The future of biothermal engineering.

This is a report on a three-day workshop held at the Allerton House of the University of Illinois. The first day consisted of invited tutorials on topics related to biothermal engineering: biological structures, analysis of microvascular heat transfer, temperature measurement, cryobiology and cryosurgery, burns, and industrial and consumer applications. The rest of the workshop consisted of discussions in small groups and in plenary sessions dealing with relevant topics. Although the discussions endeavored to be as comprehensive as possible, the specific topics were selected by the participants based on their expertise and interests. The main areas examined were: Instrumentation, Priority applications, Mathematical modeling, Thermal injury. The reliable measurement of the temperature distribution inside the living tissue is still the premier problem of instrumentation although the measurement of other parameters, such as properties, blood perfusion or heat flux, is also of great importance. The most important applications are medical, industrial, consumer, agricultural, space, and military. The degree of sophistication needed in the analysis of specific problems varies a great deal from relatively simple heat conduction models to complicated ones including blood perfusion, anisotropy, and the influence of large blood vessels. For many applications new experimental data are still needed. There have been significant advances in the modeling of living tissue with increasing understanding of its thermal behavior. The consensus was, however, that the models will always have to be tissue or organ specific and some new models are still to be developed.

Safe touch temperatures for hot plates.

A finite difference heat transfer model has been developed to predict the Safe Touch Temperatures (STT) for plates made of different materials. SST can be defined as the highest temperature at which no pain is felt when the surface is touched for a long enough period to allow safe handling of the equipment. The criterion used to quantify damage is the "damage function" that was originally proposed by Henriques and Moritz. There are several uncertainties present in the physiological and thermal properties of the skin that give rise to a solution range rather than a single solution. Certain simplifying assumptions are made that tend to yield solutions for STT that are toward the lower or "safe" end of the solution range. The model developed is a two-dimensional axisymmetric model in cylindrical coordinates. A finite difference scheme that uses the Alternating Direction Implicit method is used to solve the problem. It is a second-order scheme in both space and time domains. A parametric analysis of the model is performed to isolate those factors that affect the STT to the greatest extent. Data are presented for a variety of cases, which cover commonly observed ranges in material and geometric properties. It is found that the material properties, namely thermal conductivity and volumetric heat capacity, and the plate thickness ratio are the three most important parameters. These three parameters account for a range of STT from 56 degrees C-100 degrees C with thick metals at the low end and thin metals and plastics in the high range. This method represents a significant improvement over existing standard practices.

The measurement of temperature with electron paramagnetic resonance spectroscopy.

An electron paramagnetic resonance (EPR) technique, potentially suitable for in vivo temperature measurements, has been developed based on the temperature response of nitroxide stable free radicals. The response has been substantially enhanced by encapsulating the nitroxide in a medium of a fatty acid mixture inside a proteinaceous microsphere. The mixture underwent a phase transition in the temperature range required by the application. The phase change dramatically altered the shape of the EPR spectrum, providing a highly temperature sensitive signal. Using the nitroxide dissolved in a cholesterol and a long-chain fatty acid ester, we developed a mixture which provides a peakheight ratio change from 3.32 to 2.11, with a standard deviation of 0.04, for a temperature change typical in biological and medical applications, from 38 to 48 degrees C. This translated to an average temperature resolution of 0.2 degree C for our experimental system. The average diameter of the nitroxide mixture-filled microspheres was approximately 2 microns. Therefore, they are compatible with in vivo studies where the microspheres could be injected into the microvasculature having a minimum vessel diameter of the order of 8 microns. This temperature measuring method has various potential clinical applications, especially in monitoring and optimizing the treatment of cancer with hyperthermia. However, several problems regarding temperature and spatial resolution need to be resolved before this technique can be successfully used to monitor temperatures in vivo.

A comparison of steady state and transient thermography techniques using a healing tendon model.

Steady state and transient thermal techniques were used to define the thermal signatures of surgically sectioned and sham-operated common calcanean tendons in four dogs. All limbs were imaged from the lateral side using an Inframetrics 525 system at - 1, 2, 4, 6, and 8 weeks after surgery. Individual video frames were used to compute absolute surface temperatures and rewarm curves for five predetermined 1 cm2 skin areas. Angiography was performed at each observation period to correlate changes in vascular morphology and thermal data. Thermal signatures and angiograms were similar in all animals before surgery. At 2 and 4 weeks after surgery, the absolute surface temperatures of the entire lateral crus area were elevated in three of four animals. During weeks 6 and 8, the surface temperatures, rewarm curves, and angiograms returned to presurgical values for the controls. Skin areas over the repaired tendons remained warmer and were shown to correlate with vascular proliferation by transient but not steady state techniques. Steady state and transient thermal imaging techniques can be used to detect vascular changes in the area around a healing tendon. However, our data indicate that transient thermal techniques are more suitable than steady state methods for localizing vascular disturbances in tissues. Thermographic imaging techniques may become a reliable noninvasive method to monitor wound healing processes if starting temperatures, cool down techniques, and time intervals for data collection are fully evaluated in future studies using transient thermal imaging protocols.

Heat transfer to blood vessels.

Heat transfer to individual blood vessels has been investigated in three configurations: a single vessel, two vessels in counterflow, and a single vessel near the skin surface. For a single vessel the Graetz number is the controlling parameter. The arterioles, capillaries, and venules have very low Graetz numbers, Gz < 0.4, and act as perfect heat exchangers in which the blood quickly reaches the tissue temperature. The large arteries and veins with Graetz numbers over 10(3) have virtually no heat exchange with the tissue, and blood leaves them at near the entering temperature. Heat transfer between parallel vessels in counterflow is influenced most strongly by the relative distance of separation anad by the mass transferred from the artery to the vein along the length. These two effects are of the same order of magnitude, whereas the film coefficients in the blood flow are of significant but lesser importance. The effect of a blood vessel on the temperature distribution of the skin directly above it and on the heat transfer to the environment increases with decreasing depth-to-radius ratio and decreasing Biot number based on radius. The absolute magnitude of these effects is independent of other linear effects, such as internal heat generation or a superimposed one-dimensional heat flux.