Stretchable electronics are now being tested in the clinic for monitoring the health of particularly fragile patients—premature infants in neonatal intensive care. Such infants are require constant monitoring of their vital signs. Today that's done by taping wired sensors to the skin, but the wires obstruct movement, the sensors can be pulled off, and tape can damage fragile immature skin. Materials scientist John Rogers of Northwestern University, in Evanston, Ill., hopes to replace those wired sensors with thin wireless flexible sensing patches.
Rogers calls the technology “epidermal electronics.” Resembling a child's temporary tattoo, it would be only about five micrometers thick, imperceptible when applied to the skin, biocompatible, and as stretchable and flexible as the skin itself. He wants to use it for clinical sensing including heartbeat, blood oxygen levels, hydration, and blood flow near the surface.
Conventional wafer-based electronics are much too rigid, he told the Conference on Lasers and Electro-Optics in San Jose last Wednesday. Silicon nanomembranes only tens of nanometers thick are more flexible, and it's possible to make circuits that thin. However, that's not enough. “You can't make semiconductors thin enough to stretch,” he said.
Instead, Rogers puts small unstretchable devices on a stretchable substrate made of a complementary material. Thus the semiconductor chips become tiny islands on a rubbery film. Stretchable wavy pattens of thin metal filaments are embedded in the film, providing electrical connections to carry signals and power. Embedded conductors wound into loop antennas both pick up wireless power from magnetic fields and transmit signals to an external network.
Others also are working on stretchable skin sensors, and cosmetics brand Laroche-Posay is compiling a waiting list for a stick-on ultraviolet exposure meter called “My UV Patch”. But the UV patch is read by measuring color changes with a smartphone app. Rogers’ neonatal monitor patches instead incorporate antennas to gather power and transmit data wirelessly. The range is short, but adequate for use in neonatal intensive care.
Clinical tests are now under way at Northwestern's Feinberg School of Medicine. Rogers reports a promising sign. When researchers attached conventional sensors as controls, one child kept pulling off the wires, but the infant never even noticed the patch.