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Flexible and Compressible PEDOT:PSS@Melamine Conductive Sponge Prepared via One-step Dip Coating as Piezoresistive Pressure Sensor for Human Motion Detection Yichun Ding, Jack Yang, Charles Tolle, and Zhengtao Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 13 Apr 2018 Downloaded from http://pubs.acs.org on April 13, 2018

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ACS Applied Materials & Interfaces

Flexible and Compressible PEDOT:PSS@Melamine Conductive Sponge Prepared via One-step Dip Coating as Piezoresistive Pressure Sensor for Human Motion Detection Yichun Ding,† Jack Yang,‡ Charles R. Tolle,∥ and Zhengtao Zhu*,†,⊥ †

Biomedical Engineering PhD Program, South Dakota School of Mines & Technology, Rapid

City, SD 57701, USA. ‡

Materials Engineering and Science PhD Program, South Dakota School of Mines &

Technology, Rapid City, SD 57701, USA. ∥

Department of Electrical Engineering, South Dakota School of Mines & Technology, Rapid

City, SD 57701, USA ⊥ Department

of Chemistry and Applied Biological Sciences, South Dakota School of Mines &

Technology, Rapid City, SD 57701, USA. KEYWORDS. Piezoresistive pressure sensor, PEDOT:PSS, Melamine sponge, Conductive sponge, Dip coating, Human motion detection, Electronic skin

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ABSTRACT. Flexible and wearable pressure sensor may offer convenient, timely, and portable solutions to human motion detection, yet it is a challenge to develop cost-effective materials for pressure sensor with high compressibility and sensitivity. Herein, a cost-efficient and scalable approach is reported to prepare highly flexible and compressible conductive sponge for piezoresistive pressure sensor. The conductive sponge, PEDOT:PSS@MS, is prepared by onestep dip coating the commercial melamine sponge (MS) in an aqueous dispersion of poly(3,4ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).

Due to the interconnected

porous structure of MS, the conductive PEDOT:PSS@MS has high compressibility and stable piezoresistive response at the compressive strain up to 80%, as well as good reproducibility over 1000 cycles.

Thereafter, versatile pressure sensors fabricated using the conductive

PEDOT:PSS@MS sponges are attached to the different parts of human body; the capabilities of these devices to detect a variety of human motions including speaking, finger bending, elbow bending, and walking are evaluated. Furthermore, prototype tactile sensory array based on these pressure sensors is demonstrated.

1. INTRODUCTION Flexible and wearable strain and pressure sensors have shown great potential for applications in human-machine interfaces, soft robotics, human motion detection, artificial electronic skin, etc.1-5

In particular, highly sensitive pressure/tactile sensors are essential in detecting or

monitoring a wide range of human motions, such as subtle motions of speaking, heartbeat, and wrist pulse or large motions of joint bending, muscle movements, and walking.3,

6-7

The

traditional microelectromechanical systems (MEMS), based on metals and silicon, have enough


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sensitivity to detect the subtle motion changes; however, due to the rigidity and small strain tolerance of metals and silicon,8-9 the MEMS devices are unable to detect large strain changes in the flexible and wearable applications.

Additionally, fabrication of the MEMS devices is

typically complicated and costly. In recent years, new materials and/or structures have been explored for highly stretchable and sensitive strain/pressure sensors.10-13 There are mainly four types of transduction mechanisms (i.e., piezoresistive, capacitive, piezoelectric, and triboelectric) for converting mechanical force and/or deformation into electrical signals.10-13 Piezoelectric and triboelectric effects have been explored to develop high sensitivity and self-powered pressure/strain sensors.14 For example, vertical aligned nanowires of PVDF copolymers

were prepared by template or imprinting

methods, and then patterned and packaged into flexible tactile sensors.15-17 The piezoresistive sensor, which transduces the external pressure into a resistance signal, has the advantages of simple fabrication, low-cost, and easy signal acquisition. Flexible piezoresistive pressure sensors have been demonstrated using conductive elastomeric composites or conductive porous sponges.18-21 As an example, a highly flexible and transparent strain sensor was fabricated by embedding multiwalled carbon

nanotubes

(MWCNTs) into patterned interconnected

microtrenches of PDMS films; the sensor showed high stretchability (~10 %) and sensitivity with a gauge factor of 1140 at a small strain of 8.75%.21 In these sensors, under the compressive force, more conductive pathways are formed in the conductive elastomeric composites or sponges, resulting in decrease of resistance of the sensors; this transduction mechanism is known as the negative piezoresistive effect. Soft, spongy, and conductive materials can be used in highly sensitive pressure sensors that are conformable and compatible with wearable electronics.

Typically, these materials are


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composites of a scaffold component and a conductive component.

Superior mechanical

elasticity and high compressibility are required for the scaffold component, whereas sensitive piezoresistive changes are required for the conductive component.

Additionally, good

integration/adhesion between the two components is essential to have stable and reproducible devices.

Many materials and methods have been reported in preparation of the spongy

conductive materials, often involving multiple-steps and various new materials. The general idea is to assemble a compressible three-dimensional (3D) structure incorporating conductive materials. Conductive sponges composed of neat conductive materials such as carbon materials (e.g., CNTs, graphene, carbon nanofibers), metal nanowires, and conducting polymers can be prepared by direct carbonization of polymer sponges, template growth, and freeze-drying, etc.2230

For example, Wang et al. reported a piezoresistive pressure sensor based on a 3D porous

carbon architecture of carbonized melamine sponge and CNTs;30

Ma et al. prepared an

interconnected graphene-amorphous carbon hierarchical foam by chemical vapor deposition (CVD) growth of graphene on copper foam followed by etching away the copper skeleton.25 These conductive sponges may have limited sensitivity and range of compressive strain, because the 3D structures of the neat materials are mechanically fragile and less flexible. To improve the stability and compressibility, polymer can be infiltrated into the porous structures of the neat conductive materials to form polymer-coated/impregnated conductive sponges.31-33 Another strategy to prepare conductive elastomers/sponges is to assemble 3D composite sponges by methods such as foaming or freeze-drying after mixing the conductive components with insulating polymers.19,

34-37

For example, Liu et al. prepared porous graphene/thermoplastic

polyurethane foams with tunable conductivity by freeze-drying;19 Xu et al. reported a 3D conductive sponge prepared by assembly of shortened electrospun nanofibers of polyacrylonitrile


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(PAN), polyimide (PI), and PAN-based carbon.37 In these conductive sponges, the conductivity and sensitivity of the materials can be readily customized by varying the content of the conductive material; on the other hand, non-uniform dispersion of the conductive material, hysteresis effect, and irreversible plastic deformation may be problematic for these composite based conductive sponges.19 The 3D conductive sponges can also be prepared by coating a polymer sponge with conductive materials using methods such as dip coating and sputtering.18, 38-41

Overall, this strategy is simple, straightforward, and efficient. Solution or dispersion of

carbon and metals (such as CNTs, graphene, Au, etc.) are widely used for dip coating of sponges to make them conductive.18,

38-41

These conductive materials are typically prepared by

sophisticated and costly processes, and may be problematic to form a stable and homogeneous “ink” for dip coating. In addition, the prepared conductive sponges may have stability issues; for example, the coated carbon materials or metals might peel off from the sponge due to the weak adhesion between the conductive materials and the sponges. In this work, we are interested in developing the spongy conductive materials with the wellestablished materials and simple preparation process. For the spongy scaffold, the commercially available melamine sponge (MS) is chosen. The 3D, highly porous, and interconnected opencell structure of MS gives the material good elasticity and compressibility; more importantly, MS is intrinsically superhydrophilic and can form good adhesion with our choice of the conductive component, PEDOT:PSS. PEDOT:PSS, a highly conductive polymer commercially available as an aqueous dispersion, has been widely used in various flexible electronics and sensors.42-43 The superhydrophilic MS and the stable PEDOT:PSS aqueous dispersion are ideal for preparation of a robust conductive sponge by the simple and scalable dip coating process. Strong and uniform adhesion is expected between the PEDOT:PSS and MS. During the dip coating process, the


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hydrophilic MS surface and the aqueous solution of the PEDOT:PSS can afford intimacy wetting; additionally, both the MS and PEDOT:PSS are polymers with aromatic heterocyclic molecules, which may lead to strong intermolecular interactions (e.g., π-π stacking).44 Such good adhesion would be beneficial for overcoming some common drawbacks such as non-uniform dispersion and weak adhesion of carbon or metal nanostructures. To the best of our knowledge, conductive sponge prepared by the combination of melamine sponge and PEDOT:PSS has not been reported. In comparison with dip coating of carbon and metal nanomaterials for preparation of conductive sponges, the method reported here has several distinguished advantages.

(1) Both MS and PEDOT:PSS are stable and ready available

commercially, which eliminates the sophisticated and costly preparation of carbon and metal nanomaterials. (2) Coating of the conducting polymer (PEDOT:PSS) on a polymeric sponge (MS) leads to conformable and uniform conductive sponge, which may overcome the problems with the materials prepared by dip coating of carbon and metal nanomaterials (e.g., fragile, easily peel off, aggregation of coating materials).

(3) The good flexibility, high mechanical

compressibility, and robust porous morphology of the PEDOT:PSS coated melamine sponge result in highly sensitive and stable piezoresistive pressure sensors that respond to wide range of human motions. The morphology, mechanical property, and piezoresistive response of the conductive PEDOT:PSS coated melamine sponge (PEDOT:PSS@MS) are investigated, and the results show that PEDOT:PSS@MS has good compressibility and stable piezoresistive response under the compressive strain up to 80%. Versatile pressure sensors are fabricated based on the PEDOT:PSS@MS conductive sponge for the detection of human motions including speaking, finger bending, elbow bending, and walking, etc.

Prototype of a pressure sensory array

containing 3×3 pixel units is demonstrated.


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2. EXPERIMENTAL SECTION Materials: Melamine sponge (melamine-formaldehyde resin sponge) was purchased from SINOYQX

(Sichuan,

China).

Poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate)

(PEDOT:PSS) (CleviosTM PH1000, solid content 1.3%) was purchased from Heraeus Precious Metals North America Daychem LLC (Vandalia, OH, USA). Dimethyl sulfoxide (DMSO, 99.9%) and 4-dodecylbenzenesulfonic acid (~90%) were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, USA). Silver paste (Flash-DryTM Ag colloidal suspension) was purchased from SPI supplies (West Chester, PA, USA). Copper/Nickel tape was purchased from TED PELLA, Inc. (Redding, CA, USA). All chemicals were used as received without further purification. Preparation of PEDOT:PSS@MS and pressure sensor: A piece of melamine sponge was cut into cylindrical shaped pieces with a diameter of 8.0 mm and a height of 10.0 mm (or other dimensions); and then the cylindrical shaped melamine sponge was dipped in a PEDOT:PSS solution (prepared by mixing of 9.5 g CleviosTM PH1000 PEDOT:PSS dispersion, 0.5 g DMSO, and 0.1 g 4-dodecylbenzenesulfonic acid) for 15 min. After the excessive solution was squeezed out of the sample, the sample was dried in a vacuum oven at 100 °C for 1 h to obtain a PEDOT:PSS@MS conductive sponge. The pressure sensor was prepared by adding the adhesive tapes of copper/nickel with copper wires at the two ends of the PEDOT:PSS conductive sponge for electric contacts. To facilitate good electric contacts, the two ends of the sponge were brushed with silver paste prior to adding of the copper/nickel tape. Characterization of PEDOT:PSS@MS: The morphology and elemental composition of the samples were characterized by Field Emission Scanning Electron Microscopy (FE-SEM, Zeiss


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Supra 40 VP) equipped with energy-dispersive X-ray spectroscope (SEM/EDS); the SEM images were acquired at an accelerating voltage of 2 kV while the EDS elemental mapping was collected at an accelerating voltage of 8 kV. Water contact angle was measured by the sessile drop technique using the 500-F1 Advanced Goniometer (Ramé-Hart Instrument Co., USA). The surface energies of the PEDOT:PSS and melamine resin films were determined by the Owens/Wendt two component theory. Additional experimental details on film preparation and contact angle measurement are included in Supporting Information. The compressive stressstrain curves at the compressive strain of 20%, 40%, 60%, and 80% were collected respectively on an electromechanical testing machine (CMT-8102, Shenzhen, China) with the compress/release speed of 5 mm min-1. Keithley 2612 Source Meter was used to measure the resistance and current-voltage (I-V) response of the PEDOT:PSS@MS pressure sensor. Five samples were measured, and the average value of the resistance was reported. A custom-built computer-controlled apparatus was used for the cyclic compress-release test. Two pressure sensors that were in series with a light emitting diode (LED) and connected to a power supply were used in a simple light switch circuitry demonstration. Human motion detection: To detect/monitor the human motions of speaking, finger bending, elbow bending, and knee bending, a PEDOT:PSS@MS pressure sensor was attached to the volunteer’s throat, index finger, elbow, and knee, respectively. To maintain a stable contact with human body and good electric contacts, the pressure sensor was fixed at the position using medical tape or Band-Aid, and thus the pressure sensor was compressed to certain extent (~10%) initially. To monitor human gait, three pressure sensors were settled at three positions (toe, middle, and heel) of an insole using Band-Aid. The resistance changes were recorded on


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Keithley 2612 Source Meter using Test Script Builder program. The videos of the above human motion monitoring were recorded using a home-designed multichannel resistance tester. Prototype of sensory array: The 3×3 pixel sensory array was fabricated by embedding 9 pressure sensors in a non-conductive foam (pristine melamine sponge) that pre-punched with 9 holes. On one side, the 9 sensors had a common electrode made by copper/nickel tape, while on the other side each sensor had a separated electrode. The sensory array was packaged using PDMS thin film and tape. A weight or an “AAA” battery was placed on the sensory array, and the resistance changes of the 9 sensors were recorded.

3. RESULTS AND DISCUSSION Figure 1a illustrates schematically the preparation procedure of the conductive sponges based on melamine sponge (MS) and PEDOT:PSS. A piece of MS is readily cut into specimens of various shapes. The specimens are then immersed in a PEDOT:PSS aqueous dispersion to coat MS with PEDOT:PSS; after the excess PEDOT:PSS dispersion in the sponges is squeezed out, the PEDOT:PSS coated sponges are dried to obtain the conductive PEDOT:PSS@MS sponges. As shown in Figure 1a, the PEDOT:PSS@MS sponges retain the shapes of the original sponges after the dip coating and drying process; the color changes from the white color of MS to the dark gray color of PEDOT:PSS@MS. Figure 1b shows photos of a cylindrical sample being compressed, demonstrating the PEDOT:PSS@MS is highly compressible. Figure 1c shows photos demonstrating the PEDOT:PSS@MS sample is flexible and can subject to bending and torsion. Additionally, the PEDOT:PSS@MS conductive sponges with different shapes and sizes can be prepared by cutting the melamine sponges (Figure S1), which may enable us to develop


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wearable pressure sensors adaptable to the various shapes of human body positions/parts. These results demonstrate that a highly flexible and compressible sponge of PEDOT:PSS@MS is obtained by the simple dip coating process. Besides the simple, facile, and scalable preparation process, this method is also cost-effective; as shown in Table S1, the total material cost of preparation of PEDOT:PSS@MS is estimated to be only ~ ¢10.6 per device (for the size mentioned above).

Figure 1. a) Schematic for preparation of the PEDOT:PSS coated melamine sponge (PEDOT:PSS@MS) by a simple dip coating process.

b-c) Photos showing that the

PEDOT:PSS@MS is compressed, bent, or twisted.

Figure 2 shows the SEM images of melamine sponge, the melamine sponge after dip coating (i.e., PEDOT:PSS@MS), and PEDOT:PSS@MS under compression. As shown in Figure 2a,


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the melamine sponge has porous and cellular-like structure with the interconnected tetrapodshaped skeletons/backbones; and the skeleton length is about tens of micrometer and the pore size is about 100~200 micrometers (Figure S2). The highly porous MS is lightweight with the density of only 9.87 ± 0.10 mg cm-3 and the porosity higher than 99%,45 beneficial for making conformable and wearable devices.

After dip coating and drying, the obtained

PEDOT:PSS@MS sample retains the porous and interconnected structure (Figure 2b); the surface of the sponge becomes slightly rough with the coated PEDOT:PSS layer (inset of Figure 2b). SEM Energy Dispersive X-ray Spectroscopy (SEM-EDS) analysis further confirms the successful coating of PEDOT:PSS; as shown in Figure S3, C, N, O, S, and Na elements are identified on the surface of PEDOT:PSS@MS, where the S element is largely originated from PEDOT:PSS.

Figure 2c shows the SEM image of PEDOT:PSS@MS under compression.

When the sponge is compressed under applied force, the cellular-like pores become smaller, and the temporary contacts are created between the polymer skeletons. As being discussed later, due to the increased temporary contacts in the skeletons, more conductive pathways may be established, resulting in pressure-sensitive decrease of the electric resistance.


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Figure 2. SEM images of a) pristine melamine sponge, b) PEDOT:PSS@MS, and c) PEDOT:PSS@MS under compression.

Inset of b): the magnified SEM image of

PEDOT:PSS@MS.

The melamine sponge is highly elastic and can be compressed up to 80% strain without plastic deformation. Figure 3a shows the stress-strain curves of the pristine melamine sponge at the compressive strain up to 20%, 40%, 60%, and 80%, respectively. The compress curve exhibits three characteristic deformation regions typically observed in an open-cell foam. In the elastic region at strain less than 20%, the stress increases linearly and the compressive stress was 5.8 kPa at 20% strain; in the plateau region at the strain between 20% and 60%, the compress stress increases slowly and flats out, and the compressive stress values at 40% and 60% strains are 8.0


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kPa and 12.2 kPa, respectively; in the densification region at the compressive strain higher than 60%, the stress increases sharply due to the diminished pore volume.

The maximum

compressive stress at 80% strain was 35.0 kPa. The relatively slow increase in compressive stress at low strains (60%) when most skeletal structures are already in contact, continuous compression leads to relatively rapid increase in stress. After the dip coating process, the obtained PEDOT:PSS@MS conductive sponge shows similar mechanical properties as the melamine sponge. In comparison to the pristine MS, the PEDOT:PSS@MS has slightly higher compressive stress at the same strain; the maximum compressive stress values are 5.9 kPa, 9.6 kPa, 15.9 kPa, and 37.6 kPa, respectively, at 20%, 40%, 60%, and 80% strains. The typical pressure related to the normal human activities can be categorized into three pressure regimes: (1) subtle-pressure regime from 1 Pa to 1 kPa (e.g., human skin sensing), (2) low-pressure regime from 1 to ~10 kPa, (e.g., gentle manipulation of items), and (3) medium-pressure regime from 10 to ~100 kPa (e.g., blood pulse, human weight, and joint movements).3 Therefore, the wide compressive strain (0~80%) and stress (0~37.6 kPa) of the as-prepared PEDOT:PSS@MS conductive sponge is sufficient to detect most human motions.


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Figure 3. Compressive stress-strain curves of a) pristine melamine sponge and b) PEDOT:PSS@MS at strain maxima of 20%, 40%, 60%, and 80%. To investigate the piezoresistive response of the PEDOT:PSS@MS sponge, pressure sensors are assembled using cylindrical sponges with the dimension of 8.0 mm in diameter and 10.0 mm in height. Figure 4a shows a simple circuit, where two pressure sensors are connected in series to a LED and a power supply (battery). The LED lights up with increase of the compressive strain applied on the pressure sensors, visually demonstrating decrease of resistance with increase of pressure. This negative piezoresistive effect is attributed to the increased conductive pathways during compression of the sponge, as shown schematically in Figure 4b. During the compression, the temporary contacts between the skeleton branches of the PEDOT:PSS@MS sponge are formed, resulting in increase of the conductivity. Such temporary morphological change during compression is clearly observed in the SEM images of Figure 2c.


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Figure 4. a) LED circuit using two PEDOT:PSS@MS conductive sponges as interconnectors. b) Schematic of the structural change of PEDOT:PSS@MS under compression. c) Electric resistance variations of PEDOT:PSS@MS under different compressive strains. Inset: image of a pressure sensor device. d) Relative resistance variation ((R-R0)/R0) of PEDOT:PSS@MS under different compressive strains. e) Gauge factor variations of PEDOT:PSS@MS under different compressive strains. f) Time-dependent response of (R-R0)/R0 during compress-release cycle between 0% and 70% strain.


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The resistance of the PEDOT:PSS@MS pressure sensor is measured with a 10% stepwise increase of the compressive strain up to 80% strain. Figure S4 shows the current-voltage (I-V) curves of the PEDOT:PSS@MS pressure sensor under different strains (0~80%); the linear I-V responses indicate the contacts between the conductive sponge and the electrode (silver paste/ copper/nickel tape and copper wire) are ohmic and the contact resistances are negligible. Figure 4c shows the electric resistance (R) of PEDOT:PSS@MS under different compressive strains. The resistance decreases from 173.9±33.4 Ω to 20.6±5.1 Ω as the strain increases from 0 to 80%. The corresponding relative resistive change (∆R/R0, ∆R=R-R0, where R0 is the resistance at 0% strain, and R is the resistance under compressive strain) varies from 0 to -88.1±2.2% when the strain changes from 0% to 80% (Figure 4d). The relative resistive change shows two linear regions with slightly different slopes, possibly related to the elastic deformation and the plateau/densification of the sponge during compression (Figure 3). The sensitivity, defined as gauge factor (GF=(∆R/R0)/ε, where ε is the compressive strain), varies from -2.32 ± 0.41 to 1.10 ± 0.028 as the strain changes from 10% to 80% (Figure 4e), indicating that the pressure sensor has relatively high sensitivity at low strains. Note that the resistance change with strain can be correlated to the pressure, which may be preferable in certain practical applications, by combining the stress-strain curve with the relative resistive change-strain curve, as shown in Figure S5. Performance of devices with different sizes and shapes is shown in Figure S6. Overall, the resistance changes and gauge factors of these devices are similar when the strain changes from 0% to 80%. The response time of the pressure sensor is evaluated. When the pressure sensor is compressed and released between 0% to 70% strain at a constant speed of ~2.0 mm s-1, the device shows a response/recovery time of about 3.5 s (Figure 4f). The results indicate the response time of the


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pressure sensor follows the compress-release speed of the motor without significant delay. Furthermore, the response of the resistance change with compress and release is nearly symmetric, suggesting little hysteresis is observed during the compress-release cycle. Note that at the end of the compress cycle, the apparatus needs time to move the stage backwards for the release cycle, which causes some anomaly in the resistance change. During the compress/release of the PEDOT:PSS@MS sensor, the temporary electric contacts are formed or broken, leading to quick response of the resistance change. On the other hand, the mechanical stress-strain curve (Figure 3) shows hysteresis, which is attributed to the energy loss during compression of the polymeric sponge. The hysteresis of the strain-stress response suggests that it may not be ideal for the PEDOT:PSS@MS device to accurately determine the strain and stress simultaneously by measuring resistance change; on the other hand, since the resistance change has little hysteresis with strain, it is possible to use the PEDOT:PSS@MS sensor for measuring strain or stress separately. The stability, durability, and reproducibility of the PEDOT:PSS@MS pressure sensor are evaluated by the cyclic compress-release test using a custom-built computer-controlled apparatus. The pressure sensor is compressed and released between 30% and 10% strains for about 1000 cycles continuously. The relative resistance change ((R-R0)/R0) versus the cycle number is shown in Figure 5. The stable resistance change at the initial (i.e., 1~10 cycles), intermediate (i.e., 490~500 cycles), and final (i.e., 990~1000 cycles) stages of the cyclic test demonstrates the excellent reproducibility and durability of the PEDOT:PSS@MS pressure sensor. SEM images of PEDOT:PSS@MS after the 1000 compress-release cycles are shown in Figure S7. The PEDOT:PSS@MS sponge retains its interconnected open cell structure without any observable damage; the coated PEDOT:PSS layer is also intact. Table S2 compares the performance of the


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PEDOT:PSS@MS pressure sensor in this work and several other pressure sensors fabricated from 3D porous conductive sponges in recent publications. The PEDOT:PSS@MS pressure sensor shows excellent compressibility, sensitivity, and reproducibility among these pressure sensors; more importantly, the preparation process of the PEDOT:PSS@MS material is simple and straightforward in comparison with other preparation methods of the 3D porous conductive sponges.

Figure 5. Relative resistance variation ((R-R0)/R0) of the PEDOT:PSS@MS pressure sensor under continuous compress and release between 30% and 10% strains for ~1000 cycles. The performance of the PEDOT:PSS@MS sponge is also evaluated after the material is washed in D.I. water and detergent solution for 30 min followed by drying. Figure S8 shows the resistances of the PEDOT:PSS@MS sponges do not change after being washed. The SEM images of the sponges after washing (Figure S9) shown that the sponges do not have any morphological changes. Pressure sensors prepared from these washed sponges also have similar


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piezoeresistive response as the as-prepared PEDOT:PSS@MS sponges (Figure S10). These results demonstrate that the PEDOT:PSS@MS based piezoresistive pressure sensors not only have good stability and reproducibility but also have excellent chemical and environmental resilience. The robust and stable properties of the PEDOT:PSS@MS conductive sponge can be attributed to the strong adhesion between PEDOT:PSS and melamine sponge during dip coating. Melamine sponge is highly porous and superhydrophilic, which is favorable for intimate wetting of the aqueous PEDOT:PSS solution during the dip coating process. To further understand the adhesion interaction between the PEDOT:PSS and the melamine sponge, contact angles and surface energies of both materials are characterized. Note that the porous melamine sponge is both hydrophilic and oleophilic and the porous structure may contribute significantly to the wettability of the material.45

To characterize the surface property of the melamine-based

polymer, a flat film of the melamine-formaldehyde resin is prepared. Table S3 shows the contact angles and surface energies of the melamine resin film and the PEDOT:PSS film. The water contact angles of the melamine resin film and the PEDOT:PSS film are 48.9 ± 2.4° and 37.1 ± 3.8°, respectively. The overall surface energy, the dispersive component (ߪ௦஽ ), and the polar component (ߪ௦௉ ) of both films are calculated by the Owens/Wendt two component theory using water and formamide as probe liquids.46 The overall surface energies of the melamine ௉ resin film and the PEDOT:PSS film are 50.05 and 67.17 mJ m-2, respectively. The σୈ ୱ and ߪ௦

components of the melamine resin film are 16.42 and 33.62 mJ m-2, respectively; and the ߪ௦஽ , and ߪ௦஽ of the PEDOT:PSS film are 5.33 and 61.84 mJ m-2, respectively. The surface energies of both materials have consideration polar component, implying strong interaction and good adhesion between the PEDOT:PSS and MS.


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The capabilities of the PEDOT:PSS@MS pressure sensors for practical applications of human activity monitoring are investigated. The PEDOT:PSS@MS pressure sensors are used to detect both gentle motion (i.e., speech recognition) and large body motions (including finger bending, elbow bending, knee bending, and walking). A pressure sensor is attached to the throat of a volunteer using a medical tape to noninvasively recognize speech sounds. When a volunteer pronounces different words (such as “sponge”, “sensor”, and “melamine”), the resistance changes are recorded. As shown in Figure 6a and Movie S1, the pressure sensor recognizes the different words with good reproducibility. The signals of the different words are significant different, which may be attributed to the different muscle movements during pronouncing different words.

These results demonstrate the pressure sensor can be used in a speech

recognition system. Finger and elbow bending are common motions for manipulation of objects using hands. The PEDOT:PSS@MS pressure sensors are attached to finger and elbow to monitor the bending motions. As shown in Figure 6b, a pressure sensor is mounted on the joint of an index finger. When the finger is bent gently, small resistance change signals are generated; when the finger is bent to a large degree, significantly large signals are collected; and when the finger is bent quickly, a fast response of the pressure sensor follows (Figure 6b & Movie S2). Figure 6c shows a pressure sensor fixed on the outside of elbow; and different signals are collected reproducibly during different degrees of elbow bending (Movie S3). These results demonstrate that the PEDOT:PSS@MS pressure sensors are capable of detecting common joint bending motions, which enables the potential application in human-machine interaction, robotic arms, and athlete training data acquisition, etc.


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The PEDOT:PSS@MS pressure sensor is also used to recognize walking speed and gait of a person because of the wide compressive strain range of the sensor. The PEDOT:PSS@MS sensors are attached to a knee and an insole. As shown in Figure 6d and Movie S4, the pressure sensor attached to the knee generates similar signal intensities when a person walks slow or fast or joggles in similar gait and stride; additionally, the signals follow closely with the cadence of the walking and jogging activities.

To recognize the detailed walking gait, three

PEDOT:PSS@MS pressure sensors are fixed at three positions of an insole using Bond-Aid, as shown in Figure 6f-6g. The signals of the three pressure sensors are collected simultaneously in response to different walking gaits (Movie S5). As we know, the plantar pressures are different at the different regions of a foot.30 When a volunteer (with the weight ~70 kg) walks normally, all three sensors generate distinct signals, whereas the signal at the arch (position “2”) is apparently smaller than the signals of the sensors attached at the heel (position “1”) and the forefoot (position “3”) (green area in Figure 6e). When the volunteer is tip-toe walking, only the sensor at position “3” (i.e., forefoot) gives significant signals, whereas the pressure sensor at position “2” (arch) has some small signals and the pressure sensor at position “1” (heel) has very weak response (blue area in Figure 6e). When the volunteer walks on heel, only sensor at position “1” (heel) has significantly large signal, whereas the other two sensors at the arch and forefoot almost have no response (yellow area in Figure 6e). The zoom-in plot of the sensor response (Figure S11) shows that the response times of the three devices during walking are about 0.1 ~ 0.5 s. These results demonstrate that the PEDOT:PSS@MS pressure sensors can recognize gaits, speed, and other variations of walking when attached to the knee and the sole of a shoe, which may be useful in physical therapy (such as gait training to help stroke patients recover.)


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Figure 6. Applications of the PEDOT:PSS@MS pressure sensor for human motion detection: a) relative resistance variations of the sensor attached on the throat induced by phonation of “sponge”, “sensor”, and “melamine”; b) relative resistance variations of the sensor attached on the index finger induced by bending; c) relative resistance variations of the sensor attached on the elbow induced by bending; d) relative resistance variations of the sensor attached on the knee joint induced by walking; e) relative resistance variations of the sensors attached to an insole induced by walking, f-g) photos showing the three sensors attached to the insole and dressed on foot for testing.

The simple fabrication processes, along with the good flexibility and sensitivity of PEDOT:PSS@MS, make it possible to integrate multiple pressure sensors on flexible substrate for tactile force mapping. As a proof-of-concept, a sensory array containing 3 pixel × 3 pixel sensor units is assembled by embedding in the pristine insulating melamine sponge with prepunched holes. As shown in Figure 7a, the 9 pressure sensors share one common electrode using copper/nickel tape at the bottom substrate, and individual electrode is made on the other


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end to address each pressure sensor. Upon applying pressure on the sensory array, the resistance change of each pixel is measured. The sensory array can map the shape of an object placed on it. As shown in Figure 7b and 7c, when a balance weight or an “AAA” battery is placed on the sensory array, the resistance changes map the specific location and pressure of the object. The sensory array demonstrated here may have potential applications in artificial electronic skin and soft robotics.

Figure 7. a) Photo of a PEDOT:PSS@MS based sensory array, containing 3 pixel × 3 pixel pressure sensor units.

b) Mapping of the relative resistance changes corresponding to the

pressure applied by a balance weight placed on the sensory array; Inset: photo of the sensory array with the balance weight placed on top. c) Mapping of the relative resistance changes corresponding to the pressure applied by an “AAA” battery placed on the sensory array; Inset: photo of the sensory array with the “AAA” battery placed on top.

4. CONCLUSION In conclusion, highly flexible, compressible, and conductive PEDOT:PSS coated melamine sponge (PEDOT:PSS@MS) is prepared by a facile approach of one-step dip coating. The conductive PEDOT:PSS@MS exhibits excellent compressibility of a compressive strain up to


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80%, and a maximum compressive stress up to 37.6 kPa at 80% strain. PEDOT:PSS@MS shows the negative piezoresistive effect (i.e., the resistance decreases upon compression) with the resistance change (∆R/R0) varied from 0 to -88.1±2.2% and gauge factor (GF) varied from 232±41% to -110±2.8%. Piezoresistive pressure sensors are fabricated using the conductive PEDOT:PSS@MS sponge; and these sensors are used to detect a variety of human motions (including speaking, joint bending, and walking) reproducibly. Additionally, multiple pressure sensors are readily assembled into a prototype 3 pixel × 3 pixel array for force mapping and tactile sensing. The excellent flexibility, compressibility, and sensitivity of PEDOT:PSS@MS based piezoresistive sensors, along with the simple preparation process, suggest that the PEDOT:PSS@MS pressure sensors may have great potential applications in the fields of personal healthcare monitoring, artificial intelligence, and electronic skin, etc.

ASSOCIATED CONTENT Supporting

Information.

The

following

files

are

available

free

of

charge.

Movies of the PEDOT:PSS@MS pressure sensors for human motion detection (file type, AVI); Photo of PEDOT:PSS@MS in various shapes; Pore size measurement of melamine sponge; SEM-EDS element analysis of PEDOT:PSS@MS; I-V curves of the pressure sensor under different strains; Morphologies (SEM) of PEDOT:PSS@MS sponge after 1000 cyclic compress/release and after treatment in DI water and detergent solution; Resistance and performance of PEDOT:PSS@MS after treatment in DI water and detergent solution; Response time of pressure sensor during monitoring human walking; Cost estimation of the materials for preparation of PEDOT:PSS@MS; Performance comparison of the PEDOT:PSS@MS pressure


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sensor with other pressure sensors in recent published works; Contact angle and surface energy of the melamine resin film and the PEDOT:PSS film (file type, PDF)

AUTHOR INFORMATION Corresponding Author *Z. Zhu [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This research was supported by the National Aeronautics and Space Administration (NASA Cooperative Agreement No.: 80NSSC18M0022), and Program of Biomedical Engineering at South Dakota School of Mines & Technology.

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