A strain sensor capable of measurements ranging from the touch of a feather to hard-hitting impacts has been developed by Marcus O’Mara and colleagues at the University of Sussex in the UK. Claimed to be the world’s most sensitive strain sensor, the device was made using a processing technique that creates networks of graphene nanosheets in a highly flexible polymer matrix. This material’s sensitivity over such an extreme range of strains could make it well suited for wide-ranging uses in areas such as healthcare and robotics.
Polydimethylsiloxane (PDMS) is a biocompatible, transparent, and durable polymer that is widely used in applications ranging from healthcare to aerospace engineering. The material’s high flexibility could also make it useful in the latest generation of strain sensors, which display varying electrical resistance when under strain due to embedded conductive nanomaterials like graphene and silver nanoparticles. However, the twisting of molecules within the material makes it difficult to disperse nanostructures evenly, and this has so far prevented PDMS from finding practical use in sensing applications.
Oil and water
Now, O’Mara and colleagues have developed a processing technique that involves a mixture of oil and water that is stabilized by solid particles, which become trapped at the interfaces between droplets to form solid structures. In this case, the researchers assembled graphene nanosheets to stabilize oil droplets containing PDMS molecules. By fine-tuning the processing conditions, they could vary the molecular structure of the resulting material to produce continuous, highly elastic films containing large quantities of conductive graphene nanosheets.
Graphene-based strain sensor can detect a feather’s touch
The resulting material displayed a robust exponential relationship between strain and electrical resistance. This made it sensitive to strains ranging from under 0.1% to over 80% — which changed the material’s resistance by a factor of over one million. This represented a significant improvement over most of today’s advanced strain sensors, in which sensitivity and range often need to be sacrificed to maintain accuracy and reliability. The PDMS-based material produced by O’Mara’s team can stretch up to 80 times higher strain, and achieve resistance changes as much as 100 times greater – the largest ever reported – than current devices.
Such a significant improvement could make the material an ideal strain sensor in a wide variety of applications, particularly in healthcare – where sensitive strain measurements are crucial in monitoring heartrate, chest motion, joint bending and patient ventilation. Elsewhere, the material could be incorporated into wearable technologies for monitoring sports performance; and may lead to new advances in “soft robots” that simulate the properties of biological systems.
The strain gauge is described in Advanced Functional Materials.