Abstract:
A method and an apparatus for the manufacture of a single silk mono-filament. The single silk mono filament has a tensile strength of above 40 Newtons. The single silk mono-filament has applications as a musical string or a medical device.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    The present application claims the benefit of priority under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/106,479 filed on Oct. 17, 2008, and the priority under 35 USC 119(b) of United Kingdom Patent Application No. 0819056.3 filed on Oct. 17, 2008. The disclosures of U.S. Provisional Patent Application No. 61/106,479 and United Kingdom Patent Application No. 0819056.3 are hereby incorporated herein by reference in their respective entireties, for all purposes. 
     
    
     FIELD OF INVENTION  
       [0002]    The present invention teaches an apparatus and a method for the manufacture of a native single silk mono-filament with a tensile strength of at least 40 Newtons. The native single silk mono-filament can have applications for the use as a musical string and in medical devices. 
       BACKGROUND OF THE INVENTION 
       [0003]    The use of a silk filament as a string for a musical instrument has been known in China for more than 2000 years. A method for the manufacture of the silk filament for the use as the string for the musical instrument has remained essentially the same for many centuries. 
         [0004]    Strings used in musical instruments need to be placed under a high tension in order to ensure high quality and volume of the sound generated by the resonance of the musical string. It is known that the musical string used in a classical violin requires a tension ranging from 30 Newtons to 100 Newtons (see product brochure of string maker Thomastik Infeld GmbH, Vienna, Austria and the range of Vision Solo and Dominant strings of the product brochures of string maker Pirastro GmbH, Offenbach, Germany and the range of Evah Pirazzi, Obligato and Violino strings). 
         [0005]    Currently a native single silk mono-filament fiber can withstand a tensile strength of approximately 0.5 Newton. The tensile strength of 0.5 Newton is due to the diameter of the single silk mono-filament fiber which is in the range of 10-100 μm. The tensile strength of the native single silk mono-filament fiber is approximately 60 times less than the tensile strength of the violin string. Hence, when using silk fibers for a core of the musical string, string makers have to overcome the problem of insufficient mechanical strength of the native silk mono-filaments by combining a plurality of silk mono-filaments to manufacture the silk multi-filament fiber. The silk multi-filaments fiber, due to the combination of the plurality of silk mono-filaments fibers provides the sufficient mechanical strength that is able to withstand the tension required for use as the musical string. 
         [0006]    In general, the musical string that is comprised of silk is manufactured by collecting a large number of individual silk threads from a silkworm cocoon. The silk threads are then combined into bundles. The bundles are then twisted tightly together to form the silk multi-filament fiber. The silk multi-filament fiber is then immersed in liquid glue. The liquid glue provides improved mechanical and acoustic properties to the musical string that is comprised of silk. 
         [0007]    A detailed description for the manufacture of the musical string that comprises silk has been published by Alexander Raykov on the internet (see www.globalissuesgroup.com/silk-strings/howsilk.html). 
         [0008]    In contrast, musical strings that comprise of gut, polymers (nylon) or metal (steel) can be manufactured with the string core comprising of both mono-filament fibers or multi-filament fibers, these musical strings are able to withstand the mechanical tension required by the musical string. The choice between a string core manufactured from the mono-filament fiber or the multi-filament fiber provides string manufactures with a degree of flexibility for the development of musical strings with different musical characteristics and high volumes of sound. As a consequence the musical strings that comprise gut, polymers (nylon) or metal (steel) have now almost completely replaced silk strings for use as musical strings due to their broader range of musical characteristics and their easier handling. 
         [0009]    Today the use of the musical strings that comprise of silk are confined to niche applications such as historical and Chinese musical instruments. 
         [0010]    The state of the art for the manufacture of mechanically strong silk filaments follows two general strategies. 
         [0011]    The first strategy for the manufacture of mechanically strong silk filaments uses a method that combines individual native silk fibers into bundles. The bundles are then combined into ropes, whereby the ropes are used for making mechanically strong silk multi-filaments fibers. Such an example of this the use of silk multi-filaments fibers is in medical applications for the use in ligament replacement. Other examples of the use of silk multi-filaments fibers are in tennis racket strings. An example that discloses the combination of individual native silk fibers into bundles is described in US Patent Application Publication No. 2004/0224406 by Altmann et al. Altmann et al. teaches a method for manufacturing twisted ropes which are assembled from of a plurality of individual native silk fibers. The twisted ropes are used for manufacturing substitutes for ligaments in medical applications. The Altmann et al. document discloses (in table 1 and 4) an average tensile strength of 0.52 Newton to 0.9 Newton per native silk mono-filament. 
         [0012]    The second strategy for the manufacture of mechanically strong silk filaments uses regenerated silk. The regenerated silk is obtained by dissolving silk in a solvent and spinning the regenerated silk doped solvent by a variety of different spinning techniques. The International Patent Application, WO 02/081793 by John S. Crighton discloses the manufacture of silk filaments from regenerated silk, wherein the silk filaments have a tensile strength of 1.2 Newtons per silk filament. 
         [0013]    To date there is no published method or apparatus for the manufacture of single silk mono-filaments with a tensile strength above 10 Newton per silk mono-filament. 
         [0014]    A method and apparatus that is capable of manufacturing single silk mono-filaments with a tensile strength of 40 Newtons and higher would therefore be highly advantageous. 
         [0015]    The manufacture of silk films and silk membranes from regenerated silk fibroin or artificially-made silk proteins and peptides is known. An example for the manufacture of regenerated silk membranes is described in the International Patent Publication No. WO 2005/012606 by Kaplan et al. The Kaplan et al. document discloses the manufacture of silk fibroin films from regenerated silk by dissolving a silk protein in a protein denaturing solvent. An example of the artificial silk protein membrane is described in International Patent Publication No WO 2006/008163 by Scheibel et al. The Scheibel et al. document teaches the manufacture of silk fibroin films from artificial silk proteins. 
         [0016]    However, there is only a small amount of prior art that discloses the manufacture of silk fibroin membranes from native silk fibroin protein solutions which are manufactured without the use of protein denaturing agents such as strong salts, solvents, heat or other protein denaturing conditions. For example, U.S. Pat. No. 7,041,797 (by Vollrath) and the International Patent Publication No. WO 2007/09851 (by Rheinnecker et al.) teaches the manufacture of native silk fibroin solutions. Upon using the inventions by Vollrath and Rheinnecker, the present applicant has discovered that products manufactured from native silk fibroin solutions tend to undergo contraction during the drying process of the silk products. The contraction leads to irregular deviation from the intended shape of the silk products. For native silk protein products with a thickness of up to 0.2 mm, these deformations are less pronounced and can be reduced to edge effects. However, for silk products with a required thickness above 0.2 mm the final physical shape of those products after drying is difficult to control. For example, silk membranes with a thickness of above 0.2 mm which are casted from a native silk fibroin solution often develop irregular and uneven surfaces and shapes. These silk membranes require further mechanical treatment after casting. 
         [0017]    There is therefore a need for an improved casting technique which avoids the physical deformation which occurs during the drying of the protein solution and a need that allows the manufacture of silk protein products with a thickness of more than 0.2 mm. 
       SUMMARY OF INVENTION  
       [0018]    A casting apparatus and a method for the manufacture of an object, such as a single silk mono-filament fiber is discussed. 
         [0019]    The casting apparatus comprises a solid support with at least one permeable surface for supporting a first surface of the single silk mono-filament fiber. The solid support has an exposed region allowing a second surface of the single silk mono-filament fiber to be in contact with a gas. 
         [0020]    The water-permeable surface avoids problems associated with the physical deformation of the single silk mono-filament fibers that occurs during drying of a native silk protein solution. 
         [0021]    The use of the water-permeable surface in the casting apparatus enables the evaporation of the solvent of the native silk protein solution not only to the air/solvent interface, but also through the contact of the native silk protein solution with the first surface of the water-permeable surface by diffusion. 
         [0022]    The water permeable surface improves the drying process of the native silk protein solution which could not have been anticipated by the prior art. 
         [0023]    Products of single silk mono-filament fibers that are manufactured according to the present disclosure are described. 
         [0024]    A further object of the present disclosure teaches that naturally occurring, non-artificial silk proteins derived from Bombyx Mori silkworms can be manufactured into single silk mono-filament fibers which can withstand tensile forces of at least 40 Newtons and above. 
         [0025]    A further object of the present disclosure is a use of the manufactured single silk mono-filament fibers as a string for a musical instrument and for applications in a medical device. 
     
    
     
       DESCRIPTION OF FIGURES  
         [0026]      FIG. 1  shows a schematic for a method for the manufacture of a single silk mono-filament fiber. 
           [0027]      FIG. 2  shows an apparatus for manufacture of a single silk mono-filament fiber. 
           [0028]      FIG. 3  shows a cross sectional view of a single silk mono-filament fiber. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0029]    For a complete understanding of the present disclosure and the advantages thereof, reference is now made to the following detailed description taken in conjunction with the figures. 
         [0030]    It should be appreciated that the various aspects of the disclosure discussed herein are merely illustrative of the specific ways to make and use the technology and do not therefore limit the scope of the technology when taken into consideration with the claims and the following detailed description. 
         [0031]    A method for the manufacture of a single silk mono-filament fiber is shown in  FIG. 1 . In a first step  100 , a silk protein solution  10  is manufactured with a silk protein content of between 0.3% and 30% (w/w.). The silk protein solution  10  is manufactured as described according to the U.S. Pat. No. 7,041,797 B2, the teachings of which are incorporated herein by reference. The silk protein solution  10  is then transferred onto a water permeable surface  40  of a casting device  20 . The casting device  20  comprises at least one water-permeable surface  40 . The water-permeable surface  40  is present as a base of the casting device  20 . 
         [0032]    The casting device  20  can be made from glass, plastic or can be made from polytetrafluoroethylene (PTFE). The casting device  20  can also be made from any other material that is suitable for use with the silk protein solution  10 . 
         [0033]    The water-permeable surface  40  can be any one of a water-permeable material, such as clay or a protein compatible polymer-based water-permeable membrane. 
         [0034]    In the next step  110 , the silk protein solution  10  is dried in the casting device  20 . When dry, the silk protein solution  10  forms a silk membrane cast  30 . The duration time for drying the silk protein solution  10  depends on the protein content of the silk protein solution  10  and the rate of evaporation of the solvent of the silk protein solution  10 . The evaporation rate of the solvent of the silk protein solution  10  can be varied for example by the use of a vacuum and or an air flow. 
         [0035]    In the next step  120 , the formed silk membrane cast  30  is removed from the casting device  20 . 
         [0036]    In the next step  130 , the silk membrane cast  30  is cut to give at least one individual silk filament  50 . 
         [0037]    In the next step  140 , the silk filament  50  is stretched by a mechanical means. 
         [0038]    In the next step  150 , the silk filament  50  is polished to yield the single silk mono filament  60 . The single silk mono filament  60  has essentially a cylindrical shape (see  FIG. 3 ). 
         [0039]    In a further aspect  160 , the single silk mono filament  60  may be further optimized by coating the single silk mono filament  60  with a surface layer  70 . The single silk mono filament  60  may be coated with the surface layer  70  to improve the resistance of the single silk mono filament  60  to water. The single silk mono filament  60  may be coated with the surface layer  70  by coating with a metal wire or by coating with a polymer fiber. 
         [0040]    In a further aspect of the present invention, the material properties of the single silk mono filament  60  may be further enhanced by the introduction of an impregnated layer  80 . The impregnated layer  80  is a substance that is embedded between the single silk mono filament  60  and the surface layer  70 . The impregnated layer  80  may for example be a polymer fiber. 
         [0041]    The following example for carrying out the present disclosure is offered for illustrative purposes only and is not intended to limit the scope of the technology in any way. 
       Example 1 
       [0042]    The silk protein membrane cast  30  was made by transferring a  450  ml silk protein solution  10  with approximately a 10% silk protein content into the casting device  20  (390 mm×110 mm×20 mm). The casting device  20  comprises a base of water permeable surface  40 . The water permeable surface  40  is a water permeable modelling clay (Glorex GmbH, Art No. 68075201). 
         [0043]    The silk protein solution  10  was manufactured according to the disclosure of international patent application publication No. WO 2007/098951, the teachings of which are incorporated herein by reference. 
         [0044]    After filling the casting device  20  with the silk protein solution  10 , the casting device  20  was positioned such that air was able to circulate around the top and around the bottom of the casting device  20 . The ability of air to circulate around the top of the casting device  20  enables efficient evaporation of the solvent from the silk protein solution  10 . The ability of air to circulate around the bottom of the casting device  20  facilitates diffusion of the solvent of the silk protein solution  10  through the water permeable surface  40 . 
         [0045]    After drying the silk protein solution  10  at room temperature, a silk membrane cast  30  with a thickness of between 0.5 mm and 1.2 mm was manufactured. The thickness of the silk membrane cast  30  depends on the volume and the concentration of the silk protein solution  10 . 
         [0046]    The silk membrane cast  30  was then cut into individual rectangular silk filament  50  samples (390 mm×1 mm×1 mm). 
         [0047]    The silk filament  50  samples were then stretched manually to approximately twice their original length into the single silk mono filament  60 . 
         [0048]    Three samples of the single silk mono filament  60  were then weight tested to determine the tensile strength using a digital balance (Kern CH50 K50). The three samples of the single silk mono filament  60  showed tensile strength of 53 Newtons, 44 Newtons and 54 Newtons, respectively. 
         [0049]    The fact that such a tensile strength can be achieved with the single silk mono-filament fibers manufactured from native silk protein materials and not through bundling methods of a plurality of silk filament fibers or through use of a spinning technology was surprising and not predictable from the prior art. 
       Example 2 
       [0050]    The silk protein membrane cast  30  was made by transferring a 80 ml silk protein solution  10  with approximately a 10% silk protein content into the casting device  20  (80 mm×80 mm×20 mm). The casting device  20  comprises a base of water permeable surface  40 . The water permeable surface  40  is a water permeable modelling gypsum (Pufas Werk KG GmbH, Modellgips für Bau+Hobby). 
         [0051]    The silk protein solution  10  was manufactured according to the disclosure of international patent application publication No. WO 2007/098951, the teachings of which are incorporated herein by reference. 
         [0052]    After filling the casting device  20  with the silk protein solution  10 , the casting device  20  was positioned such that air was able to circulate around the top and around the bottom of the casting device  20 . The ability of air to circulate around the top of the casting device  20  enables efficient evaporation of the solvent from the silk protein solution  10 . The ability of air to circulate around the bottom of the casting device  20  facilitates diffusion of the solvent of the silk protein solution  10  through the water permeable surface  40 . 
         [0053]    After drying the silk protein solution  10  at room temperature, the silk membrane cast  30  with a thickness of approximately 1 mm was manufactured. The thickness of the silk membrane cast  30  depends on the volume and the concentration of the silk protein solution  10 . 
         [0054]    A listing of reference numerals and correspondingly referenced elements is set out below. 
       Reference Numerals 
       [0000]    
       
           10  Silk protein solution 
           15  Exposed surface 
           20  Casting device 
           30  Silk membrane cast 
           40  Water-permeable surface 
           50  Silk filament 
           60  Single silk mono filament 
           70  Surface layer 
           80  Impregnated layer 
           90  Second water permeable surface 
       
     
         [0065]    Having thus described the present technology in detail, it is to be understood that the foregoing detailed description of the technology is not intended to limit the scope of the technology thereof. What is desired to be protected by letters patent is set forth in the following claims.