Austrian scientists create soft actuator from biodegradable materials

Austrian researchers developed an extrusion 3D printing process that uses biodegradable ink to model complex objects with stable dimensions. The bio-ink consists of a gelatin-based hydrogel that stretches six times its original length.

This promises new applications for soft robotics and opens the door to zero-waste manufacturing. Soft robotics engineers draw inspiration from nature when developing robotic devices that interact with people and other living organisms. However, the production of these devices often leads to non-recyclable waste.

Moreover, traditional manufacturing methods such as molding do not meet the demands of complex soft robotics applications. Thus, advances in soft robotics require new production methods as well as greater environmental sustainability.

Advantages and challenges of soft robotics

Soft robots differ from rigid robots used in industrial manufacturing settings. Drawing on imitation of nature, soft robots made from soft adaptive materials help people solve complex tasks in industry, healthcare and society.

However, soft robotics devices that use sensors and actuators have a significant impact on the environment. In this context, soft robotics actuators made from biodegradable materials are desirable.

Attempts have been made to produce biodegradable soft robotic components using hydrogels, polyesters and proteins. However, traditional molding methods limit the degree of freedom achievable with modern soft robots.

Therefore, complex robotic devices with integrated flexible sensors require new manufacturing approaches, such as 3D extrusion printing.

Printing biodegradable robots

A team at Johannes Kepler University Linz in Austria developed a 3D printing method for producing soft robot actuators with embedded sensor networks. Their method is based on fused deposition modeling (FDM), in which a continuous filament of bio-gel material is fed through a heated extruder head to form the desired object. The minimum achievable feature size is 0.6 mm.

Soft and elastic materials are essential for developing soft actuators for robotics. Biopolymers such as cellulose, alginate and gelatin have been used to produce biodegradable materials for engineering applications. For its study, the Austrian group developed a gelatin-based hydrogel (bio-gel) with high elasticity (ultimate strain of 500%).

Gelatin is a hydrolyzed form of collagen, commonly used in food products. It is a tasteless and colorless polymer obtained from animal body parts. Thanks to its versatility, it is used in a wide range of engineering tasks, including tissue engineering and fiber optics.

The material properties achieved by the Austrian team resulted from adding glycerol and sugar syrup to the gelatin. In addition, citric acid controls bacterial growth. This ideal composition enables enzymatic degradation of the material in wastewater in just a few days, as well as a shelf life of more than a year under ambient conditions.

The bio-gel can be used to make soft pneumatic actuators with bending angles of up to 74 degrees in any direction. In addition, the bio-gel can be reused for printing up to five times.

The team studied the effects of heating, storage and reuse of bio-gels. They noticed the material's ultimate strain dropped to 470% after 24 hours, which exceeds that of molded bio-gels by 140%.

Their bio-gel retains an ultimate strain above 430% after 12 days of storage under ambient conditions, well above the 200% required for soft robotics applications.

The Austrian researchers also printed waveguides using the bio-gel, which detect deformation through loss of transmitted light. They could adjust the refractive index of their waveguides by controlling the water content of the bio-gel.

Finally, the scientists printed a three-chamber actuator (60 mm high and 16.6 mm in diameter). Three electropneumatic pressure regulators, a single pressure source and a parallel arrangement of inflatable chambers enabled omnidirectional movement.

By recreating a two-dimensional map of actuation states and corresponding pressure values, they could control the robot with a standard gaming controller. The team demonstrated the robot's capabilities by programming a search-and-clear routine so it could detect and remove obstacles.

By combining stretchable waveguides with soft actuators, the Austrian scientists were able to control curvature, direction and force measurement with high precision, while also reducing production costs and waste.

The researchers believe their technology will one day lead to tiny soft robots that can swim inside the body, delivering drugs or repairing damaged tissue. Also, 'gelatin robots' could be used to penetrate sensitive environments such as areas with high radiation levels. The technology could even be applied to making children's toys.