Determination of micromechanical properties of biocompatible vascular grafts by nanoindentation

In this study, it is aimed to determine and compare the mechanical strength properties in nanoscale of electrospun nanofibers which are manufactured in non-woven structure and suitable for use as vascular grafts. Vascular grafts are woven or nonwoven tubular structures that are used to surgically replace damaged vessels in traumas resulting in vessel damage, so that blood flow can reach the tissue involved. Although limited number of data, it is known in the literature that vascular bypass graft procedures are applied to approximately 600,000 patients per year in the U.S. If the heart disease occurrence frequency is approximately the same in our country, it can be estimated that potential of 100,000 patients / year in Turkey by using population ratios between two countries. Although autogenous vascular grafts are preferred in such reconstructive arterial surgeries, it is sometimes difficult to have such tissues in the patient. In addition, removal of the vascular graft requires additional cost and time. Additional time and additional surgical procedures also increase the mortality rate of the patients. Although some synthetic vascular grafts such as polytetrafluoroethylene (PTFE), polyethylene terephthalate and polyurethane are used today, clot formation is not suitable for substituting vessels smaller than 6 mm in diameter for any of these grafts due to incompatibility problems. For this reason, vessel grafts with different compositions are needed. In addition to the biocompatibility of the vascular graft, it must also meet the conditions of visceral fatigue and viscoelasticity provided by collagen and elastin in the natural vascular structure, in terms of long-term stability. In vascular applications, mechanical strength is very important and the material must withstand pulsed circular stress and 80-120 mmHg physiological pressure. In this study, poly (L-lactate) and poly (ε-caprolactone), two of the few polymers approved for medical use by the US Food and Drug Administration (FDA), were used as biocompatible vascular grafts. Poly (ε-caprolactone-co- lactate) copolymers have been synthesized using different monomer-to-dimer ratios (70-30; 60-40; 50-50 ratio by mass), with the flexibility and strength that can be used as vascular grafts. Also blends were prepared at the same ratios. Electrospinning technique, which is suitable for the production of nanofibers, has been used for the preparation of vascular grafts. Vascular grafts have been produced in such a way that suitable for vascular grafting (but not in tubular shape), by optimizing the process parameters (voltage to be applied, solution concentration, injection rate, tip-collector distance) for electrospinning and using the specified copolymers and blends. Macromechanical or micromechanical tests are not applicable to high technology products due to applied load, application area and relatively low precision results. For example, the mechanical properties (strength, stiffness, etc.) of an arterial graft cannot be accurately measured with the loads of the order of kilograms applied in conventional methods. For this reason, nanoindentation technique was used to determine the mechanical properties of the generated vessel grafts in the nano-scale. The relationship between the structure of the material and its mechanical properties is most important parameter during the selection of the material. For example, the nanoindentation process using the device performed in the form of indentation of the sample with a small load at nN and recording the amount of charge and displacement continuously during this time is an approved technique for the determination of the local mechanical properties of a material using load-displacement curves. The nanoindentation performed using atomic force microscopy (AFM) -force spectroscopy can apply a force ranging from a few nN to pN. This method has several advantages over standard methods known in the art, with the ability to perform multiple analyzes on the same sample, and with the measurements being carried out in a very narrow local area (in other words, with a small contact area between the sample and the probe). In this study, a new method for examining the mechanical properties of vascular grafts obtained by electrospinning method is presented. For this purpose, the nanomechanical test method which is an alternative to macromechanical tests, is proposed in determining the mechanical strengths of the nanofibers obtained from electrospinning. The relationship between the structural properties of the material and nanomechanical behaviors has been established. Surface and structure characterizations of the obtained samples were performed using Scanning Electron Microscopy (SEM) and image processing software. It was found that the diameter of 830 nm and a compressive elastic modulus (E) of 42 kPa is obtained for the poly(L-lactate) which is one of the main structural component of blends and copolymers in this study. Also, it was reported 450 kPa E for the poly (ε-caprolactone), which has a diameter of 135 nm. By increasing the amount of poly (ε-caprolactone) in the mixture and the copolymer, a compressive elastic modulus of 7 MPa in the blends and 17 MPa in the copolymers was obtained. The diameters of the mixture nanofibers were around 700-800 nm, while the copolymers had low diameters of 55-175 nm and higher mechanical strength values.