This Origami Implant Unfolds Under the Skin to Monitor Health

Sometimes the smartest way to make a medical device smaller isn't to shrink it—it's to fold it.

Doctors have long wanted implantable sensors that can continuously monitor a person’s health without requiring major surgery or bulky external equipment. It’s a big engineering challenge. A device must be small enough to insert without major surgery, but large enough to carry an antenna, electronic circuits and multiple sensors.

A new device called MiFi tries to escape that trade-off by changing shape.

Developed by researchers from Imperial College London and the University of Southern California, this origami-inspired implant slips beneath the skin through a tiny incision before unfolding into its full operating shape. According to Fırat Güder, one of the study authors and a professor of intelligent interfaces at Imperial College London, this is the first sensor of its type.

Folding instead of shrinking

Most efforts to make implants less invasive focus on miniaturizing their components. But sensors and electronics need physical space, and there’s only so much real-estate in our bodies. So the MiFi team took a different approach. Instead of forcing the implant to remain tiny after insertion, the researchers designed it to temporarily fold.

Before implantation, their device is folded using origami-inspired patterns that dramatically reduce its footprint, allowing it to pass through a much smaller incision. 

Once beneath the skin, it unfolds into a square measuring about 2.1 centimeters by 2.1 centimeters and just 0.3 millimeters thick. The larger operating shape provides enough room for antennas, sensors, and electronic circuits while keeping the implantation procedure minimally invasive.

The key innovation is that the device’s insertion size is no longer the same as its operating size. Conventional implantable sensors often become larger as more functions are added, making implantation more invasive. By separating these two requirements, the foldable design avoids the usual compromise between size and performance.

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The researchers report that the folded design can reduce the implant’s insertion footprint by as much as sixfold.

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No battery needed

MiFi receives power through near-field communication, or NFC, the short-range wireless technology that’s also used in contactless cards and smartphone payments.

A nearby external reader generates the electromagnetic field needed to power the implant. The same wireless connection carries measurements from the device to an external computer.

Not having a battery means more space for a suite of sensors, an approach that was also followed by NASA in some of their projects.

One configuration used a miniature accelerometer to detect the tiny movements produced by the animals’ heartbeats and breathing. Software then converted those movements into heart-rate and respiratory-rate measurements. Another configuration included electrodes for recording cardiac electrical activity.

The implant also carried a temperature sensor and electrochemical modules for measuring tissue pH and lithium.

That chemical sensing is an important part of the experiment. MiFi sits in direct contact with interstitial fluid, the liquid surrounding the body’s cells. Unlike a watch or adhesive skin patch, an implanted sensor can potentially measure molecules that are difficult or impossible to detect reliably in sweat.

“For this reason, MiFi could be used in the future to enable real-time monitoring of chemical markers – e.g., metabolites, enzymes, drugs, antibiotics – that cannot reliably be detected from sweat, for example,” Selin Olenik, first study author and a postdoc researcher at Imperial College London, said.

For now, just in rats

To evaluate the technology, the researchers implanted MiFi beneath the skin of rats and compared its measurements with those from commercial wearable sensors attached to the animals’ skin.

The implant’s estimates of heart rate, breathing rate and temperature closely followed those produced by the external monitoring system. Its position beneath the skin also appeared to produce stable mechanical measurements during the short experiment.

The researchers separately demonstrated electrochemical sensing by monitoring pH and changes in lithium concentration. Lithium is medically relevant because doctors prescribe lithium salts to some people with bipolar disorder, yet the drug has a narrow therapeutic range and requires regular blood testing.

In the experiment, the sensor detected rising lithium levels after the animals received lithium chloride. This was a limited proof of concept, but a very promising one.

Still, MiFi isn’t quite ready for use in people.

The in vivo tests were acute experiments conducted on anesthetized rats, with recordings lasting roughly an hour. That is enough to demonstrate that the folding mechanism and wireless electronics work beneath the skin, but not enough to establish long-term safety or reliability.

The electronics and encapsulating materials must also survive repeated movement and prolonged exposure to body fluids. Sterilization, manufacturing consistency, NFC reader placement and safe removal will create additional challenges. Human trials can only become realistic after those questions are addressed.

Still, MiFi demonstrates a clever alternative to relentless miniaturization. An implant does not necessarily have to remain as small as the opening used to insert it.

By folding for implantation and unfolding for operation, future devices may be able to combine less-invasive procedures with the surface area needed for sophisticated wireless monitoring. For now, though, that possibility has been demonstrated only in an early animal experiment.

The study is published in the journal Advanced Materials.