Cut-and-Paste Transferrable Pressure Sensing Cartridge Films

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Cut-and-Paste Transferrable Pressure Sensing Cartridge Films Hyejin Hwang, Song-Ee Choi, Sang Woo Han, Insang You, Eun Sook Jeong, Sinae Kim, Hakyeong Yang, Sangyeop Lee, Jaebum Choo, Jin Woong Kim, and Unyong Jeong Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b02695 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on September 1, 2018

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Chemistry of Materials

Revised: cm-2018-026958

Cut-and-Paste Transferrable Pressure Sensing Cartridge Films

Hyejin Hwang,#,† Song-Ee Choi,#,‡ Sang Woo Han,#, ‡ Insang You,† Eun Sook Jeong, ‡ Sinae Kim, ‡ Hakyeong Yang, ‡ Sangyeop Lee, ‡ Jaebum Choo, ‡ Jin Woong Kim,*, ‡,§ and Unyong Jeong*,†

†

Department of Materials Science and Engineering, Pohang University of Science and

Technology (POSTECH), Cheongam-Ro 77, Nam-Gu, Pohang, Gyeongbuk, 37673, Republic of Korea ‡

Department of Bionano Technology, Hanyang University, 55 Hanyangdaehak-ro, Sangnok-

gu, Ansan, Gyeonggi-do, 15588, Republic of Korea §

Department

of

Chemical

and

Molecular

Engineering,

Hanyang

University,

Hanyangdaehak-ro, Sangnok-gu, Ansan, Gyeonggi-do, 15588, Republic of Korea

*Corresponding author: [email protected], [email protected] #

These authors contributed equally to the work.

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Abstract Flexible tactile sensors have been intensively studied for healthcare and electronic skin devices. Currently, a sensing material, electrode, and substrate are manufactured as one set by depositing the sensing material on the electrode. For this reason, when other electrode or substrate is used in the sensor or when different sensor characteristics is required, a new sensing material must be developed and the fabrication conditions should be changed. This study proposes a novel method of manufacturing a pressure sensing material like a cartridge film. The cartridge film is made by filling the holes of a stencil film (one MP in each hole) with conductive microparticles (MPs). Using the cartridge film, the sensing material can be cut-andpasted on electrodes and transferred to other electrodes for reuse. This study analyzes the electrical responses of the sensors made of the cartridge film on the basis of the Hertzian contact theory, and also correlates the sensing performance of the sensors with the conductivity of the MPs and the degree of protrusion of the MPs from the stencil surface.


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1. Introduction Flexible tactile sensors have become an important issue for the development of electronic skin and wearable health care devices.1-7 Because the performance of a tactile sensor relies on the responsivity of the sensing material, many research groups have sought to develop materials the electrical changes of which are synchronizing with an external mechanical stimulation.2,8-12 Currently, tactile sensors are based on the change of electrical resistance or capacitance. Among the various approaches to investigate the resistive tactile sensors, the material design is based on the percolation theory and uses the composite of polymer and conductive

nanomaterials

such

as

metal

nanowires,

nanosheet,

nanoflakes,

and

nanoparticles.13-17 Structural approaches have also been attempted for high-performance tactile sensors. Park et al. demonstrated a nanoscale crack-based strain sensor with a gauge factor of over 2000 within 2 % of strain.18,19 Mannsfeld et al. reported that the pyramidal dielectric materials increases the sensitivity of the capacitive pressure sensor.20 Similarly, resistive sensors based on the change of contact resistance have been demonstrated by Shao et al.21 Porous structures have also been investigated for high performance tactile sensors.22-25 As one of the intrinsic approaches, the development of ionic gel-based tactile sensors has been reported as the capacitive sensor.26 The ionic sensors have the various advantages due to their highly stretchable, transparent, and even biocompatible properties. Very recently, conductive microparticle-based resistive tactile sensors were demonstrated and the same group presented a pressure sensor matrix using flexible diodes between the microparticles and metal electrodes.27,28 The performance of a tactile sensor is judged only after fabricating the device set and testing the sensor characteristics. Even though a sensing material used in a sensor is well designed for a specific use, the material is often not usable in other device sets. It is because the sensing material is deposited onto a certain type of electrode on a substrate so that the 3 ACS Paragon Plus Environment


sensing material cannot be separated from the electrode.29,30 When one wants a sensor using different electrode or substrate, he/she has to develop a new sensing material and change the fabrication process. If a well-designed sensing material can be repeatedly used in various sensor systems one can know the performance of the sensor before fabrication and can eliminate the inconvenience of making a new sensor every time. In this study, we propose a new concept of a tactile sensing material, a cut-and-paste transferrable pressure sensing cartridge film. The sensing cartridge film suggested in this study has following characteristics: i) it can be cut into small pieces and pasted onto various electrode pairs, ii) it is transferrable from one electrode pairs to others, iii) its sensing performance can be readily modulated so that it is easy to prepare a set of sensors with different performance, and iv) its fabrication is simple and scalable to a large size. We fabricated the sensing cartridge film by filling elastic conductive microparticles (MPs) in the holes of a perforated flexible polymer stencil film, one MP in each hole of the stencil. We coated CNTs on elastic solid MPs. We used CNTs because of the high flexibility, excellent mechanical strength, easy surface modification required for depositing on the MP surfaces, and good chemical stability.31 The process is easy and highly reproducible. This study correlates the sensor performance of the cartridge film with the design rules in the cartridge film.

2. Results and Discussion Figure 1A is a schematic illustration describing the fabrication of the cartridge film. We filled conductive elastic MPs (50 µm in diameter) in the holes of a flexible stencil (10 cm × 10 cm) made of polyurethane (PU). The PU stencil had periodic holes of 55 μm in diameter. The thickness of the stencil was varied to be 30, 38, and 45 μm. The holes in the stencil are open from the top through the bottom with a uniform diameter (Supporting Information, Figure S1). 4 ACS Paragon Plus Environment

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We followed the process in a literature to prepare the stencil.32 The PU prepolymer liquid was poured onto a pillar-arrayed PDMS stamp, and extra prepolymer was wiped out with a flat PDMS piece. UV (λ = 365 nm, 500 W) was irradiated for 20 s to crosslink the PU prepolymer, and the crosslinked PU film was peeled off from the PDMS stamp. The PU stencil was placed on a glass substrate. A small amount of a conductive MP powder was rubbed unidirectionally on the stencil by using a small piece of PDMS rubber.33,34 The mechanical rubbing process forced the MPs to be filled in the holes of the stencil, one MP in a hole.35 The extra MPs spontaneously moved to the edges of the stencil during the rubbing process. The rubbing process took about 20 s for completion. A small number of excess MPs on top of the stencil surface were removed by air blowing. The diameter of the MPs was larger than the thickness of the stencil, hence the MPs were protruding above the surface of the stencil. It is notable that efficient positioning could be obtained when the diameter of the MP was 10-20% smaller than the diameter of the hole. The images corresponding to the rubbing and the air-blowing processes are shown in the Supporting Information (Figure S2). A small piece was cut from the cartridge film and sandwiched between two electrodes. Applying a mechanical stimulus to the upper electrode or lower electrode deformed the conductive MP and changed the current through the MP. The cartridge film was very flexible and could be cut with a knife or scissors (Figure 1B). Figure 1C is a scanning electron microscope (SEM) image of the cartridge film. The thickness of the stencil in the image was 45 µm, hence the MPs were protruding by 5 µm above the surface of the stencil. Every hole was filled with one MP. The filling efficiency was more than 98.8 %. The number of the vacancy defects (not filling) was 24 in a sample of 4 mm × 5 mm area, hence the maximum vacancy density was 40 out of 2000 holes. Some particles were found on the top surface of the stencil after the blowing process. The maximum number density of



such interstitial defects was 5 out of 2000 holes. The validity of this rubbing process has already been verified in our previous paper.35

Figure 1. (A) Schematic illustration showing the fabrication of the cartridge film in which conductive microparticle (MP) is filled in each hole of the polyurethane (PU) stencil. The process includes rubbing a powder of the MPs and air blowing. The cartridge film is placed between two electrodes and used a pressure sensor. (B) Optical microscope (OM) image of the cartridge film to be cut into small pieces. (C) Scanning electron microscope (SEM) image showing the MPs in the cartridge film. (D, E) The MPs in the cartridge film before (D) and after mechanical bending at a bending radius of 5 mm (E). The particles are remained the same.


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Repeated bending tests were performed 10 times at a bend radius of 5 mm, indicating that the MP did not come out of the hole and no cracks were found in the cartridge film (Figure 1D, E and also Supporting Information Figure S3). This high positional stability of the MP is due to the void space between the MP and the hole wall. This space prevents stencil bending stress from being transferred to the MP. Because the MP does not protrude and does not slip on the bottom side of the cartridge film, a conformal contact can be made to the flat electrode and repeatedly transferred to the other electrode. The conductive elastic MPs were prepared by the glass capillary-based microfluidic technology.36 Figure 2A shows the chemical structures of the urethane precursor and the ionomer. The typical coaxial jetting generated monodisperse O/W emulsion drops via the Rayleigh-Plateau instability (Figure 2B). Oil-in-water (O/W) PU precursor emulsion drops were generated by coaxial flow-focusing of immiscible fluids. The emulsion drop consisted of a urethane precursor (20 vol%, 600 cP), toluene (51 vol%), and chloroform (29 vol%). The outer aqueous fluid contained 10 wt% PVA and 2 wt% negatively charged PU ionomer precursor. The PU ionomer reduced the interfacial tension between the drops and outer fluid and made the PU MPs negatively charged (Supporting Information, Figure S4).37 The size of the emulsion droplets was controlled by adjusting the relative ratio between the the dispersion fluid and the outer fluid (Figure 2C). After complete evaporation of solvents, the diameter of the emulsion droplets decreased by approximately 58 % (Figure 2D and also Supporting Information Figure S5). To ensure that the particle size distribution does not affect the deviation of the contact radius, the standard deviations of MPs in size were adjusted to be smaller than 2.3%. The polymerization of the remaining PU precursor droplets was conducted by UV irradiation at 365 nm for one minute. The modulus of PU MP was ~ 3.9 MPa, which was directly measured with a nanoindenter (Nanotest vantage platform, Micro Materials). It is noteworthy that the modulus of the PU flat film produced by the same crosslinking method 7 ACS Paragon Plus Environment


was 100 MPa, indicating that the MP was easily deformed under pressure than its corresponding film. Figure S5 was shown to exhibit the process of MP preparation. We used completely dried MPs as sensing powder to be filled in the holes of the stencil.

Figure 2. (A) Molecular structures of the PU precursor and ionomer precursor. (B) Schematic representation of a micro-capillary device for the generation of PU precursor drops. (C) Formation of emulsion drops at selected scaled flow rates. (D) Normalized emulsion drop sizes with varying the scaled flow rate (QOF/QDF): emulsion drop size on generation (■) and after removal of solvents (●). The dotted line is the calculate diameter with varying the scaled flow rate. The PU precursor concentration in the drop was set to 20 vol%. 8 ACS Paragon Plus Environment

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The surface of the MPs was carboxylic acid-functionalized.38 Carbon nanotubes (CNTs) were deposited on the surface of the MPs by the layer-by-layer (LbL) process to form a thin flexible conductive layer (Figure 3A).39,40 To investigate the effect of the CNT functionalization on the electrical response to the external pressure, three types of CNTs were used: multi-walled CNTs stabilized by cetyl trimethyl ammonium bromide (CTAB), aminofunctionalized CNTs (CNT-NH3+), and carboxylic acid-functionalized CNTs (CNT-COO-). The CTAB-stabilized CNTs were readily deposited on the carboxylic-functionalized MPs in an aqueous suspension (Case 1).41 Sequential deposition of the charged CNTs, CNT-NH3+ and then CNT-COO-, was carried out for comparison (Case 2). The degree of CNT deposition was dependent on the solvent species of the CNT-NH3 suspension. Aqueous CNT-NH3 suspension containing a small amount of CTAB produced a thin CNT wall while a mixture suspension of water and ethanol resulted in a thicker CNT wall (Case 2). The generated MPs were uniform in size as shown in Figure 3B. The presence of a CNT network on the MP surface was observed with transmission electron microscope (TEM) (Figure 3C) and SEM (Figure 3D). The distribution of CNTs on the surface was visualized by confocal Raman mapping of the CNTs (Figure 3E) showing the CNT shell on the PU MP. The amount of CNT on the MPs for the two cases was qualitatively compared by Raman spectroscopy (Figure 3F). The Raman spectra were obtained from the cartridge films containing the MPs. The intensities of the D band peak (1320 cm-1) and the G band peak (1580 cm-1) were clearly different, indicating that the Case 2 MPs had a thicker CNT shell. The elastic modulus of Case 1 and Case 2 were 5.1 MPa and 6.0 MPa, respectively, as measured by nanoindentation. To measure contact resistance between ITO and CNT electrode prepared by the same process applied to the MP preparation, the contact (Rc) was measured by the transmission line model (TLM)42,43 (Supporting Information Figure S6). Because the morphology of the CNT layer on the MP was homogeneous, it is reasonable to assume that the conductivity (σ) in the 9 ACS Paragon Plus Environment


spherical surface is uniform. We neglected the electrode resistance (Re) because it is much smaller than the contact resistance and the CNT resistance. The Rc values between ITO and the CNT layer was 8.14 kΩ, and the value between Au and the CNT layer was 2.72 kΩ.

Figure 3. (A) Schematic illustration for producing the CNTs/PU MPs. CNT was coated on the PU MPs in different methods (Case 1, Case 2). (B) SEM image of the PU MPs obtained by photo-polymerization. (C, D) Transmission electron microscope (TEM) image (C) and SEM image (D) showing the CNTs on the surface of the PU MPs. (E) Confocal Raman mapping image of the CNTs/PU MP. (F) Raman spectra of the CNTs/PU MPs.


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To test the sensing performance of the cartridge film, a small piece (4 mm × 5 mm) was cut from the large one and it was sandwiched between two indium tin oxide (ITO) electrodes. Figure 4A describes schematically the contact of the MP with the electrode before applying an external force (P). The MPs are protruding above the surface of the stencil by δ, hence the relative protrusion versus the MP diameter (D) is δ/D. The electrode makes a contact with a certain contact radius (a). The initial contact radius (ao) can be differed by the initial force (Po) that is the sum of the top electrode weight and a small force applied on the device to make a stable contact. The contact resistance is inversely proportional to the contact area, Rc = ρc/(πa2). The specific contact resistivity (ρc) between the CNT electrode and the ITO electrode was 75.5 MΩ cm2. The RMP can be approximately regarded as a resistance between two circular electrodes of radius a when the MP makes contacts with the circular electrodes at its top and bottom. Then, the RMP along the conductive shell of a thickness (t) and a conductivity (σ) can be presented as following,

𝑅𝑅𝑀𝑀𝑀𝑀 ≈ 𝑙𝑙𝑙𝑙 �

1+�1−(2𝑎𝑎⁄𝐷𝐷 )2 1−�1−(2𝑎𝑎⁄𝐷𝐷 )2

1

� 2𝜋𝜋𝜋𝜋𝜋𝜋

(1)

The mathematical derivation of Eq (1) is shown in the Supporting Information. When external pressure is applied on the sensor, the contact radius increases as depicted in Figure 4B. From the Hertzian contact theory,44,45 the contact radius of an elastic particle on a flat solid substrate increases with the total external force (P) applied on the MP by following the relationship

𝑎𝑎 =

1

3𝑃𝑃𝑃𝑃𝑃𝑃 3 � 8𝐸𝐸 �

(2)

, where E is the modulus of the particle and A is the device area divided by the number of MPs on the device. The conductivity of the shell may vary as the curvature of the MP changes under external pressure. Many studies have proved that the conductivity of CNT film electrodes can 11 ACS Paragon Plus Environment


maintain the same under severe bending states.46 We prepared the CNT electrode film by following the same process used for coating CNT on the MPs and confirmed the constant relative resistance of the CNT electrode at diverse bending strains (0.42% - 1.41%) (Figure 4C, D). Because the bending strains in the deformed MPs are within the electrically insensitive strain range of the CNT film, it is reasonable that the change in the conductivity of the CNTcoated layer on the MPs is negligible when pressure is applied on the sensor.

Figure 4. (A, B) Schematic illustration presenting the MP before and after being pressed by external pressure (∆P). The MP of diameter (D) is protruding by δ. The contact radius (a) between the MP and the electrode changes. The total measured resistance is the sum of the electrode resistance (Re), the contact interface resistance (Rc), and the MP resistance (RMP). (C) Changes in the relative resistance of the CNT/PU film according to the bending strain applied to the sample. (D) Camera images showing the procedure during the bending test.


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Under the constant ρc and σ, the change in the total resistance (Rtot) of the sensor under an applied external pressure (∆P = P - Po) on the sensor can be represented as a function of the contact area. Figure 5A represents an example calculation of the resistances (Re, RMP, Rtot) and the relative total resistance (∆R/Ro) through one MP as a function of the contact radius. The glass ITO electrodes were used as the top and bottom electrodes. The initial pressure (Po) applied on the sensor was 0.54 kPa obtained. The sum of the ITO weight (0.34 g) and the force (0.75 gf) applied for stable contact was divided by the area of the cartridge film (0.2 cm2). On the basis of the initial force, the initial contact radius (ao) was calculated to be 2.4 µm from the Eq. (2) and the corresponding initial resistance (Ro) of one MP was calculated to be 47.2 GΩ from Eq. (1). In Figure 3C, RMP is smaller than Rc and it is not sensitive to the contact radius, but Rc decreases sensitively as the contact radius increases, indicating the total resistance change of the sensor is governed by the change of the contact resistance. For instance, Rtot decreases to 22.0 GΩ when the contact area increases to 3.5 µm (indicated by a1). The relative resistance change (∆R/Ro) resulting from this change in Rtot was -0.53. Figure 5B shows the measured relative resistance changes (the symbols) for the Case 1 and Case 2 as a function of the applied pressure on the sensor (∆P applied through a cylindrical metal bar on 0.20 cm2). The experimental setup is shown in the Supporting Information (Figure S7). In order to investigate the effect of relative protrusion (δ/D), we used stencils of different thickness (30, 38, 45 µm) that are corresponding to the δ/D values of 0.4, 0.24, and 0.1, respectively. The sensitivity of a pressure sensor is defined as 𝑆𝑆 = 𝑑𝑑(∆𝑅𝑅/𝑅𝑅𝑜𝑜 )/𝑑𝑑𝑑𝑑.



Figure 5 (A) Predicted resistance profiles through one MP and the relative resistance change (∆R/Ro) as the contact radius changes. (B) Relative resistance changes of the cartridge film pressure sensor according to the degree of protrusion (δ/D) for the Case 2 and Case 1 MPs. The applied voltage was 1 V. (C) Dynamic mechanical test for the Case 2 (δ/D = 0.4) between 3.5 kPa and 5.0 kPa. (D) Sensor responses for the Case 2 (δ/D = 0.4) when the modulus of the stencil was varied (10, 20, 100 MPa).

For the Case 2 sensor, the sensitivity was higher when the δ/D was large, 0.65 kPa-1 for δ/D = 0.4, 0.31 kPa-1 for δ/D = 0.24, and 0.10 kPa-1 for δ/D = 0.1. The sensitivity in the Case 1 sensor at δ/D = 0.4 (hollow inverse triangles) was 0.38 kPa-1 in the small pressure range and it approached zero when the pressure was higher than 7 kPa. These results indicate that the maximum pressure that a cartridge film can sense can be determined by adjusting the relative protrusion of the MP. To achieve reliable sensing without electrical failures, it is important to set the maximum pressure by the sensing material rather than the circuit design. The dotted line is the calculated prediction for the Case 2 sensor at δ/D = 0.4. It was obtained from Eq. (2) and 14 ACS Paragon Plus Environment

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the corresponding change in Rtot. The same initial variables (ao, Ro, Po) were used in the calculation. The prediction was in an excellent coincidence with the experimental measurements, indicating the mathematical model is valid for this cartridge film sensors. We chose the Case 2 cartridge film at δ/D = 0.4 for further characterization as a pressure sensor. The sensor showed a good electrical stability during 1000 pressure cycles between 3.5 kPa and 5.0 kPa (Figure 5C). The possibility that the modulus of the stencil affects the electrical behavior of the sensor was tested with three stencils of different Young’s modulus (10, 20, 100 MPa) (Figure 5D). The diameter of the holes and the thickness of the stencils were the same (δ//D = 0.4) and the Case 2 MPs were used in common. The resistance profiles obtained from the different stencils were in a good agreement, which indicates that the modulus of the stencil does not affect on the sensor performance because the modulus of Case 2 MPs (6.0 MPa) is lower than that of stencil film in three all stencils. Notice that the initial resistance of the sensors (~ 3×104 Ω) was much higher than the resistance (~ 4.7 ×108 Ω) in Figure 3C because the prediction in Figure 3C is a resistance from one MP (there were 2000 MPs in the sensors). It is noteworthy that a pressure sensor working in a wide range of high pressure could be fabricated readily by reducing the degree of protrusion of the MP. In many pressure sensors, the upper limit pressure should be set to prevent mechanical defects in the MP. When the protrusion was small, the MP was effectively protected under extreme pressure because the pressure was distributed to the stencil matrix as the external object hit the stencil surface. An example is shown in Figure 6A, where the protrusion of the MP was 5 μm (δ/D = 0.1) and the external pressure was applied beyond 480 kPa. The OM images taken for the same position before and after 100 contacts (Figure 6B) shows no damage in the MPs or the MP array, indicating the stencil played as the protection barrier. In this study, we used uniform-sized MPs 15 ACS Paragon Plus Environment


to control the δ/D values. This size uniformity is important also to attain stability of the sensors. When the MPs have a large size distribution, some small or large MPs may come out of the holes during the repeated pressure measurement. The size tolerance was ± 10 % difference from the average diameter. In this size distribution, the sensing performance would be stable because the MPs are stable in the hole and the electrical signals from many MPs of different sizes are averaged.

Figure 6. (A) Pressure-response test at a high pressure region (>100 kPa) when the relative protrusion was (δ/D) = 0.1 (CASE 2). (B) OM images at the same position before and after 100 pressure tests.

We took a full advantage of the cartridge film by cutting a small piece and reusing it for different electrode pairs, as schematically illustrated in Figure 7A. The electrodes tested in this study were flexible ITO and Au thin film (60 nm) coated on PET substrates (150 µm in thickness). The Case 2 (δ/D = 0.4) cartridge film was used as an example. The small piece of the cartridge film first placed between the ITO electrodes and the sensor performance was 16 ACS Paragon Plus Environment

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tested. Then, the small piece was peeled off and was inserted between the flexible Au electrodes. Figure 7B represents the relative current changes of the sensors upon the external pressure. Since the moduli of the electrodes were much larger than that of the Case 2 MP, the current profiles were dependent on the deformation of the MPs in the cartridge film. Since the relative current changes obtained in the electrode pairs are nearly identical, a cartridge film can be used for various electrode pairs to produce pressure sensors with the same sensing performance.

Figure 7. (A) Schematic illustration showing the cut-and-paste of the cartridge film and the transfer to another electrode. (B) The current changes of sensor when a piece of the cartridge film (4 mm × 5 mm) was used in a ITO glass electrode pair and then transferred to a flexible Au electrode pair. (C) The electrical response to a weak mechanical stimulation (20 Pa). The response was similar to the stimulation (response and recovery times were 31 ms and 43 ms in case of the ITO electrodes, and 56 ms and 67 ms in case of the flexible Au electrodes). (D) Relative current change by a blood pulse from the pressure sensor made of the Case 2 (δ/D = 0.4) in a Au electrode pair. The applied voltage was 1.0 V. 17 ACS Paragon Plus Environment


Response time to external stimulation is an important property of a tactile sensor. Figure 7C shows the electrical response of the the Case 2 (δ/D = 0.4) cartridge film (4 mm × 5 mm) inserted in the two the ITO glass electrodes and the flexible Au electrodes. A small cyclic pressure (2 kPa) was applied on the sensors. The cartridge film with the ITO glass electrodes showed slightly faster detection than the one with the flexible Au electrodes. Both cases showed prompt response and recovery within 70 ms. The response time of the sensor is determined by the elastic dynamics of the MPs. Because the MPs are elastic solid spheres the deformation dynamics follows the mechanical dynamics of the external stimulation, hence the response times in Figure 7C actually indicate the length of time spent during applying the pressure and releasing the pressure by the testing machine. The fast response upon a small pressure enables the real-time monitoring of biological signals such as heart pulse on the wrist (Figure 7D). The sensing cartridge was placed between the flexible Au electrodes. The sensor was highly flexible. Because the bending strain causes a small pressure to the MPs, the initial current in the sensor was increased. The increase of the initial current was small, hence the decrease of the sensitivity was not considerable. The blood pulse sensor used on the wrist is an example showing the excellent sensor performance under bending status. The pulse profile exhibits the three characteristic pulse peaks with a period of 0.56 s. These results exhibit that the cartridge film sensor can be reused in many electrode pairs with the same sensing responsivity.

3. Conclusions This study has proposed a new pressure sensing material called a cut-and-paste transferrable cartridge film. The cartridge film proposed in this study is easy to change the 18 ACS Paragon Plus Environment

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sensing characteristics, possible to be cut-and-paste to electrodes, and transferrable to another electrode. As an example, a cartridge film was prepared by filling CNT-coated polyurethane (PU) microparticles in the holes of a PU stencil film (one MP in a hole). We showed that the conductivity of the MPs and the protrusion of the MPs above the surface of the stencil determine the characteristics of the pressure sensor. Flexible pressure sensors were fabricated by cutting and pasting a small piece of the cartridge film. In this study, the cartridge film was used as a pressure sensor to monitor the heartbeat, but the concept can be extended to a variety of devices such as MP-type pressure sensors or MP-type displays. This work focused on suggesting a new concept using the MP-based pressure sensor cartridge film. More studies should be carried out, including the effect of size distribution, detailed simulation on the deformation of the MPs with different modulus, the real contact area between the surfaces with roughness, the possible change in the electrical properties in the MPs and the interfaces, methods to stabilize the interface, and the impact using highly conductive MPs.

4. Experimental Section Materials. Polyurethane (PU) precursor was purchased from Topjin Chemiclas (Korea). PU ionomer precursor was kindly supplied from Miwon Specialty Chemical Co. (Korea). Thin multi-walled carbon nanotubes were purchased from Nanosolution (Korea). Multi-walled carbon nanotubes functionalized with amino group and carboxylate group, respectively, were purchased from Cheap Tubes (USA). Hexyltrimethoxysilane (TCI, Japan) and 2[methoxy(polyethyleneoxy)-propyl]trimethoxysilane (Gelest, USA) were used as received. Isopropyl alcohol (IPA), ethyl alcohol anhydrous (99.9%), dimethylformamide (DMF), toluene and chloroform were purchased from Daejung (Korea). Poly(vinyl alcohol) (PVA, Mw 1300023000 g/mol, 87-89% hydrolyzed) and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich (USA). All above chemicals were reagent grades and used without further purification.



Fabrication of glass capillary microfluidic devices. Glass capillary-based microfluidic devices were fabricated by assembling glass capillaries. First, round glass capillaries were tapered by heating and pulling a cylindrical glass capillary (outer diameter = 1.0 mm, inner diameter = 0.58 mm, World Precision Instruments, USA) with a pipette puller (Model P-97; Sutter Instruments, USA). For hydrophilic modification, the round capillary tube was treated with 2 wt% of 2-[methoxy (polyethyleneoxy) propyl]trimethoxysilane in mixture of ethanol and water at pH 5. A square capillary (inner diameter ~1 mm, Atlantic International Technology, USA) was wetted with 1 wt% hexyltrimethoxysilane in toluene to hydrophobication. The capillaries were dipped in the chemicals for 1 min at room temperature and dried at 50 °C for 6 h. For the preparation of O/W emulsions, a tapered cylindrical capillary tube, which was used as the collection tube, was inserted into a square capillary tube. The diameter of the tapered capillary inner tube was set to 200 μm. Each end of the square capillary was fit in with a needle (Cupdown needle, Korea Vaccine Co., Ltd., Korea) and completely glued with epoxy resin (Araldite5, Huntsman). Synthesis of monodisperse PU elastomer microparticles. Uniform O/W emulsions were generated through the microcapillary microfluidic device. The outer fluid was a PVA aqueous solution (10 wt%) containing 2 wt% PU ionomer precursor. The dispersion fluid was made with 20 vol% PU precursor dissolved in a mixture of toluene and chloroform in a volume ratio of 1.8:1. The flow rate of each fluid was precisely controlled using syringe pumps (Pump 11 Elite, Harvard Apparatus, USA). The generation of emulsion drops was monitored using an inverted microscope equipped with a high-speed camera (Phantom Miro eX2, Vision Research Inc., USA). The emulsion drops were then solidified by photo-polymerization by irradiation with UV light (365 nm) for 1 min (JHC1-051S-V2, A&D, Korea) after complete evaporation of solvents from the emulsion drops. After the polymerization, PVA and other additives were completely washed out by repeating centrifugation at 1500 rpm with a large amount of water. The PU elastomer microparticles were re-dispersed in deionized (DI) water. The sizes of the emulsion drops and PU elastomer microparticles were determined by analyzing the bright-field microscopy images (Axio Vert.A1, Zeiss, Germany). The modulus of the PU films was 100 MPa, however the modulus of the MPs was 3.7 MPa. LbL deposition of CNTs on PU microparticles. To uniformly coat CNTs on PU microparticles, the layer-by-layer (LbL) deposition was employed. PU elastomer microparticles (45 mg) were placed in a vial (10 mL). Thin multi-walled CNTs (200 µg/mL) were dispersed in a 0.3 wt% CTAB aqueous solution by sonication for 30 min. The PU 20 ACS Paragon Plus Environment

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microparticles were then immersed in the CNT dispersion while stirring for 24 h at room temperature (Case 1). After the deposition of CNTs, the sample was repeatedly washed with DMF and DI water for more than five times to remove unreacted materials. In the case of generating a thicker CNT layer, oppositely charged CNTs were alternately deposited on the PU microparticles (Case 2). For this, CNT-COO- (200 µg/mL) was dispersed in the mixture of DI water and ethanol in a volume ratio of 2:1. Also, CNT-NH3+ (200 µg/mL) was dispersed in a mixture of DI water and ethanol in a volume ratio of 2:1. The PU microparticles were then immersed in the CNT-NH3+ dispersion (2 mL) while sonicating for 2 h. After removal of unreacted CNTs and additives by repeated washing with DMF and DI water for more than five times, the same process was undergone in the CNT-COO- dispersion (2 ml), thus generating one more CNT layer on the CNT-coated PU microparticles. The surface morphology of CNTs/PU microparticles was observed with a scanning electron microscopy (SEM, S-4800, Hitachi, Japan) and energy-filtering transmission electron microscopy (TEM, LIBRA 120, Carl ZEISS, Germany). The presence of CNT layers on the surface of PU microparticles was confirmed by using a confocal Raman Microscopy (Alpha 300R Plus, WITec, Germany) and a Raman spectroscopy (RM 1000, Renishaw, UK). Fabrication of PU stencil. PU stencil films were molded using a pillar-arrayed PDMS which was derived from a hole-patterned SU-8 resist. First, a silicon wafer was pretreated by applying oxygen plasma (Cute-B, Femto Science, Korea) at 70 W for 90 s to make the surface hydrophilic. Then, 1mL of SU-8 per inch was poured on the silicon wafer. The SU-8 was spincoated in a vacuum by 1500 rpm for 60 s. The uniformly coated SU-8 was prebaked at 65°C for 6 min and consecutively at 95°C for 24 min. The baked SU-8 resist was photo-polymerized by irradiation of UV light (365 nm, JHC1-051S-V2, A&D, Korea) for 9 min while covered with a photomask. The photomask has hexagonal pattern of filled circles with 50 μm of diameter and 100 μm of distance between the center of each circle. After removal of the photomask, the SU-8 resist was rinsed with D.I. water for several times. The SU-8 resist was post baked at 65°C for 1 min and at 95°C for 7 min. Then, the SU-8 resist was soaked in a developer for 4 min to dissolve out the reacted SU-8 residue. The patterned SU-8 resist was rinsed with IPA and dried. After bake at 120°C for 1 min, holes-arrayed SU-8 resist of which diameter is 50 μm diameter was obtained. The pillar-arrayed PDMS was molded from the holes-arrayed SU-8 resist. The mixture of pre-polymer/cross-linker (10/1, v/v) was poured onto the SU-8 resist and heated to 85 °C for 1 h to solidify it. After separation of the SU-8 resist, pillar-arrayed PDMS was obtained. Taking the same fabrication procedure, PU precursor was 21 ACS Paragon Plus Environment


poured onto the pillar-arrayed PDMS. Consecutive UV irradiation (365 nm) for 20 s transformed the PU precursor to a flexible, transparent PU stencil with holes uniformly pattered with regular intervals. 2D colloidal array using unidirectional rubbing process. 2-D microparticle array was conducted by sitting the target microparticles on each hole of the PU stencil. For this, amounts of completely dried CNTs/PU microparticles were put on stencil. Then, using a flat PDMS piece, the particles were unidirectionally rubbed on the PU stencil for 30 sec. Excess microparticles on the stencil were carefully blew off using a dust blower. The positioning on the stencil was confirmed by direct bright-field microscopy observation. Precise detection of pressure changes. Indium tin oxide (ITO)-coated glasses were used as top and bottom electrodes (10 mm × 20 mm). The ITO-coated glasses were cleaned by sequentially sonicating with alcohol and acetone for 30 min. Then, their surface was exposed to oxygen plasma (Cute-B, Femto Science, Korea) at 70 W, 90 sec. The flexible PU stencil filled with CNTs/PU microparticles by unidirectionally rubbing was placed on the bottom electrode. The particles-arrayed ITO glass was covered with the other ITO glass as a top electrode. Each electrode was extended with an Au-Ni woven conductive fabric (SILTEX CNG Type, Solueta Co. Ltd., Korea) and fixed with a conductive copper tape. The arrangement of microparticles on the ITO glasses was observed with an optical microscopy (BX-51, Olympus, Japan). Current changes in response to the applied pressure were detected using a source meter (2400, Keithley, USA). The applied force for pressurizing the sensor was precisely controlled using a universal manipulator with 0.01 N resolution (UMP 100, Teraleader Co. Ltd., Korea).


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■ ASSOCIATED CONTENT

Supporting Information. Additional data supporting this publication including mathematical derivation, characterization, and supplementary OM, SEM, and Digit images.

This material is available free of charge via the Internet at http://pubs.acs.org. ■ AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected].

Author Contributions #

H. Hwang, S.-E. Choi, S. W. Han contributed equally to this work.

Notes The authors declare no competing financial interest. ■ ACKNOWLEDGMENT This research was supported partly by Samsung Research Funding Center of Samsung Electronics under Project Number SRFC-MA1301-07.

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Cut-and-Paste Transferrable Pressure Sensing Cartridge Films

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