PSI - Issue 28

Anurag Singh et al. / Procedia Structural Integrity 28 (2020) 2218–2227 Anurag Singh/ Structural Integrity Procedia 00 (2019) 000–000

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1. Introduction Patients who are in an immediate need for tissue replacement; tissue engineering provides them with an opportunity for faster rehabilitation. (Cascalho and Platt 2006; Ogle, Cascalho, and Platt 2004) In case of soft tissues, proposed implants consisting of a biodegradable scaffold which temporarily replaces the biomechanical functions of tissues and progressively regenerate and support the loads imposed by the body. (A. C. Vieira et al. 2011; André C. Vieira, Guedes, and Tita 2015) During the rehabilitation period, the rate of degradation of the scaffold must match the rate of recovery of tissue, as shown in figure 1; to avoid the stress shielding. Scaffold and tissue in the repair should have similar mechanical behaviour, endurance strength and creep resistance. Thus protecting the new tissue from active mobilisation and subjecting it to gradual exposure to loading so that that tissue can regenerate more effectively and naturally. The focus of this work is on ACL; ACL is one of the four ligaments of the knee, which keeps the knee stable. ACL is a dense and complex structure capable of supporting multiaxial stresses and different tensile strains (Strocchi et al. 1992). Besides helical, the collagenous network is planar, parallel or twisted from the femoral attachment site to the tibia attachment site (Silver 1994). The basic building block of the ACL matrix is composed of collagen fibres, which is responsible for the tensile strength of the ligament; figure 2 shows the structure of the ACL. Similarly, to mimic the properties of collagen fibres; this research proposes PGLA fibres as a potential element for the composite scaffold in the form of a cord. PGLA is an aliphatic polyester; aliphatic polyesters have hydrolysable functions, such as ester, amide and urethane, or polymers with carbon backbones. In the case of hydrolytic degradation, the ester group in the molecular structure is to blame. Figure 3 shows the synthesis and molecular structure of PGLA. When the water molecule attacks the ester group, it degrades into shorter chains containing carboxylic end group leading to chain scission (Anurag Singh n.d.; Gunatillake, Adhikari, and Gadegaard 2003; Pandey et al. 2005; A C Vieira, Guedes, and Marques 2009), Chain scission leads to autocatalysis and results in a reduction of the plastic flowability (Djemai et al.). This reduction of plastic flowability results in changing of ductile behaviour into the brittle behaviour or increase in the brittleness of already brittle polymer (C. Vieira A. et al. 2012). Many tissue engineering application uses biodegradable polymers (Langer 1998), (Shulamit Levenberg and Robert Langer 2004). PGA is hydrophilic and has more tendency to swell; thus, a high rate of degradation causes it to lose the mechanical properties in two to four weeks (Chen and Wu 2005; Ma and Langer 2011). PGA has a high degree of crystallinity; it degrades into glycolic acid in in-vivo condition. Glycolic acid is nontoxic and further degrades into carbon dioxide and water, which are easily excreted by the body (Middleton and Tipton 2000; Takahashi 2000). Viera et al. explain the degradation of hydrophilic polymers, as degradation first occurs in the amorphous region as the large volume of water can be swelled in these regions compared to the crystalline region due to the negative gradient that exists from the surface to centre. On reaching the saturation point, water lowers the glass transition temperature while making the polymer soft [17]. Hydrolytic degradation is dependent on the capacity of the polymer to absorb water and can significantly affect the rate of degradation.

Figure 1: Rate of decay of scaffold and rate of growth of tissue

Figure 2: Basic building blocks of ligament