Startseite The effect of thermal treatment on the mechanical properties of PLLA tubular specimens
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The effect of thermal treatment on the mechanical properties of PLLA tubular specimens

  • Daniela Arbeiter EMAIL logo , K. Schümann , O. Sahmel , T. Eickner , K.-P. Schmitz und N. Grabow
Veröffentlicht/Copyright: 30. September 2016
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Current Directions in Biomedical Engineering
Aus der Zeitschrift Current Directions in Biomedical Engineering Band 2 Heft 1

Abstract

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).

Keywords: blow molding; extrusion; PLLA; thermal and mechanical properties

1 Introduction

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 [1]. 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 [2], 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.

2.1.1 Extrusion

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 [3] for totally crystalline PLLA was used.

(1)χS=Δ⁢HSΔ⁢H100⋅100

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 Results

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).

Figure 1: PLLA tubes manufactured with extrusion process (top) and after expansion with blow molding (middle) at T = 77°C (PLLA 77). Bottom: enlarged image of the samples.
Figure 1:

PLLA tubes manufactured with extrusion process (top) and after expansion with blow molding (middle) at T = 77°C (PLLA 77). Bottom: enlarged image of the samples.

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.

Figure 2: Degree of crystallinity and glass transition temperature of extruded PLLA tubes and subsequently blow molded specimens as measured with DSC.
Figure 2:

Degree of crystallinity and glass transition temperature of extruded PLLA tubes and subsequently blow molded specimens as measured with DSC.

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.

Figure 3: Stress strain curves of blow molded PLLA samples compared with original extruded PLLA samples measured with uniaxial tensile test.
Figure 3:

Stress strain curves of blow molded PLLA samples compared with original extruded PLLA samples measured with uniaxial tensile test.

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).

Table 1:

Physicochemical and mechanical properties of the tested PLLA specimens.

Modulus (MPa)εB (%)TG (°C)χ (%)
PLLA original2166 ± 15820 ± 360.2 ± 0.318.2 ± 1.0
PLLA 672271 ± 16856 ± 562.5 ± 0.117.1 ± 0.4
PLLA 772494 ± 4679 ± 862.0 ± 0.117.7 ± 0.3
PLLA 872591 ± 72120 ± 1162.3 ± 0.332 ± 3

4 Discussion

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 [4]. 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 [2]. 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.

Acknowledgement

Financial support by the Federal Ministry of Education and Research (BMBF) within RESPONSE “Partnership for Innovation in Implant Technology” is gratefully acknowledged.

Author’s Statement

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.

References

[1] Soares JS, Moore, JE. Biomechanical challenges to polymeric biodegradable stents. Ann Biomed Eng. 2016;44:560–79.10.1007/s10439-015-1477-2Suche in Google Scholar PubMed

[2] Kumaraswamy G, Kornfield JA, Yeh F, Hsiao BS. Shear-enhanced crystallization in isotactic polypropylene. 3. Evidence for a kinetic pathway to nucleation. Macromolecules. 2002;35:1762–9.10.1021/ma0114180Suche in Google Scholar

[3] Fischer EW, Sterzel HJ, Wegner G. Investigation of the structure of solution grown crystals of lactide copolymers by means of chemical reactions. Kolloid.-Z und Z Polymer. 1973;251:980–90.10.1007/978-3-642-47050-9_24Suche in Google Scholar

[4] Renouf-Glauser AC, Rose J, Farrar DF, Cameron RE. The effect of crystallinity on the deformation mechanism and bulk mechanical properties of PLLA. Biomaterials. 2005;26:5771–82.10.1016/j.biomaterials.2005.03.002Suche in Google Scholar PubMed

Published Online: 2016-9-30
Published in Print: 2016-9-1

©2016 Daniela Arbeiter et al., licensee De Gruyter.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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https://doi.org/10.1515/cdbme-2016-0009
Schlagwörter für diesen Artikel
blow molding; extrusion; PLLA; thermal and mechanical properties
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  138. Numerical simulation of mechanically stimulated bone remodelling
  139. Conversion of engineering stresses to Cauchy stresses in tensile and compression tests of thermoplastic polymers
  140. Numerical examinations of simplified spondylodesis models concerning energy absorption in magnetic resonance imaging
  141. Principle study on the signal connection at transabdominal fetal pulse oximetry
  142. Influence of Siluron® insertion on model drug distribution in the simulated vitreous body
  143. Evaluating different approaches to identify a three parameter gas exchange model
  144. Effects of fibrosis on the extracellular potential based on 3D reconstructions from histological sections of heart tissue
  145. From imaging to hemodynamics – how reconstruction kernels influence the blood flow predictions in intracranial aneurysms
  146. Flow optimised design of a novel point-of-care diagnostic device for the detection of disease specific biomarkers
  147. Improved FPGA controlled artificial vascular system for plethysmographic measurements
  148. Minimally spaced electrode positions for multi-functional chest sensors: ECG and respiratory signal estimation
  149. Automated detection of alveolar arches for nasoalveolar molding in cleft lip and palate treatment
  150. Control scheme selection in human-machine- interfaces by analysis of activity signals
  151. Event-based sampling for reducing communication load in realtime human motion analysis by wireless inertial sensor networks
  152. Automatic pairing of inertial sensors to lower limb segments – a plug-and-play approach
  153. Contactless respiratory monitoring system for magnetic resonance imaging applications using a laser range sensor
  154. Interactive monitoring system for visual respiratory biofeedback
  155. Development of a low-cost senor based aid for visually impaired people
  156. Patient assistive system for the shoulder joint
  157. A passive beating heart setup for interventional cardiology training
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Heruntergeladen am 8.9.2025 von https://www.degruyterbrill.com/document/doi/10.1515/cdbme-2016-0009/html
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