*Review* **Hydrogels as Potential Nano-, Micro- and Macro-Scale Systems for Controlled Drug Delivery**

#### **Adam Chyzy** †**, Monika Tomczykowa** † **and Marta E. Plonska-Brzezinska \***

Department of Organic Chemistry, Faculty of Pharmacy with the Division of Laboratory Medicine, Medical University of Bialystok, Mickiewicza 2A, 15-222 Bialystok, Poland; adam.chyzy24@gmail.com (A.C.); monika.tomczyk@umb.edu.pl (M.T.)

**\*** Correspondence: marta.plonska-brzezinska@umb.edu.pl; Tel.: +4885-748-5683

† These authors contributed equally to this work.

Received: 16 December 2019; Accepted: 27 December 2019; Published: 2 January 2020

**Abstract:** This review is an extensive evaluation and essential analysis of the design and formation of hydrogels (HGs) for drug delivery. We review the fundamental principles of HGs (their chemical structures, physicochemical properties, synthesis routes, di fferent types, etc.) that influence their biological properties and medical and pharmaceutical applications. Strategies for fabricating HGs with di fferent diameters (macro, micro, and nano) are also presented. The size of biocompatible HG materials determines their potential uses in medicine as drug carriers. Additionally, novel drug delivery methods for enhancing treatment are discussed. A critical review is performed based on the latest literature reports.

**Keywords:** hydrogel; drug delivery; polymer; biocompatibility; immobilization of drug
