ARMY AL&T
Traditional MCC approaches involve constant skin cooling with liquids at low temperatures and high flow rates. As a result, MCC power, size, and weight requirements are large. A longer- term solution was needed that increased the efficiency of heat transfer from the human body to the MCC system.
Scientists at the U.S. Army Research Institute of Environmental Medicine, with the help of engineers at the Natick Soldier Research, Development, and Engineering Center (NSRDEC), discovered that over-cooling the skin can actually slow heat loss, while under- cooling the skin results in greater strain on the heart.
Both problems were minimized by allowing skin temperature to fluctuate narrowly—in other words, using skin temperature itself to automate cooling.
Intermittent vs. Continuous Cooling
The idea and the system for intermittently cooling the skin, rather than cooling it continuously, were conceived as a way to prevent the skin from constricting. The body constricts vessels in the skin to conserve heat when cold, and dilates vessels to expel heat when hot.
Although significant cooling can still occur when the skin is constricted (such as when we fall into ice water), it made sense that the MCC garment would become less efficient at removing body heat if the skin were over-cooled.
Experimentation determined that the choice of intermittent cooling paradigm did not seem to matter so long as skin
The technology is both evolutionary and revolutionary— evolutionary because it applies existing biomedical knowledge in a new way, and revolutionary because it heralds the development of new cooling vests that can sense temperature and deliver cooling to specific body areas.
temperature was kept within a nar- row range (33-35 degrees centigrade). Lower skin temperatures offered only a small cooling advantage, while warmer skin temperatures drastically increased strain on the heart.
Using skin temperature feedback to control MCC made the most sense; the research team determined that a Skin Temperature Feedback Cooler (STFC) reduced MCC power requirements by more than 40 percent.
A patent was awarded Nov. 23, 2010, for body temperature regulation using skin temperature feedback, as an MCC methodology for maximizing heat flux, minimizing physiological strain, and conserving battery power. Sensors within an MCC garment signal the need to provide or withdraw cooling based on an optimal skin temperature range, as determined empirically from the laboratory experiments. Studies demonstrated that with this approach, heat extraction is optimized (similar to constant cooling), but power consump- tion is reduced by 40-50 percent.
Temperature and Power Requirements
With STFC, application or withdrawal of cooling is determined automatically by skin temperature sensors.
The idea and the system for intermittently cooling the skin, rather than cooling it continuously, were conceived as a way to prevent the skin from constricting.
Over-cooling the skin (to less than 33 degrees centigrade) results in body heat conservation and inefficient use of MCC power. Under-cooling the skin, allowing it to heat to more than 35 degrees centigrade, increases cardiovascular strain because of increased skin blood flow and skin blood volume. When STFC is used, cooling is automatically turned on or off when these thresholds have been reached. Compared with the traditional constant-cooling approach, STFC removes body heat and reduces cardiovascular strain.
STFC also requires 40 percent less power, which could reduce the size and weight of batteries carried by dis- mounted Soldiers. The net result is that STFC feedback may allow for expan- sion and integration of personal cooling systems for dismounted or mounted Soldiers. In addition, STFC improves comfort when compared with tradi- tional systems. The initial research was funded as an Independent Laboratory In-House Research project in 1999 and then funded by a grant from the Defense Threat Reduction Agency. All of these findings have been published in peer-reviewed scientific journals.
Evolutionary and Revolutionary
The application and integration of this MCC method will decrease the size and weight of future MCC systems and make possible effective MCC for Soldiers mobilized on foot.
The technology is both evolutionary and revolutionary—evolutionary
APRIL –JUNE 2011 39
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