1.
Introduction
Flexible pressure sensors, capable of being embedded into clothes[1, 2], shoes[3, 4] or directly wrapped on skin surfaces[5, 6] have attracted profound research interest for promising applications in wearable healthcare, patient rehabilitation, and biomedical prostheses. Capacitive pressure sensors with an elastomeric dielectric layer sandwiched between two flexible electrodes have been widely adopted in the studies for their simple structure, low-cost and ease of processing, and low dynamic power consumption for power constraint applications[7–9]. When an external pressure is applied to the sensors, the induced deformation of the dielectric layer causes changes of the capacitance to measure the applied force. The sensing performance is thus determined by the mechanical properties of the elastomer dielectric layer. Polydimethylsiloxane (PDMS) elastomer is a popular material of choice, for its excellent flexible and elastic properties, biomedical compliance with human tissues and commercial availability[10–13]. However, the bulk PDMS film, typically having a modulus of ~1 MPa, cannot provide acceptable sensitivity for reliable sensing in the low pressure regime (e.g. < 1 kPa) [14, 15]. Moreover, the visco-elastic behavior of unstructured PDMS thin film can cause a slow relaxation time after removal of the pressure load[15]. To improve the sensitivity for detecting weak pressure force and reduce the relaxation time, several approaches have been developed to form air void microstructures inside the PDMS layer to reduce its modulus and suppress the influence of the intrinsic visco-elastic behavior[6, 16, 17].
On the other hand, to construct such pressure sensors, electrodes between the dielectric layer need to match the pressure induced local deformation, especially for making pressure sensor arrays. Therefore, soft conductive films having the same mechanical properties as those of the dielectric layer are demanded. Moreover, low cost large area and stable manufacturing processes with extremely low cost, high throughput and short production cycles are required. With the problems of brittleness and the high temperature processing used in production, and the rising cost of indium, the commonly used indium tin oxide (ITO) is not an ideal choice[18]. Among those new flexible transparent conductors[19–22], the silver nanowire (AgNW) mesh is considered to be one of the most promising candidates to replace ITO, for its high DC conductivity and optical transmittance, excellent mechanical flexibility, and the environmental and economic advantages for manufacturing[23]. However, large wire-to-wire junction resistance and poor air stability due to oxidation of AgNWs in air are issues to be addressed[24]. Previous work has shown that by using the modified PEDOT:PSS of neutral pH as the over-coating layer on top of the AgNW mesh, the electrical conductivity and mechanical flexibility can be improved, which is attributed to the enhanced wire-to-wire and wire-to-substrate adhesion[25]. The neutral-pH PEDOT:PSS over-coating layer is also able to prevent AgNWs from oxidation in air ambient[25].
In this work, soft conductive films composed of the AgNW network with the neutral-pH PEDOT:PSS over-coating layer and the PDMS substrate were fabricated by large area compatible coating processes. The electrical and mechanical properties were characterized. Finally, the soft conductive films were used to fabricate an 8 × 8 pressure sensor array with a simple maskless patterning approach. It is shown that such soft conductive films can help to improve the sensitivity and reduce the signal crosstalk.
2.
Experimental
2.1
Processing of the composite films
Fig. 1(a) shows the experimental setup for the coating processes in this work. An electrically controlled motorized stage is used for accurate moving speed control along the horizontal direction. For the blade-coating process, the polyethylene naphthalene-2, 6-dicarboxylate (PEN) film covered with teflon tape was used. For the bar-coating, the bar with 35 μm wire width and 76 μm wire-to-wire space was used. The fabrication procedure for the soft conductive films is illustrated in Fig. 1(b). The glass support substrate was cleaned using detergent, deionized water, acetone, and IPA, and then treated with oxygen plasma (PDC-32G-2, Harrick) for 5 min before use. A polyvinylpyrrolidon (PVP) solution of 50 mg/ml in ethanol was blade-coated on the glass substrate with a speed of 10 mm/s, and annealed at 100 °C for 10 min to obtain the separation layer. A mixed solution of PDMS pre-polymer and its curing agent (Sylgard 184, Dow Corning) with a proportion of 10 : 1 was blade-coated on top of the PVP layer with a speed of 10 mm/s. After being cured at 100 °C for 30 min, about 100 μm thick PDMS film was obtained. The AgNW network was then formed on the oxygen plasma treated PDMS substrate by bar-coating an AgNW suspension (Nanjing JCNANO) with a concentration of 3 mg/ml at different speeds twice along two perpendicular directions, followed by a drying process at 100 °C for 10 min. The ethanol diluted neutral-pH PEDOT (Shanghai OE Chemicals) with a proportion of 1 : 1 was bar-coated at different speeds on the substrate, and dried at 100 °C for 20 min to obtain the AgNW composite film.
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Figure1.
(Color online) (a) The experimental setup for the coating processes (blade-coating and bar-coating) used in this work. (b) Illustration of the fabrication procedure for the PDMS/AgNW/PEDOT:PSS soft conductive films.
2.2
Sensor array fabrication
For sensor array fabrication, patterning of the conductive film is needed. A maskless patterning approach is developed in this work as depicted in Fig. 2(a). Firstly, the PDMS substrate was treated with oxygen plasma to increase its surface hydrophilic property. A fluorinated separation layer of trimethoxy (1H, 1H, 2H, 2H-heptadecafluorodecyl) silane (FAS, Sigma-Aldrich) with a proportion of 3% in IPA was spin coated on the PDMS substrate at 3000 rpm for 40 s. The PDMS lines were dispensed on the FAS modified PDMS substrate surface at a substrate temperature of 70 °C and a speed of 2 mm/s using a motor controlled dispensing system. The PDMS lines were cured with a thickness of about 50 μm. Then, the substrate was treated with oxygen plasma to remove the FAS in the areas without coverage of PDMS lines. The AgNW and neutral pH PEDOT were then bar-coated on the substrate as described above. The patterned conductive film was finally obtained by removing the dispensed PDMS lines with the help of the weak adhesion force between the FAS treated surface and the PDMS lines.
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Figure2.
(Color online) (a) Illustration of the maskless approach for patterning the soft conductive film. (b) Illustration of the procedure for fabricating the pressure sensor array using the PDMS/AgNW/PEDOT:PSS films as the electrodes and microstructured PDMS dielectric layer.
The fabrication of the pressure sensor array is illustrated in Fig. 2(b). The microstructured PDMS dielectric layer in the sensor was fabricated using the method developed in a previous work[26]. A mold of 5 × 5 cm2 size with periodical micro-grooves on the surface was fabricated by a 3D printer (UP Plus 2, Tiertime) using acrylonitrile butadiene styrene (ABS). The PDMS was cast on the fabricated mold, followed by a curing process at 65 °C for 60 min. The free-standing PDMS film with micro-structured surface of periodical line geometries was peeled off from the mold.
The PDMS dielectric film was attached on the patterned AgNW-polymer composite films with the micro-strip structures facing outward. An 8 × 8 flexible sensor array was finally fabricated by laminating two pieces of film with the micro-strip structures of the PDMS dielectric layer perpendicularly crossing. The sensor array with PET/ITO film was fabricated for comparison.
The sheet resistance of the conductive films was measured using a 4-point probe system (ST-2258A, Suzhou JG). The capacitance of the flexible pressure sensors was measured by a WK6500B impedance analyzer with a 10 kHz and 1 V amplitude AC signal.
3.
Results and discussions
3.1
Film properties
Fig. 3(a) shows the influence of the coating speed on the sheet resistance of the formed films. The values were obtained by measuring 9 different locations over the film. It can be seen that without the PEDOT:PSS over-coating layer, the AgNW network film has relatively large sheet resistance near 400 Ω/□, which is not obviously influenced by the coating speed. The photo image of the formed AgNW network by twice coating processes is given in Fig. 3(b), showing very uniform and dense distribution of AgNWs. After coating the neutral-pH PEDOT:PSS layer on top, the sheet resistance is significantly reduced attributed to the decreased wire-to-wire junction resistance[25]. With an optimal coating speed of 10 mm/s, a low sheet resistance of about 50 Ω/□ is achieved without needing high temperature annealing or other additional complex processes[27]. Fig. 3(a) also shows the PEDOT: PSS over-coating layer can improve the uniformity of the conductive films. With higher conductive PEDOT:PSS modified by 5% dimethyl sulfoxide (DMSO), the sheet resistance of the composite film is further reduced below 28 Ω/□, as shown in Fig. 3(c). The histogram of the statistical distribution of the measured sheet resistance values of 9 locations over 6 films is shown in Fig. 3(d). The results show good uniformity and repeatability of the electrical performance of the fabricated composite films.
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Figure3.
(Color online) (a) Influence of the coating speed on the sheet resistance of the formed AgNW films without and with the neutral-pH PEDOT:PSS layer. (b) Photo image of the formed AgNW network by twice bar-coating processes. (c) Comparison of the sheet resistance of the AgNW/neutral-pH PEDOT:PSS films without and with DMSO modification. (d) Histogram of the statistical distribution of the measured sheet resistance values of 9 locations over 6 films.
The resistance changes of the soft conductive films fabricated on PDMS substrate without and with the PEDOT:PSS layer upon continuous stretching of 20% and relaxation for 6000 s are compared in Figs. 4(a) and 4(b). The conductive films were cut to 1 × 4 cm2 size for the tests. It can be seen that, without the PEDOT:PSS over-coating layer, the conductivity of the AgNW network film is very sensitive to stretching due to stretching induced poorer contacts among the AgNWs. With the conductive PEDOT:PSS overcoating layer, the contacts among the AgNWs are strengthened and become much less sensitive to stretching. Therefore, the conductivity of the film is nearly constant. The neutral-pH PEDOT:PSS over-coating layer can also help to improve the air stability[25]. As shown in Fig. 5, with the neutral-pH PEDOT:PSS over-coating layer, the long term air stability of the AgNW composite film is significantly improved, compared to that of the pure AgNW film and the AgNW film with the commonly used PEDOT:PSS (Clevios PH1000). With PH1000 on top of AgNW film, the worse stability is thought to be caused by water absorption and oxygen-consuming corrosion of AgNWs with the hygroscopic and acid PH1000 PEDOT:PSS[25].
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Figure4.
(Color online) The resistance changes of the soft conductive films fabricated on PDMS substrate (a) without and (b) with the neutral PEDOT:PSS layer upon continuous stretching of 20% and relaxation for 6000 s.
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Figure5.
(Color online) The measured sheet resistance over time for the different conductive films being exposed in the air ambient.
3.2
Sensor performance
Fig. 6(a) gives the photo image of the fabricated flexible pressure sensor with the PDMS/AgNW/PEDOT:PSS composite films as the electrodes. Fig. 6(b) compares the measured relative capacitance change (ΔC/C0) as a function of the applied pressure for this sensor with that using the PET/ITO films as the electrodes. Upon a larger applied pressure, the capacitance of the devices increased due to the reduction in the distance between the two electrodes. With the micro-structured PDMS dielectric layer, the compression of air voids inside the film might also induce an increase of the effective dielectric constant of the dielectric layer for the additional increase of the measured capacitance.
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Figure6.
(Color online) (a) The photo image of the fabricated flexible pressure sensor with the PDMS/AgNW/PEDOT:PSS composite films as the electrodes. (b) Comparison of the measured relative capacitance change (ΔC/C0) as a function of the applied pressure for the sensors using two different electrodes.
The pressure sensitivity S is defined as S = δ(ΔC/C0)/δp, where C0 is the initial capacitance of the pressure sensor and ΔC is the capacitance change with the applied pressure (p). With the same dielectric layer, the device using PDMS/AgNW/PEDOT:PSS presents improved sensitivity compared to the devices with PET/ITO electrodes in both low (0–0.2 kPa) and high (>10 kPa) pressure regimes. The improved sensitivity could be attributed to that the rough surface of the conductive films with the AgNWs creates additional air voids at the interface between the electrode and dielectric layer, which is easier to be deformed upon applied pressure[27].
Two 8 × 8 pressure sensor arrays using the PDMS/AgNW/PEDOT:PSS and the PET/ITO electrodes, respectively, were fabricated to evaluate the capability of collecting spatially resolved pressure information. The width of conductive stripes is 4 mm and the space between two adjacent conductive stripes is 1 mm. By applying a 0.2 N force (20 g counterpoise) onto the pixels of the same location on the sensor arrays as illustrated in Fig. 7(a), the mapping of the measured relative capacitance changes of the 64 pixels for the two sensor arrays was obtained as shown in Fig. 7(b). It can be seen that, for the PDMS/AgNW/PEDOT:PSS based sensor array, the pixel with the applied pressure presents a larger response due to the higher sensitivity of the sensor, and the neighboring pixels are also less influenced with lower signal crosstalk across the array. When a pressure is applied to certain pixels in the array, local deformation can also be passed to neighboring pixels through the common electrode films. With the softer PDMS/AgNW/PEDOT:PSS conductive films, it will be easier for local deformation upon the applied pressure, and less pressure induced deformation will be passed to neighboring pixels. Therefore, the crosstalk can be reduced. The results indicate an important advantage of using soft conductive films as the electrodes for the pressure sensor array: it can help to enhance the response and reduce the signal crosstalk. The reduced signal crosstalk can help to improve the capability of distinguishing the applied spatially distributed pressure.
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Figure7.
(Color online) (a) Photo images of the fabricated 8 × 8 pressure sensor arrays using the ITO/PET (left) and the PDMS/AgNW/PEDOT:PSS (right) as the electrodes, and illustration of the pixel where the pressure is applied. (b) Mapping of the measured relative capacitance changes of the 64 pixels for the two sensor arrays using the ITO/PET (left) and the PDMS/AgNW/PEDOT:PSS (right) as the electrodes, respectively.
4.
Conclusion
With the neutral-pH PEDOT:PSS over-coating layer, the fabricated PDMS/AgNW/PEDOT:PSS soft conductive film shows excellent conductivity, stretchability and air stability for flexible and conformable sensors. Another key feature of such soft conductive films for pressure sensor arrays is its easier local deformation upon applied spatially distributed pressure, and both improved response and reduced signal crosstalk over the array can be achieved. Moreover, all the fabrication processes including the coating and maskless patterning are simple and scalable for a large area, indicating potential for extremely low cost and high throughput manufacturing.
