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Precision Micro-Braiding for Implantable Devices, Part Two

Micro-Braiding in Implantable Devices

The advent of micro-braiding—ultra-high density braids made from very fine wire and polymeric filaments—opens up a new realm of possibilities for implantable device design.  It is now possible to produce fabrics with very small pore-sizes, essential in cardiovascular and neurovascular applications, where dense fabrics are required to slow down or stop turbulent blood flow.  Another critical advantage of micro-braiding, is the use of filaments that often have a diameter of 25 microns and below.  The use of these very small filaments allows for braided implants to be necked down into very small catheters, often 10F and below, and delivered through tortuous vasculature until they are deployed at the treatment site. 

One such surgical procedure that has been enabled through the use of micro-braiding is the treatment of cerebral vascular aneurysms.  Many techniques have been developed and commercialized for this application including aneurysm clipping and Platinum coil placement inside of the aneurysm sac.  A recent technique for diverting the main flow of blood away from the weakened vessel wall leverages the use of a micro-braid. 

Tiny Nitinol filaments as small as 0.0007” in diameter are braided together, with end counts ranging from 144 to 288, over small diameter mandrels to create a dense metal tube that has stent-like properties.  This dense “mesh” structure slows down blood flow and assists in facilitating clot development in the aneurysm, effectively sealing the aneurysm.  The foreshortening principle of the braid geometry allows for the implant to be delivered in a low profile catheter and expanded radially once in the proper position, over the mouth of the aneurysm.  The braid is oversized for the vessel that it is deployed in and relies on the radial forces generated by the helical orientation of the Nitinol wires in the fabric’s architecture.  The resulting micro-braid is a permanent “filter screen” that facilitates the formation of a clot to reduce the risk of the aneurysm’s rupture.

Another application for state of the art micro-braids comes from the ever-expanding use of transcatheter heart valve devices for aortic (TAVR) and mitral (TMVR) valve repair and replacement.  A wide range of implantable fabrics are commonly used as textile “skirt” materials on heart valves; typically to facilitate tissue ingrowth and prevent para-valvular leakage.  However, micro-braided structures have found a specialty niche during the surgical procedure used to implant some types of trans-catheter heart valves. 

In some heavily calcified heart valves, a large amount of vulnerable plaque debris is at an increased risk of embolization during the actual TAVR implantation procedure.  Some device designers have incorporated a micro-braided filter to be deployed distally from the implantation site.

As before, the braid is engineered to utilize a large quantity of fine filaments to create a mesh or filter like effect.  This mesh is designed to catch large particles or emboli that are sloughed off during the procedure while simultaneously remaining porous enough to allow the unrestricted passage of blood.  After the implant procedure is complete, the micro-braid is necked down, using the foreshortening principle, re-sheathed and captured in a catheter for removal from the patient.  Some studies have found that this highly engineered braided device is essential to preventing in-procedure strokes for high-risk patients. 

All micro-braided medical devices need not be porous to be effective in their end application.  There are emerging device concepts in the sports medicine field that are capitalizing on micro-braiding techniques to create interdependent layers of fabric as a means to spread axial forces over a larger surface area.  Through novel braided fabric design, small filaments of ultra-strong polymers can be braided in layers, concentrically, over top one another to create a core-sheath effect.  This enables an increasing bundle of fibers to completely envelop the previous layer creating compressive forces through an “over-braiding” technique. 

When this technique is applied, it is possible to create a braid system that translates axial loads to the inner layers of the braid system.  The use of very fine filaments at very high end counts, enables a significant amount of load bearing elements to be placed into the system.  The resultant fabric system is able to withstand a significant amount of force due to an internal truss system whereby the braid layers reinforce and buttress the adjoining braids.  This type of construct has application in tendon and ligament repair as well as joint reconstruction and offers an alternative in procedures where an allograft or autograft is typically utilized.

These few examples are but the beginning of the areas in which micro-braided structures have been explored in medical device design.  There are a myriad possible combinations of braid geometry, alpha angles, end counts, raw materials and filament orientations that can be engineered to meet the significant challenges of designing a textile for implant.  Often times, it is of benefit to the medical device designer to find a skilled team of engineers that understand the significance of the device challenges and possess the requisite skills to deliver the proper textile system.

Continue reading at Precision Micro-Braiding for Implantable Devices, Part Three

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.

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