What are hydrogels?
A hydrogel consists of a three-dimensional network of flexible polymer chains that can hold and immobilize a large amount of water or aqueous solution. The hydrogels are systems with a great scientific-technical interest, since they combine a very high porosity and a typical mechanical behavior of viscoelastic solid. Consequently, as long as they possess adequate biocompatibility, hydrogels can mimic the extracellular matrix of living tissues better than any other type of synthetic biomaterial. This has led to the development of a wide variety of biomedical applications, many of them already commercial. In regenerative medicine, hydrogels have been used as vehicles for the release of cells, drugs and growth factors and, mainly, as extracellular matrices (scaffolds) to generate artificial tissues by tissue engineering. Generally, hydrogels are formed by permanent chemical cross-links between the polymer chains via non-reversible covalent bonds. However, such hydrogels are sometimes brittle, at times opaque and without the ability to self-heal when the cross-linked network is broken, thus greatly limiting their application in various biomedical fields.
What are supramolecular hydrogels?
Supramolecular hydrogels, which perfectly combine the advantages of synthetic hydrogels with those of supramolecular polymers are a novel class of noncovalently cross-linked polymer materials. The supramolecular cross-linking by various noncovalent interactions such as hydrogen bonding, metal-ligand coordination, host-guest recognition, and electrostatic interaction remarkably reduces the structural flexibility and alters the macroscopic performance, resulting in the formation of 3D cross-linked networks. In sharp contrast, such noncovalent hydrogels show not only the moderate mechanical properties gained from polymeric building blocks, but also show reversible sol-gel transition behavior in response to a wide variety of bio-related stimuli and processability inherent to the supramolecular cross-linking units, which can serve as either intelligent carriers for delivering versatile therapeutic agents or promising matrices for repairing and regenerating tissues and organs in the human body. In recent years, a rapidly growing number of publications on supramolecular hydrogels have been reported. Considering recent advances in this emerging area, a systematic summary of the synthesis, properties, and bio applications of supramolecular hydrogels is urgently required.
What are magnetic hydrogels?
An important group within the “intelligent” hydrogels are the magnetic hydrogels also called ferrogels. Their microstructural and macroscopic properties (shape, mechanical properties) can be modified by remote action of external magnetic fields. From a microstructural point of view, magnetic hydrogels are materials composed of a polymer network containing ferrous or ferromagnetic micro- or nanoparticles. There are different methods of preparation of magnetic hydrogels. Magnetic hydrogels have remarkable advantages compared to their non-magnetic counterparts in the field of biomedicine. Recent studies showed that the presence of magnetic nanoparticles in the extracellular matrix stimulates cell adhesion, proliferation and differentiation in vitro, and even bone regeneration in vivo. In addition, an important synergy in the regeneration of bone has been demonstrated when the use of magnetic constructs is combined with the application of static magnetic fields.
Where we use hydrogels?
The answer is quite simple: in many areas of life. Since the first example of synthetic hydrogels was reported by Wichterle and Lim in 1960, hydrogels based on both natural and synthetic polymers have been of great interest for a wide range of applications ranging from industrial to biological. Excellent biocompatibility and high hydrophilicity of the hydrogels make them particularly useful in biomedical applications ranging from drug delivery to tissue engineering. The high water content of hydrogels renders them compatible with most living tissue and their viscoelastic nature minimizes damage to the surrounding tissue when implanted in the host. In addition, their mechanical properties parallel those of soft tissue, which makes them particularly appealing to tissue engineers. These amazing materials are capable of interacting with the host tissues, assisting and improving the healing process, and mimicking functional and morphological characteristics of organ tissue.