Just press a button and the 3D printer does the rest automatically. Similar to an inkjet printer, liquid is sprayed onto a surface. However, the PolyJet printer uses plastic instead of different colors: the nozzles can apply two different polymer solutions individually or mixed with pinpoint accuracy. UV light hardens the plastics before the next layer is applied. This creates a medical robot layer by layer. Once development is complete, the robot will assist doctors in taking tissue samples and in thermal tumor treatment.

"Positioning a needle or probe during a minimally invasive procedure like this is a particularly difficult task, because the doctor can best orient himself with the help of computer tomography or MRI images - and that means he has to work while the patient is lying in a narrow tube. There is hardly any freedom of movement," explains Marius Siegfarth from the Project Group for Automation in Medicine and Biotechnology (PAMB) of Fraunhofer IPA at the Mannheim Medical Faculty of Heidelberg University.

Robot fits into the tube with the patient

The robot, which his team is developing together with four other research groups from Germany, France and Switzerland in the 'SPIRITS' project, is so small and light that it can be pushed into the tube together with the patient. It can be controlled from the outside using hydraulics. The doctor can therefore sit a few meters away and even in another room, where he is protected from radiation in the event of a CT scan. The abbreviation 'SPIRITS' stands for 'Smart Printed Interactive Robots for Interventional Therapy and Surgery'. "The challenge of the project was to develop a design that could be produced in a single step using a PolyJet printer, but at the same time consisted of fully functional components - such as swivel joints with hydraulic actuators and a drive for the needle feed. All of these components have different material properties," explains Siegfarth.

In-house hydrausic drive system developed

3D-printed pistons of the hydraulic drive

© Marius Siegfarth

At the Institut 'national des sciences appliquées de Strasbourg' (INSA), where the research project is being coordinated, the PolyJet is already printing the first prototypes. These have lever arms that are connected via joints. They can be used to rotate a needle around the penetration point in all spatial directions. A specially developed hydraulic system provides the drive: tiny tubes with diameters of just 4 mm, seals and pistons. The special thing about it: the pistons could be designed using 3D production technology so that the hydraulic pressure acts on the seal and reinforces its effect.

Initial tests show that the hydraulic drive from the 3D printer works. Further components will be integrated into the prototype in the coming months: the intelligent needle with force sensor, for example, a development of the 'École polytechnique fédérale de Lausanne' (EPFL). The feed mechanism for the needle was developed by INSA researchers. Added to this is the "haptic feedback". It transforms the measurement results of the force sensor into resistances that the doctor feels when he guides the needle through softer or harder tissue. This feedback system was developed by researchers at Furtwangen University. And at the University of Applied Sciences and Arts Northwestern Switzerland FHNW, printable, non-magnetizable metal components are currently being developed for the next generation of prototypes.

The first fully printed medical robot is to be tested on dummies before the end of 2019. With a total budget of 1.67 million euros, 'SPIRITS' is co-financed by the 'INTERREG V Upper Rhine' program with 436,201 euros from the European Regional Development Fund. As part of the 'Offensive Sciences' initiative, which funds top cross-border research projects, the project is also supported by regional and cantonal partners. The project is co-financed by the Greater Region East, the state of Baden-Württemberg, the state of Rhineland-Palatinate, the Swiss Confederation, the canton of Aargau, the canton of Basel-Stadt and the canton of Basel-Landschaft.