Effect of Imposing Spatial Constraints on Low Molecular Weight Gels.

We outline the effect of imposing spatial constraints during gelation on hydrogels formed by dipeptide-based low molecular weight gelators. The gels were formed via either a solvent switch or a change in pH and formed in different sized vessels to produce gels of different thickness while maintaining the same volume. The different methods of gelation led to gels with different underlying microstructure. Confocal microscopy was used to visualize the resulting microstructures, while the corresponding mechanical properties were probed via cavitation rheology. We show that solvent-switch-triggered gels are sensitive to imposed spatial constraints, in both altered microstructure and mechanical properties, while their pH-triggered equivalents are not. These results are significant because it is often necessary to form gels of different thicknesses for different analytical techniques. Also, gels of different thicknesses are utilized between various applications of these materials. Our data show that it is important to consider the spatial constraints imposed in these situations.

Investigating Aggregation Using In Situ Electrochemistry and Small-Angle Neutron Scattering.

Using small-angle neutron scattering to investigate the aggregation of self-assembling molecules is well established. Some of these molecules are electrochemically useful, for example, in electrochromic devices. Electrochemistry can also be used in some cases to induce aggregation. Here, we describe an approach whereby electrochemistry can be directly carried out on a sample in the neutron beam, allowing us to monitor changes directly in situ. We exemplify with two examples but highlight that there are many other potential opportunities.

Mechanical Characterization of Multilayered Hydrogels: A Rheological Study for 3D-Printed Systems.

We describe rheological protocols to study layered and three-dimensional (3D)-printed gels. Our methods allow us to measure the properties at different depths and determine the contribution of each layer to the resulting combined properties of the gels. We show that there are differences when using different measuring systems for rheological measurement, which directly affects the resulting properties being measured. These methods allow us to measure the gel properties after printing, rather than having to rely on the assumption that there is no change in properties from a preprinted gel. We show that the rheological properties of fluorenylmethoxycarbonyl-diphenylalanine (FmocFF) gels are heavily influenced by the printing process.

Tuning the antimicrobial activity of low molecular weight hydrogels using dopamine autoxidation.

We present a method to trigger the formation of dipeptide-based hydrogels by the simple addition of dopamine. Dopamine undergoes oxidation in air, reducing the pH to induce gelation. The production of polydopamine and release of reactive oxygen species such as hydrogen peroxide confers antimicrobial activity. Gel stiffness can be controlled by modulating the initial starting pH of the gelator solution. We can use this method to tune the antimicrobial activity of the gels, with gels that are less stiff demonstrating increased bactericidal efficacy against Gram-positive bacteria.

Isotopic Control over Self-Assembly in Supramolecular Gels.

It is common to switch between H2O and D2O when examining peptide-based systems, with the assumption being that there are no effects from this change. Here, we describe the effect of changing from H2O to D2O in a number of low-molecular-weight dipeptide-based gels. Gels are formed by decreasing the pH. In most cases, there is little difference in the structures formed at high pH, but this is not universally true. On lowering the pH, the kinetics of gelation are affected and, in some cases, the structures underpinning the gel network are different. Where there are differences in the self-assembled structures, the resulting gel properties are different. We, therefore, show that isotopic control over gel properties is possible.

Annealing Supramolecular Gels by a Reaction Relay.

Supramolecular gels have potential in many areas. In many cases, a major drawback is that the gels are formed at a high rate. As a result, nonoptimal, kinetically trapped self-assembled structures are often formed, leading to gels that can be hard to reproduce and control. One method to get around kinetic trapping is annealing. Thermal annealing is one possibility, but it is not always desirable to heat the gels. Here, we describe a method to anneal pH-triggered gels after they are formed. We employ a reaction relay in a peptide-based hydrogel system to anneal the structures by a controlled and uniform pH change. Our method allows us to prepare gels with more controlled properties. We show that this can be used to enable homogeneous "molding and casting" of the hydrogels. This method of annealing is more effective in improving gel robustness than a conventional heat-cool cycle.

PAINT-ing Fluorenylmethoxycarbonyl (Fmoc)-Diphenylalanine Hydrogels.

Self-assembly of fluorenylmethoxycarbonyl-protected diphenylalanine (FmocFF) in water is widely known to produce hydrogels. Typically, confocal microscopy is used to visualize such hydrogels under wet conditions, that is, without freezing or drying. However, key aspects of hydrogels like fiber diameter, network morphology and mesh size are sub-diffraction limited features and cannot be visualized effectively using this approach. In this work, we show that it is possible to image FmocFF hydrogels by Points Accumulation for Imaging in Nanoscale Topography (PAINT) in native conditions and without direct gel labelling. We demonstrate that the fiber network can be visualized with improved resolution (≈50 nm) both in 2D and 3D. Quantitative information is extracted such as mesh size and fiber diameter. This method can complement the existing characterization tools for hydrogels and provide useful information supporting the design of new materials.

Controlling the properties of the micellar and gel phase by varying the counterion in functionalised-dipeptide systems.

The micellar aggregates formed at high pH for dipeptide-based gelators can be varied by using different alkali metal salts to prepare the solutions. The nature of the micellar aggregates directly affects the properties of the resulting gels.

On the Mechanical Properties of N-Functionalised Dipeptide Gels.

The properties of a hydrogel are controlled by the underlying network that immobilizes the solvent. For gels formed by the self-assembly of a small molecule, it is common to show the primary fibres that entangle to form the network by microscopy, but it is difficult to access information about the network. One approach to understand the network is to examine the effect of the concentration on the rheological properties, such that G'∝ cx, where G' is the storage modulus and c is the concentration. A number of reports link the exponent x to a specific type of network. Here, we discuss a small library of gels formed using functionalized dipeptides, and describe the underlying networks of these gels, using microscopy, small angle scattering and rheology. We show that apparently different networks can give very similar values of x.

Using cavitation rheology to understand dipeptide-based low molecular weight gels.

The study of dipeptide-based hydrogels has been the focus of significant effort recently due to their potential for use in a variety of biomedical and biotechnological applications. It is essential to study the mechanical properties in order to fully characterise and understand this type of soft materials. In terms of mechanical properties, the linear elastic modulus is normally measured using traditional shear rheometry. This technique requires millilitre sample volumes, which can be difficult when only small amounts of gel are available, and can present difficulties when loading the sample into the machine. Here, we describe the use of cavitation rheology, an easy and efficient technique, to characterise the linear elastic modulus of a range of hydrogels. Unlike traditional shear rheometry, this technique can be used on hydrogels in their native environment, and small sample volumes are required. We describe our set-up and show how it can be used to probe and understand different types of gels. Gels can be formed by different triggers from the same gelator and this leads to different microstructures. We show that the data from the cavitational rheometer correlates with the underlying microstructure in the gels, which allows a greater degree of understanding of the gels than can be obtained from the bulk measurements.

Annealing multicomponent supramolecular gels.

Annealing is widely used as a means of changing the physical properties of a material. The rate of heating and cooling used in the annealing process controls the final properties. Annealing can be used as a means of driving towards the, or at least a, thermodynamic minimum. There is surprisingly little information on annealing kinetically-trapped supramolecular gels. Here, we show that annealing multicomponent gels can be used to prepare materials with tunable mechanical properties. We show that annealing in a two-component gel leads to a self-sorted network, which has significantly different mechanical properties to the as-prepared gels. Whilst the fibres are self-sorted, we show that the annealing of this system leads to significant change in the network level of assembly, and it is this that leads to the increase in storage modulus. We also show that it is possible to selectively anneal only a single component in the mixture.

SPIONs for cell labelling and tracking using MRI: magnetite or maghemite?

Although there is extensive literature covering the biomedical applications of superparamagnetic iron oxide nanoparticles (SPIONs), the phase of the iron oxide core used is not often taken into account when cell labelling and tracking studies for regenerative medicine are considered. Here, we use a co-precipitation reaction to synthesise particles of both magnetite- (Fe3O4) and maghemite- (γ-Fe2O3) based cores and consider whether the extra synthesis step to make maghemite based particles is advantageous for cell tracking.

Optimising low molecular weight hydrogels for automated 3D printing.

Hydrogels prepared from low molecular weight gelators (LMWGs) are formed as a result of hierarchical intermolecular interactions between gelators to form fibres, and then further interactions between the self-assembled fibres via physical entanglements, as well as potential branching points. These interactions can allow hydrogels to recover quickly after a high shear rate has been applied. There are currently limited design rules describing which types of morphology or rheological properties are required for a LMWG hydrogel to be used as an effective, printable gel. By preparing hydrogels with different types of fibrous network structures, we have been able to understand in more detail the morphological type which gives rise to a 3D-printable hydrogel using a range of techniques, including rheology, small angle scattering and microscopy.