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Figure 2.10: When a deflected flexure-tip needle (i.e., the flexure is already bent) is inserted and rotated with constant velocity into tissue, the flexure angle between the tip and the needle shaft decreases toward zero as shown here. This effect enables straight trajectories with reduced tissue damage as described in [6].
Fig. 2.10 demonstrates the flexure-tip needle’s tendency to straighten the hinge by plotting the flexure angle when the needle is rotated at a constant target angular velocity of 1.5 rad/s and inserted at a constant velocity of 5.5 mm/s as plotted in Fig. 2.10(a). Fig. 2.10(b) shows that the flexure angle rapidly drops to a steady-state angle of 0.15 rad after 7.5 s. This be- havior is also predicted by the flexure-tip model as shown in Fig. 2.10(b). The error between the model and actual approaches a steady state of 0.11 rad after 7.5 s. We expect that some of the steady-state error can be attributed to misalignment of the distal-most sensor inside the flexure tip.
and can be directly applied to existing motion planners [78, 79], potentially improving the accuracy of motion planning results when using the flexure-tip needle as in [61, 79].
The question of when to use the flexure-tip needle model with these motion planners is an interesting one.
Fig. 2.9 shows that a simplified kinked bevel-tip model can predict a flexure-tip needle’s actual behavior some of the time (e.g., when inserting the needle with little to no rotation velocity as in the first trajectory). The key contribution of the flexure-tip model presented in Sec. 2.5 is to predict the transient behavior of the flexure tip when the flexure angle is within the hard-stop. We expect that the kinked bevel-tip and flexure-tip models predicted the needle’s actual behavior equally well for the first trajectory of Fig. 2.9 because the needle’s configuration is within the transient regime for a much smaller amount of time compared to the second trajectory (the flexure angle does not reach the hard-stop limit during the entirety of the second trajectory). Since the flexure-tip model presented in Sec. 2.5 is more complex than the standard unicycle/bicycle models, determining when a simpler model could be used to predict a flexure-tip needle’s behavior is an important consideration. This is especially true for motion planning applications where the model is heavily used and complexity matters [78, 79]. In general, a simple model can be used when the flexure-tip needle is expected to spend little time in the transient regime, otherwise the full flexure-tip model will be substantially more accurate.
The kinematic model developed here for the flexure-tip needle provides a good approx- imation of the behavior of the needle during insertion, although several unmodeled effects likely influence the overall accuracy of the model. For example, the model does not account for buckling of the cutout flexure joint. Buckling of the flexure can change the stiffness of the flexure joint and potentially lead to plastic deformation of the flexure joint, creating a bias in the needle trajectory. However, the use of nitinol wires to create the flexure joint as in Sec. 2.3 will likely remove this unmodeled effect. Manufacturing tolerances, whether the width and depth of the flexure cutout or the alignment and spacing of the nitinol wires
used to make the flexure joint, will also affect the trajectory of the needle in tissue. Re- gardless of the flexure-tip needle design used, there are clinical questions that should also be investigated for the flexure-tip needle.
While we note that initial anecdotal phantom tissue histological images provided in this chapter show little apparent tissue damage when using the flexure-tip needle, more in-depth histological evaluations in living biological tissues are needed to quantitatively verify re- duced tissue damage. It will also be useful to verify that the tissue damage incurred by any of these needle tip designs (including the kinked bevel-tip needle) is clinically significant.
To adapt this needle to carry specific interventional payloads (e.g., biopsy, therapy deliv- ery, brachytherapy, and others), similar to prior bevel-tip steerable needles, the flexure-tip needle can serve as a guide-wire to the region of interest, with a sheath advanced over the needle after the tip has reached the desired location. This concept is experimentally shown in Sec. 4.7. We hope that the reduced tissue damage of the flexure-tip needle and the ability to use the needle as a guide-wire for accurate targeting will help to facilitate clinical trans- lation of flexible tip-steerable needles sooner rather than later. Toward this end, we have engaged in numerous discussions with medical device companies and are in active discus- sions with a large medical device company to translate the flexure-tip needle to clinical use.
Some results from this commercialization effort are presented below.
We envision that the flexure-tip needle could be deployed by hand in a clinical setting to correct for tip error due to misalignment or deflection of the needle during insertion.
Toward clinical adoption and commercialization of the flexure-tip needle, we have built multiple variations of the flexure-tip needle to test the performance of the needles under various conditions and in various tissue mediums. We have built flexure-tip needles with stainless steel hypodermic tubing in place of nitinol for the needle shaft, showing that the stainless steel still provides appreciable curvature with the benefit of reduced cost. We have also built flexure-tip needles with different numbers of nitinol flexure wires, and have even used a thin piece of nitinol strip to make the flexure in order to show how the curvature
Figure 2.11: Multiple variations of the flexure-tip needle and corresponding insertions into a gelatin phantom. Note that needle paths shown to the right were all achieved by inserting the needle by hand, one of the deployment methods envisioned for clinical use. (a) 19 gauge stainless steel coaxial needle with a 21 gauge flexure-tip stylet. The flexure-tip stylet is made using a nitinol shaft, and three 0.125 mm nitinol wires make up the flexure joint. (b) 18 gauge nitinol shaft flexure-tip needle with a three wire flexure joint. (c) 20 gauge nitinol shaft flexure-tip needle with a two wire flexure joint. (d) 19 gauge stainless steel coaxial needle with a 21 gauge flexure-tip stylet made with a stainless steel shaft and a three wire flexure joint. (e) Flexure-tip incorporated into a clinical core biopsy gun. The flexure-tip is made with a three wire flexure joint.
of the needle can be altered by changing the geometry of the flexure. Lastly, we have integrated the flexure-tip needle into clinical biopsy needle systems. We built a flexure- tip needle that could pass through a coaxial introducer needle, and were able to vary the maximum bend angle of the flexure-tip by changing the length of the needle tip exposed from the introducer needle. This approach may be useful for correcting for misalignment or errors when targeting locations using standard tri-bevel needles that are intended to go straight and cannot steer. We also displayed the ability to steer core biopsy needles by adding a flexure-tip needle onto the end of a core biopsy needle gun. This collection of needles, built to illustrate the clinical application and usefulness of the flexure-tip needle, can be seen in Fig. 2.11.
Chapter 3
Miniature Wrists for Surgical Robots