Over decades, textile structures are applied with great success in the field of Biomedical Engineering. In combination with the development of novel polymeric materials and the progresses in the field of textile manufacturing processes textile structures will also offer great potential for the development of innovative treatment methods in the future. One goal for the development of novel mesh implants is the adaption of the implant’s mechanical properties to the physiological properties of the native tissue. Following this approach there is a multitude of interesting applications where implant structures with a defined elastic behaviour are required. Textile structures made out of elastic filaments can fulfil these requirements. One main field of application for mesh implants are tissue fractures (hernias). As a therapy, warp knitted mesh structures are applied in order to reinforce the native tissue . Currently available mesh implants made from polypropylene, polyester or PVDF are successfully applied as a reinforcement of minimal elastic tissue like in the inguinal region. The application in areas with great anatomical mobility leads to significant complications. The reason is the low elastic elongation rate of the so-far applied meshes. Concerning the elastic requirements of mesh implants, it can be found in a study by Macciej Smietanski who measured the elongation of different abdominal vectors for natural movements . According to this study, elongation rates of more than 25% occur in the abdominal region under physiological compressive and tensile loads. For current mesh implants the increase in elongation under a tensile load is almost exclusively caused by a slot-shaped deformation of the pores with a tapering in the mesh width and an elastic elongation under 10%.
The comparison between the tensile strength and the elongation at break shows that the tensile strength increases up to threefold with increasing drawing rates whereas the elongation at break decreases. This behaviour can be explained with the increasing orientation of the macromolecules along the fibre axis. The highest tensile strengths are reached for high drawing rates at drawing temperature of 80°C. For the harder PCU with a shore 65D the decrease of elongation at break is more distinctive than for the softer PCU with a shore 95A. It is not possible to adjust a drawing rate of 3.5 for the harder PCU under room conditions due to filaments breakage. For higher drawing temperatures the mobility of the molecule chains increase which enables drawing rates of 3.5 even for the harder PCU. Furthermore it is shown, that the thermosetting after drawing shows only minor influence on the tensile strength and the elongation at break. In conclusion, the mechanical properties tensile strength and elongation at break can be specifically adjusted by the process parameters drawing and drawing temperature.
Considering the test results for the cyclic tensile tests it is shown that the harder PCU has a significantly lower elastic elongation rate of only 30% (after applying 60% elongation). The softer PCU-filaments show very well the elastic properties up to 56% of elastic elongation (after applying 60% elongation). A potential explanation for this behaviour is seen in the arrangement of hard-segment-domains which is fostered with increasing hard segment ratio. By applying an elongation some of the hard segments are drawn out of the domains which results in a plastic elongation rate. In contrast to the properties tensile strength and elongation at break it must be noted that the elastic behaviour can only be influenced to a limited degree by the process parameters drawing rate and drawing temperature. However, qualitatively one result which was found when investigating the tensile strength and elongation at break also applies to the results of the elastic behaviour: The level of force increases with increasing drawing rates and the highest level of force is reached for high drawing rates at a drawing temperature of 80°C. Again, this behaviour can be explained with the increasing orientation of the macromolecules along the fibre axis with increasing drawing rates. Both in the case of the tensile tests and in case of the cyclic tensile tests, thermosetting shows minor influence on the fibre properties.
Materials and methods
The wording “elasticity” is often used in different meanings. Thus a short definition of the most important terms which are used in this study to describe the elastic behaviour of textile mesh structures will be given. Initially, the elastic and plastic elongation rates are distinguished. When a test load is applied to the sample, a length variation of the sample occurs in load direction depending on the state of the sample and the test load. This length variation is the total elongation of the sample, which consists of the elastic and plastic elongation rate. The elastic elongation rate is the part of the length variation by which the sample recovers in the direction of the original state after unloading. Therefore it is also called the reversible elongation rate. The plastic elongation rate is the part of the elongation change by which the sample stays deformed permanently after unloading.
The polymer group of polyurethanes (Figure 3) is used for the development of elastic polymer fibres. This polymer group stands out through high elastic elongation rates where the properties can be adjusted through the state and the rate of the applied monomer units. Through this it is possible to synthesize hard and inelastic polyurethanes as well as soft and elastic ones . Processes described in the literature in which polyurethanes are spun into fibres use solved polyurethanes, which can be problematic for the field of medical applications due to the harmful solvents applied. However, thermoplastic polyurethanes (TPU) can be processed solvent-free and are therefore eminently suitable for medical applications. TPU are cross-linked with allophanate groups that can be cleaved reversibly at temperatures above 150°C . They can be processed into continuous fibres through melt spinning. Since the 1960s, different TPU types have been developed, which differ from each other especially in their hydrolytic stability.
Poly(ester)urethanes were already used clinically in the 1960s. Although polyurethanes are significantly blood and skin friendly it shows that ester-based polyurethanes are extremely hydrolytically instable. The development of poly(ether)urethanes which are more stable against hydrolytic degradation enabled the usage of TPUs in different implants such as vascular implants, artificial heart valves, blood pumps and catheters [3, 4]. For some years now, poly(carbonat)urethanes (PCU) have been available that have the highest resistance against hydrolytic degradation . For this reason different PCU types are analysed in this study in respect to their processing behaviour and their elastic properties. In the first pre trials with aromatic and aliphatic PCU it was shown that aromatic PCU expose better elastic properties. In the following, the results of the trials for the soft aromatic PCU (Carbothane AC-4095A shore 95 A; Lubrizol Corp., Wickliffe, OH, USA) as well as for the hard PCU (ChronoFlex C65D shore 65 D; AdvanSource Biomaterials Corp., Wilmington, MA, USA) are presented. Both materials are USP class VI certified.
The spinning tests are performed on a single-screw extruder melt spinning machine by Fourné Polymertechnik GmbH, Alfter-Impekoven, Germany. After the fibre formation, the filament is cooled in a water bath before it is drawn between rotating cylinders (godets) of different speeds. For this study, the drawing rate as well as the drawing temperature was varied. Furthermore, the influence of thermosetting on the filament properties was investigated (Figure 4). The as-spun filaments were characterized via simple tensile tests as well as via cyclic tensile tests with an applied elongation of 60%, a deformation speed of 500 mm/min and a cycle number of 10.
The study was supported by the Federal Ministry of Education and Research – BMBF, Berlin, Germany (Grant number: 360665). The authors carry the responsibility for the content of this publication.
Smietanski M, Bury K, Tomaszewska A, Lubowiecka I, Szymczak C. Biomechanics of the front abdominal wall as a potential factor leading to recurrence with laparoscopic ventral hernia repair. Surg Endosc 2012;26:1461–7.Web of SciencePubMedGoogle Scholar
Plank H. Kunststoffe und Elastomere in der Medizin. Stuttgart, Berlin, Köln: Kohlhammer, 1993.Google Scholar
Wintermantel E, Ha SW. Medizintechnik – Life Science Engineering. Berlin, Heidelberg: Springer Verlag, 2009.Google Scholar
Wintermantel E, Ha SW. Biokompatible Werkstoffe und Bauweisen. Berlin, Heidelberg: Springer Verlag, 1996.Google Scholar
About the article
Published Online: 2014-10-08
Published in Print: 2014-09-01