Forging out-of-equilibrium supramolecular gels

Abstract

The design of supramolecular hydrogels comprising aligned domains is important for the fabrication of biomimetic materials and applications in optoelectronics. One way to access such materials is by the self-assembly of small molecules into long fibres, which can be aligned using an external stimulus. Out-of-equilibrium supramolecular gels can also be designed, where pre-programmed changes of state can be induced by the addition of chemical fuels. Here we exploit these dynamic properties to form materials with aligned domains through a 'forging' approach: an external force is used to rearrange the underlying network from random to aligned fibres as the system undergoes a pre-programmed gel-to-sol-to-gel transition. We show that we can predictably organize the supramolecular fibres, leading to controllable formation of materials with aligned domains through a high degree of temporal control.

Keywords: Biomaterials; Self-assembly; Soft materials; Supramolecular polymers.

Fig. 1. Forging supramolecular gels using external stimuli to induce alignment.

A cartoon of the work described in this Article: alignment is induced in a system undergoing a chemically triggered gel-to-sol-to-gel cycle by applying external invasive (shear) and non-invasive (magnetic field) stimuli to impart changes in the organization of the self-assembled fibres.

Fig. 2. A pre-programmed gel-to-sol-to-gel system.

a, The chemical structure of l,d-2NapFF. b, A cartoon of the gel-to-sol-to-gel process. c, Photographs of the system at 10 s, 7 min and 16 h (scale bar, 1 cm). d, The change in rheology and pH with time for a system containing l,d-2NapFF in the presence of urea, urease and GdL. The lines and circles are colour coded to match the coloured axis titles. e, Cryo-EM of l,d-2NapFF (scale bar, 50 nm). In c and d, [l,d-2NapFF] = 5 mg ml⁻¹, [urea] = 0.04 M, [urease] = 0.4 mg ml⁻¹ and [GdL] = 14.3 mg ml⁻¹. Source data

Fig. 3. Forging the system by the application of a shear force to induce alignment.

a, The change in rheology and pH with time for a system containing l,d-2NapFF in the presence of urea, urease and GdL with no unidirectional shear. The inset photograph shows the sample after 16 h using the rheo-optics system. Throughout this figure, the lines and circles are colour coded to the coloured axis titles. b, The change in rheology and pH with time for a system containing l,d-2NapFF in the presence of urea, urease and GdL with unidirectional shear starting at 7 min and finishing at 100 min. The inset photograph shows the sample after 16 h using the rheo-optics system. c,d, Multi-scale analysis for a system with no shear alignment (c) and unidirectional shear (d) starting at 7 min and finishing at 100 min. Top: photographs of the sample at specific timepoints during the shearing process, indicated with an arrow. The dashed red line in the final image highlights the perimeter of the gel as it dried. Upper middle: time–space diagram of PLI using Herman’s algorithm to detect the presence of the Maltese cross pattern (green). Lower middle: time–space diagram of azimuthally integrated SAXS data with Herman’s orientation parameter (blue). Bottom: the imposed shear rate amplitude (oscillatory shear, c) or shear rate (steady shear, d). Scalar plot scattering intensity scales can be found in Supplementary Fig. 12. A full description of the calculations to obtain the orientation parameters can be found in Methods. In all cases, [l,d-2NapFF] = 5 mg ml⁻¹, [urea] = 0.04 M, [urease] = 0.4 mg ml⁻¹ and [GdL] = 14.3 mg ml⁻¹. Ω, angular velocity of rotating geometry; ω, angular frequency; $\dot{\gamma }$, applied shear rate; P, orientation parameter; L, length of arc taken from each PLI frame; φ, azimuthal angle. Source data

Fig. 4. Forging by magnetic alignment within an MRI scanner.

a,b, A cross-polarized optical image (a) and SEM image (b) for a system containing l,d-2NapFF in the presence of urea, urease and GdL allowed to evolve overnight in an MRI magnet. The vectors labelled P and A are the polarizer and analyzer axes, respectively. c,d, A cross-polarized optical image (c) and SEM image (d) for a system containing l,d-2NapFF in the presence of urea, urease and GdL allowed to evolve overnight outside of the magnet. Note that all the samples were kept in the same room with controlled humidity and temperature. In all cases, [l,d-2NapFF] = 5 mg ml⁻¹, [urea] = 0.04 M, [urease] = 0.4 mg ml⁻¹ and [GdL] = 14.3 mg ml⁻¹.