Soft robot mimics caterpillar movement

Researchers at Tufts University in the US are using a ZwickRoell biaxial testing machine to test biological and engineered biocompatible materials for the development of the world's first soft-bodied robot. Based on the neuromechanical system of the caterpillar Manduca sexta, the softbot prototype is approximately 30 cm long and made of silicone elastomer. The mechanical material characterization is performed at the Soft Materials Characterization Laboratory at Tufts' Advanced Technology Laboratory, led by Professor Luis Dorfmann.

The ZwickRoell biaxial testing machine isbeing used to characterize isotropic and anisotropic elastomers and thus helpsthe researchers to model the behaviour of the material. "Unlike vertebrates,caterpillars don't have bones associated with their muscles to provide a systemof levers," says Professor Barry Trimmer, principal investigator on theproject. "Though we know lots about vertebrate muscles we don't know awhole lot about caterpillars' muscles. Yet caterpillars can crawl up walls,grasp narrow branches or stems with sticky Velcro-like feet, and rotate theirbodies almost full-circle as they sense their environment." 

Understanding soft materials is one of thefirst steps to building a robot that, like a caterpillar, couldn't be made outof traditional hard materials. 

Potential applications for the innovativerobots include emergency search and retrieval, medical diagnosis and treatment,manufacturing and aerospace. 

Though the soft-body robot prototypes won'tbe made with a material that can mimic all the properties of the naturalcuticle, the group is aiming to make a soft-bodied robot using a biodegradablematerial, such as silk. "By developing a soft, flexible material that candegrade, a soft-bodied robot made of this material could enter the body as adiagnostic tool and not need to be retrieved," says David Kaplan,co-principal investigator on the project and professor in the Department ofBiomedical Engineering. 

The ZwickRoell machine is a custom built biaxialmaterial testing system equipped with four linear independently controlledactuators of 2kN capacity. It is used todetermine the constitutive functions for the in-plane response of isotropic andanisotropic materials. Each actuator is fitted with a loading fixturefor applications of tensile or compressive loads to the test sample and adedicated load cell. The actuator drive system’s travel resolution is 0.1µm,while the load cells resolution is 0.001N.


ZwickRoell’s videoXtens extensometer, which isrigidly fixed to the machine frame, allows for non-contact strain measurementsof materials that undergo medium to large deformations. The extensometer isequipped to capture both axial and transverse strains simultaneously in separateinput channels. Camera lenses are interchangeable to permit different field ofviews and minimum theoretical resolution range from 0.3 µm to 6 µm. The testingsoftware testXpert® II installed on a dedicated computer allowsuniversities full customization of test procedures, loading sequences as wellas standard DIN, ISO and ASTM test programs. To accommodate diverse testingprotocols, custom-designed test fixtures can be designed by one of the labengineers, and coupled with an appropriate force transducer. A water bath forcharacterization of materials in various solutions is also available. 

"TheZwickRoell machine has proved invaluable to us in our research," says ProfessorDorfmann. "Understanding and characterization ofanisotropic materials requires testing with such specialized equipment.”  

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University enjoys a global reputation for academic excellence and for the preparation of students as leaders in a wide range of professions. A growing number of teaching and research initiatives span all Tufts campuses, and collaboration among the faculty and students in the undergraduate, graduate and professional programs across the university's schools is widely encouraged.