INFUSION FLUID WARMER COMPRISING PRINTED CIRCUIT BOARD HEATING ELEMENTS

Abstract

The present invention relates to an infusion fluid warmer comprising a heat exchanger and first and second printed circuit boards comprising respective integrally formed electrically resistive patterns acting as heating elements. The integrally formed electrically resistive patterns are heated by supply of electrical power and thermally coupled to a heat exchanger to warm an infusion fluid flowing through a fluid passage of the heat exchanger.

Claims

1-18. (canceled)

19. An infusion fluid warmer comprising: a DC power supply input, a first carrier board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a first integrally formed electrically resistive pattern, a second carrier board comprising a first surface and a second, opposing, surface, wherein the second surface comprises a second integrally formed electrically resistive pattern, a heat exchanger comprising an aluminium upper wall structure and an aluminium lower, opposing, wall structure separated by a fluid channel extending between fluid inlets and outlets of the heat exchanger; wherein the fluid channel extends substantially straight along a longitudinal axis of the heat exchanger; and wherein the respective surfaces of the aluminium upper wall structure and aluminium lower wall structure are anodized by an aluminium oxide layer to provide a bio-compatible layer in the fluid channel; and a controller for connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns, wherein an outer surface of the aluminium upper wall structure is thermally connected to the first resistive pattern and an outer surface of the aluminium lower wall structure is thermally connected to the second resistive pattern, and that the controller is configured to: connecting the DC power supply input to the first and/or second integrally formed electrically resistive patterns during a first time period to dissipate power in the first or second integrally formed electrically resistive pattern to thereby serve as a heating element during the first time period.

20. An infusion fluid warmer according to claim 19, wherein the controller is further configured to: disconnect the first and/or second integrally formed electrically resistive patterns from the DC power supply input during a predetermined delay time, determining, during a second time period, following the predetermined delay time, a resistance of the first integrally formed electrically resistive pattern or determining a resistance of the second integrally formed electrically resistive pattern, determining a temperature of the first or second integrally formed electrically resistive pattern based on the determined resistance.

21. An infusion fluid warmer according to claim 19, wherein the controller is configured for selectively connecting and disconnecting the DC power supply input to the first or second integrally formed electrically resistive patterns over time to control the temperature of the infusion fluid in accordance with a desired or target temperature of the infusion fluid.

22. An infusion fluid warmer according to claim 19, wherein the first carrier board comprises a printed circuit board (PCB) and the second carrier board comprises a printed circuit board (PCB).

23. An infusion fluid warmer according to claim 19, wherein each of the aluminium upper wall structure and the aluminium lower wall structure has a plate shaped structure defining a thickness of the fluid channel of less than 5 mm, or less than 3 mm.

24. An infusion fluid warmer according to claim 19, wherein the fluid channel comprises a cavity formed in the aluminium upper wall structure.

25. An infusion fluid warmer according to claim 19, further comprising: an outer housing or casing surrounding and enclosing at least the heat exchanger, the first carrier board and the second carrier board; and an electrically insulating frame, gasket or ring surrounding and contacting peripheral edges of the upper and lower wall structures of the heat exchanger to prevent physical contact and electrical contact between the heat exchanger and the outer housing.

26. An infusion fluid warmer according to claim 25, wherein the fluid channel comprises an inlet transition zone at the fluid inlet, a central zone and an outlet transition zone at the fluid outlet; wherein the central zone has a rectangular cross-section in a longitudinal direction of the heat exchanger along a fluid flow path of the infusion fluid.

27. An infusion fluid warmer according to claim 26, wherein a height-to-thickness ratio of the central zone of the fluid channel is at least 50:1 or at least 175:1 or at least 350:1.

28. An infusion fluid warmer according to claim 27, wherein a height of the central zone of the fluid channel is between 0.1 mm and 0.5 mm.

29. An infusion fluid warmer according to claim 19, wherein the second surface of the first carrier board is plane and arranged to contact a complementary shaped plane surface of the aluminium upper wall structure; and the second surface of the second carrier board is plane and arranged to contact a complementary shaped plane surface of the aluminium lower wall structure. $4848\text{-}5838\text{-}5138.1$

30. An infusion fluid warmer according to claim 29, wherein the first integrally formed electrically resistive pattern and said at least one additional and separate integrally formed electrically resistive patterns are arranged sequentially along the second surface of the first carrier board; and/or wherein the second integrally formed electrically resistive pattern and the one or more additional and separate integrally formed electrically resistive patterns are arranged sequentially along the second surface of the second carrier board.

31. An infusion fluid warmer according to claim 20, wherein the electronic switching circuit further comprises: a first controllable semiconductor switch coupled in series between the DC power supply input and the first integrally formed electrically resistive pattern and a first reference resistor connected across input and output terminals of the first controllable semiconductor switch; and a second controllable semiconductor switch coupled in series between the DC power supply input and the second integrally formed electrically resistive patterns and a reference resistor connected across input and output terminals of the second controllable semiconductor switch.

32. An infusion fluid warmer according to claim 31, wherein a resistance of each of the first and second reference resistors is at least 100 times larger than an on-resistance of each of the first and second first controllable semiconductor switch.

33. An infusion fluid warmer according to claim 19, wherein a resistance of each of the first and second integrally formed electrically resistive patterns is less than 11 Ω or between 1 Ω and 7Ω.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0114] Various embodiments of the invention will be explained in more detail below with reference to the accompanying drawings, wherein:

[0115] FIG. 1 shows an illustration of an infusion fluid warmer,

[0116] FIG. 2 is an exploded view of the infusion fluid warmer of FIG. 1, showing the main components,

[0117] FIG. 3 shows an exploded view of a heating unit of the infusion fluid warmer of FIG. 1,

[0118] FIG. 4 shows an illustration of a power supply unit for the infusion fluid warmer of FIG. 1,

[0119] FIG. 5 shows an exploded view of the main components of the power supply of FIG. 4,

[0120] FIG. 6 shows block diagram of the electrical circuit and the heat exchanger of the heating unit.

[0121] FIG. 7a shows the block diagram of the heating unit of FIG. 6 in heating mode,

[0122] FIG. 7b shows a diagram of the electrical circuit in heating mode,

[0123] FIG. 8a shows the block diagram of the heating unit of FIG. 6 in temperature measuring mode,

[0124] FIG. 8b shows a diagram of the electrical circuit in temperature measuring mode; and

[0125] FIG. 9 shows a state/time diagram of the switches in the heating unit.

[0126] In the following, the invention will be described in greater detail with reference to embodiments shown by the enclosed figures. It should be emphasized that the embodiments shown are used for example purposes only and should not be used to limit the scope of the invention.

DESCRIPTION OF EMBODIMENTS

[0127] In the explanation of the figures, identical or corresponding elements will be provided with the same designations in different figures. Therefore, no explanation of all details will be given in connection with each single figure/embodiment.

[0128] FIG. 1 shows an illustration of an infusion fluid warmer 100 according to the invention and FIG. 2 shows an exploded view of the main components of the infusion fluid warmer of FIG. 1.

[0129] The infusion fluid warmer 100 comprises an outer casing 200, a heating unit 300, see FIG. 2, an inlet tube unit 400, an outlet tube unit 500 and an attachment unit 600.

[0130] The infusion fluid warmer 100 is configured for being placed or attached directly onto a patients arm or other parts of the body. Therefore its size and weight is restricted by this requirement.

[0131] The outer casing 200 provides support for- and protection of the components of the infusion fluid warmer 100. Moreover, the outer casing 200 provides electrical insulation and some thermal insulation of the heating unit 300, such that the patient is protected from electrical shock and such that the electrical components are protected from damage due to for example electrostatic discharge (ESD). The heating unit 300 preferably operates at temperatures that are sufficiently low, for example below 42 degree C., to avoid burns. Therefore the thermal insulating capabilities of the outer casing 200 may be of secondary importance.

[0132] The outer casing 200 comprises an upper shell 202 and a lower shell 204. In the embodiment shown the upper shell 202 and the lower shell 204 respectively is formed with internal supports that complements the shape of the heating unit 300, the inlet tube unit 400 and the outlet tube unit 500 when installed in the outer casing 200 such that said units are held in a firm form-fit attachment to the outer casing 200, when the upper shell 202 and the lower shell 204 are mated.

[0133] In the embodiment shown the heating unit 300 is attached to the lower shell 204 by two screws 206 that are inserted through the heating unit 300 into studs 208 with holes formed in the lower shell 204.

[0134] In one embodiment the holes are threaded to match machine screws

[0135] In a further embodiment the holes are unthreaded to match self-tapping screws.

[0136] In the embodiment shown the upper shell 202 and the lower shell 204 are injection moulded plastic parts.

[0137] Alternatively the upper shell 202 and lower shell 204 can be formed in other suitable ways, for example by milling, casting or 3D printing.

[0138] The heating unit 300 is configured for heating to a required temperature, an infusion fluid that flows through the heating unit 300 to be used for intravenous therapy in a patient.

[0139] The infusion fluid enters the heating unit 300 through a fluid inlet port 302 that is in communication with the inlet tube unit 400 and exits through a fluid outlet port 304 that is in communication with the outlet tube unit 500.

[0140] The heating unit 300 is electrically powered through a DC power supply input 306 that is in communication with a receptacle 308 configured for receiving a power plug 782 of a power supply 700, see FIGS. 4 and 5.

[0141] The heating unit 300 is further described in FIG. 3.

[0142] The inlet tube unit 400 is a plastic tube 402 having a luer type connector 404 at its first end configured for connection of the inlet tube unit 400 to a supply of intravenous fluid. The second end of the plastic tube 402 is configured for fitting onto the fluid inlet port 302 of the heating unit 300. The fluid inlet port 302 comprises a barbed fitting for providing a leak tight connection to the tube 402.

[0143] The outlet tube unit 500 is a plastic tube 502 connected at its first end 504 to the fluid outlet port 304 of the heating unit 300. The fluid outlet port 304 comprises a barbed fitting for providing a leak tight connection to the tube 502. The second end of the plastic tube 502 is having a luer type connector 506 that is configured for connection of the outlet tube unit 500 to an intravenous access device, for example a catheter.

[0144] The plastic tubes 402, 502 are made of a flexible plastic material.

[0145] The attachment unit 600 is configured for attachment of the infusion fluid warmer 100 directly to the patient or a support arrangement in the vicinity of the patient.

[0146] In the embodiment shown the attachment unit 600 comprises an adhesive patch 602 that attaches the infusion fluid warmer 100 directly on the skin of the patient.

[0147] In a first embodiment of the attachment unit 600 the adhesive patch 602 comprises an upper adhesive layer 604 that adheres to the lower shell 204 of the infusion fluid warmer 100, a lower adhesive layer 606 that is configured for adhering to the skin of a patient and a carrier layer 608 separating the upper and lower adhesive layers 604, 606.

[0148] The upper adhesive layer 604 can be composed of any adhesive suitable for attaching the adhesive patch 602 to the infusion fluid warmer 100, for example an acrylic.

[0149] The lower adhesive layer 606 shall be composed of a medical grade adhesive, for example a medical grade silicone adhesive. The carrier layer is configured for preventing collapsing or wrinkling of the adhesive layers 604, 606. In an embodiment the carrier layer 608 is a foam layer. The carrier layer 608 can also be a textile or a film layer.

[0150] In a second embodiment (not shown), the attachment unit 600 comprises an adhesive patch with an outline corresponding to the outline of the lower shell 204 of the outer casing 200. The adhesive patch is composed of an upper adhesive layer for attaching the adhesive patch to the infusion fluid warmer 100 and a lower adhesive layer for attaching the adhesive patch to the patient.

[0151] The upper adhesive layer can be composed of any adhesive suitable for attaching the adhesive patch to the infusion fluid warmer 100, for example an acrylic. The lower adhesive layer shall be composed of a medical grade adhesive, for example a medical grade silicone adhesive, a hydrogel or a medical grade acrylic adhesive.

[0152] In this embodiment the lower shell 204 acts to prevent the collapsing and wrinkling of the adhesive layers.

[0153] FIG. 3 shows an exploded view of a heating unit 300 of the infusion fluid warmer 100 of FIG. 1.

[0154] The heating unit 300 comprises a receptacle 308 with connector pins 310, a heat exchanger 312, a first printed circuit board 314 hereinafter referred to as the first PCB, a second printed circuit board 316 hereinafter referred to as the first PCB, a plurality of heat exchanger assembly screws 318 and a heating unit assembly screw 320.

[0155] The receptacle 308 is described above.

[0156] The connector pins 310 interface with corresponding sockets in the power plug 782 of the power supply 700, see FIGS. 4 and 5. The connector pins 310 are in electrical contact with the DC power supply input 306 on the second PCB 314.

[0157] The heat exchanger 312 comprises an upper wall structure 322 and an opposing lower wall structure 324. A peripheral gasket 326 and a central gasket 328 is located between the upper wall structure 322 and the lower wall structure 324 to seal the fluid channel 331 and thereby prevent leakage of infusion fluid flowing through the heat exchanger 312. The infusion fluid warmer may comprise an electrically insulating frame or ring (not shown) surrounding and contacting peripheral edges of the upper and lower wall structure 322, 324. The electrically insulating frame or ring may comprise an elastomeric agent or composition such as rubber and may be arranged in-between the heat exchanger 312 and the outer casing 200 to avoid physical contact between these items. This optional arrangement of the electrically insulating frame or ring around the heat exchanger 312 provides an additional electrically insulating barrier between the infusion fluid and the outer casing 200 of the infusion fluid warmer to enhance its mains insulation. This arrangement of two separate electrically insulting barriers may be advantageous, or even mandatory, to comply with various official safety standards for medical equipment.

[0158] The upper wall structure 322 of the heat exchanger 312 is an elongate plate shaped member that has a fluid inlet port 302 at one end and a fluid outlet port 304 at its other end. A cavity 330 is formed in the upper wall structure 322. The cavity 330 extends between the fluid inlet port 302 and the fluid outlet port 304 of the heat exchanger 312. The cavity 330, together with the peripheral gasket 326 and the central gasket 328, defines the fluid channel 331 or passage, see FIG. 6-8 in the longitudinal direction of the heat exchanger 312 for the infusion fluid.

[0159] The flow path comprises three zones. The first zone is an inlet transition zone 332, where the cross section of the cavity 330 in a plane perpendicular to the longitudinal direction of the heat exchanger 312 transitions from a circular cross section to a polygonal cross section. The second zone is a central zone 334, where the cross section of the cavity 330 is unchanged from the polygonal cross section apart from the location of the heating unit assembly screw 320 and associated central gasket 328, where it changes to two separate polygons. In the embodiment shown the polygon has four sides and forms a rectangle. Cavity 330 is formed such that the polygon in the central zone has two opposing long sides facing the first and second PCBs 314, 316 respectively, and two opposing short sides where the long sides are significantly longer than the short sides. The third zone is an outlet transition zone, where the cross section of the cavity 330 transitions from a polygonal cross section to a circular cross section to interface with the fluid outlet port 304.

[0160] In the embodiment shown the thickness of the fluid channel is 0.2 mm. The width of the fluid channel is approximately 35 mm and the length approximately 60 mm. Thus, the width to thickness ratio of the fluid channel 331 in the central zone 334 is approximately 175:1.

[0161] The upper wall structure 322 has a plane bearing surface for the peripheral gasket 326 and the central gasket 328.

[0162] The lower wall structure 324 is a plane plate shaped member with an outline corresponding to the outline of the upper wall structure 322.

[0163] The lower wall structure 324 has a plane bearing surface for the peripheral gasket 326 and the central gasket 328.

[0164] The heat exchanger 312 is assembled by inserting the peripheral gasket 326 and the central gasket 328 between the upper and lower wall structures 322, 324 before said wall structures are moved together.

[0165] The heat exchanger assembly screws 318 are inserted through holes in the lower wall structure 324 into threaded holes in the upper wall structure 322. The heat exchanger assembly screws 318 are tightened to ensure a leak-proof flow path inside the heat exchanger 312.

[0166] The upper and lower wall structures 322, 324 are made of aluminium that has a high thermal conductivity.

[0167] The aluminium of the upper and lower wall structures is preferably passivated through anodizing. This adds a layer of aluminium oxide (Al.sub.2O.sub.3) to the surface of the aluminium. This aluminium oxide layer is bio-compatible and therefore the infusion fluid is allowed direct contact with the anodized surface of the upper and lower wall structures 322, 324 prior to distribution to the patient.

[0168] The anodizing provides good corrosion resistance of the aluminium. Moreover the aluminium oxide layer is electrically non-conductive. The thermal conductivity of the aluminium oxide layer is reduced in comparison to aluminium, but because the layer may be very thin, this has no noticeable effect on the operation of the heating unit 300.

[0169] The peripheral gasket 326 and the central gasket 328 are made of silicone, for example medical grade silicone.

[0170] The first PCB 314 has a first surface 338 and an opposing second surface 340. When installed into the heating unit 300 the second surface 340 is placed in contact with an outer surface 342 of the upper wall structure 322 of the heat exchanger 312. The second surface 340 of the first PCB 314 is plane to complement the shape of the outer surface 342 of the upper wall structure to ensure good contact across the surface. In addition, a layer of thermally conductive paste or film is added between the first PCB 314 and the upper wall structure 322 to ensure a good thermal connection or coupling between the two parts.

[0171] The second PCB 316 has a first surface 344 and an opposing second surface 346. When installed into the heating unit 300 the second surface 346 is placed in contact with an outer surface 348 of the lower wall structure 324 of the heat exchanger 312. The second surface 346 of the first PCB 314 is plane to complement the shape of the outer surface 348 of the lower wall structure to ensure good contact across the surface. In addition, a layer of thermally conductive paste or film is added between the second PCB 316 and the lower wall structure 324 to ensure good thermal coupling or connection between the two parts.

[0172] The first and second PCBs 314, 316 each has an electrically resistive pattern 350. In FIG. 3 the electrically resistive pattern on the second PCB 316 is hidden from view. However, the electrically resistive pattern on the second PCB 316 is similar to the electrically resistive pattern 350 on the first PCB 314 that is visible in FIG. 3.

[0173] In the embodiment shown the first and second PCBs 314, 316 the electrically resistive pattern each comprises five separate and integrally formed electrically resistive patterns formed on their respective second surfaces 340, 346.

[0174] In the embodiment shown, the electrical components that control the application of power to the electrically resistive pattern 350 is located on the first surface 344 of the second PCB 316. This includes a controller 352 in the form of an integrated circuit (IC) and a reference resistor 354.

[0175] The controller 352 is configured for controlling the application of power to the electrically resistive pattern 350 on the first and second PCBs 314, 316 and for determining the resistance in the electrically resistive pattern 350.

[0176] When power is applied to the electrically resistive pattern 350, power is dissipated in the material making up the electrically resistive pattern 350 to produce heat. Due to the resistive pattern 350 being in thermal contact with the outer surface 342, 348 of the upper and lower wall structure 322, 324 of the heat exchanger 312 the temperature of the upper and lower wall structure 342, 348 will rise in the vicinity of the powered resistive pattern 350. The temperature of the infusion fluid flowing through the fluid channel 331, see FIG. 6-8 will thus be elevated between the fluid inlet port 302 and the fluid outlet port 304.

[0177] Due to the ratio of width to height of the fluid channel of 175:1 as previously mentioned, the area of the electrically resistive pattern 350 which, when power is applied, acts as a heater or heating element is relatively high in relation to the thickness of the fluid channel. Moreover, due to the small height of the fluid channel, the heating of the infusion fluid is rapid. The skilled person will, based on the laws of thermodynamics, appreciate that it is possible to dissipate a relatively high amount of power to the infusion fluid with a relatively low temperature increase of the heat exchanger surfaces. Therefore the temperature of the heat exchanger structure is close to the temperature of the infusion fluid.

[0178] FIG. 4 shows an illustration of a power supply unit 700 for the infusion fluid warmer 100 of FIG. 1 and FIG. 5 shows an exploded view of the main components of the power supply of FIG. 4.

[0179] The power supply unit 700 comprises a power supply casing 720, a battery pack 740, a power supply controller 760 and a power cable assembly 780.

[0180] The power supply casing 720 provides support for and protection of the components within the power supply unit 700. Additionally, the power supply casing 720 provides electrical insulation, such that the personnel is protected from electrical shock and such that the electrical components are protected from damage due to for example electrostatic discharge (ESD).

[0181] The power supply casing 720 comprises a front shell 722 and a back shell 724. When assembled an eye 726 is formed by complementary apertures 728 in the front and back shell, respectively for hanging the power supply unit 700 from an IV stand, for example with a hook or a web through the eye 726.

[0182] The front shell 722 and back shell 724 are formed with cut-outs 730 that each forms half of an opening when the front and back shells 722, 724 are assembled.

[0183] The back shell 724 and optionally the front shell 722 each are formed with a non-slip material (not shown) over part of its surface. The non-slip material is a thermoplastic elastomer (TPE) or a rubber material.

[0184] The front shell 722 is formed with a plurality of small openings 732 providing a visible access to the LEDs that provide status information. A foil label 734 is provided to cover the openings 732. A recess 738 having an outline corresponding to the outline of the label 734 and a depth corresponding to the thickness of the label is formed on the outside of the front shell 722. The foil label 734 has transparent or semi-transparent windows aligned with the LEDs, such that the openings 732 into the internal of the power supply casing 720 are closed while at the same time the LEDs are visible through the windows and able to provide status information to the personnel.

[0185] In an embodiment not shown the infusion fluid warmer 100 comprise audible alarming means for audibly providing status and alarms.

[0186] In the embodiment shown the front shell 722 and the back shell 724 are injection moulded plastic parts that are moulded in a 2K moulding process where two materials are introduced during the moulding process.

[0187] First a mixture of polycarbonate and acrylnitrile-butadiene-styrene (PC-ABS) is introduced into the mould to form the shell. Then a thermoplastic elastomer is introduced to form the non-slip surface over part of the shell.

[0188] The front shell 722 and back shell 724 are connected through ultrasonic welding.

[0189] The battery pack 740 comprises a plurality of battery cells 742, for example lithium polymer (Li—Po) or lithium ion (Li-Ion).

[0190] The power supply controller 760 comprises a printed circuit board (PCB) 762 with a controller configured for providing electrical power at a specified voltage from the battery pack 740 to the power cable assembly 780 and ultimately the infusion fluid warmer 100, configured for providing status information through LEDs that are visible through the front shell 722 and configured for controlling the recharging of the battery pack 740.

[0191] The power cable assembly 780 comprises a power plug 782, a cable 784 and a connection means 786 for electrically connecting the cable to the power supply controller 760.

[0192] The power plug 782 is configured for being inserted into the receptacle 308 of the infusion fluid warmer 100. The sockets of the power plug 782, when inserted into the receptacle, mates with connector pins 310 and provides an electrical connection between the infusion fluid warmer 100 and the power supply unit 700.

[0193] FIG. 6 shows a simplified schematic block diagram of the electrical circuit 356 and the heat exchanger 312 of the heating unit 300.

[0194] In the block diagram only the positive side of the electrical circuit has been shown for simplicity. The skilled person will understand that each of the electrically resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10 are connected to the negative potential of the DC power supply V.sub.DD, for example via a ground connection.

[0195] The heat exchanger 312 is shown with the associated electrical components. The fluid channel or passage 331 is bound by an upper wall structure 322 with a first PCB 314 attached and a lower wall structure 324 with a second PCB attached 316.

[0196] A first integrally formed electrically resistive pattern R.sub.1 and four additional and separate integrally formed electrically resistive patterns R.sub.2, R.sub.3, R.sub.4, R.sub.5 or simply a plurality of integrally formed electrically resistive patterns are formed on the second surface 340 of the first PCB 314. A second integrally formed electrically resistive pattern R.sub.6 and four additional and separate integrally formed electrically resistive patterns R.sub.7, R.sub.8, R.sub.9, R.sub.10 or simply a plurality of integrally formed electrically resistive patterns are formed on the second surface 346 of the second PCB 316.

[0197] An electronic switching circuit is provided that comprises a plurality of controllable semiconductor switches G.sub.1/SW.sub.1, G.sub.2/SW.sub.2, G.sub.3/SW.sub.3, G.sub.4/SW.sub.4, G.sub.5/SW.sub.5, G.sub.6/SW.sub.6, G.sub.7/SW.sub.7, G.sub.8/SW.sub.8, G.sub.9/SW.sub.9, G.sub.10/SW.sub.10 each coupled in series with a respective resistive pattern R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10. The switches G.sub.1/SW.sub.1, G.sub.2/SW.sub.2, G.sub.3/SW.sub.3, G.sub.4/SW.sub.4, G.sub.5/SW.sub.5, G.sub.6/SW.sub.6, G.sub.7/SW.sub.7, G.sub.8/SW.sub.8, G.sub.9/SW.sub.9, G.sub.10/SW.sub.10 are coupled to a common circuit node V.sub.MES. Each of the controllable semiconductor switches G.sub.1/SW.sub.1, G.sub.2/SW.sub.2, G.sub.3/SW.sub.3, G.sub.4/SW.sub.4, G.sub.5/SW.sub.5, G.sub.6/SW.sub.6, G.sub.7/SW.sub.7, G.sub.8/SW.sub.8, G.sub.9/SW.sub.9, G.sub.10/SW.sub.10 may comprise a MOSFET such an NMOS or PMOS transistor.

[0198] A reference controllable semiconductor switch G.sub.Ref/SW.sub.Ref is coupled between the DC power supply input V.sub.DD and the common circuit node V.sub.MES. The reference controllable semiconductor switch G.sub.Ref/SW.sub.Ref may comprise a MOSFET such an NMOS or PMOS transistor. A first reference resistor R.sub.Ref is coupled across the input and output terminals of the reference switch G.sub.Ref/SW.sub.Ref. The first reference resistor R.sub.Ref may comprise a precision resistor with small tolerance, e.g. less than 1%, and preferably also small temperature coefficient.

[0199] In an alternative embodiment the switches G.sub.6/SW.sub.6, G.sub.7/SW.sub.7, G.sub.8/SW.sub.8, G.sub.9/SW.sub.9, G.sub.10/SW.sub.10 connected to the second integrally formed electrically resistive pattern R.sub.6 and the four additional and separate integrally formed electrically resistive patterns R.sub.7, R.sub.8, R.sub.9, R.sub.10 formed on the second surface 346 of the second PCB 316 are coupled to a second common circuit node V.sub.MES2 (not shown). The alternative circuit comprises a second controllable semiconductor switch G.sub.Ref2/SW.sub.Ref2 (not shown) coupled between the DC power supply input V.sub.DD and the second common junction point V.sub.MES2 (not shown). A second reference resistor R.sub.Ref2 is coupled across the terminals of the second switch G.sub.Ref2/SW.sub.Ref2.

[0200] In an embodiment the controller includes a proportional-integral-derivative controller (PID controller) for controlling the power dissipation in each of the electrically resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10.

[0201] FIG. 7a shows the block diagram of the heating unit of FIG. 6 in heating mode and FIG. 7b shows an equivalent diagram of the electrical circuit 356 in heating mode.

[0202] In FIG. 7a the reference switch G.sub.Reff is closed and the switch G.sub.1/SW.sub.1 is closed. Therefore the first resistive pattern R.sub.1 is coupled to the DC power supply input V.sub.DD during a first time period of operation.

[0203] In FIG. 7b the equivalent electrical circuit corresponding to the heating mode for the electrically resistive pattern R.sub.1 is shown.

[0204] The reference resistor R.sub.Ref is coupled in parallel with the resistance SW.sub.Ref of the switch G.sub.Ref. The resistance of the switch SW.sub.1 and the electrically resistive pattern R.sub.1 is coupled in series with the former two resistances. The electrically resistive pattern R.sub.1 is connected to the negative side of the DC power supply voltage.

[0205] The resistance of the reference resistor R.sub.Ref is 75 ohm and the resistance of each of the switches G.sub.Ref, SW.sub.1 is approximately 15 mohm. The majority of the power delivered by the DC power supply V.sub.DD is dissipated in the electrically resistive pattern R.sub.1. In an example the resistance of the electrically resistive pattern R.sub.1 is 5 ohm. With a 24 VDC of the DC power supply V.sub.DD an electric power of 114 W will be dissipated in the electrically resistive pattern R.sub.1.

[0206] FIG. 8a shows the block diagram of the heating unit of FIG. 6 in temperature measuring mode during a second time period. FIG. 8b shows an equivalent diagram of the electrical circuit 356 in the temperature measuring mode.

[0207] In FIG. 8a the reference switch G.sub.Ref/SW.sub.Ref is open or non-conducting and the switch G.sub.1/SW.sub.1 is closed or conducting. The reference resistor R.sub.Ref is coupled in series with the switch G.sub.1/SW.sub.1 and the electrically resistive pattern R.sub.1. The electrically resistive pattern R.sub.1 is connected to the negative side of the DC power supply input.

[0208] The DC voltage of the DC power supply V.sub.DD is known in advance or measured during operation of the heating unit 300 and voltage is measured at V.sub.Mes.

[0209] Based on the measured DC voltage at V.sub.Mes and the known resistance of the reference resistor R.sub.Ref the current in the circuit can be calculated. As a resistance of R.sub.SW1 is either known, or preferably insignificant compared to the resistance of R.sub.1, the resistance of the electrically resistive pattern R.sub.1 is the only unknown. Hence the resistance of R.sub.1 can easily be determined or calculated based on the known circuit variables.

[0210] The determined resistance of the electrically resistive pattern R.sub.1 allows the instantaneous temperature of the electrically resistive pattern R.sub.1 to be determined or computed based on a known temperature coefficient of the electrically resistive pattern R.sub.1. The instantaneous temperature of the electrically resistive pattern R.sub.1 is used for controlling the heating.

[0211] FIG. 9 shows a state/time diagram showing an example of the respective states, i.e. either conducting/closed or non-conducting/open, of the switches SW.sub.Ref, SW.sub.1, SW.sub.2, SW.sub.3, SW.sub.4, SW.sub.5, SW.sub.6, SW.sub.7, SW.sub.8, SW.sub.9, SW.sub.10 of the heating unit of FIG. 6 during a time period of 140 ms.

[0212] In the diagram a value of 0 indicates that a switch is open and a value of 1 indicates that a switch is closed.

[0213] In the present exemplary embodiment, the first time period is set to last 100 ms. The first time period is subdivided into 5 ms ticks or sub-intervals defining a minimum time a switch can be closed. Alternatively, the length of the first time period and the subdivision can be adjusted to other lengths as required by a particular application, for example if the precision of the temperature control needs to be improved.

[0214] The second time period is set to last for 20 ms and is subdivided into 1 ms ticks. The controller may select to close or open a switch for a minimum duration of 1 ms. However, the measurements may be completed within a much smaller duration, i.e. as little as 1.5 ms for all resistors.

[0215] The diagram shows the three distinct time periods. From −20 ms to 0 ms—a delay where all switches are open. From 0 ms to 20 ms—a second time period, where the temperature of each electrically resistive pattern R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10 is determined. From 20 ms to 100 ms—a first time period where the DC power supply is selectively connected to the resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10. From 100 ms to 120 ms—a second delay period, where all switches are open.

[0216] During the delay periods, as previously mentioned, all switches are open. Therefore no power is dissipated in any of the electrically resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10. The temperature of said electrically resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10 will therefore converge towards the temperature of the infusion fluid in the fluid channel 331 or passage in the heat exchanger 312 due to the good thermal coupling between the infusion fluid in the fluid channel 331 or passage and the PCB holding the resistive patterns via the aluminium heat exchanger.

[0217] During the second time period each of the switches SW.sub.1, SW.sub.2, SW.sub.3, SW.sub.4, SW.sub.6, SW.sub.6, SW.sub.7, SW.sub.8, SW.sub.9, SW.sub.10 is closed briefly one after the other to sequentially connect the electrically resistive pattern R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10 to the DC power supply inlet for measuring the voltage at the junction point V.sub.MES and thereby determine the temperature of each electrically resistive pattern R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10. Each switch may be closed for a brief a time period as possible to limit the power dissipation in each resistive pattern.

[0218] The controller performs power management based on the temperature measurements. The controller 352 calculates the required power to be dissipated in order to maintain or increase the temperature of the infusion fluid in the fluid channel 331 or passage. The power dissipation may be distributed between the electrically resistive patterns R.sub.1, R.sub.2, R.sub.3, R.sub.4, R.sub.5, R.sub.6, R.sub.7, R.sub.8, R.sub.9, R.sub.10 such that the coldest resistive patterns are prioritised. Moreover, the maximum current that can be drawn from the DC power supply input may also be considered.

[0219] In the present example it is assumed that the temperature determination during the second time period has found that the temperature of the infusion fluid decreases towards the outlet end. Therefore, the resistive patterns and corresponding switches closer to the outlet is prioritised during the subsequent first time period initiated at 20 ms. Switch SW.sub.Ref is closed from 20 ms to 100 ms that is the complete duration of the first time period in order to connect the DC power supply input in heating mode. Switches SW.sub.5 and SW.sub.10 that lead to power dissipation in the resistive patterns closest to the outlet end are closed as the first switches for 15 ms each. Then switches SW.sub.4 and SW.sub.9 are closed for 10 ms. The remaining switches in the following order SW.sub.3, SW.sub.8, SW.sub.2 SW.sub.7, SW.sub.1, and SW.sub.6 are closed for 5 ms each. Therefore, more power has been dissipated in the electrically resistive patterns R.sub.4, R.sub.5, R.sub.9, R.sub.10 closer to the outlet than dissipated in the other electrically resistive patterns. Therefore, a higher temperature of the infusion fluid may be seen closer to the outlet end during the next temperature measurement after the second delay period (100 ms to 120 ms).

[0220] In subsequent first time periods the order and duration in which the switches open and close may change according to the temperature determination in the second time period immediately prior to a following first time period.

[0221] It is to be noted that the figures and the above description have shown the exemplary embodiments of the infusion fluid warmer in a simple and schematic manner.