BIOMATERIALS SYNTHESIS AND CHARACTERIZATION Characterization of gelatin–agar based phase separated hydrogel, emulgel and bigel: a comparative study Senggam Wakhet • Vinay K. Singh • Saikat Sahoo • Sai Sateesh Sagiri • Senthilguru Kulanthaivel • Mrinal K. Bhattacharya • Naresh Kumar • Indranil Banerjee • Kunal Pal Received: 12 June 2014 / Accepted: 29 November 2014 / Published online: 12 February 2015 Ó Springer Science+Business Media New York 2015 Abstract The current study describes the in-depth charac- terization of agar–gelatin based co-hydrogels, emulgels and bigels to have an insight about the differences in the properties of the formulations. Hydrogels have been extensively studied as vehicle for controlled drug release, whereas, the concept of emulgels and bigels is relatively new. The formulations were characterized by scanning electron microscopy, FTIR spec- troscopy, XRD and mechanical properties. The biocom- patibility and the ability of the formulations to be used as drug delivery vehicle were also studied. The scanning electron micrographs suggested the presence of internal phases within the agar–gelatin composite matrices of co-hydrogel, emulgel and bigel. FTIR and XRD studies suggested higher crys- tallinity of emulgels and bigels. Electrical impedance and mechanical stability of the emulgel and the bigel was higher than the hydrogel. The prepared formulations were found to be biocompatible and suitable for drug delivery applications. 1 Introduction The behavior of the co-hydrogels has received a special interest in the recent times. Co-hydrogels may be defined as the crosslinked polymeric networks of two or more polymers. The modulation of the properties of the co-hy- drogels can be achieved by tailoring the composition of the co-hydrogels. In this regard, gelatin–agar co-hydrogels have received much attention due to the inherent biocom- patible nature of gelatin and agar. Both gelatin and agar are naturally occurring polymers. Also, the cost of the gelatin and agar is affordable and are easily available. Gelatin is a protein biopolymer while agar is a polysaccharide biopolymer [1]. Gelatin–polysaccharide based phase- separated hydrogels have been reported and studied ex- tensively. The phase-separated hydrogels are formed due to the thermodynamic instability of the gelatin–polysaccha- ride mixtures (solution) when the concentration of the ge- latin and the polysaccharides are high. As the hot gelatin– polysaccharide mixtures are cooled down to room-tem- perature, there is a formation of water-in-water type of emulsions. In these emulsions, the internal phase is con- centrated with polysaccharide while the external phase is gelatin rich [2]. Emulgels, also known as emulsion gels, are defined as the biphasic formulations whose internal phase is apolar liquid while the continuous phase is the semisolid in nature which holds an aqueous phase. Firoozmand and Rousseau [3] have reported gelatin–starch phase separated emulgels. They have reported that the dispersed oil phase acts as active fillers and contributes to the improvement in the mechanical properties of the emulgels [4] when the compositions of the formulations are judiciously chosen [3]. In recent years, bigels have evolved as one of the promising formulations with improved mechanical and controlled delivery properties. Bigels are also defined as biphasic formulations like emulgels but unlike emulgels the internal phase is semisolid in nature [5, 6]. Due to the increase in the viscosity of the internal phase, it is expected that the filler effect of the inner phase will be more S. Wakhet  V. K. Singh  S. Sahoo  S. S. Sagiri  S. Kulanthaivel  I. Banerjee  K. Pal (&) Department of Biotechnology & Medical Engineering, National Institute of Technology, Rourkela 769008, Odisha, India e-mail: kpal.nitrkl@gmail.com; pal.kunal@yahoo.com M. K. Bhattacharya Department of Botany & Biotechnology, Karimganj College, Karimganj, Assam, India N. Kumar Scientific and Digital Systems, IDA House, New Delhi, India 123 J Mater Sci: Mater Med (2015) 26:118 DOI 10.1007/s10856-015-5434-2