Soft, miniature implantable cooling devices designed to wrap around nerves can help relieve pain on demand without drugs, a new study finds.
Although the researchers do not envision their invention completely replacing painkilling drugs, they suggest it could offer pain relief in a number of key scenarios, such as after surgeries.
Acute pain afflicts one in five adults globally, making it one of the most critical health problems in the world. Although opioids can treat pain, they are extremely addictive. Overuse of those drugs has led to an opioid crisis; more than 70 percent of drug overdose deaths in 2019 involved an opioid.
“We started thinking about engineering approaches to pain relief some years ago, motivated by the opioid epidemic,” says John Rogers, the biomedical engineering professor at Northwestern University who helmed the research team. “We’re excited by the idea of an engineering, device-oriented approach to control over pain,” he says.
The group detailed its findings in the 1 July issue of the journal Science.
In the new study, researchers explored a drug-free way to control pain known as analgesic nerve cooling. Much as putting an icepack on a sore joint or muscle can relieve aches, so can applying cold to a nerve to block pain signals.
“We decided to focus on cooling as a pain block, mainly because the basic phenomena have been known and studied for some years. But [heretofore,] the absence of a corresponding device platform has prevented any realistic means to use this effect with patients,” Rogers says.
Conventional nerve-cooling devices rely on cooling a liquid such as methanol and delivering it into the body through a metal or silicone loop or a thermoelectric device. However, these machines are bulky, rigid, and need a lot of power, limiting their practical long-term use in the body. In addition, instead of targeting specific nerves, they take a broad-spectrum approach, cooling large areas of tissue. That could potentially lead to unwanted effects such as tissue damage and inflammation.
In the new study, Rogers and his colleagues developed a miniaturized implantable nerve-cooling system that is about as thick as a sheet of paper and made of soft, flexible, and stretchable materials. It relies on evaporation—the same phenomenon we’ve all experienced in the form of sweat evaporating and cooling the body.
The device is an elastic band that wraps around nerves. An integrated thin-film sensor monitors a nerve’s temperature in real time to ensure the device is not making the nerve too cold. An external pump helps users remotely activate the device and increase or decrease its cooling intensity on demand.
The red oval indicates pain. The device softly wraps around the peripheral nerve to silence signals to the brain.Northwestern University
How does the device sidestep the need for bulky, power-hungry machinery? It comprises a bunch of tiny microfluidic channels. Flowing through them—most of the time separately—are the coolant perfluoropentane and dry nitrogen gas. (Perfluoropentane is already clinically approved for internal use as an ultrasound contrast agent and for pressurized inhalers.) When the liquid and gas flow into a chamber together, a reaction occurs that causes the liquid to quickly evaporate, leading to more rapid cooling than conventional nerve-cooling techniques.
In addition, the device is bioresorbable, meaning it naturally and safely dissolves in the body over the course of a few months. This eliminates the need to surgically extract it when it is no longer needed.
In experiments on live rats, the device triggered cooling at a maximum rate of 3 °C per second to provide specific, reversible pain relief on demand. “Uses with humans would be easier, given the larger sizes of the nerves,” Rogers says.
The scientists envision their invention finding use “in the context of postsurgical pain, where the devices can be implanted as the final step in surgery, just prior to closing the surgical site,” Rogers says. “This technology is not a silver-bullet alternative to painkilling drugs in a general sense—it is not designed to treat chronic pain, for instance. Because it is an implantable device, its use makes the most sense in the context of a surgery that is scheduled to occur for other reasons, and the device can be inserted as a small extension of that surgical process.”
Rogers notes that he and his colleagues have not explored the limits of how long this cooling can be imposed on nerves without unwanted side effects. Future research will systematically explore the biological effects of cooling over time. “We are also working to reducing the size and the power requirements for the externalized pumping systems needed to introduce and extract the coolant from the device,” he says.
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Charles Q. Choi is a science reporter who contributes regularly to IEEE Spectrum. He has written for Scientific American, The New York Times, Wired, and Science, among others.