Tubular, bifurcated structure

Figure 7.1 Possible applications of a tubular, bifurcated structure (Schreiber et al., 2009).

Recent advances in 3D braiding have focused on the development of processes to produce tubular, bifurcated structures. Because this structure combines complex cross shapes and cross-sectional variations, it is highly interesting for pushing the limits of 3D braiding. 

The development of processes to produce a tubular, bifurcated structure requires the definition of the desired structure. In this chapter, the designated structure is thus defined by various parameters. These parameters may be subdivided into... 

Figure 7.36 Machine process for the production of a nonnear-net-shaped tubular, bifurcated structure.

Although widely being explored, the majority of the automated production of 3D braids is often limited to fabricate constant cross-sectional 3D braid geometry. However, the production of a tubular or bifurcated structure requires variations in the geometry of the cross sections. This leads to manual interference in the production process, which slows the production process and constrains the use of 3D braids to products with small quantities. Thus, development of a fuUy automated process will clear the way towards the production of 3D braids in large quantities and allow the use of 3D braids in wide areas of application. Examples are the preforms in composites, for example, stmctural stiffeners in car bodies or as stents in a medical devices. 

The bifurcation conditions and the UM method to braid a tubular stmcture can be combined to develop a process that allows for the production of bifurcated, tubular stractures. To produce a closed structure, three different cross-section geometries and subsequently two cross-sectional changes are required. In Figure 7.33, the tubular bifurcated stmcture and the cross-section geometries are depicted. These are one tube (1), a figure 8 (2) and two tubes (3). 

This is the undisputed material of choice for the replacement of medium- and large-diameter arteries. The prostheses are available in straight and bifurcated designs (Fig. 15.7). In fact the technology associated with the design and fabrication of arterial prostheses has spread across the world over the past 40 years. So it is fair to say that these woven (Fig. 15.8) and knitted (Fig. 15.9) tubular structures are now a mature medical product. 

Both weft and warp knitting machines are able to construct tubular structures. Although circular weft knitting machines can only be used in producing single tubes, flat weft knitting machines with two needle beds and double-needle bar Raschel machines are able to produce single, bifurcated and multibranched tubes. 

Figure 6.11 A schematic illustration of a warp-knitted bifurcated tubular structure.

Figure 7.33 Varying cross-section geometry in a bifurcated, tubular structure.

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