Bristleworm chemistries inspires new materials

Materials World magazine
,
27 Sep 2019

Inspired by the jaw of the bristle worm, researchers have created new materials for shape-memory and soft robotics. Idha Valeur finds out more.

A bristle worm is a segmented sea creature, generally smaller than 10cm in length, which can extend its jaw outside of its mouth to trap prey. Researchers from the University of Delaware, USA, are examining how that is achieved to make new materials for use in soft robotics.

Although stiff at the base and pliable at the top, the jaw is made of one material that contains zinc and the amino acid, histidine. The combination of the two, known as metal coordination chemistry, is what enables this behaviour.

University of Delaware Engineer, Professor LaShanda Korley, told Materials World that if they can control the supramolecular interactions via covalent crosslinking, they will be able to finely tune the responsive behaviour of materials for specific applications.

Material mechanics

To replicate the jaw mechanics, the team used a metal-coordinating supramolecular polymer in an overlaid covalently crosslinked network to create a semi-interpenetrating network material. ‘The stabilisation of the material with covalent crosslinks allowed us to chelate the coordinated supramolecules with a competitive ligand solvent front, driving the metal centres to travel with the chelating solvent,’ she said.

The resulting network material is built up of zinc and polymers. Using the metal-coordination crosslinks of the bio sample, the researchers used this natural framework in the design of their own system.

‘The use of self-assembling polymers as our platform allows us to fabricate materials with enhanced toughness, due to the sacrificial nature of non-covalent interactions, while also maintaining a stimuli-responsive handle – in this case the non-covalent metal coordination sites – for manipulation of local structure and mechanics,’ Korley said.

‘Zinc was chosen as a model divalent metal centre, allowing us to coordinate two polymer end groups. This chain-end coordination resulted in long, self-assembled polymer chains while maintaining desirable stimuli-responsive behaviour,’ she added.

The team chose to synthesise short, end-functionalised polymer chains that could be coordinated and assembled with the addition of a metal. ‘A highly crosslinked network made up of polyethylene glycol dimethacrylate was fabricated via reaction of the methacrylate end-groups with a tetrafunctional thiol crosslinker moiety initiated by ultraviolet light,’ Korley said. ‘The interactions that occurred between the supramolecular polymer and the crosslinked network could be altered by changing the reactant ratios during film fabrication.

We used this as a platform to alter both the nano and microscale supramolecular assembly and phase separation, respectively, as well as the bulk mechanical properties of the samples. The localised supramolecular interaction gradients were driven using a competitive ligand to chelate the metal centers via capillary uptake.’

Supramolecular polymeric materials has become increasingly popular for developing next-gen materials capable of self-healing and chemical sensing. ‘Much of the research into these systems has delved into altering their chemical structure to dictate shifts in their association strength or the final architecture of the self-assembled system,’ Korley said.

‘However, while it is understood that shifts in local environment heavily impact how these molecules assemble in solution, it is less clear how covalent crosslinking and microphase separation drive changes in non-covalent assembly. We aim to better understand these behaviours in our interpenetrating network-type materials.’

Towards the goal

The research team is currently exploring changes in supramolecular interactions, such as hydrogen bonding and metal-ligand coordination. ‘Beyond that, we are exploring how other stimuli can be incorporated into the network to offer finer spatial control over local dissociation behaviour in the supramolecular networks,’ Korley said.

In addition, the team aims to understand how the supramolecular interactions are impacted by their environment when confined to elastomer-like materials, where covalent crosslinking is often present. ‘We can use classical polymeric behaviour, such as microphase separation, to tune the assembly characteristics in our networks, shifting both the mechanics and stimuli-responsive behaviour of the supramolecular moieties,’ Korley said. ‘We envision that a better understanding of how supramolecular interactions are controlled by the presence of confining covalent crosslinks will expand their use both in research and industry.’

The paper, Gradient supramolecular interactions and tunable mechanics in polychaete jaw inspired semi-interpenetrating networks published in European Polymer Journal can be read here: bit.ly/2lXQhyI