A strain sensor is a device to measure strain on an object. It converts force, pressure and tension into a change in electrical resistance that can be measured. Progress in making compliant, low-weight, low-cost and long-lifetime strain sensors has been witnessed so far. A common type of strain sensors uses a metallic foil patterned on an insulating flexible substrate, resulted in a small strain limit of 0.5%\~5% due to its low mechanical compliance. Higher stretchability and sensitivity of strain sensors are achieved by patterning conductive nanoparticles (AgNP), nanowires (AuNW), nanotubes (CNT) or graphene on an elastomeric substrate utilizing the microstructure disconnection mechanism. Although some of them have high gauge factors, the maximum strain allowed is not enough for large motion detection.

Given above, researchers show interest in liquid metals (EGaln), conductive polymers (PEDOT:PSS) and hydrogels for making soft strain sensors. However, conductive polymers are not ideally stretchable (fraction strain \~ 5%), thereby ionic additives are usually incorporated to enhance the stretchability. Besides, conductive polymers need to cure at high temperature for use, and both conductive polymers and liquid metals are hazardous for human beings in case of leakage.

Hydrogels are odorless and non-staining swellable polymeric materials, which can be defined as a cross-linked polymeric network that can hold water within its porous structure. Some conductive hydrogels have been developed in labs, for example, the polyvinyl alcohol (PVA) gel with salt solution soaked, and the polyaniline (PANI) gel using electrodeposition, yet both needs sophisticated preparation. On the other hand, commercial conductive gels have been widely used in electrocardiography (ECG), where the electrode gel greatly affects the signals from the skin to electrode. These commercial conductive gels are biocompatible, soft, transparent and low-cost, and they are appropriate for strain sensing due to their resistive response to large strain. Meanwhile, we find another two types of viscous conductive carbon paint and carbon grease that are also electrically sensitive to high strain. But stretchable strain sensors using commercial conductive gels, carbon paint, or carbon grease have not been fully explored.

This paper proposes stretchable strain sensors using multiple viscous conductive materials as resistive component and introduces a facile fabrication process by sandwiching the conductive materials between two layers of VHB tapes. The working mechanism of this strain sensor is discussed, the conductivity of these viscous conductive materials and the strain gauge factors of achieved sensors are studied. Preliminary Characterization shows that the sensor with conductive gels, carbon paint, and carbon grease could subject to a large tensile strain of 100%, 100%, and 350%, respectively, and reveals a good gauge factor, 0.33, 1.7, and 6.8 at the corresponding maximal strain. Furthermore, strain sensors with conductive nanoparticles mixed into conductive gels are fabricated and tested to check if the gauge factor increases. With the optimized configuration of structure design and fabrication process from the tensile stretch tests, bending experiments are also investigated and show good repeatability.

Lastly, it is demonstrated that such a stretchable strain sensor with these conductive materials are capable of measuring finger bending or recording knee rotation for athletic assessment. With its simple and low-cost fabrication process and large strain detection range, the conductive materials-based strain sensors are promising for future biomedical, wearable electronics and rehabilitation applications.


Highly Stretchable Resistive Strain Sensors Using Multiple Viscous Conductive Materials

Division / Sponsors: 
Aerospace Division

Paper Proceedings: https://asmedigitalcollection.asme.org/SMASIS/SMASIS2020/volume/84027

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