# Use of Hydrogels to Regulate Orthodontic Tooth Movement in Animal Models: A Systematic Review
###### Abstract  
The objective of this article is to conduct a systematic review of the literature to contrast the existing evidence regarding the use of hydrogels during and after experimental orthodontic treatment in animals. An extensive search was performed through the electronic databases, Medline, Web of Science and Scopus, from December 2020 to April 2021 for in vivo animal studies. A total of 282 studies were reviewed. Eight studies were included for final revision; four studies were conducted in rats, two in rabbits, one study in mice and one study in guinea pigs. The quality assessment of the eight included studies was performed according to the ARRIVE guidelines and the risk of bias was assessed using the Center for Systematic Review of Laboratory Animal Experimentation tool; in four of the eight articles evaluated, a high risk-of-bias rating was obtained in 40% of the criteria evaluated. In the studies reviewed, the hydrogel acted as a carrier, and inhibition (post-treatment retention) or acceleration of orthodontic tooth movement was assessed according to the active substance used in each of the articles. The uses of hydrogels for transporting active substances to regulate the rate of orthodontic tooth movement remains debatable. Future studies are suggested to evaluate the feasibility of hydrogel as a transport method in humans.  
hydrogels; orthodontic tooth movement; acceleration; relapse +
Footnote †: journal: Medical Imaging  
0000-0001-6000-0001-6000][PERSON]. [PERSON]  
0000-0001-6000-0001-6000-0001-6000-0001-60001-6000-0001-60001-  
In 1960, [PERSON] et al., first used the term \"hydrogel\" to describe a three-dimensional network of hydroxyethyl methacrylate (HEMA) [8; 9; 10]. Hydrogels consist of a large amount of water and a network of crosslinked polymers. The high water content provides a physical resemblance to tissues and gives hydrogels excellent biocompatibility and the ability to easily encapsulate hydrophilic drugs [11; 12].  
Their main property lies in their absorption capacity, which is visually manifested as a \"swelling\" of the material that is due to the presence of water-like groups in their molecular structure upon contact with a thermodynamically compatible solvent. Their softness and elasticity in the hydrated state depends on the hydrophilicity of the monomers and the crosslink density [13; 14].  
Hydrogels are currently of great importance in medicine and research. Due to their diverse properties, which are similar to those of biological tissues, many advances have been made in their applications and formulations [15]. Thus, hydrogels can serve as scaffolds that act as supports for cell growth, as materials for the encapsulation of cells, proteins, or genes, as adhesives between tissues and material surfaces, as systems that allow a reversible response to an external stimulus, and as porous membranes; as well, because of their biological compatibility, hydrogels can be designed to allow the release of a drug according to therapeutic needs, making them ideal extended-release vehicles [16].  
The variety of polymers used to synthesize hydrogels provides a wide range of physicochemical properties that together allow encapsulation of bioactive molecules and a suitable profile for their release in a localized area such as the buccal mucosa, which is a desired property in orthodontic treatment acceleration [15; 16].  
A systematic review focusing on the use of hydrogels in orthodontic tooth movement would be of great benefit to dentists by providing up-to-date information on hydrogel as a vehicle for future use in accelerating tooth movement or anchorage after treatment. The purpose of this systematic review is to highlight the use of hydrogels during and after orthodontic treatment.