Dedicated Wearable Sensitive Strain Sensor, based on Carbon Nanotubes, for Monitoring the Rat Respiration Rate

: This paper presents a highly sensitive and novel wearable strain sensor using one-dimen-17 sional material for monitoring the respiration rate of an anesthetized rat. The dedicated sensitive sensor, based on carbon nanotubes mixed with poly(3,4-ethylenedioxythiophene) polystyrene sul-19 fonate, was attached above the rat chest. A Wheatstone bridge electrical circuit, associated with a 20 multifunction portable device, was connected to the strain sensor. The change of the strain sensor’s 21 resistance value, induced by the mechanical deformability during the rat respiration, was detected 22 and transformed into a voltage signal. The respiration information could be thus extracted and an-23 alyzed.


28
Cardiovascular disease is a group of disorders of the heart and blood vessels, includ-29 ing coronary heart disease, cerebrovascular disease, and rheumatic heart disease, etc.
[1] 30 It is one of the most common causes of death in the world. In 2013, cardiovascular disease 31 claimed a total of 17.3 million lives (31% of total deaths), which is a significant increase 32 from 12.3 million in 1990 (25.8% of total deaths). Since the 1970s, death rates from cardio-33 vascular diseases increases in developing countries regardless of age group, while it de-34 clines in most developed countries [2]. 35 There is an increasing demand for a reliable heart monitoring system to capture in-36 termittent abnormalities and detect serious heart behaviors, leading to sudden death in 37 extreme cases. In addition to cardiac monitoring, there is a growing need to capture the 38 respiratory function in several contexts such as sleep apneas. For example, "Internet-of- 39 Medical-Things (IoMT)" are now considered to be a good strategy for monitoring the ab-40 normalities of breathing and cardiac rate. A wearable monitoring strain sensor detects the 41 accidents and sends immediately the information to the medical staff [3]. 42 Therefore, there is a need to develop a highly sensitive, durable, and biocompatible 43 strain sensor. In this paper, a new one-dimensional nanomaterial based strain sensor will 44 be introduced for the respiratory rate monitoring, followed by the fabrication process and 45  the experimental platform. Then, we perform stretch tests on the strain sensor for the ver-1 ification of the sensor's durability. Finally, the results of the respiration monitoring of the 2 anesthetized rat will be presented. Compared to other strain sensors like capacitive, piezoelectric and triboelectric sen-5 sor, piezoresistive sensors have gained interest as they provide high sensitivity with sim-6 ple device design and readout circuits. The working principle of the piezoresistive sensor 7 is based on the strain effect: when the conductor or semiconductor material is mechani-8 cally deformed under the action of external force, its electrical properties change accord-9 ingly. The sensitivity of the strain sensor could be characterized by the gauge factor (GF) 10 [4]: where r0 represents the initial resistance of the strain sensor, Δr represents the difference 12 between its real-time resistance under stretching r and the original value r0 (Δr = r -r0). l0 13 represents the initial length of the strain sensor, Δl represents the change in the length (Δl 14 = l -l0, l the real-time length). 15 In general, metallic materials have a significantly low GF (about 2), indicating a low 16 sensitivity [5]. In addition, the appearance of the microcracking of the metallic thin-films 17 affects its stretch capability. Non-linear electromechanical behavior is the discrete defor-18 mation behavior of the conductive networks at different strain regimes. It impacts the long 19 term sensitivity and stretchability of the strain. 20 One-dimensional material was thus chosen to solve the problem of the metallic pie-21 zoresistive strain sensor. In this paper, carbon nanotubes (CNTs) [6] were mixed with 22 poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) [7] to obtain the 23 filler networks more stable to avoid the resistance value shift caused by device aging and 24 the repetitive tests. CNT network formed a thin film less compact than the metallic layer, 25 whereas the conductive polymer PEDOT:PSS filled the space in the network to ensure the 26 tolerance of the stretch. 27 To encapsulate the wearable strain sensor, instead of the traditional polymers like 28 Polydimethylsiloxane (PDMS), Dragon skin (DS FX-Pro) was considered for its good elas-29 ticity, its robustness as well as its biocompatibility.

31
The conventional method of the strain sensor fabrication was realized by the deposi-32 tion of gold evaporation on Polydimethylsiloxane (PDMS), and then covered all patterned 33 surface with another PDMS layer. The obtained GF was very high (GF from 2.5 × 10 6 to 10 34 × 10 6 ), however, two of the major limitations were: (i) the response was linear up to 0.3% 35 strain only followed by the sudden change in the resistance value, and (ii) poor recovera-36 bility. The nominal resistance of the device significantly changed after releasing the ap-37 plied strain. Therefore, the nanocomposite material like carbon nanotubes was chosen to 38 have a GF high enough (from 3 to 6) to sense subtle movements but more robust and show 39 linear change in resistance up to strain of 40%. Moreover, instead of linear shape, such 1D 40 material was encapsulated in the polymer with 'U' shape, which was very useful to in-41 crease the sensitivity of the strain sensor. 3.1. Fabrication 43 To fabricate the strain sensor by the unconventional method, here is the process to 44 follow (See Figure 1): 1. In the first step, dragon skin (DS) chip containing macro channels was realized by 1 using a standard molding process. The Kapton tape was cut with a blade to define the 2 dimensions of the macro channel.

19
The wearable strain sensor was first tested using a linear stage controlled by E-861 20 PiezoWalk ® NEXACT ® Controller (See Figure 2a). Thanks to the monitoring with Lab-21 VIEW controlled by Keithley 2400 source meter, the strain sensor was stretched between 22 0.01 mm/s to 10 mm/s. When there was no stretch force, the strain sensor's resistance was 23 equal to other three resistances with fixed value (22 kΩ). While the strain sensor was 24 stretching, the resistance value change due to the mechanical deformability was detected 25 by the Wheatstone bridge and the induced current signal was transformed into voltage 26 signal via an operational amplifier (MCP6401UT) with approximately a gain of two (See 27 Figure 2b). This allows the verification of the viability and durability of the CNTs/PE-28 DOT:PSS based strain sensor. The transformed current signal (or output voltage signal) 29 was monitored by an oscilloscope (DSOX2024A-Keysight Technologies) (See Figure 2c) or 30 a multifunction hardware (Analog Discovery 2) and recorded for data processing. Thanks 31 to Analog Discovery 2, the fast Fourier transform was obtained at the same time under 32 the output voltage signal curve (See Figure 2d).

6
With the application of a direct current (DC) source signal (1.8 V) to the dedicated 7 Wheatstone bridge circuit, the evolution of the output signal of the amplifier connected 8 by the Wheatstone bridge circuit is shown in Figure 3. When the stretch length of the strain 9 sensor is equal to 0.5 mm, the output voltage signal changed from 0.04 V to 0.14 V. As the 10 applied stretch force was cyclical, the signal curve was also repetitive. There are some 11 peaks, which were observed at the beginning and the end of the square signal. Such "in-12 stantaneous stretch-relax" phenomenon might be related to the elasticity of the strain sen-13 sor.  After the validation, such CNTs/PEDOT:PSS based strain sensor was reversibly at-18 tached above the rat chest for its respiration monitoring during anesthesia. Isoflurane 19 (2.5%) was used to make the rat sleep during all the experiment [8]. The regular respira-20 tion curve for 60 seconds is shown in Figure 4a. The output voltage evolution is between 21 0.6 and 0.7 V. For 1 min, the rat respiration rate during the rest equals to 62 times/min, 22 which corresponds with the theory value [9]. The electrical monitoring result is very reli-23 able and repetitive. The experiments were carried out by using three different rats, and 24 stable electrical sensing curves were obtained (Figure 4b-d). The rat's state was also mon-1 itored by the cardiogram, which detected the cardiac frequency during the rat anesthesia 2 (See Figure 5.).  Figure 5. Cardiogram for the rat respiration rate monitoring during its anesthesia. 9 One major drawback we found with this type of sensor is the drifting of the nominal 10 resistance that occurs after intensive stretching of the sensor. We experiment the aging of 11 the sensor by measuring its nominal resistance over several weeks and by stretching at 12 different times. We found that the nominal resistance increases slightly with time but 13 mainly after testing experiences, as shown in Figure 6. The mechanism behind this shift 14 in resistance is not well known and we think that permanent changes in the CNTs network 15 can occur with time but also when the sensor is used for large (>10%) elongations. Carbon 16 nanotubes may move within the polymer composite and contacts between CNTs may 17 break during large elongations. 1 Figure 6. Resistance value evolution of the strain sensor before and after test. 2 Finally, in order to build the sensor with common shape and functionalities, a fila-3 ment mold printed by 3D printing was realized to create the repetitive polymer substrate 4 to insert the CNTs. 5 5. Conclusions 6 We developed a new wearable strain sensor, based on one dimensional material 7 (CNTs) mixed with conductive polymer (PEDOT:PSS) and report that the sensitivity and 8 the durability of the wearable strain sensor was improved compared to other CNT based 9 strain sensors. The respiration of anesthetized rat was successful monitored using 10 CNTs/PEDOT:PSS based strain sensor. 11 In addition to many advantages such as low cost fabrication, simple fabrication pro-12 cess, ease of testing as well as the biocompatibility, stretchable strain sensors will play a 13 crucial role to the monitoring patient with cardiovascular disease as well as their breath-14 ing and heart rate. Such sensors will enable the new technological drive called "Internet-15 of-Medical-Things (IoMT)", which links wearable devices/sensors into a communication 16 network for real-time or periodic patient-doctor communications. The patient information 17 could be immediately recorded and transformed to healthcare service providers and that 18 might reduce the death rate of cardiovascular disease.