PSI - Issue 28

Shirsha Bose et al. / Procedia Structural Integrity 28 (2020) 843–849 S. Bose et. al/ Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Collagen comprises of one-third of human body proteins and contribute significantly to the mechanical strength and structural integrity of the body (Kadler et al. , 2007; Fratzl, 2008). It has a very complex hierarchical structure, with collagen molecules forming the lowest level in this hierarchy. The molecules bind up to form the next level of hierarchy – collagen fibrils – which combine to form the collagen nanofibers (Gautieri et al. , 2011). Collagen films find a wide range of applications in biomedical field such as in tissue engineering (Ber et al., 2005; Liu et al. , 2012), dural substitutes (Collins et al. , 1991), flexible electronics (Moreno et al. , 2015) etc. thanks to their biocompatibility, biodegradability, flexibility (Fratzl, 2008). Thus, while characterizing the mechanical properties of collagen, it should be tested in aqua , to reproduce some of in-vivo conditions. Various collagenous materials demonstrated a considerable change in their mechanical properties under the influence of water (Yuan and Verma, 2006; Yang et al. , 2015; Wang et al. , 2015; Bose et al., 2020a, b). The modulus obtained for dry and wet/submerged collagenous materials differed by orders of magnitude (Yuan and Verma, 2006; Bose et al., 2020a, b). Such differences in the mechanical properties suggest that designing collagenous materials based on the data for in aqua or submerged testing environment could help to define the best usability for these materials. Hence, a study of the fracture behaviour of collagen should be preferably conducted in aqua . Deformation behaviour and crack-path analysis for tough collagenous tissues such as bone (Nalla et al. , 2005; Wang et al. , 2020) and dentin (Nalla et al., 2003) differ significantly from those of soft collagenous materials (Yang et al. , 2015; Pissarenko et al. , 2020). In former, crack initiation and propagation are dominated by kinks and deflections, crack bridging or micro-cracking (Nalla et al., 2003; Wang et al. , 2020). However, for soft collagenous materials, such as skin, the crack propagation is accompanied by its opening and blunting (Yang et al. , 2015; Pissarenko et al. , 2020). Collagen films in-aqua demonstarte mechanical properties similar to those of soft materials (Bose et al., 2020b). This work aims to analyze and predict the fracture behaviour of pure collagen films using single-edged notched tension (SENT) specimens, subjected to two different environmental loading conditions – in-air and in-aqua . 2. Materials and Methods 2.1. Materials Type I collagen (bovine achilles tendon) was procured from Sigma-Aldrich (St. Louis, USA) and stored at 2-6 ⁰ C before use. Acetic acid (ACS reagent, ≥ 99.8%) was also purchased from Sigma-Aldrich. Distilled water was produced in the laboratory. 2.2. Sample Preparation Collagen films were produced following the protocol in Bose et al., 2020a. In brief, collagen solution (8 mg/ml) was prepared using 0.05M acetic acid solution and stored at 2-6 ⁰ C for 48 hours, followed by stirring and centrifugation. Next, the solution was degassed to remove the bubbles entrapped in it. Finally, it was casted in a polystyrene (PS) petri-dish and left in a fume hood to dry at ambient conditions. For fracture testing, SENT specimens were prepared by introducing a sharp notch with a scalpel on one side. The films were cut into rectangular strips with a gauge length ሺ ܮ ሻ and width ሺܹሻ of 20 mm and 22 mm, respectively, with a ligament length ሺܾሻ of 11 mm (Fig. 1). The collagen films had a thickness of about 50 ± 10 µm.