Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Development of a Highly Sensitive, Broad-Range Hierarchically Structured Reduced Graphene Oxide/PolyHIPE Foam for Pressure Sensing Lei Yang,*,†,‡ Yu Liu,†,⊥ Carlos D. M. Filipe,‡ Darko Ljubic,‡ Yingwu Luo,∥ He Zhu,§ Jiaxing Yan,† and Shiping Zhu*,‡,§ †
College of Materials and Textiles, Zhejiang Sci-Tech University, Hangzhou 310018, China Department of Chemical Engineering, McMaster University, Hamilton L8S 4L7, Canada ∥ State Key Lab of Chemical Engineering, College of Chemical and Biological Engineering, Zhejiang University, Hangzhou 310027, China § School of Science and Engineering, The Chinese University of Hong Kong, Shenzhen 518172, China
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ABSTRACT: Highly sensitive pressure sensors are usually made from soft materials that allow large deformations to be obtained when very small pressures are applied. Unfortunately, this current paradigm limits the ability to create sensors capable of high sensitivities and broad dynamic ranges as these materials are prone to saturation responses when attempting to obtain measurements involving high pressures. In this paper, we detail a piezoresistive pressure sensor that is capable of high sensitivity over a pressure range spanning from 0.6 Pa (a mosquito touching a surface) to 200 kPa (an elephant standing on the surface). The sensor’s ability to cover such a broad dynamic range is made possible by the fairly hard foam used in its construction as this material is capable of propagating strain in a highly effective manner due to its hierarchical porous structure. The material was fabricated by using high-internal-phase emulsion (HIPE) as a template to generate a highly porous material consisting of small pores packed between larger ones whose inner walls are lined with reduced graphene oxide. The developed foam exhibits very fast response times (less than 15.4 ms) and excellent cyclic stability (at least 10,000 cycles). Furthermore, it is capable of responding to the entire tactile pressure range, and it can be formatted as pixelated arrays, which makes it highly suitable for integration into wearable electronic devices. Such arrays were built and used to identify and render the shape of objects with different geometries, including a sphere, a triangle, a square, and two nearly identical rods differing only by 0.4 mm in diameter. KEYWORDS: hierarchically structured foam, effective strain propagation, broad pressure-sensing range, high sensitivity, high spatial resolution
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narrow working rangesusually below 30 kPaand they are unable to measure static pressure because the sensor’s response tends to drift over time.32 Conversely, capacitance-based sensors feature a relatively large dynamic range (below 100 kPa), but they usually provide poor sensitivity in low-pressure applications.24,33,34 Although promising strategies for overcoming these limitations (i.e., to increase pressure sensitivity) have been developed using microstructured, nanostructured, or porous elastomers, these approaches either depend on expensive microfabrication processes or are limited to very low pressure ranges (sensitivity of 0.601 kPa−1 for pressures below 5 kPa).35−40 Piezoresistive sensors have received considerably more attention than piezoelectric- and capacitance-based
uman beings rely on the detection of pressure as part of everyday life; for example, finger gaiting (pressures > 5 kPa) allows us to handle objects, and the ability to sense small pressures allows us to observe the action of airflow or to detect a heartbeat (< 50 Pa).1 To create machines and robots capable of feeling and interpreting the environment in a manner that approximates the human sensory ability, it is necessary to develop tactile sensors that feature similar pressure-sensing ranges and sensitivities. Such pressure sensors are of considerable scientific importance due to their many potential technical applications such as health care,2−5 sports-motion monitoring, 6−10 voice recognition, 11−13 and electronic skins.14−19 There are several types of pressure sensors that are based on different working principles, such as piezoelectricity,20,21 capacitance,22−24 and piezoresistivity.25−31 Piezoelectric sensors are well-known for their high-frequency/high-sensitivity dynamic pressure sensing, but they are generally only applicable to © XXXX American Chemical Society
Received: September 29, 2018 Accepted: January 7, 2019 Published: January 7, 2019 A
DOI: 10.1021/acsami.8b17020 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
polymerization (PolyHIPE) in the presence of reduced graphene oxide (rGO)are capable of detecting the entire range of pressures associated with human tactile activities with a high degree of sensitivity and accuracy. In addition, this rGO@ PolyHIPE-based sensor can be easily integrated into an artificial finger for the purpose of measuring grasping forces ranging from pinching a piece of 3.2 g candy (80 Pa stress) to picking up a full 760 g bottle of beverage (160 kPa stress). Furthermore, the sensor’s high spatial resolution enables it to identify and distinguish between different object shapes, for example, between two glass rods that only differ in diameter by 0.4 mm.
sensors due to their readout simplicity, sensitivity to both pressure and flexion, and ability to be applied in high-pixeldensity mode.1,32,41 However, planar-structured piezoresistive sensors do not offer very good performance, which has led to several attempts to improve upon them using microstructured and porous sensing materials. Recently, Pan et al. documented the development of a high-sensitivity hollow-sphere microstructure-based pressure sensor that featured a sensitivity ranging from 56 to 133 kPa−1,42 whereas Su et al. successfully utilized an irregular microstructure to fabricate a mimosainspired pressure sensor with a sensitivity of 50.17 kPa−1.43 In another study, Yu et al. developed a fractured reduced-graphene oxide−polyurethane (RGO−PU) sponge, which was then used to construct a porous-structured sensor featuring a sensitivity of 0.26 kPa−1.11 Chung et al. combined photolithography, selective wet etching, and a chemical vapor deposition process to produce a pressure sensor with a maximum sensitivity of 8.5 kPa−1 that was based on a layered polydimethylsiloxane (PDMS) decorated with graphene.44 Besides graphene and graphene oxide, other conductive materials such as carbon black28 and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)8 were also used to realize foams for effective pressure-sensing purposes. Although some of these sensors are capable of detecting very gentle vibrations (less than 1 Pa), they only work within a limited pressure range and up to 12 kPa.11,42−48 Other researchers have developed piezoresistive sensors that can operate in a broader pressure response range (up to 100 kPa), but these sensors exhibit highly nonlinear and unstable responses below 50 kPa.49 In essence, there exists a delicate tradeoff between range and pressure sensitivity among the current selection of pressure sensors.1,32,41 One of the primary reasons for this tradeoff is that it is very challenging to fabricate a sensor that offers high pressure sensitivity throughout a broad pressure-measurement range. This paper presents a hierarchically structured sensor that is capable of meeting both of these requirements. Pore hierarchy is a concept that describes a pore size distribution that is multimodal in nature.50 Hierarchical porous structures are much more effective than their monomodal counterparts in terms of load transfer and the alleviation of stress concentration.50−52 Several porous materials are found in naturefor example, wood, bamboo, and bonesand these materials consist of individual structural units that are hierarchically arranged to optimize various mechanical properties, such as strength and toughness.50 A porous material’s structural hierarchy provides the key to modifying its mechanical properties; such modifications may include improving how evenly it distributes internal stress or increasing its compression strength.50−52 In the context of piezoresistive sensors, the use of hierarchical porous structures should provide deformation homogeneity within the conductive percolation network that is superior to that offered by a monomodal system. Improving deformation homogeneity enables the enhancement of the construction of new conductive pathways and the extension of pressure detection thresholds, which are both critical to the development of a compressive piezoresistive sensor with a highpressure sensitivity and a broad pressure-measurement range. In this research, we developed a hierarchically structured and porous piezoresistive sensor that provides ultrahigh pressure sensitivity (2.53 kPa−1,
