Radiation protection with robotics

The Institute for Medical Engineering and Medical Informatics (IM2) laboratory has row after row of 3D printers. Layer by layer, they create metal or ceramic implants to support healing bones. In an international research collaboration, Michael de Wild and his team are now applying their 3D printing expertise to robotics. “We are developing a robotic support system that can help with im- age-controlled surgical procedures,” says the researcher. “For example, when taking tissue samples, doctors have to insert biopsy needles with millimetric precision into the organ or area to be examined. Using medical imaging such as computer tomography (CT), they can see where the needle is and, if necessary, correct its position. With the planned robotic system, the needle can be controlled remotely and observed on the monitor while the patient is lying in the CT scanner. This not only improves the precision of tissue extraction but is also suitable for interventional tumour radiology or for localised drug treatment, such as pain therapy with image control.”

Complex 3D-printed designs
The SPIRITS project (Smart Printed Interactive Robots for Interventional Therapy and Surgery) involves the HLS as well as companies, clinics and universities from Austria, France, Germany and Switzerland. The HLS team is constructing the motor to move the needle forward in the body. To en- able safe operation of this actuator in the operating theatre, it is driven by compressed air. “What’s special is the way the drive structure functions,” explains de Wild. “The biopsy needle is mounted in the centre of a cylindrical grid structure surrounded by four to five channels. Unlike ordinary materials, this grid structure expands transversely to the direction of pull, so when pressure inside the cylinder is increased, the structure expands along the longitudinal axis and drives the needle forward.” The HLS researchers achieve this through the auxetic geometry of the lattice. “We raise the pressure in the cylinder with four to five of the small balloons Minimally that are used in cardiac surgery to expand stents to their final size to keep blood vessels open,” says de Wild. “However, because the auxetic structure is only extended by fractions of a millimetre, the process needs to be repeated several times per second. The needle moves forward using a clamping system in very small steps, similar to a caterpillar.” This geometry is too complex and fragile for conventional manufacturing processes but not for 3D printing. IM2 at the HLS is one of the few institutes in the world that can 3D print such fine metallic structures as the drive cylinder. The researchers use titanium powder for the cylinder, applied in a thin layer to a printing platform, then fuse the areas which will form the structure. “We use a fiber laser that melts an area of around 100 micrometres in diameter with up to 200 Watts,” says the researcher.

Temperature management is key
Such energy densities require special designs, explains de Wild: “If the layer below or beside the laser target area is not melted, the gas between the particles acts as an insulator. The titanium then becomes too hot, even evaporating as a result of the laser beam, or causing stress or deformations in the manufactured object. Components are anchored to the building platform with struts so that heat can be conducted away. We therefore specifically add support structures to the final design that not only give mechanical stability during 3D printing but also dissipate the heat.” Prior to manufacturing, the researchers did extensive calculations to customise the localised laser energy, taking into account the material and shape of the object. For each of the thousands of points in the design, this heat calculation simulates how well heat can conduct through the structure and supports and calculates with how much energy and for how long the laser should stay on each point. Using this exhaustive construction and process plan, the 3D printer then produces a structure a few centimetres in size from thousands of layers over a period of several hours.

The human touch
Despite state-of-the-art 3D printing technology, finishing is currently still done by hand. The researchers first take the structure off the printing platform and remove the slender supports. This requires a delicate touch to avoid damaging the fragile structure. Then they smooth the surface using electro-polishing, mechanical polishing, chemical etching or sandblasting. Finally, the researchers subject the structures to various tests. In static mechanical tests, they pull them apart and measure how strong and elastic they are. In dynamic tests, they analyse fatigue after thousands or millions of load cycles; improvements from all these tests are incorporated in the surgical robot prototypes. On completion of the 2020 project, medical technology companies in the three countries area around Basel should complete the product development and bring it to market; a spin-off is also an option for de Wild. He and his team are already investigating alternatives to pure titanium such as the nickel-titanium alloy Nitinol; the shape memory alloy is up to five times more elastic and the first auxetic structures made of Nitinol have already been printed.