1.  Technical Field of the Invention
The present invention generally relates to the field of prostheses for surgical applications, to methods of their manufacturing and to methods of treating a patient by implanting them into a patient.  More particularly, the present invention relates to prostheses having a multi-layered sheet structure and their use in hernia repair, the repair of anatomical defects of the abdominal wall, diaphragm, and chest wall, correction of defects in the genitourinary system, and repair of traumatically damaged organs such as the spleen, liver or kidney. 
2.  Description of the Related Art
Abdominal wall repairs and especially hernia repair are among the most common surgical operations in the United States.  A hernia occurs when the inside layers of the abdominal wall weaken and then bulge or tear, causing the abdomen lining to push through the weakened area to form a balloon-like sac.  The intestines or abdominal tissue slips into the sac, causing pain and risk of damage.  Small hernias can be repaired with sutures but larger hernias are treated by surgically inserting mesh prosthesis into the peritoneum (the membrane separating the body organs from the muscles and fat layers) and securing it in place with sutures or tacks.  The prosthesis is usually inserted into an intra-peritoneal location to reinforce the weakened abdominal wall to prevent the balloon sac.  FIG.  1 shows an example of a hernia defect and the placement of a mesh to rectify the hernia defect. 
Similar mesh prostheses are commonly also used in other surgical procedures including the repair of anatomical defects of the several walls or diaphragms, correction of defects in several lumens or in the genitourinary system, and repair of traumatically damaged organs such as internal organs.  Thereby, weakened walls can be reinforced or completed by such mesh prostheses.  Sometimes, the prostheses are wound around the organ to serve as a reinforcing member.  All such prostheses are usually made from a textile material such as mesh fabrics of woven or knitted fibers or filaments.  In the last decades, a variety of different materials for the mesh fabrics have been proposed. 
Polypropylene (PP) materials have widely been used in meshes for hernia repair since the 1960's.  Incremental innovations were introduced along the way.  However, to date, polypropylene has remained unsatisfactory.  It has poor tensile strength and elongation and suffers from significant aging effects caused by the formation of microcracks in the polypropylene material which drastically reduce its strength and flexibility over time.  In the course of implantation, polypropylene also shows some degree of shrinkage.  Polypropylene also results in frequent, significant and unacceptable connective tissue adhesion, which invariably leads to inflammation.  The degree of tissue connection correlates directly to the degree of inflammation.  If the inflammation response is high, this may result in rigid scar plate formation.  Connective tissues adhesion can also cause severe discomfort and even medical trauma in the patient and can lead to the necessity for a premature surgical replacement of the mesh.  FIG.  2 shows examples of such extensive visceral adhesions surrounding a polypropylene mesh due to which an additional surgical treatment was necessary. 
Polytetrafluoroethylene (PTFE) materials were introduced in the early 1990's to separate tissue from the viscera when closing abdominal wounds.  PTFE was further adopted as an enabling technology to perform laparoscopic ventral/incisional repair procedures intra-abdominally.  There have been material and performance issues associated with PTFE materials used in hernia repair.  Most PTFE materials are extruded and made into sheets.  These PTFE sheets had been implanted into the body of a patient, i.e., seroma formation, infection, and sheet shrinkage post-implantation.  It is widely-known that a PTFE sheet shrinks on average 34% in 10 to 14 days post-implantation.  Many of these patients had to be an additional surgery in order to remove the implanted prosthesis due to these complications. 
Recently, PVDF has been shown to be fully biocompatible and can generally possess high strength and flexibility, which has been shown not to age and change over time.  It also has almost no shrinkage when used over time, and has significantly smaller tissue adhesion issues.  Thus, it has been proposed as a suitable material in textile-based meshes for surgical applications. 
In the light of the above experience with sheet-like materials, most of the current hernia meshes are made from filaments woven into a mesh.  While easy to produce by weaving, such a process limits the design possibilities of the mesh.  For example, based on the anatomical characteristics of the human abdomen, an optimally compatible mesh should have a higher degree of stretch and flexibility in one direction over the other perpendicular direction.  This is not easily achieved using a filament weaving process.  Also, when stretched, such a filament woven mesh would undergo a reduction of pore size of its open cells or pores (the space between the filaments).  It is known that such a reduction of pore size would also detrimentally increase the likelihood of visceral tissue adhesion.  However, despite of these disadvantages of woven meshes, the common prostheses used in connection with hernia repairs are made of different textile materials such as mesh fabrics.  Examples of such mesh fabrics are disclosed in US Patent Application No.  2007/0250147 A1.  Knitted and woven fabrics constructed from a variety of synthetic fibers and the use of the fabrics in surgical repair is also known (e.g., U.S. Pat.  No.  3,054,406). 
There are two key problems still unresolved in current prosthesis such as prosthesis for hernia repair.  One is the undesired viscera tissue adhesion to the prosthesis, i.e., the adhesion of the organs inside the abdomen to the prosthesis, coming as a response to the surgical procedure, and inflammation caused by the prosthesis to the peritoneum.  The occurrence is very high with existing prosthesis such as the above-described hernia meshes and results in severe continuous pain, immobility and bowel related problems.  Approximately above 30% of the patients require re-surgery due to this adhesion phenomenon.  The other key problem is that the mechanical behavior of the current prostheses changes adversely over time due to polymer swelling and/or aging, resulting in poor flexibility and strength.  Over time, the prosthesis stiffens and no longer flex in compliance to the abdominal wall movements in any directions, causing very poor anchoring and compliance.  Often, the prosthesis would completely break and need urgent surgical replacement. 
It is therefore an object of the present invention to provide a prosthesis that overcomes some of the above explained difficulties.