Responsive grippers for soft robotics

Researchers at the Adolphe Merkle Institute’s Polymer Chemistry and Materials group have developed bio-inspired thermally activated actuators that could serve as grippers for so-called soft robotics, picking up and releasing small objects.

 Mechanically morphing materials, which can change their shape or alter their mechanical properties in response to a signal, are quite common in nature and enable essential functions, especially those requiring motion. Some of the best-known examples of this are pinecones and the Venus fly trap. The design of these materials is often based upon hierarchical structures that efficiently translate chemical or physico-chemical events from a molecular to a macroscopic level, allowing for substantial mechanical effects while relying on only minor or no compositional changes.   

 Over the past two decades, scientists have investigated the underlying natural operational principles of such materials, and integrated their findings in artificial materials destined for applications as varied as architecture and soft robotics. Polymeric bilayer bending actuators represent one straightforward and widely investigated approach for creating devices that translate external stimuli into motion. Actuators are typically responsible for moving and controlling a mechanism or system in machines. Inducing movement in these actuators relies on the dissimilar thermal expansion of two different materials, which causes the bilayers to bend upon being heated as the two layers are expanding at different rates. However, since thermal expansion coefficients of polymers are generally small, the efficiency of this process is very limited.

 Graduate student Livius Muff and Professor Christoph Weder of the Polymer Chemistry and Materials group sought to overcome this problem by creating bilayer actuators with a thermoplastic polyurethane elastomer containing a crystallizable polyester segment. The domains formed by the polyester segment melt and crystallize in a reversible manner at temperatures of between 30 and 60 degrees Celsius. This reversible physical transition leads to a very large thermal expansion, and therefore actuation, in a well-defined temperature range. Importantly, the polymer as a whole does not become liquid, because the urethane hard segments guarantee both mechanical integrity and elastic behavior at temperatures where the polyester domains have melted.

 To achieve electrically-driven actuation, the researchers embedded electrodes in their devices that serve as resistive heating elements. After applying an electrical signal of several volts, the actuators would bend in a few seconds. Returning to the initial shape required more time, because the polyester segments in the specific polymer that was used displayed slow crystallization. The AMI researchers say that faster response times should be possible by changing the type of  polyester and improving the electrode design.

 These actuators could be applied as part of a novel design for soft robotics with the addition of a thermally controlled supramolecular polymer adhesive "gripper." The researchers demonstrated that a small object could be picked up, moved, and released.

 The project is part of the international consortium “Program for International Research and Education (PIRE): Bio-inspired Materials and Systems” that is jointly funded by the US and Swiss National Science Foundations and unites researchers at four American universities and AMI to work on bio-inspired materials and systems and their use in soft robotics.

 Reference: Muff, L.F.; Weder, Exploiting Phase Transitions in Polymer Bilayer Actuators, Advanced Intelligent Systems, 2000177 (2020)