Body covering and warming apparatus
United States Patent [1 1 Levin et al.  
 [ Apr. 15, 1975 1 BODY COVERING AND WARMING APPARATUS [73] Assignee: The Sierracin Corporation, Sylmar,  
 Calif.  
 [22] Filed: J l 1 197 1 1 Appl. No.: 484,441  
 Related US. Application Data [63] Continuation of Ser. No. 375,673, July 2, 1973, abandoned, which is a continuation-in-part of Ser. No. 188,312, Oct. 12, 1971, abandoned.  
 [52] US. Cl. 219/522; 219/385; 219/527;  
  219/543 [51] Int. Cl. H051) 3/34 [58] Field of Search 29/611; 117/211; 161/45,  
 [56] References Cited UNITED STATES PATENTS 2,739,083 3/1956 Brown, Jr. et al. 219/549 X 3,020,376 2/1962 Hofmann et al. 219/203 X 3,180,781 4/1965 Ryan et a1 161/45 3,288,983 11/1966 Lear, Sr. 219/522 3,296,415 1/1967 Eisler 219/385 3,299,253 1/1967 Lawson, Jr.... 219/385 3,330,942 7/1967 Whilson 2&#39;19/522 3,514,581 5/1970 Rocholl et al. 219/522 3,529,074 9/1970 Lewis 174/685 3,718,535 2/1973 Armstrong et a1 161/165 Primary Examiner-Volodymyr Y. Mayewsky Attorney, Agent, or FirmChristie, Parker &amp; Hale [57] ABSTRACT A transparent flexible warming structure particularly useful as a medical accessory such as an incubator or other individual patient warming structure is described herein. The structure comprises a pair of flexible transparent plastic sheets having an electrically conductive transparent heater layer over substantially the entire extent thereof sandwiched between the plastic sheets. A metal strip is electrically and mechanically connected to the heater layer along side edges so that passage of an electric current therebetween causes heating over the entire transparent flexible structure. In some embodiments, the transparent sheet structure is placed in a rigid frame substantially the same size as the periphery of the heater. In other embodiments, the flexible sheet is arched between a pair of rigid rails which provide mechanical and electrical contacts.  
 21 Claims, 6 Drawing Figures BODY COVERING AND. WARMING APPARATUS This is a continuation&#39;of U.S. Pat. application Ser.  
 No. 375,673, filed July 2, 1973 (now abandoned) BACKGROUND There are many situations in hospitals where it is desirable to maintain the temperature of a patients surroundings above the usual ambient, such as, for example, at nearly body temperature. This often occurs, for example, with newborn infants, particularly premature infants, and commonly such infants are maintained in an incubator for complete environmental control. In most incubators a transparent plastic or glass cover is used, and the temperature is maintained by circulation of warm air. 1  
  U.S. Pat. No. 3,299,253 by W. H. Lawson, .lr., provides a baby incubator having a transparent cover having an electrical heating layer embedded therein so that the infant is warmed by gentle irradiation. By providing such a transparent cover there is no obstruction to continual observation of the infant so that one can observe any changes in its condition. The transparent cover provided in the aforementioned patent is preferably made of transparent polyester plastics bonded together with polyvinyl butyral with a transparent vacuum deposited layer of conductive material sandwiched between the plastic layers. To obtain sufficiently high optical quality in the incubator cover, expensive tooling is required for precisely bending the plastic layers without optical distortion. The manufacturing technique involves vacuum metallizing of a flat plastic sheet, bonding of the second sheet to it, and then heating and pressing on rigid tooling to form the plastic sheets to the desired shape. A portion of the cost of such a structure is attributable to the batch metallizing needed for rigid sheets.  
  Another application of a transparent warming structure is in care of severe burn patients who often lose the ability to maintain body temperature control. The severely burned person needs to be kept warm and yet should be free of any covering. It is also highly desirable that the burn be observable continually without removing any coverings. Other similar situations arise with some severe wounds and with allergic persons who need to be kept warm without contact with coverings.  
  A transparent warming structure for such a person must be appreciably larger than the aforementioned baby incubator, and the costs of tooling for forming an optically acceptable enclosure are significantly magnifled, as are the problems of obtaining uniform vacuum metallizing of a large sheet. Optical transparency and freedom from distortion is desirable, not only to permit observation by medical personnel and visitors, but also to alleviate the psychological effect of the patient being in an enclosure, and also to permit observation of television and. the like by the patient.  
  Although suchtransparent warming structures are highly desirable, they have significant drawbacks in addition to high cost. The plastic covers are susceptible to scratching and other marring, as well as chemical attack, so that their usefullife may be limited in practical hospital situations. When a sufficient amount of chemical attack and scratching has occurred, the optical quality is diminished to the point that the structure needs to be discarded and replaced with consequent high cost. The transparent structures are heavy, particularly one large enough for an adult patient, and are therefore difficult to handle in a hospital situation. Another disadvantage can arise where close and rapid temperature control is required, since the massive structure has a high thermal inertia so that is temperature cannot be changed rapidly. Also electrical failure, e.g.. due to burn out, renders the entire assembly useless, necessitating replacement of this cost by heater.  
  Flexible warming structures which do not contact the patient have been devised in the form of tents made of fabric to which metal wires are attached to provide radiant heating, somewhat in the manner of an electric blanket on a frame. Such tents require framework supports to which the fabric is tailored and are also opaque so that to observe the patient it is necessary to remove at least a portion of the covering.  
  It is, therefore, desirable to provide a body warming apparatus having a flexible transparent warming structure that is sufficiently inexpensive that it can be readily replaced, is lightweight, and has fast thermal response and high optical quality.  
 BRIEF SUMMARY OF THE INVENTION Therefore, there is provided in practice of this invention according to a presently preferred embodiment a transparent, flexible warming structure having a pair of flexible transparent plastic sheets bonded together with an electrically conductive transparent heater layer over substantially the entire area thereof sandwiched between the two sheets. A high conductivity strip electrically and mechanically bonded to the heater layer along the side edges of the plastic sheets provides electrical contact. The transparent flexible warming structure is removably mechanically supported along at least a pair of opposed side edges in substantially fixed position with means for making electrical contact with the high conductivity strips so as to form a body warming structure.  
 DRAWINGS These and other features and advantages of this invention will be appreciated as the same becomes bettter understood by reference to the following detailed description of a presently preferred embodiment when considered in connection with the accompanying drawings wherein:  
  FIG. 1 illustrates in exploded perspective a body warming structure constructed according to principles of this invention;  
  FIG. 2 illustrates in cross section an edge of a flexible element of the structure of FIG. 1;  
  FIG. 3 illustrates semi-schematically a transparent flexible warming structure for connection to a bed or the like;  
  FIG. 4 illustrates a fragment of an edge clip for a flexible structure such as that illustrated in FIG. 3;  
  FIG. 5 illustrates another embodiment of body covering and warming apparatus; and  
  FIG. 6 illustrates still another embodiment of body covering and warming structure.  
 DESCRIPTION FIG. 1 illustrates in exploded perspective a flexible, transparent warming structure constructed according to principles of this invention. As illustrated in this presently preferred embodiment. there is a rigid frame 10 having a pair of straight side edges 11 and arched ends 12 interconnecting the side edges. The center portion of the frame is open. Such a frame can be formed of metal or preferably is injection molded or assembled of high impact resistant plastic so as to be anelectrical insulator. It will be recognized that the frame 10 is illustrated in relatively simple form and that if desired additional stiffening ribs, handles, or the like may be used as&#39;desired.  
  On one side edge 11 a conventional male plug 13 is provided for making electrical connection to a conventional power supply (not shown) the output of which may be controlled in a conventional manner. Wires 14 lead from the plug 13 to a pair of metal contact pads l6at the corners of the frame. The frame 10 is set on or mounted in any conventional manner to an incubator bed 17, which is illustrated only schematically as a box and will be understood to be any conventional incubator. The frame may be held onto the incubator bed 17 by clip slorjfit into brackets, or can be hinged along one side edge I! so&#39;that it can be raised and lowered as desired. An arcuate&#39;end piece 18 is mounted on the incubator bed fo.r,.closing the end of the arcuate frame. Such an end closure can be secured to the frame, or the ends can be left open as may be desired.  
  A transparentfflexible plastic warming element 21, hereinafter described in greater detail, is laid over the frame &#39;li soas to&#39;c&#39;onform thereto. The flexible, transparent jsheet 21 is substantially coextensive with the frame so that the side edges of the sheet overlap the &#39;edgesand&#39; ends of the frame. Along each edge of the sheet 2i lflies on the ends of 12 of the frame there is afr&#39;net&#39;al fbus bar&#34; 22, which in in practice a copper stripls u&#39;fflcie&#39;ntly thin to be flexible. The bus bar 22 is.  
 onthe downward face of the sheet 21 so that when the sheet is placed over the frame the two bus bars 22 at fth e ends are in contact with the metal contact pads 16 a: the corners of the frame. In this way, electrical contact is made between the plug 13 and the bus bars 22.  
  A second substantially rigid frame 23 complementary to the frame 10 fits overthe flexible sheet heater 21 for providing additional rigidity and holding the sheet in place. Typically bolts 24 are used to secure the two frames 10 and 23 together during use of the incubator.  
  An edge of the transparent, flexible heater 2] is seen in cross section in FIG. 2. As illustrated in this embodiment the heater comprises an outer plastic sheet 26 which in a &#39;typical embodiment is a sheet&#39;of polyethylene terephthalate about five mils thick. A conductive layer 27, such as gold, silver, copper, iron or nickel is deposited on the sheet 26 substantially throughout its extent. Such deposition is typically by vacuum metallizing according to conventional practice with a metal thickness sufficiently low that it is transparent.  
  A second plastic sheet 28 is bonded to the metal layer 27 so that the metal layer is between the two piastic sheets 26 and 28. Typcally, ths second plastc sheet which would be located on the inside of the incubator is also polethylene terephthalate about 10 mils thick. The second sheet 28 is narrower than the outer sheet 26 so that a clear space is left along each of the pair of side edges that lie at the ends 12 of the frame (FIG. 1). The copper bus bar 22 extends along this exposed side edge and is bonded to the metallized heater layer by a conventional conductive adhesive 29. The bus bar 22 has a much higher electrical conductivity than the metal layer 27 (the thickness of which is exaggerated in F IG. 2 for purposes of illustration) so that uniform electrical contact is made along each side edge of the heater. If desired, the inner plastic, sheet &#39;28 mayoverlap the bus bar 22 over a portion of its extentfor providing electrical insultion thereof or some mechanical support. At least a portion of the bus bar must be left free for making electrical contact with the metal contact pads 16 or similar conductive elements. Thus, for example, the entire bus barmay be covered by the inner sheet except for a small area left at one corner for electrical contact. Such construction minimizes electrical hazards.  
  The plastic sheets 26 and 28 are prefectly polyethylene terephthalate such as commerically available under a variety of trade marks such as Mylar, Scotchpar&#39;, and Celanar. Alternatively, the plastic sheets may be a high temperature polyamide resin, such as nylon 6 or nylon 12. Another suitable plastic sheet is polytetramethylene terephthalate. Although these &#39;rnaterialsar e preferred for the plastic sheets used in making the transparent flexible warming structure of this invention,  
 other suitable transparent synthetic resins can be selected form the group consisting of polycarbonate resin, polyimide resin, fluroinated ethylene-propylene resin, polyester resin polyacrylic resin, and cellulose triacetate resin, so long as at least one of the sheets selected for the structure has a sufficiently low vapor pressure to be vacuum metallizable and the plastics have a heat deformation of less than about one present in any direction at a temperature up to about&#39;l5 0F. Unless the sheets have less than this heat deformation, dimensional and electrical changes may occur that are unacceptable in a commerical embodiment. The plastics should also be substantially transparent over much of the visible spectrum and are preferably largely opaque in bulk in the infrared region.  
  It is preferred that the plastics be selected with a suitable composition and thickness so that they are substantially opaque to infrared radiation in the range of wavelength of about 2 to 20 microns. With such properties the heater sheet has a high infrared emittance and can deliver heat in effective rates with a lower operating temperature than would be the case if it were more transparent to infrared. This means that the en tire flexible warming structure can be operated at a sufficiently low temperature that it is completely safe to personnel, while at the same time delivering sufficient heat to maintain at a desired temperature a body irradiated by the flexible warming structure. 1  
 Y about 5 mils (0.005 inch) and that the inner sheet 28 have a thickness in the order of 7 to l0 mils&#39; or more. This arrangement provides a desired balance &#34;ofeco&#39;nomy of production andmechanical protection of the conductive coating by the sheet and&#34; relatively low&#39; infrared emittzmcefrorh the outer sheet. The  
 &#39; thicker&#39;inner sheet assures adequate strength of the assembly and a relatively high&#34; infrared emittance from the inner sheet in the wavelength range of about 2 to 20 microns. When lower stiffness is feasible, a preferred arangement has both plastic sheets of equal thickness, for example, mils for ease and economy of manufacture.  
  If a higher stiffness is desired. the plastic sheets making up the flexible transparent heater element can be somewhat thicker. Satisfactory structures have been made with a total thickness of about 30 mils. As thicknesses become larger the stiffness of the flexible heater element increases and the available radius of curvature decreases. It is important, however, that the transparent heater element remain flexible so that it can be readily made and transported and applied to various supporting structres without preforming to a desired curvature. Such preforming is expensive and has been found to be avoidable in practice of this invention.  
 , A flexible sheet as contemplated in practice of this invention lies flat or substantially flat under its own weight when laid on an ordinary flat surface. The transparent heater element is sufficiently flexible, however, so that when laid over a curved structure it sags into substanial curvature under its own weight and if need be sharper curvature can be obtained by only slight additional force. Such flexible heater elements can for example be rolled up for storage or shipment, or maintained flat as may be desired. Typically plastic sheets up to almost 100 mils thick have sufficient flexibility to be used in many embodiments of this invention. Some-- what thinner sheets are required ifa high degree of curvature is desirable. Such a sheet would normally lie flat on a flat surface and bend too a substantial degree of curvature in either direction when laid over a curved surface or nonflat support. This is to be contrasted with a rigid structure mode of glass or thick plates of plastic as have been employed in the past.  
  The flexible transparent heater element is readily manufactured at low cost since it can be made substantially continuously in a vacuum metallizing operation with conventional techniques. Thus, for example. the outer-sheet 26 is provided in large rolls which can be unreeled continuously and passed through a deposition region in a vacuum metallizing chamber and coiled up again. Typically the mating surfaces of the plastic sheets are primed with a conventional silane primer and the sheets bonded together with a conventional silicone adhesive. A variety of other adhesive systems may be employed, as will be apparent to one skilled in the art. The priming, adhesive coating, and bonding steps can be carried on continuously with long sheets of the plastics. Similarly, the adhesive 29 and the bus bar 22, which is typically a thin copper ribbon, and be applied along each edge of the roll of plastic in a&#39;continuous operation. Rolls of flexible transparent heater with integral bus bars are thus made in continuous conventional processes are extremely low cost as compared with the batch type processing of prior rigid transparent heaters. ln order to make individual heaters of these rolls, it is only necessary to cut the sheet to appropriate lengths. I  
  In the above described embodiment the flexible transparent heater sheet is about l5 mils thick and, hence, has some degree of elasticity. This stiffness of the sheet is sufficient that the sheet is well supported in its central portion when fitted within the frame illustrated in FIG. 1. It will also be apparent, of course, that a greater degree of stiffness can be provided by making one or both of the plastic sheets somewhat thicker, and greater flexiblility can be obtained by making-both the sheets somewhat thinner. It is generallyfpreferred that the total thickness of the sandwich of two plastic sheets be no less than about 10 mils in order to retain a relatively high strength and infrared&#39;emittance. Otherwise the flexible heater sheet must&#39;be heated to a higher temperature to get a desired heat flux.  
  Because of the thinness of the flexible transparent heater sheet, it is hightly advantageous as a flexible worming structure. It is light in weight so as to be easy and safe to handle in a hospital or other situation. Being flexible, a substantial number of such sheets may be stored merely by stacking them in flat layersja&#39;n d they are bent as required to fit the incubator frames. &#39;Because of the low mass of the flexible transparentsheet, the thermal inertia is very small and the sheet is rapidly responsive to changes in heating demand. Thinness also assures excellent optical quality and minimum distortion since the sheets are of consistently uniformthickness and, even when viewed at an angle, no&#39;substantial optical distortion is encountered. 7  
  Since the flexible transparent heater sheet is made in continuous processes, it cost is&#39;extremely low as compared prior rigid structures. Because of this, if scratching or other mechanical damages&#39;to the heater&#39;sheet occurs, the entire sheet can be discarded and replaced relatively economically. In a prior rigid structure, the high cost inhibits discarding the structure anditmay be used for a substantial period after the optica&#39;f&#39;qu&#39;ality has deteriorated significantly. e  
  If desired, holes can be placed through the transparent flexible heater sheet, which may be particularly advantageous where it is desired to pass a tube or the like through the sheet. Thus, for example, if intravenous fluids are delivered to aburn patient, the tubes may be passedthrough an aperture in the sheet without significantly disturbing the temperature distribution.  
  To cut the transparent flexible heater sheet is a simple matter using any sharp cutting intrument. such as a razor blade or sharp shears. It should be emphasized that a sharp instrument should be used to avoid tiny cracks along the cut edge which may interrupt te thin metal heater layer. A sharp crack in the heater layer extending transverse to the direction of current flow causes a high localized current density which may be sufficient to damage the plastic sheets, and once such localized heating hasprogressed to the point of damage, the damaged area rapidly propagates acrosss the entire sheet so that it is burned out.  
  A thin flexible sheet as provided in practice of this invention can be cut with a sharp knife or shears without introduction of tiny cracks in the metal layer, anditis therefore possible to provide any desired shape.  
 &#39;With the thick, rigid transparent heaters of the prior art, such cutting is extremely difficult without introduction of tiny cracks and cannot be accomplished except in a machine shop or the like, and even theremust be done with great care. Low replacement cost of the thin flexible sheet heater make use for abroad variety of applications feasible as compared with prior heavy, expensive structures.  
  FIG. 3 illustrates semi-schematically another arrangement for supporting a flexible warming structure constructed according to principles of this invention. As illustrated in this embodiment a conventional bed 46, such as a hospital bed or the like may be fitted with a pair of side rails 47 each of which has a- U-shaped cross section. A flexible, tr,ansparent warming sheet 48 has a sideedge inserted,into. the. U-shaped rail along each side, and because of the slight stiffness of the sheet it arches freely between the two side rails without support at the arched ends. The side rails may have the grooves vertical as in the illustrated embodiment or, if desired, the grooves may be partly tilted towards each other so that&#39;the arch is lower. A bus bar 49 is provided along each side edge of the sheet 48 in the portion that is inserted into the channel of the rails 47 for making electical contact to the heater sheet. Since the rails are separate from each other and the sheet is flexible, the entire structure can be readily stored until needed and quickly installed on a bed. Since the sheet is flexible and lightweight it can easily be moved or removed as required for providing assistance to a patient within the warming structure.  
  FIG. 4 illustrates an alternate arrangement of side rail for an arched warming structure such as illustrated in FIG. 3. As illustrated in this embodiment, the side rrail comprises an elongated plastic strip 51 having a generally T shape. One arm 52 of the T may be connected to. or form a portion of, an incubator, for example. The other arm 53 of the T forms a rigid upstanding edge against which a transparent flexible electric heater 54, such as hereinabove described. may be placed. A metal pad 56 within the T-shaped structure provides electrical contact to the bus bar (not shown) of the flexible transparent heater.  
  AT the bottom of the leg 57 of the T an elongated piano hinge 58 is formed for pivotally mounting a clip arm 59. A spring 61 between the clip arm 59 and the T-shaped membe 51 biases the clip arm so that an enlarged head 62 thereon is pressed towards the upstanding arm 53 of the T-shaped member. To insert a heater element 54 in the clip-like rail, the lower portion of the clip arm 59 is&#39;pressed, thereby opening the slot between the enlarged head 62 and the upstanding arm 53. The edge of the sheet electric heater 54 can then be inserted into the slot with the conductive bus bar (not shown) facing the electrical conact pad 56. Release of the clip arm 59 holds the plastic heater element in position and assures electrical contact between the bus bar and contact pad.  
  FIG. illustrates in perspective another embodiment of body warming and covering apparatus constructed according to principles of this invention. As illustrated in this arrangement the entire body warming apparatus may be supported on a stand 66, only the vertical leg of which is shown in FIG. 5. Such a stand may have any desired support arrangement on the floor such as a U- shaped base (not shown) which fits beneath a patients bed to prevent overturning. Such a stand may also be clamped to other supporting structures such as the side of a bed or the like. A mounting block 67 is free to slide vertically on the stand 66 and can be lockedin place by a screw 68 or the like. This permits the height of the warming structure to be adjusted as may be desired in a particular arrangement.  
 A rigid U-shaped bracket 69 extends laterally from the mounting blcok and terminates in a pair of spaced apart coaxial trunnions 71. A rigid frame 72 is connected between the trunnions and is free to tilt about the axis thereof. A locking bolt 73 in each trunnion can be loosened to&#39; permit tilting and tightened to inhibit tilting.  
 The frame 72 can be in two parts much like the arched frame illustrated in FIG. 1 so that electrical contact can be made to a flexible transparent warming sheet 74 mounted in the frame. If desired the&#39;parts of the frame can be hinged along one edge for insertion of the transparent sheet or any other convenient arrangement for removably holding the sheet in the frame can be provided. Since substantial time may elapse between instances where it is desirable to change the electrically conductive sheet the frame can be bolted together without great inconvenience.  
  An electrical connector 76 is provided on one edge of the frame so that electrical contact can be made thereto.  
  When such an apparatus is used the height of the body warming structure over the patient is adjusted by loosening the screw 68 and sliding the mounting block along the vertical leg of the stand. Further the &#34;position of the transparent electrically conductive sheet 74 in the frame can be adjusted by loosing the bolts 73 and tilting the frame in the trunnions 71. Clearly other mounting arrangemets for the flat frame with a transparent electrically conductive flexible sheet therein can be provided.  
  FIG. 6 illustrates in a view similar to FIG. 5 another embodiment of body warming and covering apparatus wherein the flexible transparent sheet is arched between a pair of supporting members along opposite edges. in this arrangement a floor stand 78 or the like supports a U-shaped bracket 79 in a manner as hereinabove described. Coaxial trunnions 81 are mounted on the ends of the U-shaped bracket so that the body warming structure can be tilted if desired. A rigid curved rail 82 is connected to each trunnion and can be tilted about its axis. The rails 82 can be clamped in place by locking bolts 83 if desired. In many embodiments the rails can be maintained in a fixed position without detriment.  
  Each of the rails 82 inlcudes means for clamping a flexible transparent electrically conductive sheet 84. The means for clamping may be a simple spring loaded clamp as hereinabove described and illustrated in FIG. 4 or the rail can be made in two pieces which can be separated for releasing a sheet held thereby. A great variety of ways of temporarily and removably securing a transparent flexible conductive sheet between the side rails will be apparent to one skilled in the art. An electrical connector 86 is provided on each rail so that electrical contact can be made to a bus bar (not shown) along the clamped edges of the flexible sheet. Such an arrangement provides means for removably supporting the two arched side edges of the flexible sheet in a fixed position. The other two edges extend between the rails without direct mechanical support.  
  When a flat frame as. illustrated in FIG. 15 is employed. the four side edges of the flexible heater are sufficiently contained that neither a patient nor attending personcan make contact therewith. ln an arrangement as illustrated in FIG. 6 wherein the flexible heater is supported in an arched position between a pair of parallel rails without intermediate support, two of the side edges are exposed.. lt is desirable to provide an electrical isolation line through the conductive film or to remove the conductive film in the portions adjacent these free edges to completely eliminate any electrical hazard. The same effect can be obtained with a nonconductive coating along the unsupported edges.  
 plastic sheet, the scope of utiliztion of the transparent warming structure can be significantly increased. Thus, for example, the transparent flexible warming structure can be used in applications where it is desired to keep food or other similar materials warm, or demonstration egg incubators can be made with such material. Economy is such that hot frames and the like for growing seedlings can be provided with transparent covers incorporating heaters to prevent frost damage. Many other applications of a transparent flexible heating structure will be apparent to one skilled in the art. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.  
 What is claimed is:  
  l. A body covering and warming apparatus comprising:  
 a transparent flexible electrical heater element having a thickness of up to about 30 mils and sufficient flexiblity to lie substantially flat on a flat surface and curve in either direction over a curved surface under its own weight comprising:  
 a first flexible transparent plastic sheet having an elastic modulus in the range of about 1.8 X 10 to 4.5 X psi,  
 a secnd flexible transparent plastic sheet having an elastic modulus in the range of about 1.8 X 10 to 4.5 X 10 psi,  
 an electrically conductive layer sandwiched between the first and second sheets, said conductive layer being sufficiently thin to be transparent and having at least a portion adjacent each of a pair of opposite side edges extending beyond the edge of one of the plastic sheets, and  
 flexible bus bar means for making substantially continuous electrical contact with the conductive layer along said opposite side edges;  
 support means for removably mechanically supporting a pair of opposed side edges of the heater element in substantially fixed position and forcing the central portion of the flexible heater element to arch therebetween said heater element being mounted on the support means in an arched shape; and  
 electrical means on the support means for electrical contact with the bus bar means.  
  2. A body covering and warming apparatus as defined in claim 1 wherein the means for mechanically supporting comprises:  
 means on the first rail for supporting a first side edge of the heaterelement;  
 a seond rail parallel to the first rail; and  
 means on the second rail for supporting a second side edge of the heater element;  
  said means for supporting the side edges including a mechnical clamp for engaging each side for retaining it in fixed juxtaposition to the rails and estabilishing electrical contact between the bus bar 5 means and an external heater power source.  
  3. A body covering and warming apparatus as defined in claim 1 wherein the means for mechanically supporting comprises:  
 a first rail;  
 means on the first rail for supporting a first side edge of the heater element;  
 a second rail parallel to the first rail; and  
 means on the second rail for supporting a second side edge of the heater element;  
 said means for supporting the side edges being positioned for forcing the heater element to arch therebetween without intermediate support.  
  4. A body covering and warming apparatus as defined in claim 3 wherein the means for supporting the side edges includes a clip for engaging each side edge, and spring means for biasing the clip towards engagement with the sheet edge.  
  5. a body covering and warming apparatus as defined in claim 1 wherein:  
 35 said plastic sheets are selected from synthetic resins that are substantially transparent in the visible spectrum and substantially opaque in bulk in the infrared spectrum, have a heat deformation of less than about 1% in any direction at about 150F. and  
 are selected from the group consisting of polycarbonate resin, polyimide resin, fluorinated ethylenepropylene resin, polyamide resin, polyester resin. polyacrylic resin and cellulose triacetate resin. at least one of said sheets havng a suffficiently low vapor pressure to be vacuum metallizable: and wherein one of said plastic sheets has a relatively higher infrared emittance and the other plastic sheet has a relatively lower infrared emittance.  
  6. A body coveing and warming apparatus as defined in claim I wherein one of the plastic sheets has a relatively higher stiffness and the other plastic sheet has a relatively lower stiffness.  
  7. A body covering and warming apparatus comprismg:  
 a transparent flexible electrical heater element comprising: a first flexible transparent plastic sheet,  
 a secnd flexible transparent plastic sheet,  
 an electrically conductive layer sandwiched between the first and second sheets, and conductive layer being sufficiently thin to be transparent and having at least a portion adjacent each of a pair of opposite side edges extending beyond the edge of one of the plastic sheets, and  
 bus bar means for making substantially continuous electrical contact with the conductive layer along said opposite side edges;  
 means for removably mechanically supporting a pair of opposed side edges of the heater element in substantially fixed position comprising:  
 an arched frame substantially the same size as the periphery of the heater element; and  
 means for removably attaching the heater element in arched form conforming to the frame; and  
 electrical means for making electrical contact with the bus bar means. i  
  8. A body covering and warming apparatus as defined in claim 7 wherein the means for removably ataching comprises a second frame complementary to the arched frame; and means for temporarily connectng the two frames together.  
  9. A body covering and warming apparatus as defined in claim 7 wherein the means for making electrical contact comprises:  
 a pair of separated metal contact pads on the arched frame; and  
 means for making electrical connection to a power supply.  
  10. A body covering and warming apparatus comprising:  
 a transparent flexible electrical heater element having a thickness of up to about lOO mils and sufficient flexibility to lie substantially flat on a flat surface and curve in either direction over a curved surface under its own weight comprising:  
 a first flexible transparent plastic sheet having an elastic modulus in the range of about 1.8 X 10 to 4.5 X psi.  
 a second flexible transparent plastic sheet having an elastic modulus in the range of about 1.8 X 10 to 4.5 X 10 psi.  
 an electrically conductive layer sandwiched between the first and second sheets, said conductive layer being sufficiently thin to be transparem. and  
 flexible bus bar means for making substantially continuous electrical contact with te conductive layer along opposite side edges thereof;  
 support means for removably mechnically supporting a pair of opposed side edges of the heater element in substantially fixed position and forcing the central portion of the flexible heater element to arch therebetween said heater element being mounted on the support means in an arched shape; and  
 electrical means on the support means for making electrical contact with the bus bar means.  
  11. A body covering and warming apparatus as defined in claim 10 wherein the means for mechanically supporting comprises:  
 a first rail;  
 means on the first rail for supporting a first side edge of the heater element;  
 means on the second rail for supporting a second side edge of the heater element;  
 said means for supporting the side edges including means for engaging each side for retaining it in fixed juxtaposition to the rails and establishing electrical contact between the bus bar means and an external heater power source.  
  12. A body covering and warming apparatus as defined in claim 10 wherein the means for mechanically supporting comprises:  
 a first rail;  
 means on the first rail for supporting a first side edge of the heater element;  
 a secnd rail parallel to the first rail; and  
 means on the second rail for supporting a second side edge of the heater element;  
 said means for supporting the side edges being posi- 6 tioned for forcing the heater element to arch therebetween without intermediate support.  
  13. A body covering and warming apparatus as defined in claim 12 wherein the means for supporting the side edges includes a clip for engaging each side edge, and spring means for biasing the clip towards engagment with the sheet edge.  
  14. A body covering and warming apparatus as defined in claim 10 wherein:  
 said plastic sheets are selected from synthetic resins that are substantially transparent in the visible spectrum and substantially opaque in bulk in the infrared spectrum, have a heat deformation of less than about 1% in any direction at about F,&#39;and are selected from the group consisting of polycar bonate resin, polyimide resin, fluorinated ehtylenepropylene resin, polyamide resin, polyester resin, polyarcylic resin and cellulose triacetate resin, at least one of said sheets having a sufficiently low vapor pressure to be vacuum metallizable; and wherein one of said plastic sheets has a relatively higher infrared emittance and the other plastic sheet has a relatively lower infrared remittance.  
  15. A body covering and warming apparatus as defined in claim 10 wherein one of the plastic sheets has a relatively higher stiffness and the other plastic sheet has a relatively lower stiffness.  
  16. A body covering and warming apparatus comprising:  
 a transparent flexible electrical heater element having sufficient flexiblity to lie substantailly flat on a flat surface and curve in either direction over a curved surface under its own weight comprising: a first flexible transparent plastic sheet.  
 a second flexible transparent plastic sheet.  
 an electrically conductive layer sandwiched between the first and second sheets. said conductive layer being sufficiently thin to be transparem, and  
 flexible bus bar means for making substantially continuous electrical contact with the coonductive layer along opposite side edges thereof;  
 an arched frame substantailly the same size as the periphery of the heater element removably mechanically supporting the heater element in substantially fixed position with the central portion of the flexible heater element arched;  
 means for temporarily attaching the heater element to the frame in the arched position; and  
 electrical means on the frame for making electrical contact with the bus bar means.  
  17. A body mvering and warming apparatus as defined in claim 16 wherein the means for removably attaching comprisies a second frame complementary to the arched frame; and  
 means for temporarily connecting the two frames together.  
  18. A body covering and warming apparatus as de fined in claim 16 wherein the means for making electrical contact comprises:  
 a pair of separated metal contact pads on the arched frame; and  
 means for making electrical connection to a power supply.  
 19. A body covering and warming apparatus comprising:  
 a transparent flexible electrical heater element comprising:  
 a first flexible transparent plastic sheet having an elstic modulus in the range of about l.8 X 10- to 4.5 X 10 psi.  
 a second flexible transparent plastic sheet having an elastic modulus in the range of about 1.8 X l to 4.5 X l0 psi,  
 an electrically conductive layer sandwiched between the first and second sheets, said conductive layer being sufficiently thin to be transparem, and  
 flexible bus bar means for making substantially continuous electrical contact with the conductive layer along opposite side edges thereof, the total thickness of the heater element being up to about 100 mils;  
 means for removably mechanically supporting a pair of opposed side edges of the heater element in substantially fixed position comprising:  
 an arched frame substantially the same size as the fined in claim 19 whrein the means for removably attaching comprises a second frame complementary to the arched frame; and  
 means for temporarily connecting the two frames together. 21. A body covering and warming apparatus as de fined in claim 19 wherein the means for making electrical contact comprises:  
 a pair of separated metal contact pads on the arched frame; and  
 means for making electrical connection to a power supply.  
  makes Column 7, line 12 eleonical&#34; should read UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION PAT ENT NO]: ,361 p 1 f 2 DATED April 5, 1975 v (5) Berton P. Levin, Darrell D. Stoddard It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
  Column 2, line 8 &#34;is&#34; should read --its--. Column 3, line 35 &#34;in&#34; (first occurrence) should read is line 60 &#34;Typcally, ths second plastc&#34; should read Typically, this second plastic line 62 &#34;polethylene&#34; should read polyethylene Column 4, line 8 &#34;insultion&#34; should read insulation; line 32 &#34;present&#34; should read percent-; line 56 &#34;harness&#34; should read hardness Column 5, line &#34;structres&#34; should read structures line 36 &#34;mode&#34; should read made line 53 &#34;and&#34; should read can line 57 &#34;are&#34; should read at Column 6, line 10 &#34;hightly&#34; should read highly line 11 &#34;worming&#34; should read warming line 24 &#34;it&#34; should read its line 44 &#34;te&#34; should read the line 61 &#34;make&#34; should read electrical line 21 &#34;rrail&#34; should read rail line &#34;membe&#34; should read member line 42 &#34;conact&#34; should read contact line 63 &#34;blcok&#34; should read block Column 8 line 20 &#34;loosing&#34; should read loosening line 22 &#34;arrangemets&#34; should read arrangements line 38 substantial should be inserted afterv &#34;without&#34; and before &#34;detriment&#34;. Column 9, line 12 &#34;utiliztion&#34; should read --utilization--; line 38, &#34;secnd&#34; should read second Column 10, line 34 &#34;havng&#34; should read having line 40 &#34;coveing&#34; should read covering line 50, &#34;secnd&#34; should read second line 52 &#34;and&#34; should read said Column 11, line 5, &#34;connectng&#34; should read connecting line 31 &#34;te&#34; should read the line 48 a second rail parallel to the first rail; and should be inserted after &#34;element&#34; on line 48 and before &#34;means&#34; on line 49; line 62 &#34;secnd&#34; should read second-- line 16 &#34;polyarcylic&#34; should read polyacrylic UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Pa e 2 of 2 PATENT NO. 3,873, 361 DATED April 15, 1975 WVENTOHS) Berton P. Levin, Darrell D. Stoddard It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 &#39; line 30 &#34;substantailly&#34; should read substantially line 40 &#34;coonductive&#34; should read conductive line 42 &#34;substantailly&#34; should read substantially line 54 &#34;comprisies&#34; should read comprises Column 13, line 2, &#34;elstic&#34; should read elastic Column 14, line 7 &#34;whrein&#34; should read wherein Signed and Sealed this A! test:  
 RUTH C. MASON C. MARSHALL DANN Arresting Officer Commissioner uflatents and Trademarks