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AZoRobotics speaks with Andreas Heiden from the Johannes Kepler University Linz about his research into a soft actuator that has been constructed using biodegradable materials. It holds promise for new applications in soft robotics while opening the door to zero-waste manufacturing.
New and rapidly changing technologies contribute to increasing amounts of tech waste, accumulating to as much as over 100,000 tons per day in 2019. That is the combined mass of almost 14 Eiffel Towers. Polymer products, in particular, end up in our ecosystem, polluting even the most remote areas on our planet.
Soft robotics, however, still needs to improve in terms of sustainability, regarding the limited lifetime of soft materials or applications where, for example, deployed robots cannot be retrieved. Thus sustainable soft actuators and sensors from renewable, biodegradable, and recyclable resources are required to avoid additional waste streams.
In our lab, we soon realized that despite soft robotics being largely inspired by nature and its soft critters, an inherent “feature” of nature’s creations was missing: biodegradability. Once a soft robot has reached its end of life, there is often no simple solution to recycle its components or treat waste in an environmentally friendly manner. By extending the fabrication of our biodegradable gels to 3D printing, we can produce versatile soft actuators in various complex shapes and even include integrated sensor networks that enable them to interact with their environment. Once they are no longer of use, they can be simply disposed of, or even eaten, as all the materials used are also edible.
Nowadays, Fused Deposition Modeling (FDM) is one of the most common 3D printing methods. Our process uses a Biogel reservoir in a medical syringe, which is heated above the gel's melting temperature. Similar to a syringe pump, our device then squeezes the liquid biogel through a heated nozzle which leads to the deposition of a biogel “thread” that solidifies quickly after the extrusion.
In this manner, several two-dimensional layers are drawn subsequently and stacked on top of each other to form the three-dimensional object.
The biogel was already developed by our group, which we published in Nature Materials. The aim was to engineer bioderived materials such as gels based on the biopolymer gelatin that can match the performance of conventional synthetic elastomers yet degrade fully after their intended use – leaving essentially no trace of having existed.
About a year ago, we optimized such gels to be of use in soft electronic patches and for soft robots. It is highly stretchable and elastic, and its thermoreversability makes it reusable and perfectly suitable for 3D printing. Based on naturally occurring materials as degradable building blocks that are durable, self-adhering, stretchable, mechanically tunable and healable, it is a broadly applicable gelatin-based biogel that unites the challenging needs of resilient yet sustainable (soft) robots and electronics in a single platform.
The main structure of our actuator, including the fiber reinforcement and waveguides, is entirely biodegradable. Additionally, the biogels’ water solubility enables a simple method to separate the robot from its non-biodegradable control parts, allowing their reuse.
Alongside this eco-friendly fabrication approach using biodegradable materials, we introduced an additional reuse cycle where the biogel is reprinted up to 5 times, maintaining more than 70% of the initial performance metrics.
After the actuator has fulfilled its designated purpose and is no longer of use, it can be disposed of. Immersing it in water triggers the swelling and dissolution of the biogel and, in the presence of enzymes, complete decomposition. The undissolved parts (e.g., air inlet tubes or cotton threads) can thus be simply taken out and reused or disposed of separately. Alternatively, the coated biogel can readily be recycled after the removal of the additional components to create a new generation of sustainable devices.
Despite being entirely degradable when disposed of in wastewater, our gels maintain their mechanical properties for more than one year under ambient conditions and enable soft actuators that operate for more than 330,000 cycles without failure. We tested this by performing tensile tests in many different ways and under continuous cyclic loading. In this manner, we also determined parameters like the elastic properties, the resilience and the drying behavior.
The material is stretchable to more than 500% of its original length, which enables pneumatically driven actuators with response times of less than one second and high bending angles up to 74 degrees. Despite the material’s water solubility, equipped with a biodegradable shellac coating, such actuators can also operate underwater for about 1.5 hours and more than 1500 actuation cycles before failure.
Biogels that are processed by conventional molding techniques only yield objects of limited geometry and feature size. This constrains their applicability, as their locomotion is often reduced to a single predefined pattern, and limited rudimentary sensing capabilities are implemented.
The controlled printing process allows finer features and fabrication of multi-degree of freedom (DOF) actuators with soft sensors while, at the same time, increasing the material’s stretchability by about 140%, which is essential for reliable soft robotic devices. Mold-free fabrication and material reusability also speed up prototyping and reduce waste generation.
The biggest challenge was finding the correct printing parameters that account for a stable three-dimensional print. Besides the gels temperature and the temperature and airflow of the cooling system, there are parameters like printing speed, extrusion rate, cooling time per layer, etc. All those parameters need to be controlled precisely, and some of them affect each other, which makes it tricky to find the correct settings.
We applied our approach to create a new set of durable biodegradable soft actuators and autonomous electronic platforms with multimodal sensing capabilities. Future Multi-material printing strategies will enable combinations of mechanically, electrically and optically tuned biogels to achieve actuation mechanisms beyond pneumatic control and enhance sensing and control capabilities for highly integrated robots.
Possible applications range from edible robotics, produce harvesting or animal behavior studies to single-use scenarios in hazardous environments or medical settings, in which tools and healthcare devices are commonly disposed of after their employment to meet stringent hygiene requirements.
Alongside the added technical improvements, end-of-life considerations are critical for future sustainable development. The presented accessible, sustainable and cost-effective fabrication strategy will likely have a positive impact on soft and collaborative robotics through a reduced ecological footprint.
Scalable production, low material costs and safety of all the fabrication steps will enable widespread employment, from industry and healthcare to education.
Increasing the level of complexity will also require more advanced actuator shapes and multi-material combinations. So far, the biogels viscosity and cooling time hinder the printing of overhanging features or cavities. Developing suitable biodegradable support materials will solve these issues in combination with multi-material printing. Moreover, implementing multi-material printing will enable combinations of mechanically, electrically, and optically tuned biogels to achieve actuation mechanisms beyond pneumatic control and enhance sensing and control capabilities for highly integrated robots.
Andreas Heiden is currently a Ph.D. student at the Institute of Soft Matter Physics of Johannes Kepler University (JKU), Linz – Austria. He received his master degree at JKU in 2020 and his bachelors degree akt JKU in 2017. His research interests include developing sustainable soft robotic devices, materials research and conceptuation and design of alternative fabrication strategies. His research was published in the Science Robotics Journal.
Disclaimer: The views expressed here are those of the interviewee and do not necessarily represent the views of AZoM.com Limited (T/A) AZoNetwork, the owner and operator of this website. This disclaimer forms part of the Terms and Conditions of use of this website.
Bethan has just graduated from the University of Liverpool with a First Class Honors in English Literature and Chinese Studies. Throughout her studies, Bethan worked as a Chinese Translator and Proofreader. Having spent five years living in China, Bethan has a profound interest in photography, travel and learning about different cultures. She also enjoys taking her dog on adventures around the Peak District. Bethan aims to travel more of the world, taking her camera with her.
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