Patent Publication Number: US-11388782-B2

Title: Heating blanket

Description:
PRIORITY CLAIM 
     This application is a continuation of U.S. application Ser. No. 14/287,292, filed May 27, 2014, which is a continuation of U.S. application Ser. No. 13/460,368, filed Apr. 30, 2012 which issued as U.S. Pat. No. 8,772,676 on Jul. 8, 2014, which is a continuation of U.S. application Ser. No. 12/050,806, filed Mar. 18, 2008 which issued as U.S. Pat. No. 8,283,602 on Oct. 9, 2012, which is a non-provisional application of U.S. Provisional Application No. 60/895,736, filed Mar. 19, 2007, which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present invention is related to heating or warming blankets or pads and more particularly to those including electrical heating elements. 
     BACKGROUND 
     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. 
     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. 
     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. 
     To overcome the aforementioned problems with FAW, several companies have developed electric warming blankets. Some of these warming blankets employ flexible heaters, the flexibility of which is desirable to maintain when employing the blankets. In many cases, an electric warming blanket employs a shell for holding the heater and for serving other purposes. For example, in some cases the shell includes layers formed of a substantially water impermeable material to help prevent fluid damage to the heater. Also, when these heaters are used for patient or other care, especially in the operating room, the shell can protect the patient and others in the vicinity from electric shock hazards. In addition to often providing a seal around the heater, the shell often contains a fastening mechanism that must reliably attach the heater to the shell to prevent electrical shorting across the heater during folding of the electric warming blanket. 
     Because the seals of the shell must be very reliable, the seals have traditionally been adhesive seals that are reinforced with combinations of sewing, rivets, and grommets. Sewing stitches, rivets, and grommets all share one characteristic—they all perforate the material layers to create a mechanical linkage between the layers. 
     While such a reinforced bond may be desirable for strength, it can create additional problems when used during surgery or medical procedures. For example, heated blankets placed over a patient during a surgery or medical procedure are frequently soiled with waste blood or other body fluids. The fluid waste can saturate the stitching and then dry and accumulate in the thread or the stitch holes. If rivets or grommets are used for reinforcement, additional crevasses are introduced that can trap waste fluids. When the outer shell of the blanket is cleaned by hospital personnel, it is nearly impossible to clean the residual contaminating materials out of the holes, crevasses, and/or stitches. Therefore, the stitching holes and thread, the grommets, rivets and snaps can all become sources of microbial contamination because they cannot be thoroughly cleaned and disinfected. 
     Accordingly, there remains a need for heated blankets and shells for flexible heaters that is readily and thoroughly cleanable. Various embodiments of the invention described herein solve one or more of the problems discussed above in addition to other problems that will become apparent. 
     SUMMARY 
     Certain embodiments of the invention include an electric heating blanket including a flexible sheet-like heating element and a shell. The shell covers the heating blanket and includes two sheets of flexible material welded together. In some embodiments the weld couples the sheets together about the edges of the heating element. In some embodiments, the weld couples the sheets about the edges of the sheets. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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. 
         FIG. 1  is a top plan view of a heating blanket, according to some embodiments of the present invention. 
         FIG. 2A  is a plan view of a flexible heating blanket subassembly for a heating blanket, according to some embodiments of the present invention. 
         FIG. 2B  is an end view of some embodiments of the subassembly shown in  FIG. 2A . 
         FIG. 3A  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. 1 . 
         FIG. 3B  is a section view of the temperature sensor assembly of  FIG. 3A . 
         FIG. 4A  is a top plan view of a heating element assembly, which may be incorporated in the blanket shown in  FIG. 1 . 
         FIG. 4B  is a cross-section view through section line  4 B- 4 B of  FIG. 4A . 
         FIG. 5A  is a cross-section of a shell containing a heating element according to some embodiments of the present invention. 
         FIG. 5B  is a top plan view of the shell of  FIG. 5A . 
         FIG. 6  is a cross-section of a shell containing an air pocket according to some embodiments of the present invention. 
         FIG. 7A  is a top plan view of a shell having straps according to some embodiments of the present invention. 
         FIG. 7B  is a cross-section of the shell of  FIG. 7A . 
         FIG. 8  is a cross-section of a shell containing a heating element secured to the shell according to some embodiments of the present invention. 
         FIG. 9A  is a top plan view of a shell containing reinforced hanger points according to some embodiments of the present invention. 
         FIG. 9B  is a cross-section of the shell of  FIG. 9A . 
         FIG. 10A  is a cross-section of a shell containing a heating element, including an attachment point secured to the shell according to some embodiments of the present invention. 
         FIG. 10B  is a cross-section of a shell containing a heating element, including an attachment point secured to the shell according to some embodiments of the present invention. 
         FIG. 11  is a cross-section of two ends of a shell containing a heating element, including a securing magnet. 
     
    
    
     DETAILED DESCRIPTION 
     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. 
       FIG. 1  shows a heating blanket  100  according to some embodiments of the present invention. As shown, the heating blanket  100  is generally rectangular. Embodiments of the present invention can be used in connection with a wide variety of heating blankets. For example, in some cases, the heating blanket can be a blanket sized and shaped for the upper body or upper body limb (e.g., a wrap-around blanket), or a blanket sized and shaped for the lower body or lower body limb. In some cases the heating blanket can be used in conjunction with a disposable cover. 
     The heating blanket  100  of  FIG. 1  includes a shell  105  that can be durable and waterproof. As shown, a portion of the shell  105  is cut away, revealing a heating element assembly  350 . The heating element assembly  350  is generally covered by the shell and can extend within the shell  105  between edge  112  and edge  114  and between edge  116  and edge  118 . An electrical connector housing  325  and a corresponding connector plug  323  can be coupled to the shell  105 , thereby enabling access to a temperature sensor assembly such as those discussed below. 
     The shell  105  can protect and isolate the heating element assembly  350  from an external environment of heating blanket  100 . The shell  105  can include a water-resistant material layer that can form a substantially hermetic seal around the heating element assembly  350 . The shell  105  can provide further protection to a patient disposed beneath heating blanket  100  against electrical shock hazards. According to preferred embodiments of the present invention, shell  105  is waterproof to prevent fluids (e.g., bodily fluids, IV fluids, cleaning fluids, etc.) from contacting the heating element assembly  350 . In some preferred embodiments, shell  105  may further include an anti-microbial element (e.g., a SILVERion™ antimicrobial fabric available from Domestic Fabrics Corporation or Ultra-Fresh™ from Thomson Research Associates). 
     According to an illustrative embodiment of the present invention, shell  105  comprises a nylon fabric having an overlay of polyurethane coating to provide waterproofing. The coating can be on at least an inner surface of each of the two sheets, further facilitating a heat seal between the two sheets, according to preferred embodiments. In other embodiments, the shell  105  comprises polyvinyl chloride (PVC) to facilitate an RF weld to bond the sheets. It should be noted that, according to some embodiments of the present invention, a covering for heating element assemblies may be removable and, thus, include a reversible closure facilitating removal of a heating element assembly  350  therefrom and insertion of the same or another heating element assembly  350  therein. In some embodiments, shell  105  comprises a PVC film of sufficient thickness to provide the necessary strength. In some such embodiments, the edge seals can be softer. 
     In some embodiments, one or more layers may be positioned between the heating element assembly  350  and the shell  105 . For example, in some embodiments, a layer of thermally insulating material (e.g., polymeric foam or high-loft fibrous non-woven material) can be included in one or more locations. In some instances, a layer of thermally insulating material can be positioned to protect a portion of the patient from the heating element assembly  350  in the event that part of the shell  105  is inadvertently placed under that portion of the patient. In such instances, a layer of thermal insulating material can be positioned between the heating element assembly  350  and the patient-contacting surface of the shell  105 . In this way, in the event that part of the shell  105  is inadvertently placed under that portion of the patient, that portion of the patient can contact an insulated portion of the shell  105  rather than a non-insulated portion of the shell  105 . 
     In some instances a layer of thermally insulating material can be positioned to make sure that a maximal amount of heat being generated by the heating element assembly  350  is transferred to the patient. In such instances, a layer of thermally insulating material can help insulate the heating element assembly  350  from the environment and provide a more uniform temperature distribution. The layer of thermally insulating material can be positioned between the heating element assembly  350  and the surface of the shell  105  that does not contact the patient. In this way, a maximal amount of heat being generated by the heating element assembly  350  can be transferred to the patient and not to the surrounding environment. 
     In some instances a layer of thermally insulating material can be positioned to prevent caregivers from experiencing unwanted contact with activated heating blankets. Other layers (e.g., an electrically insulating layer similar to those discussed elsewhere herein) can be positioned between the heating element assembly  350  and the shell  105 . 
       FIGS. 2A-2B  show an illustrative heating blanket subassembly  300  that can be incorporated into heating element assemblies (e.g., heating element assembly  350  of  FIG. 1 ) in some embodiments of the present invention. Referring again to  FIGS. 2A-2B , in many embodiments, the heating blanket subassembly  300  is flexible. The heating blanket subassembly  300  can include a flexible sheet-like heating element  310 , or heater, which can include a first side edge  301  and a second side edge  302 . According to preferred embodiments of the present invention, heating element  310  comprises a conductive fabric or a fabric incorporating closely spaced conductive elements such that heating element  310  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  313 ,  314  ( FIG. 2B ). 
     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, conductive films, or woven or non-woven non-conductive fabric or film substrates coated with a conductive material, for example, polypyrrole, carbonized ink, or metalized ink. In many embodiments, the conductive fabric is a polymeric fabric coated with a conductive polymeric material such as polypyrrole. In addition, the flexible heating element  310  may be made from a matrix of electrically resistant wire or metal traces attached to a fibrous or film material layer. 
       FIG. 2A  further illustrates subassembly  300  including two bus bars  315  coupled to heating element  310  for powering heating element  310 . Each bar  315  is shown extending between first and second side edges  301 ,  302 . With reference to  FIG. 2B , according to some embodiments, bus bars  315  are coupled to heating element  310  by a stitched coupling  345  (e.g., formed with conductive thread such as silver-coated polyester or nylon thread (Marktek Inc., Chesterfield, Mo.)). 
     As shown, insulation is provided between the bus bars  315  and the heating element  310 .  FIG. 2B  illustrates subassembly  300  wherein insulating members  318  (e.g., fiberglass material strips having an optional PTFE coating and a thickness of approximately 0.003 inch) extend between bus bars  315  and heating element  310  at each stitched coupling  345 , so that electrical contact points between bars  315  and heating element  310  are solely defined by the conductive thread of stitched couplings  345 . Alternatively, the electrical insulation material layer could be made of polymeric film, a polymeric film reinforced with a fibrous material, a cellulose material, a glass fibrous material, rubber sheeting, polymeric or rubber coated fabric or woven materials or any other suitable electrically insulating material. 
     Each of the conductive thread stitches of coupling  345  can maintain a stable and constant contact with bus bar  315  on one side and heating element  310  on the other side of insulating member  318 . The stitches produce a stable contact in the face of any degree of flexion, so that the potential problem of intermittent contact between bus bar  315  and heating element  310  (that could arise for the embodiment shown in  FIG. 2B , where bus bar  315  is in physical contact with heating element  310 ) can be avoided. The stitches are the only electrical connection between bus bar  315  and heating element  310 , but, since the conductive thread has a much lower electrical resistance than the conductive fabric of heating element  310 , the thread does not heat under normal conditions. 
     In addition to heating blanket applications described herein, such a design for providing for a uniform and stable conductive interface between a bus bar and a conductive fabric heating element material can be used in other applications. For example, such a design can improve the conductive interface between a bus bar or electrode and a conductive fabric in non-flexible heating elements, in electronic shielding, in radar shielding and other applications of conductive fabrics. 
     In some preferred embodiments, coupling  345  includes two or more rows of stitches for added security and stability. However, due to the flexible nature of blanket subassembly  300 , the thread of stitched couplings  345  may undergo significant stresses. These stresses, over time and with multiple uses of a blanket containing subassembly  300 , could lead to one or more fractures along the length of stitched coupling  345 . Such a fracture, in other designs, could also result in intermittent contact points, between bus bar  315  and heating element  310 , that could lead to a thermal breakdown of heating element  310  along bus bar. But, if such a fracture were to occur in the embodiment of  FIG. 2B , insulating member  318  may prevent a thermal breakdown of heating element  310 , so that only the conductive thread of stitched coupling  345  melts down along bus bar  315 . According to some preferred embodiments, more than two rows of stitches are applied to each bus bar  315  for added safety and stability of the bus bar/heating element interface. 
     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  315  are comprised of flattened tubes of braided wires, such as are known to those skilled in the art (e.g., 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  300 . According to alternate embodiments, bus bars  315  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  315  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  310  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  345  of a silver-coated bus bar  315 . 
     According to an exemplary embodiment, a conductive fabric comprising heating element  310  comprises a non-woven polyester having a basis weight of approximately 170 g/m2 and being 100% coated with polypyrrole (available from Eeonyx Inc., Pinole, Calif.). The coated fabric has an average resistance (e.g., determined with a four point probe measurement) of approximately 15 ohms per square inch. This average resistance is suitable to produce the preferred watt density of 0.2 to 0.4 watts/sq. in. for surface areas of heating element  310  having a width, between bus bars  315 , in the neighborhood of about 19 to 28 inches, when powered at about 48 volts. In some embodiments, the basis weight of the non-woven polyester may be chosen in the range of approximately 80-180 g/m2. However, other basis weights may be engineered to operate adequately are therefore within the scope of embodiments of the invention. 
     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. In some instances, this can be achieved by increasing or decreasing the basis weight of the nonwoven. 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 (e.g., due to (a) variation in a thickness of a conductive coating, (b) variation within the conductive coating itself, (c) variation in effective surface area of the substrate which is available to receive the conductive coating, or (d) variation in the density of the substrate itself). Local surface resistance across a heating element, for example heating element  310 , is directly related to heat generation according to the following relationship:
 
 Q (Joules)= I 2(Amps)× R (Ohms)
 
     Variability in resistance thus translates into variability in heat generation, which can ultimately manifest as a variation in 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  FIG. 3A . 
     Referring again to  FIGS. 2A-2B , the flexibility of blanket subassembly  300  can allow blanket subassembly  300  to conform to the contours of a body (e.g., all or a portion of a patient undergoing surgery). This flexibility can be provided primarily by flexible heating element  310  and can be optionally enhanced by the incorporation of flexible bus bars. Conforming to the contours of a patient&#39;s body is preferable to simply bridging across high spots of the body. Such conformance may optimize a conductive heat transfer from heating element  310  to a surface of the body. 
     The uniform watt-density output across the surface areas of preferred embodiments of heating element  310  translates into generally uniform heating of the surface areas, but not necessarily a uniform temperature. For example, at locations of heating element  310  which are in conductive contact with a body acting as a heat sink, the heat is efficiently drawn away from heating element  310  and into the body (e.g., by blood flow). At the same time, at those locations where heating element  310  does not come into conductive contact with the body, 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  310  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  310 . Since the heat generation is generally uniform, the heat flux to the patient will also be generally uniform. However, at the non-contacting locations, the temperature is higher to achieve the same flux as the contacting portions. Some of the extra heat from the higher temperatures at the non-contacting portions can therefore be dissipated out the back of the pad instead of into the patient. 
     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 generally uniform heat flux from the blanket. Therefore, by controlling the ‘contacting’ portions to a safe temperature (e.g., via a temperature sensor assembly  321  coupled to heating element  310  in a location where heating element  310  will be in conductive contact with the body), the ‘non-contacting’ portions, will also be operating at a safe temperature because of the less efficient radiant and convective heat transfer. 
     According to preferred embodiments, heating element  310  comprises a conductive fabric having a relatively small thermal mass. When a portion of such a heating element 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. 
       FIGS. 3A-3B  show a heating element assembly  350  similar to the heating element assembly  350  of  FIG. 1 . Referring again to  FIGS. 3A-3B , the heating element assembly can include a temperature sensor assembly  321 . As shown, the temperature sensor assembly  321  is coupled to heating element  310  at a location where heating element  310  would come into conductive contact with the patient. This can assist in maintaining a safe temperature distribution across heating element  310 . The more constant the temperature information, the more the temperature controller can rely on it in controlling the heater temperature. In some embodiments, the temperature sensor assembly  321  can even be provided separately from the heating blanket. 
     According to embodiments of the present invention, zones of heating element  310  may be differentiated according to whether or not portions of heating element  310  are in conductive contact with a body (e.g., a patient undergoing surgery). In some embodiments, the threshold temperature is between 37 and 43° C. In one particular embodiment, the threshold temperature is 43° C. A temperature of 43° C. has been shown to provide beneficial warming to a patient without providing excessive heat. In the case of conductive heating, gentle external pressure may be applied to a heating blanket including heating element  310 . Such pressure conforms heating element  310  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 temperatures in excess of approximately 42° C. Several studies show 42° C. to be the highest skin temperature that 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  310  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 of a heating blanket cover that surrounds heating element  310  (e.g., shell  105  of  FIG. 1 ). 
       FIG. 3B  illustrates the temperature sensor assembly  321  assembled on side  314  of the heating element  310 . As shown, the heating element  310  is overlaid on both sides  313 ,  314  with an electrically insulating layer  330 . The electrically insulating layer  330  is preferably formed of a flexible non-woven very low loft fibrous material (e.g., 1.5 ounces-per-square-yard nylon), which is preferably laminated to sides  313 ,  314  with a hotmelt laminating adhesive. In some embodiments, the adhesive is applied over the entire interfaces between insulating layer  330  and heating element  310 . Other examples of suitable materials for insulating layer  330  include, without limitation, polymeric foam, a woven fabric, such as cotton or fiberglass, and a relatively thin plastic film, cotton, and a non-flammable material, such as fiberglass or treated cotton. According to preferred embodiments, overlaid insulating layers  330  prevent electrical shorting of one portion of heating element  310  with another portion of heating element  310  if heating element  310  is folded over onto itself. Many such embodiments prevent electrical shorting without compromising the flexibility of heating assembly  350 . Heating element assembly  350  may be powered by a relatively low voltage (approximately 48V). Insulating layers  330  may even be porous in nature to further maintain the desired flexibility of assembly  350 . 
     As shown in  FIG. 3A , an assembly of leads  305 ,  306  and junctions  355  can connect the bus bars  315  and the temperature sensor assembly  321  to an electrical connector housing  325 . Leads  305  couple the connector housing  325  to bus bars  315  at junctions  355 . Lead  306  couples the temperature sensor assembly  321  to the connector housing  325 . In many embodiments, leads  305 ,  306  extend over any insulating layer (e.g.,  330  in  FIG. 3B ) and into the electrical connector housing  325 . As is noted above (see discussion in connection with  FIG. 1 ) and discussed in greater detail below (see discussion in connection with  FIG. 4A ), electrical connector housing  325  can contain a connector plug  323 . 
     Returning now to  FIG. 3B , the illustrative temperature sensor assembly  321  will be described in greater detail. The temperature sensor assembly  321  can include a temperature sensor  351  (e.g., a surface mount chip thermistor (such as a Panasonic ERT-J1VG103FA: 10K, 1% chip thermistor)) soldered to an etched metal foil. In many embodiments, a substrate  331  (e.g., of polyimide (Kapton)) surrounds the temperature sensor  351 . A heat spreader  332  (e.g., a copper or aluminum foil) can be mounted to an opposite side of substrate  331  (e.g., being bonded with a pressure sensitive adhesive). Substrate  331  can be relatively thin (e.g., about 0.0005-inch thick) so that heat transfer between heat spreader  332  and sensor is not significantly impeded. 
     In some embodiments, the temperature sensor  351  is positioned such that the regions surrounding sensor  351  will be in conductive contact with the body when a heating blanket is placed over a body. As previously described, in many instances, it is desirable that a temperature of approximately 43° C. be maintained over a surface of heating element  310  which is in conductive contact with a body of a patient undergoing surgery. An additional alternate embodiment is contemplated in which an array of temperature sensors are positioned over the surface of heating element  310 , being spaced apart to collect temperature readings. In some such embodiments, the collected temperatures can be averaged to account for resistance variance. 
       FIGS. 4A-4B  show a heating element assembly  350  that may be incorporated into a heating blanket (e.g., heating blanket  100  of  FIG. 1 ). As shown, the heating element assembly  350  includes heating element  310  overlaid with electrical insulation  330  on both sides  313 ,  314  and thermal insulation layer  311  extending over the top side  314  thereof (dashed lines show leads and sensor assembly beneath layer  311 ). 
     A heating blanket may include a layer of thermal insulation  311  extending over a top side (corresponding to side  314  of heating element  310  as shown in  FIG. 2B ) of heating assembly  350  as discussed above. According to the illustrated embodiment, layer  311  is inserted beneath a portion of each insulating member  318 . The insulating members  318  have been folded over the respective bus bar  315  (e.g., as illustrated by arrow B in  FIG. 2B ), and then held in place by a respective row of non-conductive stitching  347  that extends through insulating member  318 , layer  311  and heating element  310 . Although not shown, it should be appreciated that layer  311  may further extend over bus bars  315 . Although insulating layer  330  is shown extending beneath layer  311  on side  314  of heating element  310 , according to alternate embodiments, layer  311  independently performs as a thermal and electrical insulation so that insulating layer  330  is not required on side  314  of heating element  310 .  FIG. 4A  further illustrates, with longitudinally extending dashed lines, a plurality of optional slits  303  in layer  311 , which may extend partially or completely through layer  311 , in order to increase the flexibility of assembly  350 . Such slits are desirable if a thickness or density of layer  311  is such that it prevents the heating blanket from draping effectively about a patient. The optional slits are preferably formed, for example, extending only partially through layer  311  starting from an upper surface thereof, to allow bending of the heating blanket about a patient and to prevent bending of the heating blanket in the opposition direction. 
     Returning now to  FIG. 3A , to be referenced in conjunction with  FIGS. 1 and 4A , connector housing  325  and connector plug  323  will be described in greater detail. According to certain embodiments, housing  325  is an injection molded thermoplastic (e.g., PVC) and may be coupled to assembly  350  by being stitched into place, over insulating layer  330 .  FIG. 3A  shows housing  325  including a flange  353  through which such stitching can extend. 
     Referring to  FIGS. 1 and 4A , in some embodiments, a surface of flange  353  of housing  325  protrudes through a hole formed in thermal insulating layer  311  so that a seal may be formed (e.g., by adhesive bonding and/or welding, such as heat sealing) between an inner surface of shell  105  and surface  352 . According to one embodiment, wherein housing  325  is injection molded PVC and the inner surface of shell  105  is likewise PVC, housing  325  is sealed to shell  105  via a solvent bond. It may be appreciated that the location of the connector plug  323  is suitable to keep the corresponding connector cord well away from the surgical field. In embodiments in which the inner surface of shell  105  is coated with polyurethane and the housing  325  is injection molded PVC, an intermediate adhesive can be used to allow for a heat seal connection (e.g., a solvent bond adhesive can be applied to the housing  325 , and the polyurethane film can be heat sealed to the exposed adhesive). 
       FIGS. 4A-4B  further illustrate a pair of securing strips  317 , each extending laterally from and alongside respective lateral portions of heating element  310 , parallel to bus bars  315 , and each coupled to side  313  of heating element  310  by the respective row of non-conductive stitching  347 . Another pair of securing strips  371  is shown in  FIG. 4A , each strip  371  extending longitudinally from and alongside respective side edges  301 ,  302  of heating element  310  and being coupled thereto by a respective row of non-conductive stitching  354 . Strips  371  may extend over layer  311  or beneath heating element  310 . As shown, strips  317  preferably extend over conductive stitching of stitched coupling  345  on side  313  of heating element  310 . The strips  317  can provide a layer of insulation that can prevent shorting between portions of side  313  of heating element  310  if heating element  310  were to fold over on itself along rows of conductive stitching of stitched coupling  345  that couple bus bars  315  to heating element  310 . In some embodiments, strips  317  may alternately extend over insulating member  318  on the opposite side of heating element  310 . According to the illustrated embodiment, securing strips  317  and  371  are made of a polymer material (e.g., PVC). They may be heat sealed between the sheets of shell ( 105  of  FIG. 1 ) in corresponding areas of the heat seal zone in order to secure heating element assembly  350  within a corresponding gap between the two sheets of shell ( 105  of  FIG. 1 ). According to an alternate embodiment, for example, shown by dashed lines in  FIGS. 2A and 4B , heating element  310  extends laterally out from each bus bar  315  to a securing edge  327 , which may include one or more slots or holes  307  extending therethrough so that inner surfaces of sheets of shell ( 105  of  FIG. 1 ) can contact one another to be sealed together and thereby hold edges  327 . 
     Referring to  FIG. 1 , connector plug  323  can protrude from shell  105  of the heating blanket  100 . An extension cable may couple the heating element assembly  350  to a console  60 . The console  60  includes a shut-off timer  30  and a power source  50  each coupled to a control system (or controller)  40 . The shut-off timer  30  can be operatively coupled to the control system  40 , meaning that the shut-off timer  30  can be integrated into the control system  40 , the shut-off timer  30  can be a separate component, or the shut-off timer  30  and the control system  40  can have any other suitable functional relationship. The temperature sensor assembly  321  can be configured to provide temperature information to the control system  40 , which may act as a temperature controller. The controller 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. 
     The power source  50  and power type can be any type known in the art. In certain embodiments, the power source  50  supplies a straight-line DC voltage to the control system  40 , and the control system  40  provides a pulse-width-modulated voltage (e.g., at a 75% duty cycle) to the heating element assembly  350 . Of course, other duty cycles and/or voltage levels can be used based on the design of the blanket and its heating element in order to achieve a desired threshold temperature in a reasonable amount of time. Too high of voltage or duty cycle, while decreasing the time to reach the desired temperature threshold, may increase the amount of temperature overshoot before the control system reduces or shuts off power. Moreover, in the case of temperature sensor failure, thermal runaway presents a greater concern with relatively higher voltage or duty cycle settings. Too low of a voltage or duty cycle may cause unreasonably long warm-up times. 
     As discussed above, warming blankets in accordance with embodiments of the invention include or make use of a shell or covering, such as shell  105  shown in  FIG. 1 . Several embodiments of such shells will now be described in greater detail, although it should be understood that these embodiments are for illustrative purposes only. 
       FIG. 5A  is a cross-section of a shell  500  containing a heating element  502  in accordance with some embodiments of the invention. The shell  500  can include a top sheet  504  and a bottom sheet  506  that are welded or coupled at one or more locations in order to define a pocket or pouch  508  that can enclose the heating element  502 . Any type of suitable weld may be used, such as heat welding (heat bonding), RF welding, ultrasonic welding, etc., depending on the type of materials used in sheets  504 ,  506 . Each sheet  504  and  506  can comprise a flexible, substantially water-resistant material and include the ability to be welded together. In some embodiments, the water-resistant material includes a single layer, and in some embodiments, sheets are comprised of a laminate of two or more layers. For instance, in some embodiments one or both of sheets  504 ,  506  are comprised of a single layer of polyvinyl chloride (PVC). In such embodiments where PVC is used, high frequency or RF welding (RF heat sealing) may be used to bond the sheets  504 ,  506  together. PVC sheets also provide a water-resistant material in order to protect the heating element  502  from fluids to which the heating blanket is exposed. 
     In some embodiments, one or both of sheets  504 ,  506  include respective strengthening layers  510 ,  512  that provide strength and color to the shell  500 . For example, the strengthening layers  510 ,  512  can be a fibrous material such as woven nylon. It will be appreciated that other materials can also be used for this layer. 
     With further reference to  FIG. 5A , sheets  504 ,  506  can each also include a second layer  514 ,  516  located along an inside surface of the sheets  504 ,  506 . These second layers can in some embodiments provide a water-resistant layer in order to protect the heating element  502  from fluids to which the heating blanket is exposed. For example, the second layers  514 ,  516  can be a polymeric film attached to the strengthening layer. In some embodiments, the second layers  514 ,  516  are preferably polymeric film layers that are a durable and made of a weldable material, such as urethane or vinyl, which can be laminated or extrusion coated on to the strengthening layers  510 ,  512  and the second layers  514 ,  516  may be welded together via heating bonding along the bonding points. 
     In some embodiments, one or both of sheets  504 ,  506  include a third layer laminated to their respective outer surfaces. The third layer, in some embodiments, is a polymeric layer, which may or may not be the same material as second layers  514 ,  516  in some embodiments. For example, the third layer can comprise a polymeric layer that can substantially seal one or both of the strengthening layers so that it cannot be substantially wetted. In some embodiments, the third layer may also be somewhat tacky so that it prevents the blanket from slipping when applied over a patient. The third layer may also comprise a material with the ability to limit and/or prevent iodine and cleaning solutions from staining the blanket. Examples of materials that could serve this purpose include vinyl and silicone. 
     With further reference to  FIG. 5A , top sheet  504  and bottom sheet  506  can be positioned on opposing sides of heating element  502  to envelope the heating element. Although descriptive terms “top” and “bottom” are used herein, it will be appreciated that in some embodiments, the sheets  504  and  506  may be identical and that either sheet may be referred to as “top” or “bottom.” As shown in the embodiment of  FIG. 5A , the sheets are positioned so that the weldable layers  514 ,  516  of each sheet oppose each other. 
       FIG. 5B  is a top plan view of the heating blanket depicted in  FIG. 5A . In some embodiments, the sheets  504 ,  506  are sized to completely cover the heating element  502 , and can extend beyond all edges (e.g., top, bottom, right and left side edges in  FIG. 5B ) of the heating element  502 . In some embodiments, the heating element  502  is substantially hermetically sealed into the shell  500  formed by the two sheets  504 ,  506 . As shown in the embodiment of  FIGS. 5A and 5B , the sheets  504 ,  506  are coupled together along two welds. A first weld  518  can extend about a perimeter  520  of the heating element  502 , thus surrounding the entire periphery of the heating element. A second weld  522  can extend about a perimeter edge  524  of the sheets  504 ,  506 , thus sealing the periphery of the sheets together. In some embodiments, the space  526  between the first weld  518  and the second weld  522  may be totally or partially welded together. In alternate embodiments, the space  526  between the welds may contain other structural components of the blanket as previously described and further discussed below. For example, the space  526  can enclose weighting members, the added weight of which helps retain the blanket in position and against the patient. 
     The weld used in some embodiments to create a substantially hermetically sealed shell for protecting the heating element provides a number of advantages over traditional bonding mechanisms such as sewing, stitches, rivets or grommets that create or reinforce a seal. In certain embodiments of those that employ a heat sealed shell, the external surface of the substantially hermetically sealed shell is not punctured by needle holes, sewing, stitching, rivets, grommets or other fasteners. These traditional fasteners create holes and can accumulate contaminants from blood and body fluids. These holes, crevasses, and fibrous materials such as thread are difficult or even impossible to clean with standard cleaning methods and solutions. Exemplary heating blankets described herein can advantageously have a smooth, non-violated shell, without external attachments or physical places to trap contaminants, thus providing a readily and thoroughly cleanable heating blanket in some embodiments. As will be appreciated, the welded construction used in some embodiments can also facilitate a variety of features that would otherwise require traditional fasteners such as sewing, stitching, riveting, grommets or snaps. 
     In some embodiments, portions of the shell extending beyond the perimeter of the heating element can form non-heated edge flaps of the heating blanket, such as those described above. Exemplary non-heated edge flaps can preferably extend from 1 inch to 24 inches away from the perimeter of the heating element, although it will be appreciated that any suitable length of extension is possible. The non-heated edge flaps can be used to create a cocoon-like space that traps the heat from the heater in a space around the patient. For example, in alternative embodiments, the edges  112 ,  114 ,  116 , and  118  of the heating blanket depicted in  FIG. 1  can include non-heated edge flaps instead of lateral portions of the heating element. The non-heated edge flaps can thus create a thermal barrier between the heater edge and the operating table or bed. In some embodiments, the two sheets of the non-heated edge flaps may be partially or completely welded together between the first weld about the perimeter of the heating element and the second weld about the perimeter of the warming blanket. With reference to  FIG. 6 , in embodiments with a partial weld, the non-welded area may include an air pocket  530 . Air can be introduced into the space  526  between the first weld  518  and the second weld  522 . Embodiments with such an air pocket  530  can thus provide a thermal barrier that further limits the escape of heat from the space around the patient. 
     With reference to  FIG. 7A , some exemplary heating blankets can include one or more straps  532  extending from the blanket for securing the blanket in place over the patient. In some embodiments, the straps  532  are preferably of the same material and contiguous with the sheets making up the shell and protrude from the edges of the sheets such that there is no seam joining the straps  532  with the sheets. In some embodiments, holes  534  can be punched in the straps  532  to facilitate buckling the straps (e.g., to another blanket strap extending from a different edge of the blanket, to a protuberance extending from the blanket, etc.), hanging the warming blanket, or other common uses. With reference to  FIG. 7B , some embodiments can include a reinforcing layer  536  positioned between the sheets  504 ,  506  before they are welded in order to reinforce the straps  532 . For example, the reinforcing layer can in some embodiments comprise a plastic film such as a urethane film. The reinforcing layer may be formed in addition to strengthening layer of sheets  504 ,  506  described above. Alternatively, the reinforcing layer could be formed by the inclusion of the strengthening layer on one or both of sheets  504 ,  506  at the strap locations shown in  FIG. 7A . As will be appreciated, the straps  532  are provided with the warming blanket without the addition of sewing, stitching, grommets or other traditional fasteners, thus providing the advantages previously discussed. 
     As previously discussed with reference to at least  FIGS. 2A, 4A and 4B , securing strips  317 ,  371  or securing edges  327  can be provided in some embodiments to facilitate securing the heating element to the shell. With reference to  FIG. 8 , an exemplary securing strip  540  can comprise a weldable plastic film, for example, a urethane film. A first end  542  of the securing strip  540  can be attached to the heating element  502 , for example by sewing. A second end  544  of the securing strip  540  (or securing edge according to alternate embodiments) can be placed between the two sheets  504 ,  506  and incorporated into the welds between the two sheets. Thus the heater is held in an extended position within the shell, without using stitches, sewing, rivets or grommets that would pierce the flexible material sheets and make the shell difficult to clean. 
     With reference to  FIGS. 9A-9B , some exemplary shells provide reinforced hanger points  550  without the use of grommets or another similar mechanism for reinforcement. As shown, a reinforcing layer  552  extends between the sheets  504 ,  506  where they are welded at one end about the perimeter of the heating element  502 . The reinforcing layer  552  may be formed in addition to strengthening layer of sheets  504 ,  506 . In some embodiments more than one reinforcing layer may be utilized, for example, on opposing ends of the shell  500  or one layer integrated into one of both of sheets  504 ,  506 . The reinforcing layer  552  can in some embodiments comprise one or more pieces of thermally bondable plastic film, for example a urethane film. The reinforcing layer  552  is incorporated into a weld  554  that may extend from near the perimeter of the heater to near the perimeter of the sheets. One or more holes can be punched through both sheets and through the reinforcing layer  552  to create a hanging point  550 . The exemplary reinforcing layer  552  reinforces the hanging point  550  without the need for additional grommets that would make the blanket more difficult to clean. 
     With reference to  FIGS. 10A-10B , exemplary shells are shown with an incorporated anchor point  560 . As shown, the anchor point  560  can in some embodiments be a “ball-shaped” or a “mushroom-shaped” protuberance which can serve as an attachment post on which a strap with holes in it may be secured, for example, the straps of  FIGS. 7A-7B . The anchor point  560  can be made of plastic or some other material such as metal. As shown in  FIGS. 10A-10B , the anchor point  560  can be molded or otherwise attached to an anchoring layer  562 , which in some embodiments comprises a flat piece of thermally bondable plastic material, such as, for example, a urethane material. The anchoring layer  562  can be placed between the two sheets  504 ,  506  about the perimeter of the heating element  502  and the anchor point  560  can extend from the edges of the sheets as in  FIG. 10B  or through a hole  564  made in one of the sheets as in  FIG. 10A . The sheets  504 ,  506  can be welded to the anchoring layer  562  to anchor the anchoring layer  562  between the sheets and also to seal the cut edge of the hole  564  or edge of the sheets. 
     In some embodiments, a piece of ribbing or piping can be molded to the edge of an anchoring layer similar to that shown in  FIG. 10B . The anchoring layer can then be placed between the two sheets at their edges such that the ribbing or piping protrudes beyond the edges of the sheets. Exemplary ribbing or piping may be plastic or another suitable material such that the ribbing or piping advantageously seals the edges of the shell and creates a soft edge to the warming blanket. Portions of the ribbing or piping may include the anchor point  560 . 
     With reference to  FIG. 11 , in some embodiments, a warming blanket can be secured to a patient with one or more magnets and/or ferrous metal pieces.  FIG. 11  shows two opposing ends  570 ,  572  of a single shell  500  and warming blanket configured in a loop according to some embodiments. As shown, a magnet  574  can be fixed in position between sheets  504 ,  506  at end  570  via appropriately placed welds of sheets  504 ,  506 . Alternately a ferrous metal piece  576  or another magnet can be fixed in position between sheets  506 ,  504  at end  572  in the same manner as magnet  574 . The magnet  574  is placed in a position to mate with ferrous metal piece  576 , securing the blanket in place. The metal piece  576  and the magnet  574  are both contained between the sheets and therefore do not complicate the cleaning of the warming blanket. 
     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.