Patent Publication Number: US-2023143753-A1

Title: Leadless Active Implantable Medical Device Having Electrodes Co-Fired Onto Its Ceramic Housing

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims priority to U.S. Provisional Application Serial No. 63/276,031, filed on Nov. 5, 2021. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the field of implantable medical devices. More particularly, the present invention relates to a leadless active implantable medical device (AIMD) that is designed to deliver electrical stimulation to a patient or sense biological signals from body tissue. 
     The leadless AIMD of the present invention has an elongate ceramic housing. At least two spaced-apart superficial electrodes are supported on an outer surface of the ceramic housing. The electrodes can be ring-shaped to electrically stimulate body tissue or sense biological signals in a 360° field about the ceramic housing, or they can be discrete electrodes to provide current to body tissue and/or to sense electrical signals from body tissue in a radiating pattern that is substantially directly in front of them. As an inherently electrically insulative material, the ceramic material forming the housing electrically isolates the electrodes from each other. A charging coil housed inside the housing is configured to charge a Li-ion battery or capacitor that is connected to a printed circuit board (PCB) assembly contained inside the housing. The PCB assembly comprises a PCB supporting a plurality of electronic components that control the various functions of the medical device, including having the electrodes deliver electrical stimulation to a patient and sense biological signals from body tissue. The electrodes are connected to the PCB assembly by electrically conductive pathways comprising a via extending through the housing and containing a platinum-containing paste that is co-sintered with the housing when in a green-state. 
     2. Prior Art 
     Conventional leadless implantable medical devices have a metallic housing. The metallic housing supports at least two electrodes that are configured to send electrical pulses to the surrounding body tissue or sense biological signals from the body tissue. The stimulation and sensing electrodes are electrically energized by electronic circuits housed inside the medical device. Since the stimulation and sensing electrodes need to be in contact with the surrounding body tissue, they must be hermetically sealed in the device housing or its header and electrically isolated from the metallic housing. In that respect, including the features needed to support and hermetically seal them in the device housing, the stimulation and sensing electrodes and their associated support structures consume a relatively large amount of space in the medical device. 
     Moreover, the metallic housing can interfere with two-way communication between the medical device and an associated patient programmer or clinician programmer. These programmers are used by the patient and the clinician to configure the medical device to operate in a desired manner. A patient programmer is used by the patient in whom the medical device is implanted to adjust the parameters of electrical stimulation delivered by the device. A clinician programmer is used by medical personnel to configure and adjust stimulation parameters that the patient is not permitted to control. However, a metallic device housing may interfere with the inductive signals that are transmitted back and forth from the patient and clinician programmers to the medical device. A metallic device housing may also interfere with the inductive or RF charging signals that are used to recharge the electrical power source of the medical device. 
     To overcome these shortcomings, the communication/charging antenna, whether there are two dedicated antennas or an integrated antenna, must be placed outside the metallic housing, typically inside a polymeric header connected to the device housing. Positioning the communication/charging antenna in the device header requires a significant amount of space that, if eliminated, could greatly reduce the size of the implantable medical device. A smaller device is easier to implant in a patient and would be expected to cause less trauma to the patient. 
     Therefore, there is a desire to minimize the space occupied by the stimulation and sensing electrodes in a leadless active implantable medical device. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to reduce the size of a leadless implantable medical device by reducing the volume of the electrodes, reducing or eliminating the features needed to hermetically seal the electrodes to the housing, reducing the number of components or processes needed to insulate the electrodes from each other and from the other electrical components of the medical device, and reducing the volume of the connections between the electrodes and the electrical components supported on a printed circuit board contained inside the medical device housing. 
     In that respect, the leadless implantable medical device of the present invention has a housing that is made from a ceramic material. At least two superficial stimulation or sensing electrodes are supported on the ceramic housing, spaced-apart from each other. Since the electrodes are superficial structures, lying on the outer surface of the device housing, they occupy very little space. And, since the ceramic forming the device housing is an inherently electrically insulative material, the at least two electrodes supported on the housing are electrically isolated from each other and from the rest of the device components without any additional support or isolation structure. 
     Respective electrically conductive pathways connect from the electrodes to electronic circuits supported on a printed circuit board (PCB) contained inside the ceramic device housing. The conductive pathways are formed from an electrically conductive paste, preferably a platinum-containing paste, that is filled into vias aligned with a respective one of the electrodes and extending through the ceramic housing. With the ceramic forming the housing being in a green-state, the housing supporting the superficial electrodes and the conductive paste in the housing vias are co-fired in a sintering operation. Sintering solidifies the ceramic housing into the desired shape for the medical device, bonds the superficial electrodes to the ceramic housing and simultaneously transforms the conductive paste in the vias into solid, electrically conductive pathways. An outer end of each conductive pathway contacts an inner surface of a respective one of the electrodes while an inner end of the conductive pathway is in electrical continuity with an electrical contact supported on the PCB. The electrical contacts connect to electronic circuits of the PCB, and in turn, the electronic circuits draw power from an electrical power source connected to the device housing. In that manner, the at least two spaced-apart superficial electrodes are energized to either provide electrical stimulation to body tissue in which the medical device is implanted, sense biological signals from adjacent body tissue, or both. Thus, providing the at least two spaced-apart electrodes as superficial electrodes supported on the ceramic housing means that the amount of space the electrodes and their connections to the electrical components supported on the PCB represents a significant improvement in space savings in comparison to a convention leadless implantable medical device having a metallic housing. 
     Moreover, the ceramic housing does not interfere with the communication/charging fields that are transmitted between external devices and the medical device when it is implanted in body tissue. These include communication signals from patient and clinician programmers to the implanted medical device, and inductive or RF (radio frequency) charging signals sent from an external charging transmitter to the power source charging antenna. This means that the communication and charging antennas can be placed inside the ceramic housing, which further reduces the device’s volume and manufacturing complexity. 
     Furthermore, providing the spaced-apart electrodes as ring-shaped electrodes means that the leadless implantable medical device of the present invention can contact surrounding body tissue in a 360° geometry about the medical device. However, the present invention is not limited to a defined number of electrodes; there are at least two electrodes in a system, and they can be at least one ring-shaped electrode combined with at least one discrete electrode that is not ring-shaped. 
     These and other aspects of the present invention will become increasingly more apparent to those skilled in the art by reference to the following detailed description and to the appended drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a simplified block diagram of an exemplary medical system  10  according to various embodiments of the present invention. 
         FIG.  2    is a perspective view of an exemplary embodiment of the implantable medical device  12  shown in the medical system  10  illustrated in  FIG.  1   . 
         FIG.  3    is a side elevation view of the implantable medical device  12  shown in  FIG.  2   . 
         FIG.  4    is a cross-sectional view taken along line 4-4 of  FIG.  3   . 
         FIG.  5    is an enlarged view of the indicated area in  FIG.  4   . 
         FIG.  6    is a partially exploded view of the implantable medical device  12  shown in  FIG.  2    looking at the charging coil  50  supported on the printed circuit board (PCB)  48 . 
         FIG.  7    is a plan view of the implantable medical device  12  shown in  FIG.  2   . 
         FIG.  8    is a cross-sectional view taken along line 8-8 of  FIG.  7   . 
         FIG.  9    is an enlarged view of the indicated area in  FIG.  8   . 
         FIG.  10    is a partially exploded view of the implantable medical device  12  shown in  FIG.  2    looking at the bottom side of the printed circuit board (PCB)  48 , opposite the charging coil  50 . 
         FIG.  11    is a perspective view of another exemplary embodiment of an implantable medical device  100  that is useful in the medical system  10  illustrated in  FIG.  1   . 
         FIG.  12    is a side elevational view of the implantable medical device  100  shown in  FIG.  11   . 
         FIG.  13    is a plan view of the implantable medical device  100  shown in  FIG.  11   . 
         FIG.  14    is a cross-sectional view taken along line 14-14 of  FIG.  13   . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the drawings,  FIG.  1   , illustrates a simplified block diagram of an exemplary medical system  10  according to the present invention. The medical system  10  includes a leadless active implantable medical device (AIMD)  12 , which can be any one of various types of known implantable medical devices that can be implanted in a patient’s body tissue. The medical system  10  also has an external charger  14 , a patient programmer  16 , and a clinician programmer  18 . 
     The patient programmer  16  and the clinician programmer  18  may be portable handheld devices, such as a smartphone or other custom device, that are used to configure the AIMD  12  so that the AIMD can operate in a desired manner. The patient programmer  16  is used by the patient in whom the AIMD  12  is implanted. The patient may adjust the parameters of electrical stimulation delivered by the AIMD  12 , such as by selecting a stimulation program, changing the amplitude and frequency of the electrical stimulation, among other parameters, and by turning stimulation on and off. 
     The clinician programmer  18  is used by medical personnel to configure the other system components and to adjust stimulation parameters that the patient is not permitted to control. These include setting up stimulation programs among which the patient may choose and setting upper and lower limits for the patient’s adjustments of amplitude, frequency, and other parameters. It is also understood that although  FIG.  1    illustrates the patient programmer  16  and the clinician programmer  18  as two separate devices, they may be integrated into a single programmer in some embodiments. 
     Electrical power may be delivered to the AIMD  12  through an external charging pad  20  that is connected to the external charger  14 . In some embodiments, the external charging pad  20  is configured to directly power the AIMD  12  or it is configured to charge a rechargeable electrical energy power source  24  of the AIMD. The external charging pad  20  can be a hand-held device that is connected to the external charger  14 , or it can be an internal component of the external charger  14 . 
     Referring now to  FIGS.  2  to  7   , an exemplary embodiment of the leadless active implantable medical device (AIMD)  12  shown in  FIG.  1    is illustrated. The AIMD  12  is an elongate device that extends longitudinally along an axis A-A with an exemplary length of about 33 mm and a diameter of about 4 mm. According to the present invention, however, the shape of the AIMD  12  is not limited to the elongate shape that is shown. For example, the AIMD  12  could have a cylindrical shape or a shape that is not elongated. 
     The AIMD  12  comprises a non-conductive, elongate housing  22 , for example, of a ceramic material, connected to an electrical energy power source  24 . However, the material of the device housing  22  is not limited to ceramic materials; other non-conductive materials, for example, glass (e.g., HPFS) or plastic (e.g., PEEK) may be used as long as they are biocompatible and offer the appropriate mechanical robustness. 
     The electrical energy power source  24  can be a capacitor or a rechargeable battery, for example a hermetically sealed rechargeable Li-ion battery. However, the electrical energy power source  24  is not limited to any one chemistry or even a rechargeable chemistry and can be of an alkaline cell, a primary lithium cell, a rechargeable lithium-ion cell, a Ni/cadmium cell, a Ni/metal hydride cell, a supercapacitor, a thin film solid-state cell, and the like. Preferably, the electrical energy power source  24  is a lithium-ion electrochemical cell comprising a carbon-based or Li 4 Ti 5 O 12 -based anode and a lithium metal oxide-based cathode, such as of LiCoO 2  or lithium nickel manganese cobalt oxide (LiNi a Mn b Co 1-a-b O 2 ) . The electrical energy power source  24  can also be a solid-state thin film electrochemical cell having a lithium anode, a metal-oxide based cathode and a solid electrolyte, such as an electrolyte of LiPON (Li x PO y N z ) . 
     The AIMD  12  supports at least two spaced-apart band-shaped or ring-shaped electrodes  26  and  28 . The ring-shaped electrodes  26  and  28  are configured to provide current to body tissue and/or to sense electrical signals from body tissue. While two electrodes  26 ,  28  are shown in the exemplary embodiment of the present implantable medical device  12 , that is the minimum number of electrodes that is needed for a functioning medical device. It is contemplated that there can three, four, and possibly more electrodes comprising the AIMD  12 . 
     In more detail, the device housing  22  is formed from a ceramic material that has a relatively high dielectric constant. An essentially pure alumina is a preferred ceramic material. The term “essentially pure alumina” refers to an alumina ceramic having the chemical formula A1 2 O 3 . “Essentially pure” means that the post-sintered alumina is at least 96% alumina. In a preferred embodiment, the post-sintered alumina is at least 99% high purity alumina. 
     Prior to sintering, the alumina comprising the device housing  22  may be a paste, a slurry, or a green-state ceramic that is injection molded, powder pressed, and the like, into the desired shape of the housing as a single monolithic structure. The green-state alumina ceramic is very pliable due to the organic binders and solvents that have been temporarily added to the system. It is at this step that at least two vias  30  (only one via is shown in  FIGS.  4  and  5   ) are formed through the green-state ceramic from an outer or body fluid side surface  22 A of the housing  22  to an inner or device side surface  22 B (electronics side) defining an interior space  32  inside the alumina housing  22 . Drilling is a preferred method for forming the vias  30 , but they may also be formed by punching, laser drilling, water cutting, or any other equivalent process. The at least two vias  30  are aligned with a respective one of the at least two electrodes  26  and  28  that will subsequently be supported on the green-state ceramic comprising the device housing  22 . 
     After the vias  30  are formed, a pure platinum paste composition is injected under pressure or via vacuum into the vias. The pressure or vacuum is carefully controlled to drive the platinum paste into intimate contact with the ceramic surface surrounding the vias so that the paste conforms to and creates a mirror image of the inner surface of the ceramic material defining the vias  30  and, in so doing, the platinum paste interconnects with the already tortuous members prevalent in ceramic materials. 
     The thermal expansion of metals, for example platinum, is generally considerably greater than that of ceramics, for example alumina. For example, at a bakeout temperature of about 500° C., the CTE of alumina is 7.8x10 -6 /K and of platinum is 9.6x10 -6 /K. Historically, CTE differences within a range of from about 0.5 x10 -6 /K to about 1.0x10 -6 /K between the mating metal and ceramic materials are adequate to sustain hermetic bonding between these materials. Hence, a hermetic seal is formed between the sintered platinum  34  in the vias  30  and the ceramic material comprising the device housing  22  through the controlled fabrication process parameters of the platinum metal particle solids loading within the paste, controlled packing of the platinum paste within the vias  30 , and the controlled shrinkage of the alumina device housing  22  and platinum via paste through a prescribed co-sintering process. 
     In that respect, a highly irregular surface at the material interface between the alumina housing  22  and the platinum metal particles within the vias  30  provides a mechanical contribution to adherence and robustness of the hermetic seal between the alumina and the platinum  34 . A surface roughness produced by drill bits, sandblasting, grit blasting or chemical etching of the vias  30  through the device housing  22  helps to increase the surface area at the vias and, in so doing, provides for a stronger mechanical attachment along the mutually conformal interface. Examples of sandblasting and grit blasting media include sand, sodium bicarbonate, walnut shells, alumina particles or other equivalent media. 
     Thus, to achieve sustainable hermeticity between the alumina housing  22  and the platinum metal particles  34  within the vias  30 , the following is required. Because the CTE of platinum (9.6x10 -6 /K) is sufficiently higher than the CTE of alumina (7.8X10 -6 /K), the platinum material  34  filled into the ceramic vias  30  must be formed using a paste, a slurry, or the like, having a minimum of about 80% solids loading. A paste is defined as a flowable medium having a viscosity that ranges in centipoise (cP) from about 1 x 10 5  cP to about 1 x 10 10  cP. 
     In a preferred embodiment, the solids loading of the platinum particles within the paste is about 90%. In a more preferred embodiment, the solids loading of the platinum particles within the paste is about 95%. In addition, the vias  30  must be packed with the platinum paste to occupy at least 90% of the available space within each via. In a preferred embodiment, the platinum paste is packed within the via to occupy about 95% of the open space. In a more preferred embodiment, the platinum paste is packed to occupy about 99% of the via opening. The shrinkage of the alumina must be no greater than about 20% of that of the platinum paste filled into the vias  30 . In a preferred embodiment, shrinkage is about 14%. In a more preferred embodiment, shrinkage is about 16%. 
     Referring now to the at least two spaced-apart band-shaped or ring-shaped electrodes  26  and  28 , they are of an electrically conductive material, preferably platinum or a platinum alloy, supported in matching shaped recesses in the body fluid side surface  22 A of the green-state ceramic housing  22 . The electrodes  26  and  28  are aligned and in direct contact with the body fluid ends of the platinum paste filled into a respective one of the at least two vias  30 . Since the electrodes  26  and  28  will be exposed to body fluids, and the like, they must be of a biocompatible material. In addition to platinum and its alloys, suitable biocompatible materials include gold, gold alloys, rhodium, titanium, molybdenum, and mixtures thereof. The electrodes  26  and  28  may be applied to the outer or body fluid side surface  22 A of the green-state ceramic housing  22  by thin and thick film technologies, such as printing, screen printing, pad printing, painting, plating, brush coating, direct bonding, active metal brazing, magnetron sputtering, physical vapor deposition, ion implantation, electroplating, and electroless plating. 
     As an assembly, the green-state ceramic housing  22  supporting the ring-shaped electrodes  26 ,  28  aligned and in direct contact with the platinum paste filled vias  30  is then subjected to a controlled co-firing or co-sintering process in ambient air that comprises a binder bakeout portion, a sintering portion, and a cool down portion. The binder bakeout portion is performed at a temperature that ranges from about 400° C. to about 700° C. for a minimum of about 4 hours. A preferred binder bakeout is at a temperature that ranges from about 550° C. to about 650° C. A more preferred binder bakeout is at a temperature that ranges from about 500° C. to about 600° C. 
     The sintering portion is preferably performed at a temperature that ranges from about 1,400° C. to about 1,900° C. for up to about 6 hours. A preferred sintering profile has a temperature that ranges from about 1,500° C. to about 1,800° C. A more preferred sintering temperature ranges from about 1,600° C. to about 1,700° C. 
     The cool down portion occurs either by turning off the heating chamber and allowing the chamber to equalize to room temperature or, preferably by setting the cool down portion at a rate of up to about 5° C./min from the hold temperature cooled down to about 1,000° C. At about 1,000° C., the chamber is allowed to naturally equalize to room temperature. A more preferred cool down is at a rate of about 1° C./min from the hold temperature to about 1,000° C. and then the heating chamber is allowed to naturally equalize to room temperature. In so doing, the desired outcome of achieving robust electrical connections of the platinum  34  forming a conductive pathway connecting from each of the electrodes  26  and  28  to the interior space  32  inside the device housing and the hermetic seal formed between the mating materials of the alumina device housing  22  and the platinum  34  hermetically sealed to the ceramic material of the housing in the vias  30  is achieved. 
     Once the binders and solvents have been driven out of the system and sintering has occurred, the result is a solid monolithic high purity alumina device housing  22  supporting the at least two electrodes  26  and  28  aligned and in direct physical and electrical contact with a body fluid side end of a respective one of the at least two conductive pathways  34  in the vias  30 . The sintering process has transformed the platinum paste in the vias  30  into solid electrically conductive pathways  34  comprising platinum extending from the outer or body fluid side surface  22 A of the housing where they directly electrically and physically contact the electrodes  26  and  28  to the inner surface  22 B thereof defining the interior space  32  inside the device housing. A mutually conformal hermetic interface is thereby formed between the platinum conductive pathways  34  and the ceramic material of the device housing  22  defining the vias  30 . 
     The sintered high purity alumina device housing  22  has a surrounding sidewall extending from a proximal annular rim  36 A to a closed distal end  36 B. The proximal rim  36 A surrounds and defines an opening leading into the interior space  32  inside the housing  22 . In that respect, the device housing  22  is a hollow member that is comprised of opposed upper and lower face walls  38  and  40  that extend to and meet with opposed curved edge walls  42  and  44 . Together, the face walls  38 ,  40  and edge walls  42 ,  44  extend from the proximal rim  36 A to the curved closed distal end  36 B. 
     As shown in  FIGS.  4 ,  6  and  7   , a pair of integral side-by-side rails  46 A and  46 B resides inside the interior space  32  inside the device housing  22 . The integral rails  46 A,  46 B are aligned along, but spaced on opposite side of, the longitudinal axis A-A and extend from adjacent to the proximal rim  36 A to the closed distal end  36 B. 
     A printed circuit board (PCB) assembly  48  resides in the interior space  32  inside the device housing  22 , supported on the rails  46 A,  46 B. The PCB  48  supports at least one, and preferably a plurality of electronic components (not shown) as an assembly that controls the various functions performed by the AIMD  12 . These include, but are not limited to, receiving sensed electrical signals pertaining to functions of the body tissue in which the AIMD  12  is implanted and for delivering electrical current pulses to the body tissue through the electrodes  26  and  28 . The PCB  48  also supports a charging coil  50  connected to a charging circuit (not shown). The charging circuit is configured to convert RF or inductive energy signals received by the inductive charging coil  50  from the external charging pad  20  connected to the external charger  14  ( FIG.  1   ) into a direct current voltage to charge the electrical power source  24  to power the electronic components of the PCB  48 . 
     As shown in  FIGS.  4  and  5   , with the PCB  48  residing in the interior space  32 , supported on the rails  46 A,  46 B, inside the device housing  22 , an electrical connection is made from the electrically conductive pathways  34  at the inner surface  22 B of the device housing to the PCB by a re-flowed solder  52 . There are two solders  52 , each contacting one of the conductive pathways  34  and an electrical contact  54  supported on the PCB  48 . The electrical contact  54  is in electrical continuity with the electronic components supported on the PCB  48 . 
       FIGS.  7  to  10    illustrate an alternate embodiment for electrically connecting the electronic components supported on the PCB  48  to the electrodes  26 ,  28 . In this embodiment, the PCB  48  supports the same number of leaf springs as there are electrodes. Since the illustrated embodiment shows two electrodes  26 ,  28 , there are two leaf springs  56  and  58  corresponding to a respective one of the electrodes. The leaf springs  56  and  58  are spaced apart and configured to bias toward the PCB as the PCB  48  is moved along the rails  46 A,  46 B into the interior space  32  inside the device housing  22 . Once the PCB is properly positioned inside the device housing  22 , the leaf springs  56  and  58  bias into direct physical and electrical contact with the conductive pathways  34  in turn electrically contacting the electrodes  26  and  28 . 
     As shown in  FIGS.  2 ,  3 ,  6  to  8   , an annular flange  60  of a biocompatible material, for example titanium, is supported at the proximal rim  36 A of the device housing. To connect the ceramic housing  22  to the flange  60 , the outwardly facing edge of the rim  36 A is provided with a metallization (not shown). A suitable metallization comprises two metallization layers, a first adhesion layer that is directly applied to the outwardly facing edge of the rim  36 A, and a second, wetting layer, which is applied on top of the adhesion layer. In a preferred embodiment, the adhesion layer is titanium, and the wetting layer is either molybdenum or niobium. 
     The adhesion and wetting metallization layers may be applied to the device housing rim  36 A by thin and thick film technologies, such as printing, painting, plating, and deposition processes. Metallization processes include screen printing, pad printing, brush coating, direct bonding, active metal brazing, magnetron sputtering, physical vapor deposition, ion implantation, electroplating, and electroless plating. In an alternate embodiment, both the adhesion and wetting metallization layers may be provided by a single metallization layer. It is noted that in the present drawings, the adhesion and wetting layers are intentionally not shown for the sake of simplicity. 
     A braze pre-form, for example a gold ring-shaped preform, (not shown) is seated on the metallized rim  36 A, and the ceramic device housing  22 /gold pre-form/annular flange  60  subassembly is then subjected to a brazing process, as is well known to those skilled in the art related to brazing a ceramic material to a metallic flange. The brazing process melts the gold to thereby form a hermetic seal joining the flange  60  to the ceramic device housing  22  at the annular rim  36 A. 
     The electrical energy power source  24  is a hermetically sealed electrochemical cell or capacitor comprising a metal or a ceramic casing  62 . If ceramic, in a similar manner as with the ceramic material forming the device housing  22 , the ceramic material forming the power source casing  62  is of an essentially high purity alumina. Whether metal or ceramic, the power source casing  62  has a surrounding sidewall extending from an annular proximal rim  62 A to a closed distal end  62 B. The annular proximal rim  62 A is hermetically sealed to a metallic ferrule  64 . The power source casing  62  is comprised of opposed upper and lower casing face walls  66  and  68  that extend to and meet with opposed curved casing edge walls  70  and  72 . Together, the casing face walls  66 ,  68  and edge walls  70 ,  72  extend from the proximal rim  62 A to the curved closed distal end  62 B. 
     The ferrule  64  is of a biocompatible metal, for example titanium. If the casing  62  for the power source is metal, the ferrule  64  is attached to the proximal rim  62 A of the casing  62  by a welding process, for example by laser welding. Alternatively, if the casing  62  is of a ceramic material, the ferrule  64  is hermetically secured to the proximal rim  62 A of the ceramic casing by a brazing process, which is similar to the process that is used to braze the flange  60  to the proximal rim  36 A of the device housing  22 . 
     In any event, the ferrule  64  is sized and shaped to abut with the housing flange  60 . A welding process, for example, a laser welding process is then used to connect the device housing  22  to the power source  24  at the flange  60  and ferrule  64 . With the device housing  22  hermetically secured to the power source  24 , the face walls  38 ,  40  and edge walls  42 ,  44  of the device housing  22  align with the face walls  66 ,  68  and edge walls  70 ,  72  of the power source casing  62 . This provides the AIMD  12  comprising the device housing  22  secured to the power source  24  with a contoured exterior shape that is free of sharp edges and suitable to be implanted into a body tissue for an extended period of time without causing undue trauma to the surrounding body tissue. 
     While not shown in the drawings, the PCB  48  has opposite polarity terminal blocks that mate with opposite polarity terminal pins of the electrical energy power source  24 . When the power source  24  is hermetically connected to the device housing  22  by welding the housing flange  60  to the power source ferrule  64 , the power source is electrically connected to the PCB  48  and the electronic components supported on the PCB are electrically energized. Further, the power source  24  preferably has a third terminal pin (not shown) that is connected to the charging coil  50 . 
       FIGS.  11  to  14    illustrate an alternate embodiment of a leadless AIMD  100  according to the present invention that is useful in the medical system  10  illustrated in  FIG.  1   . As with the previously described AIMD  12 , the AIMD  100  comprises a non-conductive, elongate housing  102 , for example of a ceramic material, connected to an electrical energy power source  104 . The power source  104  can be a capacitor or a rechargeable battery, for example a hermetically sealed rechargeable Li-ion battery. 
     The AIMD  100  supports at least two spaced-apart discrete electrodes  106  and  108 . Since they are not ring-shaped, the discrete electrodes  106  and  108  are configured to provide current to body tissue and/or to sense electrical signals from body tissue in a radiating pattern that is substantially directly in front of them. That is in comparison to the previously described ring-shaped electrodes  26  and  28 , which provide current to body tissue and/or to sense electrical signals from body tissue in a 360° field about the ceramic housing. Moreover, while two electrodes  106 ,  108  are shown in this exemplary embodiment of an AIMD  100 , that is the minimum number of electrodes that is needed for a functioning active implantable medical device. It is contemplated that there can three, four, and possibly more electrodes comprising the AIMD  100 . 
     The device housing  102  is formed from a green-state ceramic material, for example essentially pure green-state alumina, having a U-shape in cross-section. The U-shaped housing sidewall  110  extends to opposed curved end walls  112  and  114 . The sidewall  110  and end walls  112 ,  114  extend to a peripheral rim  116  that is hermetically connected to an annular flange  120  of a biocompatible material, for example titanium. The rim  118  of the ceramic housing  102  is connected to the flange  120  in a similar manner as previously described for connecting the proximal rim  36 A of the device housing  22  for the AIMD  12  to the flange  60 . That is, the peripheral rim  118  is provided with a metallization (not shown) comprising an adhesion layer of titanium and the wetting layer of either molybdenum or niobium. Then, a braze pre-form, for example a gold ring-shaped preform, (not shown) is seated on the metallized rim  118 , and the ceramic device housing  102 /gold pre-form/annular flange  120  subassembly is subjected to a brazing process that melts the gold to thereby form a hermetic seal joining the flange  120  to the ceramic device housing  102  at the peripheral rim  118 . 
     Prior to sintering, at least two vias  122  (only one via is shown in  FIG.  14   ) are formed through the sidewall  110  of the green-state ceramic from an outer or body fluid side surface  124 A to an inner or device side surface  124 B (electronics side) defining an interior space  126  inside the alumina housing  102 . Drilling is a preferred method for forming the vias  122 , but they may also be formed by punching, laser drilling, water cutting, or any other equivalent process. After the vias  122  are formed, a pure platinum paste composition is injected under pressure or via vacuum into the vias. A connector pin  128  is inserted into the platinum paste so that the pin extends into the interior space  126  inside the device housing  102 . 
     Referring now to the at least two discrete electrodes  106  and  108 , they are of a biocompatible and electrically conductive material, preferably platinum, supported in matching shaped recesses in the body fluid side surface  124 A of the green-state ceramic housing  102 . The electrodes  106  and  108  are aligned and in direct contact with the body fluid ends of the platinum paste filled into a respective one of the at least two vias  122 . The electrodes  106  and  108  are applied to the body fluid side surface  124 A of the green-state ceramic housing  102  in a similar manner as the previously described electrodes  26 ,  28  are applied to the body fluid side surface  22 A of the green-state ceramic housing  22  for the AIMD  12 . 
     As an assembly, the green-state ceramic housing  102  supporting the discrete electrodes  106 ,  108  aligned and in direct contact with the platinum paste filled vias  122  is then subjected to a controlled co-firing or co-sintering process in ambient air that comprises a binder bakeout portion, a sintering portion, and a cool down portion. The sintering process results in a solid monolithic high purity alumina device housing  102  supporting the at least two electrodes  106  and  108  aligned and in direct physical and electrical contact with a body fluid side end of a respective one of the at least two conductive pathways  130  in the vias  122 . The sintering process has transformed the platinum paste in the vias  122  into solid electrically conductive pathways  130  comprising platinum extending from the outer or body fluid side surface  124 A of the housing where they directly electrically and physically contact the electrodes  106  and  108  to the inner surface  124 B thereof defining the interior space  126  inside the device housing  102 . A mutually conformal hermetic interface is thereby formed between the platinum conductive pathways  130  and the ceramic material of the device housing  102  defining the vias  122 . The connector pin  128  extends outwardly from each of the conductive pathways  130  into the interior space  126  inside the device housing  102 . 
     A printed circuit board (PCB) assembly  132  resides in the interior space  126  inside the device housing  102 . The PCB  132  supports at least one, and preferably a plurality of electronic components (not shown) as an assembly that controls the various functions performed by the AIMD  100 . As with the previously described AIMD  12 , these include, but are not limited to, receiving sensed electrical signals pertaining to functions of the body tissue in which the AIMD  100  is implanted and for delivering electrical current pulses to the body tissue through the electrodes  106  and  108 . The PCB  132  also supports a charging coil (not shown) connected to a charging circuit (not shown). The charging circuit is configured to convert RF or inductive energy received by the charging coil from the external charging pad  20  connected to the external charger  14  ( FIG.  1   ) into a direct current voltage to charge the electrical power source  104  to power the electronic components of the PCB  132 . 
     As shown in  FIG.  14   , with the PCB  132  residing in the interior space  126  inside the device housing  102 , an electrical connection is made from the connector pins  128  extending outwardly from each of the electrically conductive pathways  130  by a re-flowed solder  134 . The solder  134  connects the connector pins  128  to a respective electrical contact (not shown) supported on the PCB  132 . The electrical contacts connect to electronic circuits of the PCB, and in turn, the electronic circuits draw power from the electrical power source  104  connected to the device housing  102 . 
     In an alternate embodiment, the re-flowed solder  134  can be substituted with leaf springs (not shown) that are similar to the previously described leaf springs  56  and  58  and that bias into direct physical and electrical contact with the connector pins  128  extending outwardly from each of the conductive pathways  130  in turn electrically contacting the electrodes  106  and  108 . 
     In a similar manner as the electrical energy power source  24  for the previously described AIMD  12 , the electrical energy power source  104  is a hermetically sealed electrochemical cell or capacitor comprising a metal or a ceramic casing  136 . Whether metal or ceramic, the power source casing  136  has a surrounding sidewall extending to an annular proximal rim 136A. The annular proximal rim  136 A is hermetically sealed to a metallic ferrule  138 . The ferrule  138  is of a biocompatible metal, for example titanium. If the casing  136  for the power source  104  is metal, the ferrule  138  is attached to the proximal rim  136 A of the casing  136  by a welding process, for example, by laser welding. Alternatively, if the casing  136  is of a ceramic material, the ferrule  138  is hermetically secured to the proximal rim  136 A of the ceramic casing by a brazing process, which is similar to the process that is used to braze the annular flange  120  to the peripheral rim  116  of the device housing  102 . 
     A welding process, for example, a laser welding process is then used to connect the device housing  102  to the power source  104  at the flange  120  and ferrule  138 . While not shown in the drawings, the PCB  132  has opposite polarity terminal blocks that mate with opposite polarity terminal pins of the electrical energy power source  104 . When the power source  104  is hermetically connected to the device housing  102  by welding the housing flange  120  to the power source ferrule  138 , the power source is electrically connected to the PCB  132  and the electronic components supported on the PCB are electrically energized. Further, the power source  104  preferably has a third terminal pin (not shown) that is connected to the charging coil. 
     The elongated shape of the leadless AIMD  12  shown in  FIGS.  2  to  10    and the leadless AIMD  100  shown in  FIGS.  11  to  14    allows the implantation procedure to be performed by injecting or inserting the AIMD into body tissue using of an insertion tool, typically through a very small incision. With the AIMD  12  or  100  implanted in body tissue, the electrical power source  24 ,  104  electrically connected to the PCB  48 ,  132  provides electrical power to the spaced-apart ring-shaped electrodes  26 ,  28  or discrete electrodes  106 ,  108 . As ring-shaped members, the electrodes  26  and  28  are configured to electrically stimulate body tissue or sense biological signals in a 360° field about the device housing  22 . Alternately, the discrete electrodes  106  and  108  shown in  FIGS.  11  to  14    do not extend completely around a perimeter of the device housing. Instead, these electrodes are configured to stimulate body tissue or sense biological signals in a field that is centered directly in front of them. 
     It is noted that the elongated, annular shape of the ceramic device housing  22  for the leadless AIMD  12  means that the electrodes  26 ,  28  can be ring-shaped members that are continuously supported on the sidewall surrounding the longitudinal axis A-A of the ceramic housing  22 . That is in contrast to the leadless AIMD  100  which does not have a ceramic housing with an annular shape. While the leadless AIMD  100  itself has an annular shape extending along a longitudinal axis, a portion of the AIMD  100  is provided by the electrical power source  104 . For that reason, the electrode  106 ,  108  are discrete members that are not ring-shaped. 
     It is also within the scope of the present invention that an AIMD can have both ring-shaped electrodes and discrete electrodes. While at least two electrode are needed for a functioning device, there can be more than two electrodes of either a ring-shape or a discrete shape. In any event, the electrodes  26 ,  28  or  106 ,  108  are powered by the respective electrical power source  24 ,  104 , or they can be powered through RF or inductive energy transmitted from the external charging pad  20  connected to the external charger  14  to the charging coil  50  electrically connected to the PCB  48 ,  132 . The PCB supports electronic components that control the various functions of the medical device, including having the electrodes deliver electrical stimulation to a patient and sense biological signals from body tissue. 
     Thus, a method for powering an active implantable medical device (AIMD) according to the present invention comprises providing an active implantable medical device by providing a housing of a green-state alumina, the housing having a sidewall defining an open end and a sidewall thickness extending from a body fluid side surface to a device side surface. At least two vias are formed through the green-state housing sidewall thickness and the vias are then filled with an electrically conductive paste. At least two spaced-apart electrodes are supported on the green-state housing body fluid side surface, aligned with a respective one of the electrically conductive paste filled vias. The green-state alumina housing is then sintered to: solidify the alumina into the desired shape of a housing for the AIMD, transform the electrically conductive paste in the at least two vias into electrically conductive pathways extending from the device side surface of the housing to a respective one of the electrodes at the body fluid side surface, and secure the at least two electrodes to the body fluid side surface of the alumina housing. After sintering, a printed circuit board (PCB) assembly comprising a printed circuit board supporting at least one electronic component is moved through an open end and of the housing so that the PCB assembly resides inside the housing. The device side ends of each of the at least two platinum-containing pathways are then electrically connected to the at least one electronic component of the PCB assembly. Next, an electrical energy power source is secured to the open end of the housing so that the power source is electrically connected to the PCB assembly to provide electrical power to the at least one electronic component. Then, with the at least one electronic component being electrically energized, the at least two electrodes electrically connected to the PCB assembly by the respective platinum-containing pathways are configured to at least one of receive sensed electrical signals pertaining to functions of a body tissue in which the AIMD is implanted and deliver electrical current pulses to the body tissue. 
     It is appreciated that various modifications to the inventive concepts described herein may be apparent to those skilled in the art without departing from the spirit and scope of the present invention as defined by the hereinafter appended claims.