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3Bioprinting of hierarchical auxetic coronary stents: from design to mechanical testing
E. Mazur1, I. Shishkovsky2*
1- Skolkovo Institute of Science and Technology, 121205 Moscow, Russia 2- Lebedev Physical Institute of the RAS, 443011 Samara, Russia
Samples of coronary stents, which are widely used in cardiac surgery and have an inner lattice structure, were fabricated (Fig. 1). Hierarchical auxetic materials based on rotating units are more versatile than similar non-hierarchical lattice materials, and thus they are a promising candidate not only for stent fabrication, but also for soft robotic applications. When reviewing soft robotics applications, it is important to consider the ability of auxetic structures to change shape when moving through narrow spaces. For example, a soft robot designed like an inchworm to crawl through blood vessels using mechanical metamaterials for synchronized movement of two passive clutches has been presented in [1]. Due to thermal or electrical activation, this design allows the robot to move through a confined passage using a single actuator, similar to the movement of an inchworm. The sequence of images in Fig. 1a shows three cycles of the bellows expanding and contracting. During expansion, the auxetic structure moves upward, while the normal structure remains stationary. During contraction, the auxetic structure remains fixed, while the normal structure slides upward.
The passive stent mechanism, based on complementary material properties, helps to reduce the number of actuators to one and avoid synchronization problems, as the smart synchronization is already built into the robot's material body. This type of robot can be used for navigation within soft capillaries, cylindrical blood channels with unknown cross-section, and other related applications.
Six different models were designed in Rhino 7 and Grasshopper, varying two key parameters: the number of fibers and fiber radius. Internal structure management and/or radius gradient creation can introduce auxetic properties into these scaffolds. The stents were manufactured using Anycubic 3D printing UV-sensitive resin material and a Zortax Inkspire 3D printer (digital light processing, DLP technology), and tested using an Instron 5269 machine.
Each set of samples included the following modifications: the radii of the wires inside the coronary were 0.25 and 0.30 m, and the number of wires used in the proposed design was 24, 29, and 34. During the design process, several issues need to be addressed. First, there is an overhang angle limitation due to the chosen printing technology. Secondly, there is a connection issue between the solid and lattice parts of the sample, which is a potential weak point of the model. To solve this problem, the lattice is embedded in the solid part, increasing the cross-sectional area of their contact.
Mechanical testing has shown that the lattice design parameters have a significant impact on the mechanical behavior of the samples. The sample with a wire radius of 0.25 mm and a wire number of 29 exhibited the highest Young's modulus of 383 MPa, compared to the minimum Young's modulus of 24.2 MPa determined in the experiment for the sample with a wire radius of 0.3 mm and a number of wires equal to 29. The design lies in the middle ground between the number of wires and the wire radius in this selected set of parameters, as adding more wires and/or increasing the wire radius tends to make the specimens less stiff. During mechanical testing, most of the samples broke in the middle region of the stent's lattice (see Table). To improve the mechanical properties, such as increasing the limit load of the samples, advanced design may be needed in the future. One possible solution is the use of a gradient lattice. Two options for implementation exist: additional wires could be added to the lattice, or the wire radius could be increased in the center of the sample (see Fig. 1b).
[1] A.G. Mark, S. Palagi, T. Qiu, P. Fischer, Auxetic metamaterial simplifies soft robot design, In Proceedings of the 2016 IEEE International
Conference on Robotics and Automation (ICRA), Stockholm, Sweden, 16-21 May 2016; pp. 4951-4956.
