Development of strain/stress sensors for human-motion detection and implantable devices for human health monitoring assumed significance in recent years. Multi-functionality, poor interface stability and flexibility of such sensors still remain as a major challenge. Researchers at State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, and Engineering Research Center of Advanced Glasses Manufacturing Technology, College of Materials Science and Engineering, Donghua University, China have reported fabrication of a mechanically strong and flexible vessel-like sensor that is capable of continuously monitoring applied temperature, strain, and the frequency, pressure, and temperature of the in-vessel flow.
Braided cotton yarn served as the base and it was coated with single-walled carbon nanotubes (SWCNTs) by dip-coating (cotton@SWNTs) to function as a current collector/electrode. ZnO nanoseeds were nucleated on the surface of cotton@SWNTs to promote the growth of ZnO nanorods, which could serve as temperature sensing arrays. A polyvinylidene fluoride (PVDF) film was formed over the cotton@SWCNTs/ZnO through dip-coating. A PVDF fibrous membrane was further prepared on the PVDF surface through electrospinning. The PVDF fibrous membrane was immersed in ethyl acetate for 12 h to obtain a compact film. The PVDF membrane could serve as piezoelectric nanofibers. Finally, silver paste was uniformly coated on the surface of the PVDF membrane. The various stages involved in the fabrication are shown in Fig. 1.
Fig. 1 various stages involved in the fabrication of flexible sensor
The morphological features of cotton@SWNTs indicate a uniform distribution of SWCNTs on the surface of cotton yarn (Fig. 2(a)). Similarly, the ZnO nanorods arrays@PVDF are also uniformly developed on the surface of cotton@SWCNTs (Figs. 2(b) and 2(c)).
Fig. 2 Morphological features of (a) SWCNTs coated cotton yarn; (b, c) cotton@SWCNTs/ZnO@PVDF; (c) magnified image of ZnO nanorods
Structural representation of the tubular sensor used for monitoring temperature, motions and liquid flow is depicted in Fig 3(a). The thermosensitivity of ZnO grown between the fibers of SWCNTs helps monitoring the temperature variation, which is detected through resistance measurement of the tubular inner electrode (Fig. 3(a)). The I-V curves measured using the functionalized inner electrode of the sensor at different temperatures indicate the change in resistance with variation in temperature (Fig. 3(b)).
Fig. 3 (a) Schematic representation of the sensor used for monitoring temperature, motions and liquid flow; (b) Linear I-V curves of the sensor measured under different fluid temperature.
The sensor is also capable of monitoring human body motions by sensing both bending and pressing. When a force is applied on the surface of the PVDF nanofibers, they get polarized. The equivalent bound charges accumulated on the surface of PVDF nanofibers are measured as electrical signals with the help of bonding wires between the two electrodes – cotton@SWCNTs/ZnO layer and the silver paste layer. The output voltage is increased from 0.038V to 0.1V with an increase in bending angle from 1.4 rad to 2.1 rad (Figs. 4(a) and 4(c)). Similarly, an increase in output voltage is observed with an increase in frequency during compression (Figs. 4(b) and 4(d)). These inferences suggest that human motions such as bending and compression can be directly monitored without any external power supply.
Fig. 4 (a, b) Demonstration of the bending and pressing motion; (c) output voltage generated by the sensor under different bending angles; and (d) output voltage generated by the sensor under compression at different frequencies.
The ability of the sensor to monitor blood temperature and blood pressure is also evaluated. Fig. 5(a) shows the experimental set-up used to monitor liquid flow. The temperature of the test fluid (deionized water to mimic blood flow) that flows into the sensor is accurately controlled using microfluidic channels heated by sample heater. The variation in the temperature of the fluid under flow from 35 to 40 °C is clearly reflected by the change in resistance measured using the sensor (Fig. 5(b)). The sensor could also detect the variation in the input pressure and frequency of the pulsed flow. The increase in the output voltage with an increase in applied pressure indicates the ability of the sensor to measure the variation in pressure under flow conditions (Fig. 5(c)). Similarly, the variation in the output current is sufficient enough to qualify the sensor for its ability to measure the pulse frequency of fluid (Fig. 5(d)).
Fig. 5 (a) Experimental set-up used to monitor liquid flow using the sensor; (b) I-V curves of the sensor measured under a narrow fluid temperature range; (c) output voltage generated by the sensor under different fluid input pressure; and (d) output current generated by the sensor under different pulse frequencies.
The cotton@SWCNTs/ZnO@PVDF sensor is flexible and durable. The output signal of the sensor remains steady during 1920 circles of the bending circle test. The sensor facilities detection and continuous monitoring of the applied temperature, strain, and the frequency, pressure, and temperature of pulsed fluids. It can be used for implantable physical sensing applications and monitoring of human health.
T.S.N. Sankara Narayanan
For more information, the reader may kindly refer: Wei Zhang et al., A strong and flexible electronic vessel for real-time monitoring temperature, motions and flow, Nanoscale, 2017, DOI: 10.1039/C7NR05575G
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