Abstract:
An electric warming blanket for warming patients during surgery and other medical procedures includes a flexible heater and a temperature sensor assembly coupled thereto; a first layer of water resistant material coupled to a second layer of water resistant material, about a perimeter of the heater, forms a substantially hermetically sealed space for the heater and the temperature sensor assembly. The blanket may further include a thermal insulation layer disposed between the temperature sensor assembly and the first layer of water resistant material. The temperature sensor assembly may provide input of an average temperature over a portion of a surface area of the heater to a temperature controller, when the heater and sensor assembly are coupled to the controller.

Description:
PRIORITY CLAIM 
       [0001]    The present application is a divisional of U.S. application Ser. No. 11/537,189, which was filed on Sep. 29, 2006 and claims priority to each of the following provisional applications: Ser. No. 60/825,573, entitled HEATING BLANKET SYSTEM filed on Sep. 13, 2006; Ser. No. 60/722,106, entitled ELECTRIC WARMING BLANKET INCLUDING TEMPERATURE ZONES AUTOMATICALLY OPTIMIZED, filed Sep. 29, 2005; and Ser. No. 60/722,246, entitled HEATING BLANKET, filed Sep. 29, 2005. Each of the above-referenced applications are hereby incorporated by reference, in their entireties, herein. 
       RELATED APPLICATIONS 
       [0002]    The present application is related to the following co-pending and commonly assigned utility patent applications, all of which are hereby incorporated by reference in their entireties: A) ELECTRIC WARMING BLANKET HAVING OPTIMIZED TEMPERATURE ZONES, Ser. No. 11/537,173; B) NOVEL DESIGNS FOR HEATING BLANKETS AND PADS, Ser. No. 11/537,179; C) FLEXIBLE HEATING ELEMENT CONSTRUCTION, Ser. No. 11/537,199; D) BUS BAR ATTACHMENTS FOR FLEXIBLE HEATING ELEMENTS, Ser. No. 11/537,212; and E) BUS BAR INTERFACES FOR FLEXIBLE HEATING ELEMENTS, Ser. No. 11/537,222. 
     
    
     TECHNICAL FIELD 
       [0003]    The present invention is related to heating or warming blankets or pads and more particularly to those including electrical heating elements. 
       BACKGROUND 
       [0004]    It is well established that surgical patients under anesthesia become poikilothermic. This means that the patients lose their ability to control their body temperature and will take on or lose heat depending on the temperature of the environment. Since modern operating rooms are all air conditioned to a relatively low temperature for surgeon comfort, the majority of patients undergoing general anesthesia will lose heat and become clinically hypothermic if not warmed. 
         [0005]    Over the past 15 years, forced-air warming (FAW) has become the “standard of care” for preventing and treating the hypothermia caused by anesthesia and surgery. FAW consists of a large heater/blower attached by a hose to an inflatable air blanket. The warm air is distributed over the patient within the chambers of the blanket and then is exhausted onto the patient through holes in the bottom surface of the blanket. 
         [0006]    Although FAW is clinically effective, it suffers from several problems including: a relatively high price; air blowing in the operating room, which can be noisy and can potentially contaminate the surgical field; and bulkiness, which, at times, may obscure the view of the surgeon. Moreover, the low specific heat of air and the rapid loss of heat from air require that the temperature of the air, as it leaves the hose, be dangerously high—in some products as high as 45° C. This poses significant dangers for the patient. Second and third degree burns have occurred both because of contact between the hose and the patient&#39;s skin, and by blowing hot air directly from the hose onto the skin without connecting a blanket to the hose. This condition is common enough to have its own name—“hosing.” The manufacturers of forced air warming equipment actively warn their users against hosing and the risks it poses to the patient. 
         [0007]    To overcome the aforementioned problems with FAW, several companies have developed electric warming blankets. However, there is still a need for electrically heated blankets or pads that can be used safely and effectively warm patients undergoing surgery or other medical treatments. These blankets need to be flexible in order to effectively drape over the patient (making excellent contact for conductive heat transfer and maximizing the area of the patient&#39;s skin receiving conductive as well as radiant heat transfer), and should incorporate means for precise temperature control. 
         [0008]    Precise temperature control is important because non-uniform heat distribution can occur within an electric warming blanket. Unfortunately, many temperature sensors used to provide feedback to a temperature controller do not dependably report an accurate average temperature of the blanket because they sense temperature from too small of an area. For example, if the temperature of a measured location is cooler than the average blanket temperature, the temperature sensor will cause the controller to deliver more power to the heater and the resulting average temperature of the heater will be higher than desired. 
         [0009]    Further, an electric blanket can overheat if the temperature sensor is thermally grounded to a cool object. This condition can occur if a cool object such as a metal pan is placed on top of the heater in the area of the temperature sensor. The sensor “feels” cool and tells the temperature controller to deliver more power to the heater. 
         [0010]    Accordingly, there is a need for a blanket that utilizes a temperature sensor that takes temperature measurements that are representative of the average temperature of the blanket. Further, there is a need for a blanket with a temperature sensor that will not cause the blanket to overheat if a cool object is placed in proximity to it. Various embodiments of the invention described herein solve one or more of the problems discussed above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The following drawings are illustrative of particular embodiments of the present invention and therefore do not limit the scope of the invention. The drawings are not to scale (unless so stated) and are intended for use in conjunction with the explanations in the following detailed description. Embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, wherein like numerals denote like elements. 
           [0012]      FIG. 1A  is a plan view of a flexible heating blanket subassembly for a heating blanket, according to some embodiments of the present invention. 
           [0013]      FIGS. 1B-C  are end views of two embodiments of the subassembly shown in  FIG. 1A . 
           [0014]      FIG. 1D  is a schematic showing a blanket including the subassembly of  FIG. 1A  draped over a body. 
           [0015]      FIG. 2A  is a top plan view of a heating element assembly, according to some embodiments of the present invention, which may be incorporated in the blanket shown in  FIG. 3A . 
           [0016]      FIG. 2B  is a section view through section line A-A of  FIG. 2A . 
           [0017]      FIG. 2C  is an enlarged plan view and corresponding end view schematic of a portion of the assembly shown in  FIG. 2A , according to some embodiments of the present invention. 
           [0018]      FIG. 2D  is an enlarged view of a portion of the assembly shown in  FIG. 2A , according to some embodiments of the present invention. 
           [0019]      FIG. 3A  is a top plan view, including partial cut-away views, of a lower body heating blanket, according to some embodiments of the present invention. 
           [0020]      FIG. 3B  is a schematic side view of the blanket of  FIG. 3A  draped over a lower body portion of a patient. 
           [0021]      FIG. 3C  is a top plan view of a heating element assembly, which may be incorporated in the blanket shown in  FIG. 3A . 
           [0022]      FIG. 3D  is a cross-section view through section line D-D of  FIG. 3C . 
           [0023]      FIG. 4A  is a plan view of flexible heating element, according to some alternate embodiments of the present invention. 
           [0024]      FIG. 4B  is a top plan view, including a partial cut-away view, of a heating element assembly, according to some embodiments of the present invention, which may be incorporated in the blanket shown in  FIG. 4C . 
           [0025]      FIG. 4C  is a top plan view, including a partial cut-away view, of an upper body heating blanket, according to some embodiments of the present invention. 
           [0026]      FIG. 4D  is a schematic end view of the blanket of  FIG. 4B  draped over an upper body portion of a patient. 
       
    
    
     DETAILED DESCRIPTION 
       [0027]    The following detailed description is exemplary in nature and is not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the following description provides practical illustrations for implementing exemplary embodiments of the present invention. Examples of constructions, materials, dimensions, and manufacturing processes are provided for selected elements, and all other elements employ that which is known to those of skill in the field of the invention. Those skilled in the art will recognize that many of the examples provided have suitable alternatives that can be utilized. The term ‘blanket’, used to describe embodiments of the present invention, may be considered to encompass heating blankets and pads. 
         [0028]      FIG. 1A  is a plan view of a flexible heating blanket subassembly  100 , according to some embodiments of the present invention; and  FIGS. 1B-C  are end views of two embodiments of the subassembly shown in  FIG. 1A .  FIG. 1A  illustrates a flexible sheet-like heating element, or heater,  10  of subassembly  100  including a first end  101 , a second end  102 , a first lateral portion  11  extending between ends  101 ,  102 , and a second lateral portion  12 , opposite first lateral portion  11 , also extending between ends  101 ,  102 . According to preferred embodiments of the present invention, heater  10  comprises a conductive fabric or a fabric incorporating closely spaced conductive elements such that heater  10  has a substantially uniform watt density output, preferably less than approximately 0.5 watts/sq. inch, and more preferably between approximately 0.2 and approximately 0.4 watts/sq. inch, across a surface area, of one or both sides  13 ,  14  ( FIGS. 1B-C ), the surface area including and extending between lateral portions  11 ,  12  of heater  10 . Some examples of conductive fabrics which may be employed by embodiments of the present invention include, without limitation, carbon fiber fabrics, fabrics made from carbonized fibers, woven or non-woven non-conductive substrates coated with a conductive material, for example, polypyrrole, carbonized ink, or metalized ink. 
         [0029]      FIG. 1A  further illustrates subassembly  100  including two bus bars  15  coupled to heating element  10  for powering element  10 ; each bar  15  is shown extending alongside opposing lateral portions  11 ,  12 , between first and second ends  101 ,  102 . With reference to  FIG. 1B , according to some embodiments, bus bars  15  are coupled to heating element  10  within folds of opposing wrapped perimeter edges  108  of heating element  10  by a stitched coupling  145 , for example, formed with conductive thread such as silver-coated polyester or nylon thread (Marktek Inc., Chesterfield, Mo.), extending through edges  108  of heating element  10 , bars  15 , and again through heating element  10  on opposite side of bars  15 . According to alternate embodiments heating element  10  is not folded over bus bars  15  as shown. Alternative threads or yarns employed by embodiments of the present invention may be made of other polymeric or natural fibers coated with other electrically conductive materials; in addition, nickel, gold, platinum and various conductive polymers can be used to make conductive threads. Metal threads such as stainless steel, copper or nickel could also be used for this application. According to an exemplary embodiment, bars  15  are comprised of flattened tubes of braided wires, such as are known to those skilled in the art, for example, a flat braided silver coated copper wire, and may thus accommodate the thread extending therethrough, passing through openings between the braided wires thereof. In addition such bars are flexible to enhance the flexibility of blanket subassembly  100 . According to alternate embodiments, bus bars  15  can be a conductive foil or wire, flattened braided wires not formed in tubes, an embroidery of conductive thread, or a printing of conductive ink. Preferably, bus bars  15  are each a flat braided silver-coated copper wire material, since a silver coating has shown superior durability with repeated flexion, as compared to tin-coated wire, for example, and may be less susceptible to oxidative interaction with a polypyrrole coating of heating element  10  according to an embodiment described below. Additionally, an oxidative potential, related to dissimilar metals in contact with one another is reduced if a silver-coated thread is used for stitched coupling  145  of a silver-coated bus bar  15 . 
         [0030]    According to an exemplary embodiment, a conductive fabric comprising heating element  10  comprises a non-woven polyester having a basis weight of approximately 130 g/m 2  and being 100% coated with polypyrrole (available from Eeonyx Inc., Pinole, Calif.); the coated fabric has an average resistance, for example, determined with a four point probe measurement, of approximately 15-20 ohms per square inch at about 48 volts, which is suitable to produce the preferred watt density of 0.2 to 0.4 watts/sq. in. for surface areas of heating element  10  having a width, between bus bars  15 , in the neighborhood of about 20 inches. Such a width is suitable for a lower body heating blanket, some embodiments of which will be described below. A resistance of such a conductive fabric may be tailored for different widths between bus bars (wider requiring a lower resistance and narrower requiring a higher resistance) by increasing or decreasing a surface area of the fabric that can receive the conductive coating, for example by increasing or decreasing the basis weight of the fabric. Resistance over the surface area of the conductive fabrics is generally uniform in many embodiments of the present invention. However, the resistance over different portions of the surface area of conductive fabrics such as these may vary, for example, due to variation in a thickness of a conductive coating, variation within the conductive coating itself, variation in effective surface area of the substrate which is available to receive the conductive coating, or variation in the density of the substrate itself. Local surface resistance across a heating element, for example heater  10 , is directly related to heat generation according to the following relationship: 
         [0000]        Q (Joules)= I   2 (Amps)× R (Ohms) 
         [0000]    Variability in resistance thus translates into variability in heat generation, which is measured as a temperature. According to preferred embodiments of the present invention, which are employed to warm patients undergoing surgery, precise temperature control is desirable. Means for determining heating element temperatures, which average out temperature variability caused by resistance variability across a surface of the heating element, are described below in conjunction with  FIGS. 2A-B . 
         [0031]    A flexibility of blanket subassembly  100 , provided primarily by flexible heating element  10 , and optionally enhanced by the incorporation of flexible bus bars, allows blanket subassembly  100  to conform to the contours of a body, for example, all or a portion of a patient undergoing surgery, rather than simply bridging across high spots of the body; such conformance may optimize a conductive heat transfer from element  10  to a surface of the body. However, as illustrated in  FIG. 1D , heating element  10  may be draped over a body  16  such that lateral portions  11 ,  12  do not contact side surfaces of body  16 ; the mechanism of heat transfer between portions  11 ,  12  and body  16 , as illustrated in  FIG. 1D , is primarily radiant with some convection. 
         [0032]    The uniform watt-density output across the surface areas of preferred embodiments of heating element  10  translates into generally uniform heating of the surface areas, but not necessarily a uniform temperature. At locations of heating element  10  which are in conductive contact with a body acting as a heat sink, for example, body  16 , the heat is efficiently drawn away from heating element  10  and into the body, for example by blood flow, while at those locations where element  10  does not come into conductive contact with the body, for example lateral portions  11 ,  12  as illustrated in  FIG. 1D , an insulating air gap exists between the body and those portions, so that the heat is not drawn off those portions as easily. Therefore, those portions of heating element  10  not in conductive contact with the body will gain in temperature, since heat is not transferred as efficiently from these portions as from those in conductive contact with the body. The ‘non-contacting’ portions will reach a higher equilibrium temperature than that of the ‘contacting’ portions, when the radiant and convective heat loss equal the constant heat production through heating element  10 . Although radiant and convective heat transfer are more efficient at higher heater temperatures, the laws of thermodynamics dictate that as long as there is a uniform watt-density of heat production, even at the higher temperature, the radiant and convective heat transfer from a blanket of this construction will result in a lower heat flux to the skin than the heat flux caused by the conductive heat transfer at the ‘contacting’ portions at the lower temperature. Even though the temperature is higher, the watt-density is uniform and, since the radiant and convective heat transfer are less efficient than conductive heat transfer, the ‘non-contacting’ portions must have a lower heat flux. Therefore, by controlling the ‘contacting’ portions to a safe temperature, for example, via a temperature sensor  121  coupled to heating element  10  in a location where element  10  will be in conductive contact with the body, as illustrated in  FIG. 1D , the ‘non-contacting’ portions, for example, lateral portions  11 ,  12 , will also be operating at a safe temperature because of the less efficient radiant and convective heat transfer. According to preferred embodiments, heating element  10  comprises a conductive fabric having a relatively small thermal mass so that when a portion of the heater that is operating at the higher temperature is touched, suddenly converting a ‘non-contacting’portion into a ‘contacting’ portion, that portion will cool almost instantly to the lower operating temperature. 
         [0033]    According to embodiments of the present invention, zones of heating element  10  may be differentiated according to whether or not portions of element  10  are in conductive contact with a body, for example, a patient undergoing surgery. In the case of conductive heating, gentle external pressure may be applied to a heating blanket including heating element  10 , which pressure forces heating element  10  into better conductive contact with the patient to improve heat transfer. However, if excessive pressure is applied the blood flow to that skin may be reduced at the same time that the heat transfer is improved and this combination of heat and pressure to the skin can be dangerous. It is well known that patients with poor perfusion should not have prolonged contact with conductive heat in excess of approximately 42° C. 42° C. has been shown in several studies to be the highest skin temperature, which cannot cause thermal damage to normally perfused skin, even with prolonged exposure. (Stoll &amp; Greene, Relationship between pain and tissue damage due to thermal radiation. J. Applied Physiology 14(3):373-382, 1959. and Moritz and Henriques, Studies of thermal injury: The relative importance of time and surface temperature in the causation of cutaneous burns. Am. J. Pathology 23:695-720, 1947) Thus, according to certain embodiments of the present invention, the portion of heating element  10  that is in conductive contact with the patient is controlled to approximately 43° C. in order to achieve a temperature of about 41-42° C. on a surface a heating blanket cover that surrounds element  10 , for example, a cover or shell  20 ,  40  which will be described below in conjunction with  FIGS. 3A and 4C . With further reference to  FIG. 1D , flaps  125  are shown extending laterally from either side of heating element  10  in order to enclose the sides of body  16  thereby preventing heat loss; according to preferred embodiments of the present invention, flaps  125  are not heated and thus provide no thermal injury risk to body if they were to be tucked beneath sides of body  16 . 
         [0034]    Referring now to the end view of  FIG. 1C , an alternate embodiment to that shown in  FIG. 1B  is presented.  FIG. 1C  illustrates subassembly  100  wherein insulating members  18 , for example, fiberglass material strips having an optional PTFE coating and a thickness of approximately 0.003 inch, extend between bus bars  15  and heating element  10  at each stitched coupling  145 , so that electrical contact points between bars  15  and heating element  10  are solely defined by the conductive thread of stitched couplings  145 . 
         [0035]      FIG. 2A  is a top plan view of a heating element assembly  250 , according to some embodiments of the present invention, which may be incorporated by blanket  200 , which is shown in  FIG. 3A  and further described below.  FIG. 2B  is a section view through section line A-A of  FIG. 2A .  FIGS. 2A-B  illustrate a temperature sensor assembly  421  assembled on side  14  of heater  10 , and heater  10  overlaid on both sides  13 ,  14  with an electrically insulating layer  210 , preferably formed of a flexible non-woven high loft fibrous material, for example, 1.5 OSY (ounces per square yard) nylon, which is preferably laminated to sides  13 ,  14  with a hotmelt laminating adhesive. In some embodiments, the adhesive is applied over the entire interfaces between layer  210  and heater  10 . Other examples of suitable materials for layer  210  include, without limitation, polymeric foam, a woven fabric, such as cotton or fiberglass, and a relatively thin plastic film. According to preferred embodiments, overlaid layers  210 , without compromising the flexibility of heating assembly  250 , prevent electrical shorting of one portion of heater  10  with another portion of heater  10  if heater  10  is folded over onto itself. Heating element assembly  250  may be enclosed within a relatively durable and waterproof shell, for example shell  20  shown with dashed lines in  FIG. 2B , and will be powered by a relatively low voltage (approximately 48V). Layers  210  may even be porous in nature to further maintain the desired flexibility of assembly  250 . 
         [0036]      FIG. 2C  is an enlarged plan view and a corresponding end view schematic showing some details of the corner of assembly  250  that is circled in  FIG. 2A , according to some embodiments.  FIG. 2C  is representative of each corner of assembly  250 .  FIG. 2C  illustrates insulating layer  210  disposed over side  14  of heater  10  and extending beneath bus bar  15 , optional electrical insulating member  18 , and layer  210  disposed over side  13  of heater  10  and terminated adjacent bus bar  15  within lateral portion  12  so that threads of conductive stitching  145  securing bus bars  15  to heater  10  electrically contact heating element  10  along side  13  of heating element  10 .  FIG. 2C  further illustrates two rows of conductive stitching  145  coupling bus bar  15  to heating element  10 , and bus bar  15  and insulating member  18  extending past end  102 . 
         [0037]      FIG. 2A  further illustrates junctions  50  coupling leads  205  to each bus bar  15 , and another lead  221  coupled to and extending from temperature sensor assembly  421 ; each of leads  205 ,  221  extend over insulating layer  210  and into an electrical connector housing  225  containing a connector  23 , which will be described in greater detail below, in conjunction with  FIGS. 3A-C .  FIG. 2D  is an enlarged view of junction  50 , which is circled in  FIG. 2A , according to some embodiments of the present invention.  FIG. 2D  illustrates junction  50  including a conductive insert  55  which has been secured to bus bar  15 , for example, by inserting insert  55  through a side wall of bus bar  15  and into an inner diameter thereof.  FIG. 2D  further illustrates lead  205  coupled to insert  55 , for example, via soldering, and an insulating tube and strain relief  54 , for example, a polymer shrink tube, surrounding the coupling between lead  205  and insert  55 . 
         [0038]    Returning now to  FIG. 2B , temperature sensor assembly  421  will be described in greater detail.  FIG. 2B  illustrates assembly  421  including a substrate  211 , for example, of polyimide (Kapton), on which a temperature sensor  21 , for example, a surface mount chip thermistor (such as a Panasonic ERT-J1VG103FA: 10K, 1% chip thermistor), is mounted; a heat spreader  212 , for example, a copper or aluminum foil, is mounted to an opposite side of substrate  211 , for example, being bonded with a pressure sensitive adhesive; substrate  211  is relatively thin, for example about 0.0005 inch thick, so that heat transfer between heat spreader  212  and sensor is not significantly impeded. Temperature sensor assembly  421  may be bonded to layer  210  with an adhesive layer  213 , for example, hotmelt EVA. Although not shown, it should be noted that sensor assembly  421  may be potted with a flexible electrically insulating material, such as silicon or polyurethane. 
         [0039]    According to the illustrated embodiment, heat spreader  212  is sized to contact an enlarged surface area so that a temperature sensed by sensor  21  is more representative of an average temperature over a region of heater  10  surrounding sensor  21 , which is positioned such that, when a heating blanket including heater  10  is placed over a body, the regions surrounding sensor  21  will be in conductive contact with the body. As previously described, it is desirable that a temperature of approximately 43° C. be maintained over a surface of heater  10  which is in conductive contact with a body of a patient undergoing surgery. Other types of heat spreaders, in addition to metallic foils, include metallic meshes or screens, or an adhesive/epoxy filled with a thermally conductive material. 
         [0040]    Heat spreader  212  is a desirable component of a temperature sensor assembly, according to some embodiments of the present invention, since conductive fabrics employed by heating element  10 , such as those previously described, may not exhibit uniform resistance across surface areas thereof. Heat spreader  212 , having a surface area that does not exceed approximately four square inches, according to a preferred embodiment, may be effective in averaging out relatively small scale spatial resistance variation, for example, about 3% to 10% variability over less than about one or two inches. Such a limitation on heat spreader  212  surface area may be necessary so that heat spreader  212  does not become too bulky, since the larger the surface area, the greater the thickness of spreader  212  needed in order to maintain effective heat transfer across spreader  212  and to sensor  21 . In addition, if spreader  212  is too thick, a thermal mass of spreader  212  will cause spreader  212  to respond too slowly to changes in heat loss or gain by heating element. According to an exemplary embodiment of the present invention, spreader  212  has a surface area of no greater than approximately four square inches and a thickness of no greater than approximately 0.001 inch. Some alternate embodiments of the present invention address a non-uniform resistance across a surface area of element  10  by employing a distributed temperature sensor, for example, a resistance temperature detector (RTD) laid out in flat plane across a surface of heater  10 , or by employing an infrared temperature measurement device positioned to receive thermal radiation from a given area of heater  10 . An additional alternate embodiment is contemplated in which an array of temperature sensors are positioned over the surface of heater  10 , being spaced apart so as to collect temperature readings which may be averaged to account for resistance variance. 
         [0041]    According to a preferred embodiment, assembly  421  includes a second, redundant, temperature sensor mounted to substrate  211 , close enough to sensor  21  to detect approximately the same temperature; while sensor  21  may be coupled to a microprocessor temperature control, the second sensor, for example, a chip thermistor similar to sensor  21 , may be coupled to an analog over-temperature cutout that cuts power to element  10 , and/or sends a signal triggering an audible or visible alarm. The design of the second sensor may be the same as the first sensor and need not be described again. Another safety check may be provided by mounting an identification resistor to substrate  211  in order to detect an increase in resistance of element  10 , due, for example, to degradation of the material of element  10 , or a fractured bus bar; the optional identification resistor monitors a resistance of heating element  10  and compares the measured resistance to an original resistance of element  10 . 
         [0042]    According to some embodiments of the present invention, for example as illustrated in  FIG. 2A , super over-temperature sensors  41  are incorporated to detect overheating of areas of assembly  250  susceptible to nicking, that is areas, for example, lateral portions  11 ,  12 , where assembly  250  is most likely to be folded over on itself, either inadvertently or on purpose to gain access to a portion of a patient disposed beneath a blanket including assembly  250 . An area of assembly  250  which is beneath the folded-over portion of assembly  250 , and not in close proximity to sensor assembly  421 , can become significantly warmer due to the additional thermal insulation provided by the folded-over portion that goes undetected by sensor  21 . According to preferred embodiments, sensors  41  are wired in series, as illustrated in  FIG. 2A . Super over-temperature sensors  41  may be set to open, or significantly increase resistance in, a circuit, for example, the over-temperature circuit, thereby activating an alarm and/or cutting power to heating element  10 , at prescribed temperatures that are significantly above the normal operating range, for example, temperatures between approximately 45° C. and approximately 60° C. Alternately, sensors  41  may be part of the bus bar power circuit, in which case sensors  41  directly shut down power to heating element  10  when in an open condition or add sufficient resistance when in a high resistance condition to substantially reduce heating of element  10 . 
         [0043]      FIG. 3A  is a top plan view, including partial cut-away views, of a lower body heating blanket  200 , according to some embodiments of the present invention, which may be used to keep a patient warm during surgery.  FIG. 3A  illustrates blanket  200  including heating element assembly  250  covered by flexible shell  20 ; shell  20  protects and isolates assembly  250  from an external environment of blanket  200  and may further protect a patient disposed beneath blanket  200  from electrical shock hazards. According to preferred embodiments of the present invention, shell  20  is waterproof to prevent fluids, for example, bodily fluids, IV fluids, or cleaning fluids, from contacting assembly  250 , and may further include an anti-microbial element, for example, being a SILVERion™ antimicrobial fabric available from Domestic Fabrics Corporation. According to the illustrated embodiment, blanket  200  further includes a layer of thermal insulation  201  extending over a top side (corresponding to side  14  of heating element  10 ) of assembly  250 ; layer  201  may or may not be bonded to a surface of assembly  250 . Layer  201  may serve to prevent heat loss away from a body disposed on the opposite side of blanket  200 , particularly if a heat sink comes into contact with the top side of blanket  200 .  FIG. 3C  illustrates insulation  201  extending over an entire surface of side  14  of heating element  10  and over sensor assembly  421 . According to the illustrated embodiment, layer  201  is secured to heating element assembly  250  to form an assembly  250 ′, as will be described in greater detail below. According to an exemplary embodiment of the present invention, insulating layer  201  comprises a polymer foam, for example, a 1 pound density 30 ILD urethane foam, which has a thickness between approximately ⅛ th  inch and approximately ¾ th  inch. According to alternate embodiments layer  201  comprises any, or a combination of the following: high loft fibrous polymeric non-woven material, non-woven cellulose material, and air, for example, held within a polymeric film bubble. 
         [0044]      FIG. 3A  further illustrates shell  20  forming flaps  25  extending laterally from either side of assembly  250  and a foot drape  26  extending longitudinally from assembly  250 . According to exemplary embodiments of the present invention, a length of assembly  250  is either approximately 28 inches or approximately 48 inches, the shorter length providing adequate coverage for smaller patients or a smaller portion of an average adult patient.  FIG. 3B  is a schematic side view of blanket  200  draped over a lower body portion of a patient. With reference to  FIG. 3B  it may be appreciated that flaps  25 , extending down on either side of the patient, and foot drape  26 , being folded under and secured by reversible fasteners  29  ( FIG. 3A ) to form a pocket about the feet of the patient, together effectively enclose the lower body portion of the patient to prevent heat loss. With reference to  FIG. 2A , in conjunction with  FIG. 3B , it may be appreciated that temperature sensor assembly  421  is located on assembly  250  so that, when blanket  200  including assembly  250  is draped over the lower body of the patient, the area of heating element  10  surrounding sensor assembly  421  will be in conductive contact with one of the legs of the patient in order to maintain a safe temperature distribution across element  10 . 
         [0045]    According to some embodiments of the present invention, shell  20  includes top and bottom sheets extending over either side of assembly  250 ; the two sheets of shell  20  are coupled together along a seal zone  22  (shown with cross-hatching in the cut-away portion of  FIG. 3A ) that extends about a perimeter edge  2000  of blanket  200 , and within perimeter edge  2000  to form zones, or pockets, where a gap exists between the two sheets. 
         [0046]      FIG. 3A  further illustrates flaps  25  including zones where there are gaps between the sheets to enclose weighting members, which are shown as relatively flat plastic slabs  255 . Alternately flaps  25  can be weighted by attaching weighting members to exterior surfaces thereof. 
         [0047]      FIG. 3C  is a top plan view, including partial cut-away views, of heating element assembly  250 ′, which may be incorporated in blanket  200 ; and  FIG. 3D  is a cross-section view through section line D-D of  FIG. 3C .  FIGS. 3C-D  illustrates heating element assembly  250 ′ including heating element  10  overlaid with electrical insulation  210  on both sides  13 ,  14  and thermal insulation layer  201  extending over the top side  14  thereof (dashed lines show leads and sensor assembly beneath layer  201 ). According to the illustrated embodiment, layer  201  is inserted beneath a portion of each insulating member  18 , each which has been folded over the respective bus bar  15 , for example as illustrated by arrow B in  FIG. 1C , and then held in place by a respective row of non-conductive stitching  345  that extends through member  18 , layer  201  and heating element  10 . Although layer  210  is shown extending beneath layer  201  on side  14  of heating element, according to alternate embodiments, layer  201  independently performs as a thermal and electrical insulation so that layer  210  is not required on side  14  of heating element  10 . 
         [0048]    Returning now to  FIG. 2A , to be referenced in conjunction with  FIGS. 3A-C , connector housing  225  and connector  23  will be described in greater detail. According to certain embodiments, housing  225  is an injection molded thermoplastic, for example, PVC, and may be coupled to assembly  250  by being stitched into place, over insulating layer  210 .  FIG. 2A  shows housing  225  including a flange  253  through which such stitching can extend. With reference to  FIGS. 3A-B , it can be seen that connector  23  protrudes from shell  20  of blanket  200  so that an extension cable  330  may couple bus bars  15  to a power source  234 , and temperature sensor assembly  421  to a temperature controller  232 , both shown incorporated into a console  333 . In certain embodiments, power source  234  supplies a pulse-width-modulated voltage to bus bars  15 . The controller  232  may function to interrupt such power supply (e.g., in an over-temperature condition) or to modify the duty cycle to control the heating element temperature. According to the illustrated embodiment, a surface  252  of flange  253  of housing  225  protrudes through a hole formed in thermal insulating layer  201  ( FIG. 3C ) so that a seal  202  ( FIG. 3A ) may be formed, for example, by adhesive bonding and/or heat sealing, between an inner surface of shell  20  and surface  252 . 
         [0049]      FIGS. 3C-D  further illustrate a pair of securing strips  217 , each extending laterally from and alongside respective lateral portions  11 ,  12  of heating element  10  and each coupled to side  13  of heating element  10  by the respective row of stitching  345 . Another pair of securing strips  271  is shown in  FIG. 3C , each strip  271  extending longitudinally from and alongside respective ends  101 ,  102  of heating element  10  and being coupled thereto by a respective row of non-conductive stitching  354 . Strips  217  preferably extend over conductive stitching  145  on side  13  of heating element  10 , as shown, to provide a layer of insulation that can prevent shorting between portions of side  13  of heating element  10  if element  10  were to fold over on itself along rows of conductive stitching  145  that couple bus bars  15  to heating element  10 ; however, strips  217  may alternately extend over insulating member  18  on the opposite side of heating element  10 . According to the illustrated embodiment, securing strips  217  and  271  are made of a polymer material, for example polyurethane, so that they may be heat sealed between the sheets of shell  20  in corresponding areas of heat seal zone  22  in order to secure heating element assembly  250 ′ within the corresponding gap between the two sheets of shell  20  ( FIG. 3A ). 
         [0050]      FIG. 4A  is a plan view of flexible heating element  30 , according to some alternate embodiments of the present invention. Heating element  30  is similar in nature to previously described embodiments of heating element  10 , being comprised of a conductive fabric, or a fabric incorporating closely spaced conductive elements, for a substantially uniform watt density output, preferably less than approximately 0.5 watts/sq. inch. While a shape of the surface area of heating element  10  is suited for a lower body blanket, such as blanket  200 , that would cover a lower abdomen and legs of a patient ( FIG. 3B ) undergoing upper body surgery, the shape of a surface area of heating element  30  is suited for an upper body heating blanket, for example, blanket  300  shown in  FIG. 4C , that would cover outstretched arms and a chest area of a patient undergoing lower body surgery ( FIG. 4D ). With reference to  FIG. 4B , which shows heating element  30  incorporated into a heating element assembly  450 , it can be seen that bus bars  15  are coupled to element  30  alongside respective lateral edges  311 ,  312  ( FIG. 4A ). 
         [0051]      FIG. 4B  is a top plan view, including partial cut-away views, of heating element assembly  450 , according to some embodiments of the present invention, which may be incorporated in blanket  300  shown in  FIG. 4C .  FIG. 4B  illustrates assembly  450  having a configuration similar to that of assembly  250 ′, which is illustrated in  FIGS. 3C-D . According to the embodiment illustrated in  FIG. 4B , temperature sensor assembly  421  is coupled to heating element  30  at a location where element  30 , when incorporated in an upper body heating blanket, for example, blanket  300 , would come into conductive contact with the chest of a patient, for example as illustrated in  FIG. 4D , in order to maintain a safe temperature distribution across element  30 ; bus bar junctions  50  and connector housing  225  are located in proximity to sensor assembly  421  in order to keep a length of leads  205  and  221  to a minimum. With reference back to  FIGS. 3C-D , in conjunction with  FIG. 4B , an electrical insulating layer  310  of assembly  450  corresponds to insulating layers  210  of assembly  250 ′, a thermal insulating layer  301  of assembly  450  corresponds to layer  201  of assembly  250 ′, and securing strips  317  and  371  of assembly  450  generally correspond to strips  217  and  271 , respectively, of assembly  250 ′. 
         [0052]      FIG. 4C  is a top plan view, including partial cut-away views, of upper body heating blanket  300 , according to some embodiments of the present invention.  FIG. 4C  illustrates blanket  300  including heating element assembly  450  covered by a flexible shell  40 ; shell  40  protects and isolates assembly  450  from an external environment of blanket  300  and may further protect a patient disposed beneath blanket  300  from electrical shock hazards. According to the illustrated embodiment, shell  40 , like shell  20 , includes top and bottom sheets; the sheets extend over either side of assembly  450  and are coupled together along a seal zone  32  that extends around a perimeter edge  4000  and within edge  4000  to form various zones, or pockets, where gaps exist between the two sheets. The sheets of shell  40  may be heat sealed together along zone  32 , as previously described for the sheets of shell  20 . With reference to  FIG. 4B , securing strips  317  may be heat sealed between the sheets of shell  40  in corresponding areas of seal zone  32 , on either side of a central narrowed portion  39  of blanket  300 , in order to secure heating element assembly  450  within the corresponding gap between the two sheets of shell  40 . According to an alternate embodiment, for example, as shown with dashed lines in  FIG. 4A , lateral edges  311 ,  312  of heating element  30  extend out to form securing edges  27  that each include slots or holes  207  extending therethrough so that inner surfaces of sheets of shell  40  can contact one another to be sealed together and thereby hold edges  27 . It should be noted that either of blankets  200 ,  300 , according to alternate embodiments of the present invention, may include more than one heating element  10 ,  30  and more than one assembly  250 / 250 ′,  450 . 
         [0053]    With reference to  FIG. 4C , it may be appreciated that blanket  300  is symmetrical about a central axis  30  and about another central axis, which is orthogonal to axis  30 .  FIG. 4C  illustrates shell  40  forming flaps  35 A,  35 B and  350 , each of which having a mirrored counterpart across central axis  30  and across the central axis orthogonal to axis  30 . According to the illustrated embodiment, each of flaps  35 A, B include weighting members  305 , which are similar to members  255  of blanket  200 , and which may stiffen flaps  35 A,B (dashed lines indicate outlines of members  305  held between the sheets of cover  40  by surrounding areas of seal zone  32 ). 
         [0054]      FIG. 4C  further illustrates straps  38 , each extending between respective flaps  35 A-B. With reference to  FIG. 4D , which is a schematic end view of blanket  300  draped over an upper body portion of a patient, it may be appreciated that flaps  35 A-B and  350  extend downward to enclose the outstretched arms of the patient in order to prevent heat loss and that straps  38  secure blanket  300  about the patient. 
         [0055]    With further reference to  FIG. 4D , it may also be appreciated that, when blanket  300  is positioned over the patient, each strap  38  is positioned in proximity to an elbow of the patient so that either end portion of blanket  300 , corresponding to each pair of flaps  35 A, may be temporarily folded back, as illustrated, per arrow C, in order for a clinician to access the patient&#39;s arm, for example, to insert or adjust an IV. According to some embodiments of the present invention, super over-temperature sensors, for example, sensors  41 , previously described, are included in blanket  300  being located according to the anticipated folds, for example at general locations  410  illustrated in  FIGS. 4B-C , in order to detect over-heating, which may occur if blanket  300  is folded over on itself, as illustrated in  FIG. 4D , for too long a time, and, particularly, if flaps  35 A of folded-back portion of blanket are allowed to extend downward as illustrated with the dashed line in  FIG. 4D .  FIG. 4D  further illustrates connector cord  330  plugged into connector  23  to couple heating element  30  and temperature sensor assembly  421  of blanket  300  to control console  333 . 
         [0056]    In the foregoing detailed description, the invention has been described with reference to specific embodiments. However, it may be appreciated that various modifications and changes can be made without departing from the scope of the invention as set forth in the appended claims. Although embodiments of the invention are described in the context of a hospital operating room, it is contemplated that some embodiments of the invention may be used in other environments. Those embodiments of the present invention, which are not intended for use in an operating environment and need not meet stringent FDA requirements for repeated used in an operating environment, need not including particular features described herein, for example, related to precise temperature control. Thus, some of the features of preferred embodiments described herein are not necessarily included in preferred embodiments of the invention which are intended for alternative uses.