Porous Polydimethylsiloxane-Silver Nanowire ... - ACS Publications

Subscriber access provided by BUFFALO STATE

Article

Porous Polydimethylsiloxane-Silver Nanowire Devices for Wearable Pressure Sensors Li Dan, Sophie Shi, Hyun-Joong Chung, and Anastasia Leila Elias ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00807 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 18, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 33 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

ACS Applied Nano Materials

Porous Polydimethylsiloxane-Silver Nanowire Devices for Wearable Pressure Sensors Li Dana, Sophie Shia, Hyun-Joong Chunga, and Anastasia Elias a, * a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4 Canada

Abstract

We demonstrate a simple, non-lithographic method for fabricating piezoresistive pressure sensors with a broad range of working pressures and low detection limit. Our wearable pressure sensor is fabricated using two metallized, porous polymer layers which undergo a change in resistance as a function of pressure. This sensor has a sandwich structure comprised of top and bottom sheets of porous polydimethylsiloxane (PDMS), fabricated using a simple templating method. The inner face of each of these layers is coated with a layer of conductive silver nanowires (AgNWs). The AgNW layers are initially in light contact, and the device undergoes a change in resistance when pressure is applied perpendicular to the plane of the sheets. In these devices, the size of the pores is an important determinant of the device sensitivity and range. Powder templates of KCl, NaCl, and sugar are used to create different sized pores (60-90 µm, 200-275 µm, and 400550 µm, respectively) in the elastomeric PDMS layers, which in turn affects the mechanical deformability of the devices. The sensor fabricated with KCl porous layers (which had the smallest pores) demonstrate the best performance, with a sensitivity of 14.1 kPa-1 (up to 3.5 kPa), 4.8 kPa1

(up to 10 kPa), 1.84 kPa-1 (up to 40 kPa) and good stability over 1000 loading and unloading

cycles. Pressure sensors are also fabricated by decreasing the ratio of PDMS base to curing agent from the standard 10:1 formulation, which increases the stiffness of the viscoelastic PDMS layer, and thus shortens the response time. The stiffest formulation (5:1 (base:curing agent)) samples gives the shortest response time of ~ 47 ms. Wearable electronic applications of these devices,

ACS Paragon Plus Environment

ACS Applied Nano Materials 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

including pulse measurement, facial movements, and sound tracking, are demonstrated in this work. KEYWORDS: Wearable electronic devices, piezoresistive pressure sensor, PDMS elastomer, silver nanowires, flexible electronics

1. INTRODUCTION Wearable pressure sensors provide users with the opportunity to measure physiological data such as heart rate during physical activity continuously and in real time. Pressure sensors can be used in the fields of prosthetic skin, robotics, and flexible health monitoring devices such as smart bands and smart watches. 1–4 With increasing interest in wearable pressure sensor devices, technological challenges in the field that must be addressed include achieving high sensitivity down to a low detection limit, as well as being able to measure a wide range of pressures. Physiologically relevant pressures include low pressure changes generated due to the flow across the upper lip during inhaling and exhaling (< 1 kPa), moderate pressures associated with both the pulse and with finger tapping (1-10 kPa), and relatively high pressures such as the weight of a person as measured through their feet while standing (> 10 kPa).5–9 Therefore, having such pressure information over a broad pressure range of at least 10 kPa is beneficial for health monitoring devices. Moreover, pressure sensors with linear sensing properties across a broad pressure range are desirable for interpreting data without further complex signal processing. The aforementioned requirements are not only factors to be considered for practical applications; light weight and good elasticity are also beneficial for pressure sensors.10,11 To address these difficulties, a large number of thin film pressure sensors have been reported based on various mechanisms, including capacitive, piezoresistive, etc. types.12–14


Page 2 of 33

Page 3 of 33 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


Capacitive sensors leverage materials and structures that undergo changes in dielectric properties as a function of applied pressure. Such materials typically consist one or two pieces of dielectric materials, which are sandwiched between two electrodes. With the application of increasing pressure, distance between two electrodes decreases, causes an increase in capacitance.15,16 Micropatterned dielectric layers have been widely used to enhance the sensitivity in low pressure range.17 The air voids enable the micro-patterned surface to elastically deform under applied pressure, thereby deforming and releasing the energy reversibly. Typically, a patterned structure (e.g. microfabricated pyramids) act as a spacer, and when pressure is applied the change in distance between top and bottom electrodes leads to a change in capacitance. For example, in 2015, Zhenan Bao’s group developed a capacitive pressure sensor with a microhair structure that was capable of measuring arterial pressure.18 The sensitivity of their sensors was limited (0.56 kPa-1) due to the low dielectric coefficient of the sensing materials (elastomers).

19

The sensitivity of capacitive

sensor can be enhanced by using materials with higher dielectric constants. Cheomin Park’s group adopted ionic gels as responsive materials, as they have high dielectric constants compared to elastomers. They implemented these gels in a micropatterned pyramidal structure and the resulting devices had a high sensitivity (41 kPa-1); this sensitivity was achieved at a relatively low working pressure up to 400 Pa.20 Though capacitive sensors are promising, the manufacturing processes can be expensive, as sophisticated microfabrication techniques are often required. Capacitive sensors are additionally quite sensitive to environmental conditions regards to temperature, moisture etc., which can affect the sensing performance. Piezoresistive pressure sensors are typically engineered using a network of conductive particles in a deformable polymer matrix.21–23 Piezoresistive pressure sensors can be engineered using a broad selection of responsive materials and a simple data interpretation mechanism,



Page 4 of 33

therefore resistive sensors remain much cheaper than other type sensors (e.g. capacitive sensors). In the literature, pressure sensors have been demonstrated using responsive fillers which include carbon nanomaterials (e.g. carbon nanotubes, reduced graphene oxide, carbon black) and metallic nanoparticles;

these

fillers

are

typically

embedded

in

elastomeric

matrices

(e.g.

polydimethylsiloxane (PDMS), Ecoflex).24–26 In these composites, electrical current is carried through the network of conducting particles. When a pressure is applied, the distance between particles is reduced, and current can be transported more easily. For example, a multi-wall carbon nanotube (CNT) blend PDMS composite has been used to measure planar pressures.

27

In a

different approach, microfabricated structures that form pressure-sensitive bridged connections can be utilized. The planar surface of responsive materials with no compressible elements (e.g. pores, pillars) typically have very low pressure sensitivity and long relaxation times. Porous structures create surfaces where a change in the contact areas that occurs upon compression leads to a large change in resistance. Resistive pressure sensors which employ (porous) polymer foams have therefore been intensively reported. 28Compressive deformation of the foam structure leads to a large change in resistance while also allowing relatively high sensitivity under small deformation. In such devices the conductive particles may either be blended into the polymer foam structure or applied as a surface coating to the foam skeleton. However, foams themselves can be mechanically fragile, causing the devices to be unstable over extended cyclic testing (i.e. over thousands of cycles). 29,30 In addition, thick foams (cm scale) can be required to achieve measurable changes in conductivity upon deformation, such devices can be bulky on the surface of the skin.31,32 Therefore, simple methods to fabricate thin porous layers (< 1mm thickness) have been developed.31,33–35 Among these methods, templating is advantageous in terms of cost-effectiveness and scalability. For example, Pang et. al. used the rough surfaces of sandpaper as templates to


Page 5 of 33 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


create a random distribution spinosum microstructure which were subsequently coated with conductive reduced graphene oxide (rGO). When assembled in a sandwich structure, these devices were able to achieve a sensitivity of 25.1 kPa-1 (over the range from 0 - 2.6 kPa), which is extremely high compared to a number of other microfabricated sensors.36 Very similar structures (porous layers coated with a conductive material) have also been used extensively in a related application: triboelectric energy scavenging. Rather than using complex microfabricated structures, Cui et. al. used Na2CO3 as a sacrificial powder template to create smaller pores in PDMS. As compared with other templates (e.g. NaCl, sugar); they found out that structures formed using Na2CO3 gave better triboelectric performance due to the higher overall surface area of the smaller microstructures, which yielded a highly conductive surface when coated.37 In this work, we demonstrate a simple new templating method for fabricating a wearable pressure sensor which incorporates two thin layers (up to 150 μm) of porous PDMS coated with conductive AgNWs and sandwiched together (Figure 1). When pressure is applied perpendicular to the plane of the layers, the contact between the planar surface of the conductive layers improves, therefore increasing the conductivity of the device over low pressure ranges. At higher pressures, the pores themselves can deform, increasing the contact area even further. The pore size on device performance was investigated utilizing KCl, NaCl, and sugar as powder templates to form the porous PDMS layers; the sensing performance of devices was then tested after these layers were metallized with AgNWs and sandwiched together (Figure 1d). The size and structure of pores formed by different powders as sacrificial templates were determined through optical microscopy and scanning electron microscopy (SEM). In addition, the mechanical properties of both porous and non-porous PDMS materials were investigated by tensile tester and dynamic mechanical analysis (DMA). In order to further improve the response time, the stiffness of the PDMS was



tuned by varying the ratio of PDMS base to curing agent.38 The sensing mechanism and output performance of the devices were characterized by compressing devices while measuring their electrical characteristics.

2. EXPERIMENTAL METHODS The fabrication process and overall design concept are shown in Figure 1, and detailed photographs of the experimental process are shown in Figure S1 (Supplementary Information). The PDMS precursor (comprised of a 10:1 ratio of elastomer base to curing agent), Sylgard 184 (Sigma-Aldrich) was vacuum degassed for 3 min to remove any bubbles present, and 1.5 g of PDMS was dispensed onto a substrate of polyethylene terephthalate (PET) film (10 cm x 10 cm, Melinex 516, 3 mil), and then spin coated at 1500 rpm for 12 seconds to create a thin, evenly distributed layer (layer mass ~ 0.35 g). The resulting layer was quite viscous, and did not flow visibly during subsequent processing prior to curing. Potassium chloride (KCl) (Sigma-Aldrich, P0911-500 G, batch # SLBC7477V), NaCl (Sigma-Aldrich, 310166-1KG, batch #: 028K3402), and sugar (Redpath brand), were used as sacrificial powder templates to create pores in the PDMS. The size of each powder was observed by optical microscope (Figure S2), and was quantified by ImageJ using the Watershed method. All powder templates were somewhat cubic in structure, with the KCl having the smallest edge lengths (< 100 µm), the sugar having the largest dimensions (> 400 µm) and the NaCl falling in between (roughly between 100 µm and 200 µm). To fabricate porous layers, the powder was spread on flat surface utilizing a brown K-bar hand coater (Testing Machines Inc., USA), consisting of 1 mm wire wrapped around a stainless steel rod. These rods are normally utilized to form uniform films from solutions, where the grooves between the wires control the thickness of the solution spread (typically around 80 µm). When


Page 6 of 33

Page 7 of 33 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


using the bar to spread the powders, a relatively flat and uniform layer was formed over a 10 cm x 10 cm area (typically comprised of 1 to 2 layers of powders depending on the size of the powders themselves, Figure S2). The prepared thin layer of PDMS was placed on top of the powder, and a weight was placed on the PDMS (with pressure of 50 kPa), thereby implanting the powder into the PDMS. Excess powder was then shaken off of the sample, and the composite was cured in the oven at 80 °C for 2 hours. All powders were water soluble, and were removed after curing by dissolving in warm water while stirring for 20-30 minutes, resulting in a thin, flexible, and porous layer. Lastly, porous PDMS films were cut into 1 cm x 1 cm squares for further use. To deposit conductive layers on the porous surface of the PDMS, AgNWs 60 nm × 10 µm (diameter × length) in 0.5 % isopropyl alcohol suspension, obtained from Sigma-Aldrich (Product # 739421) were bath sonicated for 30 s and two drops (200 µl) were dispensed on each 1 cm × 1 cm square of porous PDMS and allowed to saturate the polymer. As an electric lead, a piece of aluminium foil wire (Uline, CA) was mounted on the edge of one side of each piece of PDMS with a small amount of Pelco silver paste (Ted Pella, Inc.) applied to the PDMS surrounding the wire to reduce the contact resistance. Next, double sided insulating Kapton tape was attached to the exposed area of the aluminium wires on the samples to ensure that the electrical signals created were from contact porous layers. Gentle pressure was applied to the two pieces of AgNWs coated PDMS ends to create a bridge structure, with the AgNWs surfaces facing each other and one wire extending from each side of the device.



Figure 1(a) PDMS is spin-coated on a PET substrate, and is then flipped over and pressed into a layer of powder. The composite material is then cured; (b) The PET substrate is removed and the powder is dissolved in water; (c) Silver nanowires are drop coated onto the surface of the porous layers and electrodes are attached; (d) A sandwich structure is formed; (e) The device is loaded; (f) photograph of porous layer; (g) Photograph of sandwiched device including aluminum tape wiring. 2.4 Characterization Surface Morphology The surface morphologies of porous PDMS and AgNWs coating were examined using both scanning electron microscopy (SEM, Zeiss Sigma FESEM w/EDX EBSD) and optical microscopy


Page 8 of 33

Page 9 of 33 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


(Celestron 400x Laboratory Biological Microscope). To prepare samples for SEM, samples were mounted on carbon tape and sputter-coated with 8 nm of gold; samples were then imaged with 5 kV of accelerating voltage. To collect optical images samples were mounted on glass slides and imaged using an optical microscope (Zeiss AXIO lab A1 Optical Microscope). Contact angle measurements were performed by the sessile drop method (FTA-200, FOLIO Instruments Inc., Kitchener, ON, USA) A drop of Milli-Q water was placed on the surface of each dried sample and the contact angle was measured by the installed software (FTA-200).

Tensile Testing The mechanical properties of plain PDMS and porous PDMS samples were analyzed by tensile testing. This characterization was carried out using an Instron 5943 (Instron, Norwood, MA, U.S.A.) using a 1 kN load cell and screw side action grips. Testing samples were cut into rectangular strips with a blade (dimensions: L × 500 mm, W × 10mm, T × 150 um). All samples were strained at 5 mm min-1 until breaking. From the measured raw data (i.e. load and extension), ultimate strain and stress and Young’s modulus were calculated. Young’s modulus values were calculated by fitting the linear range of the curves (typically from 0 up to 20 % strain). Each experiment was performed on five samples of a given type, and average values and standard deviations are reported.

Dynamic Mechanical Analysis The viscoelastic properties of non-porous PDMS samples prepared with different ratios of base: curing agent were measured by dynamic mechanical analysis (DMA) using PerkinElmer dynamic mechanical analyzer (DMA 8000). Non-porous PDMS samples (15 mm × 10 mm × 150



μm) were tested under compression mode, with a frequency of 1 Hz and 0.05 mm standard displacement under single strain conditions. The storage modulus (G’), loss modulus (G’’), and damping factor (tan δ) were obtained.

Electrical Characterization The change in resistance of the devices was measured by Keithley 2401 electrometer as pressure was applied using a tensile tester in compression mode. Data was recorded using KickStart Instrument Control software (Tektronic) at a sampling rate of 70 ms. The assembled pressure sensor with aluminum wires was mounted on the center of the compression platen, and an indenter with a circular diameter of 4 mm was mounted in the flat clamps of the tensile tester. The probe was then used to indent the sample at a compression rate of 0.05 mm/min. The change in resistance of the devices under a cyclic deformation was measured with a Keithley 2401 electrometer as a compressive force was applied by a dynamic mechanical analyzer (DMA, DMA 8000, Perkin-Elmer). The compressive fixtures of the DMA were utilized; the upper and lower fixtures completely covered the sample surface and the dynamic force was applied to the lower fixture. A displacement of 0.02 mm was applied at a frequency of 1 Hz.

3. RESULTS AND DISCUSSION Surface Morphology: Effect of Powder Template After templating, the resulting films – prepared using 10:1 base to curing ratio of PDMS – were thin and porous (150 µm), but not overly fragile. The films appeared cloudy due to the scattering from the network of embedded pores (Figure 1g). The films were slightly shinier on one side than the other, suggesting that one surface included a thin continuous layer of PDMS. The


Page 10 of 33

Page 11 of 33 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


morphologies of different sacrificial powder templates and resulting porous films were characterized by scanning electron microscopy (SEM), and the resulting images are shown in Figure 2. Figure 2 (a), (d), (g) show the powder templates of KCl, NaCl, and sugar, respectively, while (b), (e), (h) show the corresponding pores in the KCl-templated 10:1 PDMS, NaCl-templated 10:1 PDMS, and sugar-templated 10:1 PDMS as viewed from the top of the PDMS layer (in the sample names, the 10:1 ratio reflects the ratio of base:curing agent in the PDMS). The structure of the powder templates was well-replicated in the porous PDMS layers. For each powder template type, pores with a roughly cubic shape were randomly distributed on the PDMS surface, and the diameter of the pores was comparable size to the powder templates. Some smaller pores were observed in each porous layer due to the non-uniform size of the template powder. To quantify the pore size distribution, the Watershed method in the ImageJ software was used, and the results are shown in Figure 2 (c), (f), (i) for KCl, NaCl, and sugar templates, respectively. From the bar chart, the size of the pores in the samples templated from KCl ranged mainly from 60 to 90 μm, while those created by NaCl and sugar ranged from 200 to 275 μm and 400 to 550 μm, respectively. Therefore, the pores created by KCl were the smallest in lateral size and the pores templated from sugar were the largest. Cross-section images of each sample type of porous sample are shown in Figure S3; films template from the smallest powder (KCl) could accommodate up to two layers of pores through their thickness, while films templated from the larger powders (NaCl and sugar) could accommodate only a single layer of pores through their thickness as the size of the powder particles is comparable to or larger than the actual film thickness. Cross-sectional images of the the PDMS are shown in Figure S3, and illustrate that the pore sizes are comparable in size and shape to the powder template. Overall, the structure of the porous samples was fairly uniform across a given sample.



The water contact angle measurement was performed on plain PDMS and different porous PDMS surfaces, in Figure S4 and Table S1, and shows that the contact angle was slightly higher for the porous samples than for the non-porous PDMS. Nonetheless the difference was relatively small overall and was not expected to affect the deposition of the AgNWs from suspension in isopropyl alcohol.

Figure 2: (a), (d), (g) SEM images of KCl, NaCl, and sugar powder templates, respectively; (b), (e), (h) SEM images of porous PDMS template from KCl, NaCl, and sugar, respectively; (c), (f), and (g) pore distribution charts of each sample (shown in the same order as previously) depicting the percent of pores of each size range in the images surveyed.

SEM images depicted the porous samples after coating with AgNWs are shown in Figure S5; a spider web-like network of conductive particles can be seen on the porous surface of the PDMS. The AgNWs formed a relatively thin layer over the porous sample. The pores were


Page 12 of 33

Page 13 of 33 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


conformally coated, and therefore remain open in appearance, and have similar size and shape to the pores in the uncoated samples.

Sensitivity: effect of template Once devices were fabricated from two pieces of AgNWs-coated porous layers assembled in a sandwich structure with electrodes, they were characterized by applying a mechanical force perpendicular to the plane of the layers while measuring the resistance across the interface (as illustrated in Figure 1-e). The change in resistance as a function of applied pressure for devices template from each powder (KCl-templated 10:1 PDMS (black), NaCl-templated 10:1 PDMS (grey), sugar-templated 10:1 PDMS (orange)) is shown in Figure 3a, and an enlargement of the pressure range up to 10 kPa is shown in Figure 3b. For all devices, the resistance generally decreased as pressure was applied, due to the increased contact between the AgNWs on the surface of each porous layer. However, initially, there was little to no change in resistance as a function of pressure, as below a threshold value (the detection limit) the top and bottom layers are not in contact and current cannot travel between them. While ideally the samples would initially be perfectly flat and make light electrical contact without the application of any additional pressure, here we see that small pressures were required to overcome small separations that may result from dust or imperfect surface flatness. The detection limits of the devices templated from KCl, NaCl, and sugar were approximately 20 Pa, 350 Pa, and 700 Pa, respectively. A similar effect was observed previously by Yu et al.

35

The KCl-templated 10:1 PDMS device had the smallest

detection limit compared to other two types, although these samples exhibited the highest Young’s modulus. This could be attributed to the roughness, structure and pore sizes of the samples.



Above the minimum threshold pressure, a drop in resistance occurred for all samples as the pressure increased. The normalized change in resistance as a function of pressure was initially large (decreased rapidly), and then at higher pressures the change lessened (i.e. the slope became smaller), suggesting that the deformation that took place at higher pressures was more limited, the overall response was both larger and more consistent for the devices template from the smallest powders, i.e. KCl and NaCl (the porous layers of these devices also had higher Young’s moduli than the devices template from sugar). The sensitivity of the devices was calculated by taking the slope of the curves in initial linear portion of curves. The sensitivities were 14.1 kPa-1 (up to 3.5 kPa), 20.0 kPa-1 (up to 1 kPa) for devices templated from KCl and NaCl, respectively. The devices templated from NaCl had the highest sensitivity, however, the linear portion of the curve (up to 1 kPa) was narrower than for devices fabricated with KCl-templated 10:1 PDMS, enabling only subtle pressure detection. The lower sensitivity but larger response of the devices made from KCltemplated 10:1 PDMS may additionally be related to the fact that the porous layers of this device had the highest Young’s modulus of the three types of templated layers. Although this value was measured in tension rather than compression, the relatively high modulus of this material should enable it to undergo relatively small deformations as a function of pressure potentially over a wider range of values than for softer materials. The device templated from sugar had the lowest sensitivity overall (initially undergoing the smallest change in resistance as a function of pressure) and exhibited odd behavior whereby the change in resistance drops sharply in the range from around 700 Pa to 1 kPa, then exhibited very little change over pressures from 2 kPa and 4 kPa, and then dropped sharply again until a pressure of 5 kPa is reached, and then leveled off again. These measurements indicate that initially, the applied pressure increased the contact area between the two conductive layers, but then from 2 to 4 kPa there was very little change, and then above 4 kPa


Page 14 of 33

Page 15 of 33 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


the contact area again increased. This suggests a large-scale deformation or collapsing of the pores above 4 kPa. When characterizing the mechanical properties of the sugar-templated PDMS in compression, the compressive stress abruptly dropped 10 kPa (shown in Figure S6) at a compressive distance of 100 µm (corresponding a relative strain of 67%), which suggests that a large scale deformation such as collapsing of pores occurred. To test the reproducibility of the devices and the fabrication procedure, three devices of each type were fabricated and characterized, and the results are shown in Figure S7 (response curves and Table S2 (which summarizes the sensitivity and standard deviations within a given range for a particular type of device). The response to pressure of each device was found to be highly reproducible across independent devices prepared from the same powder template.

Response time: effect of template The response and recovery times of the devices is shown in Figure 3c through loading and unloading experiments. The relative change in resistance of the devices was measured as a pressure of 40 kPa was applied rapidly, held for about 1 s, and then removed rapidly. It can be seen that while the response of each of the three devices exhibits no obvious time delay upon initial loading, a time delay occurs for each device upon unloading. Specifically, the stiffest device – made with KCl-templated 10:1 PDMS – had the fastest recovery time (80 ± 10 ms), while devices made with sugar-templated PDMS (which had the largest pores) had the slowest recovery time (350 ± 70 ms); devices made with NaCl-templated 10:1 PDMS, which had intermediate-sized pores and Young’s modulus, exhibited intermediate recovery times (230 ± 25 ms) with respect to the other types of devices. The hysteresis of the devices was mainly caused by the stiffness of the porous layers, which was in turn influenced by the size of the pores. This is in agreement with the mechanical



test results, where KCl-templated 10:1 PDMS layers were the stiffest and sugar-templated 10:1 PDMS samples were the softest.

Figure 3. Characterization of devices fabricated using NaCl-templated 10:1 PDMS, KCl-templated 10:1 PDMS, and sugar-templated 10:1 PDMS. (a) Relative resistance change as a function of applied pressure, (b) enlargement of the low pressure range, and (c) time-response of devices upon loading and unloading of 40 kPa pressure. Device Reliability A cyclic loading and unloading test was performed to evaluate the device reliability of a KCl-templated 10:1 PDMS device (Figure 4). A constant subtle pressure (~900 Pa) was applied to and removed from the KCl-templated 10:1 PDMS device repeatedly. The output of the device varied consistently between the OFF state (corresponding to 0 change in relative resistance) and the ON state (with a relative resistance value of approximately 15). In the second part of the experiment a moderate pressure (2.8 kPa) was repeatedly applied on the same sample with same loading and unloading interval, which shows a larger change in relative resistance (R-R0)/(R0) change compared to the more subtle pressure (900 Pa). Figure 4b shows the applied pressure and relative resistance as a function of time during cyclic loading and unloading. In these cycles, the pressure was varied from 0 and 5.5 kPa in a


Page 16 of 33

Page 17 of 33 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


triangular function with a strain rate of 0.5 mm/min. A consistent relationship was achieved between pressure and resistance change, where the resistance increased/decreased almost linearly as pressure decreased/increased with no observable time delay. The resistance decreased from around 2.2 kΩ to to 900 Ω as the pressure increased from 0 to 5.5 kPa. Figure 4c exhibits the stability and reliability of the KCl-templated 10:1 PDMS device during a cyclic compression/release performed on the DMA. In this displacement-controlled test, a strain of 0.02 mm (corresponding to a pressure of around 1 kPa) was applied and removed at a rate of 1 Hz. The device showed reasonably a consistent response over hundreds of cycles. In the inset of Figure 4c, the clear sharp peaks recorded once per second under a displacement controlled test (displacement of 20 µm) confirms the reliability and reversibility of the device. Similarly, the results for a test carried out at 1 kPa over 1000 cycles is shown in Figure S8, and no obvious sensitivity degradation was observed during the test. However, for each of these cyclic tests variations in the resistance can be seen. These fluctuations indicate that the contact between the conductive layers is not consistent at low pressure. However, improving the contact (for example by bonding the layers together) would likely remove the response at low pressure altogether. At higher pressures, (e.g. up to 40 kPa), we would expect to see more consistent results over repeated cycling, although the durability of the device may decrease.



Page 18 of 33

Figure 4. Cyclic characterization of a KCl-templated 10:1 PDMS device: (a) cyclic loading and unloading (900 Pa and 2.8 kPa), (b) real time resistance changes as a function of applying pressure from 0 to 5 kPa , (c) displacement controlled compression test at 1Hz and 20 µm displacement.

Response time: effect of template on mechanical properties of PDMS matrix The mechanical properties of the pure PDMS and porous films (without AgNWs) were characterized using a tensile tester. The ultimate strain, ultimate stress, and Young’s modulus values for all samples are shown in Table 1. All porous films were found to have significantly lower Young’s modulus, ultimate strain, and ultimate stress as compared with bulk PDMS. Films templated from sugar exhibited the lowest value, undergoing a decrease in modulus of more than 50 %. This can be attributed to the large size of the resulting pores, which yield a brittle but flexible porous layer. Typical strain-stress curves and the resulting Young’s modulus of nonporous PDMS combined in different ratios of elastomer:crosslinker are shown in Figure S9 and S10, respectively. Sample

Non-porous

Young’s

Tensile

Average ultimate Thickness

Modulus (kPa)

strength (kPa)

tensile strain (%)

(µm)

1250 ± 60

420 ± 60

200 ± 14

150 ± 1

850 ± 80

108 ± 10

34 ± 7

146 ± 5

340 ± 60

136 ± 24

33 ± 2

147 ± 6

10:1 PDMS KCl-templated 10:1 PDMS NaCl-templated 10:1 PDMS


Page 19 of 33 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


Sugar-

260 ± 80

85 ± 9

28 ± 7

140 ±8

templated 10:1 PDMS

Table 1. Mechanical properties of porous films fabricated by different templates as measured by tensile testing.

To further shorten the response time of PDMS deformation under applied pressure, different ratios of PDMS base to curing agent were used. Dynamic mechanical analysis (DMA) was carried out on samples in tension mode. The storage modulus (G’, which reflects the elastic properties of the sample), loss modulus (G’’, which reflects the viscous properties) and the damping factor (tan delta, the ratio of G’ to G”) are reported in Figure 5a. The results show that, as expected, as the amount of curing agent – which acts as a crosslinker – decreases, the storage modulus decreases. For example, the PDMS sample with a 5:1 ratio (base: curing agent) had the highest G’ and 15:1 had the lowest G’. This trend agrees with the tensile test data, where 5:1 exhibited the highest Young’s modulus and 15:1 exhibited the lowest value. The loss modulus (G’’) increased only minimally as the ratio of base/curing agent was varied from 15:1 to 5:1, indicating that energy dissipation efficiency did not changed much compared to the storage modulus. Resultedly, tan delta increased dramatically with deceasing curing agent ratio (blue curve, in Figure 5a). This indicates that stiffer materials (with lowest ratios of base/curing agent) had a faster response time (shorter relaxation time) to applied stress than softer materials. This parameter is therefore expected to impact the response time of the devices. The sensing performance of KCl-templated PDMS devices with base:curing agent ratios of 5:1, 10:1, 15:1 is shown in Figure 5b. For all samples, the relative resistance generally decreased



as pressure was applied. The KCl-templated 10:1 PDMS sample exhibited the largest overall change in relative resistance, followed by the 5:1 and 15:1 samples. The sensitivity values (linear range) are 53.3 kPa-1 (up to 600 Pa), 14.1 kPa-1 (up to 3.5 kPa), and 4.8 kPa-1 (up to 2.5 kPa) for the KCl-templated 5:1, 10:1, and 15:1 device, respectively. Interestingly, the KCl-templated 15:1 PDMS initially exhibited a linear drop in resistance as the pressure increased from the lower sensitivity to around 2.5 kPa, however the relative resistance then plateaued until a particular pressure (for this device~ 10 kPa) was reached. This step-like behavior is similar to the trend observed for the sugar-templated 10:1 PDMS sample, although the KCl-templated 15:1 PDMS device could withstand a higher pressure before the sharp drop in relative resistance occurred (2.5 kPa for the KCl-templated 15:1 PDMS devices vs 1 kPa for the sugar-templated 10:1 PDMS sample). This large drop likely occurred concurrent with a sudden, large deformation of the sample. These devices are both quite soft: the Young’s modulus for the KCl-templated 15:1 and sugar-templated 10:1 PDMS layers are 500 ± 83 kPa and 262 ± 78.5 kPa. Very soft samples underwent larger deformations under same load compared to the stiff samples, quickly leading to complete contact of the flat, surface portions of the upper and lower layers. As the pressure increased further, the thin layers could deform further, allowing the coated surfaces of the deformable pores to make contact. In both cases, the abrupt change in relative resistance likely corresponded to the collapsing of the pores; the relative resistance then remained relatively flat as the pressure was increased further as the maximum contact between the pores had already been achieved. The KCl-templated 5:1 PDMS and KCl templated 10:1 PDMS devices exhibited interesting properties in terms of sensitivity. A higher sensitivity (linear range) was observed for the 5:1 device as compared with the 10:1 device at smaller pressures (hundreds Pa), however at


Page 20 of 33

Page 21 of 33 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


high pressures (around 1 kPa and higher) the 10:1 device exhibited the higher sensitivity of the two. The initial difference could be attributed to the stiffness difference, where the stiff material (of the 5:1 device) underwent the largest deformation initially in response to the external stimulus, quickly forming contact between the surfaces. Over the higher pressure range, the stiffer material deformed less as a function of pressure compared to the soft material, resulting in less contact between recessed areas such as the surface of the pores. Overall, the 10:1 device showed the most consistent performance. Relaxation time: effect of stiffness To validate the differences in time delays, loading and unloading experiments were performed on the KCl-templated sensors with PDMS base-curing ratios ranging from 5:1 to 15:1 as shown in Figure 5c. A constant pressure was quickly applied to the sensors, held for several seconds, and then quickly removed. There was not a great difference observed in the response time of sensors (upon application of the pressure), but the relaxation times (upon removal of the pressure) varied greatly. The KCl-templated 5:1 PDMS sensor has the shortest relaxation time (47 ms) while the KCl-templated 15:1 PDMS sensor required the longest to recover to the original state (160 ms). These results agree well with the DMA data, showing the addition of more curing agent could improve the response time of the sensor. This simple method is advantageous compared to adding fillers to the PDMS to enhance the response time of the sensors.38



Figure 5. (a) DMA storage and loss modulus, and damping factor (blue color, on the right y axis) of non-porous PDMS layers utilizing different ratios of base to curing agent as measured in compression (1 Hz) (a); (b) Resistance change as a function of applied pressure (up to 40 kPa), and (c) relaxation time of KCl-templated PDMS devices with varying ratios of base to curing agent under 40 kPa.

Applications Owing to the small threshold, high sensitivity and broad pressure measuring range of the KCltemplated10:1 PDMS pressure sensor, it is a good candidate device to detect physiological signals (e.g. pulse, voice, motion movement). The pulse rate is an important sign that can be used to evaluate the health status of human bodies. Figure 6a shows a KCl-templated 10:1 PDMS pressure sensor that was mounted on the wrist with 3M tape. The heart rate was measured by the KClPDMS pressure sensor from the radial artery of the wrist. Each signal had two waves, and the pulse was calculated by the interval between each of the two strong peaks, as shown on Figure 6b. Two typical peaks (P1 and P2) can be clearly distinguished (Figure 5c) each cycle, corresponding to the incident forward wave for the strong peak and late systolic wave for the smaller peak, respectively. From Figure 6c, an interval of 0.5 s can be determined. This corresponds to a heart rate of 120 beats per second, which falls in the healthy range for the tester (a 25-year-old adult female). This


Page 22 of 33

Page 23 of 33 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


demonstrates that our sensor is capable of detecting a very small range of pressure, showing stable data. The KCl-templated 10:1 PDMS pressure sensor also exhibits the ability to monitor respiration rates (i.e. inhaling and exhaling). Figure 6d shows a device attached to the upper lop, and the data shows that the person has a normal respiratory rate of 25 breaths per minute. Figure 6e shows that the device is capable of monitoring facial expressions. Here, a smiling expression was recorded by the sensor, with sharp peaks visible in the relative resistance due to the movement of the skin. Moreover, when the sensor was mounted on the throat, voice signals (which were detected as vibrations in the throat) were recorded (as shown in Figure 6f). Overall, the pressure sensor exhibits great potential in the health monitoring field.



Figure 6. (a) A KCl-PDMS (10:1) device was mounted on the wrist for pulse measurement, enabling the collection of (b) numerous pulse signals over a 12 s interval, and (c) a single pulse signal. The sensor could also be used to collect (d) exhaling signals, (e) facial expression signals, and (f) voice signals (recorded as the person said: “Hello”).

CONCLUSIONS In this work, a simple templating method and two layer structure were proposed to design a new type of resistive pressure sensor. The morphologies, surface properties, mechanical properties, sensing performance of three types powder templates (KCl, NaCl, sugar) were systematically compared. We demonstrated the size, distribution of the pores could influence the effective contact area of the sensing performance. The results show that KCl-templated 10:1 PDMS gave the best sensing performance regards to large linearity range over a broad pressure, exhibiting a sensitivity of 14.1 kPa-1 (up to 3.5 kPa), and good stability. These results are on par with current known pressure sensors. 38–44 The pores offer a larger range of resistance change under applied pressure but render the materials to be soft; therefore, varying the curing agent to PDMS was firstly proposed for the aim of shortening the response time. The 5:1 (elastomer: curing agent) gave the shortest response time (47 ms). The KCl-templated 10:1 PDMS device was used to detect physiological signals in health monitoring field. A simple, low-cost piezoresistive pressure sensor was demonstrated, and can be potentially used for wellness devices such as smart watches, smart bands. From safety viewpoint, the fabrication process and the use of materials do not involve any harmful or toxic substances and the AgNWs did not contact the skin directly, thus ensuring the resulted device has good


Page 24 of 33

Page 25 of 33 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


biocompatibility. More importantly, the simple templating method should enable scalability in terms of fabrication, thereby ensuring low device costs as compared with those fabricated using lighography-based techniques. The two layer structure is the key design to enhance the detection limit. Lastly, the thin device can be mounted on skin, to measure subtle pressure such as the pulse, and was shown to be able to detect both the major and minor peaks. Beyond pulse detection, the device can be leveraged to measure other physiological movements and pressures. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional characterization of samples (optical images of salt templates, SEM images of AgNWs distribution and cross-sectional images of created pores, stability test, mechanical properties measurements, etc.) Acknowledgments Funding from the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canada Foundation for Innovation is gratefully acknowledged.



REFERENCES (1) (2) (3) (4) (5)

(6) (7) (8) (9) (10)

(11) (12) (13) (14) (15) (16) (17) (18)

Jeong, J.-W.; Yeo, W.-H.; Akhtar, A.; Norton, J. J. S.; Kwack, Y.-J.; Li, S.; Jung, S.-Y.; Su, Y.; Lee, W.; Xia, J.; Cheng, H. Materials and Optimized Designs for Human-Machine Interfaces Via Epidermal Electronics. Adv. Mater. 2013, 25 (47), 6839–6846. https://doi.org/10.1002/adma.201301921. Takei, K.; Takahashi, T.; Ho, J. C.; Ko, H.; Gillies, A. G.; Leu, P. W.; Fearing, R. S.; Javey, A. Nanowire Active-Matrix Circuitry for Low-Voltage Macroscale Artificial Skin. Nat. Mater. 2010, 9 (10), 821–826. https://doi.org/10.1038/nmat2835. Gerratt, A. P.; Michaud, H. O.; Lacour, S. P. Elastomeric Electronic Skin for Prosthetic Tactile Sensation. Adv. Funct. Mater. 2015, 25 (15), 2287–2295. https://doi.org/10.1002/adfm.201404365. Chorsi, M. T.; Curry, E. J.; Chorsi, H. T.; Das, R.; Baroody, J.; Purohit, P. K.; Ilies, H.; Nguyen, T. D. Piezoelectric Biomaterials for Sensors and Actuators. Adv. Mater. 2019, 31 (1), 1802084. https://doi.org/10.1002/adma.201802084. Zhang, Y.; Hu, Y.; Zhu, P.; Han, F.; Zhu, Y.; Sun, R.; Wong, C.-P. Flexible and Highly Sensitive Pressure Sensor Based on Microdome-Patterned PDMS Forming with Assistance of Colloid Self-Assembly and Replica Technique for Wearable Electronics. ACS Appl. Mater. Interfaces 2017, 9 (41), 35968–35976. https://doi.org/10.1021/acsami.7b09617. Kim, Y.; Zhu, J.; Yeom, B.; Prima, M. D.; Su, X.; Kim, J.-G.; Yoo, S. J.; Uher, C.; Kotov, N. A. Stretchable Nanoparticle Conductors with Self-Organized Conductive Pathways. Nature 2013, 500 (7460), 59–63. https://doi.org/10.1038/nature12401. Trung, T. Q.; Ramasundaram, S.; Hwang, B.-U.; Lee, N.-E. An All-Elastomeric Transparent and Stretchable Temperature Sensor for Body-Attachable Wearable Electronics. Adv. Mater. 2016, 28 (3), 502–509. https://doi.org/10.1002/adma.201504441. Fan, Y. J.; Li, X.; Kuang, S. Y.; Zhang, L.; Chen, Y. H.; Liu, L.; Zhang, K.; Ma, S. W.; Liang, F.; Wu, T.; Wang, Z.L. Highly Robust, Transparent, and Breathable Epidermal Electrode. ACS Nano 2018, 12 (9), 9326–9332. https://doi.org/10.1021/acsnano.8b04245. Doshi, S. M.; Thostenson, E. T. Thin and Flexible Carbon Nanotube-Based Pressure Sensors with Ultrawide Sensing Range. ACS Sens. 2018, 3 (7), 1276–1282. https://doi.org/10.1021/acssensors.8b00378. Kim, K. L.; Lee, W.; Hwang, S. K.; Joo, S. H.; Cho, S. M.; Song, G.; Cho, S. H.; Jeong, B.; Hwang, I.; Ahn, J.-H.; Yu, Y.J. Epitaxial Growth of Thin Ferroelectric Polymer Films on Graphene Layer for Fully Transparent and Flexible Nonvolatile Memory. Nano Lett. 2016, 16 (1), 334–340. https://doi.org/10.1021/acs.nanolett.5b03882. Yin, B.; Liu, X.; Gao, H.; Fu, T.; Yao, J. Bioinspired and Bristled Microparticles for Ultrasensitive Pressure and Strain Sensors. Nat. Commun. 2018, 9 (1), 5161. https://doi.org/10.1038/s41467-018-07672-2. Kang, M.; Kim, J.; Jang, B.; Chae, Y.; Kim, J.-H.; Ahn, J.-H. Graphene-Based Three-Dimensional Capacitive Touch Sensor for Wearable Electronics https://pubs.acs.org/doi/abs/10.1021/acsnano.7b02474 (accessed Aug 7, 2018). https://doi.org/10.1021/acsnano.7b02474. Yu, L.; Yeo, J. C.; Soon, R. H.; Yeo, T.; Lee, H. H.; Lim, C. T. Highly Stretchable, Weavable, and Washable Piezoresistive Microfiber Sensors. ACS Appl. Mater. Interfaces 2018, 10 (15), 12773–12780. https://doi.org/10.1021/acsami.7b19823. Triboelectric Active Sensor Array for Self-Powered Static and Dynamic Pressure Detection and Tactile Imaging - ACS Nano (ACS Publications) https://pubs.acs.org/doi/abs/10.1021/nn4037514 (accessed Oct 5, 2018). Tee, B. C.-K.; Chortos, A.; Dunn, R. R.; Schwartz, G.; Eason, E.; Bao, Z. Tunable Flexible Pressure Sensors Using Microstructured Elastomer Geometries for Intuitive Electronics. Adv. Funct. Mater. 2014, 24 (34), 5427–5434. https://doi.org/10.1002/adfm.201400712. Mannsfeld, S. C. B.; Tee, B. C.-K.; Stoltenberg, R. M.; Chen, C. V. H.-H.; Barman, S.; Muir, B. V. O.; Sokolov, A. N.; Reese, C.; Bao, Z. Highly Sensitive Flexible Pressure Sensors with Microstructured Rubber Dielectric Layers. Nat. Mater. 2010, 9 (10), 859–864. https://doi.org/10.1038/nmat2834. Yin, M.; Zhang, Y.; Yin, Z.; Zheng, Q.; Zhang, A. P. Micropatterned Elastic GoldNanowire/Polyacrylamide Composite Hydrogels for Wearable Pressure Sensors. Adv. Mater. Technol. 2018, 3 (7), 1800051. https://doi.org/10.1002/admt.201800051. Pang, C.; Koo, J. H.; Nguyen, A.; Caves, J. M.; Kim, M.-G.; Chortos, A.; Kim, K.; Wang, P. J.; Tok, J. B.H.; Bao, Z. Highly Skin-Conformal Microhairy Sensor for Pulse Signal Amplification. Adv. Mater. 2015, 27 (4), 634–640. https://doi.org/10.1002/adma.201403807.


Page 26 of 33

Page 27 of 33 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


(19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30)

(31)

(32) (33) (34) (35) (36)

Boutry, C. M.; Kaizawa, Y.; Schroeder, B. C.; Chortos, A.; Legrand, A.; Wang, Z.; Chang, J.; Fox, P.; Bao, Z. A Stretchable and Biodegradable Strain and Pressure Sensor for Orthopaedic Application. Nat. Electron. 2018, 1 (5), 314. https://doi.org/10.1038/s41928-018-0071-7. Cho, S. H.; Lee, S. W.; Yu, S.; Kim, H.; Chang, S.; Kang, D.; Hwang, I.; Kang, H. S.; Jeong, B.; Kim, E. H.; Cho, S.M. Micropatterned Pyramidal Ionic Gels for Sensing Broad-Range Pressures with High Sensitivity. ACS Appl. Mater. Interfaces 2017, 9 (11), 10128–10135. https://doi.org/10.1021/acsami.7b00398. Lopera-Valle, A.; Elias, A. Low-Resistance Silver Microparticle-HEMA-PEGDA Composites for Humidity Sensing. Smart Mater. Struct. 2018, 27 (10), 105030. https://doi.org/10.1088/1361-665X/aad355. Dan, L.; Pope, M. A.; Elias, A. L. Solution-Processed Conductive Biocomposites Based on Polyhydroxybutyrate and Reduced Graphene Oxide. J. Phys. Chem. C 2018, 122 (30), 17490–17500. https://doi.org/10.1021/acs.jpcc.8b02515. Ahmed, F.; Azhari, A.; Marzbanrad, E.; Liravi, F.; Ali, U.; Pope, M. A.; Toyserkani, E. Graphene Oxide Humidity Sensor Built Entirely by Additive Manufacturing Approaches. J. Mater. Sci. Mater. Electron. 2019. https://doi.org/10.1007/s10854-019-01226-y. Shu, Y.; Tian, H.; Yang, Y.; Li, C.; Cui, Y.; Mi, W.; Li, Y.; Wang, Z.; Deng, N.; Peng, B.; Ren, T.L. Surface-Modified Piezoresistive Nanocomposite Flexible Pressure Sensors with High Sensitivity and Wide Linearity. Nanoscale 2015, 7 (18), 8636–8644. https://doi.org/10.1039/C5NR01259G. Luo, N.; Dai, W.; Li, C.; Zhou, Z.; Lu, L.; Poon, C. C. Y.; Chen, S.-C.; Zhang, Y.; Zhao, N. Flexible Piezoresistive Sensor Patch Enabling Ultralow Power Cuffless Blood Pressure Measurement. Adv. Funct. Mater. 2016, 26 (8), 1178–1187. https://doi.org/10.1002/adfm.201504560. Ma, Y.; Yue, Y.; Zhang, H.; Cheng, F.; Zhao, W.; Rao, J.; Luo, S.; Wang, J.; Jiang, X.; Liu, Z.; Liu, N. 3D Synergistical MXene/Reduced Graphene Oxide Aerogel for a Piezoresistive Sensor. ACS Nano 2018, 12 (4), 3209–3216. https://doi.org/10.1021/acsnano.7b06909. Gerlach, C.; Krumm, D.; Illing, M.; Lange, J.; Kanoun, O.; Odenwald, S.; Hübler, A. Printed MWCNTPDMS-Composite Pressure Sensor System for Plantar Pressure Monitoring in Ulcer Prevention. IEEE Sens. J. 2015, 15 (7), 3647–3656. https://doi.org/10.1109/JSEN.2015.2392084. Wang, Z.; Guan, X.; Huang, H.; Wang, H.; Lin, W.; Peng, Z. Full 3D Printing of Stretchable Piezoresistive Sensor with Hierarchical Porosity and Multimodulus Architecture. Adv. Funct. Mater. 2019, 29 (11), 1807569. https://doi.org/10.1002/adfm.201807569. Lv, L.; Zhang, P.; Xu, T.; Qu, L. Ultrasensitive Pressure Sensor Based on an Ultralight Sparkling Graphene Block. ACS Appl. Mater. Interfaces 2017, 9 (27), 22885–22892. https://doi.org/10.1021/acsami.7b07153. Park, H.; Kim, J. W.; Hong, S. Y.; Lee, G.; Kim, D. S.; Oh, J. hyun; Jin, S. W.; Jeong, Y. R.; Oh, S. Y.; Yun, J. Y.; Ha, J.S. Microporous Polypyrrole-Coated Graphene Foam for High-Performance Multifunctional Sensors and Flexible Supercapacitors. Adv. Funct. Mater. 0 (0), 1707013. https://doi.org/10.1002/adfm.201707013. Iglio, R.; Mariani, S.; Robbiano, V.; Strambini, L.; Barillaro, G. Flexible Polydimethylsiloxane Foams Decorated with Multiwalled Carbon Nanotubes Enable Unprecedented Detection of Ultralow Strain and Pressure Coupled with a Large Working Range. ACS Appl. Mater. Interfaces 2018. https://doi.org/10.1021/acsami.8b02322. Wang, T.; Li, J.; Zhang, Y.; Liu, F.; Zhang, B.; Wang, Y.; Jiang, R.; Zhang, G.; Sun, R.; Wong, C.-P. Highly Ordered 3D Porous Graphene Sponge for Wearable Piezoresistive Pressure Sensor Applications. Chem. – Eur. J. 2019, 25 (25), 6378–6384. https://doi.org/10.1002/chem.201900014. Ding, L.; Xuan, S.; Pei, L.; Wang, S.; Hu, T.; Zhang, S.; Gong, X. Stress and Magnetic Field Bimode Detection Sensors Based on Flexible CI/CNTs–PDMS Sponges. ACS Appl. Mater. Interfaces 2018. https://doi.org/10.1021/acsami.8b11333. Gao, Y.; Yu, G.; Tan, J.; Xuan, F. Sandpaper-Molded Wearable Pressure Sensor for Electronic Skins. Sens. Actuators Phys. 2018, 280, 205–209. https://doi.org/10.1016/j.sna.2018.07.048. Yu, G.; Hu, J.; Tan, J.; Gao, Y.; Lu, Y.; Xuan, F. A Wearable Pressure Sensor Based on Ultra-Violet/Ozone Microstructured Carbon Nanotube/Polydimethylsiloxane Arrays for Electronic Skins. Nanotechnology 2018, 29 (11), 115502. https://doi.org/10.1088/1361-6528/aaa855. Pang, Y.; Zhang, K.; Yang, Z.; Jiang, S.; Ju, Z.; Li, Y.; Wang, X.; Wang, D.; Jian, M.; Zhang, Y.; Liang, R. Epidermis Microstructure Inspired Graphene Pressure Sensor with Random Distributed Spinosum for High Sensitivity and Large Linearity. ACS Nano 2018, 12 (3), 2346–2354. https://doi.org/10.1021/acsnano.7b07613.



(37) (38) (39)

(40) (41) (42) (43) (44)

Cui, C.; Wang, X.; Yi, Z.; Yang, B.; Wang, X.; Chen, X.; Liu, J.; Yang, C. Flexible Single-Electrode Triboelectric Nanogenerator and Body Moving Sensor Based on Porous Na2CO3/Polydimethylsiloxane Film. ACS Appl. Mater. Interfaces 2018, 10 (4), 3652–3659. https://doi.org/10.1021/acsami.7b17585. Fan, S.; Dan, L.; Meng, L.; Zheng, W.; Elias, A.; Wang, X. Improved Response Time of Flexible Microelectromechanical Sensors Employing Eco-Friendly Nanomaterials. Nanoscale 2017, 9 (43), 16915– 16921. https://doi.org/10.1039/C7NR05218A. Ma, Z.; Wei, A.; Ma, J.; Shao, L.; Jiang, H.; Dong, D.; Ji, Z.; Wang, Q.; Kang, S. Lightweight, Compressible and Electrically Conductive Polyurethane Sponges Coated with Synergistic Multiwalled Carbon Nanotubes and Graphene for Piezoresistive Sensors. Nanoscale 2018, 10 (15), 7116–7126. https://doi.org/10.1039/C8NR00004B. Zeng, R.; Luo, Z.; Zhang, L.; Tang, D. Platinum Nanozyme-Catalyzed Gas Generation for Pressure-Based Bioassay Using Polyaniline Nanowires-Functionalized Graphene Oxide Framework. Anal. Chem. 2018, 90 (20), 12299–12306. https://doi.org/10.1021/acs.analchem.8b03889. Hu, Y.; Zhuo, H.; Chen, Z.; Wu, K.; Luo, Q.; Liu, Q.; Jing, S.; Liu, C.; Zhong, L.; Sun, R.; Peng, X. Superelastic Carbon Aerogel with Ultrahigh and Wide-Range Linear Sensitivity. ACS Appl. Mater. Interfaces 2018, 10 (47), 40641–40650. https://doi.org/10.1021/acsami.8b15439. Guan, H.; Cheng, Z.; Wang, X. Highly Compressible Wood Sponges with a Spring-like Lamellar Structure as Effective and Reusable Oil Absorbents. ACS Nano 2018, 12 (10), 10365–10373. https://doi.org/10.1021/acsnano.8b05763. Li, S.-X.; Xia, H.; Xu, Y.-S.; Lv, C.; Wang, G.; Dai, Y.-Z.; Sun, H.-B. Gold Nanoparticle Densely Packed Micro/Nanowire-Based Pressure Sensors for Human Motion Monitoring and Physiological Signal Detection. Nanoscale 2019, 11 (11), 4925–4932. https://doi.org/10.1039/C9NR00595A. Sun, Q.-J.; Zhao, X.-H.; Zhou, Y.; Yeung, C.-C.; Wu, W.; Venkatesh, S.; Xu, Z.-X.; Wylie, J. J.; Li, W.-J.; Roy, V. A. L. Fingertip-Skin-Inspired Highly Sensitive and Multifunctional Sensor with Hierarchically Structured Conductive Graphite/Polydimethylsiloxane Foams. Adv. Funct. Mater. 0 (0), 1808829. https://doi.org/10.1002/adfm.201808829.


Page 28 of 33

Page 29 of 33 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


Porous Polydimethylsiloxane-Silver Nanowire Devices for Wearable Pressure Sensors Li Dana, Sophie Shia, Hyun-Joong Chunga, and Anastasia Elias a, * a Department of Chemical and Materials Engineering, University of Alberta, Edmonton, AB T6G 2V4 Canada

Figure 1(a) PDMS is spin-coated on a PET substrate, and is then flipped over and pressed into a layer of powder. The composite material is then cured; (b) The PET substrate is removed and the powder is dissolved in water; (c) Silver nanowires are drop coated onto the surface of the porous layers and electrodes are attached; (d) A sandwich structure is formed; (e) The device is loaded; (f) photograph of porous layer; (g) Photograph of sandwiched device including aluminum tape wiring.



Figure 2: (a), (d), (g) SEM images of KCl, NaCl, and sugar powder templates, respectively; (b), (e), (h) SEM images of porous PDMS template from KCl, NaCl, and sugar, respectively; (c), (f), and (g) pore distribution charts of each sample (shown in the same order as previously) depicting the percent of pores of each size range in the images surveyed.

Figure 3. Characterization of devices fabricated using NaCl-templated 10:1 PDMS, KCl-templated 10:1 PDMS, and sugar-templated 10:1 PDMS. (a) Relative resistance change as a function of


Page 30 of 33

Page 31 of 33 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


applied pressure, (b) enlargement of the low pressure range, and (c) time-response of devices upon loading and unloading of 40 kPa pressure.

Figure 4. Cyclic characterization of a KCl-templated 10:1 PDMS device: cyclic loading and unloading testing (900 Pa and 2.8 kPa) (a), real time resistance changes as a function of applying pressure from 0 to 5 kPa (b), Displacement controlled compression test at 1Hz and 0.02 mm displacement (c).

Figure 5. (a) DMA storage and loss modulus, and damping factor (blue color, on the right y axis) of non-porous PDMS layers utilizing different ratios of base to curing agent as measured in compression (1 Hz) (a); (b) Resistance change as a function of applied pressure (up to 40 kPa), and (c) relaxation time of KCl-templated PDMS devices with varying ratios of base to curing agent under 40 kPa.



Figure 6. (a) A KCl-PDMS (10:1) device was mounted on the wrist for pulse measurement, enabling the collection of (b) numerous pulse signals over a 12 s interval, and (c) a single pulse signal. The sensor could also be used to collect (d) exhaling signals, (e) facial expression signals, and (f) voice signals (recorded as the person said: “Hello”).


Page 32 of 33

Page 33 of 33 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


GOC


Porous Polydimethylsiloxane-Silver Nanowire ... - ACS Publications

Recommend Documents