e-skin ruffled by a feather
Wearable electronics have been around for decades — isn’t that calculator watch of the 80s making a comeback? Fortunately, electronics are getting smarter and becoming more fashionable through the integration of sensors, wireless communication, and lightweight, flexible polymers into sleek and comfortable devices that can monitor our fitness and talk to our appliances. Such electronics may be applied in robotic surgery and prosthetics if artificial fingertips gain the ability to gently grasp and manipulate objects. For artificial tactile intelligence to be fully achieved, external stimuli in tangential and normal directions should be accurately detectable and distinguishable with high sensitivity. Or, in other words, the sense of touch needs to be enhanced. Recently, researchers at the University of Electronic Science and Technology of China in Chengdu, the Harbin Institute of Technology in China, and the University of Bristol in the UK have developed a CNTs/GO@PDMS-based electronic skin, or e-skin, that can do just that. Fabricated using a porogen-assisted self-assembly process, the artificial skin can even sense light tickling from a feather.
Naturally, e-skin designers find inspiration in nature. Motivated by a spider’s leg joint and a beetle’s wing-to-body locking microstructure, ultra-sensitive strain sensors have been developed based on microscale cracks [1-3] and arrays of interlocked nanofibers . The ciliated skin of crickets and locusts has inspired the use of carbon nanotubes (CNTs) in the development of omnidirectional air-flow sensors . The cilia are sensitive to gentle touch and air vibration, while the underlying skin senses heavier pressure. Like the ciliated skin of a caterpillar, the CNTs/GO@PDMS-based e-skin is also comprised of two layers. The outer layer mimics cilia, comprised of sheets of CNTs that are bridged by graphene oxide (GO), and the underlying GO-embedded microporous polydimethylsiloxane (PDMS) layer is analogous to the dermis.
Electrical and mechanical durability upon recurrent bending, twisting, and stretching are essential for wearable electronics. The CNTs/GO@PDMS-based skin has robust mechanical properties, with an elongation at break of 45% and an elastic modulus of 1.48 MPa, and shows resistance changing outputs upon bending, stretching and torsion stimulation. The output signals remain stable for thousands of cycles of tensile strain and bending deformation as well as pressing and shearing. Remarkably, the CNTs/GO@PDMS e-skin can detect and differentiate tangential force and normal pressure by producing unique opposite resistance changes. The e-skin can detect normal pressure below 25 Pa, with a sensitivity of -0.13 kPa-1 in the range of 0 kPa to 3.8 kPa and -0.03 kPa-1 in the range of 3.8 kPa to 6.3 kPa. The CNTs/GO@PDMS e-skin is highly sensitive to tangential force, with a high gauge factor of 2.26 and an ultralow detection limit for shear force of 0.46 N under a pre-loaded normal pressure of 1 kPa.
The e-skin is capable of differentiating surfaces of varying roughness. When placed in contact with printing paper with a surface roughness of ~0 mm, the e-skin yields a resistance increase of ~14%, while tissue paper with a roughness of ~0.13 mm yields a resistance increase of ~70% and flannelette, with a roughness of 0.7 mm, gives the highest resistance increase of ~270%. In non-contact mode, the e-skin can detect breath and music rhythms. The CNTs/GO@PDMS e-skin delivers large resistance changes in response to light “tickling” (<20 mg) from a feather or when slight friction is applied with a soft brush. The authors hope to expand this collection of interesting properties to include superhydrophobicity.
Read the article here: https://onlinelibrary.wiley.com/doi/full/10.1002/adfm.201707503
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