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Biomedical Textiles Stay in Step with the Beat of Cardiovascular Designs

Fri, 10/28/2011 - 11:02am
Jeffrey M. Koslosky

Material options for medical devices are not limited to only plastics or metals. Biomedical textiles play an important role, serving as solutions for a number of areas of the medical device community, including the cardiovascular sector. This article provides insight into how these textile solutions are fabricated, what specific materials are utilized, and what unique products are able to be realized.

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Woven tubular structure has the required porosity characteristics for catheter-based deliveries.
Since the 1960s, biomedical textiles have been used in cardiovascular medical devices, such as vascular grafts and heart valve sewing cuffs. However, the current wave of innovation is looking far beyond traditional materials and textile structures to enhance capabilities and performance in the repair of damaged or diseased cardiovascular tissue. In fact, the advent of new fabrics and geometries with greater variability of properties and performance characteristics, including the combination of resorbable and nonresorbable polymers, has enabled design developments previously unimagined.

Design Options and Applications
Implantable textiles typically contain polymeric and/or metallic filaments and yarns formed through weaving, knitting, braiding, or a non-woven process designed to manipulate the materials to achieve certain mechanics. This manipulation allows controlled entanglement or intertwining of the yarns and fibers, resulting in a high degree of design freedom and flexibility for design engineers. What is exciting about textiles as a material component is their versatility and the options available through the unique combinations of advanced biomaterials with their distinct performance characteristics, fiber architectures, and complex fabric geometries.

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Knitted tubular structure supports ingrowth of soft tissue with the necessary high burst and tensile properties.
Knitted structures are formed by interlocking loops of yarn or metal in a weft (transverse stitching) or warp (longitudinal stitching) pattern to form flat, broad, or tubular structures. Knitted materials are generally porous, highly conformable, and elastic with high burst and tensile properties. They are particularly well suited for soft tissue and areas with complex anatomies commonly found in cardiovascular applications (i.e. vascular prosthesis, hemostasis products, valve sewing cuff, and cardiac support devices) because the surface and open space areas spur tissue in-growth.

Woven structures are formed by the interlacing of two yarns or wires in a perpendicular fashion. The material used dictates the density and the physical and mechanical properties of the finished product as well as how the body reacts when the device is implanted. There is a multitude of potential shapes, including flat, tubular, tapered, or near-net shaped fabrics that often are characterized by low porosity (important for containment and cardiovascular fluid transfer), dimensional stability, high tensile strength, and other unique features, such as multilumens, fenestrations, and tube-in-tube geometries. In addition, woven structures can be manufactured in low-profile fabrics as thin as 40 to 50 microns, which is important in minimally invasive cardiovascular applications. Weaves have historically been used in endovascular aneurysm repair, transcatheter heart valves, and other catheter-based technologies.

Braided structures are created by intertwining three or more yarns in a diagonally overlapping pattern, typically at 45 degrees off the main axis, in a manner similar to a suture. A number of design elements can be manipulated, such as the angle of the braid and density of the interlacing, to achieve certain characteristics. Also, braided structures are often manufactured over mandrels to fix the fabric’s internal diameter and to create near-net shapes. In this scenario, geometric foreshortening occurs with the application of axial tension to result in radial compression, which can create a structure that is an excellent candidate for catheter-based deliveries.

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Nitinol braid provides flexibility and radial reinforcement for highly angulated applications.
Braided structures can also be kink-resistant and can easily be combined with different materials to enhance specific fabric properties. It is ideally suited for applications that need radial reinforcement and expansion, compaction, flexibility, porosity, and highly angulated vasculature, such as in neurovascular applications, cylindrical stent implants that support vein function and keep arteries open, and instrumentation in small delivery sites.

Out-of-the-Box Capabilities and Potential
The mechanics of materials, or hybrids of dissimilar biomaterials, can also be leveraged to promote certain mechanical or biological responses in the body. For example, yarn architecture can be varied to alter fiber cross-section and stiffening, and to create gradient levels of absorption. By using combinations of monofilament or multifilament non-absorbable materials, such as polypropylene, polyethylene terephthalate (polyester), polytetrafluoroethylene, and polyetheretherketone (PEEK), with absorbables, such as polyglycolides (PGA), polylactides (PLLA), and various other copolymers, engineers can design in the functionality of the device. Material type and the exponential combination of materials can also impact the biologic repair by controlling the degradation or absorption rate of the implantable textile structure to range between 60 days and 12 months.

Alternatively, by using the principals of basic fabric geometry and carefully selected materials, the fabric architecture can become a platform for composite properties. For example, regions of specialization within a tissue can be created by using texture, such as a smooth surface in one area and a rough surface in another, in prompting different tissue responses in contiguous areas of the tissue to create a biological seal. Also, biphasic mechanical properties of the composite biomaterials allow for changes in the mechanical property of a textile over periods of time during its resorption by the body.

Truly novel concepts are leveraging engineered fibers and fabrics to elicit a specific tissue response that is ordered and predictable. To accommodate the unique property characteristics and specialization of the device requires the calculated evaluation of material options, including the fiber material and structure and the overall fabric structural composition. Advances in architecting hybrid fabric structures continue to push the design bar with their tendency to accommodate the need for a lower profile or the predicted porosity, permeability, or biologic response rates required by the device.

In the cardiovascular device segment, synthetic structures and tissue hybrids have become the preferred materials for long-term repairs due to their ability to withstand the biomechanics of a specific application at the time of implantation. These become the transitional biomechanical bridge to allow for device in-growth. Fabrics are also used as conductive materials and tissue-engineered scaffolds to illicit a specific biologic response. There are opportunities to use fabrics as a drug-delivery platform by embedding the dosage into the textile layers and engineering controlled-release kinetics to affect the rate of absorption by the body. Biologic materials, such as collagen, are currently being explored as another materials platform for tissue engineering in the form of fibers and filaments that can be knitted, braided, or woven into a textile structure.

That said, "off-the-shelf" biomedical textile solutions generally do not work well. Each device application is unique and identifying the most appropriate biomaterial supplier is crucial to development. A multi-disciplinary team of engineers from both the device manufacturer and textile supplier organizations should begin collaboration early in the process and be involved when evaluating the material and design options.

Conclusion
As device designers continue to work with textile engineers to apply more creative, out-of-the-box designs using the material choices and textile geometries available, the device industry will keep in step to the beat of cardiovascular innovations in advancing medical technologies and procedures for tomorrow.

Jeffrey M. Koslosky is the vice president of advanced technologies for Secant Medical, LLC. He is responsible for the integration of business development and marketing in addition to product development, materials, biology and chemistry. Koslosky can be reached at 215-257-8680 or jeff.koslosky@secantmedical.com.

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