PORTABLE DISINFECTION SYSTEM BASED ON INDUCTION HEATING

FIELD OF THE INVENTION

The present invention relates generally to the field of periprosthetic joint infection treatment through implant disinfection. More specifically, the invention proposes a portable, sealable and sterilizable disinfection system based on induction heating, which achieves effective elimination of microorganisms and biofilms in implanted prosthesis.

BACKGROUND OF THE INVENTION

Periprosthetic infections of joints with implants constitute a relevant problem for patients, surgeons and, in general, for public health systems. Microorganisms often produce biofilms on the implant’s inert material, hindering the appropriate treatment for the patient after the implantation of a prothesis. Although current combinations of techniques, such as DAIR (debridement, antibiotic, and implant retention treatments), achieve an important reduction of bacterial loads thanks to the removal of infected tissues (associated with washing/scrubbing and the use of chemical agents, such as antiseptics), in 50% of the cases these treatments are not sufficient to completely remove the infection once biofilm formation has reached a certain bioburden threshold. Furthermore, as a result of the increasing resistance of bacteria to antibiotics, patients often require invasive surgeries in which the prosthesis needs to be removed and replaced, after long periods of pharmaceutical treatment. This situation leads to health risks for the patients, as well as social costs in terms of quality of life and dependency, with a corresponding increase of treatment costs which are, roughly, ten times higher than a prosthesis implantation without complications.

As an alternative or a supplementary technique eliminate prosthetic infections, non-contact inductive heating of prostheses and/or implants has been proposed. Studies on hyperthermia treatment for cancer have shown that it is possible to heat metal objects transcutaneously and selectively within the body, for instance with the application of pulsed electromagnetic fields (PEMF) and the induction of so-called ‘eddy currents’, i.e., electric currents within the metallic object that oppose the change in PEMF, as derived from Faraday's law of electromagnetic induction, consequently causing the heating of the metallic implant. In theory, every metal implant is suitable for induction heating depending on its anatomical situation. The major advantage of induction heating is that only the metallic implant is actively heated, while the surrounding tissue remains mostly unheated. Thus, induction heating generated in the implant can be applied mainly onto the bacterial biofilm, while the surrounding tissue is only subject to indirect heating, mainly by thermal conduction from the heated metal.

Metal implants used for in vitro studies are usually heated entirely. However, heating the metal implant in vivo in its entirety may have disadvantages, as metal implants can have complex geometrical shapes and are also fixed to bone. These disadvantages include tissue necrosis from a high thermal dose, heterogeneous heating, and possibly loss of bone fixation. However, well perfused tissue and/or irrigation may maintain the heating dispersion into the bone at an acceptable level, such as during cementing techniques which exothermic reaction up to 80°C is well tolerated by the host bone.

Studies (see, for instance, [Pijls B.G., Sanders I.M.J.G., Kuijper E.J., Nelissen R.G.H.H., Non-contact electromagnetic induction heating for eradicating bacteria and yeasts on biomaterials and possible relevance to orthopaedic implant infections: In vitro findings. Bone Joint Res. 2017 May;6(5):323-330] or [Pijls B.G., Sanders I.M.J.G., Kuijper E.J., Nelissen R.G.H.H., Segmental induction heating of orthopaedic metal implants. Bone Joint Res. 2018 Dec 1 ;7(11 ):609-619]) have shown not only that several types of bacteria and yeasts can be eliminated from the surface of a metallic prosthesis through inductive heating, but also that segmental inductive heating, i.e., heating only one portion of the implant, is safe for the surrounding tissue at certain distances from the heating center.

In the field of hyperthermia cancer treatment, several studies have shown the feasibility of induction heating of "thermal seeds" and nanoparticles. In the field of fracture healing with shape memory devices, the document [Muller C. W., EIKashef T, Pfeifer R., et al. Transcutaneous electromagnetic induction heating of an intramedullary nickel-titanium shape memory implant. Int Orthop; 2014;38:2551-2557] has shown the feasibility and safety of contact-free electromagnetic induction heating of Nickel Titanium alloy (NiTi) implants in a rat model. In this document, however, there are no reports of non-contact heating of orthopedic implants by induction heating to prevent and treat infections of orthopedic implants.

Other systems implementing inductive heating of prostheses and/or implants can be found in the art, such as the one described in patent application WO 2020/067898 A2. This document refers to an apparatus and a method for inductive heating of prostheses or implants, or at least portions thereof. The apparatus comprises an induction coil which inductively heats a portion of a prosthesis, and is arranged externally to a body or housing of the apparatus. It also comprises a low-voltage and low-powered apparatus with voltages ranging between 12 V and 60 V and operating at operating at a frequency of a Pulsed Electromagnetic Field (PEMF) of less than about 100kHz.

Patent application US 2021/0106429 A1 , on the other hand, discloses a device and a method to generate an electric current in a conductor. The device comprises a magnetic field generator element (toroidal coil), a control element and a support element. These elements provide an electric current in a conductor in a non-invasive manner, preventing and/or treating the formation of biofilm on a conductor (metallic implant).

Document WO 2018/013935 A1 refers to different methods, apparatuses, systems, and implementations of inductive heating of a foreign metallic implant in order to treat bacterial biofilms. The implant may be heated via alternating magnetic fields (AMF) that allow to heat the surface in a uniform manner. The systems disclosed in this document may comprise acoustic sensors which indicate if the tissue is being heated to an undesirable level. Thus, when this occurs, the AMF pulses may be shut down for a period of time avoiding tissue damage.

In patent application WO 2011/130104 A1 a method for modifying a medical implant having a shape-memory portion is disclosed. This method comprises the use of a probe having a tip provided with an induction coil, electrically coupled to an induction power supply. The power supply is activated at a suitable frequency to cause the induction coil to generate a magnetic field. This field induces eddy currents in the shape-memory portion, heating it to a phase transformation temperature to produce a shape change of the implant. This document also discloses an induction coil heater system for activating a shape-memory metal alloy medical implant. The system includes a handpiece that can be sterilized and that is constructed of materials that permit it to be a disposable single-use component. The handpiece includes a handle, a probe, and an induction coil.

The previous documents disclose methods and systems for implementing inductive heating of prosthetics and/or implants to be used in medical scenarios with living, human patients. However, none of those documents describes a system in which all the components of the heating device (coil, electronics, power supply, etc.) are sterilizable and insulated together in a single tool. Therefore, there is no disclosure in the art of an apparatus that allows for inductive heating within a surgical setting and, at the same time, reducing the risks of bacterial infection at the connections between the main body of the system and the induction coil, while also avoiding electrical risks for the user and providing effective thermal dissipation.

The present invention proposes a solution to the technical limitations of the methods and devices mentioned above, by means of a portable, ergonomic, sterilizable, and sealable system which eliminates bacteria by hyperthermia and which can be used in a surgical setting.

SUMMARY OF THE INVENTION

In order to mitigate the shortcomings mentioned above, the present invention proposes a portable and sealable induction heating system which allows for the surgical effective elimination of biofilm and microorganisms adhered to an implanted prosthesis, by means of localized hyperthermia of their metallic elements by induction heating. This invention can also be applied to other techniques of magnetic hyperthermia in the biomedical area, such as the ablation of tumors by means of magnetic elements or nanoparticles, among others.

A first object of the invention relates to a portable induction heating system comprising an induction coil, a DC-AC (direct current to alternate current) power electronic stage connected to the induction coil, and a DC power supply stage connected to the DC-AC power electronic stage, in which these elements are housed inside a sealable insulation housing. The insulation housing further comprises a sterilizable, electrically insulating, thermally conductive material, which reduces the risks of bacterial infection at the connections between the main body of the system and the induction coil, while also avoiding electrical risks for the user and providing effective thermal dissipation.

Advantageously in the invention, the DC-AC power electronic stage comprises a wide- bandgap semiconductor bridge-type power stage. This configuration allows for a highly compact and efficient design, since this electronic architecture has a substantially smaller size than the alternatives found in the art. Besides, the use of wide-bandgap semiconductor electronics (for example, based on SiC or GaN semiconductors) provides a very high efficiency dissipating heat compared to other electronic setups. As a result, the system of the invention can be completely sealed (i.e., without the use of ventilation orifices or grooves), thereby improving its sterilizability and, furthermore, without requiring the use of air or liquid cooling means, unlike current existing systems.

With the above features, the proposed system achieves two main advantages of great interest for its application in the medical field. First, the system allows for an effective elimination of bacteria by hyperthermia, which cannot be obtained by currently available techniques. This is achieved by increasing the temperature of the metallic implant to the range of 60-75°C, as inspired by food preserving techniques.

And second, the proposed system is portable, ergonomic, and completely sealable (and, preferably, comprises no independently moving parts or slots). This makes it easily sterilizable, and thus feasible for its use in an operating room (unlike current industrial induction heating systems). This is due to the use of new high-efficiency power electronic devices, which allow a highly compact and efficient design that does not require air or liquid cooling, unlike currently existing systems. Furthermore, this compact design allows for heating the metal implant by overlapping segments, often dubbed segmental induction heating, which represents and additional advantage of using the non-heated metal part of the prosthesis as a heat sink, thereby minimizing the thermal dose and thermal damage to soft tissue and bone. Also, segmental induction heating allows for more homogeneous heating of a metal implant. This is particularly relevant for objects with an irregular shape, such as the femoral stem of a total hip prosthesis. Homogeneous heating is also important to prevent overheating of certain areas of the implant, while other areas are underheated. In addition, segmental induction heating can be used for selective heating of one or two segments. For example, in the event of a more local infection, such as after intramedullary nailing for an open tibial fracture, only one or two segments of the nail may require heating. Also, areas of bone fixation can be avoided or heated to a lower temperature using segmental induction heating.

In another preferred embodiment of the invention, the induction coil comprises a magnetic flux concentrator, so that it can be applied externally to the prothesis by using a planar design. The magnetic flux concentrator may be composed of ferrite or other high-permeability material. Unlike other solenoidal designs, this design enables real usage of the device during a surgical intervention.

In a further preferred embodiment of the invention, the DC power supply stage comprises a grid- powered AC-DC power electronic stage, and/or a battery connected to the DC-AC power electronic stage through a DC-DC converter. The former may correspond to a rectifier AC-DC circuit so that the system can be powered via an AC current, while the latter provides an autonomous and possibly wireless powering method for the system.

In order to control the amount of power delivered to the induction coil, and, thus, the heating rate, in a further preferred embodiment of the invention the DC-AC power electronic stage is adapted to generate a variable voltage and current in the induction coil, in the range of 20 to 500 kHz. Besides, with the aim of further controlling and adjusting the delivered power, in other preferred embodiments of the invention the system comprises control means adapted for controlling the modulation of the DC-AC power electronic stage during its operation, and/or monitoring means adapted for monitoring the current generated in the induction coil, so as to control the applied power, detect the presence of external objects, and/or protect the system from overcurrent.

In other preferred embodiments of the invention, the system further comprises visual signaling means adapted for providing information about the state of operation of the system, and/or a digital control system adapted for activating and/or stopping the operation of the system.

Regarding the means for controlling and adjusting the heating power, in different embodiments of the invention they can be operated manually, automatically or through a combination of both. In particular, in a preferred embodiment of the invention, the system comprises means adapted for manually selecting the heating power to be delivered by the induction coil. In another preferred embodiment of the invention, the system further comprises digital control means adapted to automatically regulate the heating power to be delivered by the induction coil, according to one or more operating conditions of the system. These operating conditions comprise at least one of: temperature, voltage, current and operating power.

With the aim of avoiding damage to the tissues surrounding the implant, in another preferred embodiment of the invention the system further comprises digital control and temperature monitoring means, adapted for stopping the operation of the system when a desired temperature has been reached. More preferably, said temperature is the temperature of an object heated by the system. This advantageously allows to set different temperatures at which the system must stop depending on the placement of the prosthesis or implant and, thus, on the type of tissue that surrounds it. In particular, in a preferred embodiment of the invention, that desired temperature is in the range of 60-75°C, which is known to be sufficient to eliminate infectious microorganisms of the prosthesis or implant without burning adjacent bone and/or muscle. In another preferred embodiment of the invention, the desired temperature is the Curie temperature reached by an object heated by the system, that can be much higher than 60°C. In summary, the present invention proposes an induction heating system, able to eliminate biofilms and microorganisms through localized hyperthermia of the metallic elements of prostheses or implants, and which is suitable for its surgical use in operating rooms, since all its components are housed in a sterilizable, electrically insulating and thermally conductive insulation housing. This is advantageously achieved by means of its electronic setup, based on a wide-bandgap semiconductor bridge-type power stage that allows for a highly compact and efficient design, and also provides very high-efficiency heat dissipation. As a result, the system of the invention can be completely sealed (and, thus, easily sterilized) without requiring the use of air or liquid cooling means, unlike current existing systems.

BRIEF DESCRIPTION OF DRAWINGS

The foregoing and other advantages and features will be more fully understood from the following detailed description of exemplary embodiments with reference to the accompanying drawings, which should be considered by way of illustration and not limitation, in which:

Figure 1 shows a schematic view of the system, according to a preferred embodiment of the invention, before sealing the housing of said system. A lid has been removed so that the inside of the system and its different constituent elements are shown.

Figure 2 shows a schematic representation of the supporting structure of the induction coil comprised in the system, as well as of the electronics thereof, according to a preferred embodiment of the invention.

Figure 3 shows an axial cut view of the induction coil and the supporting structure, according to a preferred embodiment of the invention, where the arrangement of said coil in said structure as well as the arrangement of the magnetic flux concentrator are shown.

Figure 4 shows an example of use of the system over a non-implanted knee prosthesis, for the measurement of temperature thereon.

Figure 5 shows a full-bridge MOSFET inverter comprising four power derives corresponding to the DC-AC power electronic stage of the system, according to a preferred embodiment of the invention. Figures 6a-6c show the input control signals and the output voltages and currents for a system according to the embodiment of Figure 5, wherein the results are shown for maximum power (Figure 6a) medium power using increased switching frequency (Figure 6b) and medium power using phase shift (Figure 6c).

Numerical references used in the drawings:

DETAILED DESCRIPTION OF THE INVENTION

Figure 1 depicts a sealable induction heating system (1) according to the invention wherein, for illustrative and non-limiting purposes, said system (1) is shown unsealed so that its inner elements can be more easily identified. Under this embodiment, the system (1) comprises an induction coil (2), preferably mounted on a supporting structure (3), and a printed circuit board (PCB) (4) comprising the electronics configured for powering and controlling the application of heat. More preferably, the PCB (4) can also comprise a switch (5) adapted to turn the system (1) on and off, a wheel or knob (6) used as means adapted for manually selecting the heating power to be delivered by the induction coil (2), and/or a connector (7) to connect the system (1) to the power grid. Alternatively, the system (1) can be configured with a battery (for instance, a rechargeable battery) as a power source.

As described in preceding sections, one of the main advantages of the system (1) is that all of its components can be encapsulated inside a sealable insulating housing (8), adapted to house at least the coil (2) and the PCB (4). This allows for an easy use in operating rooms, due to its compact size, in contrast with other known systems, as well as for an easy sterilization. In order to do so, the housing comprises preferably a sterilizable material such as a biocompatible polymer, a metal, or a combination thereof. According to a preferred embodiment of the invention, the housing is preferably externally metallic (except for the part in which the induction coil is located) whereas the inner part of the housing comprises an electrically insulating material in order to insulate the internal electronics. This facilitates the disinfection of the system and shields the electromagnetic emissions of the device. According to another embodiment of the invention, the housing further comprises a plastic coating on the outside.

As seen in Figure 1 , the system (1) has preferably a size and a shape adapted to be used ergonomically with one hand (for example, an elongated size). Moreover, the position of the induction coil (2) is preferably arranged at one end of the system (1) body, such that it can be well identified for a correct positioning. In this way, the user can point the coil (2) directly and in an easy way towards an object to be heated, thus allowing for an intuitive use of the system (1) during a segmental induction heating procedure.

In Figure 1 , a portion of the housing (8) (seen in the left of the image), such as a lid (9), can be removed to access the inside of the system (1) (seen in the right part of the image), including the induction coil (2) and all the control and power supply means mounted on the PCB (4). Under this embodiment, the contact regions between said first portion or lid (9) and the rest of the housing (8) are also insulating, so that the sealing of the whole housing’s body can be efficiently performed.

Figure 2 shows a schematic view of the inner part of the system (1) of the invention, according to a preferred embodiment of the invention, in which its different constituent parts are depicted, and Figure 3 shows a schematic representation of an axial cut view of the system (1), wherein the induction coil (2) configuration is shown at front. In the figures, the induction coil (2) is supported by at least three elements, which in different embodiments can be incorporated individually or in combination in the system (1):

- A planar magnetic flux concentrator (10), preferably made of ferrite or other high- permeability material, which serves also as base for other parts of the supporting structure (3), configured to collect and point the magnetic field generated by the induction coil (2) towards an object to be heated.

- A central cylindrical piece or bobbin (11) (arranged preferably at a central region of the supporting structure (3)), around which the induction coil (2) is rolled up and that provides stability to the system (1) and to the induced magnetic field.

Two supporting pieces (12, 12’), preferably placed at the periphery of the supporting structure (3), having preferably an inner curved surface adapted to the shape of the coil (2).

With this configuration, the induction coil (2) can be applied externally to the prostheses or implants, thanks to the planar design of the magnetic flux concentrator (10) which works, in turn, as a base for the other elements, making the induction heating process more efficient since the magnetic field generated by the induction coil (2) is collected and oriented appropriately. Furthermore, the induction coil (2) is supported by the cylindrical piece or bobbin (11) and the lateral supporting pieces (12, 12’), making the system (1) more stable against sudden moves or shakes that could take place during its use. Unlike other solenoidal designs, this design enables the usage of the system (1) during a surgical intervention.

To have a better understanding of the performance of the system (1), Figure 4 shows a representation of its use over a non-implanted knee prosthesis or target (13) and the consequent measurement of the increased temperature taking place in the latter. In a similar way, the system (1) may be used over a living patient, leading to the disinfection of the already implanted prosthesis, in an easy and safe way.

In the preferred embodiment shown in Figure 4, a user holds the system (1) in one hand and points it (more specifically, points the part of the system (1) in which the induction coil (2) is housed) towards a metallic element corresponding to a knee prosthesis or target (13). Note that the system (1) must be facing towards the target (13), i.e., with the side of the system (1) in which the base of the supporting structure (3) and magnetic flux concentrator (10) is placed facing the target (13). In the figure, the system (1) is connected to the power grid through a cable (14). However, in other embodiments of the invention, the system (1) can be wirelessly powered with a battery. Infrared imaging of the target (13) can be taken by means of an infrared camera (15). Temperature readings (16) in the camera show an increase in the temperature of the prosthesis or target (13) after a few seconds of application of induction heating, and, more specifically, in the region at which the center of the induction coil (2) (corresponding also to the center of the cylindrical piece (12) of the supporting structure (3)) is directly pointing, where this increase is maximal. The temperatures reached can preferably range between 60 and 75°C, which are known to be sufficient to eliminate infectious microorganisms of the prosthesis, implant or target (13) without harming adjacent bone, muscle and/or other surrounding tissues. In other embodiments of the invention, the temperature reached is the Curie temperature of the material of which the target (13) sample is made. From the maximal point outwards, heat dissipates throughout the rest of the prosthesis and, thus, temperature readings (16) on the infrared camera (15) register lower values as they are taken further from the area at which the center of the induction coil (2) is directly pointing. This shows an example of segmental induction heating as performed by the system (1) of the invention, highly beneficial since a small area can be treated increasing its temperature while keeping the surrounding tissues and structures undamaged.

Figure 5 shows a preferred embodiment of the electronic architecture of the system (1) (which, as previously described, can be integrated in a PCB (4)). Under this embodiment, the system (1) comprises a DC power supply stage, preferably implemented as an AC-DC circuit which, in a particular form, comprises a full-bridge diode rectifier. As seen in Figure 5, the rectifier comprises a plurality of rectifier groups (17, 17’, 17”, 17”’), each of them comprising a diode (D) and one or more resistances (R). The AC-DC circuit can be optionally connected to the grid through an electromagnetic compatibility (EMC) filter. In other embodiments of the invention, the DC power supply stage can alternatively be connected to a battery.

In a preferred embodiment of the invention, the DC power supply stage is connected to a DC-AC power electronic stage, preferably configured as full-bridge inverter. More preferably, Figure 5 shows a full-bridge inverter composed of power derives QH1 , QH2, QL1 , QL2, implemented by wide-bandgap MOSFET transistors (18, 18’, 18”, 18’”), which allow for a highly compact and efficient design due to its small size. Through the full-bridge MOSFET inverter configuration, the power delivered to the induction coil (2), which in turn controls the heating rate, can be controlled either using frequency control (the higher the frequency the lower the output power) or phase-shift control (the higher the phase shift, the lower the output power). Figures 6a-6c show the input control signals (V _g ^H1 , V _g ^L1 , V _g ^H2 , V _g ^L2 ) at the rectifier groups (17, 17’, 17”, 17”’) for a given frequency (‘fsw’) and phase (‘phi’), as well as the output voltage (V _o ) and current (l ₀ ). Figure 6a shows the results for maximum power, while Figures 6b-6c show the results for medium power, using increased switching frequency and phase shift, respectively.

Experimental results

With the configurations depicted in Figures 1-6, it is then possible to eliminate infectious microorganisms through non-contact induction heating, and reducing the load of bacteria and yeasts preventing tissue burning or damage.

To test the efficiency of the system (1), experimental measurements have been made for biofilm models of S. aureus ATCC25923, E. coli ATCC25922 and S. epidermidis, cultivated over titanium-aluminum-vanadium (TiAIV) and chromium-cobalt-molybdenum (CrCoMo) disks, and exposed to stable temperatures of 70° for 210 seconds using the system (1) of the invention. Within this setup, the reduction in colony-forming units (CFUs) was quantified via a Mann-Whitney test (p < 0.05) as compared to previously stablished control disks without any thermal treatment. It was observed that all the bacterial species showed a significant statistical reduction in the average number of CFUs in both materials: 68.88% in TiAIV and 74.99% in CrCoMo for S. aureus, 99.97% in TiAIV and 99.96% in CoCrMo for E. coli, and 94.00% in TiAIV and 57.63% in CoCrMo for S. epidermidis.

These results show that the bacterial load in a mature biofilms can be substantially reduced from the surface of titanium and chromium-cobalt alloys (such as those used in medical prostheses) by inductively heating those materials using the system (1) of the invention.