کی تمام اشاعتیں Muhammad Khalil . راولپنڈی ، پاکستان
Table of Contents
Experimental setup:. 7
Materials and methods:. 7
Printing parameters and design of experiments:. 8
6.1 Design of experiments based on response. 9
Surface methodology (RSM). 9
6.2Experimental procedure. 9
6.3Analysis of printing parameters and their. 11
Printing of circuits:. 14
Direct ink writing of flexible electronic circuits and their
The present work discusses the fabrication of low-cost, reliable and durable flexible electronic circuits from conductive carbon paste on flexible polyethylene terephthalate (PET) substrate using micro dispensing direct ink write technique. Printing
Parameters such as pressure, substrate speed and gap size are optimized through Box–Menken experimental design technique
To achieve the desired quality of prints (patterns). For the nozzle with an inner diameter of 200 µm, the system is able to
Produce pattern line width ranging from 198 µm to 295 µm, respectively. Based on optimized parameters, circuits such as
Foil grid and rectangular spiral are printed on a flexible PET substrate. The deposited patterns of circuits are morphologically
Characterized by optical microscopy and scanning electronic microscopy. Current–Voltage (I–V) characteristic is performed
To evaluate the conductive performance of circuits. The sensitivity of circuits to bending is assessed by studying resistance
Response under fully bend and unbend angle of 90° and 0°. Durability and reliability of circuits are analyzed by subjecting
Circuits to continuous 500 bend cycles and study its resistance response. The analysis revealed that these direct ink write
Circuits are reliable, durable and stable in performance and are applicable to the flexible electronic application.
Keywords Micro dispensing direct ink writing · Conductive carbon paste · Flexible circuits · Polyethylene substrate · Box–
The direct ink write (DIW) technique is widely used for
Patterning and selective deposition of different types of
Materials. It has the capability to form high-resolution single and multiple layers. It allows the deposition of a variety
Of material on both planers as well as a conformal surface
. this technique has been widely studied and developed
Because it allows rapid manufacturing of different kinds of
Microelectronic devices like antennas [2, 3], capacitors [4,
5], radiofrequency electronics [6, 7], micro electromechanical system (MEMS) [8, 9] and different kinds of sensors
Such as flexible sensors  and strain sensors . Unlike
Conventional electronic fabrication, it eliminates complex
Processes such as etching and lithographic masking steps for
Shaping processes, which are time-consuming, costly and not
Different technologies have been developed in DIW. The
Most well-known is extrusion and droplet-based technologies. Functions of these technologies are the same, i.e.
Selective depositions of conductive traces, but their design
Goals may differ . Extrusion-based technologies use
Positive pressure for dispensing desired materials. The dispensing material is in the form of liquid/paste and extruded
Through a small nozzle . Extrusion-based DIW technologies are direct-write assembly [15, 16], micro-pen dispensing , rob casting  and fused deposition modeling
. Droplet-based technologies eject desired material in
The form of droplets onto target substrate, unlike extrusion based technologies in which a continuous jet is formed.
Droplet-based DIW technologies are ink-jetting ,
Electro-hydrodynamic jet printing  and aerosol jetting
. for effective dispensing, numerous conductive inks
Have been designed such as metal-based nanoparticle ink,
Organometallic ink, conductive polymers, carbon nanotubes,
Grapheme oxide, sol–gel, polyelectrolyte ink and conductive
Carbon pastes [13–23].
Abbas and Rahman  fabricated a fix sensor using a
Direct ink write technique. For fix sensors patterns of conductive carbon, the paste is deposited on a flexible polyethylene substrate. The minimum line width of patterns achieved
Is 252 µm for the 200-µm nozzle? Optimal parameters for
Printing are the pressure of 70 psi, the feed rate of 1.5 mm/s
And a standoff distance of 0.4 mm. To investigate the performance of flexible sensors, they used a custom-built bending fixture. The electrical resistance is measured as function of bend angle position from 0° to maximum of 90°.
Fourfold change in electrical resistance (i.e., 6.6–26.4 Ki)
Is observed by changing bend angle from 0° to 90° due to
Development of micro cracks in patterns. Shekel et al. 
Fabricated strain sensors from conductive carbon paste on
Polyimide substrate by employing micro dispensing direct
Ink write technique. The minimum pattern size achieved for
Strain gauge is 218 µm for 200-µm nozzle. Optimize parameters for printings are the pressure of 42 psi, the feed rate
Of 2 mm/s and a standoff distance of 0.3 mm. Say et al.
 Patterned wide range of ink in planar as well in 3D
Shapes using direct-write assembly. In this process, the ink
Is injected through the nozzle by compressed air. The wide
Range of ink processed using the direct-write assemblies
Are nanoparticle fled ink, sol–gels, colloidal inks, fugitive
Organic inks, hydrogels and polyelectrolyte inks. Nozzle
Diameter size ranges from 1 µm to 500 µm, with minimum
Pattern size achieved is 250 nm for sol–gel and 200 µm for
Ceramic colloidal inks. An et al.  patterned transparent conductive grid of silver nanoparticles on flexible polyimide and glass substrate using directly in write assembly.
The minimum width of pattern achieved is 9 µm through a
5-µm nozzle under control parameters, i.e., the pressure of
50 psi and nozzle speed of 250 µm/s. To study the effects of
Mechanical bending on electrical performance, a customized mechanical stage is fixed to a micro positioner to carry
Out bend tests. The electrical resistivity is measured as a
Function of bending radius from ± 11 to ± 5 mm. After 1000
Bend cycles, an approximately twofold change in electrical
Resistivity is observed at the smallest radius of± 5 mm due to
Formation of micro cracks in patterns. Kondo et al.  prepared nanoparticle colloidal gel for direct writing technique.
The colloidal gel is prepared from a mixture of Y2O3 stabilized ZrO2 (YSZ) powder and ethanol. A rob casting system
Is employed to successfully form two-dimensional patterns
Of YSZ. An et al.  fabricated microelectrodes using
Rob casting for solar cells, small antennas, light-emitting
Diodes (LED’S) and conductive grids. The approach was
Based on designing a wide range of inks, including metallic, polymeric and organic, sol–gel and ceramic. The patterns achieved for microelectrodes using silver nanoparticle
Ink with a solid loading of 85 wt. % have high aspect ratios
(H/w = 1) and small as 2 µm which allows the fabrication
Of 1D, 2D and 3D architecture. In another study , the
Stretchable and flexible microelectrode was fabricated using
Silver nanoparticle ink with solid loading less than 70 wt. %.
By this approach, complex interconnects are patterned for
Both LED’S and solar cells. The pattern lines achieved have
A width of 15 µm, and height of 13 µm. Kadar et al. 
Fabricated electrochemical platform using rob casting. The
Performance is then compared with electrochemical platform fabricated by screen printing. Carbon graphite is used
As ink, while polymer as substrate. The thickness of layer
Achieved is 20 µm. The advantage of direct ink write over
Screen printing is that it is easy to control and mask-less
Process. Cao et al.  used novel micro-pen direct ink
Write deposition method to generate patterns of polyimide
On a quartz glass substrate. The patterns achieved have a
Thickness of 2–8 µm and 100–400 µm width and have wide
Application in the fabrication of MEMS devices. Sotiris
Et al.  fabricated chemiresistive sensors using inkjet
Printing. A gold colloid is used as ink, while thermally oxidized silicon as a substrate. Patterned electrode has gap of
5 µm with 20 µm width line. Li et al.  printed multilayer
Micro strip fractal patch antenna using inkjet printing. The
Antenna is integrated with conformal and flexible devices.
The antenna was bent over different curvatures of radii as:
50,100, 150, 200 and 250 mm, to assess the flexibility of the
Fabricated antenna. When the antenna bent with cylindrical
Radii of 100 mm and 200 mm, individually, the maximum
Increase of 0.3 and 0.4 dB in gain is detected. So it shows
Good consistency performance and specifies it’s potential
Conformal applications. Zhao et al.  fabricated strain
Sensors by an aerosol jet printing technique. The features
Achieved have a resolution less than 10 µm. Dingeldein et al.
 Have fabricated blanket layer of waveguide material and
Multimode waveguide cores using direct-write assembly.
The thicknesses of blanket layer guide material are 25 µm
To 220 µm, while for waveguide core circular cross section
Achieved is 50 µm, with total optical losses in the range of
0.06–0.09 dB cm−1 at 850 nm.
The aim of the present work is to generate reliable, durable and stable flexible patterns of conductive carbon paste
At micron levels for flexible electronic circuits. For this purpose, printing parameters of micro dispensing DIW are analyzed and optimized through Box–Menken design. Under
Optimize, parameter circuits of different shapes are printed
On flexible polyethylene terephthalate (PET) substrate. Morphological and electrical characterizations were performed
To study surface characteristics, conductive performance and
Response toward bending.
The system comprises of computer-controlled XY translational stage. An ink-loaded syringe barrel of 5 cc is attached
To the head assembly. The nozzle of 200 µm is used to generate the desired patterns with control architecture and configuration. The current design technique depends on generating
A pattern by extruding a continuous ink filament to substrate
Under pneumatic pressure mode. The same apparatus is also adopted in previously
Reported studies [10, 11].
Materials and methods:
The conductive material used in the present study is commercial carbon paste (Guangzhou Print Area Trading Co.,
Ltd). The paste is formulated for screen printing having
A viscosity of 35,000 centipoises. For the present study,
A slight modification is done to paste rheology as the following: 8 g of polyvinyl alcohol (PVA) bead is mixed with
40 ml of deionized cold water and kept on a mechanical
Shaker at room temperature for 1 h. The temperature of the
Solution is raised to 90 °C and placed on a magnetic stirrer
For 2 h, so that the PVA beads are fully dissolved. Then the
PVA solution is added to 40 g of conductive carbon paste
At room temperature and stirred continuously for 4 h to
Get a uniform homogenous paste. Finally, the conductive
Carbon paste based on PVA is ready to be used for printing. Particles in paste at 5 µm scale before sintering. The size
Of the carbon particle calculated is 6 µm. After sintering
The paste at 135 °C in a dry oven, the particles are tightly
Packed together due to the presence of PVA.
The viscosity of paste is 25,000 cps at room temperature and 75 rpm measured by remoter (Brookfield
RST plus Controlled Stress Remoter).
The shear thinning effect of the paste at 75 rpm. The purpose of the PVA is to improve mechanical strength and
Adhesiveness to the substrate.
Printing parameters and design of experiments:
The setting of printing parameters is essential to improve
The quality, productivity and cost of production of the
Design system. Printing input parameters selected for the
Study are pressure, substrate speed and gap size (distance
Between nozzle tip and surface of the substrate), while
Output (response variable) is pattern width. Input parameters are selected based on previous studies [10, 11, 27,
28, 34, 35]. Determining the optimal parameter is one of
Fig. 1 a Schematic of DIW system, b actual DIW apparatus the challenging tasks, which is beneficial for the industry.
Fig. 2SEM of carbon paste. A
Before sintering, b after sintering at 135 °C
6.1 Design of experiments based on response
Surface methodology (RSM)
Response surface methodology (RSM) is one of the statistical
Techniques that are valuable for modeling and analysis of a
Wide range of engineering problems. It facilitates experimental design and reduces the number of experiments to evaluate
Process parameters (input parameters) and their interaction.
The main objective of RSM is to optimize the response surface (output parameter) that is influenced by process variables
(Input parameters). In the present study, due to material limitations, it is beneficial to apply RSM as it reduces the experimental runs. So Box–Menken experimental design based on
RSM is employed to study the effect of input printing parameters (pressure, substrate speed and gap size) on the response
Variable (pattern width). Three levels that were established for
Each input printing parameters are given in Table 1. Levels are
Selected based on preliminary experiments. The experiments
Are designed in Minitab 17 as given in Table 2.
Setup as is used for experiments. The 5-cc
Syringe barrel is fully loaded with conductive carbon paste.
A stainless steel nozzle of 200 µm is used for dispensing.
Polyethylene substrate is placed on the worktable in such a
Way that it is at 90° to the nozzle. The printed patterns are
Cured at 135 °C for 15 min in a dry oven to remove solvents.
For each experimental run as given in Table 2, the pattern
Levels for printing parameters.
Substrate speed (mm/s), S
Gap size (mm), G
Table 2 Experimental design based on Box–Menken
Substrate Speed (Mm/s)
Gap size (mm)
Table 3 ANOVA for the width of the pattern
Adj. MS F
P × S
P × G
6.3Analysis of printing parameters and their
4.3.1 Main effect on pattern width and analysis of variance
The effect of printing input parameters on the width of the
Pattern has been determined using Box–Menken model.
The mathematical relationship between input parameters
And pattern width is obtained by using (1) and can be used
To predict patterns width within the control range.
Additionally, the adequacy and significance of the developed model have been determined using analysis of variance
(ANOVA) with 95% confidence interval (CI). The fitness of
Model is tested based on Fisher’s statistical test (F value),
Significance probability (P value), the coefficient of determination (R-square) and adjusted R-square. F value determines
Most influencing printing input parameter on the response
Variable, and P value indicates statistical significance of
Input parameter. In the present analysis, F value larger than
5.5 And P value less than 0.05 imply statistical significance.
R-square entails total variation in response estimated by
Model; the adjusted R-square is to check fitness and adequacy of the model. ANOVA result is given in Table 3. P
Values less than 0.05 depict that model terms are statistically
Significant. So pressure and substrate speed are significant,
While the gap size, quadratic terms and interaction terms are
Insignificant as given in Table 3. R-square value calculated
For the width of the pattern is 96%, which implies that data
W = 502 − 10.94P + 14.4S − 21G + 3.10S2 + 152G2
− 0.616P × S − 1.59P × G − 8S × G
Fig. 5Optimal parameters for
Fig. 6Normal probability plot for residuals
0 2 4 6 8 10
Pattern Width, W (µm)
Fig. 7Comparison between predicted and experimental values of
Are well suited for modeling. Adjusted R-square value of
90% supports the excellent correlation of experimental and
Figure 4 shows the main effects plot of printing parameters. The plot shows that both pressure and substrate speed
Are significantly contributing factors, but the former plays
An important role in decreasing pattern width since the slope
Of the line is steeper. From the plot, it is also evident that the
Mean width increases with increase in pressure. It is due to
The higher viscosity of paste and increased discharge rate at
High pressure . Moreover, the decreasing trend for substrate speed in the plot shows that pattern width decreases
With increase in substrate speed. These results occurred
Due to plastic behavior of paste. Increasing substrate speed
Causes an increase in shear stress on paste filament which
Results in lower pattern width . The plot further illustrates that influence of gap size on pattern width is insignificant as the line not sharper. By increasing the gap size
From 0.2 to 0.4 mm, the pattern width decreases and remains
Approximately constant from 0.4 to 0.6 mm. The same phenomena are described by Papanastasiou et al.  that the
Radius of filament decreases with developing length (gap
Size) exponentially; Rαe−L/2.
Optimal printing parameters are obtained by employing
Component desirability function (D) as shown in Fig. 5. The
Optimal width of the pattern that is 199 µm is achieved by
Keeping the pressure at 59 psi, substrate speed at 4 mm/s and
Gape size at 0.5 mm.
Additionally, Anderson–Darling (AD) statistical test is
Performed to check the normality of residuals. Figure 6
Shows the normality plot as the P value for the test is greater
Than the alpha value of 0.05, i.e., 0.987, so it shows that data
Are normally distributed and the procedure developed for the
Model is adequate.
Finally, new experimental runs were carried out to
Validate the performance of the fatted model. As many as
Fig. 8Printing of patterns
Fig. 9Printed circuits. A Foil grid, b rectangular spiral, c bend position of rectangular spiral circuit (top view), d bend position of rectangular spiral circuit (side view)
Fig. 10Characterization of printed patterns for circuits. An Optical
Microscopic image of the carbon-based conductive pattern of 198 µm
Average width, b SEM image of the surface topography of pattern, c
SEM image showing distribution of carbon particles in printed patterns
Ten experiments were conducted, and their results were
Compared with predicted values as shown in Fig. 7. As
The trends for predicted and experimental values are very
Close to each other, therefore, it is concluded that developed model based on Box–Menken design successfully
Predicts the width of pattern values for any combination
Of P, S, and G within the specified range of experimentation performed.
Printing of circuits:
Based on optimized parameters, circuit patterns of different shapes have been printed on a flexible PET substrate.
Figure 8 shows the printing of circuit patterns. Figure 9a,
B shows the foil grid and rectangular spiral circuit that is
Printed on a polyethylene substrate. The length to width
Dimensions of the two circuits is 2.5 cm× 2.5 cm. Figure 9c,
D shows top view and side view of bend positions of rectangular spiral circuits. Figure 10a shows the average width of
Patterns is 198 µm. Figure 10b, c shows the SEM image of
The surface topography of conductive carbon patterns and
Distribution of carbon particles in the paste. It is observed
That lines of conductive patterns are smooth and uniform,
And also carbon particles are well packed in paste due to the
Presence of PVA.
1. Electrical characterization of printed Circuits:
Current–voltage characteristic (I–V characteristic) of
Printed circuits is studied to evaluate its conductive performance. Figure 11 shows a schematic of the I–V measurement technique. A voltmeter is connected in parallel, while
Ammeter is connected in series with the printed circuit.
By varying voltage (0 V to 10 V) from DC power supply,
The voltage drop is measured across the printed circuits
Through the voltmeter. Figure 12 shows the I–V curve for
The two printed circuits after the first bend cycle and 500
Bend cycle as shown in Fig. 9. The curves show little voltage drop across each printed circuits after first and 500
Bend cycles, and also it illustrates that with increasing
Voltage, current increases linearly. The resistance values
After first and 500 bend cycle for foil grid pattern are 6 Ki
And 6.2 Ki, while for rectangular spiral is 5.5 Ki and
The sensitivity of circuits to bending is analyzed by
Studying its resistance response. The simple bending system
Fig. 11 Schematic of I–V measurement technique
Fig. 12I–V curve for patterns
Adopted in the study by Abbas and Rahman  is used. Figure 13a shows the actual bending fixture system, which
Comprises the fixed and movable side and a protractor for
Measuring bend angle. The substrate is mounted between
A fixed and movable side, so when the movable side slides
Inward, it starts bending. The approximate bending angle is
Measured using a protractor. Figure 13b shows schematic of
Bending fixture with bend angle position of PET substrate
At 10°, Fig. 13c shows bend angle position at 30°, Fig. 13d
Shows bend angle position at 50°, Fig. 13e shows bend angle
At 70°, and Fig. 13f shows maximum bend angle position of
PET substrate at 90°.
Figure 14 shows a comparison of resistance response of
Foil grid circuit printed from the modified paste and original
Paste after the first bend cycle and 500 bend cycles at different bending angles positions (0° to 90°). The surface
Morphology of circuit patterns is examined under SEM as
Shown in Fig. 15. The results show that the average resistance response time of modified and original paste after the
First bend cycle is approximately equal, i.e., 1.5 s. After the
First bend cycle, the surfaces of both circuits show no cracks
As shown in Fig. 15a, b. By subjecting the foil grid circuit
(Both modified paste and original paste) to 500 bend cycles,
The resistance response fluctuates. The average resistance
Response of the foil grid circuit based on modified paste
Is almost little affected as evident from comparison plot
In Fig. 14. The deviation is observed due to the formation
Of small micro cracks as shown in Fig. 15d. The foil grid
Fig. 13a Actual bending fixture setup, b schematic of bend angle position of PET substrate at 10°, c bend angle position at 30°, d bend angle
Position at 50°, e bend angle position at 70°, f bend angle position at 90°
Circuit of original paste shows resistance response for 0°
To 30° with an average response of 3 s but vanishes for
Succeeding bend angles due to distortion of the circuit. Figure 15c shows large micro cracks with defections in the foil
Grid circuit of original paste and shows greater deviation
In resistance response as apparent from comparison plot in
Fig. 14. So based on above analysis, it is evident that the
Printed circuits based on modified paste are reliable and
Durable for flexible electronics applications such as wearable electronics, RFID tags, antennas and collision detection in robots and cars.
Sensitivity of printed circuits from modified paste is
Studied at different bend angle positions ranging from 0°
To 90°. Ten samples of each type of circuit having length
To width dimensions of 2.5 cm × 2.5 cm, i.e., foil grid and
Rectangular spiral, are prepared for analysis, and their
Resistance response and recovery time are studied under
Bend angle ranging from 0° to 90°. Fig. 16 shows resistance response and recovery time of the foil grid circuits.
The average response time is 1.5 s, and recovery time is
1.8 s for the bend angles from 0° to 90°, and the average
Change in resistance observed is 6 Ki to 30 Ki. The change
In resistance occurs due to micro cracks formation. These
Micro cracks separate upon the extent of bend angle, for 10°
Bend angle the separation is little and shows a low change
In resistance from 6 to 10 Ki, and for 90° bend, angle
Separation is large and shows a more significant change in
Resistance, i.e., 6 Ki to 30 Ki. Similarly, the response and
Recovery time for the rectangular spiral circuit is also studied as shown in Fig. 17. The response and recovery time
Is almost similar to the foil grid circuit, and the average
Change in resistance examined is from 5.5 to 34 Ki. So the
Above results show that the printed circuits are repeatable
Fig. 14Comparison of resistance response of foil grid circuits fabricated from the modified and original paste
Fig. 15SEM images. A Patterns
Of original paste after first bend
Cycle, b patterns of modified
Paste after first bend cycle, c
Formation of micro cracks in
Patterns of original paste after
500 bend cycles, d formation
Of micro cracks in patterns of
Modified paste after 500 bend
And stable in performance and are justifiable for bend/flexible electronics applications.
The micro dispensing DIW technique can be used for the
Fabrication of low-cost and flexible electronic devices and
PCBs. In the present study, the effect of printing parameters
Such as pressure, substrate speed and gap size on pattern
Width is analyzed and optimized based on Box–Menken
Design to achieve high-quality print. Nozzle with an internal
Diameter of 200 µm is used for printing. Printing material
Is commercial conductive carbon paste. A slight modification is done to paste rheology by first preparing polyvinyl
Alcohol (PVA) solution. The PVA solution is obtained by
Mixing 8 g of polyvinyl alcohol (PVA) bead with 40 ml of
Deionized cold water and kept on a mechanical shaker at
Room temperature for 1 h. The temperature of the solution
Is raised to 90 °C and placed on a magnetic stirrer for 2 h,
So that the PVA beads are fully dissolved. Finally, the PVA
Solution is added to 40 g of conductive carbon paste at room
Temperature and stirred continuously for 4 h to get a uniform
Homogenous carbon paste. Optimal parameters for printing
Are the pressure of 59 psi, substrate speed of 4 mm/s and a
Gap size of 0.5 mm. The minimum width of pattern achieved
Is 198 µm. ANOVA results suggested that the most influencing parameter is pressure followed by substrate speed.
Under optimized parameters, foil grid and rectangular spiral
Circuits are printed on flexible PET substrate. The resistance
Response of two foil grid circuits, one printed from the original paste and the second from modified paste, is compared
After 1 bend cycle and 500 bend cycles. The results revealed
That after one bend cycle the performance of circuits is good,
But after 500 bend cycles, the performance of foil grid circuit
Based on modified paste is much more stable in performance
Compared to the foil grid circuit based on original paste. The
Sensitivity of the foil grid and rectangular spiral circuit based
On modified paste is studied under bend angles ranging from
0° to 90° through resistance response. The average resistance response time and recovery time for the two circuits
Are 1.5 s and are 1.8 s. Therefore, the present study revealed
That the circuits fabricated from conductive carbon paste
Through micro dispensing DIW technique are economical,
Stable in performance and are suitable for flexible electronics applications.
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