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Traditional Materials, Advanced Technology

11/04/2012


Applying textile engineering concepts to biocompatible metals and polymers creates implantable medical fabrics with a broad array of features that will enable future innovations, says Stephanie Lietz of Secant Medical.

Biomedical textiles are fast-becoming a materials platform that allows device designers to push the envelope of what is possible in device design. Currently, the medical device industry relies heavily on existing rigid and semi-rigid form factors such as plates, screws, injected plastic moulding and tubular structures -for creating medical devices. Unfortunately, these materials can limit design parameters. Biomedical textile engineers have the ability to work with the same raw materials (biocompatible metals and plastics), but by applying textile engineering concepts they can create implantable medical fabrics that enable a host of features and benefits not previously possible in device design.

One of the primary benefits of textiles is they have a form factor that is conducive to promoting healing. In addition, the unique properties of textiles make them a viable technology choice for minimally invasive device design. Although textiles are being used today in minimally invasive surgery (MIS), their design benefits have not been fully leveraged. The key is maximising these inherent design benefits through strategic and creative biomedical textile structure engineering.

Textile engineering
The process for engineering biomedical textiles begins with extruding polymers into a fibre form or drawing metals into wire forms. Textile structures are created from these extruded fibres or wires using traditional textile forming techniques of knitting, weaving and braiding, which are then incorporated into devices as device components.

There is a broad spectrum of biocompatible polymer filaments from which to choose, including, polyethylene (PET), polypropylene, polyetheretherketone (PEEK), polytetrafluoroethylene (PTFE) and absorbables such as polyglycolides (PGA), polylactides (PLLA), polycaprolactone (PCL) and various co-polymers. Metals such as nitinol, cobalt-chrome, stainless steel, platinum, titanium and tantalum are among the other material choices that are available. Hybrids or combinations of these biomaterials also present an opportunity for engineering a textile-based component with specific desired physiological or mechanical responses.

The three textile forming technologies use metallic and polymeric raw materials to create device components. Each has flexible and versatile mechanical and physical properties that lend themselves to the creation of many unique structural geometries.

Woven textiles
Woven fabrics involve two independent fibre or wire systems interlacing at right angle relationships in an over–under perpendicular pattern. With this approach, there is an ability to control pore size and density while adding the unique ability to create various structures such as tapered, tubular, flat, button hole and near-net shapes. These fabrics are dimensionally stable, possess high strength and can be applied to a wide range of materials, which makes woven textiles highly versatile for device designers.

Knitted textiles
Another useful forming process is knitted textiles, which are created by the interlocking of loops of yarn or metal in a weft (transverse stitching) or a warp (longitudinal stitching) pattern. This technology creates flat, broad or tubular structures that are highly conformable and have the ability to be compressed and inserted into small incisions via a cannula or catheter. High burst and tensile properties are determined or influenced in part by biomaterial selection. For example, knits can be engineered to yield potentially fast in-growth properties by designing a high surface area geometry. All properties can be enhanced when combining the right geometric structures with the appropriate raw material, which allows the engineer to "tune” the fabric properties for the desired outcome.

Weft and warp knits both have unique properties that are beneficial for use in different therapeutic areas. The general properties of weft knits include excellent elongation and recovery, controllable thickness and consistent pore size and shape. General warp knits have controllable elasticity in both directions although within limits, good dimensional stability, and good to excellent tensile/burst strength.

Braided textiles
Braided textiles are created by interweaving three or more yarns of one or more material in a diagonally overlapping pattern. Braided textiles can be formed over mandrels (shaped metal bars) of varying cross-sections to produce near-net shape structures. These structures can utilise a wide range of biomaterials and, uniquely, can blend both polymeric and metallic materials in their designs. Braids are generally flexible, porous and kink-resistant when forced around torturous anatomy. They can be designed to have varying density with the ability to compact into catheters and to expand in dimensions as needed.

By strategically pairing raw biomaterials with textile forming technologies, textile properties can be enhanced, combined and leveraged to benefit specific device requirements, depending on the intended biological outcome. Textile engineers understand the fine nuances of blending the properties of biomaterials with the appropriate medical fabric forming technologies. Using a collaborative and evaluative approach, they can guide device engineers towards the most promising combination for the challenge at hand. Advances in architecting hybrid fabric structures allow device manufacturers to push the design bar in accommodating the need for lower profiles, desired porosities and permeabilities, as well as intended biologic responses required by today’s increasingly sophisticated device designs.

Enhancing devices across therapeutic areas
Biomedical textile engineering is both an art and a science that enables device designers to revolutionise MIS device design by creating a shift in the mind set of the designers to innovate the exacting properties required for unique therapeutic areas. There are many examples of therapeutic areas that benefit from applying biomedical textile engineering to device design; this brings benefits throughout the supply chain from manufacturer, to surgeon, to patient.

 





Woven tubular structure that has the required porosity characteristics for catheter-based deliveries










Cardiovascular. Biomedical textiles have an extensive history in the cardiovascular device sector, but are now being used in innovative ways to solve additional therapeutic concerns. The endovascular aneurysm repair (EVAR) system incorporates a biomedical textile graft, which is used to treat abdominal aortic aneurysms and thoracic aortic aneurysms. This procedure utilises a catheter-based delivery system, making textiles the optimal materials choice. A woven textile graft is used for EVAR because of its low porosity and thin wall design for easy compaction into a catheter. This textile also offers high burst strength, which makes it ideal for withstanding pulsatile forces.







A multilayered scaffold structure that can be used for engineered tissue response

  






Orthopaedics. The orthopaedics device market, in particular, can benefit from the use of biomedical textile components as device makers continue to look for solutions that involve non-rigid implants (so-called "soft” implants) and the increasing focus on less-invasive medical procedures. The functional flexibility and strength of implantable textile structures can help preserve a patient’s natural range of motion, which is a crucial aspect of orthopaedic patient recovery, especially in spinal devices. One example is the use of textiles in spine stabilisation systems, which incorporate a PET braided tether in the design as an important alternative to rods used in traditional spinal fusion procedures. Arthroscopic procedures and sports medicine also incorporate biomedical textiles. An excellent example is the use of woven structures as orthobiologic tissue scaffolds to promote growth of new tissue and encourage faster healing and increased longevity of the repair for torn rotator cuffs.

Neurology. Neurovascular applications lend themselves well to the minimally invasive properties of textiles because of the unique requirements of working with small, precision devices that are typically delivered via micro-catheters. The challenge in neurology is to provide fabric structures that maximise performance, but minimise the delivery profile. This makes textiles almost a necessity for creating the next generation of effective neurologic devices. A good example is the neurovascular stent, created with a fine metal wire such as nitinol or cobalt-chromium, which is usually braided into a tubular shape. The combination of braiding, metals and tubular fabric geometry make this an effective therapeutic option for treating neurovascular aneurysms.






Small diameter, high braid angle,
cobalt-chromium braided tube
that can be used for
neurovascular applications








Textiles have a history of being used to advance device design and those listed above are just a brief cross-section of examples. Given the right combination of biomaterials, forming technologies and fabric geometries, device designers and manufacturers have a truly broad array of choices at their disposal to help enable future innovations.

Revolutionising medical device design
The future of biomedical textiles in innovative devices is just beginning. Device engineers continue to research and explore the possible application of biomedical textile technologies with advanced biomaterials. Therapies in sports medicine and neurology are likely to see a strong focus on novel ways to incorporate textiles to achieve specialised features and benefits. Biologic materials such as collagen are also being investigated as fibres and filaments for tissue engineering. Different hybrids and combinations of advanced biomaterials are being researched today to investigate the potential performance and mechanical properties that can evolve into implantable devices of tomorrow.

Technology aside, old fashioned human collaboration is truly the essential ingredient for success. When a medical device engineer works with a textile engineer all they need to bring to the design discussion is the problem, that is, what they are seeking to solve through the use of an implantable fabric. The experienced textile engineer can serve as a design partner in determining the most appropriate biomaterial to use, the most effective textile forming technology to employ and the specific geometric structure to best meet the performance requirements of the device. The engineering of a biomedical textile component takes device design to a new plateau. It requires a deep knowledge of biomaterials, textile technologies and, most importantly, an understanding of human biology. With a creative mind, the future possibilities, applications and uses for biomedical textiles are nearly endless.

Stephanie Lietz is Programme Engineering Manager at
Secant Medical, 700 W. Park Avenue, Perkasie, Pennsylvania 18944, USA
tel. +1 215 257 8680
e-mail: stephanie.lietz@secantmedical.com
www.secantmedical.com




   

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