Polymer Rigidity Control for Endoscopic Shaft-Guide ‘Plastolock’ — A Feasibility Study Arjo J. Loeve e-mail: a.j.loeve@tudelft.nl Johannes H. Bosma Paul Breedveld Dimitra Dodou Jenny Dankelman Department of Biomechanical Engineering, Faculty of Mechanical, Maritime, and Materials Engineering, Delft University of Technology, Mekelweg 2, 2628 CD Delft, The Netherlands Flexible endoscopes are used for diagnostic and therapeutic in- terventions in the human body for their ability to be advanced through tortuous trajectories. However, this very same property causes difficulties as well. For example, during surgery, a rigid shaft would be more beneficial since it provides more stability and it allows for better surgical accuracy. In order to keep the flex- ibility and to obtain the rigidity when needed, a shaft-guide with controllable rigidity could be used. In this article, we introduce the plastolock shaft-guide concept, which uses thermoplastics that are reversibly switched from rigid to compliant by changing their temperatures from 5°C to 43°C. These materials are used to make a shaft that can be rendered flexible to follow the flexible endoscope and rigid to guide it. To find polymers that are suitable for the plastolock concept, an extensive database and internet search was performed. The results suggest that many suitable ma- terials are available or can be custom synthesized to meet the requirements. The thermoplastic polymer Purasorb ® PLC 7015 was obtained and a dynamic mechanical analysis showed that it is suitable for the plastolock concept. A simple production test indi- cated that this material is suitable for prototyping by molding. Overall, the results in this article show that the plastolock concept can offer simple, scalable solutions for medical situations that desire stiffness at one instance and flexibility at another. DOI: 10.1115/1.4002494 1 Introduction For the investigation and treatment of areas in and around the digestive tract, flexible endoscopes 1 are used for many decennia to negotiate bends in organs and to approach hard to reach areas in the human body. In natural orifice translumenal endoscopic sur- gery NOTES and colonoscopy, for example, the indispensable flexibility of these instruments causes several difficulties 2–10. An example of such difficulties during NOTES is shown in Fig. 1: A flexible endoscope is advanced through the esophagus, into the abdomen through an incision in the stomach wall, toward an organ for surgery. A grasper is introduced through a channel in the in- serted endoscope to manipulate tissue. When the grasper is used to pull tissue, the flexible endoscope bends, failing to provide the stability required for the intervention because the endoscope shaft is too compliant to provide solid stability. This situation can occur in all interventions that use flexible instruments in the human body, and it contains a conflict: There is a necessity to have a flexible endoscope shaft that enables inser- tion through tortuous body cavities, and a desire for a rigid shaft that allows greater surgical accuracy. This could be solved if the endoscope shaft had widely controllable rigidity or if it had a second shaft with controllable rigidity guiding it. The mechanism providing such functionality should retain the shaft curvature when changing rigidity. This would enable altering the shaft guide for each phase of the intervention to be rigid or compliant in any suitable shape. If surgery through flexible endoscopes is to become a good alternative for current operating techniques, stable instrument sup- port and spacing between instruments are indispensable 8. To obtain instrument spacing without extremely reducing instrument sizes or using multiple endoscopes, the rigidity control mecha- nism should occupy as little space as possible. Meanwhile, it should still support scopes ranging from pediatric endoscopes to standard colonoscopes with flexural rigidities ranging from 67 N cm 2 to 330 N cm 2 11,12. In 2005, Rex et al. 13 and Swanstrom et al. 14 demonstrated a shaft guiding overtube with a rigidity control based on friction. Its shaft is a train of nested segments with tension wires running through the segments 15. This smart and simple mechanism of- fers beneficial rigidity in its current form. However, its rigidity highly depends on the high tension forces applied to the small segments. Therefore, the segments cannot be made very thin, leaving little space for instruments. Other solutions for the same problem use vast amounts of controlled segments, using much space and increasing the complexity of the devices 16–19. Size reduction and simplification of rigidity control mecha- nisms enable the application of rigidity control in smaller endo- scopes and increase the space available for instruments and work- ing channels in flexible endoscopes. It is expected that rigidity control mechanisms could be greatly simplified and downscaled if they were not dependent on applying forces or moving mechani- cal parts. One way to loose this dependency is to use an amor- phous or semicrystalline thermoplastic polymer and heat or cool it around its glass-transition temperature T g 20. Amorphous and semicrystalline polymers will further be addressed to as “partly amorphous.” The goal of this article is to demonstrate the feasibility of a concept using such a rigidity control mechanism within temperatures that are safe for the human body. Further presented are material requirements for this concept and a search for suitable polymers. 2 Plastolock Shaft Concept 2.1 Basic Concept. At temperatures below T g , partly amor- phous thermoplastic polymers are rigid with strong bonds between macromolecules. When heated, they become compliant, with weakened bonds between macromolecules. The transition around T g is fast and reversible. This rigidity control concept has been suggested in patents for applications ranging from medical catheters to inflatable space- craft structures 21–23 but has, to our knowledge, never been demonstrated in literature for use inside the human body. The concept we investigated is called the “plastolock” shaft. It is an elongated shaft Fig. 2 made of partly amorphous thermoplastic polymer. To change the shaft stiffness from rigid to compliant repeatedly and at will, fast heating and cooling of the material must be achieved. In the current concept, this is achieved using a heat carrying fluid flowing through channels in or around the plas- tolock shaft. By passing warm fluid through the channels, the temperature rises and the polymer becomes compliant, enabling the plastolock shaft to be put to any shape desired and to be advanced along Manuscript received May 3, 2010; final manuscript received August 12, 2010; published online October 12, 2010. Assoc. Editor: Paul A. Iaizzo. Journal of Medical Devices DECEMBER 2010, Vol. 4 / 045001-1 Copyright © 2010 by ASME Downloaded 19 Nov 2010 to 145.94.179.230. Redistribution subject to ASME license or copyright; see http://www.asme.org/terms/Terms_Use.cfm