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Applications of Polymer, Composite, and Coating Materials
Paper-based Mechanical Sensors Enabled by Folding and Stacking Tong Yang, and Jeffrey M. Mativetsky ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 04 Jun 2019 Downloaded from http://pubs.acs.org on June 9, 2019
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ACS Applied Materials & Interfaces
Paper-based mechanical sensors enabled by folding and stacking Tong Yang1, Jeffrey M. Mativetsky*1,2 1
2
Materials Science and Engineering, Binghamton University, Binghamton, NY, 13902, USA Department of Physics, Applied Physics, and Astronomy, Binghamton University, Binghamton, NY,
13902, USA * Corresponding author email:
[email protected] ABSTRACT Electronics based on paper substrates can be foldable, inexpensive, and biodegradable, making such systems promising for low-cost sensors, smart packaging, and medical diagnostics. In this work, we saturate tissue paper with poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) by using a simple and scalable process, and construct pressure sensors that exhibit an enhanced response when the active material is folded or stacked. Nanoscale pressure actuation and current mapping reveals a sensing mechanism that takes advantage of the fibrous microstructure of the paper and relies on the formation and expansion of electrical contacts between fibers in adjacent paper layers as pressure is applied. The resulting paper-based pressure sensors respond to an impulse within 20 ms and are robust, showing only a 4.6% decrease in operating current after 30000 load/unload cycles. Pressure distribution mapping was achieved by using a sensor array with a stacked architecture, while folding was used to demonstrate multistate switching and to detect conformational change in a three-dimensional origami system. These strategies of folding and layering paper saturated with functional materials open new avenues for building multifunctional paper electronics.
KEYWORDS: paper electronics, foldable electronics, mechanical sensor, paper composite, PEDOT:PSS
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INTRODUCTION Flexible and low-cost electronics are of growing interest for a wide range of applications,1 such as electronic skin,2,3 robotics,4,5 energy storage,6,7 and displays.8,9 Most flexible electronics, however, relies on polymer substrates, leading to long-term concerns about the accumulation of plastic waste that persists in soil or enters the ocean.10 Paper is an attractive alternative substrate, particularly for short lifecycle or one-time-use applications, such as medical diagnostics.11 Not only does paper provide mechanical flexibility, but it is also inexpensive, biodegradable, renewable, and recyclable. Recently, flexible paper electronics12–16 has been explored for a range of applications including transistors,17 light-emitting diodes,18 thermal sensors,19 and humidity sensors.20 Paper-based pressure sensors21 in particular hold promise as a foundation for paper-based electronic skin,22 motion detection,23–25 and health diagnostics.26 Material microstructure plays a key role in the performance of flexible pressure sensors. These sensors rely on pressure-dependent changes in electrical or dielectric properties which can be manipulated through means such as the introduction of air bubbles in elastomeric foams,27–33 or the microfabrication of porous or pyramidal structures.27–29,32,34 Other strategies include manipulating the interlayer coupling in 2d nanomaterials, for example by covalently linking molecular pillars between layers of graphene derivatives,35 or by making use of the relatively wide, and pressure-dependent, interlayer spacing in MXenes.36 Paper, with its fibrous cellulose microstructure, holds promise for pressure sensing, without the need for additional micropatterning. To imbue paper with electronic function, paper composites have been formed with gold nanowires,26 graphene,37 carbon nanotubes,38 or zinc oxide39. Pressure sensing function has been achieved, for example, by sandwiching paper impregnated with gold nanowires between two PDMS layers, with one layer bearing interdigitated electrodes. The resulting electrical resistance-based wearable sensor was able to detect blood pulse.26 Despite the advantages and potential applications of paper-based electronics and sensors, prior work has mainly considered paper as a substrate for conventional device fabrication or, when impregnated with a functional material, as an active device layer. So far, studies have not considered device architectures that take advantage of paper folding or stacking as a means of mediating device function. In this work, we fold and stack an electrically conductive paper composite to produce mechanical sensors. We show that folding and stacking not only leads to greatly increased mechanotransduction sensitivity, but also enables functionality such as multistate switching, pressure distribution mapping, and the detection of conformational change in a three-dimensional origami system. Nanoscale pressure actuation and electrical mapping reveal that the paper’s fibrous microstructure plays a key role in pressure sensing, with the number and size of electrical contacts between fibers in neighboring paper layers increasing with pressure. The resulting sensors respond rapidly, in under 20 ms, and reliably, even after 30000 loading and unloading cycles.
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ACS Applied Materials & Interfaces
RESULTS AND DISCUSSION
Figure 1. (a) Photograph and (b) scanning electron microscope image of PEDOT:PSS-saturated paper. (c) Schematic illustrating the process for preparing a pressure sensor based on PEDOT:PSS-saturated paper stacks.
Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) solution was deposited onto tissue paper and allowed to dry under ambient conditions. Figure 1a shows a photograph of a sheet of paper following PEDOT:PSS saturation on the right side, leading to a pale blue color. As shown in Figure 1b, the PEDOT:PSS-saturated paper maintains the tissue paper’s fibrous microstructure. The PEDOT:PSS uniformly coats the cellulose fiber matrix, as evidenced by energy-dispersive X-ray spectroscopy (Supplementary Information Figure S1). The resulting composite has a sheet resistance of 42 kΩ/sq. As illustrated in Figure 1c, pressure-sensing devices were fabricated by stacking sheets of PEDOT:PSSsaturated paper and electrically contacting the top and bottom paper layers. Adhesive tape was used to hold the layers together and encapsulate the device.
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Figure 2. (a) Real-time response to force applied and released by hand for an 8-layer pressure sensor. (b) Current response over 10 automated cycles with a load of 167 kPa. The insets show the response kinetics upon application and release of pressure, reaching 90% of the saturation current within 20 ms and returning to original state within in 26 ms. (c) Dependence of current on/off ratio on applied pressure. (d) Current response for 100 load/unload cycles (167 kPa at 1 Hz), recorded after 5000, 10000, 20000, and 30000 cycles.
The response of an 8-layer PEDOT:PSS paper sensor to the repeated manual application of pressure is shown in Figure 2a. The current reliably increases by more than one order of magnitude when pressure is applied, and then returns to a low off-state current when the pressure is released. When controlled loading and unloading cycles are automated with a load pressure of 167 kPa and frequency of 1 Hz, a highly reproducible current response is observed (Figure 2b). The sensor responds rapidly to applied pressure, reaching 90% of the maximum current within 20 ms, and fully reaching the maximum current state within 30 ms. The current is fully restored to the off state within 26 ms of releasing the pressure. Notably, the sensor works with a low power consumption. For operation at 1V, the power consumption is
