Robotic Hand with Plastic Bones, Ligaments, and Tendons Printed in One Run

In what they claim is a world first, researchers at Switzerland’s ETH Zürich university have 3D printed a robotic hand with plastic bones, ligaments, and tendons in a single process. Slow-curing thiolene polymers are the secret sauce in this application. 3D-printing technology that uses a laser to scan each polymer layer and compensate for surface imperfections in subsequent layers rather than scraping them away is also instrumental in achieving this milestone.

Advantages of slow-curing polymers

Slow-curing polymers have decisive advantages over fast-curing plastics — elastic properties are enhanced and they are more durable and robust, according to ETH Zürich researchers. A technology developed collaboratively by researchers at the Swiss school and Medford, MA–based Inkbit, an MIT spin off, enables the use of slow-curing thi­olene poly­mers as well as combinations of soft, elastic, and rigid materials.

Thiolene polymers are ideal for printing the elastic ligaments of the robotic hand, according to Thomas Buch­ner, a doc­toral stu­dent in the group of ETH Zürich ro­bot­ics pro­fessor Robert Katz­schmann and first au­thor of the study. “They have very good elastic prop­er­ties and re­turn to their ori­ginal state much faster after bend­ing than poly­ac­rylates,” which they had been using previously in 3D printing applications, said Buch­ner. In ad­di­tion, the stiff­ness of thi­olene can be fine-tuned to meet the re­quire­ments of soft ro­bots, which are less likely to injure human co-workers than their rigid counterparts and are bet­ter suited for hand­ling fra­gile goods, Katzschmann ex­plains.

Modified 3D-printing technology

To accommodate processing of the slow-curing polymers, the researchers had to modify the 3D-printing process. Typ­ic­ally, nozzles deposit the material in layers, and each layer is cured immediately with a UV lamp. Surface irregularities are then scraped away by a device after each curing step. This only works with fast-curing poly­ac­rylates, according to the researchers, because slow-curing poly­mers would simply gum up the scraper. Instead, they use a 3D laser scan­ner that im­me­di­ately checks each layer for sur­face defects. “A feed­back mech­an­ism com­pensates for these ir­reg­u­lar­it­ies when print­ing the next layer by cal­cu­lat­ing any ne­ces­sary ad­just­ments to the amount of ma­ter­ial to be prin­ted in real time and with pin­point ac­cur­acy,” ex­plains Wo­j­ciech Matusik, a professor at MIT and co-author of the study.

Ink­bit de­vel­op­ed the new print­ing tech­no­logy while ETH Zürich re­search­ers de­veloped ro­botic ap­plic­a­tions and helped op­tim­ize the tech­no­logy for use with slow-curing poly­mers. The US and Switzerland-based re­search­ers jointly detail the tech­no­logy and sample ap­plic­a­tions in a paper published in the journal Nature.

Using biomimetics to print a fluidic pump

In addition to the 3D-printed robotic hand, the paper published in Nature describes a robotic heart with a fluidic pump and integrated valves inspired by a mammalian heart. Actuation membranes, one-way valves, and internal sensor cavities are embedded in the heart’s chamber. “The integrated valves and pumping membranes were inspired by the geometries and mechanisms in mammalian hearts, which have already been optimized by nature. Our easy-to-remove support material . . . allowed us to print several small and large cavities with thin, soft membranes and rigid walls in one process. Similar pump designs were previously only possible through the casting or injection molding of individual components, both of which were followed by time-consuming and labor-intensive assembly,” the researchers write.

At ETH Zürich, Katz­schmann’s group will use this tech­no­logy to ex­plore additional applications. In the United States, Ink­bit plans to of­fer a 3D-print­ing ser­vice that applies this technology for its customers and to commercialize the print­ers.