Hydrogels are soft objects that are mainly composed of water. The water is held together by a 3D cross-linked mesh. In our latest work we show that hydrogel beads made from the bio-sustainable polymer alginate can be loaded up with different types of molecules so that the beads can communicate via chemistry. 

Chemical communication underpins a plethora of biological functions and behaviours. Plants, animals and insects rely on it for cooperative action, your body uses it to moderate its internal environment and your cells require it to survive.

A key goal of materials science is to mimic this biological behaviour, and synthetic objects that are able to communicate with one another by the sending and receiving of chemical messengers are of great interest at a range of length scales. The most widely explored platform for this kind of communication is between nanoparticles, and to a lesser extent, vesicles, but to date, very little work explores communication between large (millimetre-sized), soft objects, such as hydrogels.

In our work published in the Journal of Materials Chemistry B, we present combinations of large, soft hydrogel objects containing different signalling and receiving molecules, can exchange chemical signals. Beads encapsulating one of three species, namely the enzyme urease, the enzyme inhibitor silver (Ag+), or the Ag+ chelator dithiothreitol (DTT), are shown to interact when placed in contact with one another. By exploiting the interplay between the enzyme, its reversible inhibitor, and this inhibitor’s chelator, we demonstrate a series of ‘conversations’ between the beads. 

The movie shows two different scenario's:

Scenario 1: When a bead containing both urease, at a concentration of 5 g/L, and silver, at a concentration of 0.2 mmol/L, is immersed in a 0.1 mol/L solution of urea, no pH increase is observed (left bead). If an identical bead (middle bead) makes contact with one containing 0.52 mol/L DTT (right bead), the contained DTT diffuses into the silver-bound ‘enzyme’ bead. This results in the chelation of silver ions from the urease and thus its reactivation. Scale bar = 5 mm. 

Scenario 2: a 1 mmol/L ‘silver’ bead sits next to a 0.125 g/L ‘urease’ bead (far left and the top middle bead, respectively) in an aqueous solution of 0.1 mol/L urea. In this case, however, a third bead is introduced, namely a 0.52 mol/L ‘DTT’ bead (bottom middle). The far-right bead, containing 0.125 g/L urease, undergoes its colour and pH change as expected (onset at 214 seconds). As the left-hand ‘enzyme’ bead makes contact with a ‘silver’ bead, a pH increase after this time period is not observed, as the silver binds to the enzyme active site. After a longer delay (420 seconds), a pH increase is observed. Scale bar = 5 mm. 

Details of the study can be found in our paper "communication between hydrogel beads via chemical signalling" which was published in the Journal of Materials Chemistry B (DOI:10.1039/C7TB02278F).

The study forms part of a wider program to develop technology to design hydrogels that can be programmed and communicate. As part of this series we reported earlier in 2017 in Materials Horizons a study on the design of hydrogel fibres and beads with autonomous independent responsive behaviour and have the ability to communicate (DOI: 10.1039/C7MH00033B) and in Journal of Materials Chemistry B technology to program hydrogels.