Light-Triggered Temperature-Responsive Hydrogel Actuator Reinforced with Bacterial Cellulose for Soft Robotics

As the application fields of robots become increasingly complex and diverse, soft robotics has attracted a great deal of interest. Although conventional robots made of rigid materials have significantly improved productivity in structured environments (e.g. in industry), they are difficult to operate in unstructured environments because they consist of multiple links and joints. In addition, they are often heavy, tethered, and different in elastic modulus compared to the tissue of living organisms. In order to precisely and locally control motion while having mechanical properties similar to those of living things, many attempts are being made to use soft materials for robotics. Functional soft materials such as stimuli-responsive hydrogels, shape memory polymers, and liquid crystal elastomers have been viewed as potential candidate materials for soft robotics. Hydrogels bring several advantages due to their high water content and large elasticity. They can respond to various external stimuli via interactions between the polymer network and water, which can endow soft robots with controlled actuation. Moreover, hydrogels have biocompatibility if their polymer networks consist of non-toxic polymers, which allow soft robots to broaden into more biological applications. Despite these advantages, hydrogels have challenges in terms of mechanical strength, response time, and responsive area control. To overcome this intrinsic problem, a well-designed hydrogel actuator system with sufficiently strong mechanical properties and selective responsive sites is envisioned. In this study, we introduced a photothermal-driven hydrogel actuator system with enhanced mechanical properties while accomplishing a precise control of response areas. For this, we used poly(N-isopropylacrylamide) (PNIPAM) as a temperature-responsive polymer network and gold nanoparticle (AuNP) as a transducer from light to heat, bacterial cellulose nanofibrils (BCNF) as a supporting material for AuNPs and an interpenetrating polymer for hydrogel. The essence of our approach is to provide BCNFs with the ability to support gold nanoparticles while boosting the mechanical properties of hydrogel. Specifically, BCNFs are physically immobilized in a covalently bonded hydrogel mesh of PNIPAM, thus producing a hydrogel composite with a semi-interpenetrating network structure. This PNIPAM/BCNF hydrogel, which acts as an active layer, is combined with a non-thermoresponsive poly(acrylamide) passive layer to generate a thermal-expansion coefficient mismatch among the interface, eventually leading a bending motion. As an energy transducer, the AuNPs embedded in the BCNF absorb the light energy and transform it into thermal energy effectively as a result of a photothermal effect. The resulting hydrogel actuators exhibit enhanced mechanical properties. The actuation of bi-layered hydrogel could be easily programmed by local irradiation, allowing finger-like one-by-one bending motions. These results highlight that our hydrogel actuator system can have significant potential for the development of smart soft robots.