Piezoresistive Elastomer Composites Used for Pressure Sensing

Pressure sensors with capability to detect small physical movements and mechanical deformations have been widely used in wearable and medical applications. However, devices that are commercially available currently require complex designs and fabrication and present only a limited force-range sensitivity. To simplify the design, a thermoplastic polyurethane (TPU)/carbon nanotubes (CNTs) composite film has been developed using a melt extrusion technique followed by compression molding. Pressure sensors were made from these films, whose piezoresistive response has been analyzed as a function of the concentrations of CNTs, around the percolation threshold. The changes in the voltage of the device with applied pressure were continuously measured using a voltage divider system coupled with an electromechanical test machine that dynamically loaded the sensors under compression. The voltage divider system was tuned to obtain the best sensitivity and signal/noise (S /N) ratio for each device tested. The results showed that sensors containing a target of 2.5 wt% CNTs had market leading sensitivity and repeatability during long-term stability testing and showed high durability during underwater testing, indicating that such devices can be used as a promising robust pressure-sensitive sensor in wearable devices.

There are already pressure sensors available in the market, which exhibit good performance in many applications [45], [46]. However, their complex design, multilayered structure, often including several printing steps, limited force-range sensitivity, and finally poor environmental stability, restricts their utilization in many potential applications.
In this work, a pressure sensor with a simple structure was fabricated based on TPU as a flexible matrix and using CNTs as the conductive network. The conductive composite films were produced through the melt extrusion technique using a twin screw compounder, followed by hot pressing into a piezoresistive film layer from which the sensors were cut. The electrical properties of the constructed sensors were analyzed using a voltage divider system, which adopts an in-house developed LabVIEW program. A specified load protocol that cyclically loaded the sensor in compression was applied using an Instron ElectroPuls universal test machine, and the force sensitivity, repeatability, long-term stability, and durability of the sensors were all characterized both in air and when exposed to water. Then, these newly developed sensors were compared to a leading commercial sensor to evaluate their sensing performance, load range sensitivity, and long-term durability. The results highlight that the volume fraction of the CNTs in the composite and the selection of suitable reference resistors in the voltage divider system were both essential elements when optimizing the piezoresistivity response.
The simple and scalable fabrication techniques that were used to develop a durable sensor with high sensitivity at different force ranges and excellent repeatability could bring a great potential for these sensors to be used in wearable sensing devices.

A. Materials
TPU, Estane 1 58437, an aromatic polyester-based TPU with shore A hardness of 85 ± 3 and density of 1.19 g/cm 3 , was purchased from Lubrizol. Multiwalled CNTs, NC7000 2 , with an average length of 1.5 µm and an average diameter of 9.5 nm were purchased from Nanocyl S.A.

B. Polymer Compounding
The samples in this work were prepared from a masterbatch of TPU containing a nominal CNTs concentration of 5 wt%. The masterbatch was prepared using a Collin ZK25 twinscrew compounder with a screw length-to-diameter ratio of 42, a screw speed of 200 r/min and a temperature profile from zone 1 to zone 8 (die) of 190/240/200/200/200/200/ 200/180 • C. The main feeder of the ZK25 compounder was used to supply TPU at a speed of 1.9 kg/h. The TPU pellets were predried at 80 • C overnight before compounding. The side feeding system was used to supply the as-received CNTs into zone 3 of the compounder at a speed of 0.1 kg/h. The extrudate was cooled down in a water bath and pelletized.
The pellets of the masterbatch were then dried at 80 • C overnight before being blended to reduce the CNTs volume 1 Registered trademark. 2 Trademarked.  fraction with predried, neat TPU and obtain final sample materials with nominal CNTs concentrations of 2.1%, 2.3%, and 2.5%. The blending was done using an Xplore MC15HT micro-compounder operating in the recirculation mode for 3 min in an argon atmosphere with a screw speed of 200 r/min and a temperature of 200 • C. The extrudates were cooled down in ambient air and pelletized.

C. Film Preparation and Sensor Construction
TPU/CNTs blend films with different CNTs proportions were prepared using a hot press machine (Collin E300). The temperature was raised to 185 • C and the compounds were kept at this temperature for 3 min without pressure; then, 100-bar pressure was applied for 30 s, followed by cooling under the same pressure. An aluminum mold of size 300 × 300 mm was used to produce films of size 100 × 100 mm. To construct the sensors, films were cut from the center into 20 × 20 mm pieces with an average of 200 µm thickness and connected to the external wire electrodes using silver paint. The schematic of different steps in sensor fabrication is shown in Fig. 1.

D. Commercial Sensor Structure
An example of a simplified commercial pressure sensor structure is shown in Fig. 2. A force-sensitive resistor (FSR) ink is printed through screen printing technique from a liquid Authorized licensed use limited to the terms of the applicable license agreement with IEEE. Restrictions apply. and cured to make a solid layer. An adhesive spacer is inserted between the FSR ink and a layer of interdigitated electrodes. The small free volume provided by the spacer allows for air to be vented in and out and reaches a pressure equilibrium with the external atmosphere. In this way, the sensor promotes a fast and complete recovery to its initial unloaded state, after each loading cycle through air ventilation. The spacer can also allow for a higher sensitivity at low forces due to the increasing electrical contact between the FSR ink and the interdigitated electrodes as a function of the applied force. A dielectric layer is printed on top of the silver electrode to avoid interference and protect against corrosion. Finally, two outer protective layers, typically made of polyethylene terephthalate (PET), are added to insulate the sensor from the environment. Due to this complex structure that involves several printing steps, which produces a limited force-range sensitivity, the use of these FSR sensors is restricted to a few limited applications. For example, if we consider the case of monitoring human walking and running activities, wearable FSR sensors are required to sense a force within the range of 0-100 N. However, existing commercial sensors do not perform well over this force range and are affected by corrosion at the interdigitated ends and are subject to wearing by the relative sliding between the FSR ink and electrode layers due to the free volume produced by the spacer.

E. Characterization Methods: Thermogravimetric Analysis (TGA) and Scanning Electron Microscopy (SEM)
TGA was used to confirm the CNTs concentrations in the polymer matrix using the TA/TGA5500 instrument. The TPU/CNTs blend pellets were heated up from room temperature to 600 • C at a heating rate of 20 • C/min under an inert nitrogen atmosphere. The tests were repeated three times for each sample. The actual filler content was estimated from the residual weight percentage at 600 • C, and the results are shown in the Supporting Information (see Fig. S1).
SEM was utilized to examine the CNT's dispersion in the TPU matrix. SEM images from cross section were obtained using field-emission scanning electron microscope (FEI Inspect F), OXFORD INSTRUMENTS, with 20-kV acceleration voltage. For cross section SEM imaging, the polymeric films were cryogenically fractured in liquid nitrogen. Before imaging, the samples were sputter-coated with about a 3-nm gold layer on the fracture surface, to reduce charging effects. To prepare the CNTs sample for imaging, a low concentration of about 10 mg/mL CNTs was dispersed in ethanol, sonicated for 3 min, and deposited on a glass slide. After evaporation of ethanol, CNTs were transferred onto a carbon tape and SEM imaging was performed under the same conditions as above.

F. Data Acquisition System
A fundamental circuit widely adopted in electronics is a voltage divider, which produces an output response based on a proportion of its input voltage [5], [47]. In this system, Schematic of the sensing measurement setup and data acquisition.
a simple and constant known resistor R ref and the sensor are connected in series, as shown in Fig. 3, and attached to a power supply (Rapid dc POWER SUPPLY HY3005D) with a certain input voltage V in (3.5 V was used throughout this work). As the voltage is distributed between the two resistors, one with a known reference resistance and the sensor with an unknown resistance, the output response V out would be a function of the input voltage. As the electrical resistance R sensor of the piezoresistive material changes as a function of the applied force F, the measured output voltage V out of the voltage divider system is also a function of F In this work, an Instron ElectroPuls E1000 machine was used to apply different loading profiles on the sensors while monitoring V out of a voltage divider system using a NATIONAL INSTRUMENTS USB (NI USB-6009) together with the Lab-VIEW program. A 250-N load cell and a half sphere-shaped soft test indenter made of rubber with 15 mm diameter attached to the actuator were used. Fig. 4(a) shows the percolation threshold trend for CNTs inside the TPU matrix. We estimated the electrical percolation threshold by fitting the conductance values of a series of TPU + x% CNTs composite materials when a very small compressive force of 0.00063 N was applied to ensure a good contact between the materials and the electrode. We could not measure any conductance for materials containing less than 1.5 wt% CNTs. Since the samples had the same thickness and the electrode was the same for every sensor, we did not convert the conductance values into conductivities. We fit the conductance (G) of the sensors with the following conductivity model: G= G 0 (w− w c )n, where G 0 is the conductivity of the composite material when the CNTs concentration approaches 100%, w is the CNTs weight concentration, w c is the percolation threshold, and n is the percolation exponent. We found that the percolation threshold was close to 2 wt% and the sensors with CNTs loading close to the electrical percolation threshold were investigated in this work. In this way, any small compressive force would have a dramatic impact on the number of conductive paths, which should allow for very high sensitivities.

A. TPU/CNTs Compound Characterization
SEM imaging was used to investigate the distribution of CNTs in the polymer matrix on a cryogenically fractured cross-sectional surface and the results are shown in Fig. 4(d).
To have a better understanding of the morphologies, the SEM image of the pure CNTs dispersed in ethanol is shown in Fig. 4(b) and a freeze-fractured unfilled TPU film is shown in Fig. 4(c). In general, strong van der Waals interactions among nanofillers, large surface area, and small dimensions are the main parameters that cause the nanoparticles' aggregations [48]. The results showed that CNTs are relatively well distributed inside the polymer matrix for all the samples, due to the energetic mixing during melt extrusion, which allowed CNTs to become well dispersed inside the molten TPU matrix.

B. Output Response Dependence Upon the Reference Resistor
It is a requirement to have the highest signal intensity over a specified force range of 0 < F < F MAX . Therefore, a suitable reference resistor with R ref > R sensor (F MAX ) should be chosen so that V out (F MAX ) approaches the applied voltage V in [see (1)] when the maximum force is applied. On the other hand, V out is required to increase as linearly as possible with F to produce increased sensitivity, which is possible only when R sensor (F) > R ref . These two opposite requirements imply the need to find a good compromise in the value of R ref used so that V out can approach V in at F MAX without reaching a plateau at lower forces. To investigate the best reference resistor for our sensors when subjected to a compressive force range of 0-100 N, 50 loading and unloading cycles were performed at 4 Hz using a wide range of R ref values from 500 to 30 k . Fig. 5 shows the relation between the applied force range and the sensor output response. For simplicity, only one loading-unloading cycle is shown in Fig. 5 for each reference resistor. (The output response over 50 cycles is shown in the Supporting Information, shown in Fig. S2.) As expected, the intensity of the output response of each sensor is increased when using higher R ref values. However, V out could reach a plateau well before the maximum force was applied if R ref was too high. Instead, when R ref was small, V out appeared to increase more linearly with the force but could reach only small values within the applied force range. From a qualitative analysis of Fig. 5, the best R ref values for TPU/2.1% CNTs, TPU/2.3% CNTs, and TPU/2.5% CNTs sensors appear to be 20, 15, and 10 k , respectively. The best R ref for the commercial sensor appears to be 1 k .

C. Sensing Performance During a Cyclic Pressure Loading Analysis
To evaluate the potential of the fabricated compounds to be used as a piezoresistive pressure sensor, the sensors were subjected to cyclic pressure loading and unloading tests. All measurements were done for 200 cycles at a frequency of 4 Hz and a constant maximum load of 100 N using the optimized R ref for each sensor. Fig. 6 shows the applied load protocol together with the output response in the form of voltage change from the reference resistor in the voltage divider circuit obtained using the in-house developed LabVIEW code.
The output response V out increased with the applied pressure due to the change in the CNTs percolation network of the piezoresistive composites that became better interconnected with more conductive paths [4]. Moreover, as expected from (1), V out could reach higher values when the sensors contained higher CNTs loadings because the corresponding sensor resistances decreased.
In addition, the enlarged images indicate very stable responses in the maximum recorded voltages throughout the tests. This reflects the repeatability in the output response of the sensors under cyclic loading.
An important observation is the size of the hysteresis loop in each loading-unloading cycle, where the smallest hysteresis is Output response of 50 loading and unloading cycles on TPU/2.1% CNTs, TPU/2.3% CNTs, and TPU/2.5% CNTs devices and the commercial sensor. (To calculate the hysteresis error, two points with letters a and b at loading and unloading cycles are selected at the midpoint of the graph.) desirable [2]. This is because a specific output voltage would represent a narrower range of applied force. The electrical hysteresis reflects the normal mechanical hysteresis behavior of the polymer and the filler-polymer interactions [30], [48], [49], [50], [51], [52]. Stronger interfacial interactions between CNTs and elastic polymer networks would possibly produce lower mechanical hysteresis and perhaps increased reversibility in the sensors [53], [54], [55], [56]. In addition, the relaxation behavior of the polymer matrix could also produce a different loading and unloading response, which will contribute to the mechanical hysteresis loop [4].
To examine more closely the hysteresis and recoverability of the sensors, Fig. 7 shows the output voltage of 50 compression loading and unloading cycles when the sensors were connected to their optimum reference resistors as mentioned previously. The hysteresis error of each individual sensor at the midpoint of the graph (at 50 N) was calculated by using (2), where a and b are the output voltage in loading and unloading cycles, respectively The commercial sensor shows a small hysteresis of about 1.88% and has good stability after each loading-unloading cycle. Our fabricated sensors, TPU/2.1% CNTs, TPU/2.3% CNTs, and TPU/2.5% CNTs, instead, have higher hysteresis of 6.22%, 9.26%, and 4.47%, respectively. These sensors also show a signal that is less reproducible after each loading-unloading cycle.

D. Sensing Measurement: Force Sensitivity Characterization
To understand the force range sensitivity and the best signalto-noise (S/N ) ratio of the sensors, a loading protocol was developed in which the load was increased from 2 to 70 N gradually [see Fig. 8(a)]. First, all the sensors were subjected to a conditioning cycle, according to which a 70-N load was applied four times with 15-N/s rate and kept for 5 s on the sensors, followed by a 3-s relaxation time in between. Afterward, each sensor was tested under the compressive loadings of 2, 5, 10, 20, 40, 50, and 70 N, which were applied for 8 s prior to being released for 17 s. Fig. 8(b) shows the output voltage of the different sensors when connected to their corresponding best reference resistors as determined previously to have their output signals maximized and their sensitivity optimized. For simplicity, the responses of the sensors during the conditioning cycle are not shown in Fig. 8(b).
The sensors show different sensitivities over different force ranges. For example, TPU/2.1% CNTs sensor does not show any significant sensitivity for small loads below 10 N, and in contrast, the commercial sensor shows a poor sensitivity for higher loads, between 20 and 70 N. Overall, the output response for TPU/2.3% CNTs and TPU/2.5% CNTs sensors cover a wider force range (between 5 and 70 N) and show better sensitivity at each load value.

E. Long-Term Stability Testing
Long-term stability tests were performed up to 50 000 cycles, to assess the performance of the sensors in scenarios relevant to applications as wearable devices and in particular the insoles of smart shoes. A test protocol that  would simulate a series of five consecutive walking-andrunning sequences was chosen (see Fig. 9). In the walking phase of each sequence, a 40-N maximum load was applied at a 2-Hz frequency over 10 000 cycles (about 1 h and 20 min). In the running phase, a 100-N maximum load was applied at a 4-Hz frequency over 200 cycles (50 s). The reproducibility of the voltage output during the five running phases is used as a benchmark to assess the quality of the sensors. As a reference, we performed a preliminary running phase before the long-term tests. Fig. 8 shows the output voltage from our sensors and the commercial device during the preliminary and the subsequent five running phases (to have a better view of the voltage profiles, only eight cycles are shown here). Fig. 10 shows the stability of the maximum output voltage (Max V out ) during the running phases of the long-term tests.
Ideally, Max V out should remain constant over time to allow for a reliable conversion of the signal into force. This happens with the commercial sensor as well as our sensor containing 2.5% CNTs. In fact, Max V out of these two sensors decreased only about 2% and 6% over time. The other two sensors containing 2.1% and 2.3% CNTs show lower stability as Max V out decreased by about 35% and 39%, respectively. The viscoelastic behavior of TPU could be the main reason for the decreasing output response during this stability test. This effect is related to the relaxation state of the polymer matrix, which means that the polymer matrix does not fully recover its initial state [4], which might be improved by crosslinking the TPU. Better filler-polymer interactions might be another parameter that would introduce a more stable response in TPU/2.5% CNTs sensor. Certainly, the improved performance of the TPU/2.5% CNTs over the TPU/2.1% CNTs and TPU/2.3% CNTs comes from the higher CNTs loading and a resulting better interconnected conductive filler network. The application of repetitive forces over time may break down the CNTs network inside the polymer matrix, which corresponds to increase R sensor , hence decreased Max V out . The smaller the CNTs concentration, the more likely the conductive network deteriorates and the higher R sensor becomes. At the end, the commercial sensor shows a stable response without any significant change over time, which is a result of the much more complex sensor design.

F. Durability Testing Under Water Ingress
To further evaluate the suitability for wearable applications, the behavior of the sensors in a wet environment where water could be entered into the sensing area over time was investigated. In this section, only the TPU/2.5% CNTs sensor was compared with the commercial sensor, as it appeared to be the most promising of the sensors tested so far. A protocol of 10 000 loading-unloading cycles at 4 Hz with 100-N maximum force was performed with the sensors being continuously wet by water, which was supplied via a two-channel Ismatec pump at a flow rate of 1.8 mL/min (the schematic of the testing setup is shown in the Supporting Information, Fig. S4). In service, the commercial sensor has an air inlet which we simulated by creating a very small hole with a needle near the sensing area, to allow air and water to pass in over time. The results of this experiment are shown in Fig. 11(a) and (b). The TPU/2.5% CNTs sensor has a much better durability in wet environments compared to the commercial sensor. The baseline of the commercial sensor gradually approaches 1.8 V after about 5000 cycles, and the sensor loses all its force sensitivity. This probably results from water entering the small free volume present inside the structure as discussed in the introduction and affecting the electrodes, which represents a challenge for these types of applications, where the commercial sensor is exposed to outdoor environments. In contrast, our TPU/2.5% CNTs sensor, which was fabricated with a much simpler and cheaper design without any free volume, did not suffer from this problem. Despite displaying some modest variations in the output voltage, the sensing response was much more stable compared to the commercial sensor.

IV. CONCLUSION
In this study, a pressure sensor was fabricated by compounding CNTs into a TPU matrix via melt extrusion. A voltage divider system coupled with an ElectroPuls machine was used to apply a compressive load on the sensor and the output voltage of a reference resistor was monitored continuously using an in-house designed LabVIEW program. The effect of various CNTs loadings and the use of different reference resistors in the voltage divider system were studied to optimize the design for maximum force sensitivity and the lowest S/N ratio. To provide a comparison, our fabricated sensors were compared with a commercial sensor in terms of force-range sensitivity, long-term stability, and environmental stability underwater immersion.
The results showed that fabricated sensors have a better sensitivity over a wider force range (5-70 N) compared to the commercial sensor, which showed poor sensitivity at higher loads (20-70 N). On the other hand, the commercial sensor showed a smaller hysteresis loop of 1.88% during loading-unloading cycles, which reflects its better sensitivity compared to the fabricated sensors, TPU/2.1% CNTs, TPU/2.3% CNTs, and TPU/2.5% CNTs, which showed 6.22%, 9.26%, and 4.47%, respectively. However, the TPU/2.5% CNTs sensor showed much better durability when subjected to a water immersion test compared to the commercial sensor. It is worth noting that the TPU/2.5% CNTs sensor displays a better stability during long-term test with respect to the other fabricated sensors, indicating that such device can be used as a promising pressure sensor in wearable devices. However, the functionality can be further improved for practical applications, potentially through crosslinking, to decrease the negative impact of viscoelastic behavior of the polymer, which can influence the hysteresis of the sensor.