FIELD OF THE INVENTION [0001] This invention relates to novel calcium phosphate-coated implantable medical devices and processes of making same. The unique calcium-phosphate coated implantable medical devices minimize immune response to the implant. The coated implantable devices have the capability to store and release one or more medicinally active agents into the body in a controlled manner. BACKGROUND OF THE INVENTION [0002] Cardiovascular stents are widely used in coronary angioplasty procedures to enlarge coronary arteries and thereby allow better blood circulation. Typically this is accomplished by a balloon angioplasty procedure wherein a contracted stent, usually in the form of a metallic mesh tube, is moved in to the site of blood vessel narrowing along a guide wire. Once the stent is in place an internally situated balloon expands it radially. After expansion the balloon is deflated and removed from vessel while the stent remains expanded in place. The stent thus provides a scaffold support for the walls of the blood vessel, enlarging the vessels aperture and increasing blood flow. This operation saves millions of lives annually around the world. Unfortunately the placement of metallic stents often leads to harmful side effects. A relatively large proportion of patients (up to half of the population, according to some statistics) experience an immune response to the implanted stent called inflammatory restenosis, and other negative effects, which lead to a re-narrowing of the vessel. This typically requires repeat surgical treatment within 1-2 years of the original balloon angioplasty operation. [0003] The mechanisms that lead to restenosis and other immune responses associated with the implantation of a medical device are initiated by damage to the vessel lining during the surgical procedure. Such damage is very difficult to avoid entirely, but its effects, i.e. inflammation and/or infection, may be diminished through modifications to the surface of metallic implantable medical devices. The most common surface modification of implanted medical devices is the application of a thin polymer film coating. These coatings are frequently impregnated with medically active agent(s) such as antibiotics, anti-inflammatory agents and other, more complex drugs. These medically active agents are released from the coating through leaching to the arterial wall and the blood stream, often aided by dissolution of the carrier film. Typically, biodegradable polymers such as polylactic acid, polyglycolic acid, and others, frequently in combination with heparin and other anti-thrombogenic agents, are selected in such drug delivery systems. A particular advantage of the polymer coatings on stents is that the coatings are flexible and generally non-thrombogenic. [0004] In the past, polymeric materials have been used for drug delivery control and have enjoyed substantial clinical success for certain drug systems. Unfortunately, even biodegradable polymers, although more bio-friendly than the native metallic surface, are still recognized by living tissue as foreign objects. Therefore the bio-degradation process is frequently accompanied by inflammatory response of the tissue. In some critical applications, such as cardiovascular stents, it has been determined that polymer coated stents do not perform according to expectations in longer term (in excess of 1 year) of use. Furthermore, in many instances relatively rapidly resorbing polymer coatings are quickly depleted, from the stent surface with concomitant loss of the long-term affects of the drug and harmful exposure of the bare metal surface to contact tissue. This may result in an adverse response of the tissue, leading to inflammation, restenosis (in the case of stents), and requiring repetitive surgical intervention. [0005] There is therefore a strong need to discover materials for coating implantable medical devices that are entirely biocompatible and thus do not cause any adverse effects in the tissue. Furthermore, ideally this coating material will be able to deliver one or more pharmaceutically active agents to a targeted site. Studies have shown that porous coatings may accept the required load of drugs through adsorption and then release the drugs in a controlled manner. The drug release process is dependant on surface properties of the coating-material and the adsorption properties, molecular size, and other characteristics of the drug. [0006] One group of materials exhibiting desired characteristics has been known for a long time, and is used extensively for the surface modification of large rigid implants such as artificial hips in the human body. These materials are members of the family of calcium phosphates (CaP) and include hydroxyapatite (HA), di- and tri-calcium phosphates, as well as partially or fully amorphous calcium phosphates. These materials are mineral components of hard tissue and as such are fully bio-compatible and bio-resorbable with no side effects. Calcium phosphate, in particular hydroxyapatite (HA), is a principal inorganic component of bone, and thus offers entirely new perspectives for coating-based drug encapsulation and drug delivery systems. [0007] Hydroxyapatite ceramics, Ca 10 (PO 4 ) 6 (OH) 2 , belong to the class of calcium phosphate (CaP) based bioactive materials that are used for a variety of biomedical applications, including matrices for drug release control [M. Itokazu et al., Biomaterials, 19, 817-819, 1998; F. Minguez et al Drugs Exp. Clin. Res., 16[5], 231-235, 1990; W. Paul and C. P. Sharma, J. Mater. Sci. Mater. Med., 10, 383-388, 1999]. Other members of the CaP family, such as dicalcium phosphate (CaHPO 4 .2H 2 O) or tricalcium phosphate (Ca 3 (PO 4 ) 2 ), have also been used for similar purposes. The CaP family of materials has been long recognized as having a high degree of biocompatibility with human tissue. [0008] The use of calcium phosphate coatings, including HA coatings, thermally deposited on implantable devices has been limited by the fact that such coatings used to date have had thicknesses of >0.01 mm and have exhibited brittle behaviour when in bulk form. This characteristic has limited their use to applications where a solid support structure, such as dental or hip implant, does not allow for much deformation of the structure. In such cases, the potential for coating damage is limited and osseo-integration with the tissue occurs in an improved manner. HA coated implants in particular have been shown to possess excellent biocompatibility and provide accelerated integration of the implant with the surrounding tissue. The bio-resorption rate of such coatings can be controlled through adjustment of their crystallinity and chemical composition, e.g. by the incorporation of carbonate groups and other methods known to those skilled in the art. [0009] A method alternative to thermal coating is the biomimetic deposition of HA films at room temperature (BM-HA). This technique has been used for a variety of biomedical applications, for example drug delivery [H. B. Wen et al, J. Biomed. Mater. Res., 41, 227-36, 1998; S. Lin and A. A. Campbell, U.S. Pat. No. 5,958,430, 1999; D. M. Liu et al J. Mater. Sci. Mater. Med., 5, 147-153, 1994; K. de Groot et al, J. Biomed. Mater. Res., 21, 1375-1381, 1987). This forming mechanism is driven by supersaturation of Ca 2+ and PO 4 3− , under appropriate solution pH, where HA is the most stable phase. As the process proceeds at or near room temperature, the apatitic crystals which form through nucleation and growth may incorporate biologically active species, such as antibiotics, anti-cancer drugs, anti-inflammatory agents, etc. The deposition rates for BM-HA are in the range of 0.05-0.5 μm/h. [0010] This relatively low deposition rate may be enhanced significantly if electric field is applied to the metallic substrate being coated, e.g. stent, in a solution containing proper concentration of calcium and phosphorous ions. This variant of coating is usually referred to as Electro-Chemical Deposition (ECD), and the resulting film termed as ECD-HA. As ECD also proceeds at (or near) room temperature, drug encapsulation is also possible in ECD-HA. The physiological solutions for BM-HA formation are naturally water-based, which makes it impossible to encapsulate hydrophobic bioactive agents into BM-HA coatings. The biomimetic HA films (both BM-HA and ECD-HA) may be deposited on implantable medical devices at room temperature, which is of great advantage for drug encapsulation during deposition. [0011] Unfortunately, the bonding strength BM-HA and ECD-HA to metallic surfaces is generally significantly lower than that of sol-gel HA (termed here SG-HA). At the same time, bonding strength of BM-HA or ECD-HA to previously consolidated hydroxyapatite is high, generally in excess of 40 MPa. In this respect building additional BM-HA or ECD-HA film on top of the already existing, well-bonded to the metallic substrate film of SG-HA provides a novel and inventive route to achieve high bonding strength, controlled porosity, and drug encapsulation capability of the films deposited at room temperature, [0012] Another alternative for room (or near-room) temperature deposition of porous calcium phosphate films, in particular hydroxyapatite, for drug impregnation and encapsulation, is so-called calcium phosphate cement (CPC) route. In this previously disclosed process (refer to U.S. Patent Application No. US2002/0155144 A1 “Bifunctional Hydroxyapatite Coatings and Microspheres for in-situ Drug Encapsulation”, by T. Troczynski, D. Liu, and Q. Yang), fine particles of calcium phosphate precursor Ca(OH)2 and calcium phosphate salt monocalcium phosphate anhydrate, are milled and mixed in ethanol, followed by film deposition and impregnation by sodium phosphate solution (refer to the Example 4 below for details of this procedure). As a result of this process, microporous, semi-amorphous CPC-HA results, suitable for delivering drugs through leaching and during film resorption. Similarly as above, CPC-HA film bonds poorly to metallic surfaces, such as those of implants or stents. However, CPC-HA film deposited on previously consolidated surface of HA, such as SG-HA, achieves high bonding strength, generally in excess of 40 MPa. In this respect building additional CPC-HA film on top of the already existing, well-bonded to the metallic substrate film of SG-HA provides a novel and inventive route to achieve high bonding strength, controlled porosity, and drug encapsulation capability of the films deposited at room temperature. [0013] Electric field-assisted thin film deposition technologies have the great advantage of the resulting film uniformity, especially for complex substrates such as stents. One such technology termed Electro-Phoretic Deposition (EPD) is well known method in ceramic processing. In this method fine particles of a ceramic (generally about a micrometer or less in size) suspended in a liquid attain electric charge through interaction with the liquid or through addition to the suspension of surface-active species. The simplest example of such EPD system is oxide (or hydroxide, such as hydroxyapatite) ceramic powder suspended in water and acid (such as nitric acid) mixture. In such environment protons will have a tendency to absorb on surface of the ceramic particles, providing positive charge to the particles. Upon application of electric field, such charged particles would migrate to the negative electrode (cathode). Exactly opposite would happen in basic environment, i.e. negatively charged particles of ceramic would migrate to the positive electrode (anode). EPD is an excellent technique for deposition of ceramic films, including calcium phosphate films, as disclosed in U.S. Pat. No. 5,258,044, dated Nov. 2, 1993 (“Electro-phoretic Deposition of Calcium Phosphate Material on Implants”, by D. D. Lee). Unfortunately, EPD films must be sintered at relatively high temperature to gain sufficient structural integrity. For example, the EPD films of calcium phosphate disclosed in U.S. Pat. No. 5,258,044, had to be sintered at between 600° C. and 1350° C. These temperatures are high enough to induce substantial change to the metallic substrate, e.g. in terms of surface oxidation or microstructural changes (e.g. grain growth). [0014] Drug encapsulation in HA has been achieved in the past by simple post-impregnation of a sintered, porous HA ceramic [K. Yamamura et al, J. Biomed. Mater. Res., 26, 1053-64, 1992]. In this process, the drug molecules simply adsorb onto the surface of the porous ceramic. The drug release is accomplished through desorption and leaching of the drug to the surrounding tissue after exposure to physiological fluid. Unfortunately, most of the adsorbed drug molecules release from such system in a relatively short period of time. Impregnation of drug material into porous sintered calcium phosphate microspheres has been reported in the patent literature. “Slow release” porous granules are claimed in U.S. Pat. No. 5,055,307 [S. Tsuru et al, 1991], wherein the granule is sintered at 200-1400° C. and the drug component impregnated into its porosity. “Calcium phosphate microcarriers and microspheres” are claimed in WO 98/43558 by B. Starling et al [1998], wherein hollow microspheres are sintered and impregnated with drugs for slow release. D. Lee et al. [WO98/16209] claim poorly crystalline apatite wherein macro-shapes harden and may simultaneously encapsulate drug material for slow release. It has been suggested to use porous, composite HA as a carrier for gentamicin sulfate (GS), an aminoglycoside antibiotic to treat bacterial infections at infected osseous sites [J. M. Rogers-Foy et al, J. Inv. Surgery 12 (1997) 263-275]. The presence of proteins in HA coatings did not affect the dissolution properties of either calcium or phosphorus ions and that it was solely dependent on the media [Bender S. A. et al. Biomaterials 21 (2000) 299-305]. [0015] Stents are disclosed in several patent publications. U.S. patent publication No. 2002/0007209 A1, published Jan. 17, 2002, de Sheerder et al., discloses an expandable metal tube prosthesis with laser cuts in the walls. The prosthesis can be coated with titanium nitride (TiN) for bio-compatibility. The holes in the walls of the prosthesis can be used to locally administer medicines and the like. [0016] U.S. Pat. No. 6,387,121 B1, issued May 14, 2002, Alt, assigned to Inflow Dynamics Inc., discloses a stent constructed with a tubular metal base. The stent can be constructed to have three layers (see FIG. 2 ). The first layer 15 is typically 316L stainless steel. The intermediate layer 50 is formed of a noble metal or an alloy thereof, preferably selected from a group consisting of niobium, zirconium, titanium and tantalum (see column 7, lines 58-61). The third or outer layer 80 is preferably composed of a ceramic-like metal material such as oxide, hydroxide or nitrate of metal, preferably iridium oxide or titanium nitrate, as a bio-compatible layer that serves as a primary purpose to avoid tissue irritation and thrombus formation. [0017] EP 0 950 386 A2, published Oct. 20, 1999, Wright et al., assigned to Cordis Corporation, discloses a thin walled stent which is formed as a cylinder with a plurality of struts. The struts have channels formed therein. Therapeutic agents can be deposited in the channels. Rapamycin specifically is mentioned as a therapeutic agent which can be deposited in the channels to prevent restenosis (re-narrowing) of an artery. SUMMARY OF THE INVENTION [0018] The invention is directed to an implantable medical device with a calcium phosphate coating comprising: (a) substrate; and (b) calcium phosphate coating on the substrate, said coating having desired bonding and porosity characteristics. [0019] The calcium phosphate coating of the device can be hydroxyapatite. The thickness of the calcium phosphate coating can be between about 0.00001 mm and 0.01 mm, and preferably about 0.001 mm to 0.0001 mm. The tensile bond strength between the substrate and the calcium phosphate coating can be greater than about 20 MPa. The calcium phosphate coating can be deposited on the device as particles having a diameter between about 1 μm and 100 μm and a thickness of between about 1 μm to 10 μm. The particles can cover about 20% to about 90% of the surface of the substrate. [0020] The implantable medical device can be constructed of stainless steel, cobalt alloy, titanium cobalt-chromium or metallic alloy. The calcium phosphate coating can be porous and the pores can retain a drug. The rate of release of the drug from the pores can be controlled in an engineered manner. [0021] The substrate can have a first calcium phosphate coating and a second calcium phosphate coating and the drug can be contained in both the first and the second coating or only in one coating. The drug can be one which inhibits restenosis. The calcium phosphate coating can be dicalcium phosphate, tricalcium phosphate or tetracalcium phosphate. The device can be a human or animal tissue implantable device. The device can be a stent which is coated with calcium phosphate. [0022] The invention is also directed to a process of coating an implantable medical device with a calcium phosphate coating comprising: (a) hydrolyzing a phosphor precursor in a water or alcohol based medium; (b) adding a calcium salt precursor to the medium after the phosphite has been hydrolyzed to obtain a calcium phospate gel; (c) depositing the calcium phosphate gel as a coating on the surface of a substrate; and (d) calcining the calcium phosphate coating at a suitable elevated temperature and for pre-determined time to obtain a crystallized calcium phosphate having desired crystallinity, bonding and porosity characteristics. [0023] The deposition of the coating on the substrate can be performed by aerosol deposition, dip-coating, spin-coating, electrophospate coating or electrochemical coating. The calcium phosphate coating can be calcined at a temperature of at least about 350° C. The calcium phospate gel can be hydroxyapatite gel. [0024] The porosity of the calcium phosphate coating can be controlled and can retain a drug. The rate of release of drug can be controlled. The calcium phosphate coating can be hydroxyapatite, dicalcium phosphate, tricalcium phosphate or tetracalcium phospate. [0025] The phosphate precursor can be an alkyl phosphite or a triethyl phosphate. The calcium precursor can be a water-soluble calcium salt. The water soluble calcium salt can be calcium nitrate. [0026] The invention is also directed to a process of coating a soft tissue implantable device with a calcium phosphate coating comprising: (a) providing a soft tissue implantable substrate; (b) depositing a calcium phosphate coating on the substrate utilizing a biomimetic deposition process; or (c) depositing the calcium coating on the substrate utilizing a calcium phosphate cement deposition process; or (d) depositing the calcium phosphate coating on the substrate utilizing an electro-phoretic deposition process; or (e) depositing a calcium phosphate coating on the substrate utilizing an electrochemical deposition process. [0027] The device can be a calcium phosphate coated stent. The calcium phosphate coating can be hydroxyapatite. The calcium phosphate coating can be deposited discontinuously on the substrate as discrete particles. [0028] A first calcium phosphate coating can be deposited on the substrate utilizing an aerosol-gel process, a sol-gel process or an electro-phoretic deposition process or an electro-chemical deposition process and a second calcium phosphate coating can be deposited on the first coating or the substrate utilizing an aerosol-gel process, a sol-gel process, a biomimetic process, a calcium phosphate cement process, an electro-phoretic deposition process or an electrochemical deposition process. [0029] The calcium phosphate coating can contain and elude a drug. The calcium phosphate coating can be coated with a hydrogel film. The calcium phosphate can be deposited on the substrate as discontinuous non-equiaxial particles. The non-equiaxial particles can have an average size of about 0.1 μm and a thickness up to about 0.01 mm. The first and second coatings can contain a drug. [0030] The ratio of calcium to phosphate in the sol-gel precursor can be engineered to enable various phosphate phases to be obtained. The calcium phosphate phase can be hydroxyapatite, dicalcium phosphate, tricalcium phosphate or tetracalcium phospate. DRAWINGS [0031] In drawings which illustrate specific embodiments of the invention, but which should not be construed as restricting the spirit or scope of the invention in any way: [0032] FIG. 1A is a micrograph of a stainless steel (316L) stent coated with discontinuous ASG-HA thin film. [0033] FIG. 1B is a magnification of the sector indicated by the rectangle of FIG. 1A . [0034] FIG. 2A is a micrograph of a stainless steel stent (316L) coated with discontinuous ASG-HA thin film and crimpled, with no damage to the coating. [0035] FIG. 2B is a micrograph of the same stent as shown in FIG. 2A after expansion showing no damage to the coating. [0036] FIG. 3A is a micrograph of a stainless steel (316L) stent coated with continuous EPD-HA thin film. [0037] FIG. 3B is an about 4×6 μm magnification of the sector indicated by the rectangle of FIG. 3A . [0038] FIG. 4A is a micrograph of a stainless steel (316L) stent coated with continuous ECD-HA thin film. [0039] FIG. 4B is an about 65×88 μm magnification of the sector indicated by the rectangle of FIG. 4A . DETAILED DESCRIPTION OF THE INVENTION [0040] Throughout the following description specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the present invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. [0041] The invention in one embodiment is directed to implantable medical devices with a flexible thin film calcium phosphate bio-compatible and bio-resorbable coating that has the ability to act as a high capacity drug carrier. Such CaP coatings have no side-effects during coating dissolution into body fluids, and can be designed with a high level of control of coating dissolution rate and microstructure, which also determine the drug retention and release characteristics. [0042] Of all the types of implantable medical devices that exist, the coronary stents utilized in balloon angioplasty procedures provide a useful model for testing the effectiveness of sol-gel deposited thin flexible CaP coatings on such stents due to the fact that such stents are designed to be flexible. The use of such stents in the examples below should not, however, be considered as limiting the application of the CaP coatings described only to stents. The invention has broad application to virtually any type of body implantable device. [0043] We have determined unexpectedly that the intrinsic brittle behaviour of CaP ceases to limit the system strain capability if the strongly bonded coating is sol-gel deposited and is thinner than approximately 0.001 mm. Experiments involving repeated contraction/expansion of such thin CaP sol-gel coated stents reveal that there is no separation of the coating from the stent, nor visible damage to the coating, if the coating is thinner than about 0.001 mm and is strongly bonded to the substrate (the tensile bond strength should be larger than about 40 MPa, as measured in model strength experiments according to ASTM C-633 standard). [0044] In addition, we have discovered that if the novel sol-gel process for deposition of calcium phosphates, in particular hydroxyapatite (HA) synthesis (as previously disclosed in our U.S. Pat. No. 6,426,114 B1, Jul. 30, 2002, “Sol-Gel Calcium Phosphate Ceramic Coatings and Method of Making Same”, by T. Troczynski and D. Liu) is used, the resulting thin flexible coating has controlled porosity which may be utilized to retain drugs within the coating, and release the drugs at a controlled rate. [0045] The invention pertains to a sol-gel (SG) process for synthesis of calcium phosphate, in particular, hydroxyapatite (HA), thin film coatings on implantable medical devices. The process allows the HA to be obtained in a controlled crystallized form, at a relatively low temperatures, i.e. starting at ≈350° C. This is an unexpectedly low crystallization temperature for HA sol-gel synthesis. The process provides excellent chemical and physical homogeneity, and bonding strength of HA coatings to substrates. The low process temperature avoids substrate metal degradation due to thermally-induced phase transformation, microstructure deterioration, or oxidation. [0046] Disclosed herein is a method wherein uniform films of hydroxyapatite by the electro-phoretic deposition (EPD) method (EPD-HA) are deposited on complex stent surface, and there is no need to pursue sintering in excess of 500° C. to achieve substantial structural integrity of the film and its high bonding strength to the metallic substrate. In this method, the first step is the well-known EPD of the HA film, for example as disclosed in U.S. Pat. No. 5,258,044, using suspension of sub-micrometer particles of HA in water. This film is dried and then heat treated at 500° C. for 10-60 minutes to initiate sintering of HA. The film is still too weak and too poorly bonded for practical use as a coating on stent or other medical device or implant, but is sufficiently strong to survive the subsequent processing step comprising impregnation by aero-sol-gel HA droplets. The droplets penetrate porosity of the previously deposited EPD-HA, strongly aided by the capillary suction. Thus, majority of the pores of the EPD-HA film are penetrated by the sol-gel precursor of HA, all the way to the metallic substrate. This composite film can be now dried and sintered at a relatively low temperature or 400-500° C., due to the very high activity of the sol-gel component of the film. The sol-gel film bonds the particles of HA deposited by EPD, and bonds well to the metallic substrate during the heat treatment Thus, both the film uniformity (due to EPD process) and low-temperature sinterability (due to sol-gel process) have been achieved. This novel and inventive hybrid technology for uniform HA coatings on stents has the ability to produce films in thickness range from about 1 micron to above 100 microns, with porosity in the range from about 10 vol % to about 70 vol %. Such porous thick HA films are excellent carriers for drugs loaded through impregnation into open porosity of the film. Details of such hybrid process, and its several variants, for preparation of HA films on stents, are given in the examples below. [0047] Problems with drug delivery in vivo are frequently related to the toxicity of the carrier agent, the generally low loading capacity for drugs, and the aim to control drug delivery resulting in self-regulated, timed release. With the exception of colloidal carrier systems, which support relatively high loading capacity for drugs, most organic systems deliver inadequate levels of bioactive drugs. Sol-gel films heat-treated at relatively low temperatures closely resemble the properties of colloidal films, in terms of accessible surface area and porosity size. [0048] The sol-gel process according to the invention allows the calcium phosphate to be obtained in a crystallized form, at relatively low temperature, i.e. approximately 350-500° C. Variation of the heat treatment temperature and time provides for control of coating crystallinity (i.e. a more amorphous, more easily resorbable coating can be processed at lower temperatures) as well as coating porosity (higher porosity and smaller average pore size at lower temperatures). Variation of Ca/P ratio in the sol-gel precursor mix allows one to obtain various calcium phosphate phases, for example, hydroxyapatite, dicalcium phosphate, tricalcium phosphate or tetracalcium phosphate. [0049] The invention in one embodiment is directed to a sol-gel process for preparing calcium phosphate, such as hydroxyapatite, which comprises: (a) hydrolysing a phosphor precursor in a water or alcohol based medium; (b) adding a calcium salt precursor to the medium after the phosphite has been hydrolysed to obtain a calcium phosphate gel such as a hydroxyapatite gel; (c) depositing the gel on the surface of an implantable medical device; and (d) calcining the calcium phosphate, such as hydroxyapatite, at a suitable elevated temperature and for pre-determined time to achieve desired crystallinity, bonding and porosity characteristics for the coating on the device. The deposition of the gel can be done by any number of methods, such as aero-sol deposition, dip-coating, spin-coating, electrophoretic deposition. [0050] In a preferred embodiment, the phosphor precursor can be an alkyl phosphite and the alkyl phosphite can be triethyl phosphite. Further the calcium precursor can be a water-soluble calcium salt and the water soluble calcium salt can be calcium nitrate. The crystallized calcium phosphate can be calcined at a temperature of at about 350° C. or higher. The metallic implantable medical device can be stainless steel, cobalt alloy, a titanium substrate or other metallic alloy substrate. [0051] We have discovered that if certain specific characteristics of the calcium phosphate coatings are maintained, the coatings become highly flexible while maintaining their chemistry, high bio-compatibility, and bio-resorbability. The most important characteristics are (a) coating thickness, and (b) the strength of the coating bonding to the metallic substrate. We have repeatedly demonstrated (refer to the examples below) that if CaP coating thickness is maintained below about 0.001 mm, and its bonding strength to the metallic substrate is above approximately 40 MPa, the substrate-coating system retains the strain capabilities of the substrate alone, i.e. the system maintains its integrity during deformation. [0052] Furthermore, we have discovered that thicker CaP coatings deposited discontinuously on metallic substrate, i.e. in the form of separate “islands” and “patches” approximately 1-100 μm in diameter, retain high resistance against substrate deformation. Our experiments have shown that stents coated with such 1-100 μm patches, about 1-10 μm thick, can be crimped and then expanded without damage to the patches of ceramic. These patches can be deposited on the substrate through a variety of methods discussed above, such as BM-HA, ECD-HA, CPC-HA (all at room or near-room temperature), or EPD-HA, SG-HA and combinations thereof (these two techniques including heat treatment at elevated temperatures). These coating deposition techniques are illustrated in the following examples. The discontinuous CaP film coated medical implant may have some fraction of an area of the metallic substrate exposed to living tissue, which may again lead to the adverse tissue reaction described above. This problem can be avoided by combining discontinuous CaP films with a continuous bio-compatible and non-thrombogenic polymer. Thus, a composite CaP-polymer coating on medical implant is the result. Furthermore, a thin (<0.001 mm) continuous CaP coating can be combined with a thicker discontinuous CaP coating. [0053] The effects of this process (described in detail in the Examples) are shown in the representative FIGS. 1 and 2 . FIG. 1A illustrates stainless steel (316L) stent coated with discontinuous ASG-HA thin film; FIG. 1B is a magnification of the sector of (A) indicated by the rectangle. FIG. 2A illustrates a stainless steel (316L) stent coated with discontinuous ASG-HA thin film and crimped, with no damage to the coating. FIG. 2B is the same stent after expansion, showing no damage to the coating. [0054] Our discovery of flexible continuous/discontinuous CaP films or CaP/polymer films opens up a range of new applications of highly biocompatible Cap coatings for medical implants, particularly, but not limited to those that require deformation capability such as coronary stents. [0055] A sol-gel (SG) process provides superior chemical and physical homogeneity of the final ceramic product compared to other routes, such as solid-state synthesis, wet precipitation, or hydrothermal formation. The SG process allows the desired ceramic phase, e.g. thin film CaP coating, to be synthesized at temperatures much lower than some of the alternate processes. In the SG coating process substrate metal degradation due to thermally induced phase transformations and microstructure modification or oxidation, is avoided. SG widens green-shaping capability, for example, and it is a very convenient method for deposition of thin ceramic coatings. [0056] Sol-Gel deposition of HA (SG-HA) films at elevated temperatures (350-500° C.) was disclosed previously in U.S. Pat. No. 6,426,114 B1. Sol-gel (SG) processing of HA allows molecular-level mixing of the calcium and phosphor precursors, which improves the chemical homogeneity of the resulting calcium phosphate. The crystallinity of the calcium phosphate phase can be enhanced by appropriate use of water treatment during processing. Variation of Ca/P ratio in the sol-gel precursor mix allows one to obtain any of a number of calcium phosphate phases, for example, hydroxyapatite, dicalcium phosphate, tricalcium phosphate or tetracalcium phosphate. The versatility of the SG method provides an opportunity to form thin film coatings, either continuous or discontinuous, in a rather simple process of dip-coating, spin-coating or aero-sol deposition. [0057] A high degree of HA crystallinity is frequently required for longer-term bioactive applications, because partially crystalline, or amorphous calcium phosphate, such as HA, coatings are rapidly resorbed by living tissue. For the presently disclosed application of thin HA films on implantable medical devices, control of crystallinity of the HA coating is possible through variation of the time/temperature history during processing. This allows control of the coating resorption rate and thus rate of release of the drugs impregnated into microporosity of the coating. [0058] Ceramics produced by sol-gel processing can be designed to include high fraction of pores, with well-defined (narrowly distributed) pore size. This is a consequence of the chemical route to the final oxide ceramic produced through SG. Only a small fraction of the original precursor mass is finally converted to the ceramic oxide, the remaining fraction being released during heat treatment, usually in the form of gas, is usually as a combination of water and carbon dioxide. Thus, the released gases leave behind a large fraction of porosity, up to 90% in some instances, depending on the drying conditions and heat treatment time and temperature. These pores can be as small as several nm in diameter, again depending on the drying conditions and heat treatment time and temperature. Effectively, the accessible surface area of such sol-gel derived oxide ceramics can reach several hundred square meters per gram of the oxide, making it an excellent absorbent of gas or liquid substances, or solutions. For example, the average pore size in sol-gel HA treated at relatively low temperature of 400° C. is about 5 nm in diameter, with 90% of pore diameters falling within the range of 1-30 nm. This unique porosity characteristic is widely utilized to produce desiccants, filters and membranes of sol-gel derived ceramic. In this respect sol-gel derived ceramic oxides have a great advantage over polymers, which are in general difficult to process to possess high porosity and high accessible surface area. In the present invention, we utilize this unique property of sol-gel derived CaP coatings on medical implants, especially stents, possessing high accessible surface area to make it a high-capacity drug carrier. [0059] In the text of this application, it is understood that when appropriate, the term “calcium phosphate” (CaP) is used generically and includes minerals such as hydroxyapatite, dicalcium phosphate, tricalcium phosphate, tetracalcium phosphate and amorphous or partially amorphous calcium phosphate. Studies on the sol-gel route to thin film calcium phosphate coatings on implantable medical devices, particularly stents, performed by the inventors have led to an unexpected break-through in process development. The method according to the invention has produced CaP coatings after heat treatment in air, starting at about 350° C. We have unexpectedly discovered that the film is highly flexible if it is thinner than about 0.001 mm, thereby allowing damage-free manipulation of a CaP coated deformable implantable medical device, for example the contraction and expansion of a CaP coated stent. Preferably, the coating has a thickness between about 0.0001 and 0.001 mm. Furthermore, in this application, we have discovered that the film can accept drugs into its fine porosity, thereby allowing it to address the adverse phenomena related to common medically implanted devices, i.e. the restenosis that occurs after placement of a coronary stent in a blood vessel. [0060] The calcium phosphate coating according to the invention has been deposited on stents and other metallic surfaces using variety of techniques, including dip-coating, spin-coating, aero-sol deposition electrophoretic deposition. The coatings were deposited on stents made of 316L stainless steel and tubes, and on other metallic substrates including cobalt-iron alloy and titanium. EXAMPLES [0061] To demonstrate the feasibility of the unique processing concepts outlined above, the following examples are described below for stainless steel substrate and coronary stents. The procedures outlined below can be applied to other implantable medical devices. Example 1 [0062] In the first stage of the process, phosphite sol was hydrolysed in a water-ethanol mixture (a concentration of 3M) in a sealed beaker until the phosphite was completely hydrolysed (which is easily recognized by loss of a characteristic phosphite odour), at ambient environment. A Ca salt (2M) was then dissolved in anhydrous ethanol, and the solution was then rapidly added into the hydrolysed phosphite sol. The sol was left at ambient environment for 8 hours, followed by drying in an oven at 60° C. As a result of this process, a white gel was obtained. For the sol containing Ca/P ratio required to produce HA, the gel showed a pure (single phase) apatitic structure with a Ca/P ratio of 1.666, identical to stoichiometric HA, after calcining at a temperature as low as 350° C. Varying the Ca/P ratio allows other calcium phosphates, such as dicalcium phosphate (Ca/P=1) or tricalcium phosphate (Ca/P=1.5), to be obtained. A coating produced using this process, and applied to 316 SS substrate, showed adhesive strength of about 40 MPa after curing at a temperature<450° C. The coating was crack-free and porous. Example 2 [0063] In another variant of the process, a pure water-based environment was used. The aqueous-based sols were prepared in the same manner as described above in Example 1 for the ethanol-based system. A higher rate of hydrolysis of the phosphite sol was observed. The mixed sol was dried while stirring. After 8 hours aging, a white gel appeared. For the sol containing a Ca/P ratio required to produce HA an apatitic structure with Ca/P ratio of 1.663, close to stoichiometric HA, resulted after calcining the gel at a temperature of 350° C. Both the ethanol-based and aqueous-based gels showed essentially the same apatitic structure at relatively low temperatures. This invention provides a method of synthesizing the HA ceramics via an aqueous-based sol-gel process. Example 3 [0064] A CaP coating was deposited on the surfaces of a group of electropolished stainless steel stents through aerosol-gel processing. The stents were first treated in 2.4 N phosphoric acid solution for 10 minutes at 70° C. to clean the surface and produce microroughness for increased bonding of the coating. The treated stents were ultrasonically cleaned and dried. The CaP sol was prepared by (a) hydrolysing a phosphor precursor (phosphite); (b) adding a calcium salt precursor to the medium after the phosphite has been hydrolysed to obtain a calcium phosphate sol such as a hydroxyapatite sol. The sol was atomized into ˜ 4 μm large particles using ultrasonically assisted atomizer, and the resulting aerosol fed into a coating chamber. This specific deposition technique is referred to as Aero-Sol-Gels (ASG) deposition and the resulting hydroxyapatite film as ASG-HA. [0065] The clean stent was inserted into the coating chamber filled with flowing CaP aerosol-gel for a period of 30 seconds, while maintaining the aerosol flow at 0.1 liter/min and chamber temperature at 50° C. The temperature of the coating chamber affects the deposition mode of the coating, producing a uniform, film like coverage of the surface as evidenced by SEM. The coating was dried at 60° C. and heat treated at 450° C. for 15 min to crystallize CaP to form hydroxyapatite thin film. The procedure produces a thin coating covering uniformly the surface of the stent. The thickness of the coating is measured using ellipsometry in the range of 50-150 nm. The subsequent SEM studies on the crimped and expanded coated stents show no evidence of cracking or delamination of the coating. This proves the reliability of the uniform, thin continuous CaP coating during the deployment and implantation of the stent into the coronary artery. Example 4 [0066] CaP coating has been deposited on the surface of an electropolished stainless steel stents through aerosol-gel processing (ASG), as described in Example 3. The chamber temperature was maintained at 25° C. The coating was dried at 60° C. and heat treated at 450° C. for 15 min to crystallize CaP to form hydroxyapatite thin film. The procedure explained above produces a coating comprising of isolated island of approximately 2-6 μm in size and 0.1-2 μm in thickness, scattered uniformly on the surface of the stent, and covering about 70% of the surface of the stent, as shown in FIGS. 1A and 1B . Subsequent SEM studies on the crimped and expanded coated stents showed no evidence of cracking or delamination of the coating, as shown in FIGS. 2A and 2B . This proves the reliability of the discontinuous CaP coating of variable thickness during the deployment and implantation of the stent into the coronary artery. Example 5 [0067] Stainless steel metallic substrates (316L) were coated with a 0.6-0.8 μm thin layer of apatite (ASG-HA) as described in Example 3. One group of samples was annealed at 400° C. for 20 min to achieve crystalline SG-HA(C) film and another group at 375° C. for 60 min to achieve amorphous SG-HA(A) film. These films were used as nucleation site for precipitation of BM-HA film. The SG-HA coated samples were immersed into “simulated body fluid” (SBF) of ionic composition (in units of mmol/l) 142 Na + , 5.0 K + , 2.5 C 2+ , 1.5 Mg 2+ , 103 Cl − , 25 HCO 3 − , 1.4 HPO 4 2− , and 0.5 SO 4 2− . The SBF was buffered at pH 7 . 4 with tris(hydroxymethyl)-aminomethane and HCl. This in-vitro static deposition (i.e. the SBF was not renewed during the deposition period) at ˜24° C. produced good quality, dense 3-5 μm thick BM-HA film deposits on flat SG-HA substrates. The crystalline SG-HA(C) film is coated with dense BM-HA, whereas amorphous SG-HA(A) film is coated with porous BM-HA. The properties of the underlying SG-HA surface modification film can be used to vary the properties, e.g. porosity, of the nucleated and deposited top BM-HA film for drug encapsulation. Example 6 [0068] Stainless steel metallic stents (316L) were coated with −0.1 μm thin CaP coatings as described in Example 3. An inorganic colloidal slurry containing calcium phosphate precursor Ca(OH) 2 and calcium phosphate salt monocalcium phosphate anhydrate, was ball milled in ethanol. The two starting inorganic ingredients had particle size 0.3-2 μm and 0.5-4 μm, respectively. The initial Ca/P ratio in the slurry was kept at 1.5. As dissolution and precipitation are the principal mechanisms for apatite development in such system, 5 wt % of submicron, crystalline hydroxyapatite powder was used as seeds for heterogeneous nucleation of CPC-HA. The thin CaP film surface-modified sample was dip coated in the ethanol suspension of the precursors. After single dip coating, an approximately 10 μm thick layer of porous precursor powder mixture developed on the substrate due to rapid evaporation of ethanol. Due to the colloidal nature of the precursors slurry, this film develops sufficient structural integrity (i.e. strength and hardness) to accept the next processing step. In this step, the film is exposed to sodium phosphate water-based solution (0.25 M), which is allowed to soak into the open pores of the film, and then placed in an incubator at 37° C., 100% relative humidity, for 24 h. During incubation, the colloidal precursors react with the phosphate liquid and precipitate HA. In order to assess the possibility of using this double-coating route for controlled drug release, amethopterin (Sigma Chemicals, USA) was employed as a model drug, in an amount of 5% based on solid phase content of CPC-HA precursors. The drug was mixed with the colloidal suspension of the precursors, before dip coating was performed. During incubation period, 20 μm thick CPC-HA coating precipitated encapsulating the drug molecules within the nanopores of the crystallizing HA. After encapsulation, a drug release study was conducted by immersion of the substrates into 20 ml of phosphate buffer saline (PBS, pH=7.4) at constant ratio of (CPC coating weight)/(volume of PBS) of 1 mg/ml. A reference sample coated with hydrogel film was also tested for drug release kinetics. The hydrogel film was prepared by dipping the CPC-HA layer containing the drug into a polymer solution containing 3% polyvinyl alcohol. After drying, the weight gain of the ˜20 mg CPC-HA layer due to the additional hydrogel coating was ˜0.5 mg, corresponding to the content of polymer film in the CPC-HA matrix of about 2.5%. The samples of PBS liquid with released drug were periodically taken out (i.e. entire liquid was emptied) and refilled with the same amount of 20 ml of PBS. The drug concentration in the supernatant was determined via an UV-Visible spectroscopy. Although a burst effect was detected for both coatings over the initial period of about 8 h, a slower release is evident for the sample post-coated with hydrogel. A linear relationship was obtained between the amount of drug released and (time) 1/2 for the release time greater than 8 h. Example 7 [0069] The stent was submerged into water-based, diluted suspension of sub-micron particles of hydroxyapatite, containing approximately 2 wt % of HA in the suspension. DC voltage of 5V was applied to the stent, for times varying from 5 seconds, to 10 minutes. As the particles of HA naturally attain positive charge in such solution, they are attracted to the stent surface which is also a negative electrode (cathode) in this system. The buildup of HA particles attracted to the stent (cathode) allows to produce an extremely uniformly coated surface, thickness of the coating varying as a function of time of application of voltage. The film uniformity is the biggest advantage of such Electro-Phoretic Deposition (EPD) processing, which is difficult to reproduce using other methods such as sol-gel processing. For the short time of 10 sec., the EPD-HA coating thickness is about 1 micrometer. This type of EPD-HA coating on 316L stainless steel stent is illustrated in FIG. 3 . For the longer times of several minutes, the coating thickness may exceed 10 micrometers. Thus, in this EPD process, a controlled thickness, uniform HA film may be produced. The as deposited film constitutes loosely bonded particles of HA, of porosity generally in excess of 50 vol %. In order to increase structural integrity and bonding strength to the substrate of such EPD film, heat treatment is necessary at temperatures at least 500° C., for times at least 10 minutes. The heat treatment of EPD films proceeds at higher temperatures and longer times than sol-gel films, because HA particles deposited in the EPD process are less reactive than those deposited in the sol-gel process. The goal of such heat treatment is to increase interparticle bonding, while providing sufficient residual porosity to maintain low stiffness and flexibility of the film, and to provide room for drug impregnation. The need for higher temperature and longer times heat treatment of EPD films is a disadvantage, as the heat treatment process may adversely affect properties of the metallic substrate of the stent. Example 8 [0070] The HA was deposited on a 316L stainless steel stent surface through EPD process as described in the Example 7. The uniformly deposited EPD film was heat treated at 500° C. for 10 minutes to achieve minimal structural integrity of the film, sufficient to survive handling and preventing re-fluxing of the film upon contact with liquid medium. Such EPD-coated stent was exposed to droplets of sol in the aero-sol-gel process described in Example 3. The sol droplets have penetrated open porosity of the EPD film, and, by capillary attraction, located themselves mostly within negative curvature of the necks between EPD deposited HA particles. Such composite coating was heat treated again at 500° C. for 10 minutes. Now the active sol-gel component of the coating allowed achieving high structural integrity of the film, while EPD component of the coating allowed achieving high uniformity of coverage by the film. A uniform, porous HA film was achieved in this novel combined process. Example 9 [0071] The electrochemical deposition (ECD) of hydroxyapatite HA has been conducted in the mixed aqueous solution of Ca(NO 3 ) 2 4H 2 O and NH 4 —H 2 PO 4 . In this process HA is deposited on the cathodic (negatively biased) surface of stent or implant by the following reaction: 10Ca 2+ +6PO 4 3− +2OH→Ca 10 (PO 4 ) 6 (OH) 2 ECD was conducted in the mixed aqueous solution of 0.02329 M Ca(NO 3 ) 2 4H 2 O and 0.04347 M NH 4 H 2 PO 4 . The stainless steel specimen, i.e. stent, was the cathode, and platinum was used as the anode. The pH was controlled at 4.0 with the addition of sodium hydroxide. The environment temperature was controlled at 40° C.±1° C. The coating morphology deposited at low current density (1 mA/cm 2 ) was a thin uniform porous structure, 1-2 micrometers thick for deposition time of 0.5-1 minute, as illustrated in FIG. 4 . Example 10 [0072] The HA was deposited on a 316L stainless steel stent surface through ASG-HA process as described in the Example 4. The discontinuous network of HA patches left some of the stent surface uncoated. 5V DC bias voltage was applied to such pre-coated stent, and the stent submerged into suspension of submicron HA particles. The uncoated metallic surface of the stent preferentially attracted HA particles leading to preferential electrophoretic deposition (EPD) of HA in these areas, to build the coating about 1 micrometer thick in about 10 seconds. The coated stent was heat treated at 500 C for 10 minutes. The EPD-HA coated areas show increased porosity as compared to ASG-HA coated areas, suitable for impregnation with drug carrying liquid. Such composite engineered HA coating shows unique properties regarding mechanical performance and drug release properties. Example 11 [0073] The HA was deposited on a 316L stainless steel stent surface through ASG-HA process as described in the Example 3, followed by the process of ECD-HA deposition as described in Example 9, but on top of the already heat treated ASG-HA. Such composite engineered coating allowed to achieve substantially higher bonding strength (as compared to ECD-HA deposited directly on metallic surface), and capability of drug encapsulation during deposition of ECD-HA on top of ASG-HA. Example 12 [0074] The HA was deposited on two 316L stainless steel stents surface through ASG-HA process as described in the Example 4. The coated stents were evaluated in the standard thromboresistance test in dogs. Minimal thrombosis with a grade of 1 (defined as thrombus found at one location only) was observed in one out of two test sites. In the second test site, no thrombosis (grade 0) was observed. [0075] The process for coating of calcium phosphate, in particular HA, bioactive ceramics, on implantable medical devices disclosed herein offers the following advantages in comparison to other processes and other coating materials on implantable medical devices: (1) The coating process, including CaP sol synthesis, can be completed in ambient environment (i.e. air), in less than 24 hours. (2) The thin (<0.001 mm) adhesive CaP coatings exhibit sufficient flexibility to survive substantial strain, e.g. during crimping and expanding of a coated stent, without coating damage or spallation (3) Porous CaP coatings can be produced, with controlled amount and size of the pores, which allows design flexibility in choice and absorption/release characteristics for the drug impregnated into the coating (4) The synthesis requires low temperature (˜350° C.) and short time (<1 hour) of calcination for formation of high quality, highly adhesive CaP coating. Low temperature calcination of the novel CaP coatings on metals permits thermal treatment in an air environment without the risk of metal oxidation and possible property degradation due to microstructural deterioration or phase transformations. [0080] It will be clear for the person skilled in the art of sol-gel processing that coating deposition parameters, such as time, the flow rate of the aerosol, temperature of the coating chamber or the concentration of the sol-gel solution can be customized for different implantable medical device materials and applications producing various degree of coverage on the surface. Similar manipulation and optimization of process parameters may be applied to other coating methods disclosed, i.e. dip- and spin-coating and electrophoresis, biomimetic coating, electrochemical deposition coating, calcium phosphate cement coating, electrophoretic deposition coating, as well as coating porosity distribution and ratio of the inorganic phase (CaP) to organic phase (biodegradable polymer). These parameters were optimized for the particular CaP coatings on the implantable medical devices described in the foregoing examples. [0081] It is well known that crystallinity and microporosity of hydroxyapatite directly affects its dissolution rate in body fluids. Different heat treatment regimes and temperatures can be adopted to produce various degrees of crystallinity and microporosity to control the degradation of the coating into the body environment. This advantage is of a great importance where drug delivery capabilities are added to the implantable medical device surface coated with sol-gel derived CaP. Similar deposition process can be applied to coating other metallic surfaces, such as Ti substrates or other alloys, such as Cobalt-Chromium-Nickel-Molybdenum-Iron. A thin uniform thin HA coating is obtained. The results of this experiment provide basic evidence of the feasibility of the as described coating on implantable medical devices composed of non-metallic materials such as polymers. [0082] The nature of the process for CaP coatings deposition according to the invention is such that it can be easily incorporated into the current production practice of metallic implantable medical devices. The water-based liquid precursors to CaP ceramic coatings, simple deposition technique (e.g. dipping or spin-coating or aerosol deposition or electrophoretic deposition, and others) and low-temperature heat treatment in air make the process not unlike simple painting-curing operation which can be commercialized with relatively small effort. [0083] As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

This invention relates to novel calcium phosphate-coated implantable medical devices and processes of making same. The calcium-phosphate coatings are designed to minimize the immune response to the implant (e.g. restenosis in stenting procedures) and can be used to store and release a medicinally active agent in a controlled manner. Such coatings can be applied to any implantable medical devices and are useful for a number of medical procedures including (but not limited to) balloon angioplasty in cardiovascular stenting, ureteral stenting and catheterisation. The calcium phosphate coatings can be applied to a substrate as one or more coatings by a sol-gel deposition process, an aerosol-gel deposition process, a biomimetic deposition process, a calcium phosphate cement deposition process, an electro-phoretic deposition process or an electrochemical deposition process. The coating can contain and elude a drug in an engineered manner.

This is a division of application Ser. No. 465,354 filed Feb. 9, 1983 now U.S. Pat. No. 4,523,695. BACKGROUND OF THE INVENTION The present invention relates to a surgical stapler for applying staples to suture or close a wound or incision, particularly a surgical skin stapler for implanting skin staples in or through the skin to suture an exterior wound or incision. Surgical staplers are used for closing or connecting conformed wound edges of tissue by implanting metal staples in the tissue. By actuation of a lever, the staple is pressed by a ram or driver against an anvil surface provided at the tip of the stapler tool and is thereby deformed, so that the parts of the staple protruding from the stapler tip are moved toward each other and penetrate into the tissue. U.S. Pat. No. 4,179,057 discloses a surgical stapler comprising a staple magazine containing a supply of staples, a spring for advancing the staples in the staple magazine, an anvil surface provided at the stapler tip, and a driver displaceable relative to the anvil surface in a staple channel which deforms a staple supported on the anvil surface. In a stapler of the type disclosed in the aforementioned patent, the staples are advanced along a straight feed path in the staple magazine. The forwardmost staple lies in the path of movement of the driver which extends at an angle which appears to be about 50° with respect to the longitudinal axis of the staple magazine. The stapler is actuated in plier fashion to advance the driver which presses the forwardmost staple protruding from the stapler tip against the anvil surface and deforms it to close the staple side portions. At this point, the staple has been implanted and it is necessary to remove from the staple the anvil surface which is fixed to the stapler tip. However, if the stapler has been improperly positioned, it is possible to pull the closed staple out of the tissue when disengaging the anvil surface from the implanted staple. U.S. Pat. No. 4,202,480 discloses a surgical stapler which also comprises a staple magazine having a straight staple feed path. The staple channel in the stapler in which the driver is displaceable and the staple magazine meet at almost a right angle. The forwardmost staple is advanced by the driver to the anvil surface on which it is deformed with its side portions protruding forwardly of the stapler tip. The anvil surface is transversely disposed at the forward end of the staple channel. It is also difficult to pull the anvil surface of this stapler out of an implanted staple. U.S. Pat. No. 3,819,100 discloses a surgical stapler comprising a removable staple cartridge which is inserted into and locked to the stapler. The staple cartridge has a straight staple feed path. Staples are advanced by a driver moved by a stepping mechanism. The forward housing portion of the stapler, into which the staple cartridge is inserted, is rotatable relative to the rear housing portion. The anvil surface is fixed at the front end of the staple cartridge. Prior art surgical staplers have the disadvantage that they did not afford a good view of the work area because the driver moved transversely to the straight staple magazine. Therefore when the stapler was positioned for use, a considerable portion of the work area was obscured. While it is possible to arrange and feed the staples laying flat one behind the other in order provide a slim tool tip affording a better view of the work area, the cost of manufacturing the parts required to accomplish this is high. Moreover, the number of staples that can be accommodated in a staple magazine if the staples lie flat one behind the other is relatively small. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the present invention to provide a surgical stapler, particularly a skin stapler, which eliminates the possiblity of tearing an implanted staple out of the tissue or substantially disturbing it when the anvil is separated from the staple, particularly if the stapler was improperly positioned. The above and other objects are achieved according to the invention disclosed herein which provides a surgical stapler having an anvil surface or nose movable transversely with respect to a staple channel between an operating position and a retracted position, and in which movement of the anvil surface is controlled as a function of the position of a driver in the staple channel which cooperates with the anvil surface to deform a staple. According to the invention, movement of the anvil surface is coupled with that of the driver. When the die is moved into its operating position, the anvil surface is also automatically brought into its operating position in which it protrudes into the staple channel in which the driver moves. Advancement of the staple in the channel is stopped by the anvil surface, and deformation of the staple takes place between the anvil surface and the driver. When the die is subsequently retracted, the anvil surface moves automatically into its retracted position, so that the closed staple does not interfere with removal of the stapler instrument. According to the invention, the anvil surface and the driver are brought into their operating positions together, the driver moving longitudinally in the staple channel while the anvil moves transversely to the staple channel. Further objects of the present invention are to provide a stapler, particularly a skin stapler, whose tool tip is narrow and in which the staples are arranged and fed upright one against the other so that the staple magazine including the advancing mechanism can be relatively simple and yet the tool tip can be narrow, thereby covering up as little of the work area as possible. These and other objects are achieved in accordance with the invention by providing a staple magazine which extends essentially parallel to the staple channel and having at its forward end a curved section opening into the staple channel. According to the invention, the staples are disposed in the magazine parallel to each other standing upright so that the side and the base or crown portions of adjacent staples are in contact, and are advanced by a spring. Since the forward end of the staple magazine is curved where the staple magazine opens into the staple channel, the forwardmost staple enters the staple channel in which the driver moves lying flat in the staple channel. When the die is moved to its operating position, it blocks the opening of the magazine into the staple channel so that the next staple can be advanced into the channel only after the driver has been brought back into its retracted position. Therefore, only the forwardmost staple in the magazine can be engaged by the driver as the driver is moved past the magazine opening. According to a preferred embodiment of the invention, the anvil surface is fastened to a leaf spring which extends in the staple channel and includes an inclined surface. The driver includes a projection which cooperates with the inclined surface so that when the projection strikes the inclined surface, the leaf spring is deformed in such a way that the anvil surface is brought into its operating position. Upon release of the driver, the tension of the deformed leaf spring is released to automatically return the anvil surface into its retracted position. For skin staplers precise guiding of the staple during the staple closing process is very important because the staple is closed as it emerges from the staple channel at the tip of the tool. According to a preferred embodiment of the invention, a notch or slot for retaining the base or crown portion of the staple during the deformation process is disposed in the anvil surface. In the initial phase of deformation, a projection or bulge in the base of the staple penetrates into the notch or slot, so that the staple is prevented from turning or pivoting. Preferably the notch or slot is located in the center of the anvil surface and the projection or bulge is symmetrically disposed in the staple. The notch or slot edges preferably dig into the staple and bring about an interlocking of the staple and the anvil surface in the central portion of the base region of the staple. According to a preferred embodiment of the invention, the staple channel comprises side, upper and lower guide surfaces which limit movement of the forwardmost staple as it is advanced lying flat in the staple channel. The guide surfaces extend forwardly to beyond the anvil surface. An embossment positions the forwardmost staple in the staple channel upon being advanced from the magazine. From there, as the driver is advanced towards its operating position, the staple is transported to the anvil surface and feeding of additional staples from the magazine is blocked. The guide surfaces provide a well-defined advance of a staple in the channel. Preferably the guide surfaces are extended in projections of relatively small dimensions protruding forwardly beyond the anvil surface. A two-part housing comprising a rear housing portion and a front housing portion which is rotatable relative to the rear housing portion facilitates use of the stapler. The rear housing portion contains the actuating mechanism for the driver and the front housing portion contains the driver and anvil surface which are rotatable together with the front housing portion relative to the rear housing portion. By making the front housing portion rotatable relative to the rear housing portion, the orientation of the staple relative to the actuating mechanism can be selected freely. Hence the physician need not align the actuating mechanism transversely to the wound or incision seam but can hold the instrument in the position most favorable for working the instrument. It is important that the stapler be actuated with little effort since the instrument can only be held steady and firmly, which is required for precise setting of the staples, if the staples can be deformed and implanted with little physical force. To achieve this, the actuating element of the actuating mechanism and a lever in the rear housing portion, and the driver are coupled in such a way that the effective leverage of the lever increases as the actuating element moves further away from its inoperative position while at the same time the advancing force transmitted to the driver increases for a constant actuating force at the actuating element. In the first phase of actuation of the actuating element, the forwardmost staple of the staple magazine is simply advanced in the staple channel until it reaches the anvil surface. In this first phase the force required is relatively low. However, the maximum force that is available is required when the staple is being deformed and this maximum force occurs when the actuating element reaches its maximum travel. The amount of force required to deform the staple is reduced by the actuating mechanism disclosed herein so that it is possible to deform the staple simply by moving the actuating element with one's index finger. Compared with known staplers, the actuating force required to operate the stapler disclosed herein is reduced by about one half. It is possible to positively couple the movement of the driver with the lengthwise movement of a slide coupled to the actuating element. However such coupling of the driver to the actuating element would be disadvantageous because the driver would follow every movement of the slide and it is possible that a second staple could enter the channel without the first staple having been deformed and released if the driver is not fully advanced to its operating position. To avoid this, according to the invention, the driver and slide are not positively coupled. Instead means are provided so that the driver is not retracted by the slide unless the driver has been advanced to its operating position. According to a preferred embodiment of the invention, a slide coupled to the actuating element is provided which includes a tongue loaded with a transverse spring action which cooperates with a control cam disposed in the housing. The tongue includes a surface which is positioned against a transverse edge of the driver and permits the driver to be retracted only after the driver has been advanced fully into its operating position. Only then can the driver be retracted and the opening of the staple magazine into the channel cleared so that the next staple can be advanced. According to a preferred embodiment of the invention, a counting mechanism is provided which is advanced by a projection on the tongue of the slide. The counting mechanism indicates the number of staples used or the number of staples remaining in the magazine. The above and other objects, features, aspects and advantages of the invention will be more readily perceived from the following description of the preferred embodiments thereof when considered with the accompanying drawings and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS The present invention is illustrated by way of example and not limitation in the figures of the accompanying drawings in which like numerals indicate similar parts and in which: FIG. 1 is a schematic, longitudinal section view taken through a stapler according to the invention; FIG. 2 is a longitudinal section view taken through the tip portion of the stapler of FIG. 1 depicting the anvil surface in its retracted position; FIG. 3 is a view similar to that of FIG. 2 depicting the anvil surface in its operating position; FIG. 4 is a section taken along line IV--IV of FIG. 2; FIG. 5 is a section along line V--V of FIG. 3; FIG. 6 is a vertical section view taken through the magazine portion of the stapler of FIG. 1; FIG. 7 is a side schematic view of a portion of the stapler of FIG. 1 illustrating the cooperation of the slide of the actuating mechanism and the driver as the driver is advanced; FIG. 8 is a side schematic view similar to that of FIG. 7 illustrating the cooperation of the slide and the driver of the stapler of FIG. 1 shortly before the driver is retracted; FIG. 9 is a plan schematic view of structure depicted in FIG. 8; FIG. 10 is a vertical section view of a stapler tip including a counting mechanism according to another embodiment of the invention; FIG. 11 is a vertical section view of a part of the rear housing of a stapler according to another embodiment of the invention depicting the actuating mechanism thereof in the retracted position of the slide; and FIG. 12 is a view similar to that of FIG. 11 depicting the actuating mechanism in the feed position of the slide. DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the invention are illustrated and described in connection with a stapler for applying staples to an exterior wound or incision across a layer of skin, although the invention is not limited to such a surgical stapler. The embodiment of the stapler illustrated in FIGS. 1-9 comprises as depicted in FIG. 1 a rear housing portion 10 and a front housing portion 11. The front housing portion 11 is mounted to the rear housing position for rotation of the front housing portion about its longitudinal axis. The mechanism 12 for actuating the stapler is contained in the rear housing portion 11. A slide 13 which advances a driver 15 is guided in the front housing portion 10 for longitudinal displacement but is prevented from rotating. The slide 13 comprises a forwardly projecting flexible tongue 14 which also cooperates with the driver 15, as described more fully below. The driver 15 comprises an elongated rigid strip of material which is displaceable in its longitudinal direction in a channel or duct 16. The strip has a central recess 17 (FIG. 4) at the forward end of which is disposed a bent-up section 18 having an enlarged head. A leaf spring 19 extends in the channel 16 substantially parallel to the driver 15. The leaf spring 19 has an inclined surface 20 (FIG. 2) and is provided with a central slot 21 (FIG. 4) closed on all sides which extends forwardly and rearwardly of the region of the inclined surface 20. The enlarged head of the bent section 18 of the driver 15 protrudes through the slot 21 and is pressed against the upper side of the leaf spring 19. The forward end of the leaf spring 19 is bent downwardly to form the anvil surface 22. The rear end of leaf spring 19 is fixed to the front housing portion 11. When the driver 15 is in its retracted position, as depicted in FIG. 2, the bent section 18 is positioned at the base of the inclined surface 20. Due to the inherent tension in the front region of the leaf spring 19, the leaf spring positions itself in the channel 16 as depicted in FIG. 2. Since the height of the channel 16 is greater than the height of the anvil 22, there is a clearance between the anvil 22 in its retracted position and the lower region 16' at the front of the channel 16. A staple magazine 26 (FIGS. 1 and 6) extends parallel to the channel 16 in the front housing portion 11. Staples 24 are arranged in the magazine standing upright side by side and extending along the feed passage of the magazine parallel to the channel 16. A helical spring 25 braced against the housing contacts the rearmost staple and urges the rearmost staple and with it the entire stack of staples forward under constant tension. The forward section 26' of the staple magazine 26 is curved upwardly at an angle of 90° and opens into the channel 16. The staples are urged into the curved section 26' of the magazine and extend along the arc of the curve as depicted in FIG. 6, with the forwardmost staple 24' being disposed lying flat in the channel 16. The lower region 16' in the forward portion of the channel 16 in which the leaf spring 19 can move vertically is of greater width than the region above it. The height of the wider, lower channel portion 16' is only slightly greater than the thickness of the staples 24 so that channel portion 16' forms a guide channel for the advance of the forwardmost staple 24' and for the driver 15. This guide channel is defined by the lower guide face 16a, the two upper guide faces 16b, (FIG. 6), and by the lateral guide faces 16c (FIG. 4). The staples 24, whose undeformed configuration is depicted in broken lines by the staple 24' in FIG. 5, have arcuate side portions 24a connected via a straight leg region 24b to a central base or crown portion 24c. The straight leg regions 24b extend obliquely outwardly from the base portion to the side portions 24a. The base portion 24c is semicircular with the circumference of the semicircle facing in the direction of the side portions. The base portion is engaged by the anvil surface 22 during forward motion of the staple. In order to insure centering of the staple 24, the anvil surface 22 is provided with a vertical slot 22'. In the arcuate section 26' of the staple magazine 26, the side portions 24a of adjacent staples 24 are spaced apart while the straight leg regions 24b are in contact with adjacent leg regions due to the difference in radii of the curves for the upper and lower surfaces of the arcuate section 26'. Thus, the force of the spring 25 can be transmitted through the staples in the arcuate section 26' to the forwardmost staple 24'. At the opening 27 (FIG. 5) of the magazine 26 into the channel 16, the underside of the upper guide face 16b is embossed (not shown) to hold the forwardmost staple 24' in a well-defined position. As the driver 15 is advanced from retracted position shown in FIG. 2; its front end abuts the forwardmost staple 24' and pushes it forward in the channel section 16'. At the same time, the bent section 18 moves along the inclined surface 20 of the leaf spring 19 so that the anvil surface 22 at the forward end of the leaf spring is brought from its retracted position into the operative position shown in FIG. 3. The staple designated 24" in FIGS. 3 and 4 is now situated between the forward end of driver 15 and the anvil surface 22 in a position in which the tips of the staple side portions protrude slightly forwardly from the instrument. As the driver 15 is advanced further, the staple side portions emerge from the front end of the instrument, with staple 24" being deformed and closed to the solid line configuration depicted in FIG. 5 in which the base 24c of the staple has been bent flat on the inner side of the anvil surface 22. To obtain as long a guide path as possible during deformation of staple 24", the guide faces 16a, 16b and 16c extend into projections 28 which define the exit gap of channel 16 out of the housing and which protrude slightly beyond the anvil surface 22. As soon as the driver 15 has carried the forwardmost staple 24' away from the opening 27 of the magazine into the channel, the opening 27 is closed by the driver so that the next staple cannot be advanced into the channel 16. The next staple can only be advanced into the channel after the driver 15 has returned to its retracted position where it is clear of the opening 27. FIGS. 7-9 illustrate control of the driver 15 by the slide 13. Slide 13, which is supported to the front housing portion 11 for longitudinal displacement but is prevented from rotating, comprises at its forward end a forwardly projecting, flexible tongue 14 which is vertically springloaded. A laterally projecting guide wing or cam surface 30 is disposed at the end of the tongue 14 and cooperates with a control cam 31 fixed to the housing portion 11. When the slide 13 is advanced by the actuating mechanism 12, its front face strikes driver 15, pushing it in the direction of the tool tip. A bevel formed on wing 30 causes wing 30 to abut on a rearward bevel of the control cam 31. The tongue 14 then flexes upwardly and wing 30 slides on the upper cam surface 32. If the slide 13 is retracted before its forward end position is reached corresponding to the operating position of the driver, the wing 30 slides back on to the upper cam surface 32, which maintains the slide and correspondingly the driver in the advanced position they assumed. Only after the slide 13 reaches the position shown in FIG. 8 and the wing 30 has gone beyond the front end of the control cam 31 is the stamping operating completed and staple 24" closed. As the slide 13 is thereafter being moved back, the rear surface of the wing 30, which is inclined, contacts the correspondingly inclined forward surface of the control cam 31. As a result, the tongue 14 is forced downward, and a projection of the tongue 14 enters into the slot 17 of the driver 15. As the slide 13 is further retracted, the wing 30 is pulled beneath the control cam 31, and the driver 15 is drawn rearward. Referring to FIG. 7, after the wing 30 has passed along the underside of the control cam 31, the tongue 14 springs upward, releasing the driver 15 at its starting position. Until the driver 15 is pulled back to its starting position, it does not clear the opening 27 of the staple magazine 26 into the channel 16. FIG. 10 depicts an embodiment in which a counting mechanism 33 is secured to the front housing portion 11. The counting mechanism is stepped by movement of the tongue 14 of the slide 13. The counting mechanism 33 comprises a hollow cylinder 34 fixed in the housing portion 11 in which is rotatably mounted a cylinder 35 having ratchet teeth 36 disposed about the periphery of the lower end thereof. A projection 37 disposed at the front end of tongue 14 engages the teeth 36 when the wing 30 is raised by the guide cam 31 during a feed movement. In this manner the cylinder 35 is rotated towards the forward end of the instrument by a predetermined angle with each feed movement of the driver 15. The top of the cylinder 35 is provided with a mark and the periphery of the hollow cylinder 34 is provided with a scale so that the mark indicates on the scale the number of staples 24 remaining in the magazine 26. At the rear end of the front housing portion 11 is disposed a cylindrical bushing 40 (FIG. 1) in which slide 13 is coaxially mounted. The cylindrical bushing 40 can be removed from the rear housing portion 10 so that the magazine can be loaded with staples. The rear end of the slide 13 is coupled to a part 39 slidably movable along a track 42 in the interior of the rear housing portion 10. The sliding part 39 includes a sleeve 43 disposed about a shank 44 of the slide 13 which is bounded on both sides by flanges. The sliding part 39 is provided with a rack 45 having teeth or serrations which are engaged by corresponding serrations on a toothed disc segment 46. The toothed disc segment 46 forms one lever arm of a two-armed lever which pivots about a pivot pin 47 in the housing portion 10. The other lever arm 48 is engaged by a pin 49 disposed in a transverse slot 50 of a trigger lever 51. The trigger 51 is guided in a recess 52 of the handle 53 extending approximately parallel to channel 16, and is urged outwardly of the handle by a spring 54. Trigger 51 is dimensioned so that it can be actuated with the index finger when the handle 53 is gripped. The trigger, upon being pushed into the handle 53, causes the lever 46, 48 to be pivoted about the pivot pin 47 so that the sliding part 39 is advanced forwardly, and with it slide 13. Near the end position of the lever 46, 48 where it extends almost at right angles with the slide 13, leverage is the greatest, and corresponds to the stamping action of the driver. Thus, for a constant actuating force, the maximum force applied to the driver occurs during stamping. An actuating mechanism 12' similar to mechanism 12 of FIG. 1 is illustrated in FIGS. 11 and 12. FIG. 11 depicts the retracted position of the slide 13 and FIG. 12 its advanced position. Spring 54 urges the trigger 51 out of the handle 53 and at the same time brings the sliding part 39, and with it the slide 13, into the retracted position. In the embodiment of FIG. 1 the transverse slot 50 of the trigger 51 has an angular shape, while in the embodiment of FIGS. 11 and 12, the transverse slot 50 is straight. Certain changes and modifications of the embodiments of the invention disclosed herein will be readily apparent to those skilled in the art. It is the applicants' intention to cover by their claims all those changes and modifications which could be made to the embodiments of the invention herein chosen for the purpose of disclosure without department from the spirit and scope of the invention.

A stapler, particularly for suturing skin wounds or incisions, is disclosed which comprises a channel in which a driver is advanced by a slide in the direction of an anvil surface. A staple magazine which extends substantially parallel with the driver includes a curved section which opens into the channel to deliver staples into the channel for engagement by the driver. During forward displacement of the driver, a projection on the driver presses a leaf spring to which the anvil surface is connected. The anvil surface at the forward end of the leaf spring is thereby brought into its operating position and is automatically moved back into its retracted position upon release of the spring after the driver is retracted. The curved section in the staple magazine enables the stapler to have a slim profile which does not obscure the working area during a stapling operation. After completion of a stapling operation, the anvil surface is automatically retracted from a closed, implanted staple.

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] The present application claims the benefit of U.S. Provisional Patent Application No. 61/838,553, which was filed on Jun. 24, 2013, and which is incorporated by reference herein. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH [0002] The present invention was supported in part by funds from the U.S. government (i.e., NIH Grant No. RO3NS058595, NIH Grant No. R15 NS074404, and the Office of the Assistant Secretary of Defense for Health Affairs through the Peer Reviewed Orthopaedic Research Program under Award No. W81XWH-13-02301), and the U.S. government may therefore have certain rights in the invention. FIELD OF THE INVENTION OR TECHNICAL FIELD [0003] The present invention relates to the field of nerve regeneration, in particular to nerve conduits for the regeneration of peripheral nerves. BACKGROUND OF THE INVENTION [0004] In the United States, each year more than 700,000 people suffer from peripheral nerve injuries (PNI) that can lead to a lifelong disability, such as paralysis. The most frequent causes include motor vehicle accidents, gunshot wounds, stabbings, and birth trauma. [0005] Currently, there are two gold standard treatments for nerve repair, which are end-to-end suturing and application of autograft or allograft biological tissue. However, each strategy suffers from a number of limitations. For example, end-to-end suturing cannot be performed when the nerve gap is larger than 1 cm. The use of autograft results in potential donor site morbidity for the patient and can potentially exacerbate the condition. The use of allograft tissue has an associated risk of immunogenicity. [0006] Recent advances in tissue engineering and biomaterials suggest that there may be other approaches to nerve repair and regeneration that may overcome the limitations associated with harvesting natural tissues. One such approach would be the use of biomaterials to produce natural or synthetic nerve guidance conduits (NGCs). These NGCs may overcome some of the limitations of nerve autograft and allograft methods. The NGCs act as an essential precursor for nerve repair, since they can reduce tension at the suture line, can protect the regenerating axons from the infiltrating scar tissue, and can exhibit a low immune response. Although FDA-approved tissue engineered nerve devices have been available in the market for several years, these implant devices do not possess the proper physical topography or chemical cues for nerve repair and regeneration. Also, most of them are currently limited to a critical nerve gap of approximately 4 cm. To design an optimal NGC for enhancing PNR still remains a challenge. [0007] Current laboratorial NGCs developed using haptotactic strategies alone are not yet comparable to autograft. For example, multichannel NGCs may have an insufficient cross sectional area and or inhibit cell-cell interaction between each of the individual channels. This may lead to functional mismatches and an insufficient level of regeneration. Controlling the position of inner filament bundles within NGCs has yet to be achieved, despite the fact that the presence of microfilaments has been demonstrated to enhance axonal regeneration and provide contact guidance for the regenerating axons in rats. Alternatively, microfilaments can mislead cell migration which can result in uneven distribution of cells within the NGC. These failures in NGCs may be attributed to the inadequate design of intra-luminal guidance channels/filament, forming incomplete fibrin cables during the initial stages of regeneration. Without the formation of this aligned bridge of extracellular material (ECM), further mechanisms for nerve repair are limited. Therefore, it still remains a challenge to design an optimal NGC for enhancing PNR, when compared to the use of autografts. SUMMARY OF THE INVENTION [0008] An embodiment of the present invention provides a fabricated implantable NGC. In some embodiments, the NGC comprises an inner spiral structured porous sheet. Such conduits have the potential to serve as medical devices to treat PNI and restore function to the site of the injury. This may be achieved by the spiral structure's ability to facilitate regeneration of nerve tissues. [0009] In another embodiment of the present invention, the NGC has an integrated spiral structured porous sheet decorated with surface channels. Such a structure increases the surface area available for cell migration and attachment, and may reduce the length of time needed for recovery. Additionally, such a structure can reduce the wear and tear that is often observed with single lumen tubular NGCs. A highly-aligned set of electrospun fibers are present within the surface channels and on the backs thereof. The presence of aligned fibers in such areas ensures that the regenerating nerve will come into contact with aligned fibers. In order to place and suture the nerve tissue without tension, there are two reserved chambers at the proximal and distal end of the conduit. The chambers allow for nerve stumps to be sutured without tension due to the fact that the chambers provide space to house the nerve in place with an optimal grip. A dense layer of randomly-oriented fibers on the outside of the spiral structure can greatly improve the mechanical properties of the NGC and provides integrated structural support for nerve regeneration. The spiral conduit is tunable such that its length and diameter can be varied controllably depending on how it is to be used. The length and the outer diameter of the conduit depend on the size of its intermediate sheet, which is the spiral structured porous layer of the NGC. The method of fabricating the conduit does not limit its length, thus enabling the application for longer gap repair/regeneration for PNI. BRIEF DESCRIPTION OF FIGURES [0010] FIG. 1 is a schematic illustration in cutaway view of a nerve guidance conduit (NGC) according to an embodiment of the present invention bridging the stumps of a damaged nerve; [0011] FIG. 2 is a schematic end-on cross-sectional view of the NGC of FIG. 1 ; [0012] FIG. 3 is a scanning electomicrograph (SEM) image of a first side of a portion of a porous polymeric sheet of a type used to fabricate NGCs according to an embodiment of the present invention; [0013] FIG. 4 is an SEM image of the side opposite the first side of the porous polymeric sheet of FIG. 3 ; [0014] FIG. 5 is an SEM image of a porous polymeric sheet having aligned nanofibers thereupon according to an embodiment of the present invention; [0015] FIG. 6 is an SEM image of a porous polymeric sheet having randomly-distributed nanofibers thereupon; [0016] FIG. 7 is a stereomicroscopic image of the exterior of a second NGC; [0017] FIG. 8 is stereomicroscopic image of the NGC of FIG. 7 after being sectioned longitudinally; [0018] FIG. 9 is a stereomicroscopic image of an end-on view of the NGC of FIG. 7 ; [0019] FIG. 10 is an SEM image of surface channels on a polymer sheet of a type used to fabricate an NGC according to an embodiment of the present invention; [0020] FIG. 11 is a schematic diagram of a polymer sheet of the type shown in FIG. 10 ; [0021] FIG. 12 is a group of stress-strain plots generated from tests performed on various NGCs which are embodiments of the present invention; [0022] FIG. 13 is a bar chart comparing cell proliferation on various NGCs which are embodiments of the present invention; [0023] FIG. 14 is a plot showing changes in sciatic functional index (SFI) over time for rats having implanted NGCs according to embodiments of the present invention; [0024] FIG. 15 is a bar chart of compound muscle action potentials (CMAP) for rats having implanted NGCs according to embodiments of the present invention; [0025] FIG. 16 is a bar chart of nerve conduction velocities (NCV) for rats having implanted NGCs according to embodiments of the present invention; [0026] FIG. 17 is a bar chart of percent of neural tissue regenerated in sciatic nerves bridged by NGCs according to embodiments of the present invention; [0027] FIG. 18 is a bar chart comparing muscle weight ratios for the gastrocnemius muscle of rats for which the sciatic nerve was bridged by NGCs according to embodiments of the present invention; [0028] FIG. 19 is a bar chart comparing muscle fiber diameter for the gastrocnemius muscle of rats for which the sciatic nerve was bridged by NGCs according to embodiments of the present invention; and [0029] FIG. 20 is a bar chart comparing muscle fiber coverage for the gastrocnemius muscle of rats for which the sciatic nerve was bridged by NGCs according to embodiments of the present invention. DETAILED DESCRIPTION OF THE INVENTION [0030] Embodiments of the present invention provide NGCs with integrated spiral structured porous sheets decorated with surface channels and electrospun fibers. Such NGCs provide superior mechanical strength compared to NGCs in the prior art, along with integrated multiple channels, stable aligned fibrous layers, good inter-cell communication, and high surface/volume ratios within the NGCs. Chambers at the distal and proximal ends of the NGC provide additional space for fitting nerve stumps in order to reduce the tension at the suture line between the NGC and the nerve stump. A dense outer fibrous tube on the outside of the spiral structured porous sheet can prevent the infiltration of scar tissue while the regeneration process takes place. One embodiment of the NGC of the present invention comprises a three-dimensional (3-D) spiral structured porous sheet having two chambers at the ends thereof. The spiral structure includes a highly porous polycaprolactone (PCL) sheet, which may be formed as a spiral-wound sheet using known methods and decorated with surface channels on a surface of the spiral wound sheet, coated with a thin layer of aligned electrospun fibers on the surface channels, and a dense randomly-oriented fibrous tube on the outside of the NGC. Other bioresorbable materials known for use in the biomedical arts may be used in place of PCL for the sheet and fibers (e.g., collagen/PCL blends for the fibers). [0031] Other embodiments of the present invention provide a process for fabricating an implantable NGC, such as the embodiment of an NGC described above, which can be used as a medical device for facilitating the repair and regeneration of nerve tissues. [0032] Several features of NGCs according to embodiments of the present invention are discussed herein below. [0000] 1. Three-Dimensional (3-D) Integrated Spiral Structured Porous Sheet with Proximal and Distal Reserved Chambers [0033] Collagen tubes, which have been approved by the FDA, lack sufficient mechanical strength to support nerve regeneration. As for multi-channel NGCs, the major drawback is that cells/axons in each channel do not interact well with those in the other channels, which adversely affects nerve regeneration and would affect nerve function recovery even if the nerve gap were bridged. In comparison, the integrated spiral structure makes the NGC of the present invention superior to those in the prior art in that mechanical properties are greatly improved and favorable for inter-cellular interaction and neural myelination. This is important for nerve regeneration because of the time required for nerve regeneration to bridge large nerve gaps. Further, a NGC should have enough mechanical strength to provide structural support to the nerve during regeneration. Also, the proximal and distal chambers in the ends of the NGC provide an optimal initial environment for nerve ingrowth. These chambers can prevent stress from accruing when the nerve tissue is sutured with the conduit in an end-to-end fashion. Moreover, the increased surface/volume ratio and the highly porous intermediate layers of the PCL sheet are preferred for cell attachment and nutrient transportation during nerve regeneration. [0000] 2. Decorated Surface Channels on the Spiral Porous Sheet with Additional Electrospun Aligned Fibers and and an Outer Fibrous Tube [0034] Electrospinning is an approach for polymer biomaterial processing that provides an opportunity to control morphology, porosity and composition of an NGC using relatively unsophisticated equipment. Unlike conventional fiber spinning processes that produce fibers with diameters in the micrometer range, electrospinning is capable of producing fibers in the nanometer diameter range, which are typically deposited in the form of nonwoven fabrics. Nanofibers provide a connection between the nanoscale and the macroscale world, since, although their diameters are in the nanometer range, the fibers are very long, sometimes having lengths of the order of kilometers. A major problem of all hollow tubes is misdirection of cellular migration: since transected axons produce axon sprouts proceeding in a distal direction, a neuroma is always formed which consists of minifascicles proceeding in an abnormal manner, proliferating Schwann cells (SCs), fibroblasts and capillaries. If there is a directional factor of any kind (e.g., an artificial nerve tube which usually provides no endoneurial structure), the neuroma proceeds in the desired direction. This phenomenon has been called “neuromateous neurotization”. In consequence, only a few dispersed axons are able to enter the right fascicle and endoneurial tube in the distal nerve stump once they have reached the end of the conduit in the interior of the NGC. [0035] One successful tissue engineering strategy for nerve repair is to create aligned features on the conduit to provide guidance for cell migration and directional axonal regeneration across the glial scar and lesion site in both central nervous system and peripheral nervous system injuries. Such features may include aligned surface channels and electrospun fiber-based conduits for nerve repair, according to embodiments of the present invention. [0036] Consequently, the construction of a spiral structure conduit with highly aligned surface channels and nano-fibers is very helpful for nerve proliferation and neurite extension. Meanwhile, the intricate aligned structure can also influence the growth and distribution of seeded SCs, which further directs the longitudinal extension of the neural axons. Further, there is a wide range of polymers available that are suitable for deposition on the spiral sheet to meet the individualized specifications for the NGC (e.g., collagen/PCL copolymer nanofibers, rather than pure PCL sheets). [0037] Fibers spun along the outside of the NGC not only assist in stabilizing the spiral structure, but also inhibit infiltration of scar tissue through the inter-connective pores. By increasing the mechanical strength of the NGC, the risk of structural failure can be minimized, promoting more uniform and natural regeneration of nerve tissue. Tunable Features of the NGC [0038] In order to solve the conflict between optimizing the mechanical properties of the NGC and maximizing its length, many techniques may be used to reinforce the NGC. In a method according to an embodiment of the present invention, a spiral conduit (e.g., a spiral structured porous sheet) is placed onto a rotator and a nanofiber is spun in random orientations along the spiral structure to form an outer fibrous tube. The thickness of the outer fibrous tube can be controlled. This dense layer of randomly-oriented fibers deposited on the outside of the spiral conduit can improve the mechanical properties of the entire structure, and meanwhile provide a stable structural support during nerve regeneration. In a method according to an embodiment of the present invention, depositing the outside layer of fibers on the spiral conduit is the final and separate step of fabricating the NGC, so it is practical to modify the polymers used to form the fibers before the electrospinning step. The outer fibrous tube can be made from polymers that are different from that of the spiral sheet or the aligned fibers. [0039] In another aspect, the process of the present invention is tunable in that the sizes of the spiral conduit are controllable, and both the length and the outside diameter are dependent on the size of the spiral-wound sheet. Therefore, in order to fabricate a spiral conduit with a particular size, (e.g., a length larger than 15 mm, which is the maximum length of nerve regeneration achieved with silicone tubes in rats), it is only necessary to cut a polymer sheet to the appropriate size. Embodiments of the Present Invention [0040] FIG. 1 is a schematic illustration in cutaway view of a nerve guidance conduit (NGC) 10 according to an embodiment of the present invention bridging the stumps 12 , 14 of damaged nerve 16 . The stumps 12 , 14 are received in reserved chambers 18 , 20 at the proximal and distal ends 22 , 24 of the NGC 10 , and held in place with sutures 26 , 28 , or by other means known in the art. The reserved chambers 18 , 20 allow the nerve stumps 12 , 14 to be placed in the NGC 10 and sutured without tension by housing the nerve stumps 12 , 14 in place with an optimal grip. [0041] FIG. 2 is a schematic cross-sectional view of the NGC 10 showing that the NGC 10 includes an outer fibrous tube 30 surrounding one or more spiral wound sheets 32 The fibrous tube 30 includes a dense structure of randomly oriented polymer fibers (not shown). The spiral wound sheets 32 define a lumen 34 inside the NGC 10 . The lumen 34 is bounded by an inner surface 36 of the spiral wound sheets 32 . The NGC 10 further includes an integrated guidance spiral 38 having a plurality of surface channels 40 . The guidance spiral 38 is are composed of multiple layers (e.g., layers 42 , 44 ), and together define a spiral guidance channel 46 within the lumen 34 . In some embodiments of the present invention, the surface channels 40 are arranged such that they are substantially parallel to each other and to a longitudinal axis (not shown) of the NGC 10 . The layers 42 , 44 may be extensions of the spiral-wound sheets 32 , or may be formed separately therefrom, then integrated with the spiral-wound sheets 32 . The plurality of surface channels 40 increases the surface area of the guidance spiral 38 that is available for cell migration and may reduce the length of time needed for nerve regeneration. Additionally, the integrated layers 42 , 44 may reduce the wear and tear that can occur in NGCs known in the art. Such wear and tear is often observed with single lumen tubular NGCs. [0042] In some embodiments of the present invention, a highly aligned orientation of electrospun nanofibers (not shown) are provided as coats on the surface channels 40 , and on both layers 42 , 44 of the spiral sheet 38 , and dense randomly-oriented fibers are provided on an outer surface 48 of the NGC 10 , which greatly improves the mechanical properties of the NGC 10 , as discussed above. In some embodiments, the aligned fibers are substantially parallel to each other. In some embodiments, the aligned fibers are substantially parallel to a longitudinal axis of the NGC 10 . The presence of aligned fibers ensures that all areas of the regenerating axon will come into contact with aligned fibers. [0043] The NGC 10 is tunable such that its size can be varied in a controlled fashion depending on how it is to be used. The length and the outer diameter of the NGC 10 are dependent on the size of guidance spiral 38 . An NGC 10 according to the present invention may have any length, thus enabling it to be used to repair long gaps in the axon for the repair or regeneration of peripheral nerves. [0044] FIGS. 3 and 4 are scanning electromicrograph (SEM) images a first side and a second side opposite the first side of a portion of a porous polymeric sheet 50 of a type that may be used to fabricate the spiral-wound sheets 32 or guidance spiral 38 of an NGC of the same type as NGC 10 , before the application of electrospun nanofibers. Interconnected pores (e.g., pores 52 ) are present throughout the polymeric sheet 50 . FIG. 5 is an SEM image of a porous polymeric sheet 54 of the same type as polymeric sheet 50 , showing aligned nanofibers 56 that have been deposited on the polymeric sheet 54 by electrospinning. FIG. 6 is an SEM image of a porous polymeric sheet 58 of the same type as polymeric sheets 50 , 54 showing randomly-distributed nanofibers 60 that have been deposited on the polymeric sheet 58 by electrospinning. [0045] FIGS. 7-9 are stereomicroscopic images of an NGC 62 according to an embodiment of the present invention. NGC 62 is of the same general type as the NGC 10 discussed with respect to FIGS. 1 and 2 . FIG. 7 is an image of the intact NGC 62 showing its outer fibrous tube 64 . FIG. 8 is an image of the interior of the NGC 62 after it has been cut lengthwise, showing an interior surface 66 of the outer fibrous tube 64 , the guidance spiral 66 , and the reserved chambers 68 , 70 . FIG. 9 is an end view of the NGC 62 showing the outer spiral wall 64 , the guidance spiral 66 and the channels 72 of the guidance spiral 66 . FIG. 10 is a SEM image of a portion of polymer sheet 74 , which is of a type for making an NGC according to an embodiment of the present invention, showing the substantially parallel alignment of channels 76 , which are separated by ridges 78 . Exemplary Fabrication Method [0046] In a method of fabricating an NGC according to an embodiment of the present invention, a polycaprolactone (PCL) sheet was fabricated using a combination of the solvent evaporation method and the salt-leaching method. An 8% (w/v) PCL solution was poured onto a glass petri dish, and acupuncture needles having a diameter of 150 μm were placed on top of the PCL solution to form multi-channels having widths of about 180 μm. The dish was moved to a hood to let it air dry. After an hour, the resulting PCL sheet was immersed into deionized water so that the salt was dissolved, producing pores in the PCL sheet. The needles were also removed, having formed multi-channels on the PCL sheet with widths of about 180 μm. After 30 minutes, the PCL sheet was taken out and dried on a paper towel. Subsequently, 2 hours later, the fully dried PCL sheet was cut into a rectangular shape having dimensions of about 12 mm by 10.5 mm to bridge a 10 mm nerve gap in an animal study. [0047] Referring to FIG. 11 , in an exemplary embodiment of the method, the cut PCL sheet 80 had opposite longer edges 82 , 84 (i.e., the 12 mm edges), and opposite shorter edges 86 , 88 (i.e., the 10.5 mm edges). It may be noted that the channels 90 are substantially parallel to the longer edges 82 , 84 . Two rectangular areas 92 , 94 were cut out from the opposite corners 96 , 98 of the edge 82 , such that edge 82 was then shorter than edge 84 . [0048] PCL aligned nanofibers were spun on the cut PCL sheet 80 using a conductible rotation disk method known in the art. A 16% (w/v) solution of PCL in 1,1,1,3,3,3 Hexafluoroisopropanol (HFIP) (Oakwood Products, Inc) was prepared for electrospinning. Aligned fibers were deposited on the 12 mm×10.5 mm PCL sheet longitudinally on the edge of the rotating disk such that the fibers were substantially parallel to channels 90 . The fibers were deposited such that they would be substantially longer than the cut PCL sheet 80 . The sheet was carefully removed from the disk to ensure the fibers deposited remained aligned. The excess lengths of fiber (i.e., the portions of the fibers that extended beyond the edges of the cut PCL sheet 80 were collected and folded onto the back of the cut PCL sheet 80 . [0049] Turning back to FIG. 11 , the cut PCL sheet 80 with the aligned nanofibers thereon was then wound in a spiral fashion from the edge 82 to the edge 84 , such that the edge 82 was in the interior of the resulting spiral NGC and the channels 90 were substantially parallel to a longitudinal axis of the spiral NGC. In the spiral NGC, the cutaway areas 92 , 94 become reserved chambers (e.g. reserved chambers 68 , 70 of spiral NGC 64 of FIGS. 7-9 , or reserved chambers 18 , 20 of spiral NGC 10 of FIG. 1 ). [0050] Random nanofibers were then spun onto the outside of the spiral NGC to form an outer fibrous tube on the spiral NGC. The thickness of the outer fibrous tube was approximately 150 μm. The outer fibrous tube is intended to secure the entire spiral structure, enhance the mechanical strength, and prevent tissue infiltration during nerve regeneration. The resulting spiral NGC with its outer fibrous tube was 1.8 mm in diameter and 12 mm in length, suitable for bridging a 10 mm nerve gap. Tensile Properties of the NGCs of the Present Invention [0051] FIG. 12 is a plot of stress versus strain for several NGCs fabricated according to a method of the present invention: an outer fiber tube comprising a dense layer of randomly-oriented nanofibers; the outer fiber tube with a spiral sheet therein, and the outer fiber tube with the spiral sheet and aligned nanofibers (“AF”). The following tensile properties were measured: Young's Modulus, percent elongation to failure, and tensile strength of the different NGCs. The Young's Modulus, calculated through the stress-strain curve shown FIG. 12 , ranged between 0.262-0.7625 Mpa. All three of the NGCs yielded a Young's Modulus that can stand force stretching and be applicable for in vivo use. The values reported for the outer fibrous tube and the other NGCs all in a useful range for use in nerve regeneration and repair. High tensile strength will provide a mechanically strong NGC that can be sutured well during coaptation of the nerve stump and NGC, and preserve the suture after surgery. The measured physical properties of the NGCs of FIG. 11 are summarized in Table 1, below. [0000] TABLE 1 Tensile Properties of Nerve Guidance Conduits Young's Tensile Modulus (MPa) % Elongation Strength (MPa) Outer Fibrous Tube 0.7625 296.4 8.98 Outer Fibrous Tube + 0.33766 171 2.08 Spiral Outer Fibrous Tube + 0.32766 301 1.78 Spiral + AF Porosities of the NGCs [0052] The measured porosity values for the outer fibrous tube (hereinafter, NGC-T), outer fibrous tube+spiral (hereinafter, NGC-T-S), and outer fibrous tube+spiral+AF (hereinafter, NGC-T-S-AF) were respectively 71.98±1.22%, 75.01±2.69%, and 78.41±3.64%. The differences in porosities for these three types of NGCs are not statistically significant (p<0.05). Cell Proliferation [0053] Schwann cells were adopted as the model for evaluation of cellular response on the fiber-based spiral NGCs. At day 4, NGC-T-S-AF showed significantly greater cell proliferation than NGC-T and NGC-T-S. The cell numbers for each type of NGC are shown in FIG. 13 . The degrees of cell proliferation for the NGC-T and NGC-T-S are significantly lower (p<0.05) than for the NGC-T-S-AF. Implantation of NGCs [0054] The NGCs were tested in a 10 mm Sprague Dawley (SD) rat sciatic nerve defect to evaluate the effect of nanofibers on peripheral nerve regeneration through porous spiral NGCs. The sciatic nerve of each rat was cut, then bridged with one of the NGCs. One group received an autograft rather than a NGC. One group received no grafts. All rats were in good condition during the survival weeks. There were no obvious signs of systemic or regional inflammation and surgical complications after implantation [0055] The recovery of motor function was assessed based on the walking track evaluation Referring to FIG. 14 , normal sciatic functional index (SFI) value of −9.4±1.4 was measured from all healthy rats (n=30) before surgery. All experimental animals had decreased SFI of values between −85.6 and −94.5 (n=30) by week 2 after surgery. During the initial 4 weeks, there was no significant improvement in any of the groups. At 6 weeks after surgery, the overall SFI reached the levels between −72.2 and −91.7, which was equivalent to an improvement of 2.8-13.4 index points from week 2. Each group's 6-week SFI value was recorded as follows: autograft (−72.2±6.6), T-S-AF (−81.5±3.2), T-S(−88.4±4.9), and T (−91.7±4.2). The autograft SFI revealed a significant difference (p<0.05) as compared to the T-S and T groups. The SFI in the T-S-AF group was significantly higher than for the T groups (p<0.05). [0056] Functional recovery was further evaluated with electrophysiological assessment to determine whether functional recovery occurred through the NGCs. Six weeks post-surgery, compound muscle action potentials (CMAP) were evoked by stimulation at the surgical limbs and recorded from gastrocnemius muscle following by measurements of amplitude and nerve conduction velocity (NCV). Signals were absent and no muscle contractions were observed in the non-grafted group. Referring to FIG. 15 , for the amplitude measurements, each group's value was recorded as follows: autograft (5.25±1.51 mV), T-S-AF (4.96±1.58 mV), T-S(3.6±1.39 mV), and T (2.0±0.64 my). Significant differences in amplitude were observed in the T group as compared to the autograft and T-S-AF groups (p<0.05). However, the difference between the autograft, T-S-AF, and T-S groups (p>0.05) was not statistically significant. Similar results were found in NCV measurement: autograft (31.57±4.13 m/s), T-S-AF (26.47±6.87 m/s), T-S (18.28±4.16 m/s), and T (13.3±5.65 m/s) (See FIG. 16 ). Significant differences in NCV were observed in the autograft group as compared to the T-S and T groups (p<0.05). The NCV result in the T group also showed a significant difference as compared to autograft and T-S-AF groups (p<0.05). However, there were no significant differences when the NCV values of the autograft group were compared to those of the T-S-AF group, which may indicate that nanofibers can accelerate the level of muscle reinnervation as well as autograft. [0057] After 6 weeks post-surgery, the distal nerve segment from each group was explored and carefully isolated from the surrounding tissues. A pinch reflex test was performed distally. A reflex movement of the back muscles indicates that the sensory fibers are positively regenerated through the NGCs, while no movement was considered as lack of sensory fibers in the NGCs. The results are presented in Table 2, below. [0000] TABLE 2 Pinch Test Results Number of rats responding to pinch test (n = 5) Autograft 5/5 T-S-AF 5/5 T-S 4/5 T 3/5 [0058] Further histological evaluations of nerve regeneration behavior with NGCs were investigated under a light microscope. The results clearly demonstrated the potential of the NGCs of the present invention to house a large number of supportive cells, both with and without nanofibers to enhance the surface area of the channel. The NGCs possessed durable mechanical strength to support the entire regeneration process. Low magnifications of micrographs showed that neural tissues, including myelinated axons and myelin sheath, were all successfully presented among the groups. Angiogenesis occurred through which new blood vessels were formed during the nerve regeneration process. Normal axons were nearly all surrounded by uniform thicknesses of myelin sheaths and presented large fiber diameters. Nevertheless, the studied groups presented premature morphologies (i.e., diverse nerve fiber sizes and thinner myelin sheaths). [0059] Quantitative analysis of the total occupied neural tissue coverage in the NGCs compared to those of normal rat nerves (70.57±3.81%) further confirmed the above findings. Referring to FIG. 17 , each group's value was recorded as follow: autograft (29.29±4.61%), T-S-AF (26.52±3.77%), T-S(17.37±2.97%), and T (5.88±1.43%). No significant differences were found among autograft and T-S-AF groups. However, the area occupied by neural tissue in T-S group showed significantly lower values than the autograft, and T-S-AF groups. High significance was observed in the T group as compared to the other groups (p<0.01). Finally, it should be noted that the cross-sectional micrograph of T group was covered with a large white area. That implied the single lumen repair limited the nerve regeneration. [0060] When severe nerve injury occurs, the muscle is denervated and the balance of muscle metabolism could be shifted from protein synthesis toward protein degradation. As a consequence, the target muscle presents a decreased muscle cell size, muscle weight loss, hyperplasia of connective tissues, and new blood vessel formation. To evaluate the reinnervation of the gastrocnemius muscle, Masson trichrome staining was applied to the section followed by measurements of muscle weight ratio, diameter of muscle fibers, and muscle fiber coverage per cross section. Referring to FIG. 18 , for comparisons of muscle weight ratio, each group's value was recorded as follows: autograft (39.73±4.19%), T-S-AF (25.64±3.01%), T-S(22.31±2.18%), and T (19.2±2.03%). The muscle weight ratio of the autograft group was greater than that of the other groups by a statistically significant amount (p<0.05). However, there were no significant differences between the T-S-AF and T-S groups (p>0.05). The T group revealed a significant lower ratio than the T-S-AF group. [0061] Referring to FIG. 19 , for comparisons of muscle fiber diameter, each group's value was recorded as follows: autograft (34.62±1.05 μm), T-S-AF (31.81±2.18 μm), T-S(25.5±6 μm), and T (21.56±2.98 μm). Although the autograft group showed a significant difference from the T-S and T groups, it was not significantly higher than the T-S-AF group. Also, there were no significant differences between the T-S and T groups (p>0.05). Further findings showed that the value for the T group was significantly lower than that for the autograft, and T-S-AF groups. [0062] Referring to FIG. 20 , for comparisons of muscle fiber coverage, each group's value was recorded as follows: autograft (96.84±4.1%), T-S-AF (93.72±4.63%), T-S (86.99±10.31%), and T (58.42±4.69%). There were no significant differences between the values for the autograft, T-S-AF, and T-S groups (p>0.05); however, they were all significantly greater than the value for the T group (p<0.05). [0063] From qualitative analyses and histological observations discussed above, spiral NGCs of the present invention, with or without nanofibers, revealed the potential to prevent muscle atrophy as well as the effect of autograft. Both the surface channels and the aligned fibers provide good topographical cues for nerve regeneration, and thus allow muscle reinnervation faster than single lumen NGCs, thus suggesting that the surface channels and nanofibers further assisted NGC structures in promoting nerve regeneration. [0064] It should be understood that the embodiments described herein are merely exemplary in nature and that a person skilled in the art may make many variations and modifications thereto without departing from the scope of the present invention. All such variations and modifications, including those discussed above, are intended to be included within the scope of the invention, as defined by the appended claims.

A nerve guidance conduit includes a spiral structured porous sheet decorated with channels on its surface and electrospun nanofibers in a parallel alignment with the channels and an outer tubular structure including randomly-oriented nanofibers. Such a structure provides augmented surface areas for providing directional guidance and augmented surfaces for enhancing and peripheral nerve regeneration. The structure also has the mechanical and nutrient transport requirements required over long regeneration periods. To prepare a nerve guidance conduit, porous polymer sheet is prepared by a solvent casting method while using a template of thin rods to form parallel channels on a surface of the sheet. Aligned nanofibers are deposited on the sheet parallel to the channels. The polymer sheet is then wound to form a spiral structure. A dense layer of randomly-oriented nanofibers may be deposited on the outside of the spiral.

FIELD OF THE INVENTION This invention is in the field of seat base support assemblies. It relates to seat base support assemblies for furniture or the like wherein the support assemblies employed are of the non-coil spring type; i.e., they comprise sinuous spring bands, wire grids or chord-rubber webbing, or are made up of flexible steel bands. The invention finds particularly advantageous application to sinuous band seat spring assemblies, however, and is discussed initially in that context. BACKGROUND OF THE INVENTION Over the past ten to twelve years furniture seat spring torsioning devices such as disclosed in U.S. Pat. No. 3,210,064, No. 3,388,904, and No. 3,525,514, met the industry's long sought need for deep-drop uplift at the back rail and also contributed in other ways to the luxury seat which evolved during that time frame. As eleven (11) gauge helical spring connectors became disproportionally more expensive during this period these devices have been used almost exclusively with SWING ANCHOR connecting links and radius links such as disclosed in U.S Pat. No. 3,790,149, and depended upon kinetic energy stored in the arced sinuous spring itself to produce all upward resilience. The upholstered furniture styles most widely sold at the time developed all the back rail uplift considered desirable using such connecting links. During the past three to four years, however, there has been a move toward the use of thicker and thicker cushions. Attractive new and thicker cushion materials, including foam rubber laminates, have necessitated the lowering of seat frame heights dramatically. As a result, an urgent need was created in such constructions for more upward resilience of a strong dynamic nature in the spring base. SUMMARY OF THE INVENTION An object of the present invention is to provide a new and improved rail connector for sinuous spring bands, wire grids, chord-rubber webbing, and flexible steel bands. Another object is to provide a rail connector which embodies the salutary features of conventional helical spring connectors while retaining essentially none of the undesirable features thereof. Still another object is to provide a rail connector which produces spring torsioning and dynamic uplift at the back rail through kinetic energy which it itself stores, and which then cooperates with any spring action in the seat base support assembly, which might be sinuous, arced, or de-arced, a wire grid, chord-rubber webbing, or flexible steel bands. Yet another object is to provide such rail connectors which give varying degrees of dynamic uplift resilience obtained by offering alternative spring action modes within themselves. The foregoing and other objects are realized in accord with the present invention by producing two related forms of rail connector. A first form uses pre-stressed, close wound coil spring with attachment arms. In one alternative the coil spring is wound on an axis transverse to the axis of spring expansion and contraction while in another alternative the coil spring is wound on an axis longitudinally arranged relative thereto. In either alternative the connector may selectively have a leverage-amplified torsioning capability. A second form uses a pre-stressed, cantilever spring configuration. This connector may selectively be used with a sinuous spring band having leverage-amplified torsioning incorporated therein. The invention for the first time provides seat spring-enhancing connectors that in themselves combine the four essential seat-force-generators; i.e., (1) torsioning; (2) dynamic uplift; (3) expansion-contraction; and (4) leverage-amplification. These, in turn, produce to the greatest degree the four most desired seat-performance characteristics; i.e., (1) initial-drop; (2) deep-drop; (3) softness without "oil canning", "bucketing", "jack-knifing", or "bottoming"; and (4) resilient uplift proportionate to load. BRIEF DESCRIPTION OF THE DRAWINGS The invention, including its construction and modes of operation, together with additional objects and advantages thereof, is illustrated more or less diagrammatically in the drawings, in which: FIG. 1 is a vertical sectional view through a portion of the back end of a furniture seat spring base, illustrating a spring band assembly including a first form of rail connector embodying features of the present invention; FIG. 2 is a view taken along line 2--2 of FIG. 1; FIG. 3 is a view similar to FIG. 1 illustrating one modification of the first form of rail connector embodying features of the invention; FIG. 4 is a view similar to FIG. 1 illustrating another modification of the first form of rail connector embodying features of the invention; FIG. 5 is an enlarged view of a portion of an alternative first form of rail connector embodying features of the invention; FIG. 6 is a view similar to FIG. 1 illustrating a second form of rail connector in a spring band assembly embodying features of the invention; FIG. 7 is a view taken along line 7--7 of FIG. 6; FIG. 8 is a view similar to FIG. 7 illustrating the second form of rail connector in a sinuous spring band assembly embodying features of the invention; and FIG. 9 is a view similar to FIG. 7 illustrating the second form of rail connector in another sinuous spring band assembly. DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawings, and particularly to FIGS. 1 and 2, a portion of the back end of a furniture seat base is illustrated generally at 10. The seat base 10 comprises spring band assemblies 15, only one of which is shown, extending in parallel relationship between the front rail (not shown) and back rail 16 of the base frame. Each assembly 15 includes a normally arced sinuous spring band 20 of standard loop size; i.e., a seven-eighths (2/3) inch interval between linear segments 25 and semi-circular segments 26 of the band. Each band 20 is connected to the back frame rail 16 by a first form of rail connector 30 embodying features of the invention. The rail connector 30 is fabricated of eleven (11) gauge wire, similar to standard helicals. It comprises a section 35 of three coils tightly wound on an axis transverse to that of the band 20 and the axis of expansion and contraction of the connector 30. Extending from the coil section 35, at their uppermost extremity, tangent to the arc of the coils and in opposite directions, are a rail-attachment leg 38 and a spring-attachment leg 39. The rail-attachment leg 38 terminates in a transversely disposed anchor section 40 which seats in a conventional "G" clip 41, while the spring-attachment leg 39 seats on and grips the spring band 20. The spring attachment leg 39 is inclined slightly upwardly from the horizontal, in contrast to the rail-attachment leg 38, and includes an upwardly formed shoulder 45 and a terminal hook 46. The shoulder is formed approximately mid-way between the hook 46 and tangency with the coil 35, seven-eighths (2/3) inches each way in the case where the band 20 is regular sinuous. As seen best in FIG. 1, the downwardly opening hook 46 is designed to seat over the penultimate linear segment 25b in the spring band 20, while the ultimate linear segment 25a seats against the shoulder 45. The result is to lock the end of the band 20 and the connector 30 together. In operation, the attachment-arm 39 reaching up into the band 20 sets up a torsioning effect in the back of the band. The arm 39 is spring loaded upwardly by the strength of the coil section 35 and produces dynamic uplift. At the same time the coil section 35 permits of longitudinal expansion-contraction of the connector 30. The coil section 35 and rail-attachment leg 38 extending outwardly of the band 20 end amplify the leverage induced torque. In an alternative construction of the first form of the invention, as seen in FIG. 3, the rail connector 130 is attached to the rail 116 through a gang bore 142. The rail-attachment leg 138 of the connector has a shorter anchor section 140 which can pass through the bore 142 from front to back of the rail 116 and then seats against the back of the rail to lock the connector 130 to the rail. The spring-attachment leg 139 in this form of the connector is much shorter and has an upwardly formed hook 146 at its inner end. The hook 146 is so formed that when it seats upwardly, onto the ultimate linear segment 125a of the spring band 120, it cannot slip off during seat base operation. The connector 130 provides both dynamic uplift and resilient expansion-contraction at the band end. It does not induce torsion or leverage amplification. The connector 130 can also be connected to the ultimate linear segment 125a of the band 120 by a conventional VLE clip, as seen at 150 in FIG. 4. As such, the single spring attachment leg 139 obtains a wider purchase area on the band 120 end. The effect is to enhance lateral stability of the spring band assembly. Turning now to FIG. 5, a modified coil section for a connector otherwise identical to that hereinbefore discussed is illustrated at 235. As illustrated, the coil section 235 is tightly wound in five (5) coils on an axis longitudinally aligned with the sinuous spring band span (not shown). This form of the connector 130 produces the same salutary effects, the dynamic uplift being produced by a torquing expansion-contraction of the coil section 235 in contrast to the loop compression-expansion of the coil section 35, however. FIGS. 6 and 7 illustrate a portion of a furniture seat base 310 comprising spring band assemblies 315 (only one shown) in which a second form of spring band 320 connector is illustrated at 330. The connector 330 uses a cantilever principle to provide dynamic uplift to the band 320 at the back rail 316. The rail connector 330 is fabricated of spring steel wire of relatively heavy gauge; i.e., eight (8) gauge or heavier. As best illustrated in FIG. 7, it includes a pair of identical connector arms 331 extending parallel to each other between the rail 316 and the band 320. As seen once again in FIG. 6, each connector arm 331 includes a generally V-shaped body 335 made up of a rail-attachment leg 338 and a spring-attachment leg 339. The legs 338 are vertically oriented and preferably four (4) inches long. The legs 338 are joined at their upper ends by a base leg 340 which seats in a conventional EKS clip 341 stapled to the top of the rail 316. Curving upwardly and inwardly from the lower end of each rail-attachment leg 338 is a corresponding spring-attachment leg 339. The spring-attachment legs 339 are approximately eight (8) inches long. Formed on the free ends of the legs 339 are attachment hooks 346 identical to the hooks 146 hereinbefore discussed. The connector 330 is a variation of the second form of the invention wherein the hooks 346 receive and seat on the ultimate linear segment 325a of the spring band 320. In operation the legs 338 are braced against the rail 316 with the spring-attachment legs 339 extending inwardly and upwardly therefrom to the hooks 346. In unloaded position the hooks are disposed approximately one (1) inch above the level of the EKS clip 341. The connector 330 thus is effective to dynamically urge the spring band 320 end upwardly when a subject is seated. At the same time longitudinal resilient expansion-contraction can and does take place in the connector 330, enhancing seat base softness. FIG. 8 illustrates a sinuous spring band assembly 415 which incorporates a connector 430 identical to the connector 330 hereinbefore discussed. In the spring band assembly 415 the connector hooks 446 are seated on a linear segment 425f of the band 420 which is sixth from the end of the band; i.e., the ultimate linear segment 425a. The linear segment 425a is connected to the rail by a SWING ANCHOR connector clip 460 such as illustrated in FIG. 1 of the aforementioned U.S. Pat. No. 3,790,149. The base of the clip 460 is seated, together with the base leg 440 of the connector 430, in the conventional EKS clip stapled to the top of the rail 416. The spring band 420 immediately inwardly of its ultimate linear segment 425a, at the penultimate linear segment, is bent upwardly for the length of one semi-circular band segment 426a and then bent back into the normal arc of the band. This creates a torsion inducing moment arm configuration in the end of the band as illustrated at FIG. 12 in the aforementioned U.S. Pat. No. 3,525,514. In operation of this spring band assembly 415 the connector 430 performs the same functions as previously ascribed to the connector 330. Further, however, its dynamic uplift is effected inwardly of the band end. This uplift, coupled with the torsion inducing band 420 configuration and the articulate connection provided by the clip 460 produces a highly sophisticated and luxurious seat base. FIG. 9 illustrates a sinuous spring band assembly 515 which also incorporates a connector 530 identical to the connector 330 hereinbefore discussed. In the assembly 515 the sinuous band 520 is a de-arced band, however; i.e., it has very little inherent upward resilience. In this assembly the connector 530 pre-loads the band 520 upwardly at the fourth linear segment 525d from the ultimate linear segment 525a. The ultimate linear segment 525a is seated in the EKS clip 540 on the rail 516, together with the base leg 540 of the connector 530. The connector leg 539 thus preloads the band 520 upwardly with the seat base 10 in its relaxed state as a subject is seated and rises, the connector provides a dynamic uplift which would otherwise not be present. All of the connectors hereinbefore discussed are also used to connect other forms of seat base support means to the frame rails. As will readily be understood, wire grids such as manufactured under the trademark PERMA-MESH by Flexolators, Inc., chord-rubber webbing such as manufactured by the Pirelli, s.p.a., of Italy, and flat steel bands, for example, do not have stored upward resilience in the sense that arced sinuous spring bands do. When connected to the back frame rail by connectors embodying the inventions disclosed herein, however, they are provided with a dynamic uplift adjacent the back rail. In this sense they are similar to a de-arced sinuous spring band. While several embodiments described herein are at present considered to be preferred, it is understood that various modifications and improvements may be made therein, and it is intended to cover in the appended claims all such modification and improvements as fall within the true spirit and scope of the invention.

A rail connector and improvement in seat base support assembly. The connector takes two basic forms. In the first a pre-stressed, close wound coil, disposed either transversely or longitudinally of the connector, is effective to continuously bias the seat base support means upwardly. In the second a cantilevered, curved spring arm serves the same purpose. The connector may be configured to reach into the body of a sinuous spring band, for example, and define a torque arm in the band, at the back rail. All forms are applicable to wire mesh, chord rubber webbing, flat steel bands and sinuous, both arced and dearced.

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application claims the benefit of and priority to U.S. Provisional Patent Application No. 62/352,683 filed Jun. 21, 2016, the entire disclosure of which is incorporated by reference herein. BACKGROUND 1. Technical Description [0002] The present disclosure is directed to an anvil assembly for use with a surgical stapling device. More particularly, the present disclosure is directed to an anvil assembly for a circular surgical stapling device including a stabilizing collet positioned to prevent damage to the anvil assembly. 2. Background of Related Art [0003] Circular staplers are commonly used to perform a variety of surgical procedures including anastomosis procedures for joining ends of tubular tissue sections and hemorrhoidectomy procedures for treating hemorrhoids. Typically, circular staplers include a stapling device and an anvil assembly. The stapling device includes a handle assembly, a body portion extending from the handle assembly, a shell assembly including a staple cartridge, and a trocar extending from the shell assembly. The anvil assembly is releasably secured to the trocar of the stapling device and includes an anvil assembly having an anvil shaft and an anvil head assembly. The shell assembly includes a circular knife. When the circular stapler is fired, the circular knife is advanced from the shell assembly and cuts tissue as staples are ejected from the staple cartridge and formed against the anvil head assembly. In use, the stapling device and the anvil assembly are delivered to a surgical site within a patient separately and coupled to each other prior to use. [0004] Typically, the stapling device and the anvil assembly are coupled together at the surgical site by a clinician using a grasper. More particularly, the clinician grasps the anvil shaft of the anvil assembly with the grasper and positions the anvil shaft about the trocar of the stapling device to couple the trocar to the anvil shaft. This coupling procedure takes place within a body lumen or orifice where visibility is limited. [0005] When a clinician applies too much pressure on the anvil shaft, the anvil shaft can be damaged, e.g., crushed or deformed, such that the anvil shaft cannot be properly coupled to the stapling device. This problem is exacerbated because due to the poor visibility at the surgical site, the clinician may be unaware that the anvil shaft has been damaged and is not properly coupled to the stapling device. As such, when circular stapler is fired, the anvil assembly may become disengaged from the stapling device such that the staples are not formed in cut tissue. [0006] Accordingly, a need exists in the surgical arts for an anvil assembly that is less susceptible to damage during attachment of the anvil assembly to the stapling device to facilitate reliable attachment of the anvil assembly to a stapling device. SUMMARY [0007] In one aspect of the disclosure, an anvil assembly includes an anvil shaft defining a first longitudinal bore and an anvil head assembly. The anvil shaft has a proximal portion and a distal portion. The proximal portion includes a plurality of flexible legs that define the first longitudinal bore. The anvil head assembly is secured to the distal portion of the anvil shaft and supports an anvil plate that defines a plurality of staple deforming pockets. A stabilizing collet defines a second longitudinal bore. The collet is supported within the first longitudinal bore and is positioned to prevent damage to the plurality of flexible legs. [0008] In another aspect of the disclosure, a surgical stapler includes a stapling device and an anvil assembly. The stapling device includes a handle assembly, a body portion that extends distally from the handle assembly, a shell assembly including a staple cartridge having a plurality of staples, and a trocar extending from the shell assembly. The anvil assembly includes an anvil shaft and an anvil head assembly. The anvil shaft has a proximal portion and a distal portion and defines a first longitudinal bore configured to receive the trocar of the stapling device. The proximal portion includes a plurality of flexible legs that defines the first longitudinal bore. The anvil head assembly is secured to the distal portion of the anvil shaft and supports an anvil plate that defines a plurality of staple deforming pockets. A stabilizing collet defines a second longitudinal bore configured to receive the trocar. The collet is supported within the first longitudinal bore and is positioned to prevent damage to the plurality of flexible legs. [0009] In embodiments, the collet is cylindrical. [0010] In certain embodiments, the collet is substantially rigid. [0011] In some embodiments, the collet has a distal end including a plurality of cantilevered fingers, wherein each of the plurality of cantilevered fingers has a protrusion configured to secure the collet within the first longitudinal bore of the anvil shaft. [0012] In certain embodiments, each of the plurality of flexible legs defines a longitudinal channel with an adjacent one of the plurality of flexible legs. [0013] In embodiments, the anvil shaft defines a hole positioned adjacent the distal end of each of the longitudinal channels. Each of the holes is configured to receive a respective one of the protrusions. [0014] In some embodiments, each of the holes is circular. [0015] In certain embodiments, the anvil head assembly is pivotally secured to the anvil shaft. [0016] In embodiments, the anvil plate is annular. BRIEF DESCRIPTION OF THE DRAWINGS [0017] Various embodiments of the presently disclosed crush resistant anvil assembly are described herein below with reference to the drawings, wherein: [0018] FIG. 1 is a side perspective view of a surgical stapler including an exemplary embodiment of the presently disclosed crush resistant anvil assembly; [0019] FIG. 2 is an enlarged view of the indicted area of detail shown in FIG. 1 ; [0020] FIG. 3 is a cross-sectional view taken along section line 3 - 3 of FIG. 2 ; [0021] FIG. 4 is a side perspective view of the anvil assembly shown in FIG. 2 ; [0022] FIG. 5 is an enlarged view of the indicated area of detail shown in FIG. 4 ; [0023] FIG. 6 is a side perspective view of a collet of the anvil assembly shown in FIG. 4 ; [0024] FIG. 7 is a side cross-sectional view of the collet shown in FIG. 6 and the anvil shaft of the anvil assembly shown in FIG. 4 with parts separated; [0025] FIG. 8 is a side cross-sectional view of the collet and anvil shaft shown in FIG. 7 as the collet is slid into the anvil shaft; [0026] FIG. 9 is a side cross-sectional view of the collet and anvil shaft shown in FIG. 8 with the collet secured within the anvil shaft; and [0027] FIG. 10 is a side cross-sectional view of the collet and anvil shaft shown in FIG. 9 as a trocar of the stapling device is positioned within the anvil shaft. DETAILED DESCRIPTION OF EMBODIMENTS [0028] Exemplary embodiments of the presently disclosed damage resistant anvil assembly will now be described in detail with reference to the drawings in which like reference numerals designate identical or corresponding elements in each of the several views. In this description, the term “proximal” is used generally to refer to that portion of the stapler that is closer to a clinician, while the term “distal” is used generally to refer to that portion of the stapler that is farther from the clinician. In addition, the term “endoscopic” is used generally to refer to procedures performed through a small incision or a cannula inserted into a patient's body including endoscopic, laparoscopic, and arthroscopic procedures. Finally, the term clinician is used generally to refer to medical personnel including doctors, nurses, and support personnel. [0029] The presently disclosed anvil assembly includes an anvil head assembly, an anvil shaft, and a stabilizing collet. In embodiments, the stabilizing collet may be formed of a substantially rigid material. Alternately, other materials of construction that provide support to the anvil shaft are envisioned. The anvil shaft includes a plurality of flexible legs that flex outwardly in response to insertion of a trocar of a surgical stapling device into the anvil shaft to releasably couple the anvil shaft to the trocar. The collet is received within a longitudinal bore defined by the flexible legs of the anvil shaft at a location to support the flexible legs and minimize the likelihood of damage to the anvil shaft caused by engagement of the anvil shaft with a grasper. The collet is also positioned in a location not to interfere with flexing of the legs during coupling of the anvil shaft to the trocar of the stapling device. [0030] FIG. 1 illustrates a manually powered surgical stapler 10 including a stapling device 12 supporting an exemplary embodiment of anvil assembly 100 . The stapling device 12 includes a handle assembly 14 , a body portion 16 that extends distally from the handle portion 14 , and a shell assembly 18 that supports a staple cartridge 20 . The staple cartridge 20 supports a plurality of staples (not shown) that are arranged in an annular configuration within the shell assembly 18 . The stapling device 12 also includes a trocar 22 that extends from the distal end of the body portion 16 through the shell assembly 18 . The trocar 22 is configured to releasably engage the anvil assembly 100 as described in further detail below. For a more detailed description of a suitable stapling device, see U.S. Pat. Nos. 7,234,624, 7,364,060 and 7,857,187 (“the incorporated patents”) which are incorporated herein by reference in their entirety. [0031] Referring also to FIGS. 2-4 , the anvil assembly 100 includes an anvil head assembly 102 and an anvil shaft 104 . Although not specifically described in this application, the anvil head assembly 102 can be pivotally or fixedly attached to the anvil shaft 104 . Examples of pivotally attached anvil head assemblies are described in the incorporated patents. [0032] The anvil head assembly 102 includes a housing 106 that supports an anvil plate 108 ( FIG. 2 ) and a cut ring assembly 110 . The housing 106 has a smoothly curved distal surface 112 that facilitates atraumatic entry of the anvil assembly 100 into and through a body orifice or lumen. A proximal side of the housing 106 defines a cavity (not shown) that is configured to receive the anvil plate 108 and the cut ring assembly 110 . For a more detailed description of the components of the anvil head assembly 102 , see the incorporated patents. [0033] The anvil shaft 104 includes a longitudinal body portion 116 that includes a tubular portion 118 and a plurality of flexible legs 120 that extend proximally from the tubular portion 118 . Each of the flexible legs 120 has a semi-cylindrical configuration such that the legs 120 cooperate to define a longitudinal bore 122 ( FIG. 3 ) that is dimensioned to receive the trocar 22 of the stapling device 12 ( FIG. 1 ) when the anvil assembly 100 is secured to the stapling device 12 . The bore 122 extends from the proximal end of the flexible legs 120 at least partially into the tubular portion 118 of the anvil shaft 104 . [0034] In embodiments, the anvil shaft 104 may include a plurality of splines 126 positioned about the anvil shaft 104 . As is known in the art, the splines 126 mate with recesses (not shown) defined within the shell assembly 16 FIG. 2 ) of the surgical stapling device 12 to properly orient the staple cartridge 20 in relation to the anvil plate 108 of the anvil assembly 100 when the anvil assembly 100 and the shell assembly 18 are approximated. The anvil shaft 104 may also include one or more stabilization rings 130 (only one is shown) positioned about the anvil shaft 104 at a position to engage the shell assembly 16 when the anvil assembly 100 and the shell assembly 18 are approximated to provide added stability to the anvil assembly 100 . For a more detailed description of an anvil assembly including a stabilization ring, see U.S. Pat. No. 8,424,535 which is incorporated herein by reference in its entirety. Although the splines 126 and the stabilization ring 130 are shown to be formed integrally with the anvil shaft 104 , it is contemplated the either or both could be formed separately from the anvil shaft 104 and secured to the anvil shaft 104 using any known fastening technique including welding, crimping gluing or the like. [0035] Referring to FIGS. 4 and 5 , each of the flexible legs 120 of the anvil shaft 104 defines a longitudinal channel 134 with an adjacent leg 120 . Each longitudinal channel 134 includes an enlarged cutout or hole 136 formed at the distal end of the longitudinal channel 134 . The holes 136 are configured to secure a collet 150 within the longitudinal bore 122 of the anvil shaft 104 . In embodiments, the hole 136 is substantially circular although other configurations are envisioned. One or more of the flexible legs 120 may also include a bore 140 which is configured to receive a suture or the like (not shown). The suture can be used to allow a clinician to retrieve or position the anvil assembly 100 from or within a surgical site. The proximal end of each of the flexible legs 120 has an inner surface that defines a recess 160 ( FIG. 7 ) such that the recesses 160 collectively define an annular recess 160 a ( FIG. 9 ). The annular recess 160 a facilitates releasable engagement of the anvil assembly 100 to the stapling device 12 . [0036] Referring also to FIG. 6 , the collet 150 may be substantially rigid and is positioned within the longitudinal bore 122 defined by the anvil shaft 104 . The collet 150 is substantially cylindrical and defines a longitudinal bore 152 ( FIG. 7 ) that is dimensioned to receive the trocar 22 ( FIG. 10 ). A distal portion 154 of the collet 150 includes a plurality of cantilevered fingers 156 . Each of the fingers 156 includes a protrusion 158 that is dimensioned and configured to be received in a respective one of the holes 136 ( FIG. 5 ) formed in the anvil shaft 104 as described in further detail below. [0037] Referring to FIGS. 7-9 , in order to assemble the collet 150 within the anvil shaft 104 , the distal end of the collet 150 is inserted into the proximal end of the longitudinal bore 122 of the anvil shaft 104 and slid distally in the direction indicated by arrow “A” in FIGS. 7 and 8 . The collet 150 is positioned to align the protrusions 158 with the longitudinal channels 134 positioned between the flexible legs 120 . When the protrusions 158 engage an inner wall of the flexible legs 120 , the fingers 156 are deflected inwardly in the direction indicated by arrow “B” in FIG. 8 to facilitate passage of the collet 150 through the longitudinal bore 122 . When the protrusions 158 are moved into alignment with the holes 136 , the fingers 156 spring outwardly in the direction indicated by arrow “C” in FIG. 9 to move the protrusions 158 into the holes 136 to secure the collet 150 within the longitudinal bore 122 . [0038] Referring to FIG. 10 , the trocar 22 includes a pointed distal end 30 and an enlarged proximal portion 32 that defines a shoulder 32 a . As known in the art, the proximal end of the trocar 22 is secured to an approximation mechanism (not shown) of the stapling device 12 ( FIG. 1 ) to facilitate movement of the trocar 22 between retracted and advanced positions. When the trocar 22 is inserted into the longitudinal bore 122 of the anvil shaft 104 and the longitudinal bore 152 of the collet 150 in the direction indicated by arrow “D” in FIG. 10 , the enlarged proximal portion 32 of the trocar 22 engages a proximal end of the flexible legs 120 of the anvil shaft 104 to urge the flexible legs 120 outwardly in the direction indicated by arrows “E”. When the enlarged proximal portion 32 of the trocar 22 is moved distally in the direction indicated by arrow “D” into alignment with the recess 160 defined at the proximal end of the flexible legs 120 , the flexible legs 120 return to their undeformed configuration to receive the enlarged proximal portion 32 of the trocar 22 . When the enlarged proximal portion 32 is received within the recess 160 , the shoulder 32 a on the enlarged proximal portion 32 of the trocar 32 engages a proximal wall 161 defining the recess 160 to secure the anvil shaft 104 to the trocar 22 . [0039] During an endoscopic surgical procedure, the anvil assembly 100 is grasped with a grasper (not shown) that is inserted through a small incision in the skin to position the trocar 22 within the longitudinal bore 122 of the anvil shaft 104 and secure the anvil assembly 100 to the trocar 22 of the surgical stapling device 12 . The collet 150 is positioned within the longitudinal bore 122 of the anvil shaft 104 and extends from a distal end of the flexible legs 120 towards the proximal end of the flexible legs 120 to support the flexible legs 120 and inhibit radial compression or other deformation of the flexible legs 120 that may result from pressure applied to the flexible legs 120 by a manipulating instrument (not shown). Collet 150 may be formed from any suitable, medical grade material having a stiffness to perform the functions described herein. Suitable materials include, for example, stainless steel or nylon. The collet 150 is secured within the longitudinal bore 122 of the anvil shaft 104 in a position that does not interfere with outward flexing of the flexible legs 120 and, thus, allows the anvil assembly 100 to be readily connected to the trocar 22 . [0040] Persons skilled in the art will understand that the devices and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments. It is envisioned that the elements and features illustrated or described in connection with one exemplary embodiment may be combined with the elements and features of another without departing from the scope of the present disclosure. As well, one skilled in the art will appreciate further features and advantages of the disclosure based on the above-described embodiments. Accordingly, the disclosure is not to be limited by what has been particularly shown and described, except as indicated by the appended claims.

An anvil assembly is disclosed that includes an anvil shaft including a proximal portion and a distal portion and defining a first longitudinal bore. The proximal portion includes a plurality of flexible legs that define the first longitudinal bore dimensioned to receive a trocar of a stapling device. An anvil head assembly is secured to the distal portion of the anvil shaft and supports an annular anvil plate that a plurality of staple deforming pockets. The anvil assembly also includes a rigid collet defining a second longitudinal bore that is configured to receive the trocar of the stapling device. The rigid collet is supported within the first longitudinal bore and is positioned to prevent crushing of the plurality of flexible legs when the anvil assembly is manipulated with a grasper.

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