Fig. 1. Cross-sectional view of the nanomesh pressure sensor. The sensor consists of (1) polyurethane nanomesh-embedded passivation layer; (2) top Au nanomesh electrode layer; (3) parylene/polyurethane nanomesh intermediate layer; and (4) bottom Au nanomesh electrode layer.
Fig. 2. The polyurethane and gold sensor can resist shear forces and rubbing.Futuristic prosthetic hands, human-machine interactions, and the restoration of hand function all require super-sensitive pressure sensors to reveal and reproduce our sense of touch. Various soft and flexible thin-film pressure sensors have been reported but it remains a challenge to make a sensor sufficiently delicate to avoid any interference with the sensitivity of the fingertip. Now researchers from the University of Tokyo led by Takao Someya and collaborators at the Technical University of Munich have come up with a nanomesh pressure sensor that can monitor finger pressure without any effect on sensation [Lee et al., Science 370 (2020) 966–970, https://doi.org/10.1126/science.abc9735].
The fingertip is so sensitive that even a very thin layer of material can interfere with and degrade our natural sense of touch, affecting the sensory information relayed from the finger to the brain. Consequently, it is very difficult to avoid this type of sensory interference in artificial systems aimed at recreating a natural sense of touch in robotic systems or prosthetic devices.
“Our fingertips are extremely sensitive, so sensitive, in fact, that a super thin plastic foil just a few millionths of a meter thick is enough to affect sensations,” says Sunghoon Lee of the University of Tokyo and first author of the study. “A wearable sensor for your fingers has to be extremely thin. But this makes it very fragile and susceptible to damage from rubbing or repeated physical actions.”
To overcome these limitations, Lee and his team developed an ultrathin nanomesh sensor that can be attached directly to the skin. The sensor consists of four electro-spun layers: a polyurethane nanomesh-embedded passivation layer; a top Au nanomesh electrode; a parylene-coated polyurethane nanomesh intermediate layer; and a bottom Au nanomesh electrode (Fig. 1). The device detects pressure exerted on or by the finger by monitoring the capacitance change between the top and bottom electrodes produced by the deformation of the intermediate layer.
The highly sensitive nanomesh sensors can accurately measure contact pressures with minimal effect on sensation, according to the grasp tests carried out by the researchers (Fig. 2). The fingertip sensors were imperceptible and had no effect on the ability of the hand to grip an object or the ‘feel’ of the object. But despite the thinness and delicacy of the sensors, functionality is maintained after repeated use and they are robust enough to withstand typical friction.
“Our new methodology provides a major advance in monitoring human interaction forces during object manipulation,” says Lee.
The combination of imperceptible operation and durability makes the sensors useful for applications where precise and continuous monitoring of pressure or motion without any interference in sensation is required. A novel application could be the recording and digital archiving of craftwork by expert artisans or intricate medical procedures by highly skilled surgeons, suggest the researchers.
The finding that very thin sensors do not interfere with our sensations of grasping objects is very interesting, believes Zhenan Bao, K.K. Lee Professor of Chemical Engineering and Director of the Stanford Wearable Electronics Initiative (eWEAR) at Stanford University.“This is good news, not only as it shows the importance of having thin and ultra-conformal sensors for high sensitivity and low crosstalk but also because this allows natural human touch and interactions with objects. This will allow the quantification of human touch, which can then be applied to robotic development.”
According to the researchers, increasing the number of sensors and developing a means of acquiring and interpreting spatial pressure measurements would be highly useful in the longer term, as well as creating water-resistant and stretchable devices.
“Ultimately, we would like to develop a whole system (including a measurement unit, power supply, and interconnections) to detect the pressure of fingers and/or other biological systems wirelessly,” says Lee.
This article originally appeared in Nano Today 36 (2021) 101068.