International Journal of Biological Macromolecules 73 (2015) 170–181 Contents lists available at ScienceDirect International Journal of Biological Macromolecules j ourna l ho me pa g e: www.elsevier.com/locate/ijbiomac Chitosan–nanohydroxyapatite composites: Mechanical, thermal and bio-compatibility studies Pratik Roy, R.R.N. Sailaja ∗ The Energy and Resources Institute (TERI), SRC, Bangalore 560071, India a r t i c l e i n f o Article history: Received 11 September 2014 Received in revised form 4 November 2014 Accepted 5 November 2014 Available online 2 December 2014 Keywords: Bionanocomposite Nanohydroxyapatite Mechanical properties Cytocompatibility Apatite formation a b s t r a c t Bionanocomposites of chitosan were prepared with nanohydroxyapatite (nHA) using 2-hydroxyethyl methacrylate (HEMA) as coupling agent. The tensile and flexural properties for 8% nHA loading showed optimal values. Compressive modulus also considerably increased from 525.16 MPa (0% nHA) to 1326.5 MPa with 10% nHA. Surface functionalization of fillers along with the addition of HEMA as coupling agent led to enhanced mechanical properties similar to human bone. The mechanical proper- ties were further analyzed using micromechanical theories which indicated good interfacial adhesion between the matrix and fillers. The composites showed cytocompatibility. Multiple layers of apatite for- mation have been observed when the nanocomposites were soaked in simulated body fluid (SBF). Hence, these composites showed potential for bone substitute applications. © 2014 Elsevier B.V. All rights reserved. 1. Introduction The need for a benign bone substitute is becoming a major need especially for elderly population with various bone defects. This requirement needs: (a) material matching in chemical composition with natural bone; (b) biocompatibility and mechanical strength similar to human bone. Extensive studies on hydroxyapatite are being carried out as it is a well-known material for biocompatibility and osteoconductivity with a chemical structure and composition matching the human bone and hence can mimic the same [1]. However, hydroxyapatite alone has certain shortcomings such as poor load bearing properties, difficulty to cast into the desired shape and its tendency to migrate from the implanted sites [2,3]. Hence, another biocompatible, natural and abundant biopolymer such as chitosan has been combined along with hydroxyapatite to develop bone substitute composites. Chitosan is a linear polysac- charide derived from partial deacetylation of chitin [4,5]. The ability of chitosan to support cell attachment and proliferation is attributed to its chemical properties. The polysaccharide back- bone of chitosan is structurally similar to glycosaminoglycans, the major component of the extracellular matrix of bone and cartilage [6]. Chitosan is considered as an appropriate functional mate- rial for biomedical applications because of high biocompatibility, biodegradability, non-antigenicity and adsorption properties [7,8]. Further, anti-inflammatory or allergic reactions have not been ∗ Corresponding author. Tel.: +91 80 25356590. E-mail address: sailaja.bhattacharya@gmail.com (R.R.N. Sailaja). observed in human subjects following topical application, implan- tation, injection and ingestion [7,8]. An updated review article by Pighinelli et al. [9] suggested the need and importance of natu- ral biopolymer such as chitosan–hydroxyapatite composites which can play a vital role in skeletal reconstruction. However, natural biopolymers such as chitosan have poor load bearing character- istics with rapid degradability. Mechanical properties especially compressive strength is important to tolerate the internal stress till tissue regeneration takes place. Han et al. [10] suggested that alginate–chitosan–hydroxyapatite composite exhibited enhanced mechanical strength due to strong ionic interactions. Further efforts to enhance mechanical strength using biocompatible materials like titania was carried out by Kavitha et al. [11]. It was found that the nanocomposites thus developed showed large surface area with good antibacterial activity. An improvement in com- pressive strength and Young’s modulus was observed by adding a small amount of citric acid due to salting out effect [12]. It has also been envisaged that enhanced chemical bonding with the inorganic material such as nHA will restrict its migration and also reduce tissue damage. Thus, nHA has been blended with chitosan and gelatin to improve mechanical properties [13]. A drastic reduction in mechanical properties was observed for chi- tosan/hydroxyapatite composites as nHA loading increased [14]. Chitosan is a brittle material and hydroxyapatite is also brittle, thus a combination of the two further reduces the mechanical properties. Ai et al. [15] studied the effect of micro and nanosized hydroxyapatite particles in chitosan–starch composites. The com- posites loaded with nanosized particles showed increased modulus values. http://dx.doi.org/10.1016/j.ijbiomac.2014.11.023 0141-8130/© 2014 Elsevier B.V. All rights reserved.