Scientists at the Max Planck Institute for Intelligent Systems in Stuttgart (MPI-IS), at Johannes Kepler University (JKU) in Linz, Austria, and at the University of Colorado Boulder in the USA have developed fully biodegradable, high-performance artificial muscles.
Artificial muscles are a progressing technology that could one day enable robots to function like living organisms. Such muscles open up new possibilities for how robots can shape the world around us; from assistive wearable devices that can redefine our physical abilities at old age to rescue robots that can navigate rubble in search of the missing. But just because artificial muscles can have a strong societal impact during use, doesn’t mean they have to leave a strong environmental impact after use.
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The new artificial muscle is based on gelatine, oil and bioplastics. They demonstrated the potential of the biodegradable technology by using it to animate a robotic gripper, which could be especially useful in single-use deployments, such as for waste collection. At the end of life, these artificial muscles can be disposed of in municipal compost bins; under monitored conditions, they fully biodegrade within six months.
‘We see an urgent need for sustainable materials in the accelerating field of soft robotics,’ said Ellen Rumley, a visiting scientist from UC Boulder working in the Robotic Materials Department at MPI-IS. ‘Biodegradable parts could offer a sustainable solution, especially for single-use applications, such as for medical operations, search-and-rescue missions and manipulation of hazardous substances. Instead of accumulating in landfills at the end of product life, the robots of the future could become compost for future plant growth.’
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The researchers dubbed their electrically driven artificial muscle HASEL. In essence, HASELs are oil-filled plastic pouches that are partially covered by a pair of electrodes. Applying a high voltage across the electrode pair causes opposing charges to build on them, generating a force between them that pushes oil to an electrode-free region of the pouch. This oil migration causes the pouch to contract, much like a real muscle. The key requirement for HASELs to deform is that the materials making up the plastic pouch and oil are electrical insulators, which can sustain the high electrical stresses generated by the charged electrodes.
One of the challenges faced by the researchers was the development of a conductive, soft and fully biodegradable electrode. Researchers at JKU created a recipe based on a mixture of biopolymer gelatine and salts that can be directly cast onto HASEL actuators. ‘It was important for us to make electrodes suitable for these high-performance applications, but with readily available components and an accessible fabrication strategy,’ said David Preninger, a scientist in the Soft Matter Physics Division at JKU. ‘Since our presented formulation can be easily integrated in various types of electrically driven systems, it serves as a building block for future biodegradable applications.’
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The next step was finding suitable biodegradable plastics. Engineers for this type of materials are mainly concerned with properties such as degradation rate or mechanical strength, not with electrical insulation; a requirement for HASELs that operate at a few thousand Volts. Nonetheless, some bioplastics showed good material compatibility with gelatine electrodes and sufficient electrical insulation. HASELs made from one specific material combination were even able to withstand 100,000 actuation cycles at several thousand Volts without signs of electrical failure or loss in performance. These biodegradable artificial muscles are electromechanically competitive with their non-biodegradable counterparts; an exciting result for promoting sustainability in artificial muscle technology.
‘By showing the outstanding performance of this new materials system we are giving an incentive for the robotics community to consider biodegradable materials as a viable material option for building robots,’ Rumley said. ‘The fact that we achieved such great results with bioplastics hopefully also motivates other materials scientists to create new materials with optimised electrical performance in mind.’
The research has been published in Science Advances.