September 2020

UT SOUTHWESTERN SCIENTISTS DEVELOP COOLING INSOLE

A new cooling insole developed by University of Texas Southwestern (UT Southwestern) scientists reduced the foot temperature of patients with diabetic neuropathy by several degrees, diminishing a significant risk factor for diabetic foot ulcers (DFUs). This device, detailed in an article in The Journal of Foot & Ankle Surgery, could eventually prevent thousands of amputations that take place worldwide annually because of this condition.

Foot temperature is 1 of the risk factors for DFUs. Animal studies have shown that skin maintained between 25 and 30 degrees C is less likely to break down under pressure than skin at higher temperatures. The feet of diabetic patients already tend to be warmer due to inflammation associated with the disease, explained Metin Yavuz, D Eng, an associate professor in the School of Health Professions’ Division of Prosthetics and Orthotics at UT Southwestern Medical Center. This is compounded by friction from walking and the stiff therapeutic shoes that patients wear, which are usually made of synthetic materials that act as heat insulators.

To address foot temperature, Yavuz and his lab developed a device, named Temperature and Pressure Monitoring and Regulating Insoles (TAPMARI), which consists of a small box strapped to the wearer’s calf that houses a cooling unit, a small water pump, a battery pack, and a thermostat. The cooling unit harnesses a type of thermoelectric cooling called the Peltier effect to chill water to a desired temperature that is then pumped into insoles placed in the wearer’s shoes. Yavuz later teamed up with the engineering company Vivonics, Bedford, MA, to improve the design.

The device was tested on 5 healthy individuals and 3 individuals with diabetic neuropathy. Using an infrared thermal camera, the researchers took photos of the subjects’ feet at baseline before wearing the insoles, then placed a cooling insole in only their right shoes. They took more thermal photos after the subjects walked 5 minutes on a treadmill and again after they wore the insoles an additional 2 hours and walked 5 minutes on the treadmill again.

Results showed that the mean baseline foot temperature in the group was 28.1 degrees C. Mean foot temperatures at the end of the study were 31.7 degrees C for the left foot and 25.9 degrees C for the right, 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 degrees C in the right foot, suggesting that the insoles could maintain temperature in a range that protects against skin breakdown.

Cool temperatures from the insoles did not cause vasoconstriction in the foot, which could have damaged tissue, Yavuz said. However, sole temperatures reached as high as 30.8 degrees C in some regions of the cooled feet, particularly in the midfoot, suggesting that the insole design needs improvement. Other design elements could also be tweaked, he said, such as reducing the size of the unit worn on the calf.

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