Application of a Precision Mobile Platform on an Innovative Intracranial Aneurysm Spherical Stent

Transcription

1 Application of a Precision Mobile Platform on an Innovative Intracranial Aneurysm Spherical Stent Bor-Hann Huang 1, Dian-Ru Li 1, Yung-Chi Chen 1 and Hao-Ming Hsiao 1,* 1 Department of Mechanical Engineering, National Taiwan University, Taipei, Taiwan, ROC * Corresponding Author / hmhsiao@ntu.edu.tw KEYWORDS : Intracranial aneurysm, Spherical stent, Finite element analysis, Pulse type fiber optic laser, Mobile platform Intracranial aneurysms are the main causes of hemorrhagic stroke. The current intravascular coil embolization has limitations when used to treat intracranial aneurysm. If the aneurysm neck is too large, surgery is problematic and sequelae are more significant. We proposed an innovative design of an aneurysm spherical stent that can be directly placed in the aneurysm, providing more even radial support to the aneurysm and thus reducing the risk of rupture. By using CAD software to sketch and create a finite element computer model, we predicted the clinical behavior and mechanical properties of the spherical stent as a reference index to evaluate whether the material of the spherical stent had the potential to rupture in its manufacturing process or during balloon dilation. Lastly, by using pulse type fiber optic laser with an XY (rotating) axes integrated mobile platform, we produced a prototype of the innovative spherical stent with length of 7.5mm, diameter of 2mm and wall thickness of 0.1mm. Then, we successfully combined it with the balloon as a schematic of a finished product. Manuscript received: December 18, 2013 / Accepted: January 06, Foreword Most intracranial aneurysms are caused by the impact of blood to the damaged vascular wall and are formed from a local protrusion expanding outwards over a long period of time. When the blood pressure rises abruptly or when the head is impacted by an external force, it is likely that the aneurysm may rupture and in turn cause subarachnoid haemorrhage (SAH), leaving a high risk of neural sequel and death (Fig. 1). In the past, there were no concrete countermeasures for intracranial aneurysm in medicine. Until the 1960s, when developments in microsurgery and an understanding of neuro-anatomic physiology as well as advances in diagnostic techniques meant that intracranial aneurysms became a treatable disease [1, 2]. Fig. 1 Rupture of intracranial aneurysm In traditional craniotomy, the physician will use micro vascular clips to clip the junction where the aneurysm and the normal vessel meet (called the aneurysm neck). Thus, the blood will no longer flow in the aneurysm and bleeding from a rupture can be avoided. However, in many cases the position, shape and size of the aneurysm may make it unsuitable for surgery. In 1991, a detachable coil was invented [3]. This coil is made of platinum and is welded to a leading wire to facilitate insertion of the platinum coil in the aneurysm. After the coil is filled in the aneurysm, it is energized with electricity to detach the platinum coil from the leading wire at the welded junction so that the leading wire can retreat (Fig. 2) [4, 5]. After the coil has been in place for a period of time, due to vascular intimal cell hyperplasia, the aneurysm will be blocked off from the normal vessel and the risk of rupture is avoided. Currently, detachable coil embolization has become one of the standard treatments of intracranial aneurysm. However, when the aneurysm neck is over 4mm or the neck/aneurysm length ratio is greater than 0.5 there is a risk that the platinum coil may fall into the blood vessel and cause a stroke. This research proposes the design of an innovative aneurysm stent that can be directly placed in the aneurysm to resolve the issue of detachable coil embolization. We innovatively design a spherical stent different from the cylindrical type after dilatation of the commercially available vascular stent. After dilatation, the spherical stent takes on a spherical shape making it easier to place in the aneurysm. It can provide the aneurysm with more even radial support to reduce the risk of rupture. Moreover, we use the finite element method to simulate the manufacturing process of the spherical stent and the process of its deployment in the aneurysm. We also calculate the force exerted on the spherical stent and its deformation at each step to evaluate its 24

2 clinical performance. Then, we use a pulse type fiber optic laser with the XY axes integrated into a mobile platform to complete laser cutting of the hypotube to produce the first spherical stent prototype. Lastly, we combine it successfully with a balloon catheter to demonstrate the feasibility of the product. Fig. 3 Parallel motion mechanism (telescoping metallic mechanism) Fig. 2 Detachable coil embolization 2. Research method 2.1 Innovative spherical stent design In general, the average diameter of the cerebral artery is only 2-3 mm [6], smaller than the vessel size of any other part of the body. Therefore, the key point of the design is for the spherical stent to be small enough to be delivered freely by use of a balloon catheter. When it arrives at the target site, it can dilate to the size of a small aneurysm (generally, the diameter of the aneurysm is 3-5mm). After dilatation, the stent will be the target diameter and will remain in a spherical state so as to provide even radial support to the aneurysm and also avoid falling and blocking of the blood vessel. We developed a spherical stent of 5mm diameter as our research goal. To meet the above conditions, the geometry of this spherical stent must be capable of withstanding substantial deformation without being damaged. We designed the spherical stent with reference to parallel motion mechanisms commonly seen in daily life as a development base (Fig. 3). By using Solidworks software, we first sketched a 2D draft of the spherical stent and then produced a 3D physical model (Fig. 4). Fig. 4 3D physical model of the innovative spherical stent design With the finite element method, we simulate the process of the spherical stent dilating from a cylindrical shape under compression (middle) into a spherical shape (right) as shown in Fig. 5. There is a cylindrical sleeve outside the spherical stent that is responsible for compression while a circular balloon inside the spherical stent is responsible for dilatation. We need to calculate the maximum strain under stress at each step to see whether it will exceed the material breakdown strength so that the feasibility of this design can be validated. For the selection of an element, the spherical stent body is a cubic element C3D8I while the surface element SFM3D4 is used for the cylindrical sleeve and the circular balloon. The grid-based setting element size is 0.25mm. Consequently, the number of the spherical stent element is The material parameter that we select is the cobalt-chromium alloy L-605 commonly seen in commercially available vascular stents. Young s modulus, Poisson s ratio, tensile strength and the maximum breakdown strain amount are 203GPa, 0.3, 1689MPa, and 60%, respectively [7-9]. 2.2 Finite element analysis After the spherical stent is cut, it will first be compressed in the balloon catheter. Then, it will be delivered to the aneurysm to dilate into a spherical shape. By using the finite element analysis software Abaqus, we created a computer model to evaluate whether the spherical stent suffers damage in the above steps and also to evaluate whether its clinical properties have met expectations. The computer model can be divided into four consecutive steps: Step 1: Step 2: Step 3: Step 4: Compress the stent diameter from 2mm to 1.5mm as a cylindrical shape. The stent rebounds by itself after compression. The balloon dilates the stent to a 5mm diameter sphere. After dilatation, the stent rebounds by itself. Fig. 5 Finite element method to simulate the spherical stent under compression (middle) and dilatation (right) 2.3 Laser cutting The stent is a 3D column shaped hollow structure, made by cutting a hypotube (Fig. 6) of a very small diameter (1-2mm) with a 25

3 laser light source from the designed stent pattern. This is more complicated than ordinary laser planar machining. Therefore, a special carrier should be constructed from which the laser light source can be directed to the machined surface to perform laser cutting on the hypotube. Fig. 6 Stainless steel 316L hypotube with a diameter of 2mm The machine used by this research is a 30W Nd-YAG pulse type fiber optic laser beam source with the Aerotech XY (rotating) dual axes integrated high-speed cutting module (Fig 7). A step motor is installed behind the laser cutter head to increase freedom in the Z-axis direction and to adjust the focus position of the laser beam (Fig. 8). This machine contains CAD/CAM software specifically designed for stents that can automatically convert the stent XY planar draft into cylindrical coordinate points, which are covered on the hypotube surface and form a laser cutting route. Then, with PSO (position synchronized output), we achieve laser cutting route optimization, thus ensuring better cutting effect for more complicated geometric shapes such as arcs, chamfers, etc. The Aerotech XY dual axes integrated high-speed cutting module combines a precision linear mobile platform with a direct-drive brushless motor. The mobile platform is a totally enclosed design. The anodized hard casing prevents ingress of dust and chips and protects internal elements such as bearings, non-contactable linear scales, etc. Both the front and rear platforms not only increase the overall rigidity of the sliding block, they also serve as an excellent inertia reference plane. The brushless motor reduces wear of conventional parts, e.g. electric brush, gear and belt. It is maintenance-free and can maintain in a high precision state for a long period of time. The pneumatic chuck can secure hypotubes of different diameters ( mm) on the Y (rotating) axis. The actual machining condition of this machine is shown in Fig. 9. The machining kinematic system specification of the high-speed cutting module is: The horizontal travel on the X-axis is 100 (mm) with the linear accuracy of ± 2 (microns) and the repetitive accuracy of ± 0.2 (microns). The maximum speed on the X-axis direction is 300 (mm/s). Continuous motions on the Y (rotating)-axis is allowable with the rotational accuracy of ± 25 (arc sec) and the repetitive accuracy of ± 4 (arc sec). The maximum revolving speed is 600 (rpm). When the tube, tube diameter or the tube wall thickness change, the relevant machining parameters should be adjusted, including the laser cutting power, cutter head blowing amount, cutting speed, and the auxiliary gas type. In this research, we use the stainless steel 316L tube with a diameter of 2mm, and the wall thickness of 0.1mm and test it with the above parameters to find out the optimum combination. Fig. 7 Aerotech XY dual axes integrated high-speed cutting module Fig. 8 High precision laser cutter head and step motor Fig. 9 Actual machining condition of a laser machine 3. Result and discussion 3.1 Finite element analysis The final confirmation of the stent design is determined by using the results of the finite element analysis with repetitive iteration. Fig. 10 shows the simulated results of continuous dilatation of the spherical stent. The spherical stent can successfully be obtained from the original cylindrical shape to the final spherical structure by 26

4 continuous dilatation, most suitable for use in the intracranial aneurysm. of the hypotube. Therefore, cutting may not be smooth and complete unless laser power is increased. Although the cutting edge is smoother and produces less slag residue when using oxygen as the auxiliary gas, oxygen can cause the heat affected zone (HAZ) to become larger and lead to a thicker oxidized layer on the surface, which has a negative effect on stent clinical behavior. It has to be further improved by posttreatment electrolytic polishing. Fig. 10 Simulated results of spherical stent by continuous dilatation It is not difficult for us to find from the simulation results that the larger plastic deformations are located at the tip of the diamond shaped acute angle. From the plastic strain gradient distribution, we see that there is a maximum value (in the red region) at the tip of the acute angle that gradually decreases on both sides (Fig. 11). The maximum plastic strain of the spherical stent after completion of dilatation is 40.8%, which is less than the maximum breakdown strain of the material. As stress concentration will occur at the acute angle, we can consider using chamfer at the tip or increasing the acute angle to solve the stress concentration problem and to lower the maximum strain value as well. For the stent rebound, the outside diameter (OD 3 ) of the spherical stent after dilatation in Step 3 is 5.238mm and after rebound with dilatation in Step 4 the OD 4 is 5.125mm. The rebound rate is calculated with the following formula: Fig. 11 Location where the spherical stent strain occurs with a maximum value R stent OD 3-OD 4 OD 3 2.2% (1) The rebound rate of the spherical stent is determined as at about 2.2%, which is within the acceptable range and will not affect the diameter of the spherical stent after dilatation. Combining the above simulation results, this innovative spherical stent design is confirmed to be a feasible concept. 3.2 Manufacture with laser cutting When we tested different auxiliary gases, we obtained the parameter combinations as shown in Table 1. With the same cutting speed and blowing amount, we found that the laser cutting power was 26% less when using oxygen vs. nitrogen as the auxiliary gas. This is because nitrogen can cause insufficient metal oxidizing fusion and is more apt to form solid fusion slag, which may attach to the inner wall Fig. 12 A spherical stent prototype (up) and its local magnification diagram (down) 27

5 In the future, if we wish to improve spherical stents, we will further research the balance between each cutting parameter, mobile platform accuracy, and errors caused by environmental factors (e.g. vibration of the work platform). The prototype of the spherical stent and the local cutting effect observed under magnification from a metallurgical microscope are shown in Fig. 12. Schematics of the finished product with the spherical stent compressed in the balloon catheter are shown in Fig. 13. Table 1 Parameter combination of different auxiliary gases Auxiliary gas Nitrogen Oxygen Laser power(%) Pulse repetitionrate(khz) Cutting speed(mm/s) 4 4 Blowing amount(bar) Conclusions Fig. 13 Schematics of a finished product We propose an innovative intracranial aneurysm spherical stent design which can be directly placed in the aneurysm to solve issues with detachable coil embolization. During the design process, we established a finite element computer model that we used to simulate the results of actual testing to save on cost of product development. From the simulation results, we found that during the manufacturing and placement processes, the site where the spherical stent has a higher deformation or where it may be damaged is located at a diamond-shaped acute angle, where the maximum strain value can be alleviated by using chamfering or increasing the acute angle. The rebound rate after expansion is about 2.2%, which is within the acceptable range. For laser cutting, we used nitrogen and oxygen, two different auxiliary gases, to compare the cutting effect. The obtained parameter combination can be used as a basis for cutting spherical stents in the future. Improving the PSO algorithm and upgrading the accuracy of the mobile platform, e.g. calibration of the Z-axis step motor position feedback system will be key points in our future research. We also successfully integrated a pulse type fiber optic laser and an XY (rotating) axes integrated platform to make a prototype of the innovative spherical stent with length of 7.5mm, diameter of 2mm, and tube wall thickness of 0.1mm and successfully combined it with the balloon catheter as a schematic of the finished product. ACKNOWLEDGEMENT This paper is a project of the National Science Council, NSC E MY3 and NSC E CC1. Thanks to the support of NSC, this project can be performed smoothly. We hereby express our appreciation. REFERENCES [1] J. M. Wardlaw, P. M. White, The detection and management of unruptured intracranial aneurysms, Brain, 123, (2000) [2] A. P. Lozier, E. S. Connolly Jr, S. D. Lavine, and R. A. Solomon, Guglielmi Detachable Coil Embolization of Posterior Circulation Aneurysms: A Systematic Review of the Literature, Stroke, 33, (2002) [3] G. Guglielmi, F. Viñuela, J. Dion, and G. Duckwiler, Electrothrombosis of saccular aneurysms via endovascular approach Part 2: Preliminary clinical experience, Journal of Neurosurgery, 75, 8-14 (1991) [4] F. Viñuela, G Duckwiler, and M Mawad, Guglielmi detachable coil embolization of acute intracranial aneurysm: perioperative anatomical and clinical outcome in 403 patients, Journal of Neurosurgery, 86, (1997) [5] A. F. Zubillaga, G Guglielmi, F. Viñuela, and G. R. Duckwiler, Endovascular occlusion of intracranial aneurysms with electrically detachable coils: correlation of aneurysm neck size and treatment results, American Journal of Neuroradiology, 15, (1994) [6] S. B. Pai, R. N. Kulkarni, and R. G. Varma, Microsurgical anatomy of the anterior cerebral artery-anterior communicating artery complex: an Indian study, Neurology Asia, 10, (2005) [7] H. M. Hsiao, A. Nikanorov, S. Prabhu, and M. K. Razavi, Respiration-induced kidney motion on cobalt-chromium stent fatigue resistance, Journal of Biomedical Materials Research Part B: Applied Biomaterials, 91B, (2009) [8] H. M. Hsiao, Y. H. Chiu, K. H. Lee, and C. H. Lin, Computational modeling of effects of intravascular stent design on key mechanical and hemodynamic behavior, Computer-Aided Design, 44, (2012) [9] H. M. Hsiao, Y. H. Chiu, T. Y. Wu, J. K. Shen, and T. Y. Lee, Effects of through-hole drug reservoirs on key clinical attributes for drug-eluting depot stent, Medical Engineering & Physics, 35, (2013) 28