Contact for an implantable medical device

Abstract

An apparatus, comprising: a housing including a hole and a housing groove, the housing groove being within the hole and located on an inner surface of the housing; and a coil spring located within the housing and mounted within the housing groove, wherein the housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force.

Description

[0001] Priority Claim

[0002] This application claims the benefit of priority under 35 U.S.C. § 119(e) to U.S. Provisional Patent Application Serial No. 62 / 796,956, filed on Jan. 25, 2019, the entire content of which is incorporated herein by reference. Field of the Invention

[0003] This disclosure relates to implantable medical devices and, more particularly, to electrical contacts for implantable medical devices. Background of the Invention

[0004] Leads implanted in or around the heart are used to reverse certain life-threatening arrhythmias or to stimulate the contraction of the heart. Electrical energy is applied to the heart via electrodes on the lead to restore normal heart rhythm.

[0005] The header on the implantable device is used to couple the conductors of the lead to the circuitry within the implantable device. For example, electrical contacts (such as coil springs) in the header are used to electrically couple a cardiac stimulator system to the lead and the electrodes to contact a portion of the heart.

[0006] Desirably, the connection between the lead and the header is mechanically and electrically reliable. Summary of the Invention

[0007] Example 1 may include a subject matter that may include an apparatus including: a housing including a hole and a housing groove, the housing groove being within the hole and on an inner surface of the housing; and a coil spring located within the housing and mounted within the housing groove, wherein the housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force.

[0008] In Example 2, the subject matter of Example 1 may optionally include a housing groove defining an outer peripheral edge having a non-circular shape.

[0009] In Example 3, the subject matter of Example 2 may optionally include a housing groove defining an outer peripheral edge having a hexagonal shape.

[0010] In Example 4, the subject matter of Example 2 may optionally include a housing groove defining an outer peripheral edge having a rectangular shape with rounded corners.

[0011] In Example 5, the subject matter of any one of Examples 1-4 may optionally include a housing groove with such non-uniform radius that physically constrains the relatively high contact force region of the coil spring to be positioned closer to the center of the hole relative to the relatively low contact force region of the coil spring.

[0012] In Example 6, the subject matter of any of Examples 1-5 may optionally include a coil spring having a non-uniform winding.

[0013] In Example 7, the subject matter of Example 6 may optionally include a coil spring having coil turns of a periodic length, the coil turns of the periodic length having a greater diameter than adjacent coil turns.

[0014] In Example 8, the subject matter of any of Examples 1-7 may optionally include a coil spring that includes a plurality of coil turns having an axially decreasing radius.

[0015] In Example 9, the subject matter of any of Examples 1-8 may optionally include a housing groove that defines a channel that is larger than the cross-section of the coil spring such that the coil spring can roll back and forth within the groove.

[0016] In Example 10, the subject matter of any of Examples 1-9 may optionally include a lubricant located on the coil spring.

[0017] Example 11 may include a subject matter that may include a head for an implantable medical device, the head including: a head body that includes a passage for receiving a terminal of an implantable lead; and a housing within the passage, the housing including a housing groove and a hole located on an inner surface of the housing; and a coil spring that is located within the housing and mounted within the housing groove and is exposed to the interior of the housing to contact a terminal of an implantable lead mounted within the hole, wherein the housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force, wherein the non-uniform radius of the housing groove defines an outer housing groove surface that physically constrains the region of relatively high contact force of the coil spring to be positioned closer to the center of the hole relative to the region of relatively low contact force of the coil spring.

[0018] In Example 12, the subject matter of Example 11 may optionally include a housing groove that defines an outer peripheral edge having a non-circular shape.

[0019] In Example 13, the subject matter of Example 12 may optionally include a housing groove that defines an outer peripheral edge having a hexagonal shape.

[0020] In Example 14, the subject matter of Example 12 may optionally include a housing groove that defines an outer peripheral edge having a rectangular shape with rounded corners.

[0021] In Example 15, the subject matter of any of Examples 11-14 may optionally include a housing groove that defines a channel that is larger than the cross-section of the coil spring such that the coil spring can roll back and forth within the groove.

[0022] Example 16 may include a subject matter that may include a device including: a housing including a hole and a housing groove, the housing groove being within the hole and on an inner surface of the housing; and a coil spring located within the housing and mounted within the housing groove, wherein the housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force.

[0023] In Example 17, the subject matter of Example 16 may optionally include a housing groove defining an outer peripheral edge having a non-circular shape.

[0024] In Example 18, the subject matter of Example 17 may optionally include a housing groove defining an outer peripheral edge having a hexagonal shape.

[0025] In Example 19, the subject matter of Example 17 may optionally include a housing groove defining an outer peripheral edge having a rectangular shape with rounded corners.

[0026] In Example 20, the subject matter of any one of Examples 16-19 may optionally include such a housing groove with a non-uniform radius that physically constrains the relatively high contact force region of the coil spring to be positioned closer to the center of the hole relative to the relatively low contact force region of the coil spring.

[0027] In Example 21, the subject matter of any one of Examples 16-20 may optionally include a coil spring having a non-uniform winding.

[0028] In Example 22, the subject matter of Example 21 may optionally include a coil spring having coil turns with a periodic length that has a larger diameter than adjacent coil turns.

[0029] In Example 23, the subject matter of any one of Examples 16-22 may optionally include such a coil spring that includes a plurality of coil turns having an axially decreasing radius.

[0030] In Example 24, the subject matter of any one of Examples 16-23 may optionally include a housing groove defining a channel that is larger than a cross-section of the coil spring such that the coil spring can roll back and forth within the groove.

[0031] In Example 25, the subject matter of any one of Examples 16-24 may optionally include a lubricant located on the coil spring.

[0032] Example 26 may include a subject matter that may include a head for an implantable medical device, the head including: a head body including a passage for receiving terminals of an implantable lead; a housing within the passage, the housing including a housing groove and a hole on an inner surface of the housing; and a coil spring located within the housing and mounted within the housing groove and exposed to the interior of the housing to contact the terminals of the implantable lead mounted within the hole, wherein the housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force, wherein the housing groove with the non-uniform radius defines an outer housing groove surface that physically constrains the relatively high contact force region of the coil spring to be positioned closer to the center of the hole relative to the relatively low contact force region of the coil spring.

[0033] In Example 27, the subject matter of Example 26 may optionally include a housing groove defining an outer peripheral edge having a non-circular shape.

[0034] In Example 28, the subject matter of Example 27 may optionally include a housing groove defining an outer peripheral edge having a hexagonal shape.

[0035] In Example 29, the subject matter of Example 27 may optionally include a housing groove defining an outer peripheral edge having a rectangular shape with rounded corners.

[0036] In Example 30, the subject matter of any one of Examples 26-29 may optionally include a coil spring having non-uniform windings with coil turns of a periodic length, the coil turns of the periodic length having a greater diameter than adjacent coil turns.

[0037] In Example 31, the subject matter of any one of Examples 26-30 may optionally include a coil spring including a plurality of coil turns having an axially decreasing radius.

[0038] In Example 32, the subject matter of any one of Examples 26-31 may optionally include a housing groove defining a channel larger than a cross-section of the coil spring such that the coil spring can roll back and forth within the groove.

[0039] Example 33 may include a subject matter that may include a method including: inserting a lead terminal into a passage of a head of an implantable device; and contacting the lead terminal with a coil spring located within the head, wherein the coil spring defines a region of relatively low contact force and a region of relatively high contact force around a radius of the coil spring, wherein the coil spring is mounted within a housing groove on a housing within the head, and wherein the housing groove has a non-uniform radius.

[0040] In Example 34, the subject matter of Example 33 may optionally include where the housing groove defines an outer peripheral edge having a non-circular shape.

[0041] In Example 35, the subject matter of any one of Examples 33-34 may optionally include applying a lubricant to the coil spring.

[0042] These examples may be combined in any arrangement or combination. This overview is intended to provide an overview of the subject matter of this patent application. It is not intended to provide an exclusive or exhaustive interpretation of the invention. The detailed description is included to provide further information about this patent application. BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Figure 1 A view of an implantable system is shown in accordance with at least one example.

[0044] Figure 2 A cross-sectional side view of a housing including a coil spring is shown in accordance with one example.

[0045] Figure 3 A front schematic view of a coil spring housing is shown in accordance with one example.

[0046] Figure 4 A front schematic view of a coil spring housing is shown in accordance with one example.

[0047] Figure 5 A front schematic view of a coil spring housing is shown in accordance with one example.

[0048] Figure 6 A side view of an expanded coil spring is shown in accordance with one example.

[0049] Figure 7 A cross-sectional side view of a coil spring within a housing is shown in accordance with one example.

[0050] Figure 8 Shows Figure 7 a view of the coil turns of a coil spring.

[0051] Figure 9 A cross-sectional side view of a coil spring within a housing is shown in accordance with one example. DETAILED DESCRIPTION

[0052] Figure 1Illustrates an implantable system 100 according to one embodiment. The implantable system 100 includes a pulse generator 105 and at least one lead 150. The pulse generator 105 includes a housing 107 and a head 104 mounted to the housing 107. The pulse generator 105 can be implanted into a subcutaneous pocket fabricated in the patient's chest wall. Optionally, the pulse generator 105 can be placed in a subcutaneous pocket fabricated in the abdomen, or in other locations. The pulse generator 105 can include a power source, such as a battery, a capacitor, and other components housed in the housing 107. The pulse generator 105 can include a microprocessor to provide processing, evaluation, and deliver shocks and pulses of different energy levels and durations to the heart in response to cardiac arrhythmias, including fibrillation, tachycardia, heart failure, and bradycardia, for defibrillation, cardioversion, and pacing.

[0053] In other embodiments, the implantable system 100 can also be adapted to be used with implantable electrical stimulators, such as but not limited to nerve stimulators, bone stimulators, central nervous system stimulators, or stimulators for treating pain.

[0054] The lead 150 includes a lead body having a proximal end, wherein the terminal 152 of the lead 150 can be coupled to the head 104 of the pulse generator 105. The lead 150 extends to a distal end, which can be coupled to a part of the heart when implanted. The distal end of the lead 150 includes at least one electrode that electrically couples the lead 150 to the heart. At least one electrical conductor is disposed within the lead 150 and extends from the proximal end to the electrode. The electrical conductor transmits current and signals between the pulse generator 105 and the electrode.

[0055] The head 104 includes one or more passages 108, 110, 112 formed within the head core 106 and configured to receive the lead terminals 152 of the lead 150. In this example, the lead terminals 152 include a proximal end contact 154 and an annular contact 156. In other examples, the lead terminals 152 can include multiple annular contacts. The terminal contacts 154, 156 can be made of stainless steel, while the insulating portion of the terminal 152 can be formed of polyurethane.

[0056] Within the head core 106, each of the one or more passages 108, 110, 112 can include one or more electrical contacts, such as coil springs 114 (schematically shown) located within the passages 108, 110, 112. The coil springs 114 can be mounted within a housing located within the passages 108, 110, and 112. The coil springs 114 can be electrically connected to feedthroughs to provide electrical communication between the lead 150 and the electronics within the pulse generator housing 107.

[0057] Currently, in some cases, the reliability of electrical contacts can be poor. A fundamental trade-off in existing designs is to obtain sufficient contact force without compromising the wire insertion experience. For example, the contact force is characterized in terms of Hertz stress. The Hertz stress window takes into account the material and geometry of the actual contact area. When the contact force falls within the Hertz stress window, contact reliability is optimized.

[0058] Current coil springs are wound to provide a large number of similar contact points around the periphery of the wire terminal to the coil spring connection portion. While it may seem intuitive that the large number of contact points provided in current coil spring designs would improve reliability, this is not the case. Instead, a few good contact points can outperform a large number of marginal contact points. This is because the sufficient force for good contact involving many contact points makes the wire insertion force too large. In other words, for a given insertion force budget, only a small number of contact points can be allocated sufficient contact force to operate the contact points within the Hertz stress window.

[0059] Figures 2-3 An example of a coil spring design that allows for fewer contact points is shown. Figure 2 A cross-sectional side view of a coil spring housing 202 including a coil spring 114 according to an example is shown. Figure 3 A front view schematic of a coil spring housing 202 according to an example is shown.

[0060] The housing 202 includes a housing groove 206 and a hole 208 on the inner surface 210 of the housing 202. The coil spring 114 is located within the housing 202 and mounted within the housing groove 206 and is exposed to the interior of the housing 202 to contact the terminal of an implantable wire mounted within the hole 208.

[0061] In Figure 3 , the coil spring 114 is not shown, but the coil spring 114 is located between the housing groove 206 and the wire contact ring 360. The redistribution of the contact force can be achieved by modifying the shape of the housing groove that captures the contact coil spring.

[0062] Here, the housing groove 206 has a non-uniform radius such that the coil spring 114 defines a region 320 of relatively low contact force and a region 322 of relatively high contact force. The number and size of the various radii can vary as design parameters. The region of low groove radius forms the region 322 of high contact force, while the region of large groove radius forms the region 320 of low contact force.

[0063] The non-uniform radius of the housing groove 206 defines the outer housing groove surface 330, which physically constrains the relatively high contact force region 322 of the coil spring to be positioned closer to the center of the hole relative to the relatively low contact force region 320 of the coil spring. In this embodiment, the housing groove 206 defines an outer peripheral edge having a hexagonal shape. In other examples, the housing groove 206 defines an outer peripheral edge having various non-circular shapes.

[0064] In this example, by forming the housing groove 206 with a non-uniform radius, the design allows for sufficient contact force while not affecting the wire insertion experience. Additionally, the housing 202 and the coil spring 114 are assembled within the mechanical enclosure of the current head design, thus allowing for minimal changes to the existing assembly process.

[0065] In one example, the housing 202 can be formed by machining or 3-D printing.

[0066] Figure 4 A front view schematic of a coil spring housing 402 according to one example is shown. Again, the coil spring is not shown but would occupy the space between the housing groove 406 and the wire contact ring 360. In this example, the housing groove 406 has a non-uniform radius such that the coil spring defines a relatively low contact force region 420 and a relatively high contact force region 422. Here, the housing groove 406 defines an outer peripheral edge having a rectangular shape with rounded corners. As in the above example, the non-uniform radius of the housing groove 406 defines an outer housing groove surface 430 that physically constrains the relatively high contact force region 422 of the coil spring to be positioned closer to the center of the hole relative to the relatively low contact force region 420 of the coil spring.

[0067] In one example, the housing 402 can be formed by machining or 3-D printing a standard circular radius groove housing and then using a tool to squeeze the housing to change the shape of the housing groove that captures the contact spring.

[0068] Figure 5 A front view schematic of a coil spring housing 502 according to one example is shown. Here, the housing groove 506 has a non-uniform radius such that the coil spring defines a relatively low contact force region 520 and a relatively high contact force region 522. As in the above example, the housing groove 506 defines an outer peripheral edge having a rectangular shape with rounded corners.

[0069] Here, the housing 502 can be formed by machining or 3-D printing a standard circular radius groove housing and then using a tool to "imprint" the housing in the housing 502 on the groove 506 to form a pit 530, thereby creating a portion of the housing groove 506 with a reduced radius.

[0070] Figure 6 A side view of an expanded coil spring 602 according to an example is shown. Here, the coil spring 602 has non-uniform windings. Specifically, the coil spring 602 has coil turns 610 of a periodic length, and the coil turns of the periodic length have a larger diameter than adjacent coil turns 620. In this example, the center of the raised turn 610 is shown as off-center, but this is not necessary. Additionally, a single raised turn is shown, but any number of consecutive raised turns or normal turns may be used. When the coil spring 602 is wound into a coil (such as Figure 2 coil spring 114), the coil turns 610 of larger diameter provide regions of relatively high contact force, while the coil turns 620 of smaller diameter provide regions of relatively low contact force. Again, this provides a balance between the total contact force between the wire and the contact spring and the required insertion force.

[0071] In various examples, the coil spring 602 may be used in a standard circular radius housing groove, or may be used in combination with a non-uniform radius housing groove, such as in the above example.

[0072] In one example, to form the coil spring 602, a standard coil spring may be formed and then mechanically deformed such that the coil turns 620 are clamped and the coil turns 610 may be raised to form regions of higher contact force. The clamped regions defined by the coil turns 620 form regions of reduced contact force depending on their orientation in the final assembly.

[0073] In another example, the deformation by clamping may also be done in the final assembly, where a grooved tool is inserted into the housing hole and the coil spring is clamped at the assembled radial dimension to form regions of reduced contact force. In another example, the coil spring 602 may be wound at a slightly larger diameter after every few turns of normal diameter.

[0074] Figure 7 A cross-sectional side view of a coil spring 714 within a housing 702 according to an example is shown. Figure 8A view of the coil turns 802 of the coil spring 714 is shown. Here, the coil spring 714 is shaped to define a concentrated contact area. In this example, the redistribution of the contact force can be achieved by inserting a tool into the housing hole 708, which forms an axial reduction in the coil turn radius (e.g., using a tapered tool), where an axial clamping point 810 is obtained. Thus, the wire cross-section of the coil spring 714 can concentrate the contact force into a smaller surface area at the axial clamping point 810. In this example, the coil spring 714 includes a plurality of coil turns 802 having coil turns with an axially reduced radius. The coil spring 714 can be used alone or in combination with any of the other examples herein. For example, the coil spring 714 can be used in a housing groove of uniform radius or a housing groove of non-uniform radius, and can be combined with the non-uniform winding of the coil spring 602( Figure 6 ).

[0075] Figure 9 A cross-sectional side view of the coil spring 114 within the housing 902 according to one example is shown. The housing groove 906 defines a channel larger than the cross-section of the coil spring 114 such that the coil spring can roll axially back and forth within the groove 906, thereby allowing the connection to "float". This embodiment takes advantage of the ability of the coil spring to roll (rather than slide with axial movement) and thus reduces wear because rolling reduces the ability of the wire to rub against the surface of the spring and break down oxides (resulting in wear). Additionally, allowing the coil spring 114 to roll within the housing can assist with insertion.

[0076] In some embodiments, this example can be combined with other previously proposed methods.

[0077] Referring back to any of the above embodiments, a lubricant can be added to the coil spring. For example, Figure 2 the coil spring 114 in [reference] can further include a lubricant 930 located on the coil spring 114. The lubricant can improve wear and insertion force. The lubricant can be applied to enhance any of the contact ideas presented herein. The lubricant works by excluding oxygen from the contact area (thereby reducing oxidation and thus wear) and reducing the sliding force. The lubricant selected can be biocompatible, such as mineral oil or silicone oil. The lubricant can be applied to the level of the contact components of the assembly, or the lubricant can be applied to the final assembly. Additionally, due to other sliding surfaces not directly related to the electrical contact (such as the seal ring in the wire hole), the lubricant reduces the friction when the wire is inserted. These other terms typically represent a major portion of the wire insertion force budget.

[0078] In any example herein, the shape of the coil spring wire can be symmetric or antisymmetric to balance the relationship between contact force and surface area. For example, the wire shape can be circular, triangular, square, elliptical, rectangular, or other shapes.

[0079] In one example usage of the coil spring discussed herein, reference will be made to Figure 1 , 2 and examples of 3. The method of inserting the wire terminal into the head can include inserting the wire terminal 152 into the passage 108 of the head 104 of the implantable device 100, and the wire terminal 152 contacts the coil spring 114 located within the head 104. The coil spring 114 defines a region 320 of relatively low contact force and a region 322 of relatively high contact force around the radius of the coil spring 114. The coil spring 114 can be mounted within the non-uniform radius housing groove 206 on the housing 202 within the head 104.

[0080] Among the advantages of the above designs, these designs help balance the trade-off between reliable contact and trouble-free insertion of the wire. Additionally, one advantage of the present design is that the designs herein are assembled within the mechanical housing of the current design and allow for minimal changes to the existing assembly process.

[0081] Additional Notes

[0082] The detailed description above includes reference to the drawings, which form a part of the detailed description. The drawings illustrate, by way of example, specific embodiments in which the invention may be practiced. These embodiments are also referred to herein as "examples". Such examples can include elements in addition to those shown or described. However, the inventors also contemplate examples in which only those elements shown or described are provided. Additionally, the inventors also contemplate examples using any combination or arrangement of those elements (or one or more aspects thereof) shown or described, or with respect to a particular example (or one or more aspects thereof), or with respect to other examples shown or described herein (or one or more aspects thereof).

[0083] All publications, patents, and patent documents mentioned in this document are incorporated herein by reference in their entirety as if individually incorporated by reference. If there is an inconsistency in the usage between this document and the documents incorporated by reference, the usage in the incorporated reference documents shall be regarded as a supplement to this document; for irreconcilable inconsistencies, the usage in this document shall prevail.

[0084] In this document, as is common in patent documents, the term "a" or "an" is used to include one or more, independent of any other instance or usage of "at least one" or "one or more". In this document, the term "or" is used to refer to a non-exclusive or, so that "A or B" includes "A but not B", "B but not A", and "A and B", unless otherwise stated. In the appended claims, the terms "comprising" and "wherein" are used as the plain English equivalents of the respective terms "including" and "wherein". Also, in the following claims, the terms "comprising" and "including" are open-ended, that is, a system, apparatus, article, or process that includes elements in addition to those listed after such terms in the claims is still considered to fall within the scope of that claim. Further, in the following claims, the terms "first", "second", "third", etc. are used only as labels and are not intended to impose numerical requirements on their objects.

[0085] The above description is intended to be illustrative, not restrictive. For example, the above examples (or one or more aspects thereof) may be used in combination with each other. Other embodiments may be used, such as by those of ordinary skill in the art upon review of the above description. The Abstract is provided to comply with 37 C.F.R. § 1.72(b) to allow the reader to quickly ascertain the nature of the technical disclosure. The submission of this application is based on the understanding that they will not be used to interpret or limit the scope or meaning of the claims. Also, in the above detailed description, various features may be grouped together to simplify the disclosure. This should not be construed to mean that an unclaimed disclosed feature is essential to any claim. Rather, the inventive subject matter may not lie in all features of a particular disclosed embodiment. Thus, the following claims are hereby incorporated into the detailed description, each claim standing on its own as a separate embodiment, and it is contemplated that these embodiments may be combined with each other in various combinations or permutations. The scope of the present invention should be determined with reference to the appended claims and the full scope of equivalents to which such claims are entitled.

Claims

1. An implantable medical device, comprising: A housing, the housing including a hole and a housing groove, the housing groove being within the hole and located on the inner surface of the housing; and A coil spring, the coil spring being located within the housing and mounted within the housing groove, the coil spring being located between the housing groove and a cylindrical wire contact ring, wherein, The housing groove has a non-uniform radius such that the coil spring defines a region of relatively low contact force and a region of relatively high contact force on the cylindrical wire contact ring.

2. The implantable medical device according to claim 1, wherein, The housing groove defines an outer peripheral edge having a non-circular shape.

3. The implantable medical device according to claim 2, wherein, The housing groove defines an outer peripheral edge having a hexagonal shape.

4. The implantable medical device according to claim 2, wherein, The housing groove defines an outer peripheral edge having a rectangular shape with rounded corners.

5. The implantable medical device according to any one of claims 1-4, wherein, The non-uniform radius of the housing groove physically constrains the region of relatively high contact force of the coil spring such that it is positioned closer to the center of the hole relative to the region of relatively low contact force of the coil spring.

6. The implantable medical device according to any one of claims 1-4, wherein, The coil spring has non-uniform windings.

7. The implantable medical device according to claim 6, wherein, The coil spring includes coil turns of a periodic length, the coil turns of the periodic length having a larger diameter than adjacent coil turns.

8. The implantable medical device according to any one of claims 1-4, wherein, The coil spring includes a plurality of coil turns having an axially decreasing radius.

9. The implantable medical device according to any one of claims 1-4, wherein, The housing groove defines a channel that is larger than the cross-section of the coil spring such that the coil spring can roll back and forth within the housing groove.

10. The implantable medical device according to any one of claims 1-4, further comprising a lubricant located on the coil spring.

11. A head for an implantable medical device, comprising: A head body, the head body including a passage for receiving a terminal of an implantable wire; and A housing within the passage, the housing including a housing groove and a hole located on the inner surface of the housing; and A coil spring, the coil spring being located within the housing and mounted within a housing groove, and being exposed to the interior of the housing to contact a terminal of an implantable lead mounted within the bore, the terminal of the implantable lead including a plurality of cylindrical annular contacts, wherein the housing groove has a non-uniform radius such that the coil spring defines regions of relatively low contact force and regions of relatively high contact force on the plurality of cylindrical annular contacts, and wherein the housing groove with the non-uniform radius defines an outer housing groove surface that physically constrains the region of relatively high contact force of the coil spring to be positioned closer to the center of the bore relative to the region of relatively low contact force of the coil spring.

12. The head according to claim 11, wherein, the housing groove defines an outer peripheral edge having a non-circular shape.

13. The head according to claim 12, wherein, the housing groove defines an outer peripheral edge having a hexagonal shape.

14. The head according to claim 12, wherein, the housing groove defines an outer peripheral edge having a rectangular shape with rounded corners.

15. The head according to any one of claims 11-14, wherein, the housing groove defines a channel that is larger than the cross-section of the coil spring such that the coil spring can roll back and forth within the housing groove.

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