Development of Artificial Joints Printed by 2-Photon-Laser-Printing and Actuated with pNIPAM

Soft actuators are flexible components that can adapt to dynamic environments based on responsive materials like polymers, fluids, or hydrogels, to name a few. Hydrogels made of e.g., thermoresponsive poly-<i>N</i>-isopropylacrylamid (pNIPAM) are excellent candidates for soft robotic applications due to their ability to change in volume depending on the surrounding temperature. Below the lower critical solution temperature (LCST), which is 32 °C, pNIPAM is in its swollen, hydrated state, while it shrinks to the dehydrated state above the LCST. With 2-photon-laser-printing (2PP), which is making use of the absorption of two near-infrared (NIR) photons, it is possible to initiate highly precise polymerization of the hydrogel ink, in our case pNIPAM. This enables the generation of complex 3D structures on the microscale. Here, we show the design and printing of artificial joints with pNIPAM components in the micrometer range by two approaches: First, the structures are printed in two-steps consisting of a solid polymer base, surrounded by pNIPAM, which is mimicking muscle tissues. Second, a structure is printed entirely out of hydrogel resulting in a fully soft joint. After printing and mechanically analyzing the joints by nanoindentation measurements, the thermoresponsive actuation was evaluated by switching the ambient temperature between temperatures below and above the LCST of pNIPAM. In the future, the design of the soft joints may be improved to give more complex structures with sophisticated functionalities on the microscale.