections, and a Ti3C2TX ink, a very promising 2-D material in the sensing and electronics fields for its extraordinary physical, electrical, chemical, and mechanical properties. A tuning approach was proposed to tune the printing parameters correctly. The profiles of the cross sections of printed lines were evaluated, including the process variability, when the values of the printing parameters were changed. In the case of Ti3C2TX, the improper setting of the printing setup caused undesired spots and irregular lines. The optimal settings for the printing setup were found for each ink, reaching a variability in the profile of 1.5%.


I. INTRODUCTION
P RINTED electronics (PE) has recently attracted the atten- tion of researchers and companies because it is potentially low-cost, can reduce material waste, can require less energy, and is easy to integrate [1], [2], [3], [4], [5].PE includes all the printing methods consisting of the deposition of functional inks or pastes to print the desired pattern on a wide range of substrates.Therefore, PE can fabricate sensors, flexible batteries, antennas, and electronics [6], [7], [8] by using substrates and functional inks or pastes with nonconventional properties, such as flexibility, stretchability, transparency, and biocompatibility [9], [10].In this way, it is possible to extend the use of electronic devices to numerous new applications, from biomedical to agricultural and industrial sectors [11], [12].Several technologies belonging to PE can fabricate electronic devices on 3-D surfaces [2], paving the way to produce smart objects, i.e., objects that integrate sensors and electronics to sense and/or modify the surrounding environment [13].Piezo-driven jet valve dispensing, also called Piezojet, is a printing method for depositing inks and pastes with a viscosity below 200 000 cP. Piezojet is a drop-on-demand and noncontact method.It is based on the ejection of droplets from the nozzle of 50-300 µm diameter due to the pressure generated by a piezo-driven tappet in contact with the ink inside of a small reservoir.Besides electronics, a smart object or device should integrate sensors to enable the monitoring and control of the processes and the environment [14].In particular, resistive sensors-such as strain sensors, temperature sensors, or gas sensors-fabricated with PE methods have been extensively studied [15], [16], [17].Piezojet is also a promising technology for fabricating sensors, even on 3-D surfaces and objects, but it is still poorly understood.In the Piezojet process, many parameters contribute to proper sensor fabrication, and they change according to the type of printhead structure (needle shape, piezo stack structure, and so on) and the ink used [18], [19], [20], [21].Properly controlling the printing process through tuning of printing parameters and defining guidelines for evaluating the influence of these printing parameters is essential to repeatably obtain printed sensors with good performance.Sohn and Choi [19] and Sohn et al. [22] tried to identify the most critical printing parameters to obtain drop with a repeatable minimum volume (and mass) by using a balance, but they did not study the process behavior during normal operation for sensor fabrication.
In this work, we studied the influence of the printing parameters on printed structures and by evaluating the printed features from both geometrical and electrical points of view.In this way, we could define and propose an approach to identify and tune the most critical printing parameters with the aim of improving the printing quality and repeatability of the printing process.In particular, we studied the performance of the Nordson PICO Jetting system, a Piezojet process, mounted on the Neotech PJ15X machine.Since the setting of the printing parameters strictly depends on the ink used, we tested the proposed method by using two conductive inks used in sensor fabrication, both as a sensing layer or for interconnects, a silver-based ink, and a Ti 3 C 2 T X -based ink.Silver ink is largely used to fabricate conductive pads for sensors, conductive interconnects, or entire sensors (i.e., strain sensors [23] and temperature sensors [24]).Ti 3 C 2 T X ink is a 2-D material analogous to graphene, belonging to the MXene family [25].MXenes are promising materials due to their extraordinary mechanical, physical, chemical, and optical properties.MXenes, and in particular Ti 3 C 2 T X , have been extensively studied and used for the fabrication of energy storage devices [26] but also of sensors, such as strain sensors [27], gas sensors [28], and biosensors [29].The two inks were also deposited on different substrates (glass, plastic, and ceramic) to measure the line profile and evaluate the process settings under different conditions.

II. FABRICATION METHOD A. Inks and Substrates
In this work, two inks were selected, one silver and one 2-D material ink.LOCTITE ECI 1011 E&C was used to fabricate highly conductive tracks.It is an ink based on small silver particles (less than 2 µm), suitable for flexographic, gravure, and screen printing.It has a viscosity of 2800 cP and shows good adhesion on PET.The recommended drying and sintering temperature in a box oven is 150 • C for 10 min [30].This ink was chosen because of its high conductivity, especially suitable for pads and conductive paths of sensors.The declared resistance sheet is lower than 0.005 /sq calculated for a film 25 µm thick.Ti 3 C 2 T X aqueous ink with concentration of 40 mg• mL −1 was used to fabricate tracks with conductive properties or sensitive layers.It was prepared according to the method proposed in [31], without any dilution.In this case, no curing process after the printing process is required.The expected viscosity is around 200 cP.Ti 3 C 2 T X was selected as a representative of 2-D materials, which exhibit interesting sensing, mechanical, and electrical properties.
The inks were printed on three different substrates: glass, ceramic, and plastic.A piece of glass (80 × 50 × 5 mm) was used to deposit the silver tracks and measure their profile.Aluminum oxide (Al 2 O 3 ) sheets (50 × 50 × 0.5 mm) were selected as highly porous substrates to deposit both silver and Ti 3 C 2 T X ink.Al 2 O 3 suffers from a high level of roughness but allowed us to evaluate the influence of the printing parameters with different substrates typically used to fabricate sensors.Novele IJ-220 sheets (60 × 60 × 0.140 mm), a transparent coated PET substrate, were used as a plastic substrate for Ti 3 C 2 T X for line profile measurements.The mesoporous silica coating promotes the adhesion of many electrically conductive inks.In a previous work [31], the adhesion of Ti 3 C 2 T X on Al 2 O 3 and IJ-220 was demonstrated when Ti 3 C 2 T X was printed by aerosol jet printing.

B. Design of the Printed Patterns and Preparation of the Printing Setup
The three patterns shown in Fig. 1 were adopted for the evaluation of the printed features from geometrical and electrical points of view.
Pattern #1 (three six 5-mm straight lines) was used to find the proper printing parameters for each ink, while pattern #1a (six 5-mm straight lines) was used to evaluate the process variability over six repeated prints.Evaluating printed straight and short lines is a representative case since sensors and electronics fabricated by using Piezojet are obtained from the combination of different lines forming polylines, such as grids (as in the strain sensors [32]), combs (as in the gas sensors [33]), or more complex structures [34].Pattern #2 (42-mm lines) was used to evaluate the printing process quality and to measure the resistance of the high conductive silver lines with good accuracy, while pattern #3 (serpentine shape), typically used for temperature and strain sensors, was used to evaluate the printing quality of more complex structures.
A software provided by the printer manufacturer was used to draw the designed patterns and convert the Computer-Aided Design and Drafting (CAD) files into motion control files for controlling the printhead movements according to the design pattern.

C. Printing and Curing Process
In this work, we used and tested the PICO Jetting system, a piezo-driven jet valve dispensing system produced by Nordoson, mounted on a PJ15X five-axis functional printer produced by Neotech AMT.The jetting system is composed of: 1) a piezo actuator, which forces the drop formation exiting from the nozzle; 2) a fluid body assembly, which includes the needle connected to the piezo actuator, the valve seat, and the nozzle; and 3) it is connected to the ink supply, a syringe barrel that contains the ink.M5T 3.0S P7 E05 (Nordson) with the nozzle in the seat orifice with a diameter of 50 µm was the fluid body assembly used in this work.A heater is integrated into the proximity of the fluid body assembly to reduce the viscosity of the ink.The tappet (needle) connected to the piezo-driven jet valve is in direct contact with ink inside the reservoir, from which comes out the drop through the nozzle [2].The mechanism for the drop formation can be divided into four phases: 1) the tappet is in close contact with the nozzle to prevent ink exit since the piezo actuator is powered on with the maximum voltage; 2) the voltage is changed to the minimum value in few milliseconds, the needle starts to rise, and the reservoir starts to fill up with the ink; 3) the needle is at the maximum height and the reservoir is filling up; and 4) the power of the piezo actuator changes again to the maximum value, the needle is lowered toward the reservoir, and a drop of ink is ejected from the nozzle toward the substrate.This mechanism is repeated cyclically every Pulse milliseconds according to the pattern to be printed.
After placing it on the printing plate, the substrate was cleaned with ethanol.The PET substrate was cleaned only with a dry wipe to prevent removal of the coating.
Then, the selected pattern was printed using the associate motion control file generated by the software.The printing process was possibly repeated several times to increase the total thickness of the deposited layers and thus to decrease the resistance of the selected conductive inks.
After printing, the printed samples were thermally cured according to the type of printed ink.In the case of the silver, the samples were cured in an oven at 150 • C for 10 min to promote the sintering of the silver nanoparticles, according to the datasheet.The samples based on Ti 3 C 2 T X were cured at 60 • C for 30 min in an oven to promote solvent evaporation.

D. Printing Parameter Setting
The printing process is regulated by the nine parameters listed as follows, whose value mainly depends on the rheological properties of the ink to be deposited.
1) Close Volts is the maximum voltage applied to the piezo actuator required to prevent ink exit during phase 1. 2) Stroke determines the maximum height of the needle reached at the end of phase 2 and kept in phase 3. It is expressed as a percentage of the Close Volts value.3) Pulse is the time required to move the needle from the open to the close position (phase 2).4) Open is the time in which the reservoir is filled (phase 3).5) Cycle is the time to complete the four phases.6) Close is the time required to move the needle from the open to the close position (phase 4).7) Pressure is the pressure inside the syringe and promotes the reservoir filling during phases 2 and 3.The greater the viscosity, the greater should be the pressure value.8) Speed is the printing speed at which the printing plate is moved.9) Temperature is the temperature inside the fluid body assembly regulated by the heater.In order to guarantee a repeatable and reproducible printing process, a fine control of the volume and shape of the ink drop is required through the tuning of the printing parameters.As confirmed in [35] and [36], the shape and size of the formed droplets depend on the piezoelectric force produced by the piezo actuator through the tappet and the pneumatic force generated by the pressure inside the syringe and thus on the energy transferred to the droplet during the printing process.
The piezoelectric force is mainly determined by Close Volts and Stroke.The greater their value, the higher the exerted force.Furthermore, in the case of low-viscosity inks, the greater their value, the smaller and more uncontrollable the droplets, generating satellites and undesired spots on the substrate, as confirmed by the experimental results.This phenomenon is typical of inkjet printing when the printing parameters are not set properly for low-viscosity inks [37].Inkjet drops are formed by squeezing fluid through a tiny hole, the harder the ink is pushed, the higher the stress on the fluid, and thus, drop formation depends on the ink's properties, namely, viscosity, surface tension, and particle content.This phenomenon is also expected in the case of Piezojet.Another parameter influencing the energy transferred to the droplet is Close time.
Pulse and Open determine the volume of the droplet and the dot size.They depend on the ink properties and on its ability to fill the reservoir under the pressure inside the syringe.
Speed and Cycle affect the edge of the printed lines.The greater the Cycle, the greater the time between two consecutive ejected drops.The greater the Speed value, the wider the spatial distance between two consecutive ejected drops.Their values are optimal if the edges of the line are straight (the line appears as a rectangle).For this reason, Speed and Cycle can be tuned in the second stage according to the diameter of the deposited droplet (it depends on the volume of the droplet) in order to obtain straight and uniform lines.
Pressure regulates the volume and the pneumatic force on the energy transferred to the droplet and its importance is defined by the ink viscosity, as confirmed in [38].Lower the viscosity (less than 150 cP), more pneumatic force is relevant.Temperature generally regulates the viscosity of the ink promoting the printability of the ink.

III. PRINTING PARAMETER TUNING METHOD A. Tuning Method
Due to the correlation of printing parameters, the setting of the printing parameters process passes through two stages.The goal of the first stage is to define the most significant parameters and the values of the others.In the first stage, Temperature, Speed, Open, and Cycle are kept constant, and their value can be defined starting from values used for other inks with similar viscosity and rheological properties.For example, while Ti 3 C 2 T X can be printed at room temperature, silver ink needs a higher temperature to lower its viscosity and increase its printability.Three levels for the other five parameters are defined starting from the literature or previous works and all the possible combinations are tested.As confirmed by the experimental results of Section IV, the three levels should differ by less than 20% to guarantee the formation of the droplet.Pattern#1 of Fig. 1 can be used to have replicate lines obtained with the same printing parameter set.
In the second stage, a tuning of the most significant parameters was performed by defining N levels for each parameter.Pattern #1a in Fig. 1 can be used to have replicate lines obtained with the same printing parameter set.In that way, a preliminary evaluation of the repeatability can be done.
To define the optimal values of the printing parameters in both stages, the printed lines were evaluated by measuring their features (width and thickness) and by inspecting the region around them for finding undesired isolated spots.

B. Viscosity Measurement Setup, Imaging, Dimensional, and Electrical Measurement
Viscotech VR 3000 MYR modelV2-L, a rotational viscometer, was used to measure the viscosity of noncommercial Ti 3 C 2 T X ink and commercial silver ink.The tests were performed at 25 • C, with a speed of 200 r/min and an accuracy of 10 cP.In this work, the effects of the printing parameters on the printed features were evaluated through the analysis of the geometrical aspects (cross section, width, and thickness of the printed lines) and the electrical performance (electrical resistance).
NB50T (Orma Scientific), an optical microscope with a 2× zoomed-in view, equipped with a camera connected to a laptop was used to inspect the printed lines and the area around them.Especially in the case of Ti 3 C 2 T X ink, the inspection of the area close to the printed lines was required to detect satellite drops and undesired spots generated when the selected printing parameters were not suitable for a controllable ink ejection.
An optical profilometer and a mechanical profilometer were used to measure the geometrical aspects and thus to preliminary evaluate the repeatability of the printing process and the influence of the printing parameters on the geometry of the printed patterns.
An Alpha-Step IQ Surface Profiler, a diamond stylusbased profilometer operating with an accuracy of 0.1%, was used to measure the profile characteristics of the printed Ti 3 C 2 T X -based lines and the printed silver lines on Al 2 O 3 .The stylus was moved along the x-axis, i.e., perpendicular to the printed edge, for 800 µm with steps of 1 µm, while the thickness was obtained by measuring the movement of the stylus along the z-axis, obtaining the curve z(y).The start position and the stop position were defined at least 50 µm before and after the printed line (i.e., in a position without ink).For each line, three profiles p(y) were acquired, one in the middle and the other two one millimeter from the ends of the line.
A Profilm3D (fabricated by Filmetrics), an optical profilometer, was used to evaluate the geometry of the printed silver lines on glass.In this case, 11 profiles z(y) were extracted from each inspected area (800 × 900 µm, resolution of 1.74 µm).The profile corresponds to the cross section of the printed lines (the printed lines are positioned parallel to the x-axis, and hence, the cross section is parallel to the y-axis).For each printed line, three portions of 800 µm were inspected.An example of an acquired portion of the printed silver line is shown in Fig. 2(a).
Once acquired or extracted the profile of the printed line, the baseline b was averaged and leveled considering the measurement points outside the printed line and the profile p(y) of the printed line was corrected in order to have p(y) = z(y) − b and filtered with a standard spline filter, with a cutoff wavelength of 50 µm.From the acquired profile, it was possible to define the following dimensions of the printed line.
1) w, the width, is the span where p(y) is nonzero (considering the roughness of the substrate).2) t max , the maximum height, is the maximum of p(y).
3) S, the cross-sectional area, is calculated by using a Riemann sum-type approach.4) t, the average thickness, is S/w.Considering lines obtained with the same materials and method, all the analyzed cross sections p(y) were overlapped to find w, t max , S, t, p(y), and their dispersion.An example of the profiles of a printed silver line extracted from the acquired surface [see Fig. 2(a)] is shown in Fig. 2(b).
Finally, a multimeter was used to evaluate the electrical properties of the printed tracks.An HP 34401A, a 61/2 digit multimeter, was used to measure the resistance, R, of the printed lines in a four-terminal configuration.The resistivity ρ of the material was obtained with the equation ρ = R•S/d, where S is the mean cross section calculated from the profile measurements and d is the distance between the probes.

A. Viscosity Measurements
The viscosity of the silver and Ti 3 C 2 T X inks was measured as (2900 ± 10) and (250 ± 10) cP, respectively.The resulting viscosity of LOCTITE ECI 1011 E&C is in accordance with the specification of its datasheet.The measurement of the viscosity of both inks helped to define the starting value for each process parameter.

B. Printing Parameter Tuning for the Silver Ink Deposition
Starting from the manufacturer's suggestions, we defined the starting value and their alternatives for each printing parameter (see Table I) and we tested all the combinations.Some combinations were immediately excluded, i.e., when Close Volts = 100 V, Pressure = 0.2 bar, and Stroke = 60%, reducing the possible combinations to 72.Fig. 3 shows the influence of the Pressure on the ink deposition.At 0.5 bar [see Fig. 3(b)], the deposition of the ink is not uniform on the substrate and sometimes no droplets are deposited on the substrate because the chamber is not correctly filled.Similar results were obtained for a lower level of Stroke; if Stroke is 60%, no droplet is formed, and if Stroke is 70%, the deposition is not uniform.The starting values of Table I ensured good control of the drop ejection of the silver ink.In that case, under the microscope, the ink deposition seems to be uniform with the starting value; the printed lines appear straight and the width constant along the line, as designed.
According to the results of the first stage, we decided to perform the second stage and study the influence of the parameters Cycle and Pulse on the quality of the printed Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.

TABLE II TESTED VALUES OF CYCLE AND PULSE FOR SILVER INK DEPOSITION
feature from the geometrical point of view, defining three different levels, as indicated in Table II.The values of Close Volts, Stroke, Open, Close, Pressure, Speed, and Temperature were kept constant throughout all the tests.Therefore, we defined seven combinations (printing settings), as reported in Table II, and tested the printed features from a geometrical point of view (profile, width, mean, and maximum height).In this test, we deposited silver ink on the glass, and the profile measurements were taken with the optical profilometer.The printed lines were designed according to pattern #1.For each setting, we printed six lines.The combinations with Pulse equal to 0.5 ms and Cycle equal to 8 or 10 ms were not considered because the width of lines increased considerably.
The cross sections of the printed lines acquired by the optical profilometer and evaluated according to the procedure described in Section III-B are shown in Fig. 4. The blue Fig. 4. Mean profile (blue line) and overlap of multiple cross sections of six printed silver lines (red area) grouped according to the used printing setting and listed in Table II (set #1-#7).For each plot, the number of the printing setting is indicated in the legend.5. Mean value and dispersion of (a) w, (b) t max , and (c) t calculated on the results shown in Fig. 4 (silver lines on glass group according to the used printing setting listed in Table II).line (inside the shadow area) represents the mean profile considering all the acquired cross sections of six printed lines, while the shadow area includes all the acquired profiles.
The repeatability of the printed process qualitatively deducted from the width of the shadow area is worse when Pulse and Cycle are equal to the lowest value (0.35 and 8 ms, respectively), as shown by a wider shadow area ( p(y) = 2.3%).The average w, t max , and t and calculated for the 66 cross sections is summarized in Fig. 5.As expected, the lower the Cycle, the greater w, t max , and t; this indicates a larger drop volume.Set #6 (Cycle = 12 ms and Pulse = 0.43 ms) represents the best compromise in terms of process repeatability; indeed, w and t are equal to (435.5 ± 5.0) and (9.9 ± 0.1) µm, respectively.Therefore, the optimization of the process was performed by studying the profile of six lines and minimizing their deviation from the average value.
Similar results (listed in Table III) were obtained when the silver ink was deposited on Al 2 O 3 , despite its greater roughness.
The calculated average roughness (Ra)-defined in ASME B46.1 as the "arithmetic average of the absolute values of the profile height deviations from the mean line, recorded within the evaluation length"-is 0.74 µm for Al 2 O 3 and it was obtained by measuring the profile height on a line 1-mm long with the mechanical profilometer.Ra of the glass can be considered negligible.The different Ra implied an average lower mean thickness and a greater variability in the width and mean thickness, as reported in Table III.

C. Dimensional and Electrical Measurements of Printed Silver Lines After Optimization Process
Once optimized printing parameters, pattern #1a was used to print straight lines on glass and Al 2 O 3 .For every six lines, we increased the number of printed overlaid layers (N layers ) from one to four to obtain four groups of six lines with N layers = 1, 2, 3, and 4, on both substrates.The results are shown in Figs. 6 and 7.As expected, the width and the maximum height increase, and the results are similar for both substrates by increasing N layers .The repeatability of the printing process, qualitatively deduced from the width and dispersion of the shadow area (see Fig. 6) and from the expanded uncertainty U -calculated as the experimental standard deviation on 18 profiles acquired from six identical printed lines and 95% level of confidence (the coverage factor is based on t-distribution) in accordance with the Guide to the expression of uncertainty in measurement (GUM)--, is better for lines deposited on glass because of the negligible Ra.Indeed, in the case of lines deposited on glass, U is 1.5% (4.8% on Al 2 O 3 ) when N layers = 1 and decreases to 0.25% (3.3% on Al 2 O 3 ) when N layers = 4. Furthermore, in the case of glass, the width and maximum height of the printed lines are greater.For example, N layers = 4 w and t are, respectively, 821 and 32 µm in the case of glass and 756 and 28 µm in the case of Al 2 O 3 .The reason is that, in the case of Al 2 O 3 , a certain amount of the deposited volume covers the pores of the substrate and this causes a loss of the expected shape of the printed line (the edges are irregular, and thus, the variability of the width along the line increases), as also confirmed in [39].The mean values of the linewidth, the area, and the maximum height as a function of N layers can be fit with a first-order line with the least squares method for both substrates, weighted with measurement uncertainty.The coefficient of determination R-squared of w is 0.99 in the case of the lines printed on glass and 0.98 in the case of the lines printed on glass Al 2 O 3 .
In the final test, we estimated the resistivity of the printed lines, by printing pattern #2 on glass and with different N layers .Indeed, long lines are required to measure the resistance between the two ends with an accuracy better than 3% since the resistivity of the silver is very low, and the resistance of Considering the area of the cross section of the lines measured with the profilometer and by applying the Ohm law (R =ρ•l/S), the resistivity ρ is confirmed constant (due to the metallic nature of the ink) and results (0.062 ± 0.002) µ •m, in accordance with the datasheet specifications.

D. Printing Parameter Tuning for Ti 3 C 2 T X Deposition
Starting from the previous results on inks with similar viscosity, we defined the starting value and their alternatives for each printing parameter (see Table IV).Ti 3 C 2 T X was less viscous than silver ink, and thus, satellites and undesired spots were expected without the optimization of the printing parameters.Indeed, Stroke was expected as one of the crucial parameters for the proper deposition of the ink, as shown in Fig. 8.When Stroke was 58%, the ink was not able to exit from the nozzle (for this reason, this value was excluded in the systematic tests), while when Stroke was 68%, the edges of the printed lines were not regular.The number of undesired isolated spots and the average diameter on the surface were at maximum when the Stroke was 68%.This result confirms our assumption that the satellites are produced when the ink has a low viscosity.The value of Pressure helps only to increase the width of the lines and does not reduce the number of undesired spots.In order to study the behavior, we tested more levels of pressure, as shown in Fig. 9.The diameter, d, of the desired spots that form the lines linearly increases with pressure p from 220 to 353 µm (d = 130 µm/bar•p + 190 µm/bar and Radjusted = 0.997), while the resistance decreases from infinite (at Pressure = 0.4 bar) to 455 .A significant difference in the printed line aspect (edge shape) was found when Close Volts decreases at 105 V (−8.7%), as shown in Fig. 10.If Close Volts is set to 120 V (+4.3%), the ink is not ejected from the nozzle.Low Close Voltage (105 V) causes uncontrolled deposition of the ink; in some cases, the amount of the ink is too high (when correlated with Stroke = 68%) and the ink is spread over the substrate or the edges are irregular (the width increases along the line).When Stroke is set to 63%, by increasing Pulse from 0.30 to 0.50 ms (steps of 0.10 ms), the width increases from 230 to 345 µm, but the incidence of Authorized licensed use limited to the terms of the applicable license agreement with IEEE.Restrictions apply.Fig. 6.Mean profile (solid lines) and dispersion (shadow area) of silver lines printed on (a) glass and (b) Al 2 O 3 , grouped according to the number of printed overlaps (N layers = 1 to 4).Each group includes identical six lines (pattern #1a in Fig. 1).isolated spots is not affected.In order to reduce the incidence of isolated spots at minimum, Close Volts should be set to 115 V, Stroke should be set to 63%, Pulse should be set to 0.40 ms, and Pressure should be set to 0.6 bar.In this way, the conduction path for the line is guaranteed.In order to eliminate the isolated spots, Close had to be set to 0.6 ms.Close is a key parameter to eliminate undesired spots and satellites.In this case, the rheology of ink (including the low viscosity) requires that the needle closes slower.
In order to optimize the printing process, Speed and Cycle were tuned defining two levels: 600 and 900 mm/min and 12 and 16 ms.The Speed and Cycle set to 900 mm/min and 16 ms, respectively, significantly affect the edges of the  lines because the distance between two consecutive dots is higher than the radius of the dots (see Fig. 11); it is a typical phenomenon when the ratio between Speed and the frequency  The best compromise is obtained when all the printing parameters assume the values shown in Fig. 12.In this case, the overlap of the dots is enough to have straight edges of the line, as well as a narrow enough linewidth.A short width (less than 370 µm) indicates the possibility to obtain fine features and reduced overall dimensions of the printed sensor.According to the manufacturer specifications, the linewidth cannot be lower than 300 µm.

E. Dimensional and Electrical Measurement of Printed Ti 3 C 2 T X Lines After Optimization Process
After optimizing the printing parameters, a matrix of 6 × 7 lines (seven columns of pattern #1) was printed both on Al 2 O 3 and PET.Each column has six identical lines (same N layer ) and N layer increases from one to seven across the columns.The geometrical characterization was performed only on the lines deposited on PET because the height of deposited Ti 3 C 2 T X is comparable to the roughness of Al 2 O 3 and the processing of the acquired data is difficult for the lines with N layer equal or less than six, especially in the width and area measurements.The mean profile and its variability calculated on six identical lines with N layer = 7 are shown in Fig. 13.
The mean values (calculated on six identical lines) related to the cross sections of the Ti 3 C 2 T X lines printed on PET as a function of the number of overlap depositions (N layer ) are shown in Figs. 14 and 15.As expected, the area and the maximum height of the mean cross section increase linearly with N layer (R-squared = 0.999), whereas the width increases only for N layer ≤ 4. The mean width can be considered constant for N layer ≥ 4, and it is (477 ± 14) µm.Grouping the  measurement according to N layer , the expanded uncertainty U -calculated from the experimental standard deviation on 18 profiles acquired from six identical printed lines and 95% level of confidence-is 4% for N layer = 4 and of less than 1.7% for N layer ≥ 5 (±8 µm).These results are also confirmed by area and height measurements.For example, U on the "maximum height" is minimum for N layer = 5 and 6 (±1.2%) and maximum for N layer = 3 (±3.7%).According to these findings, N layer = 5 could be considered the best compromise in terms of repeatability process, ink volume, and line dimensions.
As for silver ink, the substrate affects only the width, thickness, and area of the cross section of the printed lines.For example, when N layer = 7, the width is 415 µm in the case of Al 2 O 3 -smaller than 479 µm in the case of PET-while the expanded uncertainty U -calculated from the experimental standard deviation on 18 profiles acquired from six identical printed lines and 95% level of confidence (the coverage factor is based on t-distribution)-is 5% in the case of Al 2 O 3greater than 1.5% in the case of PET.
The resistance of all the lines printed on PET and Al 2 O 3 was measured (six identical lines for each N layer = 1, . . ., 7).The ratio of the measured resistance to the length (i.e., the distance of the two probes) as a function of N layer and according to the substrate is summarized in Fig. 16.As for silver lines, the resistance decreases as a function of N layer .The maximum deviation from the mean value calculated on the six identical 3-mm lines is less than 16% when N layer = 1.In the other cases, the deviation is less than ±3.5% for N layer = 2,   In the final test, the ability to print more complex geometries was tested.Pattern #3 was printed with Ti 3 C 2 T X ink on PET.A detail of the printed serpentine is shown in Fig. 17.The technology is also able to print long lines and patterns that required direction changes of the printhead of 90 • , contrary to other technologies that require smoother direction changes, such as aerosol jet printing [14].To obtain these results, abrupt decelerations/accelerations in correspondence with the corners are required and this causes an increase in the ink amount deposited close to the corner: near the corners, the line is visible wider.The measured resistance is (2612.00± 60.00) , calculated on six printed serpentine and considering the maximum deviation from the average value.Starting from the Ohm law (R = ρ•l/S), measuring the area of the cross section, knowing the average path length l and the resistivity of Ti 3 C 2 T X found in the previous test, the resistance is in accordance with the results in the previous tests where straight lines were evaluated.Furthermore, the resistance deviation from the average value calculated on six serpentines is less than 1.5% and this value can be used to estimate the process repeatability.

V. CONCLUSION
In this work, the performance of the piezo-driven jet valve dispensing (called also Piezojet) was studied for the fabrication of resistive sensors by depositing silver ink and Ti 3 C 2 T X ink on different substrates to provide a method for detecting and tuning the most critical printing parameters.The proposed method is divided into two stages.The evaluation of the best printing parameter set is performed through optical inspection and dimensional measurements of printed lines.The first stage of the optimization defines the setting of most of the parameters to obtain the ink ejection and to have a uniform deposition (regarding the deposited volume and the regular edges), while in the second stage, only two parameters were tuned to optimize the process.The results demonstrated that the critical printing parameters depend on the but not on the substrate, as expected.The substrate influences the final aspect of the printed lines due to its roughness and surface tension.For example, silver lines on glass and Ti 3 C 2 T X lines PET have a greater width and maximum than on Al 2 O 3 but a lower variability (<±1.5%),considering the uncertainty calculated from the experimental standard deviation.
In the case of silver ink, while Close Volts and Stroke should set to a particular value for the correct formation of droplet, Pressure is critical to obtain a line with regular edges.The optimization was performed in the second stage by changing Cycle by ±33% (starting value 12 ms) and the Pulse by ±8% (starting value = 0.43 ms) to reduce the variability in the area of the printed line cross section.The linewidth and the average thickness were found to be almost constant regardless of the value of the parameters, while the deviation increased when the value of the parameters was different from the starting value.We also found that the average thickness of the lines can be increased from 8.7 to 20 µm by depositing several overlapped ink layers from one to four, with a resulting increase in the width and variability.In the case of silver ink, one deposition is enough to obtain good conductance of the printed line.The resistivity calculated on a 42-mm line is 0.062 µ •m.
In the case of Ti 3 C 2 T X ink, we found that the low viscosity of the ink affects the printing quality when the printing parameters are not optimized; Piezojet, as in inkjet, produces satellites and undesired spots when the viscosity is low, as seen in Ti 3 C 2 T X ink (equal to 250 cP), while in the case of silver ink, this did not occur.Close was found to be a critical parameter for Ti 3 C 2 T X : shorter Close causes undesired spots, while Stroke and Close Volts must assume a specific value to deposit lines with straight edges.Pressure and Temperature are not as critical as for silver ink because the viscosity is lower.Also, in this case, it is possible to deposit more overlapped layers to reduce the resistance of the printed line, considering that the resistivity of Ti 3 C 2 T X (equal to 9.1 µ • m) and the thickness of the printed line (equal to 1.5 µm with two layers).
In particular, at least five layers are recommended to reduce the variability among the printed lines in terms of geometrical dimensions and electrical resistance.
The repeatability of the printing process, expressed here as the standard deviation calculated on the profile of six printed lines, is similar for both inks on glass (±5 µm) or PET (±12 µm).
In the ongoing work, we are fabricating and testing strain sensors based on Ti 3 C 2 T X , to define the performance of the sensors in of sensitivity to the induced strain and drift due to the temperature.The reproducibility of the printing process with the optimized parameter has been preliminary tested by using the same printing system but in different laboratories and at separate times.Preliminary tests have already been performed with good results confirming the reproducibility.Further analysis can still be done, and the scientific community is sensitized to deepening the reproducibility of the printing process and the proposed tuning method.

Fig. 1 .
Fig. 1.Patterns and dimensions used for the optimization of the printing process and the evaluation of the printed features.

Fig. 2 .
Fig. 2. Surface elaboration of a printed silver line acquired by optical profilometer.(a) 3-D profile and 11 analyzed cross sections (red lines).(b) Overlap of the 11 cross sections and parameters considered for the analysis of the printing quality.The red rectangle has the same area (w•t) of the average profile p(y).

Fig. 3 .
Fig. 3. Influence of Pressure on the printed silver line aspect.The printing parameters values are reported in Table I except for Pressure.(a) Uniform ink deposition due to the correct setting of printing parameters (Pressure is 1 bar).(b) Nonuniform ink deposition due to the wrong setting of printing parameters (Pressure is 0.5 bar).

Fig. 7 .
Fig. 7. Mean value (dots) and dispersion (bars) of (a) w, (b) S, and (c) t max calculated on the results shown in Fig. 6, grouped according to the type of substrate and as a function of the number of printed layers N layers .The bars are the expanded uncertainties calculated as experimental standard deviation calculated on 18 measurements and the coverage factor equal to 2.11 (based on t-distribution, level of confidence = 95%), in accordance with the GUM.The dashed lines are the linear regression weighted with measurement uncertainty.

Fig. 12 .
Fig. 12. Aspect of Ti 3 C 2 T X lines printed on PET.On the left are the optimized printing parameters.

Fig. 13 .
Fig. 13.Mean Profile (solid lines) and dispersion (shadow area) of Ti 3 C 2 T X lines (N layer = 7) printed on Al 2 O 3 .The roughness of the substrate makes difficult the detection of the line profile.

Fig. 14 .
Fig. 14.Mean profile of Ti 3 C 2 T X lines (calculated on six identical lines) as a function of N layer (overlap depositions) printed on PET.

Fig. 15 .
Fig. 15.Mean values (dots) and uncertainties (bars) of (a) w, (b) S, and (c) t max calculated on the results shown in Fig. 14, grouped according to the type of substrate and as a function of the number of printed layers N layers .The bars are the experimental uncertainties calculated as experimental standard deviation calculated on 18 measurements and the coverage factor equal to 2.11 (based on t-distribution, 95% level of confidence), in accordance with the GUM.The dashed lines are the linear regression weighted with measurement uncertainty.

Fig. 16 .
Fig. 16.Mean values (dots) and dispersion (bars) of the ratio of the measured resistance to the length of Ti 3 C 2 T X lines on Al 2 O 3 and PET.

3
and ±1.5% for N layer > 3 both on PET and Al 2 O 3 .The resistivity is (9.0 ± 0.4) µ •m in all the cases, considering the resistance R and the profile measurements S and using the Ohm law R = ρ•l/S.The same results in terms of resistance and profile were obtained when Pattern #2 was printed on PET.According to these results, despite the long length of the line (42 mm), the deposition can be considered uniform along it.

Fig. 17 .
Fig. 17.Details of Pattern #3 printed with Ti 3 C 2 T X ink on PET.

TABLE IV TESTED
VALUES OF THE PRINTING PARAMETERS FOR THE DEPOSITION OF A TI 3 C 2 T X INK short lines (5 mm) is expected to be in the low hundreds of milliohms and the accuracy of the multimeter in that range is 16% of the reading in the worst case.As expected, the resistance measured on the length of 40 mm (l) is 559, 280, 192, and 142 m for N layers = 1, 2, 3, and 4, respectively.