Individualized, complex braided vascular implants with a large number of wires pose greater challenges to current manufacturing processes. In addition to urgently needed flexibility in machine braiding, it must be possible to carry out the production process in a time-efficient manner. Both requirements – highest flexibility combined with high production speed – are technologically mutually exclusive. Until now. By applying modified physical principles of action to control the braiding paths and intelligent production equipment, the production process of braiding has been further developed in a forward-looking way. The production of patient-specific vascular implants (stents) is thus possible economically and in the highest quality.
Various processes are used to manufacture stents. Admedes specializes, among other things, in braided vascular implants and is constantly expanding this expertise through innovative development activities.
The preferred starting material for braided stents is nitinol wire, which can be supplemented with textile and polymer fibers as a hybrid solution in special applications. After appropriate surface treatments, the material nitinol is characterized by its high biocompatibility and corrosion resistance. Braided stents offer the advantages of smaller wall thicknesses and the associated smallest possible braid diameters. The resulting soft and pliable braided structure combined with low radial forces enables the implants to be used, for example, in the highly complex neurovascular area, which is characterized by the finest blood vessels.
Conventional circular braiding machines
Due to high production speeds, classic circular braiding machines offer optimum conditions for production of cylindrical braids. Bobbin carriers are moved by rotational movements of individual horn gears on a fixed track and continuously transferred to the adjacent impeller. The typical braid structure is created by the wires of 2 counter-rotating bobbin series crossing each other regularly and being placed on the braid core in the further course of the process.
The classic circular braiding machine usually produces dense, cylindrical braids such as hoses, ropes and shoelaces. However, by placing fine wires on the braiding core with a larger diameter, this machine design is also suitable for producing open braids for medical devices. These open braids are characterized by the fact that the individual wires only touch in the crossover area. Due to their design and mechanics, classic circular braiding machines are designed and manufactured for a predetermined number of wire ends. A simple extension by, for example, 2 additional bobbin carriers or wire ends is not possible and requires a further, separate braiding machine. As a result, this type of braiding machine is only worthwhile for high quantities and is conditionally suitable for prototypes and small series.
Discrepancy of braiding machine technologies
In addition to classic circular braiders, other machine variants can be used to produce cylindrical round braids. These differ in the form of movement of the bobbin carriers. The aim of the development is to combine the existing advantages of available braiding machines, thereby increasing the flexibility while maintaining a high production speed. This is intended to enable efficient production of individual braids based on batch size one.
Process and machine requirements
To ensure the required flexibility and production speed, several requirements must be met.
On the one hand, it must be possible to produce different braids with different numbers of wires on a single machine. Secondly, in addition to the flexible batch size one, efficient series production must be possible. Therefore, the production speed of the braids should correspond to the parameters of current, conventional braiding machines. In this case, the number of crossovers per minute is considered which corresponds to a horn gear speed of about 150 rpm.
Thanks to its compact size, the system should be able to be operated in common spaces and ensure maximum accessibility. In addition, the machine design should offer scalability or a possibility to expand the number of horn gears.
Individual bobbin carriers should be able to move flexibly in different paths and change their direction. In addition, the wire tension should be dynamic, infinitely adjustable and able to be kept constant, which mechanical, available bobbins currently do not offer.
Machine design and realization
The drive technology of the bobbins by using a horn gear is retained. The machine geometry provides for a radial arrangement of the horn gears in rings and scalability of the braiding rings, with each horn gear being driven and controllable individually. This combination results in further technical challenges: A machine design with only one ring results in a radial pathway of the bobbins. If the machine is still to be compact with a large number of horn gears, the number of horn gears must be distributed over several braiding rings. This results in a further axial direction of movement. The bobbin transfers must therefore be made possible in both directions of movement without interruption and in a stable manner. Driving the horn gears in several groups up to individual drives creates collision hazards, since the individual bobbins are not linked to fixed paths. High horn gear speeds as well as a synchronized motion sequence require a powerful control system to evaluate the large number of position sensors for collision avoidance.
However, these objectives alone do not lead to the full desired flexibility. Radial designs of braiding machines already exist today. To achieve the required flexibility, it must be possible to selectively manipulate the bobbin carrier path between adjacent horn gears. The highly flexible switch system is realized by a suitable arrangement of opposing electromagnets and their interaction with up to 192 simultaneously moving bobbin carriers. The resulting flexible traversing paths allow the operator to influence the required changes in braiding structure and weave at any time during the production process. The high production speed is maintained without loss of positioning time of mechanical elements. This new switch concept also provides for the temporary removal and re-participation of individual bobbins in the braiding process. The newly developed machine technology enables braid structure changes, layer changes, layer formation and 3D braiding. Furthermore, flat braiding processes with identical impeller geometry can be realized with dynamic adjustment of horn gear speed and the highly flexible switch system.
The innovative development is complemented by newly designed electronic bobbins. These ensure a significantly better influence on the parameters of the braiding process and thus on the execution quality of future braided products. For this purpose, the braiding material tension is controlled in a process-optimized manner at any given time and depending on the position, movement speed and individual tensile strength. The wire tension can be adjusted continuously during operation without having to replace mechanical components. At the same time, the tension value is continuously monitored and detects any wire breaks that may occur.
An additional braiding logic and learning module performs a target actual comparison regarding the correctness of the braiding pattern by applying image recognition and image evaluation at any point in the production process. For this purpose, braiding pattern images are continuously divided into segments by a form of artificial neural network (ANN) especially suited for image recognition. The braiding pattern is optimized by learning in the event of an error.
Research project Stents4Tomorrow
All development activities are carried out as part of the Stents4-Tomorrow research project.
The project is funded by the German Federal Ministry of Education and Research and is integrated into the research program ProMed – Production for Medical Technology – economically and with the highest quality. At this point, thanks for the realization of the project go to the Federal Ministry of Education and Research, the project management organization PTKA Karlsruhe, and the entire project consortium, consisting of 7 project partners.