![]() ![]() First, a single layer of a prehydrogel solution whose concentrations of hydrogel precursors or soluble protein could be changed for subsequent layers was added into a mold. The photocrosslinking process of PEGDA or GelMA prehydrogel solution containing photoinitiator was initiated by ultraviolet (UV) irradiation. The gradients were created using two different types of photocrosslinkable hydrogel-poly(ethylene glycol) diacrylate (PEGDA) and gelatin methacryloyl (GelMA). Ko and co-workers formed three-dimensional concentrations and mechanical gradients by means of a layer-stacking technique in order to generate 3D mechanical and biomolecular gradients. The layered structure forms the space gradient in these properties usually due to their sharp change between layers. ![]() Perhaps the most widely used techniques are based on the successive formation of layers of a common basic hydrogel that differ in some specific properties. ![]() Here we only note that the most often approaches of checking the gradient include electron and light microscopy, mechanical tests performed on several sections cut from the whole material. The interesting question of the detection of formed gradients is out of scope of this brief review and deserves a special paper. Some studies combined techniques from two groups but are classified under a single group selected on the basis of the main preparation principle used. The techniques are divided into several groups according to the common features indicated by the group names. In this short review, we present an overview of the techniques used to prepare gradient hydrogels which have been reported in literature over the last few years. The field of gradient hydrogels is progressing rapidly and admirable creativity can be seen in the variety of approaches to gradient formation. In contrast, conventional hydrogels can be viewed as materials with uniform properties. The gradient may refer not only to a spatial characteristic but also to time or to both, giving thus a spatiotemporal property change. These “gradient properties” may include the following: some types of mechanical properties, crosslinking density and mesh size or the porosity of the hydrogel network, chemical composition, and biochemical properties. Gradient hydrogels possess a gradual or abrupt change in one or some of their properties (physical properties mostly) when moving through a single piece of hydrogel body. Hydrogels with gradient properties, or, more simply, gradient hydrogels, are examples of hydrogels that have been intentionally modified to achieve soft materials with specific properties. More sophisticated hydrogel materials with tailored properties can be obtained by modifying the chain, incorporating additives, or manipulating the network structure including the formation of supramolecular assemblies. ![]() Basically, their properties are determined by the nature and structure of the parent polymeric chain, the number and character of crosslinks, and the content of water. For instance, as cell support or cultivation media, delivery systems, tissue substitutes, or universal sorbents. Hydrogels resemble biological environments and thus can be used in a variety of biological, biomedical, or environmental applications. Hydrogels can contain a huge amount of water (even more than 90% by weight) while retaining their shape as a solid body. The network is usually formed by hydrophilic polymer chains interconnected by means of either chemical or physical crosslinks. Hydrogels are well-known materials formed by a hydrophilic network imbibed by water, which actually plays the role of a dispersion medium. ![]()
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