Getting the Inside Scoop

The latest sensing technologies, typically in conjunction with miniaturized processing units and advanced machine learning algorithms, are enabling all manner of new wearable electronic devices to be developed. These wearables are increasingly being used as health monitoring devices, which can keep a close eye on physiological parameters that could be indicative of certain medical disorders. In this way, problems can be detected early, and more effective and customized treatment plans can be developed by health care providers.

These systems most frequently measure biomarkers in noninvasive ways, like by testing for certain chemicals that are excreted in sweat, by testing the skin surface with an electrocardiogram sensor, or by reflecting light off of subcutaneous tissue. But these surface-level tests can only provide just so much information. When more biomarkers, or more precision, is required, sensors have to get beneath the surface of the skin.

A common way to collect data from beneath the skin’s surface involves the use of microneedle electrodes. These systems can capture physiological parameters that are not available at the surface of the skin, and they can do so with high levels of accuracy. However, the rigid design of these microneedle arrays make them uncomfortable for the wearer. Moreover, they cannot easily conform to the curves of the body, especially as it is in motion. For these reasons, this technology is often undesirable, or otherwise impractical, for many applications.

Recent advances made by a research team centered around the University of Southern California, Los Angeles could soon make microneedle electrode arrays more practical. They have developed a soft microneedle array that is much more comfortable for the wearer. It is also capable of bending and stretching to conform to a body in motion such that it can collect accurate data even under otherwise challenging conditions.

For maximal flexibility, the array was built into a stretchy silicone elastomer. Highly deformable serpentine interconnects were embedded within the elastomer to both connect the microneedles to one another and to external electronics. The microneedles are covalently bonded to the silicone and the interconnects to keep them in place as the array is deformed.

The manufacturing method developed by the researchers utilizes laser micromachining, microfabrication, and transfer printing to produce the microneedle arrays in a low-cost and scalable manner. Tests demonstrated that the arrays exhibit a stretchability of 60 to 90 percent, which is the highest level that has ever been reported.

Not only does the fabrication process make it simple and inexpensive to produce the arrays, but it also offers a great deal of versatility. The electrode geometry, recording sites, and mechanical and electrical properties can be altered as needed for each use case.

Initially, the team tested their system in a series of experiments with sea slugs. It was shown that electrical activity inside the muscles of these slugs could accurately be captured as they moved about normally. But ultimately the goal is to adapt this system for use in humans, where the team believes it will be useful in the electrochemical sensing of skin interstitial fluids, diagnosis of neuromuscular disorders, and possibly even in precision drug delivery.