Mechanical response of bioresorbable poly (Glycolide-co-caprolactone) suture under monotonic and non-monotonic loadings / Low Yan Jie

Low , Yan Jie (2024) Mechanical response of bioresorbable poly (Glycolide-co-caprolactone) suture under monotonic and non-monotonic loadings / Low Yan Jie. PhD thesis, Universiti Malaya.

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Abstract

Bioresorbable sutures are crucial to alleviate any forces acting on the wound edges and to provide sufficient mechanical support to the injured tissues throughout the wound healing process. Choosing the right suture can improve wound recovery and the quality of life for patients. However, the failure of surgical sutures can occur due to suture breakage, knot breakage, knot untying, and knot slippage, which are related to the stress state in the suture. Therefore, it is necessary to understand the mechanical behavior of sutures to accurately predict their stress state during service. Along these lines, the present work investigates the mechanical responses of non-knotted and knotted poly(glycolide-co-caprolactone) (PGCL) surgical sutures under monotonic and non-monotonic loadings. The first part of this study focuses on the stress-strain response of the PGCL suture under uniaxial monotonic, cyclic, and stress relaxation loadings. Results indicated that the PGCL suture exhibits non-linear stress-strain response and strong inelastic behaviors such as hysteresis, stress-softening, permanent set, and viscoelasticity. Increasing the strain rate from 0.0001 s-1 to 0.1 s-1 resulted in notable improvements: load at break from 22.47 N to 30.44 N, overall stiffness from 3.15 N/mm to 3.92 N/mm, ultimate tensile strength from 288.50 MPa to 383.48 MPa, initial modulus from 232.98 MPa to 367.34 MPa. The gauge length was observed to influence both monotonic and cyclic responses but did not impact the stress relaxation response. In the case of knotted samples, the results showed that the mechanical behavior of all knotted samples depended on the strain rate, regardless of the number of throws. Knots significantly affected the stress-strain response of the suture, resulting in lower load and strain at break. Decreasing the strain rate in knotted samples resulted in increased elongation at break, load at break, ultimate tensile strength, strain at break, and toughness, but decreased overall stiffness and initial modulus. Additionally, an increased number of throws resulted in decreased elongation, load, stiffness, ultimate tensile strength, strain at break, and toughness, indicating the role of stress concentration in the suture due to the presence of a knot. X-ray powder diffraction (XRD) spectra analysis showed that PGCL sutures undergo strain-induced crystallization, which is dependent on the strain level and loading and relaxation durations. The inelastic behaviors observed in PGCL sutures are due to the combined effect of stress relaxation and SIC. A schematic illustration of the conformational change of polymer chains in PGCL sutures was proposed to describe the relationship between stress relaxation and SIC during loading and relaxation processes. The second part of the work focuses on the stress analysis of the PGCL surgical suture using finite element simulation. PGCL suture was assumed to obey linear viscoelastic constitutive behavior, thus can be described by a simple Prony series. The relevant material parameters were subsequently identified from relaxation data, and two sets of Prony series parameters were used to study the phenomenon of creep in surgical sutures, which leads to knot untying and slippage. A single strand solid model was employed to simulate the creep of PGCL suture under different sustained loads, enabling a comparison of creep strain and rate. The results highlighted the significance of these factors, even in non-knotted configurations.

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