Hydrogels are composed of hydrophilic polymers, whose three-dimensional network structure can not only absorb a large amount of water, but also be used to carry drugs. Hydrogels prepared with suitable materials have the characteristics of high biocompatibility, mechanical and viscoelastic control. Since the term was coined in the late 19th century, hydrogels have been widely used in drug delivery, wound dressing, tissue engineering, and hygiene products. This article mainly introduces the development history of hydrogels.
The term hydrogel dates back to 1894, when it was first used to describe colloids made of inorganic salts. Over time, the meaning of hydrogels has changed completely. The world's first mature hydrogel product Ivalon (the crosslinking of formaldehyde ester and ethylene) came out in 1949, and PHEMA (polyhydroxyethyl methacrylate) came out in 1960, which pushed the market of hydrogel to prosperity. Looking back, the history of hydrogel can be roughly divided into three generations.
1. First-genaration Hydrogels
The first generation of hydrogels is mainly divided into three categories. The first category is polymers of vinyl monomers induced by free radical chain addition reactions. The main representatives are polyacrylamide (PAM) and polyhydroxyethyl methacrylate. (pHEMA). Although it has been invented for more than 70 years, it is still an important biomaterial. The second category is covalently cross-linked hydrophilic polymers, mainly represented by polyvinyl alcohol (PVA) and polyethylene glycol (PEG), mainly It is used in tissue engineering. And the third category is cellulose-based hydrogel, which is mainly used as a drug dispersion matrix for drug delivery.
2. Second-generation Hydrogels
The second generation of hydrogels are mainly PEG/ polyester block copolymers. Compared with the first generation, the second generation is characterized by the ability to convert the chemical energy of hydrogels into the mechanical energy of hydrogels to achieve the specified function.
This class of stimulus-responsive hydrogels appeared on the market in the 1970s. These hydrogels can respond to changes in the external environment, such as temperature or pH. Stimulus-response hydrogels can be broadly divided into three categories:
The first type is temperature-sensitive hydrogels responsive to temperature, which can show the phase transition from gel state to sol state from low temperature to high temperature. The main representatives are Pluronics of BASF or Poloxamers of British Imperial Chemical Industry Group.
The second major ph-sensitive hydrogels, these polymers hydrolyze at either high or low pH.
The third is biomolecular sensitive hydrogels that can respond to changes in concentration of specific biomolecules through conformational changes. Such as glucose oxidase hydrogels, which can be used to deliver insulin. The basic principle is that when glucose diffuses in the hydrogel matrix, it will be converted into gluconic acid by the glucose oxidase in the hydrogel, which will lead to the decrease of the pH value of the environment, and then the protonation of the amine functional group of the hydrogel will result in the swelling increase, so that insulin can be released from the matrix, forming a set of insulin self-regulating release system.
3. Third-generation Hydrogels
The main feature of the third generation of hydrogels is "crosslinking", which mainly regulates the mechanical properties and degradation properties of hydrogels by means of stereoscopic complexation, inclusion complexes, metal-ligand coordination and synthetic peptide chain. For example, one of the main applications of the stereocomplex method is the preparation of injectable hydrogels from two amphiphilic copolymers, PLLA (polylactic acid) and PDLA (polylactic acid enantiomer). There are also studies on hydrogels that use cyclodextrin inclusion complexes to construct hydrophobic cavities that can accommodate different molecules. In genetic engineering, there are also studies on synthetic peptide (or protein) hydrogels that use the folding structure of peptides to construct, but such hydrogels are mainly reflected in the research.
Although hydrogels have been around for more than half a century, there's no doubting their appeal. Due to its unique characteristics, hydrogels have always attracted the attention of the pharmaceutical industry. Although relevant theoretical studies have dug deeply into its entry, there are still many prescriptions that have not entered the market, and hydrogels are still promising in the field of drug delivery.
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