Pressure sensor with the soft touch
Pressure sensor with the soft touch

Composite nanofibers of carbon nanotubes and graphene can be used to make a pressure sensor that bends the rules and is insensitive to mechanical deformation and so could be used to make a detector that could palpate soft tissues, such as those of the breast, and provide a digital readout if any putatively malignant lumps are present.

According to Takao Someya of the Department of Electrical and Electronic Engineering, at The University of Tokyo, Japan, and colleagues making a sensor for measuring small normal on curved and dynamic surfaces such as natural tissues is difficult, because pressing the sensor against the tissue often leads to it bending. [Nature: 10.1038/nnano.2015.324] "Most tissue surfaces, whether internal or external, are inherently soft and constantly in motion," the team explains. "Thus, sensors that directly touch the surface of a living body should ideally be soft so as to establish good mechanical contact between the electronic device and biological tissue." Nevertheless, the mechanical deformation gives a false reading as to the pressure being applied and the reactionary force and this has limited the applications of efforts to make flexible sensors so far.

A device that can bend but the response of which is insensitive to this bending would be a boon for many applications. For example in building human-like robotic hands that can manipulate soft or fragile objects without damaging them or in medical diagnostics where a non-subjective measure of the stiffness or hardness of a normally soft tissue is needed.

The team has devised a novel fibrous composite and simulated its behavior to show how the relative alignment of the fibers can change to accommodate bending deformation. The bending of the composite precludes inducing strain in individual fibers thus allowing pressure sensitivity that is unaffected by the deformation and so could fit the tissue or object to which it is applied. A bending radius of as small as 80 micrometers has negligible impact. In real-world tests with a fabricated sensor matrix just 2 micrometers thick, the team then successfully demonstrated large-area normal pressure monitoring with a time lag of just 20 milliseconds under various complex bending conditions, such as wrinkling and twisting on an inflated balloon being pressed by a finger.

The team explains that such sensors could be used on injection needles for monitoring, in heart sensors and to palpate and examine soft tissues for problems, such as the aforementioned breast diagnostics. The composite is also transparent, which will widen its potential applications.

David Bradley blogs at Sciencebase Science Blog and tweets @sciencebase, he is author of the bestselling science book "Deceived Wisdom".