Wet and dry flexural high cycle fatigue behaviour of fully bioresorbable glass fibre composites: In-situ polymerisation versus laminate stacking

Menghao Chen, Jiawa Lu, Reda M. Felfel, Andrew J. Parsons, Derek J. Irvine, Christopher D. Rudd, Ifty Ahmed
2017 Composites Science And Technology  
15 Fully bioresorbable phosphate based glass fibre reinforced polycaprolactone (PCL/PGF) 16 composites are potentially excellent candidates to address current issues experienced with use of 17 metal implants for hard tissue repair, such as stress shielding effects. It is therefore essential to 18 investigate these materials under representative loading cases and to understand their fatigue 19 behaviour (wet and dry) in order to predict their lifetime in service and their likely mechanisms of 20
more » ... failure. This paper investigated the dry and wet flexural fatigue behaviour of PCL/PGF composites 21 with 35% and 50% fibre volume fraction (V f ). Significantly longer flexural fatigue life (p<0.0001) and 22 superior fatigue damage resistance were observed for In-situ Polymerised (ISP) composites as 23 compared to the Laminate Stacking (LS) composites in both dry and wet conditions, indicating that 24 the ISP promoted considerably stronger interfacial bonding than the LS. Immersion in fluid (wet) 25 during the flexural fatigue tests resulted in significant reduction (p<0.0001) in the composites fatigue 26 life, earlier onset of fatigue damage and faster damage propagation. Regardless of testing 27 conditions, increasing fibre content led to shorter fatigue life for the PCL/PGF composites. 28 Meanwhile, immersion in degradation media caused softening of both LS and ISP composites 29 during the fatigue tests, which led to a more ductile failure mode. Among all the composites that 30 were investigated, ISP35 (35% V f ) composites maintained at least 50% of their initial stiffness at the 31 end of fatigue tests in both conditions, which is comparable to the flexural properties of human 32 cortical bones. Consequently, ISP composites with 35% V f maintained at least 50% of its flexural 33 properties after the fatigue failure, which the mechanical retentions were well matched with the 34 properties of human cortical bones. 35 Introduction 43 In recent decades, fully bioresorbable polymer composites with appropriate biocompatibility 44 and mechanical properties has provided an exciting opportunity to replace conventional 45 metal alloy implants, and it has become an active research field because of its excellent 46 potential in the field of hard tissue repair [1, 2]. Bioresorbable fibre reinforced composites 47 have been extensively studied utilising different glass fibre compositions, polymer matrices, 48 fibre architectures and volume fractions. Their mechanical properties range between 200-49 700 MPa flexural strength and 15-25 GPa flexural modulus [3-9]. 50 Polycaprolactone (PCL)/Phosphate based glass fibre (PGF) composites have been 51 investigated for developing fully bioresorbable implants [10-13]. PGFs have the ability to 52 fully dissolve within aqueous media and with adjustable degradation rate [10]. Both PGF 53 and PCL have also been proven to have good biocompatibility and are considered 54 favourable materials for biomedical application [3, 4, 14]. However, achieving satisfactory 55 fibre-matrix interface adhesion and retention, thus appropriate composite mechanical 56 properties for bone fracture fixation has been the main restriction [3, 5, 6, 15]. Studies have 57 recognised the weak fibre-matrix interface is mainly due to poor fibre impregnation, which 58 can result from the high viscosity of the melted matrix involved in traditional laminate 59 stacking (LS) and hot press moulding [5, 13, 16]. To promote a more durable fibre-matrix 60 interface, a novel in-situ polymerisation (ISP) technique has been developed to 61 manufacture PCL/PGF composites in the group [16, 17]. There is solid evidence that 62 suggested that ISP can promote a significantly stronger and more robust fibre-matrix 63 interface than LS, with accordingly higher mechanical properties and prolonged retention of 64 properties during degradation [13, 16-19]. Furthermore, the biocompatibility of the ISP PCL 65 was investigated via Alamar blue assay using osteoblasts derived from human craniofacial 66 bone cells. The results indicated that ISP PCL was highly biocompatible, and the residual 67 monomer (Measured by Nuclear Magnetic Resonance) did not significantly affect the 68 biocompatibility of the composites [19]. 69 The flexural fatigue properties are of vital importance for bone fracture fixation implant 70 applications. The main application for the PCL/PGF composites is as bone fracture fixation 71 devices and has initially focused on bone repair plates, for which the primary loading would 72 the authors knowledge, cyclic fatigue response of the fully resorbable composites has not 105 been explored yet. 106 In this study, PCL/PGF composites with V f of 35% and 50% were produced using LS and 107 ISP techniques. Environmental flexural fatigue tests were performed on these composites 108 in both dry conditions at ambient temperature and in wet conditions immersed in phosphate 109 buffered saline (PBS) solution at 37 °C. The wet conditions were intended to mimic the 110 physiological nature of the human body. Both stiffness reduction and SDC were used as 111 sensitive indicators to monitor the damage initiation of the composites during cyclic loading. 112 The influences of solution immersion, fibre content and fibre-matrix adhesion on the 113 performance of the PCL/PGF composites were investigated by comparing their fatigue 114 behaviour. 115 2. Materials and Methods 116 2.1. Materials 117 Monomer ɛ-caprolactone (97% purity), Sn(Oct) 2 (92.5%-100% purity) and benzyl alcohol 118 (99.8% purity) were obtained from Sigma Aldrich (UK). PCL pallets were also obtained from 119 Sigma Aldrich (UK) and with a molecular weight (M w ) of ~65,000 g/mol. 120 2.2. Phosphate based glass and glass fibre 121 Phosphate based salts were used to produce fully bioresorbable phosphate glass (Sigma 122 Aldrich, see Table 1 for detailed glass formulation). A specific salt mixing and melting 123 scheme was used to produce the phosphate based glass. PGFs were made using in-house 124 designed melt-draw equipment comprised of a furnace (Lenton Furnaces, UK) with a Pt/10% 125 Rh crucible (Johnson Matthey, UK). Phosphate glass and glass fibre mats were stored in a 126 desiccator to minimise any potential moisture adsorption. The fibre mats were then dried in 127 a 50 °C vacuum oven for 24 hours before use. For the detailed manufacturing process and 128 parameters, please refer to the authors' previous paper [13]. 129 2.3. PCL/PGF composites 130 Composites with 35% and 50% fibre volume fraction (V f ) were manufactured via both ISP 131 and LS. Respective composite codes and specifications are listed in Table 2 . 132 2.3.1. Laminate stacking (LS) technique 133 PCL thin films were manufactured and PGF mats were stacked with the thin films to hot 134 compress into composites. Detailed processes can be found in the authors' previous paper
doi:10.1016/j.compscitech.2017.07.006 fatcat:67r44v2otbcvhdqxseomof3nqq