Chapter 4 Compliant Laparoscopic Grasper
4.5 Testing
4.5.2 Pull Force
Tests were performed to determine the maximum pull force the graspers were capable of producing. Porcine liver samples 2x7x0.5 cm in size were clamped in the grasper on one end with the trigger fully squeezed (Figure 4-13). Weight was then added incrementally on the other end until the tissue slipped out of the jaws. Thirteen and nine
39 trials were done with the compliant and rigid graspers respectively, with new samples
used for each trial. Each trial lasted approximately one minute and the tissue samples were kept wet during until they were used.
Figure 4-13 Pull Test Configuration
4.6 Results 4.6.1 Pinch Test
Pinch force data for the compliant grasper were recorded as a function of the linear displacement of the push rod. The trigger was squeezed to translate the rod in 1
40 mm increments. Figure 4-14 shows the relationship between the push rod displacement
and the pinch force of the compliant grasper.
Figure 4-14 Compliant and Rigid Grasper Pinch Force
It was not possible to record the pinch force for the rigid grasper as a function of the displacement of its push rod since there was a considerable amount of play between the trigger, push rod and jaws due to the pin-in-slot joints at the jaws-and-rod and trigger-and-rod joints. The maximum pinch force of the rigid grasper was recorded over 10 trials with an average of 8.7N.
The pinch force data for the compliant grasper shows an interesting and possibly useful behavior. The relationship between the rod displacement and the pinch force is generally sigmoidal in shape, with the initial portion being fairly linear, a rise around the 9 mm mark, and a plateau around 3.5N at 10 mm. With the trigger fully squeezed, the
41 useful in mitigating tissue trauma. By having a “built-in” maximum pinch force a surgeon
can fully squeeze the trigger and not worry about applying excess force that can damage the tissue.
It was observed during testing that in the linear region, the contact area between the grasper and FSR sensor was increasing as the jaws closed around the sensor, and at 9 mm of displacement the jaws were fully in contact with the sensor. During the later stages of closure the jaws also pulled the sensor towards the grasper as it pinched. This behavior was different than what was observed with the rigid grasper. Since the rigid jaws pivot at their base, the resulting grasping motion was a combination of pinching and pushing the sensor away from the grasper. This pinching/pushing behavior is counterintuitive to what would constitute efficient grasping since it moves the tissue in the opposite direction of the intended stretching.
4.6.2 Pull Force
The results for the pull tests are summarized in Table 4-1. For each trial both graspers were closed as tight as possible to achieve the maximum pull force. In the case of the rigid grasper which had 1 mm teeth, significant trauma was observed in the tissue samples following each trial as seen in Figure 4-15.
42 Figure 4-15 Example of Tissue Damage as a Result of Applying Excessive Pinch Force
Table 4-1 Pull Test Data
Avg. Max. Pull Force [N] Std. Dev. [N]
Compliant 1.4 0.6
Rigid 8.1 0.6
It was expected that the prototype compliant jaw would have an inferior maximum pull force compared to the rigid grasper since the interface between the jaws and tissue is toothless. It was found that laparoscopic graspers should be able to achieve at least 5N of pull force [4-10] without causing damage. It was observed that the compliant graspers caused minimal to no trauma to the tissue following each pull test trial, while significant damage (i.e., large perforations and torn tissue) was observed on samples grasped by the rigid graspers.
4.7 Increasing Traction
The traction between the jaws and tissue needed to be improved to make the compliant grasper useful in a surgical setting. One option was to add teeth features
43 similar to rigid grasper used for testing, but that would entail adding a small wave-like
pattern to the jaw profile (Figure 4-16) that would not be physically obtainable using the heat treatment method due to the stiffness of the Nitinol. Another option would be to add a rough texture to the surface of the Nitinol through a secondary process such as knurling.
This option was not pursued since the thickness of the Nitinol was only 0.2 mm and the integrity of the ribbon would be at risk.
Figure 4-16 Conceptual Wave Pattern for Jaw Teeth
Treads made of polydimethylsiloxane (PDMS) have been shown to improve traction and mobility in wheeled crawler-type in vivo robots for medical applications [4-13] with the use of circular micro-treads 140 μm in diameter, 70 μm tall, and spaced 105 μm apart (Figure 4-17) showing an increase in traction of 50 – 100% [4-12]. From this it was believed that applying PDMS treads to the jaws of the compliant grasper would improve the pull force it could apply to tissue without damaging the tissue.
Figure 4-17 Tread Pattern From [4-13] (Units in μm)
44 To make the treads, a mold was needed to pour the liquid PDMS into for curing.
In photolithography micro-molds are made on silicon wafers with a layer of photoresist serving as the mold negative. The process to fabricate this mold is quite intricate [4-14]
and requires nanofabrication equipment which is not commonly available. As a result, a cheaper alternative was pursued that involved laser machining a mold negative into ¼”
acrylic sheet using an Epilog Mini 18 laser system with a 30 Watt CO2 laser. It was unknown how small of a hole this laser system could make so test cuts were made to determine what combination of cutting parameters, laser power and cutting head speed, produced the smallest and shallowest blind holes.
Using CorelDraw software, different hole sizes of 127, 102, 76, 51, 25, and 13 microns in diameter were drawn to determine if the laser system could make holes this small. Recommended speed, power, and frequency settings for cutting through ¼” thick acrylic were 5%, 100%, and 5000 Hz respectively [4-15]. The speed setting was increased to 30% since through holes were not wanted as they would serve no purpose in the mold. This speed setting proved to be too slow since the holes that were created came out very distorted and non-uniform when viewed under a microscope as seen in Figure 4-18.
Figure 4-18 Magnification (10x) of Hole Cut at 30% Speed
45 The speed setting was increased incrementally and the holes were inspected until uniform
holes were cut. Figure 4-19 shows the results from speed settings of 80%, 90%, and 100%.
At 80% the holes are still distorted but starting to resemble a circle. There is also an overlap of the heat-affected zone (HAZ) of the base material between the holes, which is revealed by the many air bubbles in the acrylic. At 90% the holes are becoming less distorted and the HAZ is reduced. A speed setting of 100% produced the best results with the least amount of distortion and a very small HAZ that does not seem to overlap. At higher speeds the laser-cut holes became more uniform because the reduced exposure to the laser beam prevented the area surrounding the laser from melting. The 100% setting was selected because it produced the most uniform hole and had the smallest HAZ.
Figure 4-19 Hole Cut with 80%, 90%, and 100% Speed Setting (L-R)
In verifying the hole sizes it was found that the laser system could not achieve the small hole sizes that were desired. A 25 micron diameter hole drawn in CorelDraw ended up having an actual hole diameter of 254 microns which appeared to be the smallest possible hole size since a desired hole size of 51 microns ended up being 305 microns. Therefore it was not possible to make the pattern shown in Figure 4-17. As a result, the following pattern was used for the mold, which kept the 1.75 aspect ratio of hole diameter to hole spacing.
Figure 4-20A shows the pattern that was used for the in vivo robot application and
46 Figure 4-20B was the pattern that would be used for this application.
Figure 4-20 Mold Pattern for PDMS Tread for In Vivo Robot (A) and Grasper (B) To make the mold an 89 x 12 mm piece of ¼” acrylic was cut out and three rectangles measuring 25 x 8 mm were repeatedly engraved to form three 0.5 mm deep recesses where the PDMS would be poured into. The hole pattern shown in
Figure 4-20 was cut into the recesses as shown in Figure 4-21. Sylgard® 184 Silicone Elastomer (Dow Corning, Midland MI), a two-part kit, was used to make the PDMS treads. One mL of the base material was mixed with 0.1 mL of the curing agent for a 10:1 mixing ratio. Once the two parts were thoroughly mixed and the air bubbles were removed, the liquid PDMS was poured into the acrylic mold and cured on a 75°C hot plate for 1 hour until the PDMS had fully cured.
47 Figure 4-21 Acrylic Mold for PDMS Treads
After peeling the treads from the mold the frictional properties of the PDMS treads alone were tested. It was desired to see how much frictional force was generated between the treads and tissue for varying amounts of normal force. The range of normal forces tested remained under the 3.7 N maximum the compliant grasper was capable of producing. The testing setup can be seen in Figure 4-22. An 8 mm x 27 mm strip of tread was cut from the PDMS and adhered to a piece of 0.040” shim stock of the same dimension using transfer tape. A paper clip was partially straightened and glued to the top of the shim stock using Loctite 4014 instant adhesive. Pig liver samples 3 mm x 7 mm x 5 mm in size were placed under the treads and kept wet for the duration of testing.
Weights were placed on top of the shim stock so the normal force was evenly distributed to the PDMS. Weight was then incrementally added below the pivot point to pull the PDMS to the point of a loss of grip.
48 Figure 4-22 Traction Test Setup for PDMS Treads (not to scale)
Ten trials were performed at two different levels of normal force, 1.47 N and 1.76 N. In the trials of 1.47 N there was an immediate loss of grip in all ten trials when the minimum weight of 50 grams or 0.5 N was loaded. When the normal force was increased to 1.76 N the average maximum pull force was 0.9 N. Therefore it could be extrapolated that mounting the treads onto both jaws would give an average maximum pull force of approximately 3.78 N because the maximum pinch force the grasper is capable of producing is 3.7 N which is 2.1 times of the 1.76 N normal force, and which would then be multiplied by a factor of two since there would be two friction surfaces.
Strips of PDMS were then mounted onto the jaws using transfer adhesive (Figure 4-23) for traction improvement testing. The testing procedure from Section 4.5.2 was repeated. By adding the treads to the jaws the average maximum pull force barely increased to 1.5 N, which is about 40% of the theoretical pull force of 3.78N. This lack of traction increase could be attributed to a few factors. One being that there was a noticeable amount of tissue residue (“gunk”) on the treads after the previously mentioned traction testing. This may have prevented the treads from getting a good grip on the tissue surface due to the buildup in between the treads. Another factor was that the size of the
49 treads was too big. The pattern used in the mold for these treads was about 80% larger
than one used in the in vivo robot application [4-13].
Figure 4-23 Jaws with PDMS treads
Further efforts were made to increase the pull force of the Nitinol jaws. As a proof-of-concept to illustrate the premise that adding a textured surface to the jaws would increase traction, 100-grit sandpaper was glued to the jaws with Loctite 4014 and pull tests were repeated using the same protocol outlined in Section 4.5.2. Ten trials were performed with an average pull force of 2.0 N which is close to the 2.5 N average pull force reported for laparoscopic grasper [4-10]. The last trial was not counted because the tissue slipped out of the jaws when the minimum load of 0.5 N was applied. This occurred because the sandpaper had built up a substantial amount of residue on its surface, reducing the traction between the jaws and the tissue. While some laparoscopic graspers do have teeth smaller than 1 mm teeth used in the AutoSuture™ Endoclinch™ II that was used for testing, they do not have the same issue of residue build up because the channels between the teeth are not irregular as they are in sandpaper. Having regular recesses in the teeth can allow the residue to be flushed out easily whereas the opposite is true for sandpaper. Future work could address other approaches to improve the traction
50 with teeth or tread sized on the order of the 100-grit sandpaper and using a material with
a higher stiffness than PDMS for this application.
51
Chapter 5 Conclusions
This thesis presents two projects that involved the design, analysis, and testing of two grasping tools intended for two types of surgical procedures, NOTES and laparoscopic surgery. Both of these projects used superelastic Nitinol ribbon to allow large deformations and satisfy the goal of compliant grasping. The profile shape of the Nitinol was strongly linked to the grasping behavior in both applications. For the NOTES retainer assembly, simulation data led to the decision that a plateau shaped retainer proved better than a double-peak or single-peak shaped retainer because it displayed a stable grasping behavior with lower magnitudes of insertion forces. Trends in the clamping and insertion forces generated by the retainer were established to help identify regions of stable and unstable grasping and the geometric features that caused them.
Qualitative testing was performed successfully, but to fully determine the success of the retainer design cooperative testing with the in vivo robots should be further pursued.
Future work to improve the retainer design would include an optimization of the retainer shape to lower the insertion force needed to extract the payload from the retainer and meet the 1 N requirement.
Quantitative testing of the compliant laparoscopic grasper showed that an upper limit of applicable pinch force exists and that further closing of the jaws did not result in an increase in pinch force. By having a “built-in” maximum pinch force, a surgeon can fully squeeze the trigger of the grasper handle and not worry about applying excess force that can damage the tissue. Traction between the Nitinol jaws and tissue needed to be improved for the compliant grasper to become useful in a surgical setting and to reach the 5 N requirement. Micropatterned PDMS treads offered some traction improvement but
52 not enough to reach the desired levels and more appropriate methods of increasing the
traction between the Nitinol jaws and soft tissue need to be explored. Additionally fatigue testing of the jaw should be performed to determine the risk level of the jaws fracturing under stress which could result in violent tissue laceration.
53
References
[2-1] Duerig, T., Pelton, A., & Stöckel, D. (1999). An overview of nitinol medical applications. Materials Science and Engineering: A, 273, 149-160.
[2-2] Stoeckel, D. (2000). Nitinol medical devices and implants. Minimally Invasive Therapy & Allied Technologies, 9(2), 81-88.
[2-3] Graesser, E. J., & Cozzarelli, F. A. (1991). Shape-memory alloys as new materials for aseismic isolation. Journal of Engineering Mechanics, 117(11), 2590-2608.
[2-4] De Lange, R. G., & Zijderveld, J. A. (1968). Shape-Memory Effect and the Martensitic Transformation of TiNi. Journal of Applied Physics, 39(5), 2195-2200.
[2-5] Pelton, A. R., Stöckel, D., & Duerig, T. W. (2000, January). Medical uses of nitinol. In Materials Science Forum (Vol. 327, pp. 63-70).
[2-6] Stoeckel, D., Pelton, A., & Duerig, T. (2004). Self-expanding nitinol stents:
material and design considerations. European radiology, 14(2), 292-301.
[2-7] Civjan, S., Huget, E. F., & DeSimon, L. B. (1975). Potential applications of certain nickel-titanium (nitinol) alloys. Journal of dental research, 54(1), 89-96.
[2-8] Shabalovskaya, S. A. (2002). Surface, corrosion and biocompatibility aspects of Nitinol as an implant material. Bio-medical materials and engineering, 12(1), 69-109.
[2-9] Ratner, B. D. (2001). A perspective on titanium biocompatibility. In Titanium in medicine (pp. 1-12). Springer Berlin Heidelberg.
[2-10] Morgan, N. B. (2004). Medical shape memory alloy applications—the market and its products. Materials Science and Engineering: A, 378(1), 16-23.
[2-11] Callister Jr., W. D. (2007). Materials science and engineering: An introduction.
(7th ed. ed., pp. 348-350). New York: John Wiley & Sons, Inc.
[2-12] Johnson Matthey Medical Components. How Does Nitinol Work? All About Nitinol Shape Memory and Superelasticity. http://jmmedical.com/resources/122
/How-Does-Nitinol-Work%3F-All-About-Nitinol-Shape-Memory-and-Superelasticity.html. Accessed 3-20-2013.
[2-13] Barras, C. D. J., & Myers, K. A. (2000). Nitinol–its use in vascular surgery and other applications. European Journal of Vascular and Endovascular Surgery, 19(6), 564-569.
54 [2-14] Bishara, S. E., Barrett, R. D., & Selim, M. I. (1993). Biodegradation of
orthodontic appliances. Part II. Changes in the blood level of nickel. American Journal of Orthodontics and Dentofacial Orthopedics, 103(2), 115-119.
[2-15] Prince, M. R., Salzman, E. W., Schoen, F. J., Palestrant, A. M., & Simon, M.
(1988). Local intravascular effects of the nitinol wire blood clot filter.
Investigative Radiology, 23(4), 294-300.
[2-16] Cragg, A. H., De Jong, S. C., Barnhart, W. H., Landas, S. K., & Smith, T. P.
(1993). Nitinol intravascular stent: results of preclinical evaluation. Radiology, 189(3), 775-778.
[2-17] Zhao, S., Gu, L., & Froemming, S. R. (2011). Assessment of shape memory alloy stent deployment in a stenosed artery. Biomedical Engineering Letters, 1(4), 226-231.
[2-18] Rosenthal, D., Wellons, E. D., Levitt, A. B., Shuler, F. W., O'CONNER, R. E., &
Henderson, V. J. (2004). Role of prophylactic temporary inferior vena cava filters placed at the ICU bedside under intravascular ultrasound guidance in patients with multiple trauma. Discussion. Journal of vascular surgery, 40(5), 958-964.
[2-19] Kourambas, J., Delvecchio, F. C., Munver, R., & Preminger, G. M. (2000).
Nitinol stone retrieval-assisted ureteroscopic management of lower pole renal calculi. Urology, 56(6), 935.
[2-20] Electro Medical Systems. Lithobasket® Kit. http://www.ems-company.com/?
plugin=cms&webpage_id=2305. Accessed 3/39/13
[3-1] ASGE and SAGES. 2006. ASGE/SAGES working group on natural orifice translumenal endoscopic surgery, white paper October 2005. Gastrointestinal Endoscopy, 63(2): 199-203.
[3-2] Midday, J. (2012). Material Handling System for Robotic Natural Orifice Surgery. Thesis.
[3-3] Lehman, A.C., Wood, N.A., Dumpert, J., Oleynikov, D., Farritor, S.M., 2008,
"Dexterous Miniature in vivo Robot for NOTES," 2nd IEEE RAS & EMBS International Conference on Biomedical Robotics and Biomechatronics, 2008.
BioRob 2008, pp.244-249.
[3-4] Hawks, J. A. (2010). Improved mobile wireless in vivo surgical robots: Modular design, expe6rimental results, and analysis. Thesis.
[3-5] Rentschler, M. E., Dumpert, J., Platt, S. R., Oleynikov, D., Farritor, S. M., &
Iagnemma, K. (2006, May). Mobile in vivo biopsy robot. In Robotics and
55 Automation, 2006. ICRA 2006. Proceedings 2006 IEEE International Conference
on (pp. 4155-4160). IEEE.
[3-6] LSTC. (2007). LS-DYNA Keyword User's Manual. (Version 971, p. 140 (MAT)).
[4-1] Westebring-van der Putten, E.P., van den Dobbelsteen, J.J., Goossens, R.H.M., Jakimowicz, J.J., Dankelman, J., 2009, “Effect of Laparoscopic Grasper Force Transmission Ratio on Grasp Control,” Surgical Endoscopy, 23(47), 848-824.
[4-2] Heijnsdijk, E.A.M., deVisser, H., Dankelman, J., Gouma, D.J., 2004, “Slip and Damage Properties of Jaw of Laparoscopic Graspers,” Surgical Endoscopy, 18(6), 974-979.
[4-3] Patterson, E. J., & Nagy, A. G. (1997). Don't cry over spilled stones?
Complications of gallstones spilled during laparoscopic cholecystectomy: case report and literature review. Canadian journal of surgery, 40(4), 300-303.
[4-4] Cullis SN, Jeffery PC, McLauchlan G, Borman PC. Intraperitoneal abscess after laparoscopic cholecystectomy. Surg Laparosc Endosc 1992;2(4):337-8.
[4-5] Kraft K, Butters M, Bittner R: [The lost gallstone -- complication after laparoscopic cholecystectomy] [see comment]. Chirurg 1994;65(2):142-3.
Comment in: Chirurg 1994;65(4):410
[4-6] Shakeshaft, A. J., Cartmill, J. A., Walsh, W. R., Martin, C. J., 2001, “A Curved Edge Moderates High Pressure Generated by a Laparoscopic Grasper,” Surgical Endoscopy, 15(10), 1232-1234.
[4-7] Marucci, D. D., Cartmill, J. A., Martin, C. J. and Walsh, W. R., 2002, “A Compliant Tip Reduces the Peak Pressure of Laparoscopic Graspers,” Australian and New Zealand Journal of Surgery, 72(7), 476–478.
[4-8] Cartmill, J. A., Shakeshaft, A. J., Walsh, W. R., & Martin, C. J. (1999). High pressures are generated at the tip of laparoscopic graspers. Australian and New Zealand journal of surgery, 69(2), 127-130.
[4-9] Schostek, S., Schuur, M.O., Buess, G.F., 2009, “Review on Aspects of Artificial Tactile Feedback in Laparoscopic Surgery,” Journal of Medical Engineering &
Physics, 31(8), 887-898.
[4-10] deVisser, H., Heijnsdijk, E.A.M., Herder, J.L., Pistecky, P.V., 2002, “Forces and Displacements in Colon Surgery”, Surgical Endoscopy, 16(10), 1426-1430.
[4-11] Interlink Electronics . FSR® Force Sensing Resistor® Integration Guide and Evaluation Parts Catalog. http://resenv.media.mit.edu/classes/MAS836/Readings/
fsrguide.pdf. Accessed 3-17-2013.
56 [4-12] Karagozler, M. E., Cheung, E., Kwon, J., & Sitti, M. (2006, February). Miniature
endoscopic capsule robot using biomimetic micro-patterned adhesives. In Biomedical Robotics and Biomechatronics, 2006. BioRob 2006. The First IEEE/RAS-EMBS International Conference on (pp. 105-111). IEEE.[4-13]
Sliker, L. J., Wang, X., Schoen, J. A., & Rentschler, M. E. (2010).
Micropatterned treads for in vivo robotic mobility. Journal of medical devices,4(4).
[4-14] Lim, Dr. Jung Yul. Personal Interview. 27 February 2013.
[4-15] Epilog Laser. Owner’s Manual for Epilog Mini/Helix – Model 8000. 22 April 2010. http://www.epiloglaser.com/downloads/pdf/mini_helix_4.22.10.pdf. Page 196.
57