Materials Science and Engineering A273 – 275 (1999) 780 – 783 www.elsevier.com/locate/msea
Flexible distal tip made of nitinol (NiTi) for a steerable endoscopic camera system H. Fischer a,*, B. Vogel a, W. Pfleging b, H. Besser a a
Central Engineering Department (HIT), Karlsruhe Research Center, 76021 Karlsruhe, Germany b Institute for Materials Research I (IMF-I), PO Box 3640, D-76021 Karlsruhe, Germany
Abstract In laparoscopic surgery the endoscopic camera is mounted to the end of an integrated rod lens system and, therefore, the whole endoscopic camera system is rigid. To obtain a better overview of the operation area, different lens systems with certain angles of view (0, 32, 45 and 70°) are used. To avoid changing the camera system during an operation, a flexible distal end for a single chip camera was built. Therefore an exchange of cameras during an operation is no longer necessary. The flexible tip is made of a superelastic Nitinol (NiTi) tube and can be angled up to 90° inside the human body. The outer diameter of the superelastic tube is 10.5 mm and, as there are no tubes of that size commercially available on the market, it was manufactured using NiTi sheet material. To enable angles of up to 90°, new structures were cut into the NiTi tube with a Nd-YAG laser. To reduce the induced tension at the bending zones, shape optimization was performed using the ABAQUS FEM software. This allowed a reduction in tension of about 40% for the optimized structure. Consequently, bending cycles of up to 105 are now possible. The angles are adjusted with the thumb on the proximal end of the endoscope and a locking device at this end enables the angle to be kept at any position for any time. © 1999 Elsevier Science S.A. All rights reserved. Keywords: NiTi; Steerable endoscopes; Mechanical applications; Laser structures
1. Introduction In minimally invasive surgery mostly rigid, straight optics are applied together with CCD-cameras. To cover various angles of view, optics with different lens systems are employed. Doing this, angles of view from 0° up to 90° can be achieved. To avoid the exchange of the optics during an operation, a flexible distal end was made of superelastic NiTi for accommodation of the camera unit. It allows to angle the modular camera unit under 90°. By turning the angled unit around the axial direction, a sphere of 180° can be covered. To keep the camera in the angled position, a locking device was developed. It can be operated with the thumb. An exchange of the optics, as previously required for the inspection of the abdominal cavity is no longer necessary. FEM calculations were performed for the optimization of the laser-cut structures. As a result, * Corresponding author. Tel.: +49-7247-823750; fax: +49-7247822289. E-mail address:
[email protected] (H. Fischer)
maximum strains could be reduced and the service life of the flexible tube was increased.
2. Setup of the system The system consists of three major units: the modular camera unit, the superelastic flexible end for the accommodation of the camera unit and the remaining operating unit with the manual locking device for positioning the camera under certain angles. The major component is the flexible NiTi tube with its special structure, which allows angling up to 90°. The outer diameter of the tube is 10.5 mm. Tube length amounts to about 60 mm. As tubes of such diameters are not commercially available, they had to be manufactured using NiTi sheets. For this purpose, a special bending facility was developed. The sheet was subjected to deformation in two steps and eventually given its ultimate tubular shape. Subsequently, the thus formed tube is welding by means of a Nd-YAG-laser. This laser also serves to structure the tube and hence, to make it deflectable
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H. Fischer et al. / Materials Science and Engineering A273–275 (1999) 780–783
(Fig. 1). The weld beam should not run along an edge fiber which is subjected to high stress during angling. The length of the flexible range amounts to 25.6 mm and the structural shape has to be selected such that angling starts in the distal range and ends in the proximal range. For load transmission a bowden wire is applied. To prevent the bowden wire from yielding laterally, a guiding system is required. This allows the bowden wire to transmit not only tension, but also compressive forces. As a result, functionality of the endoscope is improved considerably. For guiding, NiTi sleeves with an outer diameter of 1.6 mm are used. They are welded to the inner surface of every second rib. The structured distal end with the Bowden wire inside is shown in Fig. 2. The camera unit has not yet been integrated into the tube. Fig. 3 shows the ready-for-use endoscope camera with the handle. The distal end can be angled manually by means of the thumb. By a locking device, it can be positioned in any angle between 0 and 90°.
3. Measurements and results For the fabrication of the NiTi tubes, two different material thickness were applied: 170 and 500 mm. For both material thickness, studies were performed with regard to the fatigue of the tube under permanent bending load. The studies were carried out at the FATIBEND facility developed by the Institute for Materials Research at the Forschungszentrum Karlsruhe [1]. Using this setup, two types of fixation can be achieved. The bending moment characteristic of the tube of 170 mm in thickness is shown in Fig. 4. The maximum strain is approximately about 1.9%.
Fig. 1. Welding and cutting the structure of the tube with a Nd-YAG laser.
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Fig. 2. Tube with guiding sleeves and bowden wire.
The bending moment characteristic of the tube with 500 mm in thickness is represented in Fig. 5. The maximum strains is about 2.4%. The test rig was not sufficiently stable for the 500 mm tube. For this reason, the values dropped to the negative after unloading. Actually, an additional torsion was caused by fixation in these tests. Consequently, higher values were obtained. Furthermore, the test rig had been designed for a maximum of 200 Nmm only. This value was exceeded for the 500 mm tube. In this case, the values obtained by FEM calculation have to be preferred. Subsequent optical and mechanical testing did not reveal any change in elasticity. For measuring bending-induced fatigue, the tubes were studied under cyclic loading. The bending moment was determined and represented under three angles (0, 43 and 86). Fatigue of the material would have caused the moment to drop much more rapidly, above all in the plateau region. According to Fig. 6, no fatigue occurs up to 200 cycles. Scattering of the 0° values can be attributed to the low sensitivity of the sensor at small bending loads. When testing the 500 mm thick tube, only four load cycles could be performed as the sensor was not de-
Fig. 3. Endoscope camera ready for use.
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H. Fischer et al. / Materials Science and Engineering A273–275 (1999) 780–783
Fig. 4. Bending moment characteristics of the 170 mm tube.
signed for such large moments. Also here, no drop of the moment can be noticed at the 0° position (Fig. 7) and the maximum moments remain constant. The structure was optimized by means of computeraided shape optimization based on FEM computations [2]. Here, the stress at the bottom of the notch is reduced. As a result, overloaded areas experience a material growth, while unloaded areas shrink. For the 170 mm tube, a maximum strain of 1.97× 10 − 2 was obtained in tangential direction to the circumference prior to optimization. After optimization this value amounted to 1.13×10 − 2 [3].
Fig. 6. Bending-induced fatigue of the 170 mm tube.
4. Conclusions and outlook It was demonstrated by the experiments performed up to now that a flexible distal end for accommodation
Fig. 5. Bending moment characteristics of the 500 mm thick tube.
Fig. 7. Bending-induced fatigue of the 500 mm tube.
of the modular endoscopic camera unit can be fabricated from varying material thickness. The stiffness against torsion and the restoring force of the distal end depend on the material thickness which has to be selected accordingly for each specific use. As far as the 500 mm thick tubes are concerned, the material-dependent restoring force is too large for the material being suitable for use as distal ends. In this case, the force to be applied for angling is too high. However, stiffness against torsion is best at this thickness. During the studies it became obvious that NiTi of an adequate form may well be applied for generating a simple flexible joint. These simple structures may be used e.g. for guiding flexible endoscopic instruments. At present, further flexible ends are being developed by the Forschungszentrum Karlsruhe (FZK) for use in minimally invasive surgery.
H. Fischer et al. / Materials Science and Engineering A273–275 (1999) 780–783
References [1] R. Plietsch, P. Schloßmacher, T. Stober, K. Ehrlich, Actuator 96, fifth International Conference on New Actuators, AXON Technologie Consult GmbH, Bremen, Germany.
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[2] C. Mattheck, Design in der Natur, Rombach Verlag, Freiburg, 1993. [3] A. Gru¨nhagen, Festigkeitstheoretische Untersuchungen einer flexiblen Hu¨lse aus NiTi, wissenschaftliche interne Berichte, Forschungszentrum Karlsruhe, 1998
