Thermoresponsive hydrogels based on renewable resources Elif Isikci Koca, 1 Gülnihal Bozdag, 2 Gökhan Cayli, 3 Dilek Kazan, 2 Pinar Cakir Hatir 1 1 Istanbul Arel University, Faculty of Engineering and Architecture, Biomedical Engineering Department, 34537, Istanbul, Turkey 2 Marmara University, Faculty of Engineering, Bioengineering Department, 34722, Istanbul, Turkey 3 Istanbul University-Cerrahpasa University, Faculty of Engineering, Department of Engineering Sciences, 34320, Istanbul, Turkey Correspondence to: P. Cakir Hatir (E-mail: pinarcakir@arel.edu.tr) ABSTRACT: This work aims to synthesize novel thermoresponsive hydrogels from renewable resources, bacterial cellulose (BC), and cas- tor oil (CO), and to investigate the effect of CO on physical and thermal behaviors of BC/Poly(N-isopropylacrylamide) (PNIPAM) hydrogels. The structural properties of the hydrogels are analyzed by Fourier-transform infrared (FTIR) spectroscopy. Differential scan- ning calorimeter (DSC) technique and thermogravimetric analysis (TGA) are also performed to examine the thermal properties of the hydrogels. The morphological differences of the hydrogels are analyzed by scanning electron microscope (SEM). The thermoresponsive performances of the hydrogels are examined by swelling and deswelling behaviors. The hydrogel with CO is found to be more sensitive to temperature changes than the one without CO. Deswelling study demonstrates 91 and 25% of water loss for hydrogels with and with- out CO, respectively. The present study shows a novel approach to synthesize thermoresponsive hydrogels with renewable resources for biomedical applications. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019, 137, 48861. KEYWORDS: biomedical applications; biomaterials; biopolymers and renewable polymers Received 23 September 2019; accepted 11 December 2019 DOI: 10.1002/app.48861 INTRODUCTION Polyurethanes (PUs) are polymeric materials frequently used in many applications such as fiber, coating, foam, biomedical, con- struction, and automotive industries. 1 High mechanical strength, abrasion, and chemical resistance make PUs special materials for polymer industry. 1 The properties of PUs can be modified according to the purpose of use. Complexity of reacting mono- mers, the type and content of the reactive groups, as well as the degree of the crosslinking can affect the characteristics of PUs. 2 The traditional synthesis of PUs is based on the use of petrochemical-based materials. There has been a trend toward the use of renewable resources in PU synthesis. Additionally, in terms of the seventh principle among the 12 “Green Chemistry” principles, chemicals are being produced from renewable sources to protect human health and the environment. 1,3 Therefore, in near future the biodegradability of materials to be synthesized with renewable resources will increase and materials with high environmental sustainability at low cost will be obtained. One of the most widely used renewable raw materials to produce green PUs is vegetable oils (VOs), triglycerides of fatty acids. They are considered to be reliable materials in PU synthesis due to their multistructural characteristic, low toxicity, and biode- gradable properties. 1 Because of their lower cost, they can com- pete with petroleum-derived compounds. Fatty acids can be modified by chemical processes to obtain bonds that are easily hydrolysable. 4 In polymerization reactions, they can be used as monomers after polyol functionalization. 2 Depending on the presence of double bonds, they can be classified as saturated or unsaturated fatty acids. Unsaturated fatty acids such as oleic/ linoleic/ricinoleic acid have at least one double bond while satu- rated ones such as stearic/palmitic do not have any double bonds. Especially with unsaturated ones; different reactions can be car- ried out to obtain the bio-based polyol and reactions with isocya- nates can be achieved to obtain PUs. 1 Castor oil (CO), one of the non-volatile VOs and a member of the Euphorbiaceae family, 3 is used in the synthesis of different biodegradable polymers (Figure 1). CO is a glycerol triester of ricinoleic acid (RA) (12-hydroxy-cis-9-octadecenoic acid), which is a hydroxyl containing fatty acid and chain lengths of these fatty acids are generally 18 and 20 carbon atoms 5 (Figure 1). RA is the major constituent of CO and its ratio in CO is approxi- mately 90%. These properties make it unique among other vege- table oils in polymer synthesis, so CO/RA-based hydrophobic polymers have flexible structures. 4,6 Because of its low toxicity, CO is one of the excellent renewable agricultural resources. 3 Fur- thermore, it has been known for its medicinal value since ancient days. 4 As CO is the only commercially available natural VO that is obtained directly from plant source with natural hydroxyl © 2019 Wiley Periodicals, Inc. 48861 (1 of 10) J. APPL. POLYM. SCI. 2019, DOI: 10.1002/APP.48861