Patent Application
20100312314
1. The Field of the Invention
This invention relates to improving the heat-sinking properties of living biological tissue through which blood is passing by controlling the temperature, which in turn affects local blood perfusion.
2. Background and Relevant Art
In some applications, one or more properties of living tissue are measured by mounting some device or ensemble of devices onto the tissue, wherein some components, which the device comprises, dissipate heat. If it is necessary to heat sink the thermally dissipative components, it may not be advantageous to do so using free convection of air, because the heat sinking capacity may not be adequate. It also may not be advantageous to use forced air convection because of increased power consumption, which is particularly a concern for battery-operated devices. In some circumstances, the biological tissue must be relied upon to provide an advantageous heat sink for heat dissipating components.
The thermal conductivity of skin is low unless it is well-perfused with blood, hence it may not be suitable for heat sinking unless perfusion is adequate at the heat sink site.
Also, in some applications where a property of the biological tissue is being measured, it is advantageous to control the perfusion of blood within the tissue for purposes of measurement accuracy. An example of such a case is the measurement of glucose in human skin, wherein adequate blood perfusion is necessary for the local concentration of glucose in blood and interstitial fluid to reach equilibrium with the average glucose concentration in the body's total blood volume.
In addition, in applications where low dark current optical detectors are required, it is advantageous to maintain the temperature of the detectors at a value as low as possible consistent with power consumption limitations for cooling. Thus, the sink temperature for the detector or for a cooler for the detector, if one is employed, should be as low as possible, whereas for example, the temperature of the biological sample in the neighborhood of the measurement may advantageously be higher in order to increase blood perfusion. Hence, it is useful to have some means of providing a temperature difference between the heat sinks for the different heat-dissipating components and for the measurement site on the biological sample.
Blood perfusion can rise by as much as one order of magnitude in a biological sample if it is heated from room temperature to the neighborhood of 40° C. as is demonstrated in “Effect of high local temperature on reflex cutaneous vasodilation,” W. F. Taylor et. al., J. Appl. Physiol.: Respirat. Environ. Exercise Physiol. 57 (1): 191-196, 1984. In U.S. Pat. No. 7,509,153, Blank et al. discloses an apparatus for controlling skin perfusion wherein the temperature of the site at which glucose is measured is controlled. The above patent is a continuation-in-part of U.S. Pat. Nos. 6,640,117 and 7,039,446. Blank teaches a means of controlling perfusion of blood in skin by control of temperature and of monitoring the effects thereof spectroscopically, in which monitoring and temperature control can be parts of a closed loop system. Consideration is not given to heat sinking to the skin, of heat dissipative components that may be part of the apparatus or of maintaining temperature differentials between the component-heated skin temperatures and the temperature of the measurement site. Neither is there an arrangement that assures adequate blood perfusion in the neighborhood of the heat sinks for thermally dissipative components.
These and other limitations are addressed by the present invention, which discloses an apparatus whereby the thermal conduction of the heat sinks for the thermally dissipative components which are on the surface of the biological sample can be enhanced by increasing local blood perfusion. In addition, it is possible to maintain a different temperature for the component heat sinks and for the site at which a property of the biological tissue is measured. From the requirement of continuity on fluid flow it can be seen that perfusion must be increased in areas neighboring the heated site, thereby improving thermal conduction at these neighboring locations while not increasing their temperature to the same degree as that of the heated site. It is shown how thermal isolation between the different regions can be maintained, where the apparatus makes contact with the biological sample. It is also shown how this thermal isolation can be beneficially used to isolate a heat sink associated with an optical detector from that associated with an optical source, both of which are thermally isolated from the site at which a property of the biological sample is measured.
When the temperature at the measurement site 15 is increased, the flux of blood into and out of the biological tissue 10 in the neighborhood of the measurement site 15 is increased. In one example, biological tissue is heated in an area that is at least 20 mm2. The increased flux of blood persists to some distance from the heated area, and the zone of substantially increased flux 75 is shown in
The heater 20 is suitably chosen to be a resistive heater, which in a particularly preferred embodiment can be fabricated in a flex circuit using a nickel-chromium resistance conductor, or a conductor from some other resistive alloy. Temperature sensor 30 can be chosen to be a thermistor or a thermocouple, for example. In one embodiment, the optical window 80 should be chosen from a material whose thermal conductivity is much greater than that of skin, to assure a uniform temperature distribution at the measurement site 15. Good choices are silicon-carbide single-crystal material, sapphire, or diamond for visible or near-infrared radiation. Low-doped silicon is an excellent choice for radiation in the 1-6 um wavelength region.
The measurement apparatus, if optical, can be suitable for Raman spectroscopy, near-infrared spectroscopy, mid-infrared spectroscopy, optical coherence tomography, and diffuse reflectance measurement, but is not limited to these applications.
Properties that can be measured include but are not limited to the concentration of an analyte such as glucose, hemoglobin, water, triglycerides, or electrolytes. Additionally properties such as temperature, pulse rate, and blood perfusion can be included.
The insulator 50 can be chosen to be a polymer or air. Silica aerogel is also a good choice to lessen the heat transport by convection.
Heat sinks 60 and 70 are suitably chosen from the high thermal conductivity metals such as aluminum or copper.
The heater 160 heats the window 170 which is advantageously fabricated in silicon carbide, sapphire, or diamond for visible and near-infrared applications, in one embodiment. The window retainer 175 is fabricated from a low thermal conductivity material, such as a polymer, and thermally isolates the heated window 170 from the base 220 assuring that the heat is applied only in the area desired. The base 220 is in contact with the skin on the side opposite of top side 180. The base 220 is advantageously fabricated in aluminum to achieve good thermal conductivity. The thermoelectric cooler 200 cools detector 190 and the heat from its hot side is deposited in heat sink 210 and then flows into the base 220. The location of the heat sink 210 is well-spaced away both from heated window 170 and the assembly containing the laser 120 such that the skin is not elevated in temperature either by the heat flow from heated window 170 or the heat dissipated by the laser 120. This allows the thermoelectric cooler 200 to achieve a lower temperature on its cold side for fixed power consumption because the temperature of the heat sink 210 to the hot side of the cooler 200 is minimized. If cooling is not required, the thermoelectric cooler 200 can be omitted and the detector 190 can be mounted directly to heat sink 210.
Blood perfusion will be high if skin temperature of about 40° C. is maintained in the neighborhood of the heated window 170 which is in contact with the skin. In some embodiments, the portion of the biological tissue that is heated has a temperature greater than 20° C. and less than 50° C. The block 140 conducts heat from the laser 120 to the base 220 in the neighborhood of the heated window 170 where blood perfusion is still high, but the temperature is not as highly elevated as at the heated window 170 because of the insulation provided by the window retainer 175. This arrangement provides both low thermal impedance and a lower heat sink temperature for the laser 120.
In one embodiment,
In another preferred embodiment, window retainer 175 can be made with high thermal conductivity, for example, exceeding 40 W/m° K. The heat sink for the laser 120 and the heated window 170 would then be thermally connected. This arrangement can be advantageous when the blood perfusion is required to be increased for other reasons besides improving heat sinking and it is desired to do so by heating with minimum power consumption. The proposed arrangement would then utilize the heat generated by the laser 120 to heat the skin by means of thermal conduction through items 130, 140, and 220, allowing reduced heating from the heater 160 and lower net power consumption.
Although the detailed description contains many specifics, these should not be construed as limiting the scope of the invention, but merely as illustrating different examples and aspects of the invention. It should be appreciated that the scope of the invention includes other embodiments not discussed in detail above. Various other modifications, changes and variations which will be apparent to those skilled in the art may be made in the arrangement, and details of the apparatus of the invention disclosed herein without departing from the spirit and scope of the invention.
This application claims priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application Ser. No. 61/184,056, filed Jun. 4, 2009, entitled “Apparatus For Increasing Blood Perfusion And Improving Heat Sinking To Skin,” which is incorporated herein by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 61184056 | Jun 2009 | US |
