**Abstract**

Biopolymer based hydrogels are three-dimensional physically or chemically crosslinked polymeric networks based on natural polymers, with an intrinsic hydrophilic character due to their functional groups. They display high water content, softness, flexibility, permeability, and biocompatibility and possess a very high affinity for biological fluids. These properties resemble those of many soft living tissues, which opens up many opportunities in the biomedical field. In this regard, hydrogels provide fine systems for drug delivery and sustained release of drugs. Moreover, biopolymer based hydrogels can be applied as coatings on medical implants in order to enhance the biocompatibility of the implants and to prevent medical conditions. In this chapter we review the latest achievements concerning the use of biopolymeric physical and chemically crosslinked hydrogels as well as hydrogel coatings as sustained drug release platforms.

**Keywords:** sustained release, drug delivery, biopolymers, hydrogels, hydrogel coating

### **1. Introduction**

Gels can be defined as three-dimensional cross-linked polymeric networks which swollen in contact with a liquid. When the polymers forming the gel contain mainly hydrophilic functional groups, the liquid that causes the swelling is water, and the gel is called hydrogel [1]. Biopolymers are often used for the synthesis of hydrogels as the natural composition of the polymer leads to extremely high biocompatibility and potential applications in the biomedical field [2].

Hydrogels can be classified as physical hydrogels when the properties of the gel depend on chain entanglements and other interactions, mainly hydrogen bonds or hydrophobic interactions [3]. In this case, properties are highly dependent on chain molecular weight as well as concentration, as mobility of the chains modifies the structure of the hydrogel and therefore its physical properties. Water temperature, salt content, and pH can also affect the mobility of the chains and interactions and must be controlled [4].

Chemically crosslinked hydrogels present a much more stable structure than physical hydrogels. In chemical hydrogels, the polymeric chains are covalently bonded using one or more crosslinking agents, using a chemical process [5]. In general, crosslinked hydrogels are less biocompatible than physical hydrogels, but this is compensated by other advantages: cross-linked hydrogels are insoluble, more stable, and rheological properties such as elasticity or viscosity, their pore size, and their degradation rate can be more optimized than with physical hydrogels.

Hydrogels can present different physical forms: from macrogels to micro and nanogels, which are particulate systems with similar chemical structure but different macroscopic size; implantable gels, with strong physical properties, or injectable gels, more fluids or composed of nano-microparticles which can pass through a needle; hydrogel coatings, where hydrogel nano or microlayer is immobilized on a surface; thermoresponsive or pH-responsive gels, where a trigger modulates the sol–gel properties, can be easily injected in a liquid form before gelation in physiological conditions [6].

Physical properties of physically and chemically crosslinked hydrogels, are similar to several soft biological tissues, and therefore they can be used as substitutes or supplements when the biological function of these soft tissues is compromised. Such hydrogels have been widely used as medical devices for different applications: in traumatology, as substitute or supplement of synovial fluid in osteoarthritis; in ophthalmology, as a substitute of aqueous humor during in cataract surgery; in esthetics and reconstructive surgery, as dermal fillers for rid correction and lipoatrophy for patients with VIH; in wound healing, as wound dressings to promote regeneration and healing of wounds [7].

The biomedical use of the hydrogels can be expanded by the employ of the hydrogels as a sustained release system. Concerning this the controlled release of pharmaceutical ingredients leads to important advantages as a control of the biodisponibility, dose control, local delivery and less side effects [8]. This chapter aims to cover a general overview concerning the sustained drug release from hydrogels and hydrogel coatings. In that regard, *Section 1 Introduction* presents the topic and outlines the content of the chapter; *Section 2 Mechanism of drug release form hydrogels*, reviews the most significant theories on drug release mechanism; *Section 3 Drug release from physical hydrogels* summarizes the recent advances on the area; *Section 4 Drug release from chemically cross-linked hydrogels* revises the latest works on sustained release from chitosan, hyaluronic acid and other biopolymers; *Section 5 Drug release from hydrogelbased bioactive coatings* introduces the most relevant concepts on drug release form coatings; finally *Section 6 Conclusion* synopsizes the content of this chapter.
