You can learn a lot about the goings-on of the human body just through the skin. The electrical activity of the heart, for example, is reflected in tiny electrical changes on the skin, which can be observed through electrocardiography. Electromyography reveals the electrical activity of muscles, which may then reveal neuromuscular diseases. Electroencephalography (EEG) gives us a viewport into the workings of the brain. All are powerful tools, but every one of them involves the pasting of unwieldy pads to the skin in a hospital setting.
In a paper published Monday in Nature Nanotechnology, Akihito Miyamoto and colleagues offer an alternative in the form of ultrathin meshes that offer direct integration with the soft surface of the skin. They involve virtually no mechanical footprint while allowing skin to breathe and sweat as normal. Medical uses aside, the new nanomesh technology offers a crucial advance in wearables, generally—a seamless interface between skin and and electronics. In other words, where skin essentially becomes electronics.
Miyamoto's nanomesh is hardly the first push into skin-based interfaces. Over the past several years, light-based and biochemical sensors have been laminated onto human skin for a variety of purposes, including skin-based displays and electrical, chemical, and physical sensors. Some implementations are even designed for long-term use, offering, for example, the possibility of persistent brain-machine interfaces via soft, foldable electrodes.
A key limitation of prior efforts has been the necessity of a substrate, a thin base layer that connects electronics to skin. Substrates limit things in several ways, including overall softness, weight, and gas permeability. They just don't breathe very well.
The nanomesh described in the new paper offers a substrate-free interface. That's the advance. The mesh here is constructed on a basis of polyvinyl alcohol (PVA), a synthetic water-soluble polymer that's already used in a variety of medical applications. The result is gas-permeable, doesn't block sweat glands, and stretchable enough to be worn for long periods without discomfort.
"Touch, temperature and pressure sensors placed on the fingertips are connected by mesh conductors with an e-textile system using a wireless module," the paper explains.
Miyamoto's group performed experiments in which patches were worn for one week with no resulting inflammation. Moreover, the mesh is able to retain its electrical conductivity even after being stretched and flexed up to 10,000 times.
"Our inflammation-free sensor can be used for continuous monitoring of vital signals under normal, everyday conditions over long periods of time," Miyamoto and co. write.
To understand the structure of the new nanomesh it's probably easiest to just see it (above). First, PVA fibers are created through a process called electrospinning. Basically, you can imagine drawing thin threads of polymer material out of a solution using electric force. (It's actually really cool but a bit beyond the scope of this blog post.) The polymer fibers are then patterned out and coated in a thin layer of gold. Finally, the resulting mesh is affixed to a patch of skin and sprayed with water. The PVA dissolves, leaving only fine interwoven threads of gold. The substrate is gone.
To test out their nanomesh, Miyamoto and co. constructed a system consisting of fingertip sensors connected to a fingerless glove containing a battery and wireless module, which itself was connected to a laptop computer. As the electrical resistance of the fingertip nanomesh changed as it came into contact with a metal plate, the resulting data was transmitted to the laptop. In further tests, the nanomesh successfully took the place of electrodes in EMG monitoring.
It's not crazy to imagine such an interface going beyond the skin and into the body itself, perhaps directly monitoring internal organs. But challenges remain, such as constructing a similar nanomesh that is also resilient against the physical and chemical damage likely to occur in the wild.