Abstract:
This application discloses a composite material comprising one or more silk elements in an acrylic or cross-linked protein matrix. The silk elements are made from the group of silk elements consisting of domestic silkworm silk, wild silkworm silk, spider dragline silk, and filaments spun from recombinant silk protein or protein analogues. The composite material is particularly useful for use in-surgical-implants.

Description:
FIELD OF THE INVENTION  
       [0001]     The invention relates to a composite material comprising one or more silk elements in an acrylic or cross-lined protein matrix, its method of manufacture and its use in surgical implants.  
       BACKGROUND  
       [0002]     Biodegradable polymer materials made of silk fibroin elements-are known, for example, from US-A-2004/0005363 (Tsukada et al.). This patent document teaches a composite material made from silk fibroin and another secondary substance, such as cellulose, chitin, chitosan (or derivatives), Keratin-or polyvinyl alcohol. The composite material can be used as a sustained release substrate for medicines, a biological cell growth substrate, a metal ion-absorbing material and a biodegradable water-absorbing material.  
         [0003]     Another example of a composite material in which proteins coat the surface of a surgical device is disclosed in WO-A-94/22584 entitled “Chronic Endothelial Cell Culture under flow”. In this patent application, the inner lumen of a hollow polypropylene fibre was coated with a synthetic protein polymer Fibronectin F which contains multiple repeats of the RGD fibronectin binding site. This produced an inner lumen surface which was substantially uniformly coated with the synthetic protein polymer on which cells could be grown. The polypropylene fibres can be used in vascular grafts. The strength of the vascular graft is as a result dependent on the tensile strength of the polypropylene fibres.  
         [0004]     The surgical device (stent grafts and vascular grafts) described in WOA-01/38373 (Boston Scientific) exploits the strength of spider silk by providing an outer or interstitial sheath over the outside surface or the luminal surface of an inner stent The inner stent is made from a multitude of materials such as synthetic textile materials, fluoropolymers and polyolefines. Nylon, polyester and polyurethane are often used. These-materials which can not be resorbed by the body. The material of the outer sheath is non-resorbable conventional man-made polymeric material or a combination of spider silk and man made polymeric material.  
       SUMMARY OF THE INVENTION  
       [0005]     These and other objects of the invention are solved by providing a composite material having one or more silk elements in an acrylic or cross-linked protein matrix. This material is highly biocompatible. Preferably the silk elements are made from wild silkworm silk, domestic silkworm silk spider-dragline silk, and filaments spun from recombinant silk protein or protein analogues or mixtures of these. A protein matrix is preferred because this means that both the protein matrix and the silk elements are resorbable.  
         [0006]     In one embodiment of the invention, the silk elements are embedded in an acrylic matrix made of a cyanoacrylate, as this acrylate is known to be biocompatible. Alternatively, the silk elements can be incorporated into a matrix made of cross-linked fibroin or cross-linked casein.  
         [0007]     In a particularly advantageous embodiment of the invention, the silk elements are made from the silk derived from wild silkworms (i.e. Tussah silk) as the resorbtion rate of the composite material formed from the cross-lined protein matrix and wild silkworm silk elements is slower than that of composite material with silk elements derived from spider silk.  
         [0008]     In one embodiment of the invention, the silk elements formed from a plurality of short filaments with a staple-length less than 120 mm are from carded filaments (i.e. filaments drawn to lie approximately parallel to one another by a combing process). In another embodiment of the invention, the silk elements are twisted into a thread which call also be further twisted into a cord or rope or woven, braided, embroidered, wound, stitched or knitted.  
         [0009]     The composite material can be formed into a substantially cylindrical form with the silk elements wound at an angle in excess of 40 degrees to the long axis of the substantially cylindrical form or circumferentially to the long axis. On a luminal surface of the form, the silk elements can be wound parallel to the long axis of the substantially cylindrical form. This latter construction has the advantage that it should stimulate longitudinal movement of nerve processes in a sleeve and therefore promote nerve regeneration.  
         [0010]     Mineralisation of the composite material is advantageous as this allows the composite material to be used as a bone substitute and stimulates regrowth of the bone material.  
         [0011]     The principal silk protein used in the silk elements contains at least eight repeats of the triplet RGD. The eight repeats of the triplet RGD are located immediately adjacent to turns or predicted turns of a structure of the principal silk protein. This is advantageous as this sequence when next to a turn specifically recognises and holds the fibronectin binding site of integrin molecules anchored to the surface of most metazoan cell types. In turn this leads to excellent cell adhesion and advantageous changes in cell physiology including polarisation of function, cell differentiation and changes in the cell cycle.  
         [0012]     The composite material is in one use formed into surgically implantable devices, such as sutures, artificial ligaments and tendons, endoluminar devices, anastomosis devices, and sleeves to aid in the regeneration of nerve cells  
         [0013]     The invention also comprises a method for the manufacture of a compound material with a first step of providing one or more silk elements and a second step of embedding the silk element in an acrylic or protein matrix. The silk elements can be obtained by degumming and unwinding or combing silk from a cocoon.  
         [0014]     Fibroin can be prepared for the protein matrix by dissolving domestic or wild silkworm silk or cocoons in one or more chaotropic agents, such as calcium nitrate solution and lithium thiocyanate solution.  
         [0015]     The protein matrix is cross-linked using a cross-linking agent such as formaldehyde vapour, glutaraldehyde, polyglutaraldehyde, carbodiimide and genipin.  
         [0016]     The acrylic or fibroin material is applied to the silk elements by dipping, painting, spraying or casting.  
         [0017]     An apparatus for the manufacture of an object made from the composite material is also part of the invention. The apparatus has a storage region having silk elements,-a substantially cylindrical former and finally a feeding means to place said silk elements about the former.  
         [0018]     The feeding means comprises rollers and tension is maintained between the rollers and the former to pre-stress and straighten the silk elements so as to ensure a good final product.  
         [0019]     In one embodiment of the invention, a take up drum is used to form an object which is then removed as a substantially cylindrical form from the take up drum. Advantageously, cared silk fibres are fed to a cylindrical former and continuously coated onto the substantially cylindrical drum and finally a composite sheet emerges continuously from the substantially cylindrical drum. 
     
    
     DESCRIPTION OF THE DRAWINGS  
       [0020]      FIG. 1  shows a composite material according to this invention.  
         [0021]      FIG. 2  shows a first manufacturing method for the composite material.  
         [0022]      FIG. 3  shows a second manufacturing method for the composite material.  
         [0023]      FIG. 4  shows the distribution of the fibronectin binding triplet RGD in the repetitive part of the sequence of Antheraea pernyi (left column) and A. yamamai (right column) heavy chain fibroin.  
         [0024]      FIG. 5  shows the consensus repeat sequence for the glycine-rich domains for the first 900 repetitive amino acids of the repetitive region of A. pernyii heavy chain fibroin.  
         [0025]      FIG. 6  shows the position of a putative turn immediately adjacent to the RGD triplet in the consensus sequence from the glycine-rich domain of A. pernyi heavy chain fibroin.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0026]      FIG. 1  shows a composite material  30  comprising one or more silk elements  10  in a matrix material  20 . The matrix material  20  can be an acrylic matrix, made for example from a cyanoacrylate, or a protein matrix. The protein matrix can, for example, be a fibroin matrix or a casein matrix. Other highly soluble proteins could also be used. The protein matrix is cross-lined using a cross-liking agent such as formaldehyde vapour glutaraldehyde, polyglutaraldehyde, formaldehyde carbodiimide and genipin. In one embodiment the protein matrix is cross-lined by heating substantially dry formaldehyde vapour generated by heating paraformaldehyde in a sealed container to 80-100 degrees centigrade for 5 minutes to 3 hours.  
         [0027]     The silk elements  10  are made from domestic silkworm soil, wild silkworm silk or spider dragline silk The silk elements  10  could also be made from recombinant silk protein or protein analogues. The silk elements  10  produced from wild silkworm in general means those elements produced by  Antheraea pernyi, Antheraea yamamai, Antheraea militta, Antheraea assama, Philosamia Cynthia ricini  and  Philosamia Cynthia pryeri . The silk of other Saturnid moths such as those of the genus Actias or Cecropia though not generally defined as wild silk worms yield a closely similar silk element and can be used in this invention as the silk elements  10 .  
         [0028]     In  FIG. 1 a  simple block of composite material  30  is known. However, other shapes are possible such as fibres, rods, sheets or tubes as will become clear from the discussion below.  
         [0029]     The silk elements  10  can be made up of a plurality of silk elements which have been twisted together to form a thread. The silk elements  10  can be further made of a plurality of short filaments of silk, for example with a staple length as long as possible and at least 20 mm. The silk elements  10  can be twisted into pairs or multiples to form a thread which can also be further twisted into a cord or rope or woven, braided, embroidered, wound, stitched or knitted to form devices. In one advantageous embodiment of the invention, the silk elements  10  comprise carded filaments.  
         [0030]     In one embodiment of the invention, the silk elements  10  are degummed from the silk cocoon. However, other methods of extracting the silk elements  10  can be used.  
         [0031]     The fibroin for the protein matrix  20  is extracted by dissolving domestic or wild silkworm silk or cocoons in one or more chaotropic agents, such as calcium nitrate solution and lithium thiocyanate solution. In the case of wild sills, it is necessary to heat the chaotropic agent. The silk elements  10  are placed in the resultant solution-which is then dialysed to remove the chaotropic agent or agents and concentrated or dried by evaporation or reverse dialysis. The concentrated fibroin solution can be dried into films or flakes for storage and redissolved in distilled water or other solvents when required.  
         [0032]     US Patent U.S. Pat. No. 5,245,012 (US Army) also teaches a method of dissolving silk protein extracted from different arachnid species using varying solvent systems. The teachings of this patent are incorporated herein by reference.  
         [0033]     The composite material  30  of the invention can be used to make a variety of objects. One example of an apparatus for the manufacture of the objects  100  is shown in  FIG. 2  in which the silk elements  10  are initially held in a storage region  200  and are spooled (or otherwise fed) to a drum  210  over rollers  220  and wrapped circumferentially or helically about the drum  210 . The function of the rollers  220  is to ere that the silk elements  10  are pre-stressed and therefore firmly placed on the drum  210 . The matrix material  20  is then coated by an application device  250  over the outer surface  240  of the drum  210  with matrix material  20  to affix and embed the silk elements  10 . It will be understood that the drum  210  could also be a mandrel or more generally a former. It will be understood that the silk elements  10  can be laid down on the drum  210  or mandrel in many different patterns of orientation by rotating the drum  210  or mandrel and changing the spatial relationship between the drum  210  and the rollers  220 .  
         [0034]     In a further embodiment a composite rope or cable structure is formed as follows. Two or more lengths of single, double or multiple ply silk threads-are twisted into a thin rope. The acrylate or fibroin solutions for the matrix can be applied before or after twisting. After drying or partial drying, the matrix is then cross-linked by chemical treatment, preferably by hot formaldehyde vapour. It will be understood that by successive twisting varying hierarchical levels of construction to give cords, ropes or cables of thicknesses appropriate to end use. The resulting materials can be incorporated into a variety of prosthetic devices including-tendon and ligament prostheses or embroidered or knitted devices.  
         [0035]     The object  30  constructed in accordance with the teas of  FIG. 2  can be advantageously made with silk elements  10  formed from carded silk filaments derived from silk brins. Carded silk filaments are very thin (30 μm diameter) allowing the manufacture of thin-walled devices or composite sheets.  
         [0036]     The drum  210  can be so arranged that a continuous sheet made of composite material  30  emerges from the drum  210  as shown in the  FIG. 2 .  
         [0037]     As the composite material  30  is biocompatible, it can be used to make an implant, such as an endolumnal device, a stent device, an anastomosis is device, a sure, or a device to promote nerve regeneration. It is known that the silk elements  10  provide an excellent substrate for cell or tissue growth. Therefore some of the matrix material  20  of the object  100  can be abraded away to reveal some of the silk elements  10  on either an inner (or endolumnal) surface  230  of the object  30  or on an outer surface  240  of the object  30 .  
         [0038]     Another method for the manufacture of the object  30  in a tubular structure is shown in  FIG. 3  in which silk elements  10  are wrapped substantially longitudinally along the surface of a former  300  on both an inner surface  310  and an outersurface  320  of the former  300 . The former has typically a diameter between 0.5 mm but can have a diameter up to 10 m and has a long axis  325 . The former  300  could also be tapered. The matrix material  20  is applied over the silk elements  10  and allowed to dry and thus embed the silk elements  10  in the object. Application of the materials by dipping, painting, spraying or casting. The ends  330   a  and  330   b  of the object  30  can be cut away and the former  300  slipped out. A tubular object is thus created. Formers coated with a layer of wax or which can be caused to collapse into a smaller diameter can be used to facilitate removal of the object  30  from the former  300 .  
       EXAMPLES  
     Example 1  
       [0000]     Preparation of Composite Materials from Carded Antheraea Pernyii Silk Filaments.  
         [0039]     Antheraea pernyii silk filaments prepared from degummed silk cocoons were obtained from a commercial supplier. The carded silk from the sepin was smoothed into flat sheets and gently tensioned to pull the silk filaments parallel. Keeping the silk tensioned they were wound on to cylindrical formers 0.25 mm to 30 mm in diameter. The largest cylinders were made of glass and had a substantially smooth surface. The Smallest cylinders 0.25 mm were prepared by coating a thin stiff wire in low melting point wax. Cylindrical formers with intermediate diameters were made from low melting point wax. The silk filaments were generally laid circumferentially on the largest formers but were laid in a helical fashion on the smaller cylindrical formers.  
         [0040]     Care was taken to ensure a uniform dense lay of filaments was achieved on the cylindrical formers. The silk filaments at the ends of the thin cylindrical formers were secured with cyanoacrylate adhesive from Loctite. For the larger cylindrical formers the silk filaments at the ends were secured by a thin line of superglue running parallel to the long axis of the cylindrical former. This allows the removal of the composite material from the cylindrical former making a cut along the thin line of adhesive.  
         [0041]     The composite material was formed either by painting a thin coat of either cyanoacrylate adhesive or concentrated regenerated fibroin solution on to the silk filaments. The regenerated fibroin solution was prepared by dissolving commercial degummed  Bombyx mori  fibroin powder in aqueous 63 M lithium bromide with gentle stirring at room temperature. The silk solution was then dialysed in 18-20 kDa MWCO Visking tubes with two changes for a total of 6 hours at 4 degrees-centigrade against aqueous 0.5M lithium bromide solution with the intention of allowing the protein to refold after dissolution in the concentrated lithium bromide. Thereafter water was removed from the fibroin solution by reverse dialysis against aqueous 40% aqueous—or dry-powdered polyethylene glycol-(MW 15-20 kDa), pooling the partially concentrated fibroin after 12 hours—24 hours of dialysis to a fresh dialysis tube to obtain a final highly concentrated solution. Alternatively the dialysed silk fibroin solution was allowed to partially dry by exposing the sealed the dialysis bag to dry air. The matrix material was applied by painting the concentrated fibroin onto the silk filaments and allowed to dry. Thereafter the matrix was cross-linked by treatment with substantially dry formaldehyde gas by heating to 80-100 degrees centigrade for 5 minutes to 3 hours in a sealed vessel containing substantially dry paraformaldehyde. Unreacted formaldehyde was removed by heating the material to 100 degrees centigrade in a stream of air or by exhaustive washing.  
         [0042]     Testing of the composite materials for mechanical strength was carried out by cutting silk acrylate strips made using the larger cylindrical formers as described below in Example 3. An Instron universal mechanical testing instrument fitted with pneumatic grips was used to test composite strips to failure in uniaxial tension parallel to the lay of the silk filaments.  
       Example 2  
       [0000]     Demonstration of RGD Putative Integrin Binding Sites on Antheraea spp Heavy Chain Fibroins and their Location Adjacent to Turns.  
         [0043]     The published sequences of silk proteins were searched on the TrBMBL and SwissPROT data bases for the well established cell binding sequence RGD and the putative cell binding sites PPSRN and KNEED. The sequence triplet RGD is of considerable interest as it forms part of the mechanism used in multicellular organisms to stick many types of cells to the connective tissue framework of the body. The RGD triplet on the silk-like connective tissue protein fibronectin binds specifically to integrins. Integrins are a class of cell adhesion molecules. They are found intercalated into the lipid bilayer of the cell membrane with the fibronectin binding (RGD recognition) site protruding into the extracellular space. Thus the RGD recognition site of integrins is available-to bind cells to the connective tissue-framework via RGD triplets on fibronectin molecules.  
         [0044]     The putative cell binding sites PPSRN and KNEED were absent from all silks examined. However, multiple copies of the triplet RGD were found in the glycine rich domains of thee heavy chain fibroins of Antheraea pernyi (TrEMBL O76786), A. yamamai (TrEMBL Q964F4) and A. militta (Q8ISB3). There were 12-and 14 repeats respectively in the complete sequence of A. pernyii and A. yamana and an even higher density of repeats (7) in the partial sequence-of A. miitta. Then remarkably repetitive location of the RGD repeats is shown for A. pernyii in  FIG. 4 . The RGD motif recurs in a constant location, in the glycine-rich repeat immediately separated by one polyalanine repeat before the sequence SAARRAGHDRAAGS (sometimes truncated to RRAGHDRAAGS) or a closely similar sequence.  
         [0045]     In all cases the RGD triplet immediately preceded the motif GGYG. A. militta differed from the two other Antheraea sp in having two RGD&#39;s instead of one in one of its glycine-rich domains.  
         [0046]     The RGD triplet was absent from all other silk protein sequences examined with the exception of that for Samia ricini heavy chain fibroin which contained a single copy immediately preceding a YGSD motif in a glycine-rich domain close to the end of this very long sequence.  
         [0047]     Thus the cell-binding sequence RGD is found in multiple copies in the heavy chain fibroins of three Anthemea species.  
         [0048]     We then sought to discover if the RGD sequence is present in locations within the silk molecule likely to be accessible to cell surface integrins. To seek an answer to this we first used RADAR (Expasy tools) to determine the consensus sequence for the repetitive glycine rich domains of Antheraea pernyii heavy chain fibroin (see  FIG. 5 ).  
         [0049]     This showed that the sequence GGYGXGDGGYGSDS (where X=W or R) was well conserved within the glycine-rich domains. We then used a sophisticated secondary structure prediction tool, SCRATCH (Expasy tools) to determine the location of a putative turn within the consensus sequence (see  FIG. 6 ), as previous research (Peng et al. 2004 in preparation) has shown that tons are present within silk molecules as part of a tertiary structure defined by the existence of antiparallel β-sheets. A turn in the A. pernyi consensus sequence was found to be centered on a glycine residue immediately adjacent to the RGD. As each RGD triplet in fibronectin are located immediately adjacent to a turn (Leahy, Hendrickson et al. 1992; Leahy, Aukbil et al. 1996). We conclude that the RGD in Antheraea silks is likely to be available for binding to integrins and is therefore likely to strongly promote cell adhesion to this silk  
       Example 3  
       [0050]     Tensile Data on Antheraea Pernyi Cyanoacrylate Composite  
                                                           TABLE 1                           Tensile test data (n = 6) for wet strips of composite material containing       a high density of well-oriented  Antheraea pernyii  filaments set in a       cyanoacrylate matrix. The strips (50 mm gauge length, average width       10.5 mm and thickness 0.4 mm) were strained       parallel to the filament orientation                UTS                       (Mpa)   Strain to failure   Energy/kg (JKg)   Modulus (Mpa)                        Average:   31   0.150   1900   1000       SD   8.080   0.016   590   227                  
 
         [0051]     For comparison, the ultimate tensile stress of a typical-low alloy steel is in the region of 830 Mpa and a modulus of 200 Mpa but steel has a density of approximately 6 times that of the silk composite thus weight for weight this steel is less than 4 times as strong as the silk composite. The tensile data for the silk composite are comparable with that of that of two synthetic implantable materials (table 2).  
                                 TABLE 2                           Tensile data for wild silk cyanoacrylate composite compared       with two synthetic implantable materials.                    Poly ε     A. pernyi  acrylate       Property   Poly-L-lactide   caprolactone   composite               Ultimate tensile   45 MPa   22 MPa   31 MPa       strength       Ultimate extension   3%   500%   16%       Initial modulus   2.7 GPa   0.4 GPa   1 GPa                  
 
         [0052]