Med-Tech Innovation Materials
polypropylene with a sheath of an absorbable to lessen the amount of remaining mass of polypropylene following tissue in-growth. Other permanent polymer fibre options include polytetrafluoroethylene and ultra high molecular weight polyethylene. The potential for regenerative medicine in orthopaedic
and cardiovascular device applications has also grown exponentially. Replacing cartilage or bone with synthetic materials has been of interest and practice for decades, but companies are now recognising the benefits of not only building artificial implants, but also of using materials that enable the regrowth of damaged tissue and help the body heal itself. Absorbable fibres such as polyglycolic acid (PGA), poly-L-lactic acid (PLLA), polydioxanone (PDO) and other copolymers help to aid in repair and regrowth of native tissue before they are absorbed by the body and completely replaced with natural cells. For absorbable applications in particular, correctly selecting the most effective polymer is critical to device performance. Developers must be able to engineer a degradation profile and total lifespan alongside characteristics, including strength, abrasion resistance, elongation and pore size to ensure the successful growth of new tissue with the correct mechanical properties. Important considerations when selecting a fibre include characteristics such as • Denier: linear density, which also includes the filament count in a multifilament fibre bundle and can have an effect on density and the resulting possibilities for processing
• Tenacity: the strength per denier • Elongation: the fibre’s tendency to stretch or maintain its shape over time
• Postprocessing such as heat shrink: the amount of shrinkage at a certain time and temperature and determinative of possibilities for further downstream processing, including sterilisation. The benefits and drawbacks of each of these mechanical properties will differ depending on the end use and in vivo placement of the implant.
Hybrid composite fabric: Knit
combined with a non-woven
improved mechanical performance, anatomic accuracy and subsequent clinical benefit are virtually limitless. Braiding. This is one of the most commonly used
processing techniques for creating fabric-based implants. Braided structures are easily customisable, which means tubes, flat braids and other geometries with highly specific dimensions are possible. The braiding process produces textiles with great strength in a small surface area, which makes it ideal for orthopaedic applications such as ligament and tendon repair. For example, in small, inter- spinal cavities or joint spaces, braided structures are used to repair or replace a torn or floating tendon by providing strength within a compact space. Knee, shoulder and other small joint arthroscopic procedures in particular take advantage of braids, thanks to their strength, ability to expand and compress, and the ability to customise other performance capabilities according to the needs of the particular joint. For cardiovascular applications such as sewing
threads for aortic repair grafts, strength requirements are usually more precise. Small diameter braids are particularly useful for these kinds of applications, because they can be created in an extremely thin form factor without sacrificing strength. A secure stitch is critical to graft success, which means braids must meet precise specifications to deliver the necessary performance. Knitting. Orthopaedic meshes for indications such as lumbar and cervical disk devices are good examples of knitted textiles that provide strength and suppleness for devices that must accommodate movement at the site of the defect and in the affected soft tissue areas around it. Specifically, repair applications in parts of the body that undergo more severe instances of movement and stretch can benefit most significantly from the knitting process. Namely, containment sleeves for spinal disk repair, as well as implants and procedural assistance pieces for joints such as the knee and shoulder that move most frequently, are some of the most common types of structures that take advantage of the concentration of power provided by knitted textiles. A number of cardiovascular applications such as valve
repair patches and rings also utilise flat knit fabrics and tubular conduits. This is because their defined apertures and conformability fit the low-profile, high strength requirements necessary for device function. Weaving. This produces structures that provide the
Textile engineering for clinical precision Specialised textile engineering techniques allow device developers to take advantage of these kinds of unique properties for biomimetic construction and performance for a growing variety of orthopaedic and cardiovascular applications. Processes such as braiding, weaving, knitting and needle-punching nonwovens help magnify strength, texture, flexibility and many other performance characteristics for customised device requirements. For engineers, the possibilities for
24 ¦ September/October 2013
same strength of braids and knits without their stretch. With high tenacity, they hold their shape for support, repair and replacement functions that must retain their original form. Spinal restoration and tendon repair applications particularly benefit from the strength and stability of woven fabrics. Although lightweight, weaves are often extremely dense and can provide strength without losing their shape over time. This is a critical biomechanical property for ligaments and tendons that have to maintain strength through excessive use without turning loose or showing excessive creep with age. Possible in a variety of geometric styles, woven tapes and tubes can also be designed to meet specific requirements of strength and porosity. Vascular grafts, for example, require strength, but must keep from
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