Highly stretchable conductive polypyrrole film on a three dimensional porous polydimethylsiloxane surface fabricated by a simple soft lithography process

Abstract

Highly stretchable conductive materials provide unique advantages for flexible electronics and many other advanced fields. In this study, we create an elastic porous polydimethylsiloxane (p-PDMS) highly stretchable conductive substrate. The p-PDMS surface is fabricated by a simple soft lithography process that replicates the 3D corrugated porous microstructures from a low-cost commercially available abrasive paper. Conductive polypyrrole (PPy) is polymerized on the p-PDMS surface by UV/Ozone (UVO) surface treatment to create the highly stretchable conductive PPy/p-PDMS film. The PPy/p-PDMS film shows a high stretchability maximum up to 80% strain. PPy/p-PDMS electrical properties based on the effects of critical PPy/p-PDMS process parameters such as UVO treatment time, deposition time, and abrasive paper grit size are evaluated in this paper. Results indicate that the highest electrical conductivity of 34.9 S m⁻¹ is produced from the optimized PPy/p-PDMS process conditions. Reliability testing expressed through cyclical bending and stretching of PPy/p-PDMS films, up to 1000 cycles, is also reported as good PPy/p-PDMS repeatability with maximum 5% (bending) and 36% (20% strain stretching) resistance increases after 1000 repeating cycles.

---

1 Introduction

Conductive materials with high flexibility and stretchability have recently gained much attention in both academia and industry. These stretchable conductive materials have broad applications that can be used in flexible displays or stretchable electronics¹ and also enable many new applications such as high sensitive pressure sensors,² energy harvesting,³ microelectromechanical system (MEMS),⁴ dielectric elastomer actuators,⁵ neuronal/muscular surface interfacing,⁶ biofuel cells⁷ and cell mechanobiology.⁸ Various conducting materials have been demonstrated in stretchable conductive applications. Metals (i.e., gold,^6,9–14 silver^15–17 and silver nanowires (AgNWs)^18–20), inorganic materials (i.e., carbon nanotube (CNT)^21–24 and graphene^22–25), and conductive polymer materials (i.e., polypyrrole (PPy), poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS)^8,26 and polyazomethine (PAZ)²⁷) have been demonstrated, for example, in stretchable conductive material applications. These materials show good electrical conductivity, however, they are less resistant to high stretching conditions unless they are specially designed or engineered for enhanced stretchability.

Polydimethylsiloxane (PDMS) is an elastic polymer that has been widely used in many applications such as protective insulation coating, microfluidics, and polymer MEMS. Although PDMS itself is not a conducting material, the low-modulus PDMS material offers an excellent substrate choice for highly stretchable electronic applications. PDMS has been demonstrated as a stretchable substrate by several approaches. One straightforward method to create a stretchable conductive PDMS device is to embed the gold electronic pads between the PDMS layers.^6,9 Additionally, using the sandwiched PDMS structure to support a conductive layer, conductive films can also directly attach to the 2-dimensional (2D) PDMS surface by surface modification technique. Several surface modification techniques are proposed to create the stretchable conductive device on PDMS. Zhou et al. demonstrate this using a 3-mercaptopropyl-trimethoxysilane modification to covalently bond a gold layer to a PDMS substrate.¹⁰ Gold track,¹¹ gold/chrome metallized mesa features,¹² and screen printed conductive silver paste¹⁵ have been successfully attached to the PDMS surface as stretchable conductive material by plasma treatment. AgNWs can attach to the PDMS surface by silane treatment,¹⁸oxygen plasma modification¹⁹ or coating aerogel on the PDMS surface²⁰ as an AgNWs/PDMS based stretchable electrode. Gold layers also have been directly deposited onto the CNT/PDMS composite substrate.¹³

Another method to create a conductive stretchable layer using PDMS is by composite approach.²⁸ The polymer composite consists of PDMS base and conductive material in order to combine the functions of stretchability and conductivity. Multi-walled carbon nanotubes (MWCNT),²¹ sliver micropowder,¹⁶ PEDOT:PSS polymer,²⁶ or PAZ micropowder²⁷ can be mixed with PDMS to form stretchable conductive polymers. Graphene/MWCNT aerogel backfilled into the PDMS matrix for stretchable composite is also demonstrated.²² Liquid metal can also be used by filled into the PDMS microchannel²⁹ forming the stretchable microstructure.

Those surface modifications or composite type methods are an effective way to fabricate the stretchable conductive material and can normally reach a maximum stretchability normally reach around 50%. This value meets most stretchable electronic needs (<30%),³⁰ for high stretchability requirement, a 3-dimensional (3D) method is developed by constructing a 3D structure to provide high stretchability of conductive layer. The 3D surface provides extra freedom of movement during stretching and prevents conductive films fracture in 2D flat surface. For example: Chou et al. fabricate gold electrode out-of-plane in 3D space to enhance the flexibility and stretchability on a PDMS substrate.¹⁴ Chen and Duan et al. fabricated a 3D scaffold filling with CNT/graphene by nickel foam replication²³ or 3D printing²⁴ approach. Creating corrugated structures is also an effective way to fabricate 3D stretchable structures. The corrugated structures are generally created by releasing a prestretched material to generate micro-wrinkled structures. Thus, an intact conductive pathway can be maintained under stretching. Graphene oxide,²⁵ PEDOT:PSS⁸ and sliver thin film¹⁷ corrugated structures are demonstrated on the PDMS substrate as stretchable material. High stretchability (>80%) for a wrinkled Au electrode,¹⁷ corrugated graphene oxide,²⁵ and 3D CNT/graphene^23,24 have been reported.

Polypyrrole (PPy) has been used recently in stretchable electronics applications with good electrical properties and stability in air.^31,32 Thus, in this research, instead of creating a corrugated 3D structure on the conductive material, we demonstrate creating the 3D porous PDMS substrate by a simple soft lithography using low-cost conventional available abrasive paper. The conductive polymer (PPy) is directly polymerized onto the 3D porous PDMS (p-PDMS) as corrugated polypyrrole porous polydimethylsiloxane (PPy/p-PDMS) surface producing a highly stretchable and conductive film. Critical process parameter effects and repeated stretching/bending tests of the PPy/p-PDMS surface are presented in the paper.

2 Experimental section

2.1 Materials and reagents

PDMS (Sylgard 184 silicone elastomer kit) was purchased from Dow Corning Corp. (MI, USA). A 10 cm diameter P-type (100) silicon wafer was purchased from Summit-Tech Resource Corp. (Hsinchu, Taiwan). Pyrrole (Py, 99% extra pure) and ammonium persulfate (APS) were purchased from Acros (Geel, Belgium). P180, P400, and P600 grit size abrasive papers were purchased from Sankyo-Rikagaku Co., Ltd. (Okegawa, Japan).

2.2 SEM analysis

The PPy/p-PDMS and p-PDMS sample surface morphology were characterized by scanning electron microscope (SEM) images. The top and cross-sectional SEM images were obtained using Hitachi S800 at Instrumentation Center at National Central University, Taiwan. The samples were cut into 5 × 5 mm size by razor blade and load into the SEM. The SEM was operated under a working pressure of 4 × 10⁻⁶ torr.

2.3 Contact angle measurement

The water contact angle measurement in this research was conducted by contact angle measuring system (OCA 15 EC, DataPhysics Instruments GmbH, German). The p-PDMS sample was placed on the measurement stage and a 5 μL microdroplet was deposited on the sample. The contact angle value was recorded by the automatic measurement system (SCA software for OCA, DataPhysics Instruments GmbH, Germany).

3 Results and discussion

3.1 Preparation of PPy/p-PDMS film

Schematic illustration of the PPy/p-PDMS fabrication procedure was illustrated in Fig. 1.

Fig. 1 Fabrication process chart of PPy/p-PDMS film. (a) Attach abrasive paper to silicon substrate (b) spin coat PDMS on abrasive paper (c) cut and release the PDMS to generate porous morphology PDMS surface (p-PDMS) (d) UVO treatment followed by Py polymerization (e) form stretchable conductive PPy/p-PDMS film.

The p-PDMS layer replication was based on the standard soft lithography process³³ which was prepared by mixing the PDMS base with curing agent at a 10 : 1 weight ratio and degassed in a vacuum desiccator below 70 cm-Hg gauge pressure (PC-250, Yeong-shin, Taiwan) to remove the air bubbles. The PDMS reagent was then spin coated onto the abrasive paper at 100 rpm for 20 seconds (SPC-703, Yi Yang Co., Taiwan) and cured on a hot plate at 70 °C for 4 hours (Super-Nuova, Thermo Scientific Inc., USA) to cast the abrasive paper porous microstructures onto the PDMS surface as p-PDMS surface. The p-PDMS layer was removed from the abrasive paper and cut by razor blade to obtain a 76.2 mm (L) × 25.4 mm (W) PDMS sample. Lastly, an N₂ gun was used to clean the surface. The thickness of the p-PDMS layer was measured to be around 600 μm. It was noted that the abrasive paper was tape-fixed to a silicon wafer to ensure surface rigidity during spin coating and curing process. After PDMS soft lithography procedure, the p-PDMS sheet was placed in the UV/Ozone (UVO) chamber (PSD-UV, Novascan Technologies, IA, USA) to be treated for subsequent PPy deposition. PPy was synthesis by the Py monomer and APS. Different Py concentrations of 0.1 M, 0.3 M, 0.5 M and 1.0 M to 0.1 M APS have been tested. Results showed that the appearance of PPy film became darker with lower electrical resistance in higher Py concentrations.³⁴ However, we also observed excessive PPy particles resides on the PDMS surface at highest Py concentration of 1.0 M. Therefore, optimized Py concentration of 0.5 M was selected in the following experiments. The UVO treated p-PDMS sheet was immersed in the 0.5 M Py and 0.1 M APS mixture (1 : 1 v/v) for 40 minutes to polymerize the Py onto the p-PDMS layer as PPy/p-PDMS. Finally, the PPy/p-PDMS sheet was put in the ultrasonic sonicator DIW bath (Ultrasonic cleaner D150, Hi Sun Instrument Co. Ltd., Taiwan) for 10 minutes to remove the unreacted reagent and clean the PPy/p-PDMS surface. An N₂ blow-dry clean followed to ensure full removal of water moisture from the PPy/p-PDMS surface.

Replication of p-PDMS from commercially available abrasive paper presents a rapid, simple and low-cost approach to fabricate 3D corrugated microstructures without the need for high-end microfabrication facilities. PDMS is also a high resolution elastic material that can replicate feature sizes down to nanoscale.³⁵ Fig. 2 shows p-PDMS SEM images casting from P180, P400, and P600 grit size abrasive paper prior to PPy coating. A rough and corrugated 3D p-PDMS surface is successfully created by the soft lithography method from the abrasive paper. From the SEM images in Fig. 2, it is observed that the pore size of P180 p-PDMS surface (Fig. 2a and d) is measured approximately 50–100 μm in diameter and ∼22 μm in depth. The p-PDMS surface replicated from finer abrasive papers reveals smaller pore sizes. Grit size P400 p-PDMS (Fig. 2b and e) shows smaller 25–50 μm diameter pore size and ∼13 μm in depth. Grit size P600 p-PDMS (Fig. 2c and f) shows smallest pore size of 10–50 μm in diameter and ∼8 μm depth. We also observe several 1–5 μm “rough” PDMS features uniformly distributed on the p-PDMS as indicated in the enlarged SEM images in bottom-left corner of Fig. 2. These rough features are replicates from the pin-holes or voids of the abrasive paper. In the Fig. 2 cross-sectional images, the surface morphology of p-PDMS shows “wavy” surface profile (as plotted in red dash-lines) for all grit sizes of P180, P400 and P600. The p-PDMS surface height variation decreases for finer grit size abrasive papers from ∼22 μm (P180) to ∼8 μm (P600). All of the p-PDMS film present 3D corrugated surfaces that provide freedom for deformation when the p-PDMS is subjected to lateral stretching.

Fig. 2 Top (a–c) and 30° angle cross-sectional (d–f) SEM images of p-PDMS surface prior to PPy deposition replicate from P180 (a and d), P400 (b and e) and P600 (c and f) grit size abrasive paper. Bottom-left corner shows enlarged SEM images.

The p-PDMS structures replicated from the abrasive paper with microscale porous surface morphology result in a higher surface hydrophobicity. Compared to a native flat 2D PDMS surface of 97°, the p-PDMS surface exhibits a higher hydrophobicity around 130°. Such high hydrophobicity hinders the ability of the Py monomers diffuse into the micropores. Thus, prior to the PPy deposition, the p-PDMS sheet is treated with UVO to render the surface hydrophilic. Even with UVO treatment, we found the PPy can't be effectively deposited onto the native 2D PDMS surface without porous microstructures. As shown in the top SEM image of a native 2D flat PDMS surface without porous microstructures (Fig. 3a), no PPy films is observed on the native 2D PDMS surface, only several 1–8 μm size PPy particles residue on the PDMS surface. When the Py monomers dip into the p-PDMS surface with UVO treatment, there is an improved deposition of the PPy layer on the p-PDMS surface. As shown in the top and 30° angle cross-sectional SEM images in Fig. 3b and c, it is clearly observed that PPy successfully coats onto the p-PDMS surface. The PPy layer exhibits a flake-like morphology uniformly polymerized on the p-PDMS surface. The appearance of the PPy/p-PDMS surface that exhibits a black color is shown in Fig. 3d. These results suggest that the porous microstructures on p-PDMS surfaces likely not only enhance the PDMS stretchability for PPy/p-PDMS film but also increase their contact with Py monomers and physically trap the polymerized PPy, which eventually stably maintain PPy coating. To prove it, the coated PPy film was also scratched from PDMS substrate for FTIR analysis, and the spectrum of it demonstrated typical absorbance as those we investigated in our previous report (data not shown).³⁴ It supports our hypothesis that PPy film should not be covalently bound onto PDMS but immobilized through physical entanglement.

Fig. 3 SEM images of PPy deposition on the native PDMS and p-PDMS surface. (a) Shows top SEM view of native 2D flat PDMS surface. (b) and (c) show top and 30° angle cross-sectional SEM images of PPy-coated p-PDMS surface. And (d) shows photo images of PPy/p-PDMS film.

3.2 Electrical behavior of PPy/p-PDMS under stretching

The experimental setup used for measuring the resistance under stretching was shown in Fig. 4. The PPy/p-PDMS film was firmly clamped to the caliper. Insulating tape also covered the clamp/film/caliper interfaces to ensure electrical isolation of the PPy/p-PDMS film from the caliper. A digital multimeter (FLUKE True 115 RMS, Singapore) was used to measure the electrical resistance of the PPy/p-PDMS film. An LED light was connected to verify continuous electrical conductivity of PPy/p-PDMS film during stretching. In the stretching experiments, as shown in the bottom-left corner in Fig. 4, the LED maintained continuous illumination under PPy/p-PDMS stretching and the LED brightness decays at higher strain (ε) until the PPy/film breaks. The brightness of the LED was also an indicator for the PPy/p-PDMS film conductivity. The current and the LED light decreased with the increasing PPy/p-PDMS film resistivity because the voltage applied to the PPy/p-PDMS film was constant. Electrical conductivity (σ) is defined as: , where ρ is electrical resistivity, A is the cross-sectional area of the PPy layer, and l is the length of the tested PPy/p-PDMS sample. Resistance measurements were recorded at a constant distance of 55 mm for all stretching experiments.

Fig. 4 Experimental setup for PPy/p-PDMS film resistance measurement with strain range from 0 to 80% and film breaking at strain >85%. The enlarged LED images shown in bottom-left corner indicate the LED illuminance decay with increasing strains.

In order to understand the PPy/p-PDMS film deposition behavior as well as find the optimum processing conditions, it is important to explore the effects of PPy/p-PDMS process parameters on the electrical properties under stretching. The PPy/p-PDMS conducting and coating performance is influenced by several critical factors including the PPy deposition time, deposition temperature, UVO treatment time and the abrasive paper grit size. The PPy deposition temperature is found to be an important variable affecting the PPy conductivity and report suggests lower PPy deposition temperature promotes better electrical conductivity.³⁶ It is well-known that oxidation of Py leads to both α–α and α–β linkages during PPy formation (Fig. 5a). Because α–α linkage is thermodynamically favorable, it can be formed in low temperature. In contrast, higher temperature causes more α–β linkage.³⁷ Different from α–α linkage which can lead to linear PPy chains, α–β linkage usually results in branches in PPy chains (Fig. 5b). Such structural defects reduce not only PPy chain length but also the order of PPy molecules. Therefore, lower temperature allows the formed PPy more efficient electrical charge transportation to increase conductivity. In our research, we test the PPy deposition at 4 °C and 25 °C temperature that represent the conditions in a conventional refrigerator and room respectively. Results show that PPy/p-PDMS coated at 4 °C exhibits better electrical properties with resistance of 56 kΩ than the 25 °C of 98 kΩ for the p-PDMS sheet replicated from P400 grit size abrasive paper at 80% strain. Although −20 °C optimized PPy deposition temperature results have been reported,³⁶ a 4 °C PPy deposition temperature is selected for current tests to maintain ease of accessibility of PPy/p-PDMS process equipment under conventional refrigerating condition.

Fig. 5 (a) The APS treatment can oxidize Py to form cationic radicals. Their coupling through α–α or α–β linkages can be repeated many times to form final PPy. (b) In contrast to α–α linkage which can form linear PPy chains, the α–β linkages lead to branches to increase structural defects.

3.2.1 Effects of UVO treatment for PPy/p-PDMS film. The p-PDMS exhibits high hydrophobicity that requires UVO treatment for effective PPy deposition. Fig. 6a shows the water contact angle on p-PDMS surfaces with 15, 40 and 90 minute UVO treatment time.

Fig. 6 (a) Water contact angle of p-PDMS replicated from P180, P400 and P600 grit size abrasive paper under 5, 40 and 90 minute UVO treatments (b) electrical resistance variation of the PPy/p-PDMS film with 15, 40 and 90 minute UVO treatments under 0–80% strain stretching. PPy was deposited for 40 minutes and the p-PDMS surface was replicated from P400 grit size abrasive paper.

The p-PDMS surfaces exhibit high hydrophobicity of 139.6°, 138.8°, and 127.4° for the p-PDMS replicated from P180, P400, and P600 grit size abrasive papers respectively. The water contact angle on the p-PDMS surface becomes more hydrophilic under UVO surface modification. The water contact angle slightly reduces by ∼5 degrees with the 15 minutes UVO treatment. With increased UVO treatment time up to 90 minutes, the p-PDMS the surface wettability further reduces to 121.9° (P180), 117.9° (P400), and 115.2° (P600). Fig. 6b shows the PPy/p-PDMS electrical resistance under stretching (ε: 0–80%) for 15, 40 and 90 minute UVO treatment conditions replicated from a P400 grit size abrasive paper. Results indicate that 90 minutes UVO-treated PPy/p-PDMS film provides lowest resistance value of 8.4 kΩ equivalent to an electrical conductivity of 23.18 S m⁻¹ at ε = 0%. Under stretching, the resistance value increases (curve fitting value: R² = 0.9741) with increasing strain. The resistance increases approximately 6.5 folds to 55 kΩ at highest strain of 80%. Other UVO treatments conditions have similar tendencies but present higher resistances than the 90 minutes PPy/p-PDMS case. Resistance is measured at 24 kΩ and 17 kΩ for 15 and 40 minute treatments prior to stretching at ε = 0%. These results indicate that longer UVO treatments result in a higher p-PDMS wettability, which assists the diffusion of Py monomers into the pores. As these monomers are polymerized by APS, the formed PPy chains may interact with the texture of the PDMS surface to increase physical entanglement. On the other hand, UVO treatment can also increase the surface energy of the p-PDMS. Because different radicals and reactive groups can be introduced on UVO treated p-PDMS surfaces, covalent bonds formed between PPy and p-PDMS can increase their binding strength.³⁸ Consequently, UVO treatment is beneficial to PPy adhesion and improves conductivity. The p-PDMS surfaces replicated from P180 and P600 abrasive paper have similar results and an optimized process condition of 90 minutes UVO treatment time is used.

3.2.2 Effects of deposition time and abrasive paper grit size for PPy/p-PDMS film. The PPy/p-PDMS film's resistive behavior for P180, P400, and P600 grit sizes abrasive paper with respect to 5 minute, 40 minute, and 2 hour PPy deposition time under straining up to 80% was investigated and presented in Fig. 7. For all grit size abrasive papers, 5 minutes deposition showed higher electrical resistance than the 40 minute and 2 hour conditions. Electrical resistance of PPy/p-PDMS films with 5 minute deposition under P180, P400, and P600 grit sizes abrasive paper was measured at 18.8 ± 1.3, 18.9 ± 7.2 and 17.9 ± 6.3 kΩ respectively at zero strain. Lower electrical resistance ranging from 9.6–12.4 kΩ was observed at longer deposition conditions of 40 minute and 2 hour. Improved electrical conductivity for longer deposition time was due to the PPy layer thickness on the p-PDMS surface. From the top and cross-sectional SEM images of PPy/p-PDMS surfaces in Fig. 8, the 5 minute PPy layer was measured around 4.5 μm (Fig. 8a). PPy film grew thicker to 7.5–15 μm at 40 minute and 2 hour (Fig. 8b and c) deposition conditions. The 40 minute and 2 hour conditions presented similar electrical behavior for all P180, P400, and P600 PPy/p-PDMS conditions (Fig. 7). In this research, we have performed thermogravimetric analysis (TGA) analysis to evaluate the coating capacity of the PPy/p-PDMS film. However, because the PPy film material is relatively think compare to the bulk p-PDMS layer (600 μm). Therefore, we use SEM images to estimate the PPy coating. In the SEM images in Fig. 8, it was observed that instead of generating continuous PPy films on the p-PDMS surface, the 2 hour PPy deposition condition generated thicker (∼15 μm) PPy microparticles partially distributed on the p-PDMS surface. Those PPy particles were not interconnected for electrical conduction pathway. As a result, 2 hour deposition condition didn't show substantial electrical improvement for the PPy/p-PDMS.

Fig. 7 Variation of electrical resistance with 5 minute (black solid line), 40 minute (red dash line) and 2 hour (blue dot line) PPy deposition time on the PPy/p-PDMS surface replicated from (a) P180, (b) P400, and (c) P600 grit size abrasive paper.

Fig. 8 Top (a–c) and cross-sectional (d–f) SEM image views of PPy/p-PDMS surface with 5 minute (a and d), 40 minute (b and e), and 2 hour (c and f) deposition conditions. The p-PDMS is replicated from P400 grit size abrasive paper.

For the PPy/p-PDMS film stretching tests at different grit size abrasive paper, the PPy/p-PDMS films present increasing electrical resistance with increasing strains as expected. For P180 PPy/p-PDMS films, electrical resistances increase from 18.8 to 147.6, 12.4 to 92.3 kΩ, and 12.2 to 107.3 kΩ with 5 minute, 40 minute and 2 hour deposition respectively (Fig. 7a). Both P400 and P600 PPy/p-PDMS films also show increased electrical resistance tendencies with increasing strain but smaller increase in slopes when compared to the P180 case, especially in the high strain region when ε > 30%. While all P180, P400, and P600 PPy/p-PDMS surfaces show a corrugated 3D wavy surface profile (Fig. 2), the P180 p-PDMS film shows higher hydrophobicity with the PPy coating than other cases and results in a higher electrical resistance. Therefore, p-PDMS replicated from P400 or P600 grit size abrasive paper is a preferable choice for the stretchable PPy/p-PDMS.

From the PPy/p-PDMS process parameter investigations. The optimized process condition of PPy/p-PDMS surface was replicated from P400 abrasive paper with 90 minute UVO treatment and 40 minute PPy deposition. This process exhibited the lowest electrical resistance of 8.4 kΩ. With the PPy layer thickness measured at approximately 7.5 μm, the PPy/p-PDMS showed a good electrical conductivity of 34.9 S m⁻¹ that was comparable to many other advanced stretchable conductive materials such as micro-wrinkled graphene oxide composite of 25 S m⁻¹ (ref. 25) or CNTs/graphene porous PDMS²³ of 27 S m⁻¹.

3.3 PPy/p-PDMS stretching and bending reliability test

Although we proved electrical contact under high strain, stretchable electronic devices would more likely be subjected to frequent bending or stretching at low strains values. Thus, the fatigue effects of repeatable stretching and bending on the electrical properties of PPy/p-PDMS layer were also studied. Repeatable 10% (black line) and 20% (red line) stretching experiments as well as repeatable bending (green line) were shown in Fig. 9. The repeatable bending test was performed, where bent the PPy/p-PDMS film was bent over a 15 mm radius plastic rod. Both repeatable stretching and bending tests were performed up to 1000 times and the electrical resistance value was recorded at each 10 stretching/bending cycles. In 10% strain tests, the PPy/p-PDMS electrical resistance increased with the stretching. At 500 cycles the electrical resistance only increased 15%. For more than 500 cycles and up to 1000 cycles, the electrical resistance increased slightly to 19%. For 20% strains tests, the electrical resistance varied more than that for the 10% stretching and bending conditions. Normalized resistance increased 21% at 500 repeating cycles and continue increasing to a maximum 36% increment at 1000 cycles. Bending of the PPy/p-PDMS film revealed the best fatigue resistance. The resistivity value remained almost constant with only a maximum 5% of resistivity increase at 1000 repeating cycles.

Fig. 9 Repeatable stretching and bending tests for PPy/p-PDMS films. X-Axis present normalized resistance defined as the electrical resistance divided by electrical resistance at zero strain. Y-Axis present stretching/bending cycles up to 1000 times.

Reasons for the PPy/p-PDMS film fatigues and electrical resistance increments at high strain values were further evaluated. In the SEM images of the PPy/p-PDMS surface after stretching, shown in Fig. 10, microcracks were found on the PPy/p-PDMS surface that were not observed on the PPy/p-PDMS surface before stretching. Fig. 10a showed the PPy/p-PDMS surface morphology after a maximum 80% strain stretching. 50–60 μm long microcracks were found on the PPy/p-PDMS surface. Similar microcracks were also observed in the 500 cycle 10% stretching case, shown in Fig. 10b. Those microcracks blocked the electrical pathways on the PPy/p-PDMS surface and therefore increased the electrical resistance after stretching at high strain conditions or high cycle counts.

Fig. 10 SEM images show PPy/p-PDMS surface morphology after stretching at (a) 80% strain and (b) 10% strain, 500 repeating cycles.

4 Conclusions

3D corrugated porous microstructures on PDMS surfaces replicated from commercial abrasive paper and coated with a conductive PPy layer were successfully demonstrated as highly stretchable conductive PPy/p-PDMS polymer films in this paper. The 3D p-PDMS microstructures provided an extra degree of freedom for movement when the PPy/p-PDMS surface was subjected to stretching. All PPy/p-PDMS film exhibited high stretchability with a high ε value of 80%. Effects of major PPy/p-PDMS process parameters were investigated. Results showed that increased UVO treatment assisted the rendering of the p-PDMS surface, making it more hydrophilic for an effective PPy coating. An optimized PPy deposition time of 40 minutes produced a uniformly thick conductive PPy layer on the p-PDMS surface while still maintaining good electrical properties. The highest PPy/p-PDMS conductivity of 34.9 S m⁻¹ was reported for the optimized condition of a 90 minute UVO treatment time, 40 minute PPy deposition time, and replicated from P400 grit size abrasive paper.

The PPy/p-PDMS film exhibited high stretchability (ε up to 80%) and conductivity that was comparable to other advanced stretchable conductive materials. The abrasive paper used in this research was also demonstrated as a simple and low-cost template for fabricating corrugated porous microstructures using standard PDMS soft lithography process. Other porous templates such as porous silicon or porous aluminum can also be applied as potential micromolds for a more rigid and mass-producible approach when fabricating 3D corrugated highly stretchable conductive PPy/p-PDMS films.

Acknowledgements

The authors thank the Ministry of Science and Technology, Taiwan and National Central University–Cathay General Hospital joint research program, for financially supporting this project under Grant No. MOST 105-2221-E-008-061.

References

1. J. A. Rogers, T. Someya and Y. G. Huang, Science, 2010, 327, 1603–1607.
2. H. Park, Y. R. Jeong, J. Yun, S. Y. Hong, S. Jin, S. J. Lee, G. Zi and J. S. Ha, ACS Nano, 2015, 9, 9974–9985.
3. S. Il Han, K. S. Hwang, Y. K. Yoo, S. M. Lee and J. H. Lee, Jpn. J. Appl. Phys., 2014, 53, 08NC01.
4. J. Ruhhammer, M. Zens, F. Goldschmidtboeing, A. Seifert and P. Woias, Sci. Technol. Adv. Mater., 2015, 16, 015003.
5. S. Rosset and H. R. Shea, Appl. Phys. A: Mater. Sci. Process., 2013, 110, 281–307.
6. L. Guo, G. S. Guvanasen, X. Liu, C. Tuthill, T. R. Nichols and S. P. DeWeerth, IEEE Transaction on Biomedical Circuits and Systems, 2013, 7, 1–10.
7. Y. Fujimagari, Y. Fukushi and Y. Nishioka, J. Photopolym. Sci. Technol., 2015, 28, 357–361.
8. I. Bernardeschi, F. Greco, G. Ciofani, A. Marino, V. Mattoli, B. Mazzolai and L. Beccai, Biomed. Microdevices, 2015, 17, 46.
9. T. Adrega and S. P. Lacour, J. Micromech. Microeng., 2010, 20, 055025.
10. C. Zhou, S. Bette and U. Schnakenberg, Adv. Mater., 2015, 27, 6664–6669.
11. S. Befahy, S. Yunus, V. Burguet, J. S. Heine, M. Troosters and P. Bertrand, J. Adhes., 2008, 84, 231–239.
12. R. Seghir and S. Arscott, J. Appl. Phys., 2015, 118, 045309.
13. M. U. Manzoor, P. Lemoine, D. Dixon, J. W. J. Hamilton and P. D. Maguire, AIP Adv., 2015, 5, 107237.
14. N. Chou, J. Lee and S. Kim, Appl. Phys. Lett., 2014, 105, 241903.
15. C. Y. Li and Y. C. Liao, ACS Appl. Mater. Interfaces, 2016, 8, 11868–11874.
16. A. Larmagnac, S. Eggenberger, H. Janossy and J. Voros, Sci. Rep., 2014, 4, 7254.
17. J. Tang, H. Guo, M. M. Zhao, J. T. Yang, D. Tsoukalas, B. Z. Zhang, J. Liu, C. Y. Xue and W. D. Zhang, Sci. Rep., 2015, 5, 16527.
18. H. Lee, K. Lee, J. T. Park, W. C. Kim and H. Lee, Adv. Funct. Mater., 2014, 24, 3276–3283.
19. D. B. Zhou, H. P. Wang, J. Bai and J. G. Cao, Smart Mater. Struct., 2014, 23, 104001.
20. J. Kim, J. Park, U. Jeong and J. W. Park, J. Appl. Polym. Sci., 2016, 133, 43830.
21. J. B. Lee and D. Y. Khang, Compos. Sci. Technol., 2012, 72, 1257–1263.
22. M. T. Chen, T. Tao, L. Zhang, W. Gao and C. Z. Li, Chem. Commun., 2013, 49, 1612–1614.
23. M. T. Chen, L. Zhang, S. S. Duan, S. L. Jing, H. Jiang and C. Z. Li, Adv. Funct. Mater., 2014, 24, 7548–7556.
24. S. S. Duan, K. Yang, Z. H. Wang, M. T. Chen, L. Zhang, H. B. Zhang and C. Z. Li, ACS Appl. Mater. Interfaces, 2016, 8, 2187–2192.
25. C. F. Feng, Z. F. Yi, L. F. Dumee, C. J. Garvey, F. H. She, B. Lin, S. Lucas, J. Schutz, W. M. Gao, Z. Peng and L. X. Kong, Carbon, 2015, 93, 878–886.
26. J. S. Noh, RSC Adv., 2014, 4, 1857–1863.
27. C. Racles, V. E. Musteata, A. Bele, M. Dascalu, C. Tugui and A. L. Matricala, RSC Adv., 2015, 5, 102599–102609.
28. M. Park, J. Park and U. Jeong, Nano Today, 2014, 9, 244–260.
29. K. Kim, D. Lee, S. Eom and S. Lim, Sensors, 2016, 16, 521.
30. D. C. Hyun, M. Park, C. Park, B. Kim, Y. Xia, J. H. Hur, J. M. Kim, J. J. Park and U. Jeong, Adv. Mater., 2011, 23, 2946–2950.
31. F. M. Guo, R. Q. Xu, X. Cui, L. Zhang, K. L. Wang, Y. W. Yao and J. Q. Wei, J. Mater. Chem. A, 2016, 4, 9311–9318.
32. T. G. Yun, B. i. Hwang, D. Kim, S. Hyun and S. M. Han, ACS Appl. Mater. Interfaces, 2015, 7, 9228–9234.
33. D. Qin, Y. N. Xia and G. M. Whitesides, Nat. Protoc., 2010, 5, 491–502.
34. W. W. Hu, Y. T. Hsu, Y. C. Cheng, C. Li, R. C. Ruaan, C. C. Chien, C. A. Chung and C. W. Tsao, Mater. Sci. Eng., C, 2014, 37, 28–36.
35. M. C. Cheng, A. T. Leske, T. Matsuoka, B. C. Kim, J. Lee, M. A. Burns, S. Takayama and J. S. Biteen, J. Phys. Chem. B, 2013, 117, 4406–4411.
36. B. Sun, J. J. Jones, R. P. Burford and M. Skyllas-Kazacos, J. Electrochem. Soc., 1989, 136, 698–701.
37. G. R. Mitchell and A. Geri, J. Phys. D: Appl. Phys., 1987, 20, 1346.
38. S. Garg, C. Hurren and A. Kaynak, Synth. Met., 2007, 157, 41–47.

---