Soft electronics for noninvasive health care—From the skin to below the skin
Written by Don Monroe
Flexible electronic systems have gotten a lot of attention in recent years for their ability to monitor mechanical, electrical, and chemical signals on curved, flexible, or rough surfaces, for example for biomedical applications. In his Outstanding Young Investigator talk, Sheng Xu, of the University of California, San Diego, showed devices that extend this sensing ability several centimeters into the body.
Specifically, Xu showed a flexible patch containing an array of ultrasound transducers. When applied to the skin over a major blood vessel, for example in the neck, this device provides a continuous record of the vessel dimensions and thus of the blood pressure. In addition to providing greater temporal detail, the patch can be used during exercise and away from the clinic, in part because the flexibility allows for intimate skin contact without the usual ultrasound gel. “It enables a new sensing modality that gives us an overwhelming abundant information that is not accessible by conventional modalities,” for blood-pressure monitoring, Xu said, and “effectively opened up a new sensing dimension for the wearable community.”
The ultrasound device takes advantage of versatile platform for flexible electronics. One key enabler is that even stiff materials like metals and semiconductors become highly flexible when they are thin enough, as well as being less prone to fatigue because defects can easily find their way to the surface.
Xu and others have shown that such flexible structures can also be integrated with stiff components, including commercial chips, batteries, and other devices, to leverage their functions and low costs. In particular, stiff islands can be connected by thin, wavy bridges that absorb any external strain. Following encapsulation, for example with a soft polymer, the resulting sheets of “hybridized material” can be strained by 50% or more without losing their functionality. Moreover, they can form intimate contact with complex, flexible surfaces like skin with reduced irritation, better signal quality, and robust operation outside the clinic.
Another important feature Xu’s group developed is “mechanically invisible” integration. This technique bonds large rigid structures only at a small footprint, avoiding the strains that would otherwise concentrate around large-area devices.
Complex electronic systems often include multiple functional layers, with electrical communication provided by vertical vias that connect specific layers, but most flexible electronics schemes provide only a single layer. Xu and his colleagues developed selective laser ablation techniques to open vias, and he showed a four-layer flexible device that allows “new functions that are not possible in single-layer devices,” he said.
In addition to the ultrasound device, Xu showed several other devices that he and his collaborators built with the flexible electronic platform. These included a flexible bio-fuel cell, a wearable chemical sensor, and a patch that measures and wirelessly transmits electrocardiograms or other physiological signals. He also showed a device that measures the orientation of a person’s arm and uses that control a prosthesis.
Xu emphasized that soft electronics importance goes beyond new fabrication and materials strategies. “It is truly a field driven by the unmet need, especially in this clinical, medical area.”
The MRS Outstanding Young Investigator Award recognizes outstanding, interdisciplinary scientific work in materials research by a young scientist or engineer. The award recipient must also show exceptional promise as a developing leader in the materials area.
Xu’s award citation is “for materials and device designs in biointegrated electronics and stretchable energy systems."