Surgery: "The robot can follow any space curve"

Interview with Dr.-Ing. Jessica Burgner-Kahrs, Head of DFG Emmy Noether Research Group "CROSS – Continuum Robots for Surgical Systems", Hannover Center of Mechatronics, Leibniz Universität Hannover

An elephant’s trunk, tentacles, a snake’s body – animals show how effective flexible shapes and extremities are for moving and grasping.


Photo: Jessica Burgner-Kahrs

Dr.-Ing. Jessica Burgner-Kahrs; ©Klaus Michelmann

Robotic specialists copy these examples to make them available for medicine: in the future, continuum robots made of flexible tubes could perhaps perform surgery where conventional robots made of fixed components are not able to reach.

In the interview with, Jessica Burgner-Kahrs (D.E.Sc.) explains the importance of the shape-memory alloy Nitinol and its superelasticity for this new type of robot and where it could be used.

Doctor Burgner-Kahrs, what are continuum robots?

Jessica Burgner-Kahrs: Continuum robots are entirely different from the conventional robots that we know. They are not made of discrete joints and rigid connections. Instead, they feature a continuous structure whose flexibility can be adjusted due to its set-up. The main principle is bionics-inspired, since we also give the robots a kind of backbone like our spine, and where flexible motion is possible emulating the motions of an elephant’s trunk or a snake.

What significance does this technology have at the moment?

Burgner-Kahrs: You could say it is on the upswing. The big conferences on robotics say that the significance of continuous robotics is increasing. It also has advanced significantly over the past years in terms of mechanics and miniaturization. The trend moves towards bendable instruments and manipulators, which is something you can see in medical technology.

Photo: Continuum manopulator next to a coin

Prototype of a tubular continuum robot built from three NiTi tubules (diameter of the innermost tubule 0,8mm); ©Leibniz Universität Hannover

Why do you specifically use Nitinol as a material?

Burgner-Kahrs: Firstly, Nitinol is a biocompatible material we know from medicine. Today, stents and catheters are made from it for instance. Secondly, Nitinol has a very special characteristic for us: it is a shape-memory alloy with super elastic properties. This means we can elastically shape a small tube, but once the deforming power decreases, the small tube returns to its original shape. Ultimately, we take advantage of this elasticity for the movement of the robot. When you nest the tubes, they interact with one another. In doing so, the robot can ultimately follow any space curve.

How flexible can such an arm be, for instance during an intervention in the sinuses?

Burgner-Kahrs: Very flexible. We can design the arm or rather the tubes, as the application requires. For sinuses, the tubes would have to be very slim so the robot is very flexible and does not cause any injuries. This would also make it inherently safe for use in patients. In another procedure, we could advance transnasally through the sphenoid bone and nasal cavity to the pituitary gland. In this case, you would make the tubes more rigid in the beginning, so you could be very precise in moving to the back and then make them more elastic for the use at the operation site.

We are developing algorithms with which you can ultimately adapt the robot for the application. The physician would first have to determine based on imaging data what he or she would like to achieve with the robot and the instruments. The algorithms then calculate how the individual tubes need to be designed in terms of length, curve and rigidity.

Looking at the removal of hemorrhages in the brain, we are currently researching how many different tubes you actually need to be able to treat the largest possible patient population. Of course, we also want to keep the complexity as minimal as possible and prefer to provide ten instead of 100 different tubes.
Photo: Laboratory scene with anatomic model and robot

Project staff discusses possible ways for the robot to access the brain using an anatomic model and medical image data; ©Leibniz Universität Hannover

With this kind of flexibility, how can you prevent the robot from accidentally getting stuck and damaging tissue?

Burgner-Kahrs: The physician always checks the motion of the robot tip on the input device. He has an endoscopic view on the manipulator. On the other hand, we can also limit the motions the robot can perform during the surgical intervention based on imaging data. We can also define areas where the robot is not allowed to move or only, when the physician gives extra permission to do so. This is a programming issue, which we are currently working on, namely algorithms for the human-computer interaction.

Can you describe the applications the device is currently being developed and tested for in greater detail?

Burgner-Kahrs: We are primarily dealing with subcranial surgery and applications for the brain. There is the removal of hypophyseal tumors for example that are unfortunately very common. Even though they are not malignant, they need to be removed if they cause hormonal imbalances or press on the optic nerve. This application is perfectly suited for this, because based on anatomy, so far only 10-15 % of patients can have surgery using a transnasal approach. We are also researching the removal of hematomas in the brains of stroke patients. In this case, we are collaborating with the Vanderbilt University in the U.S. in a project funded by the National Institutes of Health.

Many other applications are also essentially conceivable, such as bronchoscopy of the lung for instance. You could advance deeper into the lung using continuum robots than is possible with today’s bronchoscopes, detect cancer early and perform a biopsy. Then there are other study groups, for instance in the U.S. that research use in prostates. There are still other ideas in orthopedics to perform hip endoscopies for instance.

Photo: Cross-section of skull, filled with gelatine

The image shows a brain mock-up made of gelatine in the skull of an anatomic model. Using real patient data, a hematoma from red gelatin has been created. The robot is coming from the left to suck off the hematoma; ©Vanderbilt University

Which research stage are you currently in?

Burgner-Kahrs: Here in Germany, we are still in the early stages in the laboratory. We work extensively with anthropomorphic phantoms, which strongly resemble the human being and human tissue. We already conducted early experiments with animals and human preparations at the Vanderbilt University, because authorizations are easier to obtain in the U.S. The projects in the next few years will then show whether we can also conduct studies with patients.

Video of the Vanderbilt University "Robot treats brain clots with steerable needles"
Photo: Timo Roth; Copyright: B. Frommann

© B. Frommann

The interview was conducted by Timo Roth and translated from German by Elena O'Meara.