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p. 62-66 / SUPRAMOLECULAR CHEMISTRY
Dynamic supramolecular hydrogelators
A biomedical perspective

VINCENT LE SAGE, VASUDEVAN LAKSHMINARAYANAN, EDUARDO MENDES, RIENK EELKEMA, JAN VAN ESCH*
*Corresponding author
Delft University of technology, Department of Chemical Engineering, Advanced Soft Matter, Julianalaan 136, 2628BL Delft, The Netherlands

KEYWORDS: supramolecular, hydrogels, self-assembly, bio-responsive, drug delivery, tissue engineering
ABSTRACT: Supramolecular hydrogels as opposed to polymeric systems are intrinsically dynamic because of their non-covalent nature, which give them distinct advantages in biomedical applications that require adaptive and responsive materials. The ease of chemical functionalization and well-defined supramolecular structures allow excellent control over morphology, display of chemical cues, drug encapsulation, and responsiveness, and makes them ideally suited for application as scaffold for cell growth and tissue engineering, and as platform for drug delivery and injectable therapeutics.

INTRODUCTION
‘Science for society’ is the motto of the biomedical industry. Hydrogels have been in the biomedical arena for more than 50 years in numerous applications including but not limited to contact lenses, electroencephalography (EEG) electrodes, drug delivery systems, cell culture platforms, wound dressings, and personal hygiene products (1-2). A typical hydrogelator consists of a three-dimensional cross-linked network of natural or synthetic compounds, with water and solutes occupying the interstitial space. The coexistence of a cross-linked network structure and a liquid make the gels macroscopically behave as viscoelastic solids with a finite yield stress while preserving the liquid character at the microscopic level. At typically 99 percent water by weight, they offer excellent biocompatibility and integration into biological systems. The aqueous reservoir offers numerous opportunities to carry and release payload while the viscoelastic properties make them pliable and mimic natural tissues in many ways.
Synthetic polymeric hydrogels with highly defined chemical composition and microstructure have rapidly progressed in the past decade to mimic biological systems. The static nature of their covalently cross-linked network however, does not comply with the complex and continuously developing biological environment (3). The complexity of biological systems, consisting of multiple hierarchical pathways and feedback mechanisms, ideally requires hydrogel systems that are bio-responsive and can be addressed by external stimuli. Dynamic properties that can be addressed are (local) degradability, the ability to release trapped substances, a change of the mechanical properties, and inducing a sol-gel phase transition. Natural systems such as collagen (4) or Matrigel™ (5) only partly exhibit these properties. Moreover, they suffer from batch variability and their xenogeneic origin raises immunogenic concerns (6).
Supramolecular hydrogels are a promising alternative for covalently cross-linked polymer gels in biomedical applications. They are formed by low molecular weight hydrogelators that reversibly assemble via non-covalent interactions into fibres and ultimately a three-dimensional fibrous network, thereby forming the gel (Figure 1) (7). The non-covalent nature of the interactions allows for remodelling of network and formation of very defined architectures similar to naturally obtained designs. In this article we highlight the potential of these dynamic supramolecular gels in several biomedical applications viz. tissue engineering, cell culture scaffolds, drug delivery, and injectable in-vivo systems.

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