Figure 1. A representative result of a subject with diabetic neuropathy. Hyperspectral (left row) and temperature (middle row) images collected before and after treadmill walking. TRI in-use (right). Bilateral temperature reduction is 1.9°C, StO2 reduction is 2%. R: Right, with TRI, L: Left, Control shoe. Photo credit: UT Southwestern.
Researchers at the University of Texas Southwestern (UT Southwestern) in Dallas have developed a new cooling insole reported to reduce the foot temperature of patients with diabetic neuropathy, diminishing one of the risk factors for diabetic foot ulcers.
In an article published online ahead of print in The Journal of Foot & Ankle Surgery (May 6, 2020), authors detail how the device could prevent thousands of amputations that take place due to these ulcers.
According to Dr. Metin Yavuz, an associate professor in the school of health professions’ division of prosthetics and orthotics at UT Southwestern Medical Center, pressure-relieving insoles—the most commonly prescribed preventive treatment for diabetic foot ulcers—have not been sufficient in accomplishing the goal of reducing diabetic amputation rates.
“Even when patients receive therapeutic shoes and insoles, education and close monitoring,” said Dr. Yavuz in a press release, “thirty to forty per cent of patients who have had one diabetic foot ulcer will still develop another within a year.”
In an effort to reduce the number of diabetic amputations, Dr. Yavuz and his UT Southwestern colleagues decided to focus on foot temperature. Animal studies have shown that skin maintained between 25 C and 30 C is less likely to break down under pressure compared to skin at higher temperatures.
Researchers note that the feet of diabetic patients already tend to be warmer due to inflammation associated with the disease. Additionally, friction from walking and the stiff therapeutic shoes that patients wear, which are generally made from synthetic materials that act as heat insulators, compound the issue.
“We thought, why don’t we break that vicious cycle by cooling the foot?” Dr. Yavuz said.
Dr. Yavuz and his lab developed a system that circulates cool water into pressure-relieving insoles. The device, which researchers called Temperature and Pressure Monitoring and Regulating Insoles (TAPMARI), consists of a small box strapped to the user’s calf that houses a cooling unit, a small water pump, a battery pack and a thermostat.
The cooling unit uses a type of thermoelectric cooling called the Peltier effect to chill the water to a desired temperature which is then pumped into the insoles in the user’s shoes.
Dr. Yavuz also teamed up with Vivonics Inc., an engineering company, and obtained funding from the U.S. National Institutes of Health to improve the design. Researchers tested an improved design on eight volunteers—one man and seven women—with a median age of 45 years. Five of the volunteers were healthy and three had diabetic neuropathy.
With an infrared thermal camera, the researchers took photos of the volunteer’s feet at baseline before wearing the insoles, then placed a cooling insole in only their right shoe. More thermal photos were taken after the subjects walked for five minutes on a treadmill and again after they wore insoles for an additional two hours and walked five minutes on the treadmill again.
Results showed that the mean baseline foot temperature in the group was 28.1 C. Mean foot temperatures at the end of the study were 31.7 C for the left foot and 25.9 C for the right foot, which was cooled by TAPMARI.
Although the diabetics’ feet got warmer than those of the healthy volunteers during walking, they still maintained a mean temperature of 27.5 C in the right foot, suggesting the insoles could maintain temperature in a range that helps to protect against skin breakdown.
Dr. Yavuz said the cool temperatures from the insoles did not cause vasoconstriction in the foot that could have damaged tissue. Sole temperatures did reach as high as 30.8 C in some regions of the cooled feet leading researchers to believe the design of the insole needs to be improved. Dr. Yavuz suggested other design elements such as reducing the size of the unit worn on the calf could also be tweaked.
The UT Southwestern researchers believe these devices could change the course of disease for patients with diabetes by preventing this common and often serious complication.
“Diabetic foot ulcers can be a major burden on patients, their families, caregivers and the health system,” Dr. Yavuz said. “What we are doing now to prevent these ulcers or simply maintain the status quo is not working. TAPMARI could be the start of a whole new approach.”