DragonFlex – Smart Steerable Laparoscopic Instrument (2013)

Laparoscopic instruments can be differentiated into rigid and steerable. The rigid ones comprise an inflexible shaft equipping them with four degrees of freedom (DOF): axial sliding, axial rotation and pivoting in two perpendicular planes around the incision point. This fulcrum effect greatly restricts the range of motion and limits the surgeon mainly to frontal or sideways approach to the tissue. In contrast, steerable instruments have additional degrees of freedom due to one or more joints in the tip, enabling the surgeon to reach behind or over obstacles [1].A state-of-the-art example of a steerable instrument is Intuitive Surgical's EndoWrist made for the da Vinci surgical robot and favorably used in prostatectomy procedures, Fig. 1. EndoWrist has two rotational DOF acting in two perpendicular planes in a form of pulleys, each individually controlled by a looped cable. The stiffness of the tip is dependent on the tensile strength of the cables and stiffness of the rigid bodies within the tip. Since both the steel cables and the rigid bodies are highly stiff, EndoWrist can be considered stiff as well. Despite its high stiffness, EndoWrist's lifespan is limited to only ten procedures [3]. The limiting factor is the fatigue resistance of the steel cables, which are guided over pulleys whose diameter is smaller than half of the tip's width. Hence, after repeatedly bending and rolling over a very small bending radius, the steel cables inevitably fail resulting in expensive replacement and maintenance costs. Moreover, the tip features a complex design of numerous miniature elements, including pulleys and rivets, leading to high manufacturing and assembly costs as well as potential sterilization issues [1].Taking into consideration the scope of the potential improvements, one would desirably aim for a design of a structurally simple steerable laparoscopic tip free from cable fatigue, while attaining sufficient bending stiffness for the surgical environment and possibly even improving on EndoWrist's maneuverability and dimensions. Considering all these requirements, at TU Delft we designed a smart laparoscopic instrument prototype named DragonFlex.In order to improve the reliability and the lifespan of cable-driven joints, their bending radius has to be maximized to the achievable limits. One straightforward approach, implemented in conventional flexible endoscopes, is guiding the cables smoothly along a series of hinged elements, together creating a sufficiently large bending radius as to avoid fatigue. Yet, such mechanisms are complex, bulky and lack stiffness due to underactuated DOF.The opposite approach, implemented in EndoWrist and in our DragonFlex design, is decreasing the number of steerable elements to the bare minimum i.e. two mutually rotatable elements per one planar rotational DOF. Once a 90 deg bend is made with only two elements, the cable bending radius gets critically small resulting in cable fatigue. However, this can be easily prevented, provided the cables are leaning over pulleys, like in EndoWrist, yet of a sufficiently large radius. The question of maximizing the pulley diameter, restricted by a constant instrument width can be resolved by splitting the circular pulley profile along the instrument's longitudinal axis into two equal axisymmetric non-tangential arcs, thus creating a leaf or drop-like shape, whose diameter can be increased beyond the instrument's width, as illustrated in Fig. 2.In our DragonFlex design, each tip element features four of these arced guides, two inner and two outer, in order to support both driving cables during the joint rotation to either side. The steerable tip elements interface in a rolling joint [4], Fig. 3, providing a moving point of rotation and thus moment arms for both cables anywhere within ±90 deg. In order to equalize the tensile forces in both cables, their moment arms were equalized as well at both straight and bent positions. A fully constrained SolidWorks drawing revealed that, at 5 mm element width, the maximum realizable arced guide radius is 4 mm. Once equipped with Ø0.4mm cable channels, this further reduces to 3.75 mm, compared to only 1–1.5 mm in the case of EndoWrist.The planar joint construction was repeated and rotated over 90 deg to generate a second steerable joint, thus allowing full two DOF motion. The tip also features a novel hinge-free grasping forceps copying the basic element design. Being the tip's almost identical counter-copy, the handle comprises two individually controllable orthogonal rolling joints as well and ends with a scissor-like openable handgrip. The joint elements in the handle are equipped with angled protrusions on their sides that stop the individual joint rotation at the maximum of ±90 deg. The two cables, driving the whole construction in a parallelogram fashion, are looped in the forceps and fixed in the handle where they can be manually re-tensioned.With all its miniature features and curved embedded holes for the cables, DragonFlex would be hard to manufacture via conventional machining. Hence, additive manufacturing technology [5] was utilized, easily resolving these issues and generating relatively inexpensive prototypes.As a design feasibility test, a large-scale DragonFlex prototype of 15 mm shaft width was first made by 3D printing from PLA plastic at TU Delft, later assembled and fitted with Ø0.3 mm steel cables. After a successful functionality test, a real-scale DragonFlex prototype of 5 mm shaft width was manufactured in close collaboration with Dutch Organization for Applied Scientific Research (TNO), in Eindhoven, Fig. 4. The real-scale prototype was made by micro-stereo lithography, from a ceramic-filled epoxy resin at 30 μm resolution. The prototype was assembled with Ø0.2 mm Dyneema cables. Functionality tests revealed no cable fatigue or any other structural failure. Grasping and omnidirectional steering proved to work successfully and the prototypes exhibited a promisingly high bending stiffness. The large- scale prototype experienced a little wear, due to the use of steel cables on plastic material and some oiling was needed. Dyneema cables in the real-scale prototype proved to decrease friction and minimize the material wear. Upon simple force testing and observation of several spare parts, the ceramic prototype material turned out to be slightly brittle yet flexible at the same time. The prototypes continue to fully function even after numerous cycles of careful usage.Having equal joint functionality and dimensions to EndoWrist, DragonFlex's tip contains the theoretical minimum of only four parts, compared to more than ten parts in EndoWrist, driven and bound by two cables fixed in the handle. Two orthogonal planar joints feature an innovative rolling link mechanism allowing the cables to follow circular arc profiles of a diameter 1.5 times larger than the width of the instrument shaft; whereas EndoWrist uses round pulleys of less than half of the shaft width. Besides maximizing the cable lifespan, the rolling joint was designed to equalize the forces on both cables throughout the joint rotation, making the handling fluid and effortless without the need for a cable tensioning system. Length difference between the two driving cables was minimized thanks to the mirrored handle-tip joint design. The smart joint design enables control of seven DOF with a total of only seven structural components of the entire instrument. DragonFlex concept sheds a new light on the possibilities of additive manufacturing of surgical tools, allowing for a feature-packed design, simple assembly, suitability for a disposable use and even a potential MRI compatibility. Last but not least, it shows that the steerable MIS instruments could be designed stacked, featuring no permanent joints, thus simplifying the design, manufacture, assembly, sterilization and as a result the overall costs.This research was performed within the framework of CTMM, the Center for Translational Molecular Medicine, project MUSIS (grant 03O-202). Many thanks to the Dutch Organization for Applied Scientific Research (TNO) for their collaboration.