Conventional permanent stent systems for vascular applications are associated with long-term risks, such as restenosis and thrombosis. To overcome these limitations, novel approaches using various biodegradable materials for stent construction have been investigated. In this context, thermal treatment of polymer materials is investigated to adjust the mechanical properties of biodegradable stents. In this work polymeric tubular specimens of biodegradable poly(L-lactide) (PLLA) were extruded and subjected to a molding process using different temperatures above glass transition temperature TG. Physicochemical properties of the molded samples were analyzed using DSC measurements and uniaxial tensile tests. The molding process resulted in a weakening of the PLLA tubular specimens with a simultaneous increase in the degree of crystallinity (χ).
Stents made of biodegradable polymeric materials are currently being investigated as a future alternative to permanent metallic stents. Insufficient mechanical properties of biodegradable polymer stents are a major challenge, as they may lead to mechanical failure or high elastic recoil after implantation into the vessel . Glass transition TG and crystalline microstructure of the applied polymeric materials influence the mechanical properties of the stents. Polymers with TG higher than human body temperature may show little or no plastic deformation prior to failure, although they potentially offer high stiffness. In opposition, polymers with TG below human body temperature could exhibit high plastic deformation associated with low stiffness. The crystallinity and crystalline microstructure of polymers, and thus the mechanical properties, can be altered by subjecting the polymers to deformation processes , which was the focus of the present study.
PLLA mini tubes were extruded and expanded with a blow molding process using different temperatures above TG. This temperature was necessary to get the samples more flexible during expansion. The morphology of the specimens was investigated, the degree of crystallinity χ and TG were determined, and the mechanical properties were subsequently assessed at 37°C in uniaxial tensile tests.
2 Material and methods
2.1 Sample preparation
Stent tubes were manufactured from PLLA (Resomer L210, Evonik Industries, Germany, TG = 60°C, TM = 178°C) with a molecular weight of ∼400.000 g/mol.
PLLA tube samples (nominal diameter d = 1.7 mm, wall thickness 300 μm) were extruded using a MiniLab II HAAKE (Thermo Fisher Scientific, Karlsruhe, Germany). For extrusion a temperature of T = 215°C and a pressure of p = 3.5 bar were adjusted.
2.1.2 Blow molding
Blow molding was performed by means of an air current with a preset pressure of p = 6 bar in a tubular mold to a nominal diameter of d = 2.3 mm.
During blow molding, heating of the PLLA tubes above glass transition temperature TG was performed to improve flexibility of the PLLA molecules. Different temperatures above TG (67/77/87°C) were realized (PLLA 67/77/87). Holding time was t = 30 s. Subsequent to the blow molding process the expanded tubes were quenched to a temperature of T = 25°C.
2.2 Thermal characterization
Melting behaviour of the blow molded samples was measured using a DSC 1 system (Mettler Toledo, Switzerland). During the scans, the specimens were exposed to nitrogen gas. The sample weights were in the range of 2–5 mg, and the heating rate was 10 K/min. The data were analyzed with respect to glass transition (TG), melting temperature (TM) and degree of crystallinity (χ) (see eq. 1). For the latter a value of ΔH100 = 93.7 J/g  for totally crystalline PLLA was used.
2.3 Mechanical characterization
Uniaxial tensile testing was performed with a Zwicki ZN 2.5 (Zwick, Ulm, Germany). Tests were conducted with a 100 N load cell and a crosshead speed of 50 mm/min. During the procedure samples were kept at a constant temperature of 37°C. The tensile force as a function of elongation was measured. Based on these data the elastic modulus (E) was calculated in the range of 0.05–0.25%. Furthermore, the elongation at break (εB) was determined.
3.1 Sample treatment
PLLA tubular specimens were succesfully expanded with all three temperature settings. Figure 1 illustrates the extruded PLLA samples before (top) and after (middle) blow molding. The expanded tubes showed a clear enlargement in radial direction (Figure 1, bottom).
No geometric imperfections or other detrimental defects on the PLLA specimens could be observed.
3.2 Effect of blow molding on physicochemical properties of PLLA tubes
3.2.1 Thermal characterization
DSC measurements were conducted with extruded specimens without molding treatment (PLLA original) in comparison to the blow molded samples PLLA 67, PLLA 77, and PLLA 87 (Figure 2). From the extruded PLLA tubes a degree of crystallinity of χ = 18.2 ± 1.0% was determined. Glass transition temperature TG of PLLA original was TG = 60.0 ± 0.3°C. In all samples cold crystallization was observed (data not shown). The crystallinity of PLLA 67 and PLLA 77 did not differ from χ of the PLLA sample without treatment.
However, for PLLA 87 a χ = 32.4 ± 3.0% was observed, an almost two-fold increase compared to PLLA original. No change in TG was detected in any sample.
3.2.2 Mechanical characterization
From the uniaxial tensile tests stress strain curves were derived as shown in Figure 3. Without thermal treatment using blow molding, an elongation at break of εB = 20 ± 3% for PLLA could be determined. With increase of the heating temperature of blow mold specimens the value of εB increased.
PLLA 67 reached a value of εB = 56 ± 5% and the value of εB of PLLA 87 increased to εB = 120 ± 11%. However, a slight tendency to advanced values in the elastic modulus could be seen. With increasing of the blow mold temperature the modulus increases (Table 1).
|Modulus (MPa)||εB (%)||TG (°C)||χ (%)|
|PLLA original||2166 ± 158||20 ± 3||60.2 ± 0.3||18.2 ± 1.0|
|PLLA 67||2271 ± 168||56 ± 5||62.5 ± 0.1||17.1 ± 0.4|
|PLLA 77||2494 ± 46||79 ± 8||62.0 ± 0.1||17.7 ± 0.3|
|PLLA 87||2591 ± 72||120 ± 11||62.3 ± 0.3||32 ± 3|
PLLA is a semicrystalline polymer consisting of amorphous and crystalline regions. Renouf-Glauser et al. showed that εB of the amorphous part is much higher than in the crystalline phase . Hence, the amorphous areas are responsible for flexible behavior. In the study we expanded tubular specimens above TG. Thus, the amorphous regions of the samples were in the liquid phase, and the tubes could be molded without the formation of cracks.
The increase of elongation at break εB may be caused by the orientation of the molecular chains during the expansion process . This is confirmed by the DSC measurements where no change in crystallinity occurred for any of the blow molded samples (Table 1).
Nevertheless, the modulus increased, indicating an increased stiffness of the specimens. The thermal properties also confirmed this interpretation, as the crystallinity is higher along with elevated heating temperatures. Here, annealing of the samples predominates, so crystal growth of the PLLA molecular chains occurred (Table 1).
With the extrusion and blow molding process cold crystallization could not be eliminated. Manufacturing of PLLA tubes with reproducible mechanical properties remains challenging.
Financial support by the Federal Ministry of Education and Research (BMBF) within RESPONSE “Partnership for Innovation in Implant Technology” is gratefully acknowledged.
Research funding: Financial support by the Federal Ministry of Education and Research (BMBF) within RESPONSE “Partnership for Innovation in Implant Technology”. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent is not applicable. Ethical approval: The conducted research is not related to either human or animal use.
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