Low friction, light and made to measure - quickly printed components for therapeutic applications

• What was needed: Finger joints for an exoskeleton
• Manufacturing process: selective laser sintering with SLS powder
• Requirements: low coefficient of friction, wear-resistant, low weight, precision
• Material: iglidur® i6
• Industry: Medical industry
• Success through collaboration: Fast delivery, cost-effective production of customised functional parts

According to the German Stroke Society, one person in Germany suffers a stroke every two minutes. The Swiss Federal Institute of Technology in Zurich (ETHZ) has developed a hand exoskeleton called RELab tenoexo, which covers up to 80 per cent of daily activities, to make it easier to learn to grasp again after a stroke. 3D printed finger joints made from the high-performance plastic iglidur® i6 ensure optimum power transmission.

Manufacturing the finger joints with a classic 3D printer proved to be difficult, as the resolution of the device is not high enough to realise the structure of the finger joints. These components not only hold the leaf spring together, but also have a filigree locking mechanism for a leather strap. The buckle into which the strap is threaded is barely wider than a millimetre. ABS filament proved to be unsuitable as a printing material, as the friction between the joints and the leaf springs was too high, resulting in a lot of energy being lost.

With iglidur® i6, ETHZ found a tribologically optimised plastic that proved to be ideally suited for the required components: The SLS powder was specially developed to reduce friction in moving applications. Laser sintering enables a high degree of precision, which means that the filigree structure of the joint can be realised without any problems. With the fast 3D printing service from igus®, the finger joints could be produced quickly and cost-effectively and were ready for immediate use.

The fingers were designed by Japanese professor Jumpei Arata from Kyushu University: three thin stainless steel leaf springs lie on top of each other and are connected by four plastic links. A Bowden cable is attached to the centre spring - if it is moved forwards, the fingers close, if it is pulled back, the hand opens. DC motor stretches and flexes the leaf springs and supports the patient in gripping movements. "The exoskeleton applies a force of six newtons per finger at"," says Jan Dittli, researcher at the ETHZ Department of Health Sciences and Technology. "The three implemented grips are sufficient to lift objects weighing up to approx. 500 grams - such as a 0.5 litre water bottle."

The exoskeleton is put on using a sensor wristband and attached to the fingers using leather straps. When the patient makes a movement with their hand, the wristband transmits electromyographic (EMG) signals to a mini-computer. This is located in a rucksack together with the motors, batteries and control electronics, which is connected to the hand module. If the wearer intends to make a gripping movement, this is recognised by the computer, which in turn activates the DC motor.

During development, the researchers encountered a challenge: the fine finger joints. These elements not only hold the leaf springs together, but also have a filigree closing mechanism for the leather strap. The buckle into which the strap is threaded is barely wider than a millimetre. A 3D printer with an ABS filament was used to produce the back of the hand - both the manufacturing process and the material proved to be unsuitable for producing the finger joints. "The friction between the joints and the leaf springs would have been too high with this material"," says Dittli. "As a result, we would have lost too much energy when moving the fingers." The resolution of a conventional 3D printer also turned out not to be high enough to realise the detailed structure of the finger joints.

The solution to this problem was found in the additive manufacturing of igus®: The self-lubricating SLS material iglidur® i6, specially developed for the production of friction-stressed parts, could be successfully used for the production of the finger joints. iglidur® i6 was originally developed for the production of worm gears for robot joints. It is particularly suitable for the production of components with fine details and precise surfaces and is characterised by high toughness and abrasion resistance. iglidur® i6 proved its suitability as a durable functional part in the igus® test laboratory: A sintered gear made of this abrasion-resistant iglidur® plastic was tested for two months under the same conditions as a milled gear made of POM. The gear made of POM already showed heavy wear after 321,000 cycles and failed after 621,000 cycles, undefined.

In contrast to metal, the plastic iglidur® i6 is particularly light, which makes it ideal for use in applications where low weight is important. An important advantage for the ETHZ researchers, because only exo-skeletons that are light and compact enough can also prove themselves in everyday life. With the finger joints made of iglidur® i6, the hand module weighs only 148 grams. The solid lubricants incorporated in the plastic make lubrication of the elements unnecessary and thus support the easy handling of the advanced therapy application.

Laser sintering as a manufacturing method is not only ideally suited to the reproduction of complex geometries and delicate structures, but also offers the possibility of cost-effectively producing small batches and one-offs. This is the case with the RELab tenoexo exoskeletons, as they can be customised for individual patients. "We have developed an algorithm to adapt the digital model of the exoskeleton to the size of the patient's hand with just a few clicks."

Whether in product development or in the production of functional parts: Speed brings companies market advantages and customers faster solutions to their problems. By uploading the 3D model of the required finger joints to our online 3D printing service, ETHZ scientists can order the required parts within a few minutes. The actual production usually takes place overnight and the finished finger joints can be installed and used for therapeutic purposes after just a few days. No other manufacturing process comes close to the speed and cost efficiency of 3D printing when it comes to the production of customised small series.

But are 3D printed parts suitable as functional parts in the end application or do they still have to be content with the modest role of quickly available prototypes? We are convinced of the performance of our materials: additively manufactured components made of iglidur® plastic are used as functional series parts in numerous other applications by our customers.