Omni-Purpose Stretchable Strain Sensor Based on a Highly Dense

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Omni-purpose stretchable strain sensor based on a highly dense nanocracking structure for whole-body motion monitoring Hyungkook Jeon, Seong Kyung Hong, Min Seo Kim, Seong J. Cho, and Geunbae Lim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14153 • Publication Date (Web): 13 Nov 2017 Downloaded from http://pubs.acs.org on November 17, 2017

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ACS Applied Materials & Interfaces

Omni-purpose stretchable strain sensor based on a highly dense nanocracking structure for whole-body motion monitoring Hyungkook Jeon, a Seong Kyung Hong, a Min Seo Kim, b Seong J. Choc,* and Geunbae Lima,b,* a

Department of Mechanical Engineering, Pohang University of Science and Technology

(POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, the Republic of Korea b

Department of Integrative Bioscience and Biotechnology, Pohang University of Science and

Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, the Republic of Korea c

School of Mechanical Engineering, Chungnam National University (CNU), 99 Daehak-Ro,

Yuseong-Gu, Daejeon 305-764, the Republic of Korea Corresponding Author *Geunbae Lim, Department of Mechanical Engineering, Pohang University of Science and Technology (POSTECH), 77 Cheongam-Ro, Nam-Gu, Pohang 790-784, the Republic of Korea, Tel. +82-54-279-2186, Fax +82-54-279-0479, e-mail: [email protected] *Seong J. Cho, School of Mechanical Engineering, Chungnam National University (CNU), 99 Daehak-Ro, Yuseong-Gu, Daejeon 305-764, the Republic of Korea, Tel. +82-42-821-5648, email: [email protected] 1

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ABSTRACT Here, we report an omni-purpose stretchable strain sensor (OPSS sensor) based on a nanocracking structure for monitoring of whole-body motions including both joint-level and skin-level motions. By controlling and optimizing the nanocracking structure, inspired by the spider sensory system, the OPSS sensor is endowed with both high sensitivity (gauge factor ~30) and wide working range (strain up to 150%) under great linearity (R2 = 0.9814) and fast response time (> R1 , R2 , the total resistance can be simplified by dividing the numerator and the denominator of the Equation (1) by RC as follows: R=

R1 R2 / RC + 2 R1 RC / RC + RC R2 / RC = 2 R1 + R2 R1 / RC + 2 R2 / RC + 1

= 9.87 (k Ω) + 2.96 (k Ω / %) × strain (%)

12


(2)

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By fitting the equation into the experimental results of the 50% strain test (Figure 2b), we obtained values of R1 and R2 ; R1 ~ 4.94(k Ω) and R2 ~ 2.96(k Ω / %) × strain (%) . We analyzed the linear change in resistance depending on strain based on Equation (2) with values

R1 and R2 ; see Figure S6 of Supporting Information for comparison of analytical and experimental results.

Optimization of cracking structure by controlling grain size The cracking structure of the Pt layer is a critical factor in the electrical connection of the sensor, so its optimization is necessary to enhance the sensing performance, especially to achieve a wide working range. Cracks of nanocrystalline materials tend to form and propagate from grain boundaries because of the low atomic density and weaker bonding between atoms compared to the bulk phase.34 Therefore, the phase of the grain boundary, which is determined by grain size, markedly affects the fracture process and crack propagation.34 Specifically, in the case of nanocrystalline materials with grain size < 100 nm, the materials show brittle rather than ductile intergranular fracture.34 In this regime, increases in grain size induce longer grain boundaries, resulting in larger intergranular cracks.35,36 Many parameters can affect the grain size, such as temperature, nucleation rate, and film thickness.37,38 In this study, we controlled the grain size to optimize the cracking structure of the Pt layer by varying its thickness, which is determined by the sputtering time; the relationship between the Pt layer thickness and the sputtering time is shown in Figure S7, Supporting Information. As the grain generally grows during the deposition process, the grain size increases with increasing Pt thickness (Figure 4a and Figure 4d).39–41 As 13



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explained above, the increase in grain size induces larger cracks and islands under conditions of constant strain.39 Therefore, the cracking structure on the Pt layer becomes more oriented toward larger cracks and islands with lower crack density as Pt thickness increases, inducing the increase of grain size (Figure 4b and Figure 4d). Figure 4d shows HR-SEM images according to Pt thickness under 50% strain. As explained by the paper model in Figure 1c, the failure strain increases with increasing crack density due to high spatial capacity. As crack density decreases, the size of each crack increases, causing the disconnection of mechanical interactions between islands under conditions of lower strain, which accounts for the relatively lower failure strain. Similarly, as the change in cracking structure on the Pt layer significantly affects the electrical connection on the Pt layer, the sensing performance also depends on the cracking structure on the Pt layer. Figure 4c shows the changes in maximum measurable strain depending on the grain size, which in turn was controlled by controlling the Pt thickness. In the first stage of Pt thickness (< 10 nm), the maximum measurable strain increased as the Pt thickness increased due to the enhanced conductance of the metallic layer. However, in the second stage of Pt thickness (> 10 nm), the maximum measurable strain decreased as the Pt thickness increased, thus generating larger nanocracks, which affected the maximum measurable strain to a far greater extent than the conductance increase in the metallic layer. Therefore, we controlled the cracking structure of the Pt layer by regulating the Pt thickness and the grain size, and determined the optimal Pt thickness and grain size (~10 and ~3 nm, respectively) for a wide sensing range. In the case of the gauge factor, the change depending on Pt thickness did not show a pronounced tendency even though the gauge factor increased as Pt thickness increased in a certain regime (details are presented in Figure S8, Supporting Information). 14


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Furthermore, to improve the sensor performance, we attempted to enhance the adhesion between the Pt layer and PU membrane by depositing silane vapor on the PU membrane before sputtering the Pt layer.42,43 The results indicated that the silane-treated sensors worked at higher strain than untreated sensors, but the improvement was limited only to the high-Pt thickness regime (> 10 nm) (Figure S9, Supporting Information). The cases where different metals and stretchable substrates other than Pt and PU comprised the sensor showed early electrical disconnection (strain < 5%) (details are presented in Figure S10, Supporting Information).

Whole-body motion detection To confirm the practical applicability of the OPSS sensor, we demonstrated two-level monitoring systems: joint level (large displacement and large strength) and skin level (small displacement and small strength). As shown in Table S2 of Supporting Information, there are six basic movements in joint-level motion, i.e., flexion, extension, abduction, adduction, medial rotation, and lateral rotation, which occur in varying combinations in the joints of the body. Figure 5a shows the results of measuring abduction and adduction movements using a sensor attached to the joint between the shoulder and arm. In Figure 5a, abduction, reference, and adduction refer to the states with the arm held up, level at the shoulder, and held down, respectively. As the strain applied to the sensor changed depending on the state, as shown in Figure 5a, the resistance change in the sensor varied depending on the state, with the highest value in abduction, middle value in the reference state, and the lowest value in adduction (Video S1, Supporting Information). Figure 5b shows the results of measuring rotation movements by two sensors attached to the left and right sides of the abdomen, parallel to the external oblique, 15



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with strain initially applied to both of the sensors. In the case of right rotation, additional strain was applied to the right sensor, while the strain applied to the left sensor decreased. As a result, the resistances of the right and left sensors increased and decreased, respectively (Video S2, Supporting Information). Figure 5c shows the results of measuring flexion and extension movements with a sensor attached to the surface over the knee. The results showed that the resistance increased as the knee bent (flexion), and the resistance decreased as the knee straightened (extension state) (Video S3, Supporting Information). In addition, the results indicated that the resistance increased more as the folding angle increased (hyperflexion), which sufficiently demonstrated the motion detecting ability of the OPSS sensor. In the case of skin-level motion detection, we measured three representative motions, eye blinking, pulsation, and breathing. Figure 5d shows the measurement of eye blinking through a sensor attached to the skin above the eyebrow in the vertical direction. The resistance of the sensor increased due to extension of the sensor with eye blinking motion. As the eye reopened, the sensor returned to its original length and thus recovered its initial resistance. Figure 5e shows the measurement of pulsation through a sensor attached to the wrist. The pulse can be monitored easily with the human sense of touch by pressing the fingers onto the wrist. In the same manner, the sensor could pick up the pulse, as it can measure subtle pressure change that causes the sensor to stretch slightly. The results show that it can even measure pressure indirectly through the slight strain change caused by the pressure change due to its high sensitivity. However, it has limitations in usage for precise pressure measurement compared to recently developed pressure sensors; W.Cheng’s group have recently proposed a highly sensitive pressure sensor, which can analyze the wrist pulses with clear detection of specific features of pulse waves.44 Figure 5f 16


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shows the measurement of breathing. On inhalation, the chest rises causing the sensor attached to the chest to stretch. In contrast, with exhalation, the chest drops down causing the sensor to return to its original length. The results of two-level monitoring applications show that the OPSS sensor is applicable to whole-body motion detection due to its high sensitivity and wide strain sensing range with great linearity and fast response. Because the whole-body motion detection is essential for various applications such as personal health monitoring,1,45 rehabilitation of patients,46 athletic performance monitoring,47,48 and human motion tracing for entertainments (e.g., games and animation),16,25 we expect that the OPSS sensor can play a pivotal role and give advances in these applications.

Hand motion detector As we can pattern the Pt layer on the desired location by the masking method, the Pt layer could be simultaneously sputtered to five different regions of the PU membrane to measure the movement of five fingers individually, as shown in Figure 6a. When each finger folds, the surface of the PU membrane undergoes linear extension, applying strain to the Pt layer. The strain applied to the Pt layer results in an increase in resistance, as shown in Figure 6b. As also shown by the resistance drops in the graph, the membranes gradually recover their electrical conductivity when the fingers unfold. With the combination of the five membranes tracking the movement of each finger, we successfully constructed a system that can detect hand motion.

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Real-time Morse code communication To demonstrate further the practicality of the OPSS sensor, we applied the sensor for communication purposes. Patients suffering from general paralysis, such as locked-in syndrome (LIS) and amyotrophic lateral sclerosis (ALS), have difficulties in communication as they are limited to highly restricted movements, e.g., moving a fingertip or blinking an eye.49 Previous communication technologies that help patients with the general paralysis can be categorized into two cases: technologies that utilize direct neural signals from the brain to obtain the patient’s thoughts50 and technologies that monitor subtle human movements such as eye blinking or fingertip motion in real time to use them as a tool of writing words.51 The first case of technologies has a great advantage of not requiring any effort (any kind of motion that can be noticed) other than just making a thought. However, these technologies have a critical problem of having a bulky system of machines that has terrible portability and affordability. The other case, where subtle human movements are monitored for communicating, has an advantage of being simple and intuitive compared to the previous case mentioned above. However, when this monitoring is carried out with optical processing methods, it is inevitable to be spatially restricted for accurate optical measurements as anything that comes in between the optical sensor and the patient will interfere accurate tracing of the motion. To overcome this limitation, we have developed a real-time Morse code communication system, based on the OPSS sensor. By taking advantage of the high sensitivity of our fabricated sensor, we were able to physically measure the minimal movements in the human body and turn them into resistance changes of the sensor, which were then converted into Morse code. When the resistance of the sensor is quickly brought up and down, the signal is regarded as a dot. When 18


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the resistance is maintained at a high level then brought down, the signal is regarded as a dash. Figure 7 shows the actual application of the sensor for writing the string “POSTECH” in Morse code, one through fingertip motion (Video S4, Supporting Information) and the other through eye blinking motion (Video S5, Supporting Information).

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CONCLUSIONS Here, we proposed an omni-purpose stretchable strain (OPSS) sensor utilizing a highly dense nanocracking structure (crack density ~107/m), which is highly sensitive over a wide sensing range. We investigated the sensing mechanism and the performance of the OPSS sensor along with observation of its cracking structure. In particular, we controlled the cracking structure on the Pt layer to optimize the sensing performance by controlling the grain size of the sputtered Pt layer through regulating its thickness. As a result, the OPSS sensor showed both high sensitivity (gauge factor ~30) and wide working range (strain up to 150%) with great linearity (R2 = 0.9814) and fast response time (

Omni-Purpose Stretchable Strain Sensor Based on a Highly Dense

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