Adhesive Stretchable Printed Conductive Thin Film Patterns on PDMS

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Adhesive Stretchable Printed Conductive Thin Film Patterns on PDMS Surface with an Atmospheric Plasma Treatment Chun-Yi Li, and Ying-Chih Liao ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02844 • Publication Date (Web): 15 Apr 2016 Downloaded from http://pubs.acs.org on April 23, 2016

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ACS Applied Materials & Interfaces

Adhesive Stretchable Printed Conductive Thin Film Patterns on PDMS Surface with an Atmospheric Plasma Treatment Chun-Yi Li and Ying-Chih Liao* Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, 10617, R.O.C

Abstract In this study, a plasma surface modification with printing process was developed to fabricate printed flexible conductor patterns or devices directly on polydimethylsiloxane (PDMS) surface. An atmospheric plasma treatment was first used to oxidize the PDMS surface and create a hydrophilic silica surface layer, which was confirmed with photoelectron spectra. The plasma operating parameters, such as gas types and plasma powers, were optimized to obtain surface silica layers with the longest life time. Conductive paste with epoxy resin was screen-printed on the plasma-treated PDMS surface to fabricate flexible conductive tracks. As a result of the strong binding forces between epoxy resin and the silica surface layer, the printed patterns showed great adhesion on PDMS and were undamaged after several stringent adhesion tests. The printed conductive tracks showed strong mechanical stability and exhibited great electric conductivity under bending, twisting, and stretching conditions. Finally, a printed

* Author to whom the correspondence should be addressed. Telephone: 886-2-3366-9688, e-mail: [email protected]. ACS Paragon Plus Environment


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pressure sensor with good sensitivity and a fast response time was fabricated to demonstrate the capability of this method for the realization of printed electronic devices. Keywords: adhesion, stretchable electronics, conductive thin film, PDMS, atmospheric plasma treatment, flexible printed electronics

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Introduction The recent advancements in electronic materials on unconventional forms of foldable and stretchable substrates have opened a new prospect in the field of future electronics. These new functional devices, such as flexible circuitries,1,

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sensors,3-5

displays,6 and artificial skins,7-9 can provide reliable performance under bending, stretching, or twisting conditions with large deformation. To achieve high flexibility, this new class of electronic devices is fabricated on plastic substrates, such as polyethylene (PET), polyurethane (PU) and polydimethylsilane (PDMS). Among these flexible materials, PDMS shows a great potential due to its chemical inertness and is soft enough to fit smoothly onto surfaces of objects with complicated shapes. Moreover, the good elasticity of PDMS can also allow the fabrication of stretchable electronic devices with complex geometries.10-13 With the help of recent advances in nanotechnology, these flexible or stretchable sensors not only can be used in portable electronic devices,

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but are also applicable to biomedical diagnoses.3 On the other hand, to fabricate the electronic devices on these flexible plastic materials, recently, printing methods, or socalled printed electronics,14 are widely adopted for their fast fabrication speed at low costs. Because conductive tracks are essential in all electronic circuits, conductive thin film patterns on flexible substrates are required for the realization of printed flexible electronic devices. However, because PDMS is a hydrophobic and chemically inert material, printed materials cannot adhere well on PDMS without pre-treatments and therefore it is difficult to print electronic circuits directly on PDMS. To improve the adhesion between metal inks and plastic surfaces, three main strategies are commonly used: (i) print a primer ink layer as a glue to adhere the substrate


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and the conductive ink,15 (ii) change the binder in conductive inks,16 and (iii) apply surface modification, such as plasma treatments.17 Primer and binder formulation have been extensively studied in the literature.18-20 However, complicated synthetic procedures are usually needed to prepare these materials. Furthermore, a large-area coating facility is usually necessary to spray primers over PDMS before printing the conductive thin films. Thus, surface modification, which can quickly result in effective affinity for printed layers on PDMS, has recently received much attention.21 To print conductive tracks, silver or carbon pastes mixed with binders or adhesives are usually applied to substrates so that the printed thin films can adhere firmly on the applied surfaces. One of the most commonly-used materials is epoxy resin. After thermal post-treatments, epoxy starts to polymerize and can react quickly with hydroxyl or carbonyl groups22 to provide strong bonding on substrates, and create strongly adhesive conductive patterns on glass or plastic surfaces. Therefore, to apply conductive pastes on PDMS directly without ink formula changes, the PDMS surface needs to be modified with the aforementioned functional groups. Among numerous surface modification methods, plasma treatments have shown their great capability and effectiveness to readily prime surfaces for better adhesion.23 The oxidation power provided by plasma treatments can help generating various functional groups on plastic surfaces, and thus provide anchoring points for the printed materials.24 Regular plasma treatments, however, usually need expansive and inconvenient vacuum facility. To reduce the cost while maintaining similar surface improvement ability, recently, an atmospheric-pressure plasma jet (APPJ) device based on dielectric barrier discharge have been developed to effectively functionalize PDMS


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surfaces.25 With the help of a motorized stage, APPJ can be scalable not only to large areas but also for a selected patterned area without using masks. In this study, we take the advantage of surface functionalization ability of APPJ to modify PDMS substrates at regular lab conditions. The surface characteristics of the modified PDMS surfaces will be carefully examined to optimize plasma operational parameters. Screen printing process is then used to fabricate conductive patterns on the modified PDMS surfaces. The adhesive strength and interfacial morphology between the conductive layer and the modified PDMS substrates will be carefully examined. The electrical and mechanical stability of the printed conductors will be characterized to evaluate the possibility of this method on printing flexible electronic devices.

Experimental section Surface modification of PDMS PDMS (Dow Corning, Sylgard 184A) solution was prepared by mixing 10:1 (w/w) monomer/curing agents. 20 g of the mixture was then poured onto an A4-size PET thin film (universal film Japan). The sample was then degased for 40 minutes under vacuum, and subsequently cured in an oven at 70oC for 2 hours. The resulting PDMS sheets (thickness of ~100µm) were peeled off from the PET back sheet. A commercial atmospheric pressure plasma system (AP Plasma Jet, Feng Tien Electronic Co. Ltd., Taiwan) was used to modify the PDMS surface with an operating power of 1 kW for 3 seconds. The modified PDMS samples were then used in the screen printing process (Figure 1).


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Contact angle measurement Static contact angle measurements were done on polymer surfaces before and after plasma treatment to determine the changes in the surface wettability by the pendant drop analysis at 25 oC. To avoid inaccurate results from the fast hydrophobic recovery of plasma-treated PDMS, a 5 µL water droplet was placed on the measured surfaces for 5 minutes, and the contact angle was observed with in-house goniometer. Each contact angle measurement reported in this work is the average of at least five different positions on the PDMS sample surface.

Adhesion tests To test the adhesion of printed silver tracks on the different PDMS substrates, 3 methods were applied in parallel to the patterns on untreated or plasma-treated PDMS sheets: (i) soaking test (put samples in a cup of water for 2 hours at room temperature), (ii) ASTM D3359 tape test (tape and peel with 3M scotch tapes) and (iii) an ultra-sonication test (put samples in an ultrasonicator (DC300H, Delta, Taiwan) for 10 minutes).

Surface characterization The chemical compositional changes on PDMS surfaces after the plasma treatment were analyzed by X-ray photoelectron spectroscopy (XPS) measurements with an ESCA-2000 Multilab apparatus (VG Microtech) equipped with Mg–Al anode. The analysis was performed using Mg kα (1253.6 eV) excitation source at a take-off angle of 30°. The surface phase images were checked by atomic force microscopy (Bruker, Dimension Edge) in tapping mode using a cut-off length of 3 µm and a lateral resolution. The


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microstructures of the conductive thin films were examined with scanning electron microscopy (Nova NanoSEM 230, FEI, USA) with an accelerating voltage of 5 kV and 10 kV.

Screen Printing A commercial screen printer (Chi Long Technology Co., Taiwan) was used to print conductive patterns on the PDMS surfaces at a printing speed of 2 cm/s. The stainless steel screen printing mesh has a size of 26 cm × 32 cm, 400 mesh counts, mesh tension at 26±2 N, and a mesh angle of 22.5o. A silver paste (Dupont PE-872) was printed and dried at 100oC for 30 minutes. The sheet resistance of the printed patterns on both treated and un-treated PDMS after drying was about 20 Ω/sq.

Electrical characterization The electrical resistance of the printed tracks was measured at room temperature in a DC regime by a Keithley 2635A multimeter. At least three samples were measured under the same conditions to obtain sufficient data.

Results and Discussion Contact angle analysis The plasma treatment shows strong effects on modifying PDMS surfaces, which can be observed from the wettability differences. After plasma treatments, the chemical composition of the PDMS surfaces are modified dramatically and thus result in much smaller contact angles with water (Figure 2(a)). The pristine PDMS surface shows a


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water contact angle of 95 degrees. After the plasma treatment, the contact angle reduces quickly to less than 10 degrees. Several key parameters in the plasma treatments, such as gas types, scanning times, and power, have been altered separately to obtain the optimum plasma condition for the surface modification. Figure 2(b) reports the contact angle evolution after plasma treatments with different gases. Both nitrogen and oxygen plasma strongly modified the hydrophobic PDMS into hydrophilic state due to strong surface oxidation. Besides the chemical structural changes on PDMS polymeric surfaces given by the plasma ablation, cracks after plasma treatment (Figure S1), although invisible to naked eyes, could also improve the wettability when compared with untreated substrate, 26

despite the slight decreases in roughness (Table S1).

However, the polymeric

structural changes are temporary, as shown in Figure 2(b), the contact angle starts to increase with time and returns to its original hydrophobic state (~95o) after a few hours. Between the three gases used, the hydrophilicity of PDMS after nitrogen plasma can last longer possibly due to the higher oxidation power of nitrogen plasma.22, 26 The contact angle on nitrogen plasma treated samples remains at 35o after 2 hours, while the one with oxygen or argon plasma treatment returned to its original hydrophobic state (95o). The plasma power and scanning times, on the other hand, show little effect on surface modification. As depicted in Figure 2(c), the surface hydrophilicity evolution is virtually the same regardless of more applied plasma power or longer plasma treatments. This also indicates that this plasma treatment can only effectively oxidize the PDMS surfaces without further decomposing the bulk materials.

Surface chemistry


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The drastic changes in contact angle after plasma treatment are attributed to the changes in the elemental composition variations on the modified PDMS surfaces. To detect the material differences, the surface of a PDMS sheet, which is partly treated by the nitrogen plasma jet, is probed by AFM in tapping mode (Figure 3(a)). The phase lag image shows the elastic modulus contrast at the PDMS surface, and one can easily observe a clear demarcation between the plasma-treated (on the top) and pristine PDMS (bottom part of the first image), indicating that there is a local change in the material properties at the plasma-treated surface. As time progresses, the PDMS starts to recover the hydrophobic nature of higher phase lag, and also reflects in the roughness as well (Figure S2).

The

chemical composition of this layer is further assessed by X-ray photoelectron spectroscopy.26 The presence of methyl groups on the pristine PDMS surface is the major reason for the surface hydrophobicity. After plasma treatments, the progressive plasma oxidation leads to a thin silica-like layer on the PDMS surface,26 which can be detected by the Si-2p peak shift (Figure 3(b)). For pristine PDMS, the Si-2p peak has a peak value of 101.5 eV and a half-maximum width of 2.6 eV. After the plasma, the peak value shifts to 103.5 eV, which is consistent with that of silica, and the half-maximum width broadens to 4.0 eV. Besides, the elemental analysis of the modified PDMS surfaces (Table 1) also shows that the carbon content decreases after plasma treatment while oxygen content increases. The increased O/C ratio indicates that oxygen containing groups formed on the surface of PDMS as a result of plasma treatment. This thin silicalike layer, however, cannot last for a long time due to the migration or re-orientation of untreated polymer chains from the bulk.26 As depicted in the XPS composition analysis (Table 1), the oxygen content reduced from 40% to the original level of ~30% after 6


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hours in the ambient air, showing the disappearance of the silica-like properties. This surface compositional change leads to hydrophobic recovery and reflects in the increasing contact angles in Figure 2. In particular, the oxygen content on the N2 plasma treated surface is the highest, indicating a higher coverage of silica-like layer is generated. Hence, it takes a longer time for the silica layer disappearance and a longer hydrophobic recovery is observed than that on O2 plasma treated PDMS, as indicated in Figure 2.

Adhesion test The thin silica-like layer produced by plasma treatment can provide active sites for binder polymers in the silver paste, and improve the surface adhesion of the printed conducting patterns. In the commercial silver paste, one of the major components is epoxy resin, which can bind strongly on silica surfaces.27 For pristine PDMS, most of the surface is covered with methyl group and is hydrophobic. This results in barren printed patterns. As shown in Figure 4, the printed silver patterns have bad adhesion on the untreated PDMS surfaces, and can be easily peeled off in the soaking, tape, or sonication tests. On the other hand, with plasma treatments, the adhesion between the PDMS substrates and printed silver patterns are strong enough so that the printed patterns not only show precise patterning with excellent continuity but also remain undamaged in all the tests. The adhesion also have a prolong stability due to the effective chemical bonding. The printed patterns on plasma treated PDMS are stored for a week, and show no damages after adhesion tests, same as those in Figure 4. On the other hand, if one prints the patterns after silica layer disappears from PDMS surface, i.e. the PDMS recovers to its hydrophobic state, the adhesion of printed patterns behaves just exactly the same as those


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on pristine PDMS surfaces. However, the printed patterns show good adhesion on the PDMS which is stored for 2 hours after N2 plasma treatment, indicating that silica-like layer still exists before total hydrophobic recovery. For the sake of good adhesion and long-time duration, in the following experiments, the nitrogen plasma-treated PDMS was used for device fabrication.

Cross-sectional morphology To further investigate the interface between the printed silver pattern and PDMS, the conductive thin film was examined by SEM. From the SEM images (Figure 5), Ag nanoparticles are closely stacked to form compact thin films over both untreated and treated PDMS. However, there was the interspace between the silver pattern and untreated PDMS, indicating little adhesion on the untreated PDMS surface. In contrast, the sample on plasma-treated PDMS surfaces shows no interspace between the PDMS substrate and silver pattern. Moreover, because the hydrophilic silica layer produced by strong plasma oxidation, the silver nanoparticles surrounded by epoxy resin bind pretty well to the PDMS surfaces, and the printed layer stacks adhesively on the PDMS substrate (Figure 5(c)) with a seamless boundary.

Electrical performances and applications The printed silver conductive tracks exhibit great mechanical stability. The resistance of the printed tracks shows no variation after bending for 300 cycles and taping for 10 times (Figure 6(a-b)), indicating great adhesion of the printed layers on the plasma-treated


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PDMS substrates. Moreover, as shown in Figure 6 (c), the printed stretchable tracks can be stretched up to 100% with a resistance increase of around 2 times with little hysteresis in electrical conductivity. The good mechanical stability of the printed conductive tracks can be utilized for various electronic applications. As shown in Figure 7, the good mechanical stability of the printed conductive tracks can be utilized for various electronic applications. As shown in Figure 7, the printed tracks are connected to LEDs. The tracks remain highly conductive no matter being bent, twisted, or even stretched, and therefore the LEDs show nearly the same brightness in these deformation processes. Besides acting as conductive tracks, the printed conductive patterns can also serve as pressure sensors by stacking two printed pads with silver tracks (Figure 8(a). Because the contact area between the two pads increases after pressurization,28 the overall resistance of the sensor decreases linearly to the applied pressure with a fast response time of 0.5 ms and a sensitivity of 0.32 percent per kPa (Figure 8(c)), which are close to those reported in the literature.29 The sensor can also be used multiple times with fast and reliable responses (Figure 8(d)). With an applied pressure of 10 N/cm2, the measured current jumps quickly from the background current of 0.05A to a higher level of 0.27A within 0.5 ms, and can be repeated for many times. This fast response time and reliable repeatability indicate that the printed sensors produced by this technique can be further applied to wearable electronics.

Conclusions In this study, a simple surface modification approach to improve the printed layer adhesion on PDMS is developed with an atmospheric plasma treatment. After the plasma


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treatment, a thin silica-like layer is produced on the PDMS surface and leads to a much better wettability. The effect of plasma treatment can last for hours so that conductive pastes can be directly printed on modified PDMS in before the hydrophobic recovery. The epoxy binder in the silver paste can adhere well to the surface silica layer, and thus the printed patterns on PDMS show great adhesion and preserve original shapes after several stringent adhesion tests. The printed conductors show great mechanical stability, and remain conductive under bending, twisting, and stretching conditions. An exemplar printed pressure sensing device is also fabricated to demonstrate the capability of this method. In summary, this study shows a simple way to modify PDMS surface for printing conductive thin film with specific pattern, and can be employed for pressure sensing application. The surface modification with printing process for flexible conductors exhibit the possibility of printed devices on PDMS elastomers and opens a new avenue to realize future wearable electronic devices.30

Acknowledgements The authors are grateful for the financial support to this research from Ministry of Science and Technology (MOST) in Taiwan.

Supporting information Surface topology, phase images, and surface roughness from AFM analyses. This information is available free of charge via the Internet at http://pubs.acs.org/.


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FIGURE CAPTIONS Figure 1 The schematic diagram of the printing process with an atmospheric pressure plasma system.

Figure 2 (a) A 5-µL water droplet on PDMS surface before and after plasma treatment. (b) Time evolution of water contact angles on plasma-treated PDMS surface with various gases. (c) Time evolution of water contact angles on nitrogen-plasma-treated PDMS surface under various operational conditions.

Figure 3 (a) AFM phase images of the PDMS surface after plasma treatment with nitrogen gas. (b) The Si-2p photoelectron spectra of the PDMS surface before and after plasma treatment.

Figure 4 Adhesion tests of 0.5 cm square silver patterns on PDMS with various treatments. The plasma treatment conditions are 1 kW power with 10 sccm gas flow rate.

Figure 5 SEM images of printed silver paste layers on (a) untreated and (b-c) nitrogen plasma treated PDMS substrates.

Figure 6 Resistance changes in printed tracks with (a) bending test (radius curvature of 5 mm), (b) tape test, and (c) strain test (with a stretching velocity of 1 mm/s).

Figure 7 Demonstration of printed conductive tracks under (a) bending, (b) twisting, and (c) stretching conditions.

Figure 8 (a) Pressure sensors assembled with two printed pads with silver tracks. The size of the sensor is 5 mm × 5 mm. (b) Schematic diagram for pressure sensing measurements: the sensor is put under a load cell for pressure measurement and connected to a multimeter for resistance measurements. (c) Resistance variation of the pressure sensor with applied pressure. (d) Response test of the pressure sensor: the current increases once the sensor is pressed and drops to background current right after release. The response time is 0.5 ms with an applied pressure of 10 N/cm2.


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Figure 1


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Figure 2 (a)

(b)

Contact Angle(degree)

100 80 60 40 Nitrogen-treated Oxygen-treatdd Argon-treated Untreated

20 0 0

2

4

6

8

10

Time after Plasma Treatment(hr) (c)

100

Contact Angle(degree)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 60 40 20

Nitrogen-plasma(1times 1kW) Nitrogen-plasma(2times 1kW) Nitrogen-plasma(1times 2kW)

0 0

2

4

6

8

10

Time after Plasma Treatment(hr)


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Figure 3 (a)

(b)


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Figure 4


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Figure 5


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Figure 6


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Figure 7


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Figure 8


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TABLE CAPTIONS Table1. Atomic composition of untreated and plasma-treated PDMS from XPS analysis


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Table1. Atomic composition of untreated and plasma-treated PDMS from XPS analysis Sample

Silicon(%)

Oxygen(%)

Carbon(%)

O/C

O/Si

Untreated

21.09

31.42

47.49

0.66

1.48

N2 plasma

20.45

40.63

38.92

1.04

1.98

O2 plasma

20.10

34.51

45.39

0.76

1.71

Recovery

20.85

31.43

47.72

0.65

1.50


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TOC


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Adhesive Stretchable Printed Conductive Thin Film Patterns on PDMS

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