NANOSCALE SURFACE ACTIVATION OF SILICONE VIA LASER PROCESSING

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0001] This application was made with government support under Grant Nos. DOE- (Artificial Retina program) and NSF - EEC-0310723(BMES ERC), awarded by The United States Department Of Energy and the National Science Foundation, respectively. The government has certain rights in the invention.

BACKGROUND OF THE INVENTION

[0002] Silicones are polymeric materials that have one characteristic in common: the polymer backbone is made of an alternate succession of Si and O atoms, joined together via strong, covalent inter-atomic bonds. The Si atoms are coupled to two adjacent O atoms and two organic radicals, i.e., C-H or C-R, where R is an organic group or moiety. The various silicones only differ from each other via these organic radicals, e.g. methyl (-CH ₃ ), vinyl (- HC=CH ₂ ), or other organic functional group. Silicones are variously referred to as "polymerized siloxanes," "polysiloxanes," and "silicone polymers." "Silicone rubbers" are included in this definition but typically include one or more additives, such as fillers, plasticizers, and crosslinkers. We use the term "silicone" in its broader sense to refer to silicone polymers, whether or not modified with one or more additional components. [0003] Silicones are notably neutral to the environment, asserting in particular no chemical interaction with foreign molecules. They also exhibit very low electrical conductivity and are fully transparent to visible or infra-red light. They absorb light photons in the UV range, typically at and below 280nm wavelength (i.e. at and above 4.4eV photon energy).

[0004] In order to allow chemical coupling of silicone materials to foreign species, it is necessary to "open" their structure, i.e. to modify irreversibly their atom assembly via breaking irreversibly some of the inter-atomic bonds. Unfortunately, this may not be accessible to mechanical action. In effect, given that these materials are elastic, they may change considerably their configuration through pulling without breaking bonds, this being in particular a consequence of the rotational symmetry of the Si-O bonds. Similarly, opening the silicone structure may not be feasible via thermal means. Silicone does not melt, sublimate or evaporate but rather condenses and transforms in a glassy and extremely fragile network at temperatures exceeding 230°C. ■ '

[0005] Due to their chemical inertness, silicones are recognized as biocompatible materials and are widely used in practical medical implants. For example, an epiretinal visual prosthesis (a microelectrode array (MEA) imbedded into or onto a silicone substrate, or

applied using photolithography) is a device that can be implanted on the retina and converts images into electrical signals that stimulate the retina. The images are received from an external camera and transfer the visual information to the MEA. [0006] Unlike cells, which attach to their extracellular environment via integral membrane proteins called integrins, MEAs and other medical implants are generally affixed to adjacent tissue using surgical tacks, or adhesives, which may be actually or potentially harmful to the tissue and, therefore, may limit the actual lifetime of the implant function. For example, a method currently used to fix an epiretinal visual prosthesis in place utilizes surgical tacks secured to the retina, which cause local pressure effects, local tissue destruction, and vascular leakage. Pressure is a crucial component of the cellular environment and can lead to pathology if it varies beyond the normal range. Disorders of this relationship can lead to disease states, such as glaucoma, in which retinal ganglion cells undergo apoptosis and necrosis. [0007] A major obstacle faced by bioengineers has been the ability to attach proteins to biocompatible substrates, and there is a continuing need for biocompatible materials and less destructive methods of attaching them to tissues. If silicone implants are to be fixed in place in the body, a way must be found to "activate" the silicone polymers to permit them to bond more readily to one or more compounds, such as cellular or extracellular proteins.

SUMMARY OF THE INVENTION

[0008] The present invention allows silicone to be activated and coupled to other compounds, and allows a new generation of biomaterials to interface with human tissue. In one aspect, the present invention provides a method of activating a silicone surface to facilitate its bonding to another compound or compounds, and an activated silicone article. According to one embodiment of the invention, the method comprises the steps of providing a silicone article having at least one surface; and irradiating at least a portion of one of the surfaces with laser light at a wavelength and power sufficient to eject organic species from the silicone article, thereby forming an activated silicone surface. In a second aspect, the invention provides a silicone article having at least one activated surface formed by irradiation with laser light at a wavelength and power sufficient to eject organic species from the silicone article.

[0009] As used herein, the term "silicone article" means a physical item that contains, as a major component, one or more silicone polymers. Other components, such as fillers, crosslinkers, plasticizers, other polymers, etc., may also be present The article can be a self- supporting object, such as a film, protective sleeve or jacket, component of a larger assembly, etc., or a coating on another object. The term "activated surface" is described below in detail. [0010] In one embodiment, the invention is used in the manufacture of a biocompatible, implantable prosthesis that can be affixed to living tissue through one or more functional

compounds (e.g., RGD peptides, discussed below) coupled to the activated surface of the silicone article. In other embodiments, the invention is used to facilitate the bonding of silicone substrates to other compounds, not necessarily biological in nature. The invention offers essential advantages over current practice and may be applied to any type of silicone- containing implant. In particular, it shows how a specific laser-activated silicone surface may be utilized in strongly fixing an implant on a living tissue without interfering with (i) that tissue, and (ii) the function of the implant.

BRIEF DESCRIPTION OF THE DRAWINGS [0011] Various features and embodiments of the invention will become more clear when reference is made to the appended drawings, wherein:

[0012] Figure 1 is a schematic drawing of polymeric SiO, created by laser activation according to one embodiment of the invention;

[0013] Figure 2 is a schematic drawing showing the interaction of RGD segments of proteins with integrins, according to one embodiment of the invention;

[0014] Figure 3 is a schematic drawing showing how proteins bind integrins via an RGD segment, according to one embodiment of the invention;

[0015] Figure 4 is a photograph of a Contortrostatin drop on a laser-activated surface according to one embodiment of the invention; [0016] Figure 5 is a photograph illustrating how Contortrostatin adheres to silicone debris according to one embodiment of the invention;

[0017] Figure 6 is a photograph of a porcine retina being torn from its aluminum base by

Contortrostatin coated silicone according to one embodiment of the invention;

[0018] Figure 7 is graph of the force needed to tear Contortrostatin coated silicone from an aluminum base; and

[0019] Figure 8 is a graph of the force needed to tear uncoated silicone from an aluminum base.

DETAILED DESCRIPTION OF THE INVENTION [0020] According to a first aspect of the invention, a method of activating a silicone surface to facilitate its bonding to another compound or compounds is provided and comprises the steps of providing a silicone article having at least one surface, and irradiating at least a portion of one of the surfaces with laser light at a wavelength and power sufficient to eject organic species from the silicone article, thereby forming an activated silicone surface. Thus, the method provides a method of treating the surface of a silicone substrate to increase its chemical reactivity.

[0021] Nonlimiting examples of silicone articles include silicone films, substrates, bulk objects, and silicone coatings. The silicone may be present as substantially pure silicone

polymers or, more typically, silicone polymers containing one or more additives to enhance the article's mechanical, thermal, or other physical characteristics. Nonlimiting examples of such additives include fillers, such as silica entities (e.g., foamed, granular, fibrous, etc; optionally, the silicone polymers are coupled to these silica entities via grafting), plasticizers, and crosslinkers, which can be admixed with the silicone-silica compounds to ensure lateral coupling between polymeric chains that are attached (i.e. grafted) to the same silica piece; etc. The whole of a silicone/silica/crosslinker assembly constitutes a silicone rubber. t Varying any of the individual constituents in quality and quantity provides a nearly infinite range of silicone rubbers that can be activated according to the invention. [0022] Laser light of sufficient wavelength and power is directed at one or more surfaces of the silicone article, such as the top of a silicone film, a portion of a surface of a silicone prosthesis, etc., which causes chemical bond breaking and formation of unpaired electrons, as described below. This "activates" the surface of the silicone article in and around the areas that have been irradiated, making that area more chemically reactive toward other compounds.

[0023] The use of a monochromatic, intense UV light source can, under specific conditions, allow substantially instant light absorption and drive the silicone structure to destabilize its atom configuration. This can be achieved with a laser source working in the UV range and under a pulsed regime, such as an excimer laser. [0024] After investigating the actual optical absorption of a given silicone or silicone rubber, a UV light wavelength (or photon energy) is chosen that allows the material to absorb the UV photons selectively and exclusively on the Si-C bond electrons. Above a given power of the light source at that wavelength (of the order of 100MW), all Si-C bond electrons that are present in the silicone volume that is traversed by the laser beam may be brought to absorb these UV photons quasi-simultaneously, over a very short period of time (on the order of l-2ns). That absorption produces the quasi-simultaneous breaking of these Si-C bonds, thus separating the corresponding organic species, e.g., organic radicals from the original silicone structure. While these radicals form a gas that disperses in the environment, the Si-O backbones of the now partially decomposed polymer remain as the sole part of the silicone that has not absorbed the UV photons. Meanwhile, each of the Si atoms in the polymer backbones is no longer fully interlinked except to two adjacent O atoms. This leaves two unpaired electrons per Si atom. Each of these electrons remains coupled to a corresponding positron in the atom nucleus and occupies a so-called orbital that is attached to the atom site. After laser irradiation of the original surface, these "dangling" bond electron orbitals constitute a dense one-dimensional network along each backbone on the actual silicone surface.

[0025] That network materializes the chemical "activation" of the processed silicone surface. In effect, and as a result of the laser-processing, the surface is no longer neutral, but

is negatively charged. Eventually, an electric field is established that stems from these orbitals and tends to attract (i) positively charged species to form covalent bonding, or even (ii) neutral species that come to settle on the silicone surface and adhere to the Si-O backbones via electrostatic forces. [0026] The end product of the laser-processed silicone surface is partially ablated and, therefore, engraved (i.e. recessed) down to some lOμm or more below the original surface plane, depending on the number of super-imposed irradiations. The activated surface is, therefore, originally localized in the recessed area but is not limited to it, as explained by the discussion. [0027] As noted above, C-H or other organic radicals are liberated during irradiation as free entities. The cloud of chemical species that is formed by these radicals tends to project outwards nanometer-scale particles (or nano-particles) of the silicone (Si-O) backbones. These nano-particles land on and populate the silicone surface area that is adjacent to the recessed laser-irradiated parts, thus contributing to the formation of a laser-activated silicone surface. Over that area, they form a dense layer of active species, since they contain those unpaired dangling bond electrons on each Si atom as mentioned above. Eventually, these species do react to the underlying virgin silicone surface, resulting in a strongly adherent, active cover. As a result, activation of the silicone surface is no longer restricted to the recessed laser-processed surface but extends eventually far beyond it. [0028] This extended activation is conformal to the un-recessed, original silicone surface. The geometry of the conformal activated surface that surrounds the laser-recessed parts may be tailored through the actual geometry and distribution of these laser-processed recessed areas. Since the latter may be monitored by precisely positioning and/or scanning the laser beam onto the silicone surface, the entire conformal activated surface may be designed through computer-monitoring of the laser positioning on the silicone surface.

[0029] All silicones (including silicone rubbers) are accessible to the above-described laser-induced selective decomposition and activation. Such materials may differ by the type of organic-radicals that they contain. However, because each radical is connected to a single Si atom by a normal Si-C bond, different organic-radicals may be identically separated from their silicone backbone via identical irradiation conditions, irrespective of the individual identity of the organic-radicals and silicone formulation.

[0030] Three types of bonds are present in every silicone: Si-O, Si-C and C-H. The weakest of these bonds is Si-C (at 318 kJ/mol), the strongest is Si-O (at 452kJ/mol), and C-H is intermediate in strength at 411 kJ/mol. Along with that bond hierarchy, optical absorption starts at 4.3, 5.3, and 5.5eV, for Si-C, C-H, and Si-O bond (valence) electrons, respectively. Choosing a monochromatic beam working at 5eV photon energy (i.e., 248nm wavelength) restricts exclusively optical absorption to electrons belonging to Si-C bonds.

[0031] Increasing the actual power of a laser beam working at 5eV should therefore allow the selective decomposition of silicone that preserves the original Si-O backbone and produces the formation of the dangling bond electrons that materialize the activation of the material. Comparatively, such 5eV photons are not absorbed by silica additive parts. In contrast, they may be absorbed by crosslinker molecules, whether these are a silicone polymer or siloxane. In that case, again C-H and other organic radicals are selectively separated from the backbone of these molecules, without affecting their inter-linking function. [0032] The preferred laser source that promotes this selective optical absorption to the most appropriate power is an excimer laser source working at 248nm wavelength, i.e. 5.0OeV photon energy. Its actual instant power (i.e. beam energy/ pulse duration) may vary in the range of 50 to 200MW. Alternatively, another laser source is utilized, though not necessarily with the same effectiveness. For example, a pulsed, quadrupled-YAG laser beam would likely operate less efficiently. [0033] According to one embodiment of the invention, the irradiation is a pulsed one

(pulse duration being variable in the range 5 to 40ns, full width, depending on manufacturer). Pulses are usually repeated several times along a train, at fixed time intervals. The processed material may be maintained fixed during irradiation, and the train of pulses processes the same area until a specific amount of ablated (activated) matter is produced. While being irradiated (i.e. during laser-scanning), the target polymeric material may also be displaced in front of the laser source on an X-Y table, moving perpendicularly to the laser beam axis. An appropriate combination of pulse repetition rate and scan velocity would ensure the required ablation per unit area. Material displacement is computer-controlled to any geometry and scan-speed velocity. [0034] The ablated species scatter around the laser-ablated area and establish the laser- activated silicone surface. Optionally, the extent of the scatter may either be limited to a few μm or expanded to several hundred μm, using a gas jet (e.g., an inert gas, such as He) that drifts the emitted species away from the irradiated area, and the scan geometry can be adapted to account for that scatter. In contrast, a monochromatic beam working at a photon energy exceeding 5.5eV induces absorption from all valence electrons, irrespective of the bond type from which they originate. At and above an appropriate instant power level, this would eventually drive the full ablation of silicone with no activation of the remaining silicone surface, either of the irradiated part of it or of the surface area surrounding it. [0035] Excimer lasers have been used to irradiate plastics to form metallized plastics. See U.S. Patent No. 5, 599, 592 to L. Laude, entitled "Process For The Metallization of Plastic Materials and Products Thereto Obtained," the entire contents of which are hereby incorporated by reference.

[0036] Figure 1 schematically depicts a conceptualization of a laser-activated silicone surface according to the invention. As shown, chemically reactive, dangling unpaired electrons bound to the Si-O backbone are exposed at the surface. The surface is thus "activated," and can react with other compounds of interest. [0037] In addition to the described method, the invention also provides a silicone article having at least one activated surface formed by irradiation with laser light at a wavelength and power sufficient to eject organic species from the silicone article. In one embodiment, such an article is prepared according to the method described above. In another embodiment, the article further comprises one or more compounds bound to the activated surface. The nature and identity of such compounds are nearly limitless. Any substance that will react with the exposed unpaired electrons can be applied to the activated surface and thereby bind to it.

[0038] A convenient way to apply the foreign compound to the activated surface is to provide it as a gas or liquid, the latter being particularly suited for introducing large molecular structures, such as peptides and proteins that are otherwise difficult to manipulate. If these are contained in a liquid solution, coating may be done by hand (e.g.,), disposing a drop of the solution on the irradiated surface(s) of the silicone article. [0039] Advantageously, coupling of the foreign compounds to the silicone surface is generally restricted to the laser-activated areas as described above. When these structures are contained in a liquid solution, a drop of that solution may be disposed (e.g., manually) on the silicone surface. Only the parts of the surface that have been activated would retain the incoming species and ensure substantial adhesion and bonding. On non-activated surface areas, foreign species do not adhere to the virgin silicone surface and may, therefore, be removed by washing in water, gentle scrubbing, or tapping out without affecting those species that are strongly fixed on the activated silicone surface. Other means of disposing these foreign species may be practiced depending on the type and size of the species. For example, disposal may also be performed by evaporation in a vacuum chamber, and other physical or chemical means may be practiced as well without affecting the particular adhesion of these species to the laser-activated silicone surface alone. [0040] The type and extremely dense distribution of the laser-generated activated parts of the silicone polymer (namely, the Si-O backbones) on a laser-processed silicone surface allow the surface to size, and keep at once, large molecules of varied formulation and shape. This is demonstrated, for example, in disposing protein molecular structures onto an activated silicone surface. [0041] One type of silicone article that can be prepared according to the invention is a silicone implant, i.e., an implantable medical device made, in whole or in part, of silicone. (In other words, silicone may constitute substantially the entire implant, or just a part of it, such as an outer coating, sleeve, jacket, or other protective barrier.) In one embodiment, the

silicone implant is a silicone article treated with a biocompatible compound that facilitates bonding to living tissue, and comprises a silicone substrate having at least one activated surface formed by irradiation with laser light at a wavelength and power sufficient to eject organic species from the silicone substrate, and at least one compound capable of binding to one or more integrins, coupled to the activated surface.

[0042] Integrins are integral membrane proteins used by cells to attach to their extracellular environment. Treating an activated silicone surface with a compound capable of binding to one or more integrins makes it possible to attach a silicone article, such as an implant, directly to tissue, without resort to surgical tacks, toxic adhesives, or other potentially destructive means.

[0043] One type of compound capable of binding to integrins is an arginine-glycine- aspartate (RGD) peptide, or a protein containing at least one RGD segment (Figure 2). Extracellular matrix (ECM) proteins can be used to bind integrins via RGD segments at the cellular interface (Figure 3). Nonlimiting examples include fibronectin, laminin, and collagen. Non-ECM proteins that contain one or more RGD segments are another example of compounds capable of binding to integrins; specific examples include Contortrostatin (CN), a low molecular weight protein found in snake venom, and Vicrostatin, the monomer of CN. Both of these two proteins stick to integrins on tissue previously occupied by ECM proteins. CN works well (i.e., it is sticky) as it is small and has two RGDs per molecule. Extracellular matrix proteins (such as fibronectin, laminin and collagen) do not adhere as well to the retina as they are large molecules with only one RGD; however they do adhere well to the activated silicone.

[0044] Because retinal cells can bind to RGD peptides and proteins containing RGD segments, the present invention can be used to make an epiretinal visual prosthesis — a silicone-coated microelectrode array (MEA) to be implanted in the eye. The internal limiting membrane of the retina (the inner-most layer) contains laminin, fibronectin, collagen type I and IV, protecglyeans and vitreous fibrils.

[0045] Biocompatibility of an epiretinal positioned electrode array is an important consideration when choosing the materials for the MEA. Additionally, the surgical techniques also play a role in the success of the implanted array. See Long-term Histological and Electrophysiological Results of an Inactive Epiretinal Electrode Array Implantation in Dogs, Invest. Ophthalmol. Vis. ScL, vol. 40, no. 9, pp. 2073-2081, August 1999 by A. B. Majji, the entire contents of which are hereby incorporated by reference. [0046] Techniques for attaching arrays to ocular tissue using biological glues, retinal tacks, and magnets are known in the art. See Bioadhesives for Intraocular Use. Retina, vol. 20, pp. 469-477, 2000, by E. Margalit et al., the entire contents of which are hereby incorporated by reference. Fabricating silicone microelectrode arrays is also known in the art. See Retinal Prosthesis for the Blind. Sury. Ophthalmology, 47 (2002), pages 335-356 by

E. Margalit, et al., the contents of which are hereby incorporated by reference. See also, U.S.

Department of Energy document UCRL-LR- 153347, entitled Microfabrication of an Implantable Silicone Microelectrode Array for and Epiretinal Prosthesis by M. N. Maghribi, dated June 10, 2003; Batch-fabricated thin-film Electrodes for Stimulation of the Central Auditory System. IEEE Trans. Biomed. Eng., vol. 36, o. 7, pp. 693-704, July 1989 by D. J. Anderson, et al.; An Integrated-circuit Approach to Extracellular Microelectrodes, IEEE Trans. Biomed. Eng., vol. BME- 17, pp. 238-247, 1970 by K. D. Wise et al.; Implantable Microsystems. Polyimide-based Neuroprostheses for Interfacing Nerves. Med. Device Tech. , vol. 10, no. 6, pp. 28-30, July 1999, by T. Stieglitz et al.; the entire contents (of all of the prior references) of which are hereby incorporated by reference.

[0047] In one embodiment, implanted components can include a multi-channel electrode array as well as bi-directional telemetry and hermetically packaged micro-electronics. These components can perform power recovery, management of data reception and transmission, digital processing, and analog output of stimulus current. [0048] In one embodiment, for a silicone implant comprising a microelectrode array

(MEA), and if an extracellular matrix (ECM) protein is selected as the compound coupled to the activated silicone surface, it is advantageous if the ECM protein has at least one of the following characteristics: (i) an RGD (arginine-glycine-aspartate) amino acid segment to enable it to interact with retinal integrins (see Figures 2 and 3), (ii) disulfide bonds to allow covalent interaction with silicone, (iii) enzyme-cleavable regions to facilitate removal of the MEA.

[0049] A non-limiting list of polymers useful for creating flexible, micro-electrode arrays are silicone, polyimide, polydimethylsiloxane, and parylenes, such as parylene N and C, and copolymer blends of silicone and non-silicone polymers. Note that non-silicones like the polyimides and parylenes, without being combined with a silicone based polymer, may not have activated surfaces when subjected to the excimer laser process, but are still useful polymers for retinal implants.

[0050] In one embodiment, the activated silicone may be used for long or short-term medical devices such as implants and drug delivery devices, and in a number of tissues, including brain (e.g., cortex), heart, liver, and eye (e.g., retina). A non-limiting list of medical devices includes cardiac pacemakers, cochlear implants, deep brain stimulators for Parkinson's disease, and epiretinal visual prostheses. For these devices, establishing good contact with the surrounding tissue is important and thus the attachment methods of the present invention may be used. The use and implanting of cochlear implants is known in the art. See Cochlear Prosthetics. Ann. Rev. Neurosci.. vol 13. pp. 357-371. 1990. bv G. E. Loeh. the entire contents of which are hereby incorporated by reference. Using implants to treat Parkinsonian tremors is also known in the art. See High-frequency Unilateral Thalmic

Stimulation in the Treatment of Essential and Parkinsonian Tremor, Ann. Neurol., vol. 42, no.

3, pp. 292-299, Sept. 1997, the entire contents of which are hereby incorporated by reference.

[0051] If it becomes necessary to remove a medical implant from tissue to which it has been attached, an enzyme such as plasmin can be used cleave RGD peptides, thereby breaking the bond between the implant and adjacent integrins.

[0052] Examples, Tests, and Discussion

[0053] Protein Attachment to a Silicone Surface

[0054] Snake venom disintegrin (Contortrostatin) is a homodimeric protein that contains an RGD amino acid segment and disulfide bonds (Represented in Figure 2) in order to attach the protein to silicone. An excimer laser was used to physically break the molecular bonds and produce dangling free bonds on the silicone surface. Using a pipette, the Contortrastatin was dropped onto the lased silicone surface and allowed to dry.

[0055] Preparation of Retinal Tissues

[0056] Postmortem porcine eyes were prepared by removing the vitreous humor with a vitreous cutter (Bausch and Lomb). The posterior segment of the eye was flattened by making four cuts in four different quadrants from the pars plana to the equator. The eye was pinned out onto a polystyrene surface and quadrants of the retina were delicately removed.

Each piece of retina was glued (Adhesive Systems RP 1500 USP) face up (i.e. internal limiting membrane up) to a piece of aluminum and allowed to dry for 10 minutes. During this time the retina was kept moist with drops of saline.

[0057] Protein Adhesive Strength

[0058] The adherence forces between the Contortrostatin-coated silicone and the retina were measured by dynamic mechanical analysis, using a Bose ElectroForce 3100.

Contortrostatin-coated silicone was glued (Adhesive Systems RP 1500 USP) to a piece of plastic and lowered onto the prepared retina. The silicone piece was raised 4 mm over 10 seconds and the adhesive forces resulting from the separation of retina and aluminum were recorded.

[0059] Results

[0060] After the excimer laser was used to physically break molecular bonds, photos were taken of the silicone surface during the attachment process. The Contortrostatin drop can be seen absorbing into the lased areas (Figure 4) and later extending over the silicone debris on the surface (Figure 5). To test the adhesive strength of the protein to the silicone, a simple scotch tape test was performed. The scotch tape could not be removed from the activated surface. [0061] Adhesive Strength to the Retina

[0062] Dynamic mechanical analysis of Contortrostatin-coated silicone and non-laser processed silicone is graphically presented in Figures 7 and 8. The silicone in each case was removed from the retina at 0.4 mm/second. Figure 7 shows the adhesive force of the

Contortrostatin-coated silicone is approximately 34OmN, at which point the retina was torn away from the aluminum surface (See Photo, Figure 6). Figure 8 shows the plain (non- activated) silicone is easily detached from retina after just 1OmN. The green line represents a force of 1OmN, and the blue line is a displacement of 4mm over 10 seconds. [0063] While this invention has been described in connection with reference to what are considered exemplary embodiments, the invention is not limited to the disclosed embodiments, dimensions, and configurations but, on the contrary, also extends to various modifications and equivalent arrangements. The invention is limited only by the appended claims and their equivalents.