Context

During minimally invasive procedures, most of the devices (endoscopes, catheters, guidewires, etc.) need to be sufficiently flexible to avoid damaging patient tissues or causing pain, but have to be stiff enough to transmit force for support or for puncture. In the case of vascular stenosis, the guidewire has to be flexible to reach the stenosis (through the blood vessels), but it requires a rigid support to pass through the occlusion for treatment, to avoid buckling or deformation due to the force application. In order to solve this duality on the rigidity, controllable stiffness mechanisms can be used. Various mechanisms to control the stiffness can be found in the literature [1]. One of the promising solutions to achieve this objective is based on granular material jamming [2]. This research aims at studying the scaling laws of such solutions for miniaturized applications (with diameters below 3mm), the mechanical rules of design and the optimization based on the stiffness performances.

Granular jamming

The granular jamming is based on the locking of granular material. In this study, a flexible membrane is filled with a granular material (glass beads). When the pressure difference between outside and inside the membrane is low, the grains are free to move with respect to each other. In this configuration, the system is very flexible. Once the difference of pressure is increased, the grains are locked to each other due to the inter-grain friction. In this configuration, the system is more rigid. It is possible to adjust the stiffness of the device by controlling the pressure difference across the membrane.

Materials and methods

In this work, the performances of the stiffness change thanks to the granular jamming are quantified by mechanical testing. On the one hand, three point bending and cantilever beam tests are performed to quantify the flexural stiffness EI (product of the Young Modulus, E, and the second moment of area, I) of the solutions. Various granular materials and diameters of the samples are studied. On the other hand, triaxial compression tests are performed to observe the influence of the pressure difference on the rigidity obtained via granular jamming, using different granular materials.

Results and discussion

The tests described previously provide information on the performances of the granular jamming solution as well as an indication of the most important parameters to optimize. An optimal size of grains is highlighted by the results of these mechanical tests. The results of the triaxial compression tests show that the pressure difference is the most important parameter influencing the Young Modulus. The bending tests show that the second moment of area greatly impacts this stiffness. Removing the influence of the geometry, the equivalent Young Modulus is positively influenced for smaller diameters which is promising for the applications targeted in this work. Some of these results, as well as pictures and conclusions are illustrated in the poster file available in the following link : https://dipot.ulb.ac.be/dspace/bitstream/2013/239703/3/20161125PosterNCBMElblanc.pdf.

One of the perspectives of this work is to develop a model for linking the results obtained from the different mechanical tests and to observe the optimization of the grains (shape and size) and cross-section of the samples with respect to the change of stiffness obtained. Further studies on stimulation method and on materials should be performed.

References

- [1] Kuder, I. K. et Al., “Variable stiffness material and structural concepts for morphing applications”, Progress in Aerospace Sciences, pp.33-55, 2013.

- [2] Loeve, A. J. et Al., “Vacuum packed particles as flexible endoscope guides with controllable rigidity”, Granular Matter, pp. 543-554, 2010.