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Wu M, Sun D, Zhang T, Zhou C, Zhang B. Study on the Function of Conveying, Kneading Block and Reversing Elements on the Mixing Efficiency and Dispersion Effect inside the Barrel of an Extruder with Numerical Simulation. Foods 2023; 12:3503. [PMID: 37761212 PMCID: PMC10528309 DOI: 10.3390/foods12183503] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/19/2023] [Revised: 09/15/2023] [Accepted: 09/18/2023] [Indexed: 09/29/2023] Open
Abstract
In order to better understand the extrusion process mechanism of plant protein inside a barrel, the parameter changes and flow characteristics of fluids under conveying, kneading block and reversing elements were investigated with numerical simulation. The results showed that the shear rate increased obviously with the increase in pitch; the shear rate value of the reversing element was larger, while that of the kneading block was the opposite. The screw combinations of conveying, kneading blocks and reversing elements all have a certain degree of mixing effect on the particles, and the reduction in pitch can effectively increase the mixing effect of the particles. The conveying element can provide a relatively constant acceleration for the particles, due to the pumping capability and pressure buildup as the pitch increases. The kneading block and the reversing element can increase the leakage flow between the discs and backflow, resulting in an extension of the residence time distribution that facilitates fluid interaction in the barrel and improves the dispersion of the particles. The restraint by the reversing element on the particles is obviously weaker than that of the kneading block and shows a higher particle mixing degree. Overall, the influence of different elements on the flow condition, mixing degree and residence time is significantly different, which improves the process controllability and provides references for potential applications to meet multiple demands.
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Affiliation(s)
- Min Wu
- College of Engineering, China Agricultural University, No. 17 Qinghua East Road, Haidian District, P.O. Box 50, Beijing 100083, China
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Matić J, Stanković-Brandl M, Bauer H, Lovey J, Martel S, Herkenne C, Paudel A, Khinast J. Pharmaceutical hot melt extrusion process development using QbD and digital twins. Int J Pharm 2023; 631:122469. [PMID: 36509223 DOI: 10.1016/j.ijpharm.2022.122469] [Citation(s) in RCA: 1] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/21/2022] [Revised: 11/28/2022] [Accepted: 12/04/2022] [Indexed: 12/14/2022]
Abstract
Pharmaceutical product development guided by Quality by Design (QbD) is based on a complete understanding of the critical process parameters (CPPs) that are important for achieving the desired product critical quality attributes (CQAs). The effect of process settings, such as the screw speed, the throughput, the barrel temperature, and the screw configuration, is a well-known factor in the setup of pharmaceutical hot melt extrusion (HME) processes. A CPP that has not yet been extensively researched is the type of cross-section geometry of the screw elements. Typically, pharmaceutical extruders have double-flighted screw cross-sections, with some elements having a single- or triple-flighted element section. The exception is a NANO16 extruder from Leistritz, with all screw elements having a triple-flighted screw geometry. We investigated the process setup and scale-up to a double-flighted extruder experimentally and in silico via a digital twin. Two formulations were processed on a NANO16 extruder and virtually transferred to a ZSE18 double-flighted co-rotating twin-screw extruder. Detailed smoothed particle hydrodynamics simulations of all screw elements available from both extruders were performed, and their efficiency in conveying, pressure build-up, and power consumption were studied. Reduced-order 1D HME simulations, which were carried out to investigate the process space and scalability of both extruders, were experimentally validated.
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Affiliation(s)
- Josip Matić
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria.
| | | | - Hannes Bauer
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Jessica Lovey
- Debiopharm Research and Manufacturing, Rue du Levant 146, 1920 Martigny, Switzerland
| | - Sophie Martel
- Debiopharm Research and Manufacturing, Rue du Levant 146, 1920 Martigny, Switzerland
| | - Christophe Herkenne
- Debiopharm Research and Manufacturing, Rue du Levant 146, 1920 Martigny, Switzerland
| | - Amrit Paudel
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria; Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
| | - Johannes Khinast
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria; Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
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3
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Cheng W. Comparison of mixing mechanism under different rotating directions in a horizontal self-cleaning differential rotating twin-shaft kneader. CHEMICAL ENGINEERING JOURNAL ADVANCES 2023. [DOI: 10.1016/j.ceja.2023.100462] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/30/2023] Open
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4
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Detecting mixing barriers in Twin-Screw extruder elements via Lagrangian Coherent Structures. Chem Eng Sci 2022. [DOI: 10.1016/j.ces.2022.118069] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
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5
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Shamsuri AA, Abdan K, Md. Jamil SNA. Polybutylene succinate (PBS)/natural fiber green composites: melt blending processes and tensile properties. PHYSICAL SCIENCES REVIEWS 2022. [DOI: 10.1515/psr-2022-0072] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
Abstract
An increase in the environmental consciousness at present has enhanced the awareness of researchers in utilizing biodegradable materials for the production of environmentally friendly products. Currently, biodegradable polymers, for example, polylactic acid, polybutylene succinate, polycaprolactone, etc., can be utilized as matrices to produce green composites. Meanwhile, natural fibers have been used as fillers for green composites as they are biodegradable and renewable. In this brief review, the physicochemical properties of selected biodegradable polymer, specifically polybutylene succinate, are demonstrated. Moreover, examples of natural fibers that are usually used to produce green composites are also shown. Additionally, practical methods employed for the preparation of green composites were exposed. The tensile properties of green composites, such as the tensile strength, tensile modulus, and elongation at break at different loadings of natural fibers, are also briefly reviewed. The information obtained in this review provides detailed differences in the preparation methods of green composites. In addition, this brief review supplies a clearer comprehension of the tensile properties of green composites for the usage of semistructural and packaging applications.
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Affiliation(s)
- Ahmad Adlie Shamsuri
- Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products , Universiti Putra Malaysia , UPM Serdang 43400 , Selangor , Malaysia
| | - Khalina Abdan
- Laboratory of Biocomposite Technology, Institute of Tropical Forestry and Forest Products , Universiti Putra Malaysia , UPM Serdang 43400 , Selangor , Malaysia
| | - Siti Nurul Ain Md. Jamil
- Department of Chemistry, Faculty of Science , Universiti Putra Malaysia , UPM Serdang 43400 , Selangor , Malaysia
- Centre of Foundation Studies for Agricultural Science , Universiti Putra Malaysia , UPM Serdang 43400 , Selangor , Malaysia
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6
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Dong T, Wu J. Simulation of
non‐Newtonian
flows in a partially filled twin‐screw extruder by smoothed particle hydrodynamics. POLYM ENG SCI 2022. [DOI: 10.1002/pen.25886] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/07/2022]
Affiliation(s)
- Tianwen Dong
- Jiangxi Province Key Laboratory of Polymer Preparation and Processing Shangrao Normal University Shangrao China
| | - Jianchun Wu
- Jiangxi Province Key Laboratory of Polymer Preparation and Processing Shangrao Normal University Shangrao China
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7
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Cheng W, Xin S, Chen S, Zhang X, Chen W, Wang J, Feng L. Hydrodynamics and mixing process in a horizontal self-cleaning opposite-rotating twin-shaft kneader. Chem Eng Sci 2021. [DOI: 10.1016/j.ces.2021.116700] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022]
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Bordawekar S, Diwan M, Nere NK. Positioning for a sustainable future—Role of chemical engineers in transforming pharmaceutical process development. AIChE J 2021. [DOI: 10.1002/aic.17364] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/31/2022]
Affiliation(s)
| | - Moiz Diwan
- Process R&D, AbbVie North Chicago Illinois USA
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9
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Matić J, Alva C, Eder S, Reusch K, Paudel A, Khinast J. Towards predicting the product quality in hot-melt extrusion: Pilot plant scale extrusion. INTERNATIONAL JOURNAL OF PHARMACEUTICS-X 2021; 3:100084. [PMID: 34159312 PMCID: PMC8193368 DOI: 10.1016/j.ijpx.2021.100084] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 05/16/2021] [Accepted: 05/17/2021] [Indexed: 11/25/2022]
Abstract
Following our study on the impact of hot melt extrusion (HME) process conditions on the product quality, we expanded our investigation to assessing the effect of scale-up on the product quality. To this end, we studied the influence of process settings and different scale-up variants on the active pharmaceutical ingredient (API) degradation in a pilot plant scale extruder. Six scale-up variants were investigated and none of them could replicate the product quality from the original process setup on a lab-scale extruder. By analyzing several process-dependent and -independent variables and cross referencing them to the experiments in the lab-scale extruder, we identified certain patterns. The results of the reduced order mechanistic 1D HME simulation of various process states made it possible to establish a correlation between the achieved API degradation and the local melt temperature and the exposure time in specific zones along the screw configuration. Since the same melt temperature and exposure time correlations were also valid for the lab scale-extruder, such an approach could be used in the future to predict the product quality as a function of processing conditions fully in silico prior to the first extrusion trials.
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Affiliation(s)
- Josip Matić
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Carolina Alva
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Simone Eder
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Kathrin Reusch
- Leistritz Pharma Extrusion, Markgrafenstraße. 29-39 1, 90459 Nürnberg, Germany
| | - Amrit Paudel
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria.,Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
| | - Johannes Khinast
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria.,Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
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Matić J, Alva C, Witschnigg A, Eder S, Reusch K, Paudel A, Khinast J. Towards predicting the product quality in hot-melt extrusion: Small scale extrusion. INTERNATIONAL JOURNAL OF PHARMACEUTICS-X 2020; 2:100062. [PMID: 33299982 PMCID: PMC7704403 DOI: 10.1016/j.ijpx.2020.100062] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 09/16/2020] [Revised: 11/06/2020] [Accepted: 11/08/2020] [Indexed: 12/02/2022]
Abstract
In product development, it is crucial to choose the appropriate drug manufacturing route accurately and timely and to ensure that the technique selected is suitable for achieving the desired product quality. Guided by the QbD principles, the pharmaceutical industry is currently transitioning from batch to continuous manufacturing. In this context, process understanding and prediction are becoming even more important. With regard to hot melt extrusion, the process setup, optimization and scale-up in early stages of product development are particularly challenging due to poor process understanding, complex product-process relationship and a small amount of premix available for extensive experimental studies. Hence, automated, quick and reliable process setup and scale-up requires simulation tools that are accurate enough to capture the process and determine the product-process relationships. To this end, the effect of process settings on the degradation of the active pharmaceutical ingredient (API) in a lab-scale Leistritz ZSE12 extruder was investigated. As part of the presented study, the limitations of traditional process analysis using integral process values were investigated, together with the potential that simulations may have in predicting the process performance and the product quality. The results of our investigation indicate that the average melt temperatures and the exposure times in specific zones along the screw configuration correlate well with the API degradation values and can be used as potent process design criteria to simplify the process development.
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Affiliation(s)
- Josip Matić
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Carolina Alva
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Andreas Witschnigg
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Simone Eder
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria
| | - Kathrin Reusch
- Leistritz Pharma Extrusion, Markgrafenstraße, 29-39 1, 90459 Nürnberg, Germany
| | - Amrit Paudel
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria.,Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
| | - Johannes Khinast
- Research Center Pharmaceutical Engineering GmbH, Inffeldgasse 13, 8010 Graz, Austria.,Institute for Process and Particle Engineering, Graz University of Technology, Inffeldgasse 13, 8010 Graz, Austria
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Ohara M, Tanifuji S, Sasai Y, Sugiyama T, Umemoto S, Murata J, Tsujimura I, Kihara S, Taki K. Resin distribution along axial and circumferential directions of self‐wiping co‐rotating parallel twin‐screw extruder. AIChE J 2020. [DOI: 10.1002/aic.17018] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/11/2022]
Affiliation(s)
- Masatoshi Ohara
- Graduate School of Natural Science Kanazawa University Kanazawa Ishikawa Japan
- Shibaura Machine Co., Ltd. Shizuoka Japan
| | | | - Yuya Sasai
- Graduate School of Natural Science Kanazawa University Kanazawa Ishikawa Japan
- Shibaura Machine Co., Ltd. Shizuoka Japan
| | - Takemasa Sugiyama
- Graduate School of Natural Science Kanazawa University Kanazawa Ishikawa Japan
| | - Sho Umemoto
- Graduate School of Natural Science Kanazawa University Kanazawa Ishikawa Japan
| | | | | | - Shin‐ichi Kihara
- Graduate School of Advanced Science and Engineering Hiroshima University Higashi Hiroshima Japan
| | - Kentaro Taki
- School of Mechanical Engineering Kanazawa University Kanzawa Ishikawa Japan
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Novel Cleaning-in-Place Strategies for Pharmaceutical Hot Melt Extrusion. Pharmaceutics 2020; 12:pharmaceutics12060588. [PMID: 32599822 PMCID: PMC7356020 DOI: 10.3390/pharmaceutics12060588] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/29/2020] [Revised: 06/17/2020] [Accepted: 06/20/2020] [Indexed: 02/06/2023] Open
Abstract
To avoid any type of cross-contamination, residue-free production equipment is of utmost importance in the pharmaceutical industry. The equipment cleaning for continuous processes such as hot melt extrusion (HME), which has recently gained popularity in pharmaceutical applications, necessitates extensive manual labour and costs. The present work tackles the HME cleaning issue by investigating two cleaning strategies following the extrusion of polymeric formulations of a hormonal drug and for a sustained release formulation of a poorly soluble drug. First, an in-line quantification by means of UV–Vis spectroscopy was successfully implemented to assess very low active pharmaceutical ingredient (API) concentrations in the extrudates during a cleaning procedure for the first time. Secondly, a novel in-situ solvent-based cleaning approach was developed and its usability was evaluated and compared to a polymer-based cleaning sequence. Comparing the in-line data to typical swab and rinse tests of the process equipment indicated that inaccessible parts of the equipment were still contaminated after the polymer-based cleaning procedure, although no API was detected in the extrudate. Nevertheless, the novel solvent-based cleaning approach proved to be suitable for removing API residue from the majority of problematic equipment parts and can potentially enable a full API cleaning-in-place of a pharmaceutical extruder for the first time.
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Matić J, Paudel A, Bauer H, Garcia RAL, Biedrzycka K, Khinast JG. Developing HME-Based Drug Products Using Emerging Science: a Fast-Track Roadmap from Concept to Clinical Batch. AAPS PharmSciTech 2020; 21:176. [PMID: 32572701 PMCID: PMC7308264 DOI: 10.1208/s12249-020-01713-0] [Citation(s) in RCA: 17] [Impact Index Per Article: 4.3] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/09/2020] [Accepted: 05/13/2020] [Indexed: 12/11/2022] Open
Abstract
This paper presents a rational workflow for developing enabling formulations, such as amorphous solid dispersions, via hot-melt extrusion in less than a year. First, our approach to an integrated product and process development framework is described, including state-of-the-art theoretical concepts, modeling, and experimental characterization described in the literature and developed by us. Next, lab-scale extruder setups are designed (processing conditions and screw design) based on a rational, model-based framework that takes into account the thermal load required, the mixing capabilities, and the thermo-mechanical degradation. The predicted optimal process setup can be validated quickly in the pilot plant. Lastly, a transfer of the process to any GMP-certified manufacturing site can be performed in silico for any extruder based on our validated computational framework. In summary, the proposed workflow massively reduces the risk in product and process development and shortens the drug-to-market time for enabling formulations.
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Affiliation(s)
- Josip Matić
- Research Center Pharmaceutical Engineering GmbH, Graz, Austria
| | - Amrit Paudel
- Research Center Pharmaceutical Engineering GmbH, Graz, Austria.
- Institute for Process and Particle Engineering, Graz University of Technology, Graz, Austria.
| | - Hannes Bauer
- Research Center Pharmaceutical Engineering GmbH, Graz, Austria
| | | | | | - Johannes G Khinast
- Research Center Pharmaceutical Engineering GmbH, Graz, Austria.
- Institute for Process and Particle Engineering, Graz University of Technology, Graz, Austria.
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16
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Cheng W, Ye Y, Jiang S, Wang J, Gu X, Feng L. Mixing intensification in a horizontal self-cleaning twin-shaft kneader with a highly viscous Newtonian fluid. Chem Eng Sci 2019. [DOI: 10.1016/j.ces.2019.03.005] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/24/2022]
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Holistic QbD approach for hot-melt extrusion process design space evaluation: Linking materials science, experimentation and process modeling. Eur J Pharm Biopharm 2019; 141:149-160. [PMID: 31132400 DOI: 10.1016/j.ejpb.2019.05.021] [Citation(s) in RCA: 15] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/12/2019] [Revised: 05/22/2019] [Accepted: 05/24/2019] [Indexed: 11/20/2022]
Abstract
The aim of this work was to investigate the relationship between formulation material properties, process parameters and process performance for the manufacturing of amorphous solid dispersions via hot-melt extrusion (HME) using experimentation coupled with process modeling. Specifically, we evaluated the impact of the matrix copovidone melt rheology with and without the addition of a plasticizing surfactant, polysorbate 80, while also varying the process parameters, barrel temperature and screw speed, and keeping fill volume constant. To correlate the process performance to a critical quality attribute, we used telmisartan as an indicator substance by processing at temperatures below its solubility temperature in the polymeric matrix. We observed a broader design space of HME processes for the plasticized formulation with respect to screw speed than for the copovidone-only matrix formulation. This observation was determined by the range of observed melt temperatures in the extruder, both measured and simulated. The reason was not primarily linked to a reduced shear-thinning behavior, characterized by the power law index, n, but instead more to an overall reduced melt viscosity during extrusion and zero-shear rate viscosity, η0, accordingly. We also found that the amount of residual crystallinity of telmisartan correlated with the simulated maximum melt temperature in the extruder barrel. This finding confirmed the applicability of the temperature-dependent API-matrix solubility phase diagram for HME to process development. Given the complex inter-dependent relationships between material properties, process and performance, process modeling combined with reduced laboratory experimentation was established as a holistic approach for the evaluation of Quality-by-Design-based HME process design spaces.
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Wesholowski J, Berghaus A, Thommes M. Inline Determination of Residence Time Distribution in Hot-Melt-Extrusion. Pharmaceutics 2018; 10:pharmaceutics10020049. [PMID: 29662034 PMCID: PMC6027324 DOI: 10.3390/pharmaceutics10020049] [Citation(s) in RCA: 18] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/28/2018] [Revised: 03/29/2018] [Accepted: 04/11/2018] [Indexed: 11/16/2022] Open
Abstract
In the framework of Quality-by-Design (QbD), the inline determination of process parameters or quality attributes of a product using sufficient process analytical technology (PAT) is a center piece for the establishment of continuous processes as a standard pharmaceutical technology. In this context, Twin-Screw-Extrusion (TSE) processes, such as Hot-Melt-Extrusion (HME), are one key aspect of current research. The main benefit of this process technology is the combination of different unit operations. Several of these sub-processes are linked to the Residence Time Distribution (RTD) of the material within the apparatus. In this study a UV/Vis spectrophotometer from ColVisTec was tested regarding the suitability for the inline determination of the RTD of an HME process. Two different measuring positions within a co-rotating Twin-Screw-Extruder were compared to an offline HPLC-UV as reference method. The obtained results were overall in good agreement and therefore the inline UV/Vis spectrophotometer is suitable for the determination of the RTD in TSE. An influence of the measuring position on repeatability was found and has to be taken into consideration for the implementation of PATs. An effect of the required amount of marker on process rheology is not likely due to the low Limit-of-Quantification (LoQ).
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Affiliation(s)
- Jens Wesholowski
- Institute of Solids Process Engineering, TU Dortmund University, Dortmund 44227, Germany.
| | | | - Markus Thommes
- Institute of Solids Process Engineering, TU Dortmund University, Dortmund 44227, Germany.
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