biomedical textiles, specializing in braided and
nonwoven components for medical devices.
Precision Micro-Braiding for Implantable Devices, Part One
Braided textile structures are among the most common fabric forming methods used in medical devices because of the unique properties that they exhibit. Braided textiles, in the form of sutures have been well exploited in applications such as wound closure. Sutures may be designed from a wide variety of polymeric materials specifically optimized for strength, visibility, intended longevity and tissue dynamics. More recently, modern medical device applications utilizing braided fabrics have leveraged unique properties of Nitinol wire in a device designed to close an atrial septal defect in the heart. While these fabrics have advanced the materials technology available to biomedical engineers and device designers, they fall short of maximizing the potential benefits of a braided architecture.
In recent years, leading biomedical textile firms have pushed the boundaries of traditional braiding technology to give rise to precision micro-braiding. Simply put, micro-braiding is a significant increase in the density of a braided fabric while using ever finer metallic and/or polymeric filaments. The resultant structures enable medical device companies to have biomaterial components at their disposal having very unique physical and mechanical properties for next generation implantable devices.
Braiding Overview and Engineering Features
Braiding is defined as the intertwining of 3 or more filaments, commonly referred to as ends, in a diagonally overlapping pattern. Most braids utilize an even number of filaments resulting in a round braid while those created with an uneven number yields a flat braid. The filaments that are used in medical braiding include a wide range of biomaterials including polymers such as polyester, polypropylene, polytetrafluoroethylene, and a growing list of various resorbable materials. Metallic filaments and alloys such as Nitinol, stainless steel, Cobalt-Chromium, Platinum and others are used as braid elements as well.
The engineering behavior of the braid becomes a function of numerous variables that the skilled textile designer has control over. Central among these is the density of the braid filaments and the amount of intersections they make in a given unit length. Many common braid machines have a fixed capacity of carriers that direct and tension the ends during the braiding process. Braiding machines that have an end capacity of at least 144 elements are thought to be high density and capable of micro-braiding.
The ends are loaded onto braider carriers and are spaced uniformly around the circumference of the braiding machine with half rotating clockwise and the remainder moving counter-clockwise in an alternating over and under pattern. This undulating effect creates the braid pattern that can be manipulated to allow for closer packing of the filaments by creating fewer filament crossings.
Engineers have utilized the unique geometry of braided structures to achieve certain physical and mechanical properties when used in implantable devices. Many braids are formed over a “mandrel” to create a hollow lumen with a specific cross-sectional shape and size. In hollow lumen and flat braids, another important variable is the braid angle. The alpha angle is the measurement of the included filament angle against the central axis of the braid. This angle cannot be directly measured, unless there is a visible central axis element. The beta angle can be directly measured, as it is the angle between two intersecting filaments in the braid structure. The axial density of the braid can be changed to alter the braid angle, and the braid angle can be manipulated to demonstrate a braid’s most interesting geometric feature, foreshortening.
Many are familiar with the “finger trap” style of a child’s toy whereby fingers become locked inside the toy and are unable to be removed because pulling increases inward tension on the fingers. In cylindrical, hollow-lumen braids, the ends combine to form an interdependent system that results in axial forces being translated to radial compressive forces. The inverse is also true. And so the trick of a finger-trap is to push from both ends to release your finger.
This engineering feature can be used in many ways in medical device design. Dynamic behaviors can be engineered that cause the structure to collapse radially for easy loading into a catheter for minimally invasive delivery. Once delivered to the correct location by a surgeon, a braided device can be unsheathed or actuated to recover its original size, effecting an in situ shape transformation not possible with many other types of implants. It is useful to explore how these braid mechanics can be utilized in high-density micro-braiding applications.
Continue reading at Precision Micro-Braiding for Implantable Devices, Part Two
For more information on how US BioDesign can accelerate your next medical device development project, please contact us at http://www.usbiodesign.com/contact-us.