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Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process
Journal Pre-proof Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process Changyoon Jeong, Hangil Ko, Hoon Eui Jeong, Young-Bin Park PII:
S0266-3538(19)31815-9
DOI:
https://doi.org/10.1016/j.compscitech.2019.107841
Reference:
CSTE 107841
To appear in:
Composites Science and Technology
Received Date: 27 June 2019 Revised Date:
25 September 2019
Accepted Date: 28 September 2019
Please cite this article as: Jeong C, Ko H, Jeong HE, Park Y-B, Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process, Composites Science and Technology (2019), doi: https://doi.org/10.1016/ j.compscitech.2019.107841. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process
Changyoon Jeong, Hangil Ko, Hoon Eui Jeong*, Young-Bin Park*
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea
*To whom correspondence should be addressed: Phone: +82-52-217-2314, Fax: +82-52-217-2439, Email: [email protected]
Abstract Measuring shear displacement and pressure simultaneously is essential for various applications, such as tactile sensors for robotic finger tips, shoe soles for gait monitoring, etc. We present a simple means of transducing shear displacement and pressure change to flexible composite sensor. The presented sensor consists of an array of cylindrical pillars standing on a flat substrate, which is composed of carbon nanotubes (CNTs) and polydimethylsiloxane. The sensing mechanism is based on changing CNT network in pillar and flat structure under shear and pressure. When a shear displacement change occurs in the pillar array, which transfers shear and pressure to flat structure in the sample, the CNT network in the sample is changed due to bending of the pillars. Under pressure, the load is transferred from the pillar array to flat structure inducing changes in relative resistance. Load transfer through this hierarchical structure enabled measurement of shear displacement and pressure up to 5 mm and 1200 kPa, respectively. Therefore, it shows great potential applications in monitoring or even recognizing various human physiological activities.
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Keywords: A. Carbon nanotubes; A. Nano composites; B. Electrical properties; C. Deformation
1. Introduction Pressure and shear sensors with flexible, stretchable, and wearable characteristics are of paramount importance for applications in reality, such as tactile sensors for robotic finger tips, shoe soles for gait monitoring, and health care monitoring/diagnosis. In general, there are various mechanical sensors using nanomaterials such as carbon nanotubes[1-18], graphene[19-31], nanowires[32-39], and metal nanoparticles[40-46]. Most of these nanomaterials’ sensors are based on capacitance[32, 42, 43, 47-50], piezoelectricity[49, 5154] and piezoresistivity[8, 10, 20, 44, 55, 56]. Especially, piezoresistive sensors have various advantages in high sensitivity, large measurement range, and simple structures. However, the majority of the piezoresistive sensors concentrate on pressure and strain sensing and have limits of multiple sensing capabilities which needed to detect physiological activities[8, 10, 12, 15, 20, 44, 55-61]. It is very important to have shear sensing capabilities in multiple mechanical stimuli and there has been attempts to able simply shear force measurements[6266], but shear displacement change sensing capabilities and measuring method of flexible electronic skins for detection of shear movement remain challenging. There have been few reports on the multiple mechanical stimuli detection sensors[32, 40, 67, 68]. Although multiple mechanical stimuli are possible, and their sensitivities are high, the low measuring range limits the range of applications. In addition to this, in the fabrication method such as photolithography and CVD method, the difficulties of process and mass production are preventing practical applications. In this study, we developed pressure and shear measurable design and demonstrate a very simple fabrication process for our sensor design, resulting in flexible and combination of
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cylindrical pillar and flat structure composite sensors with large measuring ranges in pressure (~1200 kPa) and shear (0~5mm shear displacement). Our sensors show multiple mechanical stimuli sensing capabilities to detect sweeping of hand, wrist movement and walking monitoring of different sole position. In addition, the sensor has high sensitivity of 0.075 up to 800 kPa in large measuring ranges for the pressure sensor and 2
in the
most sensitive section for the shear sensor to recognize human physiological activities.
2. Experimental 2.1. Fabrication of cylindrical pillar and flat structured carbon nanocomposites For the fabrication of nanocomposites, multi-walled carbon nanotubes (MWCNTs) synthesized by catalytic CVD process were supplied from Hanwha Nanotech (Incheon Korea). The MWCNTs with 95wt.% purity, 3.5-4 nm inner diameter, 60-100 nm outer diameter, 70-80 µm length were first dispersed using a past mixer for 30 min at 500 rpm with PDMS base (Sylgard 184, Dow Corning), followed by calendaring using a three-roll mill (EXAKT 80, Germany) to further disperse MWCNTs in PDMS with 10 repetitions. The PDMS curing agent (1:10 ratio for curing agent to base) were mixed with MWCNT-PDMS composites. To obtain combined cylindrical and flat structures, an aluminum mold featuring an array of round holes (3 mm in diameter, 5 mm deep) and a flat cavity (5 mm deep) connected to the holes was machined using a 5-axis CNC machining center (C40U, Hermle, Germany). To fabricate cylindrical pillar arrays, the MWCNT-PDMS mixture was poured into the aluminum mold. After curing for 2 hr at 70°C while applying 5 MPa of pressure to remove air bubbles in the sample using a hot press, the mold was slowly cooled for 1 hr before the sample was demolded. To minimize the contact problems under external stimulus, a silver paste was applied to attach the electrode to the composite. They were covered with
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PDMS to completely fix the electrode and composite, then they were cured at 70°C for 2hr. 2.2. Characterizations The cross-sectional morphologies of the cylindrical pillar structure and flat structures were characterized by a field-emission SEM (FE-SEM; S-4800, Hitachi). The electrical signals of the sensors were measured using Keithley 2002 multimeter. To apply compressive forces on the sensors, a high-precision universal material testing system (Instron 5982) was used and acrylic plate was bonded to compression jig using an electrical insulating tape to minimize disturbance of electrical signal. Compression test was conducted at a constant speed of 0.5 mm/min. For the shear displacement measurement and simply normal force measurements, Kapton tape was attached on flat structure and equipment to fix between sensor and equipment and shear displacement changes and normal force were then applied on the sensor using Reciprocating Friction Wear Test System (NEO-TRIBO RFW16).
3. Results and discussion 3.1. Cylindrical pillar structures nanocomposite There has been a number of studies on nanocomposite sensors fabricated by mixing conductive fillers, such as CNTs with matrix to generate piezoresistive behavior [3, 4, 7, 10, 12, 13, 15]. In particular, piezoresistive elastomeric composites (e.g., PDMS) hold substantial promise for realization of sensor design, owing to their inherent flexibility, chemical stability, and simple, scalable and low-cost fabrication process. Taking advantagef these combined benefits, elastomeric composites consisting of CNTs and PDMS can transfer loads (pressure and shear) most efficiently due to their flexibility and excellent electrical and chemical properties. These composites were very simple fabricated by molding a highly-dispersed mixture of CNTs and polydimethylsiloxane (PDMS) which were mechanically mixed using 4
three-roll mill (Fig. 1a). The fabricated sample has a flexibility and conductivity due to CNTs-PDMS nanocomposite (Cross sectional SEM image in Fig. 1b) and combination of cylindrical pillar and flat structure, characterized by height of 5mm, a diameter of 3mm, and a flat height of 5mm. The basic working principles of our sensors are illustrated in Fig. 2a and b. The internal stress distribution is estimated within the Euler Bernoulli beam theory. In this theory, when the external shear force induces deformation of sensor, the normal stress component distribution on the cross section of nanocomposite is expressed as =−
(1)
where E is modulus of material, X is distance between Y-axis and arbitrary point,
is
distance between Y-axis and neutral surface. The internal stress field induces a decreasing CNT network density at left parts of neutral surface (tension) and an increasing CNT network density at right parts of neutral surface (compression). In particular, the internal stress field induces a deformation of flat structures, which in turn causes a variation of strain on a given cross section. When pressure is applied to the sensor, the cylindrical pillars transmit pressure into flat structure, in which the internal stress intensity increases with increasing pressure and affects the tunneling resistance at flat structures. Therefore, it is possible to monitor the magnitude and classification of shear displacement change and pressure through variation of resistance change on the sensor. 3.2. Performance of the shear and pressure measurable sensor Shear displacement and pressure play an important role in the medical field, robotics and human physiological activities. To detect various mechanical stimuli using a single sensor has long been a significant challenge. Our sensor shows outstanding performance for detection of shear displacement change and pressure because the cylindrical pillar structure transmits external force into flat structure and thus enables the CNT network change in flat structure.
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The direction of resistance changes was opposite in response to the pressure and shear displacement change, leading to classification between pressure and shear displacement change. In addition, when both shear and pressure are applied simultaneously, an increase in each load can be detected by observing the electric signal. To test cyclic and static shear displacement changes and classify different pressures under the same shear displacement changes, the all CNT-PDMS configured and 4 cylindrical pillar structures aligned (4×1) samples were preloaded with different pressures (8.842,17.684, and 26.526Pa) and were subsequently subjected to cyclic shear displacement changes in the range from -3 to 3mm as shown in Fig. 3a-c.When the reciprocating shear motion is applied to the entire cylindrical structure, the resistance changes are observed through the electrodes installed at the ends of the flat structure. The preload immediately induced the deformation in the flat structure, in which pressure was transmitted by cylindrical pillars, resulting in dense CNT network and thus a decrease in relative resistance in the flat structure. The reciprocating cyclic shear movement is matched well with the relative resistance change, which also illustrates their sensing reliability. Fig. 3d shows static shear displacement response of samples with different pressure. The change in relative resistance as function of shear displacement change is the largest value for a pressure of 8.842Pa. Thus, shear-displacement sensitivity increased with a decrease in normal pressure because the decreased initial resistance changes under a lower normal load led to further resistance changes by applying shear displacement. To investigate the effects of pillar height on the shear-sensing ability, we fabricated samples with two different aspect ratios – the high aspect ratio (HAR) being twice as high as the low aspect ratio (LAR). Samples consisting of cylindrical pillar structure with LAR have higher shear displacement sensitivity than those with HAR in Fig. 4a. The shear force required to move the same shear displacement in samples with LAR is greater than that in samples with HAR in Fig. 4b. Although the samples consisting of HAR cylindrical pillar
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structure shows shear-pressure sensing capabilities, they are easily broken under large shear and pressure due to HAR structures. This difference can be explained by the moment required to induce the same bending angle by following equation. τ= F×L
(2)
where τ is the moment, F is the shear force, and L is the distance between loading point and the bottom of the sample. In addition to their shear sensing capabilities, the capabilities of LAR samples to detect combinations between pressure and shear and distinguish the changes in each load are important because external stimuli’s such as robotic fingertips and shoe soles are usually a combination of two or more forces in reality. To measure pressure and shear simultaneously, various combinations of pressure (8.842, 17.684 and 26.526 Pa) and shear displacement (1, 2 and 3 mm) were applied and the resistance change was measured using the electrodes attached to the ends of the flat structure. As shown in Fig. 5a, the relative resistance changes of sample with LAR and 4×1 cylindrical pillar structure increased with increasing shear displacement. To bend cylindrical pillar structures and increase shear displacement changes, the minimum normal pressure is required to bend samples with LAR. As change in shear displacement increased in the range from 1 to 3mm, the required minimum pressure is ranges from 8.842Pa to 26.526Pa. Fig. 5b shows the change in relative resistance of samples with LAR and 4×1 cylindrical pillar structures under different pressure (8.842Pa,17.684Pa and 35.368Pa) and applying same shear displacement change (1mm). Increased pressure can be detected by observing electric signal in the same shear displacement changes. So far, we have seen shear and pressure sensing capabilities in samples with small arrays (4×1). To apply and test the sensing capabilities of a large area samples (7×7), shear and pressure are applied to sample and we observe the electric signal changes. Fig. 6a shows the sensing capabilities of our sensor under low-high pressure. Our sensor can be measured by approximate 60% and 5% resistance decrease for the pressure 7
ranges of 0-800 kPa and 800-1200 kPa, respectively. For the quantitative analysis in shear displacement change, we define a shear sensitivity S=
∆ / (
!
(3)
)
where ∆# is relative resistance change, #$ is original resistance, and γ is the shear stress. Fig. 6b shows three linear ranges, 5, 45, and 5% resistance increase for the shear displacement change ranges of 0-1,1-3.5, and 3.5-5 mm, respectively. At the initial shear displacement change range from 0 to 1mm, the surface of the sensor moves with the surface of the shear measuring equipment, but after that the shear stress causes the cylindrical contact area to be reduced and curved in range from 1 to 3.5mm. The shear sensitivity(S) of the most sensitive section exhibited 2 kPa
)
in 1-3.5mm. The rate of
change of resistance decreases in the 3.5-5mm region because the shear displacement change reached the peak and the speed of shear force machines come to zero. Although our sensor shows pressure sensitivity of 0.076 kPa
)
and is less sensitive in the previously reported low
pressure range measurable sensor but they have high sensitivity compared to sensitivity of 0.09 kPa ) of recently reported pressure sensors with sensing capability of high pressure range[10]. In addition to the high-pressure sensitivities in large measurement range, our sensor exhibited a shear sensing capability which detect range from 0 to 5 mm shear displacement change. 3.3. Application of wearable sensor with measurable shear and pressure In order to maximize the ability of our sensors and investigate practical application in human activities, we have used the square shaped and large-scaled sensor to monitor sweeping the palm of human hand, human wrist movement and to endure and detect the walking activity at various sole position. Fig. 7a shows the change in relative resistance change with 8 repetitions and sweeping over sensor with the palm of a human hand in every
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1.5 s. Fig. 7b shows the relative resistance change corresponding to wrist-bending motions with 4 repetitions, one bending motion per second. The relative resistance increases in response to changes in wrist bending, in which squeezing and stretching in nanocomposite induce the overall resistance change in sensor (Fig. 7c). The wearable strain sensors in human walking were simply to measure pressure values and cannot measure shear displacement. When human walks in reality, the forces exerted on the soles are combinations of shear and pressure. Not only we have to detect two types of mechanical stimuli’s but also the magnitude and direction of forces depending on the position of the sole are different because surface of sole is arched shape. As shown in Fig. 7d-f, a sensor was put on the front, middle and back of sole to detect human walking. Our sensor directly responded to the human walking and shows an increased resistance, resulting from bended cylindrical pillar structure, applying stress field in flat structures. To detect human walking motions, e.g., for gait monitoring, by obtaining the load distribution over the sole of a foot, sensors were attached to the ball, arch and heel of the foot using a double-sided adhesive tape. Walking motion was generated multiple times, while the electrical resistance was recorded real-time. The relative resistance change shows different magnitude and shape in electric signal which can differentiate sole positions. The magnitude of relative resistance changes in back of sole (Fig. 7f) are ~7 times and ~90 times higher than those in front of sole (Fig. 7d) and in middle of sole (Fig. 7e). To investigate the shear sensing capability of our sensor to measure the spatial distribution and magnitude of applied shear displacement change, shear stress was applied to the sensors via rod shaped acrylic plate placed on different position of the cylindrical pillar arrays (Fig. 8). The contact regions under rod shaped acrylic plate receive shear strain by hand sweeping resulting in increased resistance in left sided whole area (Fig. 8a), left area (Fig. 8b), right area (Fig. 8c), bottom area (Fig. 8d) and upper area (Fig. 8e).
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4. Conclusions In conclusion, we have reported simple method to fabricate combination of cylindrical pillar and flat structure contained in the PDMS-MWCNT composite, which has shear and pressure sensing measurable design. This sensor exhibited a large pressure sensing range up to 1200kPa and detected shear displacement sensing range from 0 to 5mm. Our sensor showed that when combination between shear and pressure is applied to sensor, the CNT networks in the sensor were changed due to bending of the cylindrical pillar and flat structure resulting in detection of these external stimuli’s. The presented sensors consisting of an array of cylindrical pillars standing on a flat substrate are far from an ideal geometry. Therefore, further optimization of pitch, diameter, position and various dimension will be necessary to improve the sensitivity of sensor and can be possible to apply practical applications in monitoring human motion activities. These sensors will also improve sensitivities of sensor using other types of filler materials and could expand sensing capabilities. Finally, we anticipate that our sensor can apply various external stimulus such as robotic skins, sole pressure and shear detection for medical purpose, sweeping and grasps of materials in tactile sensor.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT, Korea (NRF-2017R1A5A1015311).
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Fig.1. Flexible, conductive composite with cylindrical pillar structures. (a) Schematic of simple fabrication process of CNT-PDMS nanocomposite elastomers with cylindrical pillar structures. (b)SEM image of the MWCNT in PDMS. The inset photo highlights the flexibility of sample (scale bar: 150nm).
Fig.2. Sensing mechanism of composite with cylindrical pillar structure. (a) Schematic illustration of distributed force (tension and compression) of sensor under shear force. (b) Schematic showing the working principle of cylindrical pillar structure sensor. The external force induced bending, deforming the flat structures, which in turn causes CNT network changes.
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Fig. 3. Cycling shear displacement change of a MWCNT-PDMS nanocomposite under different pressure (a) 8.842Pa, (b) 17.864Pa, (c) 26.52Pa and (d) static shear displacement response of a MWCNT-PDMS nanocomposite under different pressure.
Fig. 4. Comparison between high aspect ratio(black) and low aspect ratio(red) cylindrical pillar structure sensor; (a) Relative resistance change under shear displacement change and (b)shear force vs. shear displacement change.
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Fig. 5. Combination of shear and pressure sensing capabilities of low aspect ratio (LAR) pillar structure samples consisting 4colum and 1row (4×1). (a) Relative electrical resistance of LAR 4×1 pillar structure sample under different combination of pressure and shear displacement. (b) Comparison of shear sensitivities of nanocomposite with low aspect ratio 4X1 pillar structures under different pressure.
Fig. 6. Pressure and shear sensing capabilities of cylindrical pillar structure samples. (a) Relative resistance changes under applied static compressive loading. (b) Relative electrical resistance and shear stress of cylindrical pillar structure samples as a function of shear displacement.
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Fig. 7. Change in relative resistance corresponding to different repetitive sweeping(a) and wrist movement(b) of sensor and showing the mechanism of CNT network change under bending(c). Relative resistance change response to walking in different sole position;(d) sole front, (e) sole middle, (f) sole back.
Fig. 8. Spatial shear displacement-mapping capability of the 4X4 electrodes arrays. The shear displacement is applied to (a) left-sided whole area, (b) left area, (c) right area, (d) bottom area and (e) upper area.
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Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process
Changyoon Jeong, Hangil Ko, Hoon Eui Jeong*, Young-Bin Park*
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea
*To whom correspondence should be addressed: Phone: +82-52-217-2314, Fax: +82-52-217-2439, Email: [email protected]
Authors confirm that there is no conflict of interest.
1
S0266-3538(19)31815-9
DOI:
https://doi.org/10.1016/j.compscitech.2019.107841
Reference:
CSTE 107841
To appear in:
Composites Science and Technology
Received Date: 27 June 2019 Revised Date:
25 September 2019
Accepted Date: 28 September 2019
Please cite this article as: Jeong C, Ko H, Jeong HE, Park Y-B, Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process, Composites Science and Technology (2019), doi: https://doi.org/10.1016/ j.compscitech.2019.107841. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.
Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process
Changyoon Jeong, Hangil Ko, Hoon Eui Jeong*, Young-Bin Park*
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea
*To whom correspondence should be addressed: Phone: +82-52-217-2314, Fax: +82-52-217-2439, Email: [email protected]
Abstract Measuring shear displacement and pressure simultaneously is essential for various applications, such as tactile sensors for robotic finger tips, shoe soles for gait monitoring, etc. We present a simple means of transducing shear displacement and pressure change to flexible composite sensor. The presented sensor consists of an array of cylindrical pillars standing on a flat substrate, which is composed of carbon nanotubes (CNTs) and polydimethylsiloxane. The sensing mechanism is based on changing CNT network in pillar and flat structure under shear and pressure. When a shear displacement change occurs in the pillar array, which transfers shear and pressure to flat structure in the sample, the CNT network in the sample is changed due to bending of the pillars. Under pressure, the load is transferred from the pillar array to flat structure inducing changes in relative resistance. Load transfer through this hierarchical structure enabled measurement of shear displacement and pressure up to 5 mm and 1200 kPa, respectively. Therefore, it shows great potential applications in monitoring or even recognizing various human physiological activities.
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Keywords: A. Carbon nanotubes; A. Nano composites; B. Electrical properties; C. Deformation
1. Introduction Pressure and shear sensors with flexible, stretchable, and wearable characteristics are of paramount importance for applications in reality, such as tactile sensors for robotic finger tips, shoe soles for gait monitoring, and health care monitoring/diagnosis. In general, there are various mechanical sensors using nanomaterials such as carbon nanotubes[1-18], graphene[19-31], nanowires[32-39], and metal nanoparticles[40-46]. Most of these nanomaterials’ sensors are based on capacitance[32, 42, 43, 47-50], piezoelectricity[49, 5154] and piezoresistivity[8, 10, 20, 44, 55, 56]. Especially, piezoresistive sensors have various advantages in high sensitivity, large measurement range, and simple structures. However, the majority of the piezoresistive sensors concentrate on pressure and strain sensing and have limits of multiple sensing capabilities which needed to detect physiological activities[8, 10, 12, 15, 20, 44, 55-61]. It is very important to have shear sensing capabilities in multiple mechanical stimuli and there has been attempts to able simply shear force measurements[6266], but shear displacement change sensing capabilities and measuring method of flexible electronic skins for detection of shear movement remain challenging. There have been few reports on the multiple mechanical stimuli detection sensors[32, 40, 67, 68]. Although multiple mechanical stimuli are possible, and their sensitivities are high, the low measuring range limits the range of applications. In addition to this, in the fabrication method such as photolithography and CVD method, the difficulties of process and mass production are preventing practical applications. In this study, we developed pressure and shear measurable design and demonstrate a very simple fabrication process for our sensor design, resulting in flexible and combination of
2
cylindrical pillar and flat structure composite sensors with large measuring ranges in pressure (~1200 kPa) and shear (0~5mm shear displacement). Our sensors show multiple mechanical stimuli sensing capabilities to detect sweeping of hand, wrist movement and walking monitoring of different sole position. In addition, the sensor has high sensitivity of 0.075 up to 800 kPa in large measuring ranges for the pressure sensor and 2
in the
most sensitive section for the shear sensor to recognize human physiological activities.
2. Experimental 2.1. Fabrication of cylindrical pillar and flat structured carbon nanocomposites For the fabrication of nanocomposites, multi-walled carbon nanotubes (MWCNTs) synthesized by catalytic CVD process were supplied from Hanwha Nanotech (Incheon Korea). The MWCNTs with 95wt.% purity, 3.5-4 nm inner diameter, 60-100 nm outer diameter, 70-80 µm length were first dispersed using a past mixer for 30 min at 500 rpm with PDMS base (Sylgard 184, Dow Corning), followed by calendaring using a three-roll mill (EXAKT 80, Germany) to further disperse MWCNTs in PDMS with 10 repetitions. The PDMS curing agent (1:10 ratio for curing agent to base) were mixed with MWCNT-PDMS composites. To obtain combined cylindrical and flat structures, an aluminum mold featuring an array of round holes (3 mm in diameter, 5 mm deep) and a flat cavity (5 mm deep) connected to the holes was machined using a 5-axis CNC machining center (C40U, Hermle, Germany). To fabricate cylindrical pillar arrays, the MWCNT-PDMS mixture was poured into the aluminum mold. After curing for 2 hr at 70°C while applying 5 MPa of pressure to remove air bubbles in the sample using a hot press, the mold was slowly cooled for 1 hr before the sample was demolded. To minimize the contact problems under external stimulus, a silver paste was applied to attach the electrode to the composite. They were covered with
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PDMS to completely fix the electrode and composite, then they were cured at 70°C for 2hr. 2.2. Characterizations The cross-sectional morphologies of the cylindrical pillar structure and flat structures were characterized by a field-emission SEM (FE-SEM; S-4800, Hitachi). The electrical signals of the sensors were measured using Keithley 2002 multimeter. To apply compressive forces on the sensors, a high-precision universal material testing system (Instron 5982) was used and acrylic plate was bonded to compression jig using an electrical insulating tape to minimize disturbance of electrical signal. Compression test was conducted at a constant speed of 0.5 mm/min. For the shear displacement measurement and simply normal force measurements, Kapton tape was attached on flat structure and equipment to fix between sensor and equipment and shear displacement changes and normal force were then applied on the sensor using Reciprocating Friction Wear Test System (NEO-TRIBO RFW16).
3. Results and discussion 3.1. Cylindrical pillar structures nanocomposite There has been a number of studies on nanocomposite sensors fabricated by mixing conductive fillers, such as CNTs with matrix to generate piezoresistive behavior [3, 4, 7, 10, 12, 13, 15]. In particular, piezoresistive elastomeric composites (e.g., PDMS) hold substantial promise for realization of sensor design, owing to their inherent flexibility, chemical stability, and simple, scalable and low-cost fabrication process. Taking advantagef these combined benefits, elastomeric composites consisting of CNTs and PDMS can transfer loads (pressure and shear) most efficiently due to their flexibility and excellent electrical and chemical properties. These composites were very simple fabricated by molding a highly-dispersed mixture of CNTs and polydimethylsiloxane (PDMS) which were mechanically mixed using 4
three-roll mill (Fig. 1a). The fabricated sample has a flexibility and conductivity due to CNTs-PDMS nanocomposite (Cross sectional SEM image in Fig. 1b) and combination of cylindrical pillar and flat structure, characterized by height of 5mm, a diameter of 3mm, and a flat height of 5mm. The basic working principles of our sensors are illustrated in Fig. 2a and b. The internal stress distribution is estimated within the Euler Bernoulli beam theory. In this theory, when the external shear force induces deformation of sensor, the normal stress component distribution on the cross section of nanocomposite is expressed as =−
(1)
where E is modulus of material, X is distance between Y-axis and arbitrary point,
is
distance between Y-axis and neutral surface. The internal stress field induces a decreasing CNT network density at left parts of neutral surface (tension) and an increasing CNT network density at right parts of neutral surface (compression). In particular, the internal stress field induces a deformation of flat structures, which in turn causes a variation of strain on a given cross section. When pressure is applied to the sensor, the cylindrical pillars transmit pressure into flat structure, in which the internal stress intensity increases with increasing pressure and affects the tunneling resistance at flat structures. Therefore, it is possible to monitor the magnitude and classification of shear displacement change and pressure through variation of resistance change on the sensor. 3.2. Performance of the shear and pressure measurable sensor Shear displacement and pressure play an important role in the medical field, robotics and human physiological activities. To detect various mechanical stimuli using a single sensor has long been a significant challenge. Our sensor shows outstanding performance for detection of shear displacement change and pressure because the cylindrical pillar structure transmits external force into flat structure and thus enables the CNT network change in flat structure.
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The direction of resistance changes was opposite in response to the pressure and shear displacement change, leading to classification between pressure and shear displacement change. In addition, when both shear and pressure are applied simultaneously, an increase in each load can be detected by observing the electric signal. To test cyclic and static shear displacement changes and classify different pressures under the same shear displacement changes, the all CNT-PDMS configured and 4 cylindrical pillar structures aligned (4×1) samples were preloaded with different pressures (8.842,17.684, and 26.526Pa) and were subsequently subjected to cyclic shear displacement changes in the range from -3 to 3mm as shown in Fig. 3a-c.When the reciprocating shear motion is applied to the entire cylindrical structure, the resistance changes are observed through the electrodes installed at the ends of the flat structure. The preload immediately induced the deformation in the flat structure, in which pressure was transmitted by cylindrical pillars, resulting in dense CNT network and thus a decrease in relative resistance in the flat structure. The reciprocating cyclic shear movement is matched well with the relative resistance change, which also illustrates their sensing reliability. Fig. 3d shows static shear displacement response of samples with different pressure. The change in relative resistance as function of shear displacement change is the largest value for a pressure of 8.842Pa. Thus, shear-displacement sensitivity increased with a decrease in normal pressure because the decreased initial resistance changes under a lower normal load led to further resistance changes by applying shear displacement. To investigate the effects of pillar height on the shear-sensing ability, we fabricated samples with two different aspect ratios – the high aspect ratio (HAR) being twice as high as the low aspect ratio (LAR). Samples consisting of cylindrical pillar structure with LAR have higher shear displacement sensitivity than those with HAR in Fig. 4a. The shear force required to move the same shear displacement in samples with LAR is greater than that in samples with HAR in Fig. 4b. Although the samples consisting of HAR cylindrical pillar
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structure shows shear-pressure sensing capabilities, they are easily broken under large shear and pressure due to HAR structures. This difference can be explained by the moment required to induce the same bending angle by following equation. τ= F×L
(2)
where τ is the moment, F is the shear force, and L is the distance between loading point and the bottom of the sample. In addition to their shear sensing capabilities, the capabilities of LAR samples to detect combinations between pressure and shear and distinguish the changes in each load are important because external stimuli’s such as robotic fingertips and shoe soles are usually a combination of two or more forces in reality. To measure pressure and shear simultaneously, various combinations of pressure (8.842, 17.684 and 26.526 Pa) and shear displacement (1, 2 and 3 mm) were applied and the resistance change was measured using the electrodes attached to the ends of the flat structure. As shown in Fig. 5a, the relative resistance changes of sample with LAR and 4×1 cylindrical pillar structure increased with increasing shear displacement. To bend cylindrical pillar structures and increase shear displacement changes, the minimum normal pressure is required to bend samples with LAR. As change in shear displacement increased in the range from 1 to 3mm, the required minimum pressure is ranges from 8.842Pa to 26.526Pa. Fig. 5b shows the change in relative resistance of samples with LAR and 4×1 cylindrical pillar structures under different pressure (8.842Pa,17.684Pa and 35.368Pa) and applying same shear displacement change (1mm). Increased pressure can be detected by observing electric signal in the same shear displacement changes. So far, we have seen shear and pressure sensing capabilities in samples with small arrays (4×1). To apply and test the sensing capabilities of a large area samples (7×7), shear and pressure are applied to sample and we observe the electric signal changes. Fig. 6a shows the sensing capabilities of our sensor under low-high pressure. Our sensor can be measured by approximate 60% and 5% resistance decrease for the pressure 7
ranges of 0-800 kPa and 800-1200 kPa, respectively. For the quantitative analysis in shear displacement change, we define a shear sensitivity S=
∆ / (
!
(3)
)
where ∆# is relative resistance change, #$ is original resistance, and γ is the shear stress. Fig. 6b shows three linear ranges, 5, 45, and 5% resistance increase for the shear displacement change ranges of 0-1,1-3.5, and 3.5-5 mm, respectively. At the initial shear displacement change range from 0 to 1mm, the surface of the sensor moves with the surface of the shear measuring equipment, but after that the shear stress causes the cylindrical contact area to be reduced and curved in range from 1 to 3.5mm. The shear sensitivity(S) of the most sensitive section exhibited 2 kPa
)
in 1-3.5mm. The rate of
change of resistance decreases in the 3.5-5mm region because the shear displacement change reached the peak and the speed of shear force machines come to zero. Although our sensor shows pressure sensitivity of 0.076 kPa
)
and is less sensitive in the previously reported low
pressure range measurable sensor but they have high sensitivity compared to sensitivity of 0.09 kPa ) of recently reported pressure sensors with sensing capability of high pressure range[10]. In addition to the high-pressure sensitivities in large measurement range, our sensor exhibited a shear sensing capability which detect range from 0 to 5 mm shear displacement change. 3.3. Application of wearable sensor with measurable shear and pressure In order to maximize the ability of our sensors and investigate practical application in human activities, we have used the square shaped and large-scaled sensor to monitor sweeping the palm of human hand, human wrist movement and to endure and detect the walking activity at various sole position. Fig. 7a shows the change in relative resistance change with 8 repetitions and sweeping over sensor with the palm of a human hand in every
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1.5 s. Fig. 7b shows the relative resistance change corresponding to wrist-bending motions with 4 repetitions, one bending motion per second. The relative resistance increases in response to changes in wrist bending, in which squeezing and stretching in nanocomposite induce the overall resistance change in sensor (Fig. 7c). The wearable strain sensors in human walking were simply to measure pressure values and cannot measure shear displacement. When human walks in reality, the forces exerted on the soles are combinations of shear and pressure. Not only we have to detect two types of mechanical stimuli’s but also the magnitude and direction of forces depending on the position of the sole are different because surface of sole is arched shape. As shown in Fig. 7d-f, a sensor was put on the front, middle and back of sole to detect human walking. Our sensor directly responded to the human walking and shows an increased resistance, resulting from bended cylindrical pillar structure, applying stress field in flat structures. To detect human walking motions, e.g., for gait monitoring, by obtaining the load distribution over the sole of a foot, sensors were attached to the ball, arch and heel of the foot using a double-sided adhesive tape. Walking motion was generated multiple times, while the electrical resistance was recorded real-time. The relative resistance change shows different magnitude and shape in electric signal which can differentiate sole positions. The magnitude of relative resistance changes in back of sole (Fig. 7f) are ~7 times and ~90 times higher than those in front of sole (Fig. 7d) and in middle of sole (Fig. 7e). To investigate the shear sensing capability of our sensor to measure the spatial distribution and magnitude of applied shear displacement change, shear stress was applied to the sensors via rod shaped acrylic plate placed on different position of the cylindrical pillar arrays (Fig. 8). The contact regions under rod shaped acrylic plate receive shear strain by hand sweeping resulting in increased resistance in left sided whole area (Fig. 8a), left area (Fig. 8b), right area (Fig. 8c), bottom area (Fig. 8d) and upper area (Fig. 8e).
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4. Conclusions In conclusion, we have reported simple method to fabricate combination of cylindrical pillar and flat structure contained in the PDMS-MWCNT composite, which has shear and pressure sensing measurable design. This sensor exhibited a large pressure sensing range up to 1200kPa and detected shear displacement sensing range from 0 to 5mm. Our sensor showed that when combination between shear and pressure is applied to sensor, the CNT networks in the sensor were changed due to bending of the cylindrical pillar and flat structure resulting in detection of these external stimuli’s. The presented sensors consisting of an array of cylindrical pillars standing on a flat substrate are far from an ideal geometry. Therefore, further optimization of pitch, diameter, position and various dimension will be necessary to improve the sensitivity of sensor and can be possible to apply practical applications in monitoring human motion activities. These sensors will also improve sensitivities of sensor using other types of filler materials and could expand sensing capabilities. Finally, we anticipate that our sensor can apply various external stimulus such as robotic skins, sole pressure and shear detection for medical purpose, sweeping and grasps of materials in tactile sensor.
Acknowledgements This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Ministry of Science and ICT, Korea (NRF-2017R1A5A1015311).
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Fig.1. Flexible, conductive composite with cylindrical pillar structures. (a) Schematic of simple fabrication process of CNT-PDMS nanocomposite elastomers with cylindrical pillar structures. (b)SEM image of the MWCNT in PDMS. The inset photo highlights the flexibility of sample (scale bar: 150nm).
Fig.2. Sensing mechanism of composite with cylindrical pillar structure. (a) Schematic illustration of distributed force (tension and compression) of sensor under shear force. (b) Schematic showing the working principle of cylindrical pillar structure sensor. The external force induced bending, deforming the flat structures, which in turn causes CNT network changes.
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Fig. 3. Cycling shear displacement change of a MWCNT-PDMS nanocomposite under different pressure (a) 8.842Pa, (b) 17.864Pa, (c) 26.52Pa and (d) static shear displacement response of a MWCNT-PDMS nanocomposite under different pressure.
Fig. 4. Comparison between high aspect ratio(black) and low aspect ratio(red) cylindrical pillar structure sensor; (a) Relative resistance change under shear displacement change and (b)shear force vs. shear displacement change.
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Fig. 5. Combination of shear and pressure sensing capabilities of low aspect ratio (LAR) pillar structure samples consisting 4colum and 1row (4×1). (a) Relative electrical resistance of LAR 4×1 pillar structure sample under different combination of pressure and shear displacement. (b) Comparison of shear sensitivities of nanocomposite with low aspect ratio 4X1 pillar structures under different pressure.
Fig. 6. Pressure and shear sensing capabilities of cylindrical pillar structure samples. (a) Relative resistance changes under applied static compressive loading. (b) Relative electrical resistance and shear stress of cylindrical pillar structure samples as a function of shear displacement.
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Fig. 7. Change in relative resistance corresponding to different repetitive sweeping(a) and wrist movement(b) of sensor and showing the mechanism of CNT network change under bending(c). Relative resistance change response to walking in different sole position;(d) sole front, (e) sole middle, (f) sole back.
Fig. 8. Spatial shear displacement-mapping capability of the 4X4 electrodes arrays. The shear displacement is applied to (a) left-sided whole area, (b) left area, (c) right area, (d) bottom area and (e) upper area.
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Shear-pressure multimodal sensor based on flexible cylindrical pillar array and flat structured carbon nanocomposites with simple fabrication process
Changyoon Jeong, Hangil Ko, Hoon Eui Jeong*, Young-Bin Park*
Department of Mechanical Engineering, Ulsan National Institute of Science and Technology, UNIST-gil 50, Ulju-gun, Ulsan 44919, Republic of Korea
*To whom correspondence should be addressed: Phone: +82-52-217-2314, Fax: +82-52-217-2439, Email: [email protected]
Authors confirm that there is no conflict of interest.
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