Imperceptible Sensors Printed Directly on Human Skin

Andrew Corselli

Sensors printed on human fingers. (Image: Huang Lab, Cambridge)

Researchers have developed a method to make adaptive and eco-friendly sensors that can be directly and imperceptibly printed onto a wide range of biological surfaces, whether that’s a finger or a flower petal.

The method, developed by researchers from the University of Cambridge, takes its inspiration from spider silk, which can conform and stick to a range of surfaces. These “spider silks” also incorporate bioelectronics, so that different sensing capabilities can be added to the “web.”

“One of the biggest challenges was to step out of the existing fabrication methods and come up with a spinning mechanism to produce the fiber sensors,” Research Lead and Professor Yan Yan Shery Huang told Tech Briefs in an exclusive interview.

The fibers, at least 50 times smaller than a human hair, are so lightweight that the researchers printed them directly onto the fluffy seedhead of a dandelion without collapsing its structure. When printed on human skin, the fiber sensors conform to the skin and expose the sweat pores, so the wearer doesn’t detect their presence. Tests of the fibers printed onto a human finger suggest they could be used as continuous health monitors.

“The initial idea was kind of inspired by the combination of electronic skins and spider silks,” Huang added.

This low-waste and low-emission method for augmenting living structures could be used in a range of fields, from healthcare and virtual reality to electronic textiles and environmental monitoring. The results are reported in the Nature Electronics .

“First an aqueous solution consisting of conducting particles, hyaluronic acid, and a polymer binder is made,” Huang explained of the process. “This solution is then fed into a syringe tip, where a tiny pendant drop is created at tip. A rotating arm then comes into contact with the pendant drop, which stretch the drop into a sensing fiber.”

Although human skin is remarkably sensitive, augmenting it with electronic sensors could fundamentally change how we interact with the world around us. For example, sensors printed directly onto the skin could be used for continuous health monitoring, for understanding skin sensations, or could improve the sensation of ‘reality’ in gaming or virtual reality application.

While wearable technologies with embedded sensors, such as smartwatches, are widely available, these devices can be uncomfortable, obtrusive and can inhibit the skin’s intrinsic sensations.

There are multiple methods for making wearable sensors, but these all have drawbacks. Flexible electronics, for example, are normally printed on plastic films that don’t allow gas or moisture to pass through, so it would be like wrapping your skin in cling film. Other researchers have recently developed flexible electronics that are gas-permeable, like artificial skins, but these still interfere with normal sensation, and rely on energy- and waste-intensive manufacturing techniques.

3D printing is another potential route for bioelectronics since it is less wasteful than other production methods, but leads to thicker devices that can interfere with normal behavior. Spinning electronic fibers results in devices that are imperceptible to the user, but don't have a high degree of sensitivity or sophistication, and they’re difficult to transfer onto the object in question.

Now, the Cambridge-led team has developed a new way of making high-performance bioelectronics that can be customized to a wide range of biological surfaces, from a fingertip to the fluffy seedhead of a dandelion, by printing them directly onto that surface. Their technique takes its inspiration in part from spiders, who create sophisticated and strong web structures adapted to their environment, using minimal material.

The researchers spun their bioelectronic ‘spider silk’ from PEDOT:PSS (a biocompatible conducting polymer), hyaluronic acid, and polyethylene oxide. The high-performance fibers were produced from a water-based solution at room temperature, which enabled the researchers to control the ‘spinnability’ of the fibers. The researchers then designed an orbital spinning approach to allow the fibers to morph to living surfaces, even down to microstructures such as fingerprints.

Tests of the bioelectronic fibers, on surfaces including human fingers and dandelion seedheads, showed that they provided high-quality sensor performance while being imperceptible to the host.

“The immediate next step is to establish application-based scenarios, to determine what part of the sensor system should be made with imperceptible fibers, and the rest can use existing microfabricated devices/e-textiles,” Huang noted.