The novel biomimetic composite can be both flexible and conductive, but at microscopic scales its electrical conductivity depends on direction. Image: Mert Vural, Penn State.
The novel biomimetic composite can be both flexible and conductive, but at microscopic scales its electrical conductivity depends on direction. Image: Mert Vural, Penn State.

Sometimes, breaking the rules is not a bad thing. Especially when the rules are apparent laws of nature that apply to bulk materials, but might not apply at the nanoscale.

"Nature knows how to go from the small, atomic scale to larger scales," said Melik Demirel, professor of engineering science and mechanics at Penn State. "Engineers have used mixing rules to enhance properties, but have been limited to a single scale. We've never gone down to the next level of hierarchical engineering. The key challenge is that there are apparent forces at different scales from molecules to bulk."

Composites are, by definition, composed of more than one component. Mixture rules say that while the ratios of one component to another can vary, there is a limit on the physical properties of the composite. According to Demirel, his team has broken that limit, at least at the nanoscale.

"If you have a conducting polymer composite, the amounts of polymer and metal compound are limited by the rule of mixtures," said Demirel. "The rules govern everything about the matrix and filler. We took materials – a biopolymer and an atomically thin conducting material – let them organize by self-assembly, and broke the rule of mixtures."

The team's materials comprise a biomimetic polymer based on tandem repeat proteins produced by gene duplication and inspired by the structure of squid ring teeth proteins, and titanium carbide MXene, a conducting 2D material. As this layered composite self-assembles, the polymer mediates the distance between the MXene layers. By genetically engineering tandem repeat proteins, which repeat a conserved sequence, the researchers were able to control the inter-layer distance of the conducting MXene layers without changing the composite fractions. The researchers' goal was to use synthetic biology to create self-assembling materials with unprecedented control over their physical properties.

Because the polymer self-assembles into a cross-linked network, the ratio between the polymer matrix and MXene filler can break the mixture rules in tiny areas, thereby changing the electrical properties of the layered material. The researchers report the results of their work in a paper in ACS Nano.

The biomimetic polymer-metal composite can be both flexible and conductive in the proper bulk mixtures. At the microscopic scale, when the structural symmetry is broken, the electrical conductivity depends on direction. "What is unique is that now you can get in-plane electrical conductivity that differs from out-of-plane conductivity," said Demirel.

As long as the current is going along the plane of the 2D material layers, the conductivity is linear, but if the current is directed across the layers, the conductivity becomes nonlinear. "Now we can make a storage device," said Demirel. "We could also make diodes, switches, regulators and other electronic devices. We want to make materials that are designed with desired properties for building novel functionalities, which are difficult to achieve or previously unattainable."

This story is adapted from material from Penn State, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.