Belousov–Zhabotinsky Hydrogels: Relationship between Hydrogel Structure and Mechanical Response

Abstract

The novel chemo-mechanical feedback within autonomic Belousov-Zhabotinsky (BZ) hydrogels mimics the complex adaptivity found in natural systems, and inspires soft device concepts for chemical computing, sensing and actuation. A quantitative relationship between the dynamic response, strain, BZ reagents and hydrogel constituents however, is still evolving due to a limited material suite. Using a modular synthesis strategy for free-radical BZ-catalyst monomers, we compare the impact of cross-linker, catalyst, and total polymer concentration on the cyclic strain of PNIPAm and PAAm based BZ hydrogels. The oscillator strain of the hydrogel in the BZ solution relative to the difference between equilibrium swelling of the fully oxidized and reduced states highlights the trade-off between BZ reaction kinetics, hydrogel elasticity, and catalyst concentration. For a PNIPAm-based BZ gel, a maximum strain of 20% occurs at a total polymer and [Ru] concentration of 4.8 ± 0.5 μg/mm 3 and 1.5 mM due to a complementary balance of Ru content, extended BZ period (33 ± 8 min), and modest network crosslinking. The modular synthesis approach enables formulation studies to identify the BZ hydrogel architecture that provides maximum strain response and robustness, as well as elucidating the interrelationship between hydrogel structure, composition and reaction conditions.

Under Diffusion Control: from Structuring Matter to Directional Motion

“…When immersed in the BZ medium, the rhythmic contractions emerge due to the oscillating swelling‐deswelling process causing a periodical buckling instability of the vesicle structure. Generally, the implementation of robust, directional and synchronous actuation mechanisms relies on a) the optimization of both gel geometry and size to match the wave amplitude, b) the selection of a suitable point of reaction initiation, c) the maximization of the ratio between the volume change and the catalyst redox oscillation and d) on a match of the swelling/deswelling kinetics to the wave propagation rate . Recently, new studies have also discussed the importance of heterogeneous BZ catalyst embedding, thus resulting in a nonuniform distribution of active areas with high concentration of the catalyst (nodes) and nonactive areas.…”

Organometallic Hydrogels

“…Yoshida and co‐workers attached Ru II (trisby) 3 on crosslinked poly‐NIPAm to fabricate the first successful self‐oscillatory hydrogel in BZ solution. Since then, scientists discovered that chemomechanical behaviors of hydrogel highly associate with a network architecture (e.g., polymer composition, polymer concentration, catalyst concentration, and cross‐link density) . In this aspect, hydrogel architecture research was performed on the association of mechanical strain and the length of the linkers of the BZ catalyst on the polymer backbone (Figure A) .…”

“…Since then, scientists discovered that chemomechanical behaviors of hydrogel highly associate with a network architecture (e.g., polymer composition, polymer concentration, catalyst concentration, and cross‐link density) . In this aspect, hydrogel architecture research was performed on the association of mechanical strain and the length of the linkers of the BZ catalyst on the polymer backbone (Figure A) . Through structure modification, the Ru II ‐catalyst‐based hyper‐cross‐linker was also made to create novel type of active gels by the Xu group .…”

Belousov–Zhabotinsky reaction in liquid marbles

“…To provide insights into these processes and the use of BZ media for the applications above, previous studies utilised the BZ reaction in spatially confining media such as thin hydrogel films [41], small droplets in microfluidic devices [32,[42][43][44][45][46][47][48][49][50][51][52], vesicles [53][54][55][56][57], emulsions [58], BZ-AOT emulsions [9-11, 17, 59-64], cation exchange resins [65][66][67][68][69][70][71] and 3D printed structures [42,72,73].…”