Patent Publication Number: US-2023158316-A1

Title: Fixation mechanisms for a leadless cardiac biostimulator

Description:
This application is a continuation of co-pending U.S. patent application Ser. No. 16/541,025, filed on Aug. 14, 2019, which claims the benefit of U.S. Provisional Patent Application No. 62/719,954, filed on Aug. 20, 2018, and these applications are incorporated herein by reference in their entirety to provide continuity of disclosure. 
    
    
     FIELD 
     The present disclosure relates to leadless cardiac pacemakers and similar biostimulators, and more particularly, to features and methods by which such biostimulators are affixed within a heart. More specifically, the present disclosure relates to features and methods for resisting dislodgment of a leadless biostimulator following implantation within the heart. 
     BACKGROUND 
     Cardiac pacing by an artificial pacemaker or similar leadless biostimulator provides an electrical stimulation to the heart when a natural pacemaker and/or conduction system of the heart fails to provide synchronized atrial and ventricular contractions at rates and intervals sufficient for a patient&#39;s health. Such antibradycardial pacing provides relief from symptoms and even life support for hundreds of thousands of patients. Cardiac pacing may also provide electrical overdrive stimulation to suppress or convert tachyarrhythmias, again supplying relief from symptoms and preventing or terminating arrhythmias that could lead to sudden cardiac death. 
     Cardiac pacing by currently available or conventional pacemakers is usually performed by a pulse generator implanted subcutaneously or sub-muscularly in or near a patient&#39;s pectoral region. Pulse generator parameters are usually interrogated and modified by a programming device outside the body, via a loosely-coupled transformer with one inductance within the body and another outside, or via electromagnetic radiation with one antenna within the body and another outside. The generator usually connects to the proximal end of one or more implanted leads, the distal end of which contains one or more electrodes for positioning adjacent to the inside or outside wall of a cardiac chamber. The leads have an insulated electrical conductor or conductors for connecting the pulse generator to electrodes in the heart. Such electrode leads typically have lengths of 50 to 70 centimeters. 
     SUMMARY OF THE DESCRIPTION 
     Although more than one hundred thousand conventional cardiac pacing systems are implanted annually, various well-known difficulties exist, of which a few will be cited. For example, a pulse generator, when located subcutaneously, presents a bulge in the skin that patients can find unsightly, unpleasant, or irritating, and which patients can subconsciously or obsessively manipulate or “twiddle”. Even without persistent manipulation, subcutaneous pulse generators can exhibit erosion, extrusion, infection, and disconnection, insulation damage, or conductor breakage at the wire leads. Although sub-muscular or abdominal placement can address some concerns, such placement involves a more difficult surgical procedure for implantation and adjustment, which can prolong patient recovery. 
     A conventional pulse generator, whether pectoral or abdominal, has an interface for connection to and disconnection from the electrode leads that carry signals to and from the heart. Usually at least one male connector molding has at least one terminal pin at the proximal end of the electrode lead. The male connector mates with a corresponding female connector molding and terminal block within the connector molding at the pulse generator. Usually a setscrew is threaded in at least one terminal block per electrode lead to secure the connection electrically and mechanically. One or more O-rings usually are also supplied to help maintain electrical isolation between the connector moldings. A setscrew cap or slotted cover is typically included to provide electrical insulation of the setscrew. This briefly described complex connection between connectors and leads provides multiple opportunities for malfunction. 
     Other problematic aspects of conventional systems relate to the separately implanted pulse generator and pacing leads. By way of another example, the pacing leads, in particular, can become a site of infection and morbidity. Many of the issues associated with conventional pacemakers are resolved by the development of a self-contained and self-sustainable pacemaker, or so-called leadless pacemaker, as described in the related applications cited below in the Detailed Description. 
     The problematic aspects of conventional systems described above have been addressed by self-contained or leadless pacemakers or other biostimulators. Such biostimulators are typically fixed to an intracardial implant site by an actively engaging mechanism such as a screw or helical member that screws into the myocardium. There is a need in the art, however, for improved leadless biostimulator fixation features. 
     In a first embodiment of the present disclosure, a leadless biostimulator is provided. The leadless biostimulator includes a housing sized and configured to be implanted within a heart of a patient, a primary fixation feature attached to the housing and configured to affix the housing to a wall of the heart by rotating in a screwing direction, and a secondary fixation feature disposed on, mounted on, or otherwise coupled to the primary fixation feature. The secondary fixation feature includes a sleeve disposed about the primary fixation feature. The sleeve has an outer surface tapering radially outward to an apex. For example, the sleeve can include a barb, e.g., a flexible barb, extending from a first end of the sleeve to a barb tip at the apex. The flexible barb is angled in a direction opposite the screwing direction of the primary fixation feature such that rotation of the primary fixation feature in an unscrewing direction causes the flexible barb to engage the wall of the heart so as to reduce a likelihood that the primary fixation device will disengage from the wall of the heart. 
     In certain implementations, the secondary fixation feature may be formed from one or more materials including polyimide, polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. In another implementation, the secondary fixation feature may be formed from a material having a Young&#39;s modulus from and including 0.5 gigapascals (GPa) to and including 10 GPa. 
     In certain implementations, the secondary fixation feature may be formed from one or more bioabsorbable materials. For example, the bioabsorbable material(s) may include a magnesium alloy, a polyglycolide (PGA), polylactide (PLA), or a combination bioabsorbable material such as Vicryl® (PGA-LPLA). 
     In another implementation, the flexible barb of the secondary fixation feature is one of several flexible barbs. Each of the barbs may similarly extend from the sleeve and be angled in the direction opposite the screwing direction of the primary fixation feature. For example, in some implementations, the several flexible barbs may include four barbs. Regardless of the number of barbs, each barb may be from and including 0.010 inches to and including 0.200 inches in length. 
     In certain implementations, the secondary fixation feature is formed directly onto the primary fixation feature. Alternatively, the secondary fixation feature is formed separately from the primary fixation feature and adhered to the primary fixation feature. 
     The sleeve of the secondary fixation feature may generally include a first end and a second end opposite the first end. The outer surface may extend and taper from the first end, e.g., the flexible barb can extend from the first end, and the second end can have a taper. The sleeve may also have a thickness from and including 0.001 inches to and including 0.010 inches. 
     In certain implementations, the primary fixation feature is a helical wire having several turns and the secondary fixation feature is disposed or mounted on a first distal turn of the helical wire. 
     In another embodiment of the present disclosure, a leadless biostimulator is provided. The leadless biostimulator includes a primary fixation feature attached to a distal end of the leadless biostimulator and configured to affix the leadless biostimulator to a wall of a heart by rotating in a screwing direction. The leadless biostimulator further includes a secondary fixation feature coupled to the primary fixation feature. The secondary fixation feature is configured such that, when implanted within the wall of the heart, a first torque opposite the screwing direction causes the secondary fixation feature to engage the wall of the heart, thereby providing a first resistance to rotation of the leadless biostimulator in the direction opposite the screwing direction. The secondary fixation feature is further configured such that, when implanted within the wall of the heart, a second torque opposite the screwing direction and greater than the first torque causes deformation of the secondary fixation feature such that the secondary fixation feature is at least partially disengaged from the wall of the heart, thereby providing a second resistance less than the first resistance to rotation of the leadless biostimulator in the direction opposite the screwing direction. 
     In certain implementations, the first torque is up to and including 0.5 ounce-inches (oz-in) and the second torque is from and including 0.5 oz-in to and including 2.0 oz-in. Such values for the first torque and the second torque are offered by way of example only. In an embodiment, the second torque is higher than the first torque, and may be higher by a scale factor. The scale factor can be a multiplier that provides more resistance to disengagement under torque. For example, the scale factor may be at least five, e.g., the second torque may be at least 5 times the first torque. In an embodiment, scale factor is ten or more, e.g., the second torque is 10 times the first torque. 
     In another implementation, the secondary fixation feature includes one or more flexible barbs extending in the direction opposite the screwing direction and the deformation of the secondary fixation feature includes a deformation of the one or more flexible barbs. In such cases, the one or more flexible barbs may extend from a first end of the secondary fixation feature and the secondary fixation feature may further include a tapered surface on a second end opposite the first end. 
     In another aspect of the present disclosure a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and a planar fixation feature coupled to the housing. The planar fixation feature includes several arms extending along a lateral plane of the housing. For example, the arms can extend to a lateral location that is radially outward (radially more distant) from a primary fixation feature. Each arm includes a primary fixation feature configured to affix the housing to a wall of the heart by rotating the housing in a screwing direction and a secondary fixation feature configured to engage the wall of the heart to resist rotation of the housing in a direction opposite the screwing direction. 
     In certain implementations, the arms include from and including two arms to and including six arms. Each of the arms may extend along a circular, spiral, or straight path along the lateral plane. 
     The planar fixation feature may be formed from a unitary sheet, such as by at least one of trimming, die cutting, or laser cutting the unitary sheet. The unitary sheet may have a thickness from and including 0.001 inches to and including 0.02 inches and may be formed from at least one of polyimide, polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. 
     The secondary fixation feature may be a barb disposed at an end of each arm. The barbs may, in certain implementations, have a length from and including 0.01 inches to and including 0.05 inches. The barbs may also have a width from and including 0.01 inches to and including 0.05 inches. 
     The leadless biostimulator may further include a distal cap coupled to a distal end of the housing such that the planar fixation feature is disposed between the distal cap and the housing. In such implementations, the distal end of the housing may include a distal protrusion and each of the planar fixation feature and the cap may define respective through holes. The planar fixation feature and the distal cap may then be coupled to the housing, at least in part, by disposing the distal protrusion through each of the respective through holes. 
     Each arm of the several arms may be configured to resist rotation of the housing in the direction opposite the screwing direction when a first torque is applied in the direction opposite the screwing direction and to deform when a second torque is applied in the direction opposite the screwing direction. The second torque may be greater than the first torque. The first torque may, in some cases, be up to and including 0.5 oz-in. The second torque may, in some cases, be from and including 0.5 oz-in to and including 2.0 oz-in. To facilitate flexing and deformation of the arms, the planar fixation feature may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. 
     In another aspect of the present disclosure a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and defining a longitudinal axis and a planar fixation feature extending along a plane lateral to the housing. The planar fixation feature is adapted to engage a wall of the heart when rotated in a first direction and, when engaged with the wall of the heart, to resist disengagement of the wall of the heart when rotated in a second direction opposite the first direction. 
     In certain implementations, the planar fixation feature may be configured such that, when engaged with the wall of the heart, a first torque in the second direction causes the planar fixation feature to engage the wall of the heart, thereby providing a first resistance to rotation of the leadless biostimulator in the second direction. The planar fixation feature may be further configured such that, when engaged with the wall of the heart, a second torque in the second direction, which is greater than the first torque causes deformation of the planar fixation feature such that the planar fixation feature is at least partially disengaged from the wall of the heart. By doing so, a second resistance to rotation is provided that is less than the first resistance to rotation of the leadless biostimulator in the direction opposite the screwing direction. The first torque may, in some cases, be up to and including 0.5 oz-in. The second torque may, in some cases, be from and including 0.5 oz-in to and including 2.0 oz-in. 
     In another aspect of the present disclosure, a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and defining a longitudinal axis and a fixation feature coupled to the housing. The fixation feature includes several arms, each arm of the several arms extending distally about the longitudinal axis. Each arm includes a primary fixation feature configured to affix the housing to a wall of the heart by rotating the housing in a screwing direction and a secondary fixation feature configured to engage the wall of the heart and to resist rotation of the housing in a direction opposite the screwing direction. In certain implementations, the arms include from and including two arms to and including four arms. 
     The planar fixation feature may be formed from a unitary tube, such as by at least one of trimming, die cutting, or laser cutting the unitary tube. In certain implementations, the unitary tube may have a thickness from and including 0.004 inches to and including 0.020 inches. The unitary tube may be formed from at least one of polyimide, polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. 
     In certain implementations, the secondary fixation feature is a barb disposed at an end of the arm. In such cases, the barb may have a length from and including 0.010 inches to and including 0.200 inches. The barb may also have a thickness from and including 0.010 inches to and including 0.200 inches. 
     Each of the arms may, in certain implementations extend distally about the longitudinal axis at a pitch angle up to 90 degrees. 
     The leadless biostimulator may further include a distal header assembly including a header body extending along the longitudinal axis and the fixation feature, with the fixation feature disposed about the header body. 
     In another implementation, each arm of the several arms is configured to resist rotation of the housing in the direction opposite the screwing direction when a first torque is applied in the direction opposite the screwing direction and to deform when a second torque is applied in the direction opposite the screwing direction, the second torque being greater than the first torque. In such implementations, the first torque may be, in some cases, up to and including 0.5 oz-in. The second torque may, in some cases, be from and including 0.5 oz-in to and including 2.0 oz-in. To facilitate deformation of the arms, the fixation feature may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. 
     In yet another aspect of the present disclosure, a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and defining a longitudinal axis and a fixation feature extending from a distal end of the housing. The fixation feature includes several arms extending about the longitudinal axis, each of the arms configured to engage a wall of the heart when rotated in a first direction and, when implanted, to resist rotation in a second direction opposite the first direction. The fixation feature is configured such that, when engaged with the wall of the heart, a first torque in the second direction causes the fixation feature to engage the wall of the heart, thereby providing a first resistance to rotation of the leadless biostimulator in the second direction. The fixation feature is further configured such that, when engaged with the wall of the heart, a second torque in the second direction and greater than the first torque causes deformation of the fixation feature such that the fixation feature is at least partially disengaged from the wall of the heart, thereby providing a second resistance less than the first resistance to rotation of the leadless biostimulator in the second direction. 
     In certain implementations, the first torque may, in some cases, be up to and including 0.5 oz-in. The second torque may, in some cases, be from and including 0.5 oz-in to and including 2.0 oz-in. Each arm of the several arms may include a tip that extends, at least in part, in the first direction and a barb that extends, at least in part, in the second direction. Also, the fixation feature may be formed from a unitary tubular structure. 
     In yet another aspect of the present disclosure, a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and defining a longitudinal axis. The leadless biostimulator further includes a primary fixation feature attached to the housing and configured to affix the housing to a wall of the heart by rotating in a screwing direction and an anti-unscrewing feature. The anti-unscrewing feature includes a planar arm disposed proximal to at least a portion of the primary fixation feature and extending laterally relative to the longitudinal axis. The planar arm extends in a direction opposite the screwing direction of the primary fixation feature such that rotation of the primary fixation helix in an unscrewing direction causes the planar arm to engage the wall of the heart so as to prevent the primary fixation feature from disengaging the wall of the heart. 
     In some implementation the planar arm is one of several planar arms, each of the planar arms disposed proximal to at least a portion of the primary fixation feature and extending laterally relative to the longitudinal axis. 
     In certain implementations, the planar arm may conform to various predefined dimensions. For example, the planar arm may have a thickness from and including 0.001 inches to and including 0.02 inches. As another example, the planar arm may have a length from and including 0.002 inches to and including 0.01 inches. As yet another example, the planar arm may have a width from and including 0.001 inches to and including 0.005 inches. The planar arm may also be formed from materials having particular predetermined properties. For example, the planar arm may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. 
     The leadless biostimulator may have a header assembly that includes a header body, a header cap coupled to a distal end of the header body and a shim disposed between the header body and the header cap and that includes the planar arm. The shim may, in certain implementations, be formed from one of several materials. For example, the shim can be formed from flexible biocompatible materials including, without limitation, one or more of polyimide, polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. Alternatively, the shim can be formed from a bioresorbable polymer such as polyglycolide (PGA), polylactide (PLA), polycaprolactone (PCL), polydioxanone (PDO), polytrimethylene carbonate (TMC), and co-polymers thereof, or bioresorbable metals such as magnesium alloys, iron alloys, zinc alloys, and combinations thereof. 
     The shim may include a circular body having an outer edge and the planar arm may extend from the outer edge of the circular body. For example, the planar arm may extend from a point on the outer edge located at an intersection between the outer edge and a line extending tangentially from a circle having a radius extending from a center of the circular body. The radius may, in certain implementations, be from and including 0.050 inches to and including 0.0100 inches. In certain implementations, the planar arm may extend from the outer edge along the tangential line. Alternatively, the planar arm may extend from the outer edge at an angle relative to the tangential line. 
     The leadless biostimulator may include an additional planar arm extending from the outer edge of the circular body. In such implementations, the second planar arm may be offset along the outer edge by an angle from and including 90 degrees to and including 270 degrees relative to the first planar arm. 
     In another aspect of the present disclosure a leadless biostimulator is provided that includes a housing sized and configured to be implanted within a heart of a patient and defining a longitudinal axis. The leadless biostimulator further includes a primary fixation feature attached to the housing and configured to affix the housing to a wall of the heart by rotating in a screwing direction and a header assembly disposed at a distal end of the housing. The header assembly includes a header body, a header cap coupled to a distal end of the header body, and a shim disposed between the header body and the header cap, the shim including several laterally extending planar arms extending opposite the screwing direction of the primary fixation feature. 
     In certain implementations, the shim includes one of a shim notch or a shim protrusion that mates with a corresponding header body protrusion or a header body notch, respectively, to align the shim relative to the primary fixation feature. 
     In another implementations, the primary fixation feature is a fixation helix coupled to and extending distally from the header body. In such implementations, the shim is disposed such that each planar arm of the planar arms extends between a respective pair of adjacent turns of the fixation helix. 
     The shim may have a thickness from and including 0.001 inches to and including 0.02 inches and may be formed from a converted film. The shim may also be formed from one or more of polyimide, polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of the invention are set forth with particularity in the claims that follow. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description that sets forth illustrative embodiments, in which the principles of the invention are utilized, and the accompanying drawings of which: 
         FIG.  1    illustrates a leadless cardiac pacemaker or biostimulator, in accordance with an embodiment. 
         FIG.  2    is an isometric view of a first biostimulator, in accordance with an embodiment. 
         FIG.  3 - 4    are isometric views of a second biostimulator including a secondary fixation feature in the form of a sleeve, in accordance with an embodiment. 
         FIGS.  5 A- 5 B  are isometric and side elevation views, respectively, of the sleeve of  FIGS.  3  and  4   , in accordance with an embodiment. 
         FIGS.  5 C- 5 D  are isometric and side elevation views, respectively, of an alternative sleeve, in accordance with an embodiment. 
         FIGS.  6 - 7    are isometric views of a third biostimulator including a planar fixation feature, in accordance with an embodiment. 
         FIG.  8    is an exploded view of a distal assembly of the biostimulator of  FIGS.  6 - 7   , in accordance with an embodiment. 
         FIG.  9 A  is an isometric view of the planar fixation feature of  FIGS.  6 - 8   , in accordance with an embodiment. 
         FIG.  9 B- 9 C  are distal and side elevation views, respectively, of the planar fixation feature of  FIG.  9 A , in accordance with an embodiment. 
         FIGS.  10 - 11    are isometric views of a fourth biostimulator including a forward facing fixation feature, in accordance with an embodiment. 
         FIG.  12    is an exploded view of a distal assembly of the biostimulator of  FIGS.  10 - 11   , in accordance with an embodiment. 
         FIG.  13 A  is an isometric view of the forward facing fixation feature of  FIGS.  10 - 12   , in accordance with an embodiment. 
         FIG.  13 B- 13 C  are distal and side elevation views, respectively, of the planar fixation feature of  FIG.  13 A , in accordance with an embodiment. 
         FIGS.  14 - 15    are isometric views of a fourth biostimulator in accordance with the present disclosure and including a primary fixation helix and a planar secondary fixation feature, in accordance with an embodiment. 
         FIG.  16    is an exploded view of a distal assembly of the biostimulator of  FIGS.  14 - 15   , in accordance with an embodiment. 
         FIG.  17 A  is an isometric view of the forward facing fixation feature of  FIGS.  14 - 16   , in accordance with an embodiment. 
         FIG.  17 B- 17 C  are distal and side elevation views, respectively, of the planar fixation feature of  FIG.  17 A , in accordance with an embodiment. 
         FIGS.  18 A- 18 B  are schematic illustrations of a patient heart in which biostimulators according to the present disclosure are fixed, in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description refers to the accompanying drawings that illustrate exemplary embodiments. Other embodiments are possible, and modifications may be made to the disclosed embodiments within the spirit and scope of the invention. Therefore, the following detailed description is not meant to limit the invention. Rather, the scope of the invention is defined by the appended claims. 
     Various embodiments of a system including one or more leadless cardiac pacemakers or biostimulators are described. An embodiment of a cardiac pacing system configured to attain these characteristics includes a leadless cardiac pacemaker that is substantially enclosed in a hermetic housing suitable for placement on or attachment to the inside or outside of a cardiac chamber. The pacemaker can have two or more electrodes located within, on, or near the housing, for delivering pacing pulses to muscle of the cardiac chamber and optionally for sensing electrical activity from the muscle, and for bidirectional communication with at least one other device within or outside the body. The housing can contain a primary battery to provide power for pacing, sensing, and communication, for example bidirectional communication. The housing can optionally contain circuits for sensing cardiac activity from the electrodes. The housing contains circuits for receiving information from at least one other device via the electrodes and contains circuits for generating pacing pulses for delivery via the electrodes. The housing can optionally contain circuits for transmitting information to at least one other device via the electrodes and can optionally contain circuits for monitoring device health. The housing contains circuits for controlling these operations in a predetermined manner. 
     In some embodiments, a cardiac pacemaker can be adapted for implantation into tissue in the human body. In a particular embodiment, a leadless cardiac pacemaker can be adapted for implantation adjacent to heart tissue on the inside or outside wall of a cardiac chamber, using two or more electrodes located on or within the housing of the pacemaker, for pacing the cardiac chamber upon receiving a triggering signal from at least one other device within the body. 
     Self-contained or leadless pacemakers or other biostimulators are typically fixed to an intracardial implant site by an actively engaging mechanism such as a screw or helical member that screws into the myocardium. Examples of such leadless biostimulators are described in the following publications, the disclosures of which are incorporated by reference: (1) U.S. application Ser. No. 11/549,599, filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker System for Usage in Combination with an Implantable Cardioverter-Defibrillator”, and issued as U.S. Pat. No. 8,457,742 on Jun. 4, 2013; (2) U.S. application Ser. No. 11/549,581 filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker”, and issued as U.S. Pat. No. 9,358,400 on Jun. 7, 2016; (3) U.S. application Ser. No. 11/549,591, filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker System with Conductive Communication” and issued as U.S. Pat. No. 9,216,298 on Dec. 22, 2015; (4) U.S. application Ser. No. 11/549,596 filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker Triggered by Conductive Communication” and issued as U.S. Pat. No. 8,352,025 on Jan. 8, 2013; (5) U.S. application Ser. No. 11/549,603 filed on Oct. 13, 2006, entitled “Rate Responsive Leadless Cardiac Pacemaker” and issued as U.S. Pat. No. 7,937,148 on May 3, 2011; (6) U.S. application Ser. No. 11/549,605 filed on Oct. 13, 2006, entitled “Programmer for Biostimulator System” and issued as U.S. Pat. No. 7,945,333 on May 17, 2011; (7) U.S. application Ser. No. 11/549,574, filed on Oct. 13, 2006, entitled “Delivery System for Implantable Biostimulator” and issued as U.S. Pat. No. 8,010,209 on Aug. 30, 2011; and (8) International Application No. PCT/US2006/040564, filed on Oct. 13, 2006, entitled “Leadless Cardiac Pacemaker and System” and published as WO07047681A2 on Apr. 26, 2007. 
       FIG.  1    shows a leadless cardiac pacemaker or leadless biostimulator  100 . The biostimulators can include a hermetic housing  102  with electrodes  104  and  106  disposed thereon. As shown, electrode  106  can be disposed on or integrated within a fixation device  105 , and the electrode  104  can be disposed on the housing  102 . The fixation device  105  can be a fixation helix or other flexible or rigid structure suitable for attaching the housing to tissue, such as heart tissue. In other embodiments, the electrode  106  may be independent from the fixation device in various forms and sizes. The housing can also include an electronics compartment  110  within the housing that contains the electronic components necessary for operation of the biostimulator. The hermetic housing can be adapted to be implanted on or in a human heart, and can be cylindrically shaped, rectangular, spherical, or any other appropriate shapes, for example. 
     The housing can include a conductive, biocompatible, inert, and anodically safe material such as titanium, 316L stainless steel, or other similar materials. The housing can further include an insulator disposed on the conductive material to separate electrodes  104  and  106 . The insulator can be an insulative coating on a portion of the housing between the electrodes, and can include materials such as silicone, polyurethane, parylene, or another biocompatible electrical insulator commonly used for implantable medical devices. In the embodiment of  FIG.  1   , a single insulator  108  is disposed along the portion of the housing between electrodes  104  and  106 . In some embodiments, the housing itself can include an insulator instead of a conductor, such as an alumina ceramic or other similar materials, and the electrodes can be disposed upon the housing. 
     As shown in  FIG.  1   , the biostimulator can further include a header assembly  112  to isolate electrode  104  from electrode  106 . The header assembly  112  can be made from tecothane or another biocompatible plastic, and can contain a ceramic to metal feedthrough, a glass to metal feedthrough, or other appropriate feedthrough insulator as known in the art. 
     The electrodes  104  and  106  can include pace/sense electrodes, or return electrodes. A low-polarization coating can be applied to the electrodes, such as platinum, platinum-iridium, iridium, iridium-oxide, titanium-nitride, carbon, or other materials commonly used to reduce polarization effects, for example. In  FIG.  1   , electrode  106  can be a pace/sense electrode and electrode  104  can be a return electrode. The electrode  104  can be a portion of the conductive housing  102  that does not include an insulator  108 . 
     Several techniques and structures can be used for attaching the housing  102  to the interior or exterior wall of the heart. A helical fixation device  105 , can enable insertion of the device endocardially or epicardially through a guiding catheter. A torqueable catheter can be used to rotate the housing and force the fixation device into heart tissue, thus affixing the fixation device (and also the electrode  106  in  FIG.  1   ) into contact with stimulable tissue. Electrode  104  can serve as an indifferent electrode for sensing and pacing. The fixation device may be coated partially or in full for electrical insulation, and a steroid-eluting matrix may be included on or near the device to minimize fibrotic reaction, as is known in conventional pacing electrode-leads. 
     Various anti-unscrewing features (also referred to herein as secondary fixation features or mechanisms) can be included on the biostimulator to provide a feature that requires that the torque necessary to unscrew the biostimulator from tissue is greater than the torque necessary to unscrew the biostimulator without such a feature. In some embodiments, the torque necessary to unscrew the biostimulator from tissue is greater than the torque necessary to further screw, engage, or re-engage the biostimulator into tissue. When an anti-unscrewing feature provides this function, the chances of a biostimulator accidentally unscrewing or disengaging itself from the tissue is reduced. It should be noted that the torque necessary to initially insert a biostimulator into tissue is greater due to the puncturing or piercing of tissue and the formation of a helical cavity. Thus, in some embodiments, the anti-unscrewing features need only provide that the torque necessary to unscrew the biostimulator from tissue be greater than the torque necessary to unscrew the biostimulator from tissue after the biostimulator has already been implanted in tissue (i.e., after the tissue has been pierced). 
       FIG.  2    is an isometric view of a biostimulator  200  in accordance with the present disclosure. The biostimulator  200  includes a housing  202  and a header assembly  204  coupled thereto. Coupling of the housing  202  to the header assembly  204  may be accomplished in various ways including, without limitation, one or more of a biocompatible adhesive, a threaded connection, and ultrasonic welding. 
     The header assembly  204  generally includes a primary fixation device  205  and one or more forward facing anti-unscrewing features  212 A,  212 B. More specifically, the primary fixation device  205  can be a primary helix  205  pointing in a first direction, and the forward facing anti-unscrewing features  212 A,  212 B can be several forward facing sutures  212 A,  212 B extending from a forward face of the biostimulator  200  in a second direction opposite the first direction. 
     The primary helix  205  may be a helical wire. The helical wire can be a wire substantially formed of any suitable biocompatible material including, without limitation, one or more of stainless steel, nickel-titanium alloys (such as Nitinol), nickel-chromium allows (such as Incoloy®), titanium, or multiphase nickel alloys (such as MP35N®). In certain implementations, the substrate material of the primary helix  205  may also be conductive such that the primary helix  205  may be used as an electrode for sensing and/or pacing of cardiac tissue. 
     The primary helix  205  is preferably sized to couple the biostimulator  200  to cardiac tissue while minimizing damage to the cardiac tissue. In certain implementations, for example, the primary helix  205  extends from and including 0.25 turns to and including 3 turns from the helix mount  206 , has a wire diameter from and including 0.003 inches to and including 0.03 inches, has a pitch diameter from and including 0.06 inches to and including 0.3 inches, and has a pitch from and including 0.01 inches to and including 0.05 inches. While the implementations illustrated herein include a single primary helix  205 , other implementations of the present disclosure may include multiple fixation helices, each extending in the same direction and each adapted to engage cardiac tissue in response to rotation of the biostimulator  200 . Such multi-helix implementations may include biostimulators with multifilar helices in which multiple wires are conjoined, e.g., jointly wound, or biostimulators including multiple offset helices. 
     Functionality of the sutures  212 A,  212 B depends, at least in part, on their flexibility. Suture flexibility may be controlled by, among other things, material selection, and suture dimensions while the overall counter rotational resistance provided by the forward facing sutures may be further modified by, among other things, the quantity of sutures employed and the positioning of the sutures relative to each other or relative to the primary helix  205 . Regarding materials, the sutures  212 A,  212 B may be formed of various flexible biocompatible materials including, without limitation, one or more of polypropylene, polyethylene, polyester, nylon, polyurethane, silicone, poly(lactic acid) (PLA), poly(glycolic acid) (PGA), polyimide, polyether ether ketone (PEEK), and polycarbonate. Other biocompatible materials that may be used to form the sutures  212 A,  212 B include natural materials including one or more of hair, horse hair, nail, hide, horn, or plant fibers, such as horsetail or thistle. 
     Dimensionally, the length and diameter of the sutures  212 A,  212 B may vary depending on the specific configuration of the biostimulator  200 , however, in certain implementations the sutures  212 A,  212 B have a length from and including 0.003 inches to and including 0.2 inches and a diameter from and including 0.003 inches to and including 0.03 inches. In certain implementations, the flexibility of the sutures  212 A,  212 B is sufficiently high to resist counter rotation caused by general cardiac activity and movement of the patient, but low enough such that removal and/or repositioning of the biostimulator  200  is possible without significant damage to the cardiac tissue. For example, each of the sutures  212 A,  212 B may have a stiffness (Young&#39;s Modulus) from and including 0.5 gigapascals (GPa) to and including 10 GPa. In certain implementations, the sutures  212 A,  212 B may include tips that are configured to improve engagement with cardiac tissue. For example, the suture  212 A,  212 B may be trimmed or otherwise formed to have sharpened tips. 
     The header assembly  204  may include multiple components including a helix mount  206 , a cap  208 , and a flange  210 . Generally, the helix mount  206  couples to and retains the primary helix  205  while the cap  208  retains each of the forward facing sutures. The flange  210  couples the header assembly  204  to the housing  202  and provides a central structure to which each of the helix mount  206  and the cap  208  are mounted. The flange  210  may further include an electrode  211  that contacts tissue when the biostimulator is implanted and through which electrical stimulation may be delivered. The example biostimulator  200  further includes several laterally extending anti-unscrewing features in the form of lateral sutures  214 A- 214 C (lateral  214 C being hidden in  FIG.  2   ). As illustrated in  FIG.  2   , such lateral sutures may be coupled to and extend from the helix mount  206 . 
     Portions of the header assembly  204  may be coated or filled with a biocompatible epoxy or similar material. For example, in certain implementations, a gap  250  may be present between the flange  210  and the helix mount  206  and may be filled with a biocompatible adhesive or epoxy such as one of NuSil™ medical adhesive 6219 and Hysol® M31-CL. Such adhesives and epoxies may be used to reinforce coupling between components of the header assembly  204  and protect the components from wear and corrosion. 
     One or more surface modification technologies may also be applied to contact surfaces of the biostimulator  200 . In general, such contact surfaces may correspond to any component of the biostimulator  200  that contacts or otherwise interacts with tissue of the heart when the biostimulator  200  is implanted. Examples of contact surfaces of the biostimulator  200  include, without limitation, the face of the cap  208  and the exterior surface of the primary helix  205 . For example, a surface modification treatment may be applied to the cap  208 , in whole or in part (e.g., only a specific portion of the face  208 ), to modify the properties of the cap  208  as compared to the substrate from which the cap  208  is substantially formed. 
     Such technologies may include technologies to, among other things, change one or more of the surface energy, the surface charge, the surface chemistry, or the surface morphology of the contact surface. Such modifications may be applied to promote a more organized, thinner fibrous capsule forming about the contact surface when the biostimulator  200  is implanted, thereby reducing the effects of such a capsule on pacing thresholds. For example, implantation of the biostimulator  200  into the heart may cause the body&#39;s natural foreign body response (FBR) to form thick scar tissue around or near a distal end of the biostimulator  200  or around specific components of the biostimulator  200 , such as the cap  208  and the primary helix  205 . This scar tissue may ultimately impede pacing by the biostimulator  200 . By altering the properties of the contact surface between the biostimulator  200  (or a specific component thereof) and the heart through the application of surface modification technologies, the FBR may be controlled or directed to promote a more predictable tissue reaction. For example, surface modification technologies may be applied to promote the formation of a relatively thin and even tissue capsule around the biostimulator  200 . Surface modification may also be used to promote improved substrate-to-tissue adhesion, thereby improving fixation of the biostimulator  200  within the heart tissue. 
     Various surface modification technologies may be applied to the contact surface using different techniques. For example, surface energy of the contact surface may be modified by, among other things, glow discharge or plasma treatment of the contact surface. As another example, surface charge may be modified by material selection or deposition of polymers or other materials that may be electrically charged or conductive onto the contact surface. Examples of such materials include, without limitation, piezoelectric polymer films and polyvinylidene fluoride (PVDF) films. Surface chemistry may be modified by, among other techniques, one or more of radiation grafting, protein patterning with soft lithography or micro-contact printing, and immobilization of peptides or proteins in specific micro patterns on the material surface. As yet another example, surface morphology may be modified by topographical patterning of the contact surface. Such patterning techniques may include, without limitation, one or more of laser micromachining and micromolding, such as micromolding using polydimethylsiloxane (PDMS). 
     As described above, biostimulators in accordance with this disclosure can include one or more sutures disposed on a forward face of the biostimulator adjacent a primary fixation feature, such as a helical screw. The sutures can be oriented in a direction opposite the primary fixation feature such that after fixation of the biostimulator by rotation in a first direction, counter rotation causes the sutures to engage tissue adjacent to the primary fixation feature, thereby resisting further counter rotation. In certain implementations, the sutures are formed of a flexible material such that sufficient counter torque applied to the biostimulator may cause the sutures to bend and disengage from the tissue adjacent to the primary fixation. As a result, the biostimulator may be removed or repositioned from the fixation site with minimal damage to tissue at the fixation site. Disengagement of one or more of the sutures may also be controlled by positioning the sutures such that bending of the sutures during counter rotation is obstructed by the primary fixation feature/helical screw. 
     Other biostimulators in accordance with this disclosure include various non-suture features/mechanisms for providing anti-unscrewing functionality. In one implementation, an elastomeric or otherwise flexible sleeve is disposed on a primary helix of the leadless pacemaker. The flexible sleeve can include an apex, e.g., barb tips of a set of barbs that extend in a direction opposite that of the primary helix or similar primary fixation feature, such that the apex engages adjacent tissue when the leadless pacemaker is unscrewed. In other words, the sleeve functions as a secondary fixation feature that resists rotation of the leadless pacemaker in a direction opposite that of the primary fixation feature. The resistance provided by the apex generally resists the gradual unscrewing caused by regular movement of the patient and/or the patient&#39;s heart, however, the sleeve, e.g., one or more of the barbs, is sufficiently flexible such that the sleeve may be deformed and/or made to disengage cardiac tissue if a sufficient counter-torque is applied. By doing so, the leadless pacemaker may be removed and/or repositioned. 
     The present disclosure is also directed to lateral fixation features (a type of secondary fixation feature) that similarly provide anti-unscrewing functionality. The lateral fixation features are formed from a thin sheet and extend laterally from a location adjacent a distal end of the leadless pacemaker. In certain implementations, the lateral fixation features are used in conjunction with a primary fixation helix or similar primary fixation feature. In such cases, the lateral fixation features function as secondary fixation features that extend in a direction opposite the primary fixation feature and resist counter-rotation of the leadless pacemaker. In other implementations, the lateral fixation features incorporate both primary and secondary fixation features and, as a result, may obviate the need for a primary fixation helix. For example, the lateral fixation features may each include a body extending in a first direction and that acts as the primary fixation feature for implanting the leadless pacemaker into cardiac tissue. One or more barbs, prongs, spurs, or similar anti-rotation structures may be coupled to or integrally formed with the body and may extend from the body in a second direction opposite the first direction. As a result, the anti-rotation structure functions as a secondary fixation feature that resists unscrewing of the leadless pacemaker. 
     A. Leadless Biostimulator Including an Anti-Unscrewing Sleeve 
     As previously discussed, leadless biostimulators, such as leadless pacemakers, may include a fixation feature to ensure that the sensing/pacing electrode of the leadless biostimulator maintains good electrical contact with the cardiac tissue within which the leadless biostimulator is implanted. In certain leadless biostimulators, such fixation mechanisms may include a helical screw. In addition to the helix a secondary fixation or “anti-rotation” mechanism, such as angled sutures, may be implemented to prevent the leadless biostimulator from rotating opposite the screwing direction of the helix and potentially counter-rotating out of implantation. 
     Fixation of leadless biostimulators within certain areas of the heart may present particular challenges. For example, fixation in the right atrium is made difficult due to the shape of the right atrium. More specifically, the shape of the right atrium generally precludes the use of laterally extending secondary fixation features. As a result, secondary fixation features for use in such applications may generally extend in a substantially distal direction. However, in leadless biostimulators such as the leadless biostimulator  200  of  FIG.  2   , in which distally extending sutures are implemented as secondary fixation features, such sutures may be undesirably close to the pacing/sensing electrode. As a result of such proximity, the sutures may cause the formation of scar tissue adjacent to the electrode resulting in an increased pacing threshold. In light of this issue, it would be advantageous to have alternative secondary fixation features that are substantially displaced relative to the electrode and, as a result, minimize the formation of scar tissue adjacent to the electrode. 
     In addition to issues related to scar tissue formation, the relatively small scale of leadless biostimulators significantly limit the types of processes available for manufacturing and assembling leadless biostimulators. For example, many features of leadless biostimulators are on the order of 0.001 inches to 0.010 inches and, as a result, are unable to hold tolerances when using conventional machining or molding processes. Other manufacturing processes more suited to the scale of leadless biostimulator components can be prohibitively costly. 
     In one implementation of the present disclosure, a leadless biostimulator is provided that includes a primary fixation feature, such as a primary fixation helix. The primary fixation feature can be attached to a distal end of the leadless biostimulator for use in securing the leadless pacemaker to a wall of the heart. The leadless biostimulator further includes a secondary fixation feature in the form of a thin-wall sleeve coupled to the primary fixation feature adjacent to or around the tip of the primary fixation feature. The secondary fixation feature may be formed by, among other things, extrusion, casting, or coating and may be formed in situ on the primary fixation feature. The secondary fixation feature may be formed to include a barb or similar counter-rotation feature that is configured to resist counter rotation of the leadless biostimulator after implantation. More specifically, the primary fixation feature is generally adapted to engage the wall of the heart by being rotated in a first direction (e.g., a screwing direction). The secondary fixation feature is shaped and disposed on the primary fixation feature such that, once implanted, the counter-rotation feature resists rotation of the leadless biostimulator in a second direction opposite the first direction, (e.g., an unscrewing direction) by engaging the wall of the heart. 
     The secondary fixation feature is generally placed adjacent to the tip of the primary fixation feature such that the secondary fixation is optimally placed to prevent rotation of the leadless biostimulator in the second direction. The secondary fixation feature is also generally disposed at or near the distal extent of the primary fixation feature such that the secondary feature does not interfere with a stimulation electrode of the leadless biostimulator. 
     Implementations of the present disclosure solve the issues related to the relatively small scale of leadless biostimulators by enabling the use of extrusion, casting, or coating processes to produce a very thin wall tube. The tube is then trimmed, die-cut, laser-cut, or otherwise processed to produce the final secondary fixation feature. By doing so, the secondary fixation feature may be manufactured in a cost-effective yet precise manner despite its small size. 
     In certain implementations, the geometric profile, wall thickness, material, and other aspects of the secondary fixation feature are chosen such that the secondary fixation feature resists counter-rotation of the leadless biostimulator up to a first predetermined torque. If a torque is applied that exceeds the first predetermined torque, the secondary fixation feature is adapted to bend, flex, or otherwise deform, backing against itself and allowing the leadless biostimulator to be unscrewed and subsequently removed or repositioned without severely damaging the tissue at the original implantation site. More particularly, the secondary fixation feature is configured such that, when implanted within the heart, a first torque in the unscrewing direction opposite the screwing direction causes the secondary fixation feature to engage the heart and provide a first resistance to rotation of the leadless biostimulator in the unscrewing direction. The secondary fixation feature is further configured, however, such that, when implanted within the heart, a second torque in the unscrewing direction greater than the first torque causes deformation of the secondary fixation feature to at least partially disengage the secondary fixation feature from the heart and provide a second resistance less than the first resistance to rotation of the leadless biostimulator in the unscrewing direction. 
       FIGS.  3  and  4    are isometric views of a biostimulator  300  in accordance with an embodiment. The biostimulator  300  includes a housing  302  and a header assembly  304  coupled thereto. The housing  302  may be sized to be implanted within a heart of a patient. Coupling of the housing  302  to the header assembly  304  may be accomplished in various ways including, without limitation, one or more of a biocompatible adhesive, a threaded connection, or ultrasonic welding. 
     The header assembly  304  generally includes a primary fixation feature  305  and a secondary fixation feature  306 . The primary fixation feature  305  can be similar or identical to the primary fixation device  205  described above, e.g., the primary helix  205 . Accordingly, when the header assembly  304  is mounted on the housing  302 , the primary fixation feature  305  is coupled to the housing  302 . Likewise, the secondary fixation feature  306  can be coupled to the primary fixation feature  305 . For example, the secondary fixation feature  306  may be a separate component disposed or mounted on the primary fixation feature  305 . Alternatively, the secondary fixation feature  306  can be integrally formed with and disposed on the primary fixation feature  305 . In general, the secondary fixation feature  306  functions as an anti-unscrewing feature that resists unscrewing of the biostimulator  300  after implantation. 
     In the specific example of  FIGS.  3  and  4   , the primary fixation device  305  is a primary helix  305  pointing in a first direction, e.g., spiraling in a clockwise direction. The secondary fixation feature  306  includes a sleeve  306  disposed or mounted near a distal extent of the primary helix  305 . For example, the sleeve  306  can be disposed about a helical wire of the primary fixation device  305  by sliding the sleeve  306  onto the primary helix  305 . The sleeve  306  can have one or more anti-rotation features facing a second direction opposite the first direction, e.g., in an anticlockwise direction. 
     The primary helix  305  may be substantially formed of any suitable biocompatible material including, without limitation, one or more of stainless steel, nickel-titanium alloys (such as Nitinol), nickel-chromium allows (such as Incoloy®), titanium, or multiphase nickel alloys (such as MP35N®). In certain implementations, the substrate material of the primary helix  305  may also be conductive such that the primary helix  305  may be used as an electrode for sensing and/or pacing of cardiac tissue. 
     The primary helix  305  is preferably sized to couple the biostimulator  300  to cardiac tissue while minimizing damage to the cardiac tissue. In certain implementations, for example, the primary helix  305  extends from and including 0.25 turns to and including 3 turns from the helix mount  206 , has a wire diameter from and including 0.003 inches to and including 0.03 inches, has a pitch diameter from and including 0.06 inches to and including 0.3 inches, and has a pitch from and including 0.01 inches to and including 0.05 inches. While the implementations illustrated herein include a single primary helix  305 , other implementations of the present disclosure may include multiple fixation helices, each extending in the same direction and each adapted to engage cardiac tissue in response to rotation of the biostimulator  300 . Such multi-helix implementations may include biostimulators with multifilar helices in which multiple wires are conjoined, e.g., jointly wound, or biostimulators including multiple offset helices. In implementations in which multiple helices are implemented, any or all of the helices may include a respective secondary fixation feature  306  to resist counter-rotation. 
     As illustrated in  FIGS.  3 - 4   , the sleeve  306  is generally disposed at or near the distal end of the primary fixation feature  305 . Coupling of the sleeve  306  to the primary fixation feature  305  may be achieved in various ways. In certain implementations, the sleeve  306  is directly molded or otherwise formed on the primary fixation feature  305 . For example, the sleeve  306  may be overmolded, cast, or extruded directly onto the primary fixation feature  305 . In other implementations, the sleeve  306  may be separately formed and then subsequently disposed on the primary fixation feature  305 . For example, the separate sleeve  306  can be mounted on and attached to, e.g., adhered to, the primary fixation feature  305 . In such implementations, the sleeve  306  may be coupled to the primary fixation feature  305 , among other ways, by a biocompatible adhesive or by heat-shrinking the sleeve  306  onto the primary fixation feature  305  (when sleeve  306  is fabricated from a heat-shrink material). To facilitate coupling between the primary fixation feature  305  and the sleeve  306 , one or both of a surface of the primary fixation feature  305  and an inner surface of the sleeve  306  may be textured or otherwise roughened to improve adhesion. 
     As illustrated in  FIG.  3   , the sleeve  306  may be disposed at or near a tip of the primary fixation feature  305 . In other implementations, the sleeve  306  may be disposed at other locations along the primary fixation feature  305 . For example, in certain implementations the sleeve  306  may be disposed or mounted on the first quarter, first half, first three-quarter, or first full distal turn of the primary fixation feature  305 . 
     As described below in the context of  FIGS.  18 A- 18 B , implantation of the leadless biostimulator  300  is generally accomplished using a delivery catheter or similar delivery catheter that may be used to guide the leadless biostimulator  300  to an implantation location adjacent a wall of a chamber of the heart. Once located, the delivery catheter may be rotated in a screwing direction, thereby causing the primary fixation feature  305  to engage the wall of the heart. More particularly, rotation of the primary fixation feature  305  in the screwing direction affixes the housing  302  to the heart of the patient. As the leadless biostimulator  300  is further rotated, an electrode  311  of the leadless biostimulator  300  is brought into contact with the wall of the heart such that the leadless biostimulator  300  can deliver electrical impulses to the adjacent heart tissue. 
     As the primary fixation feature  305  is implanted into the wall of the heart, the secondary fixation feature  306  is brought into proximity with the wall of the heart as well. More specifically, the secondary fixation feature  306  is disposed adjacent to the wall of the heart such that the secondary fixation feature is able to resist counter-rotation of the implanted biostimulator  300 . Such counter-rotation may be the result of, among other things, movement of the patient or beating of the heart. As described below in more detail in the context of  FIGS.  5 A- 5 D , the sleeve  306  may include barbs, such as barbs  360 A- 360 B, or similar anti-rotation features that extend in a direction substantially opposite the screwing direction of the primary fixation feature  305 . Accordingly, as counter-torque is applied to the biostimulator  300 , the anti-rotation feature engages the wall of the heart to resist counter rotation of the biostimulator  300 . 
     In certain implementations, one or more of the secondary fixation features (also referred to herein as anti-rotation features) may be adapted to resist counter torque up to a predetermined limit but to disengage the wall of the heart when the predetermined limit is exceeded. For example, the barbs  360 A- 360 B may be formed of a flexible material capable of resisting a first counter torque but if a second counter torque is applied that is greater than the first counter torque, the barbs  360 A- 360 B may deflect, bend, compress, buckle, or otherwise deform such that the barbs  360 A- 360 B are no longer pointed in a direction substantially opposite the primary fixation feature  305 . When so deformed, the biostimulator  300  may be counter rotated to disengage the primary fixation feature  305 , thereby enabling removal and/or repositioning of the biostimulator  300 . In certain implementations, the first counter torque may generally correspond to the anticipated counter torque that may be experienced during regular patient activity plus a predetermined safety factor. For example, the first counter torque may be up to and including 0.5 ounce-inches (oz-in). In certain implementations, the second counter torque may generally correspond to a predetermined force required to be applied by a retrieval catheter or similar retrieval system that may be used to retrieve/remove the biostimulator  300  following implantation. In such implementations, the second counter torque may be from and including 0.5 oz-in to and including 2.0 oz-in, for example. 
     The first and second counter torque values above are provided by way of example. In an embodiment, the second counter torque is higher than the first counter torque, and may be higher by a scale factor. The scale factor can be a multiplier that provides more resistance to disengagement under torque. For example, the second counter torque may be at least 5 times the first counter torque, e.g., the second counter torque may be equal to the first counter torque times the scale factor of 10. 
       FIGS.  5 A- 5 B  are an isometric view and a cross-sectional side view of the sleeve  306  of  FIGS.  3 - 4    and are intended to illustrate aspects of the sleeve  306  in further detail. 
     As illustrated in  FIGS.  5 A- 5 B , the sleeve  306  generally includes a body  502  from which one or more anti-rotation features extend. The body  502  can be a tubular body, e.g., a body having a cylindrical outer surface  503  and a lumen  505  extending longitudinally through the body. An annular cross-section of the body  503  can be disposed about a conforming cylindrical outer surface of the primary helix  305 . 
     One or more anti-rotation features can extend from the body  503  of the sleeve  306 . For example, a barb  360  can extend at an angle from a first end  507  of the body  503 . The barb  360  can be one of several flexible barbs  360 A- 360 D, each of which can include respective barb tips  509 . The outer surface  503  can extend over the barb(s) from the first end  507  to the barb tips  509 . Given that the barbs  360 A- 360 D can extend at an angle, e.g., in the unscrewing direction, from the first end  507 , the outer surface  503  can similarly taper radially outward to the barb tip  509 . In an embodiment, the barb tip  509  is a radially outward limit of the sleeve  306 , and accordingly, the barb tips  509  are at an apex  511  of the sleeve  306 . More particularly, the sleeve  306  include outer surface  503  tapering radially outward to the apex  511  at a radially outward-most location. 
     The barbs  360 A- 360 D can be distributed about the circumference of the tubular body  502 . For example, four barbs may be distributed about the circumference. In other implementations, other anti-rotation features may be implemented. For example, and without limitation, such anti-rotation features may include barbs having shapes that are other than triangular. By way of example, the anti-rotation features may be elongated cylinders, e.g., include sutures or threads extending from the body. Moreover, the number of anti-rotation features may also vary in other implementations. Although illustrated as including four anti-rotation features  360 A- 360 D in  FIGS.  5 A- 5 B , in other implementations, other numbers of anti-rotation features may be used. For example, and without limitation, implementations may include any of one, two, three, or any number greater than four barbs. 
     As previously discussed, the anti-rotation features  360 A- 360 D may be flexible such that counter-rotation of the leadless biostimulator  300  after implantation is resisted. When sufficient counter-torque is applied, however, the anti-rotation features  360 A- 360 D may flex or otherwise deform, thereby enabling disengagement of the leadless biostimulator  300 . To achieve such flexibility, the sleeve  306  may be formed from a flexible plastic, such as polyimide. For example, the sleeve  306  may be formed by extruding or otherwise manufacturing a polyimide tube that is then cut (such as by die cutting, waterjet cutting, laser cutting, or a similar cutting method) or similarly processed to form the anti-rotation features  360 A- 360 D. 
     Materials for the sleeve  306  may also be selected based on particular properties or characteristics. For example, in certain implementations, the sleeve  306  may be formed from a flexible material, e.g., polyimide as described above, or another flexible biocompatible material including, without limitation, one or more of polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. Sleeve  306  may be formed from one or more flexible materials, and the term “flexible” may be (although not necessarily) defined as having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. Such material characteristics provide for adequate flexibility to allow the sleeve  306  to yield to a predetermined counter-torque, as described above. Material selection for the sleeve  306  may alternatively be based on material toughness which may be associated with specific tensile and compression strengths of the material. 
     In an embodiment, one or more portions of the secondary fixation feature, e.g., the sleeve  306 , may be formed from a bioabsorbable and/or bioresorbable material. Examples of suitable bioresorbable polymers include polyglycolide (PGA), polylactide (PLA), polycaprolactone (PCL), polydioxanone (PDO), polytrimethylene carbonate (TMC), and co-polymers thereof. Examples of suitable bioresorbable metals include magnesium alloys, iron alloys, zinc alloys, and combinations thereof. The bioresorbable material may be a magnesium-rare earth alloy with dysprosium as the main alloying element. For example, the bioresorbable material may be RESOLOY®. The bioabsorbable secondary fixation feature may exhibit surface erosion and/or bulk degradation during absorption into the heart following implantation. 
     A bioabsorbable secondary fixation feature may be tuned to absorb into the target tissue over a predetermined time range. The absorption profile can be tuned by the composition of the bioresorbable material, e.g., the monomers selected, the ratio of monomers in a co-polymer, the polymer chain length, and/or by a geometry of the secondary fixation feature. For example, in the embodiment of  FIGS.  5 C- 5 D , a shape and/or cross-sectional area, such as the tapers at either ends of the sleeve  360 , can be selected to control a rate of absorption. The geometry can cause the sleeve to absorb over a predetermined period of time. The time range may be selected to allow for complete absorption to occur after tissue has endothelialized around the secondary fixation feature. By choosing such an absorption profile, the tissue fibers can form a matrix around the primary fixation feature to secure the leadless pacemaker and prevent backout after the secondary fixation feature is fully resorbed. 
     The sleeve  306  may, in certain implementations, be formed from multiple materials. For example, the tubular body  502  and barbs  360 A- 360 D may be formed from different materials, with the tubular body  502  being relatively more rigid that the barbs  360 A- 360 D. 
     Various aspects of the sleeve  306  may conform to predetermined dimensional ranges. For example, the tubular body  502  may have a thickness  504  from and including 0.001 inches to and including 0.010 inches and each of the triangular barbs  360 A- 360 D may have a length (such as barb length  506  of barb  360 C) from and including 0.005 inches to and including 0.200 inches. Each of the barbs  360 A- 360 D may also be biased to extend at an angle  508  relative to a longitudinal axis  510  of the sleeve  306 , the angle  508  being up to and including 90 degrees. As described above, the body  502  can include a first end  507 , and the outer surface  503  can taper from the first end  507  to the apex  511  at the barb tip(s). 
     As illustrated in  FIG.  5 B , the body  502  may also include a second end  512  opposite the anti-rotation features  360 A- 360 D. More particularly, the second end  512  can be on an opposite end of the body  502  from the first end  507 . In an embodiment, the second end  512  include a taper  514 . For example, the taper  514  can be a portion of the body  502  that narrows or tapers toward the longitudinal axis  510  of the sleeve  306 . Alternatively, the second end  512  may have an external edge that is radiused, filleted, or similarly profiled. The outer surface  503  can have an outer dimension that decreases in the screwing direction over the taper  514 . Such a feature on the second end  512  may generally prevent the second end  512  of the sleeve from catching or otherwise engaging adjacent heart tissue during implantation of the leadless biostimulator  300  because the taper  514  can wedge along, rather than catch on, the tissue during implantation. Accordingly, the taper  514  can reduce the likelihood of unintentional damage during implantation. 
       FIGS.  5 C- 5 D  are isometric and side elevation views, respectively, of the sleeve, in accordance with an embodiment. The sleeve  306  may have an alternative anti-rotation feature, as compared to the barbs  360 A- 360 D. In any embodiments, the sleeve  306  can provide resistance to movement in the unscrewing direction, and the anti-rotation features can be shaped to facilitate such a function. The anti-rotation feature may, however, be barbless. 
     In an embodiment, a barbless sleeve  306  may be asymmetrically shaped to preferentially move in the screwing direction. More particularly, the sleeve  306  may move more easily in the screwing direction than in the unscrewing direction. The sleeve  306  can include a ferrule having the body  502 . More particularly, the ferrule can include an annular body  502  extending from the first end  507  to the second end  512 . The annular body  502  may include one or more tapers extending from respective ends to the apex  511  at a radially outward-most location. For example, a first taper  520  can taper radially outward in a first direction, e.g., the screwing direction, and a second taper  514  can taper radially outward in a second direction opposite to the first direction, e.g., the unscrewing direction. The tapers  514 ,  520  can meet at a ridge extending along the apex  514 , or the apex  511  can be a cylindrical portion of the body  502  separating the outermost points on the tapers  514 ,  520  as shown. The apex  511  can be longitudinally between end  507 ,  512 . 
     The length and/or angle of each of the tapers can affect an amount of torque required to move the ferrule against tissue. For example, the steeper the taper, the more torque that is required to wedge the taper along tissue when the taper is in contact with the tissue. In an embodiment, the first taper  520  faces the unscrewing direction, and thus, the first taper  520  can have a higher angle relative to the longitudinal axis of the sleeve  306  as compared to the second taper  514 . Accordingly, more torque is required to move the ferrule in the unscrewing direction than in the screwing direction. Movement in the screwing direction is relatively easier because the second taper  514  has a smaller angle relative to the longitudinal axis, and thus, wedges more gradually along the tissue. Similar to the taper  514  of  FIG.  5 A , the second taper  514  of  FIG.  5 C  facilitates movement in the screwing direction. Likewise, similar to the barbs  360 A- 360 D of  FIG.  5 A , the first taper  520  of  FIG.  5 C  resists movement in the unscrewing direction. 
     Referring to  FIG.  5 D , it can be seen that in cross-section there may be no sharp edges on the sleeve  306  that includes the ferrule configuration. A lack of sharp edges, such as the barb tips  509 , can reduce the likelihood of causing tissue trauma when the primary fixation feature  305  is unscrewed. The tapers of the ferrule, however, provide sufficient resistance to unscrewing that the sleeve  306  prevents disengagement of the primary fixation feature  305  from the heart. Accordingly, barbless secondary fixation features  306  can achieve resistance to unscrewing while reducing the likelihood of tissue trauma. 
     A method of manufacturing the leadless biostimulator  300  can include forming the secondary fixation feature  306 , which includes the sleeve. Forming the sleeve  306  can include one or more operations. For example, forming the sleeve  306  can include forming the tubular body  502 , and cutting the tubular body  502  to form one or more barbs  360 . Alternatively, forming the sleeve  306  can include a single operation, e.g., fabricating the ferrule from a bioabsorbable material in a machining or molding operation. The method can include disposing the secondary fixation feature  306  on a distal portion of the primary fixation feature  305 . For example, the secondary fixation feature  306  can be mounted on the primary fixation feature  305 , or the secondary fixation feature  306  can be directly formed onto the primary fixation feature  305 . 
     B. Leadless Biostimulator Having a Planar Fixation Feature Including Primary and Secondary Fixation Features 
     As previously discussed, various issues may arise when fixing a leadless biostimulator within the heart and, in particular, when requiring both a primary fixation feature for securing the biostimulator to the wall of the heart during implantation and a secondary fixation feature to reduce or prevent the leadless biostimulator from unscrewing or otherwise detaching once implanted. Among other issues, the placement of such fixation features relative to an electrode of the biostimulator may be problematic in that if there is insufficient spacing, the fixation features may cause the formation of scar tissue adjacent to the electrode, thereby increasing pacing and sensing thresholds. Another issue arises from the general scale of leadless biostimulators and the ineffectiveness of conventional manufacturing techniques in maintaining the required tolerances for such fixation features. 
     To address these issues, among others, another implementation of a leadless biostimulator is provided in which the primary fixation helix of the previously discussed examples is omitted. Instead, each of primary and secondary fixation are achieved using a planar fixation feature having laterally extending arms. In certain implementations, the planar fixation feature is disposed proximal to an electrode of the biostimulator, thereby reducing the likelihood that the planar fixation feature will form interfering scar tissue. The planar fixation feature may also be formed from converted or extruded thin-wall sheeting, thereby improving overall manufacturability of the planar fixation feature. 
     As discussed below in more detail, the planar fixation feature may include a body from which a series of arms extend. Each of the arms extends in the same direction such that by rotating the leadless biostimulator in the direction of the arms, pointed tips of the arms may be inserted into the wall of the heart, thereby implanting the leadless biostimulator. Each of the arms further includes a respective secondary fixation feature adjacent to the pointed tip that extends opposite the direction of the arm. Each secondary fixation feature may, for example, be in the form of a hook, a barb, or a similar protrusion. By extending in a direction opposite the arm, the secondary fixation features resist counter-rotation of the leadless biostimulator once implanted. 
     The geometric profile and the wall thickness of the sheeting from which the planar fixation feature is formed, is chosen such that the arm can easily pierce the endocardium and engage with tissue securely with a reasonable amount of forward pressure and torsion. However, the sheeting is also chosen such that the counter-rotational resistance provided by the secondary fixation feature may be overcome should a change in placement or removal of the biostimulator be required. More specifically, the sheeting is chosen such that by applying an overload torque (e.g., a torque that generally exceeds that which would be experienced by the biostimulator during normal cardiac activity) in the counter-rotational direction, the secondary fixation feature may be made to bend back on itself, give, or otherwise deform allowing the biostimulator to be unscrewed without severely damaging the tissue adjacent to the initial implantation site. 
     Implementations of the present disclosure including planar fixation features take advantage of film converting processes or similar manufacturing techniques to tightly control a first dimension (i.e., the thickness) of the fixation feature. For example, depending on the particular material used, such manufacturing techniques can achieve consistent thicknesses in the range of 0.001 inches to 0.02 inches with significantly tighter tolerances than comparable machining or molding processes. The planar fixation feature, and more specifically the arms and bars, may then be trimmed or cut from the formed sheet, such as by using a blade, a die, a waterjet, or laser. 
     Placing the secondary fixation features (e.g., the barbs) immediately adjacent to the primary fixation features (e.g., the tips of the arms), can optimally achieve both primary and secondary fixation because the counter-rotational resistance provided by the secondary fixation features is directed in the immediate vicinity of the primary fixation location. In other words, since the secondary fixation barbs are integrated with the arms, secondary fixation is guaranteed once the arms are engaged with tissue. 
     As illustrated by the implementations described herein, the planar fixation feature may include multiple, integrated arms, each of which includes a corresponding secondary fixation feature. This redundant design enables for more reliable engagement of the biostimulator to the wall of the heart as there are more chances for tissue engagement and only one engagement is generally needed for adequate fixation of the biostimulator. Including multiple points of engagement also compensates for greater variance in tissue morphology. By including multiple fixation points, the amount of rotation required to engage the wall of the heart is also generally reduced, leading to a simpler delivery experience. Moreover, by integrating primary and secondary fixation features into one formed feature, the overall fixation design is greatly simplified while still meeting design requirements. Among other benefits, the simplified design increases manufacturing efficiency, decreases part costs, provides an improved fixation delivery experience for the end user, reduces the potential damage to tissue (thus improving pacing thresholds), and generally provides a more reliable fixation function. 
       FIGS.  6  and  7    are isometric views of a biostimulator  600  in accordance with the present disclosure. The biostimulator  600  includes a housing  602  and a header assembly  604  coupled thereto. Coupling of the housing  602  to the header assembly  604  may be accomplished in various ways including, without limitation, one or more of a biocompatible adhesive, a threaded connection, or ultrasonic welding. 
     The header assembly  604  includes a planar fixation feature  605  extending laterally from the header assembly  604 . The planar fixation feature  605  includes several arms  660 A- 660 F for fixation of the leadless biostimulator  600  to a wall of the heart. More specifically, the arms  660 A- 660 F provide both primary fixation functionality by enabling implantation of the leadless biostimulator  600  into the wall of the heart and secondary fixation functionality by resisting counter-rotation of the leadless biostimulator  600  following implantation. To do so, each of the arms  660 A- 660 F extends in a first or screwing direction, terminating in a sharpened point  662 A- 662 F (indicated in  FIGS.  9 A- 9 B ). Accordingly, when the biostimulator  600  is brought into contact with a wall of the heart and rotated in the first direction, the arms  660 A- 660 F engage and implant into the wall of the heart. 
     Each of the arms  660 A- 660 F further includes a barb  664 A- 664 F (indicated in  FIGS.  9 A- 9 B ) or similar anti-rotation feature extending in a second or unscrewing direction opposite the screwing direction. Accordingly, once implanted, the barbs  664 A- 664 F provide resistance to counter-torques that may arise from movement of the patient or cardiac activity and that may otherwise cause loosening and/or disengagement of the leadless biostimulator  600  from the wall of the heart. 
       FIG.  8    is an exploded view of the header assembly  604  of  FIGS.  6  and  7   . As illustrated, the header assembly  604  may include a header body  670  and a header cap  672  between which the planar fixation feature  605  is disposed. The header assembly  604  may further include an electrode  674  for delivering pacing or other impulses to the heart tissue. 
     As shown in  FIG.  8   , the planar fixation feature  605  may be retained between the header cap  672  and the header body  670 . In certain implementations, for example, the planar fixation feature  605  may be coupled to one or both of the header body  670  and the header cap  672  using ultrasonic welding, an adhesive, or other coupling method. The header body  670  may also include a key or similar alignment feature  676  that may be used to facilitate alignment of one or both of the header cap  672  and the planar fixation feature  605 . For example, one or both of the header cap  672  and the planar fixation feature  605  may include a notch or similar indentation (not illustrated) corresponding to the key  676  such that, when assembled, one or both of the header cap  672  and the planar fixation feature  605  are in a predetermined orientation. 
     During implantation, the leadless biostimulator  600  is disposed in proximity to an implantation location and then rotated in a screwing direction (which, in the case of the leadless biostimulator  600  is a clockwise direction but may be counterclockwise in other implementations) causing one or more of the arms  660 A- 660 F to engage the wall of the heart. During rotation in the screwing direction, the barbs  664 A- 664 F of the arms  660 A- 660 F are angled away from the screwing direction of rotation and, in certain implementations, may flatten against the arms  660 A- 660 F so as to not obstruct implantation of the leadless biostimulator  600 . As one or more of the arms  660 A- 660 F engages and penetrates the endocardium, the corresponding barb similarly penetrates into the wall of the heart. As the length of the barbs  664 A- 664 F is only a fraction of the total length of their respective arms  660 A- 660 F, full engagement of a curvate arm results in the corresponding barb being fully inserted into through the endocardial layer. Following implantation of the leadless biostimulator  600 , counter rotation of the leadless biostimulator  600 , such as resulting from natural heart movement, results in the barb engaging the tissue to resist the counter rotation and maintain the leadless biostimulator  600  in engagement with the wall of the heart. 
     The barbs  664 A- 664 F are generally configured to resist regular counter-torques applied to the leadless biostimulator  600  during normal cardiac activity. However, the barbs  664 A- 664 F may also be designed to deform in the event that removal or repositioning of the leadless biostimulator  600  is required. In other words, while the barbs  664 A- 664 F are sufficiently rigid to oppose regular counter torques, they are also sufficiently pliable such that by applying a sufficient counter-torque (such as by using a delivery or retrieval catheter), the barbs  664 A- 664 F may be made to disengage from the wall of the heart with relatively minimal damage to the surrounding tissue. 
     In certain implementations, the first counter torque (i.e., the counter torque that the barbs  664 A- 664 F are designed to substantially withstand) may be in a range up to and including 0.5 oz-in. The second counter torque, in contrast, may generally correspond to a predetermined force required to be applied by a retrieval catheter or similar retrieval system that may be used to retrieve/remove the biostimulator  600  following implantation. In such implementations, the second counter torque may be from and including 0.5 oz-in to and including 2.0 oz-in, for example. As described above, the second counter torque can be higher, e.g., by a scale factor, than the first counter torque. 
       FIGS.  9 A- 9 C  illustrate the example planar fixation feature  605  in further detail. More specifically,  FIG.  9 A  is an isometric view of the planar fixation feature  605 ,  FIG.  9 B  is a distal view of the planar fixation feature  605 , and  FIG.  9 C  is a side elevation view of the planar fixation feature  605 . 
     As illustrated in  FIGS.  9 A- 9 C , the planar fixation feature includes a circular body  680  from which arms  660 A- 660 F extend. Each of the arms  660 A- 660 F extends in a first, screwing direction and terminates in a respective barb  664 A- 664 F extending in a second direction opposite the first direction. The circular body  680  further defines a through hole  682  for coupling the planar fixation feature  605  to other components of a header assembly of a leadless biostimulator, such as the header assembly  604  illustrated in  FIG.  8   . For example, the through hole  682  may be shaped to receive a protrusion or extension of the header body  670 . As previously discussed, the edge of the through hole  682  may further define a notch or protrusion shaped to mate with a corresponding protrusion or notch, respectively, of the header body  670 . By doing so, the planar fixation feature  605  may be placed in a predetermined orientation relative to the header body  670  during assembly. 
     The arms  660 A- 660 F of the planar fixation feature  605  may conform to a predetermined shape or arrangement and have a predetermined geometry. For example, as illustrated in  FIG.  9 B , each of the arms  660 A- 660 F extends from the circular body  680  along a circular path. With specific reference to arm  660 B, each of the arms  660 A- 660 F may be defined by a radius r 1  extending from a respective origin O. As shown in  FIG.  9 B , the origin O may be disposed on a circle defined by a second radius r 2  extending from a center C 1  of the circular body  680 . In certain implementations, the radius r 1  may be constant such that the arm  660 B extends along a circular arc. In other implementations, the radius r 1  may increase along the length of the arm  660 B such that the arm  660 B follows a spiraling path instead. In one specific example of a planar fixation feature, each arm may have a value of r 1  from and including 0.05 inches to and including 0.10 inches and a value of r 2  from and including 0.02 inches to and including 0.03 inches. Each of the barbs  664 A- 664 F may also conform to one or more predetermined dimensions. For example, with reference to barb  664 A, each barb may have an extension length L 1 , corresponding to the maximum distance the barb extends from its respective arm, and a width W 1 . In certain implementations, the extension length L 1  may be from and including 0.002 inches to and including 0.01 inches and the width W 1  may be from and including 0.001 inches to and including 0.005 inches. Notably, the arms  660 A- 660 F need not be curvate provided they extend in a screwing direction of the leadless biostimulator  600 . For example, in contrast to the foregoing implementations, the planar fixation feature  605  may instead include substantially straight arms that extend from the circular body  680 . Such straight arms may extend, for example, at an angle tangential to the outer extent of the circular body  680 . 
     As illustrated in  FIG.  9 C , the planar fixation feature  605  is substantially flat and is generally formed from a thin sheet of material. In certain implementations, the planar fixation feature  605  may be formed from a sheet or film having a thickness  684  from and including 0.001 inches to and including 0.010 inches. For example, in one method of manufacturing the planar fixation feature  605 , a sheet or film may be formed using a film converting process. The sheet/film may then be punched, cut, trimmed, or otherwise processed to produce the planar fixation feature  605 . 
     To achieve the required characteristics of the barbs  664 A- 664 F, the planar fixation feature  605  may be formed from a flexible plastic material, such as polyimide. In other implementations, the planar fixation feature  605  may instead be formed of other flexible biocompatible materials including, without limitation, one or more of polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. Material selection for the planar fixation feature  605  may alternatively be based on particular properties or characteristics of the material. In certain implementations, the planar fixation feature may be formed from one or more bioabsorbable materials, as described above. For example, the bioabsorbable material(s) may include a magnesium alloy. In certain implementations, the planar fixation feature  605  may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. Material selection for the planar fixation feature  605  may alternatively be based on material toughness which is often associated with specific tensile and compression strengths of the material. 
     As illustrated in the preceding figures, the planar fixation feature  605  may include six arms  660 A- 660 F. In alternative implementations, however, the planar fixation feature  605  may include more or fewer than six arms. For example, and without limitation, implementations of planar fixation features according to the present disclosure may include from one to six or more arms. Also, while each of the arms  660 A- 660 F is illustrated in  FIGS.  6 - 9 C  as being substantially the same, each arm may vary in its length and shape. For example and without limitation, some or all of the arms of planar fixation features according to this disclosure may have different lengths, different barb shapes and/or barb lengths, follow different curvatures, or have no curvature at all in the case of straight arms. 
     C. Leadless Biostimulator with a Forward-Facing Fixation Structure Including Integrated Primary and Secondary Fixation Features 
     As previously discussed in the context of  FIG.  2   , certain implantation locations within the heart (such as in the vicinity of the apex of the heart) have geometries that may present challenges to proper implantation of a leadless biostimulator. To overcome this issue, biostimulators in accordance with this disclosure may include forward-facing primary and secondary fixation features. In the implementation of  FIG.  2   , for example, the primary fixation feature is in the form of a primary wire helix while the secondary fixation feature, which resists counter-rotation following implantation, is in the form of one or more forward-facing sutures extending from a distal end of the leadless biostimulator. More specifically, the forward-facing sutures extend from a distal end of the leadless biostimulator about which the primary fixation helix extends. 
     To improve engagement of the secondary fixation feature, implementations of the present disclosure also include designs in which the secondary fixation feature is in proximity to the engagement point of the primary fixation feature. In the implementation of  FIGS.  4 - 5 B , for example, the secondary fixation feature is a sleeve disposed near the tip of the primary helix. In the implementation of  FIGS.  6 - 9 C , the primary and secondary fixation features are integrated into a unitary planar fixation feature that extends laterally from the distal end to the leadless biostimulator. Notably, the unitary design of  FIGS.  6 - 9 C  provides benefits related to manufacturing efficiency and reduced costs. 
     The following disclosure is directed to yet another implementation of a leadless biostimulator that includes primary and secondary fixation features. Similar to the previous example implementations, the primary fixation feature generally extends in a first, screwing direction to fix the leadless biostimulator to a wall of the heart. Once implanted, the secondary fixation feature resists counter rotation of the leadless biostimulator such that regular cardiac activity does not result in dislodgment of the leadless biostimulator. In contrast to the previous designs, however, the following disclosure is directed to a fixation structure that provides the implantation advantages of a forward-facing fixation feature arrangement with the improved manufacturability and engagement provided by a unitary fixation structure that incorporates both primary and secondary fixation features. 
     More specifically, a leadless biostimulator is provided that includes a forward-facing fixation structure including several arms that extend from the distal end of the leadless biostimulator. Each of the arms extends in a first or screwing direction. For example, the arms may be biased at an angle or extend helically about a longitudinal axis of the leadless biostimulator. Implantation is therefore achieved by disposing the distal ends of the arms in contact with the wall of the heart and rotating the leadless biostimulator in the screwing direction. 
     Disposed at the end of each arm is a hook, barb, or similar secondary fixation feature that points in a direction substantially opposite the screwing direction. Accordingly, after the leadless biostimulator has been implanted, counter-torques experienced by the leadless biostimulator (such as those resulting from normal cardiac activity) are resisted by the secondary fixation features. 
     The following fixation structure has various advantages. Among other things, the placement of the secondary fixation features adjacent to the tips of the primary fixation ensures that the secondary fixation features are able to engage the wall of the heart with relatively minimal engagement of the primary fixation features. Also, the unitary design of the fixation structure simplifies manufacturing of the fixation structure and improves manufacturing tolerances. For example, the fixation structure may be machined or otherwise cut from a tubular structure that may be made by extrusion or a similar process. By doing so, the manufacturing process is less complicated as compared to conventional fixation mechanisms and the thickness of the tubular structure can be tightly controlled to impart specific performance characteristics on the fixation structure. Additional implementations and benefits of those implementations will become apparent in light of the following disclosure, which provides an example leadless biostimulator and fixation structure according to the present disclosure. 
       FIGS.  10  and  11    are isometric views of a biostimulator  1000  in accordance with the present disclosure. The biostimulator  1000  includes a housing  1002  and a header assembly  1004  coupled thereto. Coupling of the housing  1002  to the header assembly  1004  may be accomplished in various ways including, without limitation, one or more of a biocompatible adhesive, a threaded connection, and ultrasonic welding. 
     The header assembly  1004  includes a fixation feature  1005  extending from a distal end of the header assembly  1004 . In contrast to the laterally extending fixation feature  605  of  FIGS.  6 - 9 C , the fixation feature  1005  of the leadless biostimulator  1000  extends in a longitudinal direction from a distal end of the leadless biostimulator  1000  about a longitudinal axis  1001  of the leadless biostimulator  1000 . The fixation feature  1005  includes several arms  1060 A- 1060 D for fixation of the leadless biostimulator  1000  to a wall of the heart. 
     The arms  1060 A- 1060 D provide both primary fixation functionality by enabling implantation of the leadless biostimulator  1000  into the wall of the heart and secondary fixation functionality by resisting counter-rotation of the leadless biostimulator  1000  following implantation. To do so, each of the arms  1060 A- 1060 D extends in a first or screwing direction and terminates in a respective point  1062 A- 1062 D (indicated in  FIG.  13 A ). Accordingly, when the biostimulator  1000  is brought into contact with a wall of the heart and rotated in the first direction, the arms  1060 A- 1060 D engage and implant into the wall of the heart. 
     The arms  1060 A- 1060 D further include respective barbs  1064 A- 1064 D (indicated in  FIG.  13 A ) or similar anti-rotation feature extending in a second or unscrewing direction opposite the screwing direction. Accordingly, once implanted, the barbs  1064 A- 1064 D provide resistance to counter-torques that may arise from movement of the patient or cardiac activity and that may otherwise cause loosening and/or disengagement of the leadless biostimulator  1000  from the wall of the heart. 
     The barbs  1064 A- 1064 D are generally configured to resist regular counter-torques applied to the leadless biostimulator  1000  during normal cardiac activity. However, the barbs  1064 A- 1064 D may also be designed to deform in the event that removal or repositioning of the leadless biostimulator  1000  is required. In other words, while the barbs  1064 A- 1064 D are sufficiently rigid to oppose regular counter torques, they are also sufficiently pliable such that by applying a sufficient counter-torque (such as by using a delivery or retrieval catheter), the barbs  1064 A- 1064 D may be made to disengage from the wall of the heart with relatively minimal damage to the surrounding tissue. 
     In certain implementations, the first counter torque (i.e., the counter torque that the barbs  1064 A- 1064 D are designed to substantially withstand) may be in a range up to and including 0.5 oz-in. The second counter torque, in contrast, may generally correspond to a predetermined force required to be applied by a retrieval catheter or similar retrieval system that may be used to retrieve/remove the biostimulator  1000  following implantation. In such implementations, the second counter torque may be from and including 0.5 oz-in to and including 2.0 oz-in, for example. As described above, the second counter torque can be higher, e.g., by a scale factor, than the first counter torque. 
     To achieve the required characteristics of the barbs  1064 A- 1064 D, the fixation feature  1005  may be formed from a flexible plastic material, such as polyimide. In other implementations, the fixation feature  1005  may instead be formed of other flexible biocompatible materials including, without limitation, one or more of polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. Material selection for the fixation feature  1005  may alternatively be based on particular properties or characteristics of the material. In certain implementations, the fixation feature may be formed from one or more bioabsorbable materials, as described above. For example, the bioabsorbable material(s) may include a magnesium alloy. In certain implementations, the planar fixation feature  1005  may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa. Material selection for the fixation feature  1005  may alternatively be based on material toughness which is often associated with specific tensile and compression strengths of the material. 
       FIG.  12    is an exploded view of the header assembly  1004  of  FIGS.  10  and  11   . As illustrated, the header assembly  1004  may include a header body  1070  about which the fixation feature  1005  is disposed. The header assembly  1004  may further include an electrode  1074  for delivering pacing or other impulses to the heart tissue. The fixation feature  1005  may be coupled to the header body  1070  using various methods including, without limitation, one or more of a threaded connection, an adhesive, ultrasonic or other welding, a fastener (such as a set screw), or any other suitable coupling method. The header body  1070  and the fixation feature  1005  may also include mating features, such as corresponding slots and keys, to ensure alignment of the header body  1070  and the fixation feature  1005 . 
       FIGS.  13 A- 13 C  illustrate the example fixation feature  1005  in further detail. More specifically,  FIG.  13 A  is an isometric view of the fixation feature  1005 ,  FIG.  13 B  is a distal view of the fixation feature  1005 , and  FIG.  13 C  is a side elevation view of the fixation feature  1005 . 
     As illustrated in  FIGS.  13 A- 13 C , the fixation feature  1005  includes a cylindrical body  1080  from which arms  1060 A- 1060 D extend. Each of the arms  1060 A- 1060 D extend in a first, screwing direction and terminates in a respective barb  1064 A- 1064 D extending in a second direction opposite the first direction. As previously discussed in the context of  FIG.  12   , the cylindrical body  1080  is shaped to be disposed about and coupled to a header body of the biostimulator. As previously discussed, the interior surface of the cylindrical body  1080  may include a protrusion, slot, or similar feature, that engages with a corresponding feature of the header body  1070  to align the cylindrical body  1080  relative to the header body  1070 . 
     The tubular structure of the fixation feature  1005  is most evident in  FIG.  13 B , which is a distal view of the fixation feature  1005 . As illustrated, each of the cylindrical body  1080  and each of the arms  1060 A- 1060 D are formed from a uniform tubular structure having a thickness t 1 . For example, the fixation feature  1005  may be formed from an extruded or similarly formed tube that is then cut (such as by die cutting, laser cutting, and the like) to form the arms  1060 A- 1060 D and their respective primary and secondary fixation features. Using the extrusion process, the thickness t 1  can be tightly controlled as can the flexibility of the arms  1060 A- 1060 D. 
     As shown in  FIG.  13 C , the arms  1060 A- 1060 D of the fixation feature  1005  may conform to a predetermined shape or arrangement and have a predetermined geometry. With reference to  FIG.  13 C , various dimensional aspects of the arm  1060 B are illustrated that are representative of the other arms of the fixation feature  1005 . A first parameter of the arm  1060 B that may be controlled to vary performance of the fixation feature  1005  is the pitch angle θ of the arm  1060 B. The pitch angle θ generally dictates the “aggressiveness” of the arm  1060 B and how readily the arm  1060 B engages the wall of the heart. In certain implementations, the θ of the arm  1060 B may be from and including 15 degrees to and including 60 degrees. The arm thickness t 2  may also be varied to change the rigidity and corresponding performance characteristics of the arms  1060 B. In certain implementations, the arm thickness t 2  may be from and including 0.005 inches to and including 0.030 inches. The arm  1068 B may be further defined by an arm length L 2 . In certain implementations, the arm length L 2  may be from and including 0.010 inches to and including 0.200 inches. 
     The barbs  1064 A- 1064 D may also conform to predetermined dimensions and geometries. For example, as illustrated in  FIG.  13 C , each barb may have a barb length L 3  that from and including 0.005 inches to and including 0.200 inches. The barbs  1064 A- 1064 D may also have a barb thickness t 3  from and including 0.004 inches to and including 0.030 inches. 
     As shown in  FIG.  13 C , the distal extent of the fixation feature  1005  is substantially flat. More specifically, each of the tips  1062 A- 1062 D and the barbs  1064 A- 1064 D extend in a substantially lateral direction. In other implementations, however, each of the tips  1062 A- 1062 D and the barbs  1064 A- 1064 D may instead extend at an angle relative to a lateral plane of the leadless biostimulator. For example, in certain implementations, each of the tips  1062 A- 1062 D may extend in a partially distal direction and each of the barbs  1064 A- 1064 D may extend in a partially proximal direction. 
     As illustrated in the preceding figures, the fixation feature  1005  may include four arms  1060 A- 1060 D. In alternative implementations, however, the fixation feature  1005  may include more or fewer than four arms. For example, and without limitation, implementations of fixation features according to the present disclosure may include from one to four or more arms. Also, while each of the arms  1060 A- 1060 D is illustrated in  FIGS.  10 - 13 C  as being substantially the same, each arm may vary in its length, shape, or other characteristics. For example and without limitation, some or all of the arms of planar fixation features according to this disclosure may have different lengths, different barb shapes and/or barb lengths, or have different pitch angles. 
     D. Leadless Biostimulator with Anti-Rotation Shim 
     In another implementation of the present disclosure, a leadless biostimulator is provided that includes a conventional primary fixation feature (e.g., a helical wire) but further includes an anti-rotational shim disposed proximal to the tip of the primary fixation feature. The shim provides secondary fixation by resisting counter-rotation of the leadless biostimulator following implantation. 
     The anti-rotational shim may be formed from any of the biocompatible materials described above, including bioabsorbable polymers or metals. In certain implementations, the shim may be formed from converted plastic thin film sheets or other thin film material. The sheet is then cut or otherwise shaped to form flexible barbs that extend laterally from the biostimulator in a direction opposite that of the screwing direction of the primary fixation helix. In certain implementations, for example, the barb features are cut out of the sheet in a circular disc pattern to form a shim. The cut shim is then placed over a helix mount and held in place by a helix mount cap. The cap may be held in place by an adhesive, ultrasonic welding, ultrasonic staking, or other bonding method. In such an implementation, the shim may be disposed between windings of the primary fixation helix. By doing so, the barb features are optimally placed to prevent counter-rotation of the leadless pacemaker (i.e., rotation in an unscrewing direction) after implantation and to also be displaced relative to a stimulation electrode of the leadless biostimulator. 
     The geometric profile of the shim, including the thickness of the sheet from which the shim is formed, is chosen such that the barb can pierce the endocardium when the leadless biostimulator is subjected to a relatively small counter-torque. However, by applying reasonable overload torque the barbs give way and bend back against themselves, allowing the leadless biostimulator to be unscrewed and repositioned without severely damaging tissue in the implantation area. 
       FIGS.  14  and  15    are isometric views of a biostimulator  1400  in accordance with the present disclosure. The biostimulator  1400  includes a housing  1402  and a header assembly  1404  coupled thereto. Coupling of the housing  1402  to the header assembly  1404  may be accomplished in various ways including, without limitation, one or more of a biocompatible adhesive, a threaded connection, and ultrasonic welding. 
     The header assembly  1404  generally includes a primary fixation feature  1405 , and a secondary fixation feature  1406  laterally extending from the leadless biostimulator  1400 . In general, the secondary fixation feature  1406  functions as an anti-unscrewing feature that resists unscrewing of the biostimulator  1400  after implantation. In the specific example of  FIG.  14   , the primary fixation device  1405  is a primary helix  1405  pointing in a first direction. For clarity and to illustrate other components of the biostimulator  1400 , the primary fixation device  1405  is removed in  FIG.  15   . The secondary fixation feature  1406  is a shim  1406  that includes barbs or arms  1460 A- 1460 B that laterally extend from the biostimulator  1400 . As illustrated, the shim  1406  is disposed relative to the primary helix  1405  such that the shim  1406  extends between adjacent windings of the primary helix  1405 . In certain implementations, for example, the shim  1406  is positioned relative to the primary helix  1405  such that the barbs  1460 A- 1460 B protrude between the first half most distal turn and the second most distal turn of the primary helix  1405 . Characteristics of the primary helix  1405  may be substantially similar to the primary helix  305  discussed in the context of the biostimulator  300  of  FIG.  3   . 
     In an embodiment, an outer dimension of the shim  1406  may be larger than an outer dimension of the primary helix  1405 . For example, an outer tip of the barbs  1460 A- 1460 B may be radially separated from a central axis of the leadless biostimulator  1400  by a radial distance that is greater than a radial distance separating the primary helix  1405  from the central axis. Accordingly, the shim  1406  can contact tissue radially outward from the helix  1405  when the leadless biostimulator  1400  is engaged with the heart tissue. Such contact allows the shim  1406  to move in one direction and resist movement in another direction, as described below. 
     As described below in the context of  FIGS.  18 A- 18 B , implantation of the leadless biostimulator  1400  includes positioning the leadless biostimulator  1400  at an implantation location adjacent a wall of a chamber of the heart and rotating the leadless biostimulator  1400  in a screwing direction to cause the primary helix  1405  to engage the wall of the heart. After initial insertion of the primary helix  1405 , the leadless biostimulator  1400  may be further rotated, such that an electrode  1411  is brought into contact with the wall of the heart. During this process, the barbs of the shim  1406  are brought into proximity with the wall of the heart as well. As a result, when a counter-torque is applied to the leadless biostimulator (such as may occur during normal cardiac activity), the barbs  1460 A- 1460 B may engage the wall of the heart and prevent the leadless biostimulator from becoming dislodged. 
     The barbs  1460 A- 1460 B may be further adapted to disengage the wall of the heart when a predetermined counter-torque is exceeded. For example, the barbs  1460 A- 1460 B may be formed of a flexible material capable of resisting a first counter torque but if a second counter torque is applied that is greater than the first counter torque, the barbs  1460 A-B may deflect, bend, compress, buckle, or otherwise deform such that the barbs  1460 A-B are no longer pointed in a direction substantially opposite that of the primary helix  1405 . When so deformed, the biostimulator  1400  may be counter rotated to disengage the primary helix  1405 , enabling removal and/or repositioning of the biostimulator  300 . In certain implementations, the first counter torque may generally correspond to the anticipated counter torque that may be experienced during regular patient activity plus a predetermined safety factor. For example, the first counter torque may be from and including 0 oz-in to and including 0.5 oz-in. In certain implementations, the second counter torque may generally correspond to a predetermined force required to be applied by a retrieval catheter or similar retrieval system that may be used to retrieve/remove the biostimulator  1400  following implantation. In such implementations, the second counter torque may be from and including 0.5 oz-in to and including 2.0 oz-in, for example. 
       FIG.  16    is an exploded view of the header assembly  1404  of  FIGS.  14  and  15   . As illustrated, the header assembly  1404  may include a header body  1470  and a header cap  1472  between which the shim  1406  is disposed. The header assembly  1404  further includes the electrode  1411  used to delivering pacing or other impulses to the heart tissue. The shim  1406  may be retained between the header cap  1472  and the header body  1470  and may be coupled to one or both of the header body  1470  and the header cap  1472  using ultrasonic welding, an adhesive, or other coupling method. The header body  1470  may also include a key or similar alignment feature  1476  that may facilitate alignment of one or both of the header cap  1472  and the shim  1406 . For example, one or both of the header cap  1472  and the shim  1406  may include a notch  1474  or similar indentation corresponding to the key  1476  such that, when assembled, one or both of the header cap  1472  and the shim  1406  are in a predetermined orientation. 
       FIGS.  17 A- 17 C  illustrate the example shim  1406  in further detail. More specifically,  FIG.  17 A  is an isometric view of the shim  1406 ,  FIG.  17 B  is a distal view of the shim  1406 , and  FIG.  17 C  is a side elevation view of the shim  1406 . 
     As illustrated in  FIGS.  17 A- 17 C , the shim  1406  includes a substantially circular body  1480  (although other shapes of shims are possible and contemplated) from which barbs  1460 A- 1460 B extend. Each of the barbs  1460 A- 1460 B extends in a counter-rotational direction opposite the primary helix  1405  of  FIG.  14   . The circular body  1480  further defines a through hole  1482  for coupling the shim  1406  to other components of a header assembly of a leadless biostimulator, such as the header assembly  1404  illustrated in  FIG.  16   . For example, the through hole  1482  may be shaped to receive a protrusion or extension of the header body  1470 . As previously discussed, the edge of the through hole  1482  may further define a notch  1474  or a protrusion shaped to mate with a corresponding protrusion or notch, respectively, of the header body  1470 . By doing so, the shim  1406  may be placed in a predetermined orientation relative to the header body  1470  during assembly. 
     The barbs  1460 A- 1460 B of the shim  1406  may conform to a predetermined shape or arrangement and have a predetermined geometry. For example, as illustrated in  FIG.  17 B , each of the barbs  1460 A- 1460 B extend outwardly from the circular body  1480  along a substantially straight path and terminate in a sharpened point. Referring to the barb  1460 A, for example, the barb  1460 A is offset from a center C 2  of the shim  1406  by a radius r 3  and extends perpendicular to the radius r 3  until the barb  1460 A extends from the outer edge of the circular body  1480  by a barb length L 4 . Phrased differently, the barb  1460 A extends a distance of the barb length L 4  from a point P on the outer edge of the circular body  1480  and the point P corresponds to an intersection of the outer edge of the circular body  1480  and a line that is tangential to a circle  1490  defined by the radius r 3 . As shown in  FIG.  17 B , the barb  1460 A may be further defined by a barb width W 2 . In certain example implementations, the radius r 3  may be from and including 0.030 inches to and including 0.090 inches, the barb length L 4  may be from and including 0.002 inches to and including 0.01 inches, and the barb width W 2  may be from and including 0.001 inches to and including 0.005 inches. 
     Although illustrated in  FIGS.  17 A- 17 C  as being substantially straight, barbs of shims according to the present disclosure may have alternative shapes provided they extend from the circular body  1480  to resist counter rotation of a leadless pacemaker in which they are incorporated. For example and without limitation, such barbs may follow a circular curvate path or a spiraling curvate path. Also, while barbs  1460 A- 1460 B are shown as extending substantially perpendicular to the radius r 3  from the outer edge of the circular body  1480 , in other implementations, the barbs of the shim may instead extend at an angle from the outer edge of the circular body  1480  (e.g. from the point P from which the barb  1460 A extends from the outer edge of the circular body  1480 ). 
     As illustrated in the preceding figures, the shim  1406  includes two barbs  1460 A- 1460 B that are disposed on one side of the circular body  1480  and offset approximately 90 degrees from each other. In alternative implementations, however, the shim  1406  may include fewer or more than two barbs. For example and without limitation, implementations of shims according to the present disclosure may include from one to six or more arms. In implementations in which a shim includes multiple barbs, the barbs may be evenly or unevenly distributed about the circular body  1480 . For example, in certain implementations, a second barb may be disposed at an offset from and including 90 degrees to and including 270 degrees relative to a first barb. Also, each barb may vary in its length and shape. For example and without limitation, some or all of the barbs of shims according to this disclosure may have different lengths, different widths, follow different curvatures, or have no curvature at all in the case of straight barbs. 
     As illustrated in  FIG.  17 C , the shim  1406  may be substantially flat and may be formed from a thin sheet of material. In certain implementations, the shim  1406  may be formed from a sheet or film having a thickness  1484  from and including 0.001 inches to and including 0.010 inches. For example, in one method of manufacturing the shim  1406 , a sheet or film may be formed using a film converting process. The sheet/film may then be punched, cut, trimmed, or otherwise processed to produce the shim  1406 . 
     To achieve the required characteristics of the barbs  1460 A- 1460 B, the shim  1406  may be formed from a flexible plastic material, such as polyimide. In other implementations, the shim  1406  may instead be formed of other flexible biocompatible materials including, without limitation, one or more of one or more of polyester, polyethylene, polypropylene, polyurethane, polyether ether ketone (PEEK), or polyvinylidene fluoride. Material selection for the shim  1406  may alternatively be based on particular properties or characteristics of the material. In certain implementations, the shim may be formed from one or more bioabsorbable materials, as described above. For example, the bioabsorbable material(s) may include a magnesium alloy. In certain implementations, the shim  1406  may be formed from a material having a Young&#39;s modulus from and including 0.5 GPa to and including 10 GPa, Material selection for the shim may alternatively be based on material toughness which is often associated with specific tensile and compression strengths of the material. 
     E. Implantation of Biostimulators 
       FIGS.  18 A- 18 B  illustrate endocardial implantation of biostimulators  1802 A,  1802 B in accordance within chambers of a patient heart  1800 . As shown in  FIG.  18 A , a first biostimulator  1802 A is implanted within an atrium  1804  of the heart  1800  while a second biostimulator  1802 B is implanted within a ventricle  1806  of the heart  1800 . Implantation of each of the first and second biostimulators  1802 A,  1802 B may be achieved, in part, by insertion of the biostimulators  1802 A,  1802 B endocardially through a guiding catheter. A torqueable catheter can be used to rotate the respective housings of the biostimulators  1802 A,  1802 B and force the respective primary fixation features  1805 A,  1805 B of the biostimulators  1802 A,  1802 B into corresponding heart tissue, affixing the primary fixation features  1805 A,  1805 B and corresponding electrodes into contact with stimulable tissue. 
     Similarly, and as illustrated in  FIG.  18 B , removal and retrieval of the biostimulators  1802 A,  1802 B may also be accomplished endocardially through a guiding catheter  1808 . In the example of  FIG.  18 B , the second biostimulator  1802 B is in the process of being removed from the heart  1800 . To remove the second biostimulator  1802 B, a torqueable catheter  1810  may be inserted into the heart  1800  through the guiding catheter  1808  and coupled to the biostimulator  1802 B. The torqueable catheter  1810  may then be counter rotated to disengage the biostimulator  1802 B. A similar process of inserting guide and torque catheters may also be used for epicardial fixation and removal of biostimulators in accordance with this disclosure. 
     For illustration purposes, the primary fixation features  1805 A,  1805 B are illustrated in  FIGS.  18 A and  18 B  as being helices extending from the distal ends of the biostimulators  1802 A,  1802 B. However, any primary fixation feature disclosed herein may be similarly implemented. For example, the biostimulators  1802 A,  1802 B may instead include any of the primary fixation features discussed in  FIGS.  1 - 17 C  of the present disclosure. 
     As for additional details pertinent to the present invention, materials and manufacturing techniques may be employed as within the level of those with skill in the relevant art. The same may hold true with respect to method-based aspects of the invention in terms of additional acts commonly or logically employed. Also, it is contemplated that any optional feature of the inventive variations described may be set forth and claimed independently, or in combination with any one or more of the features described herein. Likewise, reference to a singular item, includes the possibility that there are plural of the same items present. More specifically, as used herein and in the appended claims, the singular forms “a,” “and,” “said,” and “the” include plural referents unless the context clearly dictates otherwise. It is further noted that the claims may be drafted to exclude any optional element. As such, this statement is intended to serve as antecedent basis for use of such exclusive terminology as “solely,” “only” and the like in connection with the recitation of claim elements, or use of a “negative” limitation. Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The breadth of the present invention is not to be limited by the subject specification, but rather only by the plain meaning of the claim terms employed.