An implant which can cool nerves to block pain signals has been unveiled by researchers who say the device could offer an alternative to drugs such as opioids.

The team behind the device say it could bring benefits for management of acute pain such as that experienced after amputations, nerve grafts or spinal decompression surgeries.

“We are optimistic that this represents a very promising starting point for an engineering approach to treating pain,” said Prof John Rogers of Northwestern University in Illinois, US, a co-author of the research.

But, Rogers cautioned, it may be some time before the implant is available to patients. “As with any implantable device, the regulatory process can be slow, typically involving much more extensive animal model studies over a period of years,” he said.

Writing in the journal Science, the team report how the device, which so far has only been tested on rats, involves a pump, external control system and an implant made from a soft, rubbery polymer. The latter incorporates a sealed collection of tiny channels, which form a serpentine path in the part of the implant that sits around the target nerve like a cuff.

When liquid coolant and dry nitrogen are allowed to flow through the winding path of the implant, the liquid evaporates, resulting in a drop in temperature. This temperature change is tracked by an electronic sensor in the device, allowing the flow to be controlled and the temperature at the nerve kept constant.

“All body processes are based on metabolic chemical reactions, motions of ions and flows of fluids [such as blood] – all of which slow down as a result of cooling, ”said Rogers.

“The net effect when cooling is applied to a nerve is in blocking of electrical signals,” he added, noting similarities to the numbing sensation in fingertips that can occur in cold weather.

Among their experiments, the team tracked two rats with an injury that caused sciatic pain, recording over a three-week period the minimum force that had to be applied to the hind paw to cause the animal to retract the paw. This data was then compared against that from three rats who were similarly injured but had the implant. The results suggest bouts of cooling of the injured nerve from 37C to 10C led to a reduction in pain, with a seven-fold increase in the force that could be applied to the paw.

The team say the implant has a number of benefits – including that in contrast to opioid drugs, it is not addictive. What’s more, as the implant is made using water-soluble and biocompatible materials, it can break down in the body after use.

“Typical operating lifetime is in the range of weeks and the corresponding time for complete dissolution is in the range of months,” said Rogers, who said the implant could be inserted as an extension of a patient’s initial surgery.

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Speaking in a personal capacity, Prof David Bennett, an expert in pain at the University of Oxford, said that there is a need to find alternatives to painkillers like opioids and praised the engineering of the implant.

But, he said, the approach has potential drawbacks, including that while it may reduce pain, the device would block all types of nerve fibers including those serving other sensations such as touch and those to muscles, potentially resulting in significant weakness.

“The issue is that as far presented there is very little specificity for pain,” said Bennett. “Finally there may be a proportion of people in whom cooling could exacerbate pain and we would also need to consider the side-effects of long-term cooling.”

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