Electrochemically assisted hydrogel deposition, shaping and detachment

3D-printed gelled electrolytes for electroanalytical applications

In this work, several gelators were employed to formulate a conducive gel phase (ionic conductivity) compatible with direct ink writing/bioprinting/robocasting (different names in the literature describe the same printing technology). The main goal of this work was to evaluate gelled phases being a mixture of background electrolyte (NaCl), redox probe (Fe(CN)63-/4-), and gel precursor (guar gum, gelatine, agarose, and agar-agar). The studied concentration of gelators ranged from 0.1 to 4% depending on the employed system. Each gelator required a customized formulation protocol. We have found that guar gum exhibits the best printing properties (lack of aggregates blocking the printing nozzle) while giving the least reproducible electrochemical results (when a glassy carbon electrode was employed as the working electrode). The study of two other gelators (agarose and gelatin) indicated significant changes in the electrochemical properties of the investigated surface as their concentration and number of voltammetric scans were varied. The best electrochemical performance was obtained for agar-agar however, this was also a gelator causing the most problems during 3D printing. Finally, we have employed six screen-printed electrodes displaying approximate properties, that were further covered with a 3D-printed conductive gelled cube (direct printing over the electrode surface). We have found that such a system allowed for a surprisingly good electroanalytical response when the model redox probe (Fe(CN)63-/4-) was considered. This work is a prelude to 3D-printed gel-based detection devices we are currently developing in our team.

Janus hydrogels: merging boundaries in tissue engineering for enhanced biomaterials and regenerative therapies

In recent years, the design and synthesis of Janus hydrogels have witnessed a thriving development, overcoming the limitations of single-performance materials and expanding their potential applications in tissue engineering and regenerative medicine. Janus hydrogels, with their exceptional mechanical properties and excellent biocompatibility, have emerged as promising candidates for various biomedical applications, including tissue engineering and regenerative therapies. In this review, we present the latest progress in the synthesis of Janus hydrogels using commonly employed preparation methods. We elucidate the surface and interface interactions of these hydrogels and discuss the enhanced properties bestowed by the unique "Janus" structure in biomaterials. Additionally, we explore the applications of Janus hydrogels in facilitating regenerative therapies, such as drug delivery, wound healing, tissue engineering, and biosensing. Furthermore, we analyze the challenges and future trends associated with the utilization of Janus hydrogels in biomedical applications.

Electrogelation: Controlled Fast Formation of Micrometer-Thick Films from Low-Molecular Weight Hydrogelators

The controlled electrochemical deposition of hydrogels from low-molecular weight hydrogelators (LMWHGs) allows for the defined formation of thin films on electrodes. Here, the deposition of fibrillar networks consisting of N,N',N″-tris(4-carboxyphenylene)-1,3,5-benzenetricarboxamide (BTA) onto ultraflat gold electrodes has been studied. This process, also termed electrogelation, is based on a local change in the pH due to electrolysis of water at the electrode. The protonation of the BTA sodium salt leads to self-assembly into supramolecular fibrillar structures mainly via hydrogen bonding of the uncharged molecules. The resulting hydrogel film was characterized in terms of its thickness by atomic force microscopy (AFM). Two different AFM-based techniques have been used: ex situ imaging of dried films and in situ nanoindentation of the hydrated hydrogel films. The deposition process was studied as a function of gelator concentration, applied potential, and gelation time. These parameters allow control of the film thickness to a high degree of accuracy within a few tenths of nanometers. Film formation takes place in a few seconds at moderate applied potentials, which is beneficial for biomedical applications. The results obtained for the BTA presented here can be transferred to any type of pH-responsive LMWHG and many reversibly formed hydrogel films.

Personal Perspective on Understanding Low Molecular Weight Gels

Low molecular weight gels are formed by the self-assembly of small molecules into anisotropic structures that form a network capable of immobilizing the solvent. Such gels are common, with a huge number of different examples existing, and they have many applications. However, there are still significant gaps in our understanding of these systems and challenges that need to be addressed if we are to be able to fully design such systems. Here, a number of these challenges are discussed.

Stimuli responsive dynamic transformations in supramolecular gels

Supramolecular gels are formed by the self-assembly of small molecules under the influence of various non-covalent interactions. As the interactions are individually weak and reversible, it is possible to perturb the gels easily, which in turn enables fine tuning of their properties. Synthetic supramolecular gels are kinetically trapped and usually do not show time variable changes in material properties after formation. However, such materials potentially become switchable when exposed to external stimuli like temperature, pH, light, enzyme, redox, and chemical analytes resulting in reconfiguration of gel matrix into a different type of network. Such transformations allow gel-to-gel transitions while the changes in the molecular aggregation result in alteration of physical and chemical properties of the gel with time. Here, we discuss various methods that have been used to achieve gel-to-gel transitions by modifying a pre-formed gel material through external perturbation. We also describe methods that allow time-dependent autonomous switching of gels into different networks enabling synthesis of next generation functional materials. Dynamic modification of gels allows construction of an array of supramolecular gels with various properties from a single material which eventually extend the limit of applications of the gels. In some cases, gel-to-gel transitions lead to materials that cannot be accessed directly. Finally, we point out the necessity and possibility of further exploration of the field.