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Synlett 2021; 32(19): 1939-1942
DOI: 10.1055/a-1637-8678
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Flexible Resistance-Type Strain Sensors toward Monitoring Finger Movements

Chao Lu∗
,
Xi Chen  ∗

This work was supported by the Earth Engineering Center, Columbia University and by the Center for Advanced Materials for Energy and Environment, Columbia University.
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Abstract

Flexible strain sensors with superior flexibility and high sensitivity are critical to artificial intelligence, and it is favorable to develop highly sensitive strain sensors by simple and cost-effective methods. We have prepared carbon-nanotubes-enhanced thermal polyurethane nanocomposites with good mechanical and electrical properties for the fabrication of highly sensitive strain sensors. The nanomaterials were prepared through a simple but effective solvent-evaporation method, and cheap polyurethane was used as the main raw material. Only a small quantity of carbon nanotubes (mass content 5%) was doped into a polyurethane matrix with aim of enhancing the mechanical and electrical properties of the nanocomposite. The resulting flexible nanocomposite films show a highly sensitive resistance response under an external strain stimulus. Strain sensors based on these flexible composite films deliver excellent sensitivity and conformality under mechanical conditions, and can detect finger movements precisely at various bending angles.


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Key words

nanocomposites - flexible films - strain sensors - polyurethanes - carbon nanotubes - electrical properties

With the rapid development of artificial intelligence, strain sensors that convert mechanical stimuli into sensing signals have great potential for military applications, wearable health devices, and human–machine interactions.[1] Flexible polymer materials have been intensively explored as sensing materials for resistance-type strain sensors by virtue of their biocompatibility and good flexibility.[2] Some polymer materials prepared by various methods have been developed for use as high-performance strain sensors, such as poly(vinyl alcohol),[3] perfluorinated sulfonic acids,[4] and poly(vinylidene fluoride).[5] However, pristine polymer materials readily break down under stretching conditions because of inadequate mechanical properties, limiting their range of practical applications.[6] Moreover, pristine polymers are usually insulators with high electrical resistances and cannot display obvious resistance variations in response to external mechanical stimuli.[7] Some polymer materials suitable for the preparation of strain sensors either require synthesis by complicated methods or command high commercial prices, which greatly impedes their large-scale application.[8] Therefore, it is essential to develop robust and resistance-sensitive polymer materials through simple and cost-effective methods for the fabrication of highly sensitive resistance strain sensors.

In this work, we prepared carbon nanotubes (CNTs)-doped thermal polyurethane (TPU) nanocomposites, through a simple and cost-effective solvent-evaporation method, as good candidates for use in resistance-type strain sensors. CNTs with a low mass ratio of 5% were used as dopants in the TPU matrix, which greatly enhanced the mechanical properties of the polyurethane material because of the high modulus of the CNTs. In addition, the high electrical conductivity of the CNTs increased the resistance sensitivity of the polymer nanocomposites under an external strain stimulus, since the one-dimensional CNTs readily formed continuous and conductive pathways in the polymer matrix. As a result, the polymer nanocomposites presented stable and sensitive responses over a wide strain range from 10 to 50%, which makes them promising candidates for use in resistance-type strain sensors. The as-prepared strain sensors based on the nanocomposite films were attached to human fingers, and could detect finger movements precisely at various bending angles from 30 to 90°. Inexpensive polyurethane was used as the main raw material in the preparation of the nanocomposite films, and only a small quantity of expensive carbon nanotubes was doped into the polymer matrix. This work therefore presents a simple and cost-effective strategy for producing highly sensitive materials for strain sensors, and will accelerate the practical application of strain sensors in smart fields.

Figure [1] presents a schematic of the process for preparing the flexible TPU/CNTs nanocomposite film, including mixing the precursors and solvent evaporation. Typically, CNTs were dispersed into DMSO solvent with aid of ultrasonication in an ice–water bath. Meanwhile, the TPU was dissolved in DMSO solvent by vigorous stirring. Subsequently, the two solutions were mixed together, after stirring and ultrasonication, to form a uniformly mixed TPU/CNTs precursor. this was then was poured into a Teflon mold with heating to remove residual solvents. Finally, a flexible TPU/CNTs nanocomposite film was obtained by peeling off from the mold after sufficient drying. TPU was selected as the substrate because of its good flexibility and compatibility with CNTs. The preparation procedures do not require any sophisticated equipment or expensive raw materials. The main instrument used in the solvent evaporation is a heating stage, while the main raw material used is a cheap TPU. The expensive CNTs materials are used in only a low mass content of 5%, which does not markedly increase the cost of preparation of the nanocomposite film. Thus, it is expected that the simple and cost-effective method adopted in this study could be applied in the mass production of flexible multifunctional nanomaterials in industry.

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Figure 1 Schematic showing the preparation of the flexible TPU/CNTs films

The morphology of the CNT/TPU nanomaterials was analyzed by scanning electron microscopy (SEM) and transmission electron microscopy (TEM), and the relevant SEM and TEM images are presented in Figures [2a] and 2b. From the SEM image, it can be clearly seen that the CNTs nanomaterials have a one-dimensional morphology, and the three-dimensional close-packed structure forms continuous conductive pathways in the material. The high-resolution TEM image confirms the one-dimensional structure with a diameter of around 25 μm. The high-quality CNTs are believed to form highly conductive pathways in the TPU/CNTs nanocomposites. To confirm the presence of interfacial interactions resulted from incorporation of the CNTs dopant, FTIR spectra of TPU/CNTs nanocomposites were recorded and are shown in Figure [2c]. The peaks at 1726 cm–1 and 1702 cm–1 are ascribed to carbonyl C=O groups and to hydrogen-bonded carboxyl groups, respectively.[9] This result shows that the incorporation of CNTs dopants has a major influence on interactions in the TPU matrix. Results of a differential scanning calorimetric (DSC) study of TPU and TPU/CNTs materials are presented in Figure [2d] for the evaluation of their thermodynamic properties. The glass-transition temperatures (T g) of TPU and TPU/CNTs materials were –43 and –45 °C, respectively, indicating that incorporation of the CNTs enhances the freedom of movement of the polymer segments. The peak at about 35 °C is ascribed to the melting enthalpy of soft segments in polyurethane matrix. The melting temperatures (T m) of TPU and TPU/CNTs materials were evaluated as 122 and 127 °C, respectively, which directly reflects the crystallization properties of the two materials. The nanocomposites possess a slightly higher melting temperature resulting from interactions between functional groups of the CNTs and the TPU segments.

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Figure 2 (a) SEM image of CNTs nanomaterials. (b) TEM image of CNTs nanomaterials. (c) FTIR spectra of TPU/CNTs films. (d) DSC curves of TPU and TPU/CNTs films.

To characterize the resistance response of the flexible TPU/CNTs film, a film was fabricated into a resistance-type strain sensor through integration with metal collectors and conducting wires, as shown in Figure [3a]. When the strain sensor device is deformed under an external mechanical stimulus, the conductive CNTs networks in the polymer matrix are also deformed to another state with a change in the resistance of the TPU/CNTs film. The change in resistance of the strain sensor can be used to reflect real-time changes in the sensing signal under different mechanical conditions. A stress–strain curve for the TPU/CNTs film is shown in Figure [3b], and the tensile modulus and breaking elongation of the film were 8.83 MPa and 37.15%, respectively. These results show that the composite film possesses good flexibility and is promising for applications in strain sensing. As shown in Figure [3c], the resistance response of the strain sensor was tested under a bending strain of 10%. The strain sensor gave a stable sensing signal output after several mechanical strain cycles. The sensing signal of the sensor is defined as the ratio of the resistance change before and after mechanical deformation,[10] and the value of the sensing signal in Figure [3c] was evaluated as 17%. We also tested the sensing performances of the strain sensor under various mechanical strains from 10 to 50% and the results are shown in Figure [3d]. It is obvious that larger mechanical deformations always led to higher sensing signals, corresponding to larger variations in the resistance of the composite films. The large detection limit of strain sensors should permit the realization of real-time detection of complicated strain forms.

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Figure 3 (a) Schematic showing the structure of TPU/CNTs film-based strain sensors. (b) Stress–strain curve of a strain sensor. (c) Resistance response of a strain sensor at a bending strain of 10%. (d) Variation in the resistance response of the strain sensors as a function of the bending strain.

To demonstrate the potential of the TPU/CNTs-based strain sensor for practical applications, a sensor was attached to the surface of a human finger for in situ monitoring of its physical movement. As shown in Figure [4a], the strain sensor worked like human skin and could detect finger touching sensitively, producing real-time sensing signals. The finger touched the sensor surface several times lightly, and each time the sensor detected slight differences by outputting specific values of the sensing signals. Moreover, the sensing results shown in Figure [4b] display resistance responses of strain sensors attached to a finger bent at angles of 30°, 45°, 60°, and 90°. The sensor clearly and precisely detected the bending state of the finger, and the flexibility of the device guarantees good conformality when it is integrated with various testing surfaces. The good flexibility and high sensitivity of the strain sensor demonstrated here indicate that it has great potential for practical applications in smart fields.

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Figure 4 (a) Real-time resistance response of TPU/CNTs film-based strain sensors touched by a finger. (b) Resistance response of strain sensors attached to a finger being bent at angles of 30°, 45°, 60°, and 90°.

In summary, we have produced flexible TPU/CNTs nanocomposite films through a solution-evaporation method that is simple and cost-effective. The CNTs form continuous and conductive networks in the TPU matrix and thereby enhance its electrical conductivity, which is favorable to the resistance response under external mechanical stimuli. Furthermore, the improved mechanical and thermodynamic properties are beneficial to the flexibility and conformality of the polymer films. Resistance-type strain sensors based on TPU/CNTs nanocomposites deliver a high sensitivity and large detection limit. Moreover, the sensors function on human skin and can detect slight differences in finger touching. Additionally, when attached to fingers, the sensors are capable of in situ detection of various bending states, indicating their great potential for practical applications. The materials could also potentially be applied as solid-state electrolytes for lithium-ion batteries, because they could potentially alleviate dendrite effects. This work presents an insight into the design of highly efficient sensing materials by simple and cost-effective methods.


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Conflict of Interest

The authors declare no conflict of interest.


Corresponding Authors

Chao Lu
Earth Engineering Center, Center for Advanced Materials for Energy and Environment, Department of Earth and Environmental Engineering, Columbia University
New York, NY 10027
USA   

Xi Chen
Earth Engineering Center, Center for Advanced Materials for Energy and Environment, Department of Earth and Environmental Engineering, Columbia University
New York, NY 10027
USA   

Publication History

Received: 06 August 2021

Accepted after revision: 04 September 2021

Accepted Manuscript online:
06 September 2021

Article published online:
24 September 2021

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Zoom Image
Figure 1 Schematic showing the preparation of the flexible TPU/CNTs films
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Figure 2 (a) SEM image of CNTs nanomaterials. (b) TEM image of CNTs nanomaterials. (c) FTIR spectra of TPU/CNTs films. (d) DSC curves of TPU and TPU/CNTs films.
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Figure 3 (a) Schematic showing the structure of TPU/CNTs film-based strain sensors. (b) Stress–strain curve of a strain sensor. (c) Resistance response of a strain sensor at a bending strain of 10%. (d) Variation in the resistance response of the strain sensors as a function of the bending strain.
Zoom Image
Figure 4 (a) Real-time resistance response of TPU/CNTs film-based strain sensors touched by a finger. (b) Resistance response of strain sensors attached to a finger being bent at angles of 30°, 45°, 60°, and 90°.