Traditional robots, such as those used in manufacturing, can lift heavy loads and precisely repeat automated processes. However, they are too rigid and bulky for delicate work and human interaction.
Soft robotics research focuses on developing robots made of soft, organic materials and flexible technical components. Kiel University materials scientists have created a new soft conductive material that could greatly help.
Unlike conventional soft conductors, it exhibits remarkable electrical stability even when deformed. This is due to the material’s unique structure and a nanoscopic insulating thin film coating.
Elastic Electrical Conductors
A) Schematic depiction of the synthesis process for the multi-layered foam-like framework structure. B) Photographs showing the infiltration of a ZnO template with an aqueous PEDOT:PSS solution. C) Photograph of Aero-PEDOT:PSS hanging from a paper tissue, highlighting the ultralow density of the synthesized material. D) Photograph of resulting Aero-PEDOT:PSS samples with different geometries after etch-removal of ZnO. E) Specific conductivity as a function of the Aero-PEDOT:PSS density. The corresponding conductivity of samples prior to etching is indicated as PEDOT:PSS-ZnO. Inset: Normalized Sulfur 2p spectra of PSS and PEDOT components for Aero-PEDOT:PSS and PEDOT:PSS-ZnO measured by XPS. The increase in relative height of the PEDOT peak at 164 eV indicates removal of PSS through etching. F) Schematic illustration of the iCVD coating process of an Aero-PEDOT:PSS microtube (not to scale).
Credit: Igor Barg et. al, Adv. Funct. Mater. 2023, 2212688 CC BY
Humans and animals can perform fluid and fine movements and adapt them to their surroundings. Soft robotics, which is inspired by nature, uses elastic, organic carbon materials rather than conventional, rigid metals. Furthermore, soft robots require elastic electrical conductors to communicate with sensors and actuators.
“Conventional conductors made of metal conduct electricity well, of course, but they are too rigid for flexible components. Upon deformation they change their electrical resistance and this affects their use in soft robotics,”
said Dr. Fabian Schütt, the head of the junior research group Multiscale Materials Engineering at the Chair of Functional Nanomaterials at Kiel University.
In contrast, the resistance of the material developed by Schütt and colleagues at Kiel University’s Institute of Materials Science remains constant when deformed.
“Both the initial electrical and mechanical properties are retained during long-term cycling, even after 2000 cycles at 50% compression,”
said first author Igor Barg, a doctoral researcher.
Piezoresistive Effect
The researchers created a material made of fine wires that looks like a dark sponge by combining different expertise within Kiel University’s Priority Research Area KiNSIS (Kiel Nano, Surface, and Interface Science).
Interconnected microtubes made of an electrically conductive polymer make up the wires. This delicate network structure makes the material both ultralight and extremely elastic.
“Stretchable, sponge-like conductors have already been researched for several years. But as soon as they are deformed, their resistance also changes due to the so-called piezoresistive effect,”
Barg explained.
Nanoscopic Insulation Film
To counteract this resistance effect, the researchers coated their material with a non-conductive, nanoscopic thin layer of Polytetrafluorethylene (PTFE).
“You can think of it as a classic power cable,”
said Barg. The layer prevents the wires from coming into direct contact with each other and forming new electrically conductive paths during compression.
As a result, even with large deformations, the resistance remains constant. The insulation also improves the wires’ mechanical stability and protects their electrical properties from outside influences such as moisture.
Initiated Chemical Vapor Deposition
A unique technique is required to coat this highly porous framework structure. Dr. Stefan Schröder is the head of the Chair for Multicomponent Materials’ junior research group Functional CVD Polymers and works with the initiated chemical vapor deposition (iCVD) process.
This allows for the conformal coating of materials with complex structures and surfaces: When different gases are mixed in a reactor, a chemical reaction occurs, and a thin polymer film forms on the material to be coated.
“Since this coating is only a few nanometers thin, the wires remain elastic and the total weight of the material hardly increases,”
Schröder said.
This example demonstrates very well how nanoscale coating can be used to specifically change the properties of framework structures ranging in size from a few cubic centimeters to several cubic centimeters, and even create entirely new functions.
“By combining our methods, other applications, including commercial ones, might be possible in the future, for example, in medical technology or energy storage,”
added Schröder. Now, they want to do more joint research projects to look into these possibilities.
Reference: Barg, I., Kohlmann, N., Rasch, F., Strunskus, T., Adelung, R., Kienle, L., Faupel, F., Schröder, S., Schütt, F. Strain-Invariant, Highly Water Stable All-Organic Soft Conductors Based on Ultralight Multi-Layered Foam-Like Framework Structures. Adv. Funct. Mater. 2023, 2212688.
