Publications by authors named "Frank Clemens"

14 Publications

  • Page 1 of 1

Analysis of Styrene-Butadiene Based Thermoplastic Magnetorheological Elastomers with Surface-Treated Iron Particles.

Polymers (Basel) 2021 May 15;13(10). Epub 2021 May 15.

Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland.

Magnetorheological elastomers (MRE) are increasing in popularity in many applications because of their ability to change stiffness by applying a magnetic field. Instead of liquid-based 1 K and 2 K silicone, thermoplastic elastomers (TPE), based on styrene-butadiene-styrene block copolymers, have been investigated as matrix material. Three different carbonyl iron particles (CIPs) with different surface treatments were used as magneto active filler material. For the sample fabrication, the thermoplastic pressing method was used, and the MR effect under static and dynamic load was investigated. We show that for filler contents above 40 vol.-%, the linear relationship between powder content and the magnetorheological effect is no longer valid. We showed how the SiO and phosphate coating of the CIPs affects the saturation magnetization and the shear modulus of MRE composites. A combined silica phosphate coating resulted in a higher shear modulus, and therefore, the MR effect decreased, while coating with SiO only improved the MR effect. The highest performance was achieved at low deformations; a static MR effect of 73% and a dynamic MR effect of 126% were recorded. It was also shown that a lower melting viscosity of the TPE matrix helps to increase the static MR effect of anisotropic MREs, while low shear modulus is crucial for achieving high dynamic MR. The knowledge from TPE-based magnetic composites will open up new opportunities for processing such as injection molding, extrusion, and fused deposition modeling (FDM).
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http://dx.doi.org/10.3390/polym13101597DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8156738PMC
May 2021

Sensorized Robotic Skin Based on Piezoresistive Sensor Fiber Composites Produced with Injection Molding of Liquid Silicone.

Polymers (Basel) 2021 Apr 10;13(8). Epub 2021 Apr 10.

Department of Functional Materials, Empa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland.

Soft robotics and flexible electronics are rising in popularity and can be used in many applications. However, there is still a need for processing routes that allow the upscaling in production for functional soft robotic parts in an industrial scale. In this study, injection molding of liquid silicone is suggested as a fabrication method for sensorized robotic skin based on sensor fiber composites. Sensor fibers based on thermoplastic elastomers with two different shore hardness (50A and 70A) are combined with different silicone materials. A mathematical model is used to predict the mechanical load transfer from the silicone matrix to the fiber and shows that the matrix of the lowest shore hardness should not be combined with the stiffer fiber. The sensor fiber composites are fixed on a 3D printed robotic finger. The sensorized robotic skin based on the composite with the 50A fiber in combination with pre-straining gives good sensor performance as well as a large elasticity. It is proposed that a miss-match in the mechanical properties between fiber sensor and matrix should be avoided in order to achieve low drift and relaxation. These findings can be used as guidelines for material selection for future sensor integrated soft robotic systems.
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http://dx.doi.org/10.3390/polym13081226DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8070002PMC
April 2021

Antistatic Fibers for High-Visibility Workwear: Challenges of Melt-Spinning Industrial Fibers.

Materials (Basel) 2020 Jun 10;13(11). Epub 2020 Jun 10.

Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, 8600 Dübendorf, Switzerland.

Safety workwear often requires antistatic protection to prevent the build-up of static electricity and sparks, which can be extremely dangerous in a working environment. In order to make synthetic antistatic fibers, electrically conducting materials such as carbon black are added to the fiber-forming polymer. This leads to unwanted dark colors in the respective melt-spun fibers. To attenuate the undesired dark color, we looked into various possibilities including the embedding of the conductive element inside a dull side-by-side bicomponent fiber. The bicomponent approach, with an antistatic compound as a minor element, also helped in preventing the severe loss of tenacity often caused by a high additive loading. We could melt-spin a bicomponent fiber with a specific resistance as low as 0.1 Ωm and apply it in a fabric that fulfills the requirements regarding the antistatic properties, luminance and flame retardancy of safety workwear.
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http://dx.doi.org/10.3390/ma13112645DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7321615PMC
June 2020

Effect of the Elastomer Matrix on Thermoplastic Elastomer-Based Strain Sensor Fiber Composites.

Sensors (Basel) 2020 Apr 23;20(8). Epub 2020 Apr 23.

Department of Mechanical Engineering (MECH), Vrije Universiteit Brussel (VUB), and Flanders Make Pleinlaan 2, B-1050 Brussels, Belgium.

In this study, a thermoplastic elastomer sensor fiber was embedded in an elastomer matrix. The effect of the matrix material on the sensor properties and the piezoresistive behavior of the single fiber-matrix composite system was investigated. For all composites, cycling test (dynamic test) and the relaxation behavior at different strains (quasi-static test) were investigated. In all cases, dynamic properties and quasi-static significantly changed after embedding, compared to the pure fiber. The composite with the silicone elastomer PDMS (Polydimethylsiloxane) as matrix material exhibited deviation from linear response of the resistivity at low strains and proved an unsuitable choice compared to natural rubber. The addition of a spring construct in the embedded sensor fiber natural rubber composite improved the linearity at low strains but increased the mechanical and electrical hysteresis of the soft matter sensor composite. Using pre-vulcanized natural rubber improved linearity at low strains and reduced significantly the stress and relative resistance relaxation as well as the resistance hysteresis, especially if the resistance remained low. In both cases of the pre-vulcanized rubber and the spring structure, the piezoresistive behavior was improved, and at the same time, the stiffness of the system was increased indicating that using a stiffer matrix can be a strategy for improving the sensor properties.
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http://dx.doi.org/10.3390/s20082399DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7219333PMC
April 2020

Draw-spun, photonically annealed Ag fibers as alternative electrodes for flexible CIGS solar cells.

Sci Technol Adv Mater 2019 30;20(1):26-34. Epub 2018 Nov 30.

Laboratory for Thin films and Photovoltaics, Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Switzerland.

We explore the feasibility of Ag fiber meshes as electron transport layer for high-efficiency flexible Cu(In,Ga)Se (CIGS) solar cells. Woven meshes of Ag fibers after UV illumination and millisecond flash-lamp treatment results in a sheet resistance of 17 Ω/sq and a visible transmittance above 85%. Conductive Ag meshes are integrated into flexible CIGS cells as transparent conductive electrode (TCE) alone or together with layers of Al-doped ZnO (AZO) with various thickness of 0…900 nm. The Ag mesh alone is not able to function as a current collector. If used together with a thin AZO layer (50 nm), the Ag mesh markedly improves the fill factor and cell efficiency, in spite of the adverse mesh shadowing. When Ag mesh is combined with thicker (200 nm or 900 nm) AZO layers, no improvements in photovoltaic parameters are obtained. When comparing a hybrid TCE consisting of 50 nm AZO and Ag fiber mesh with a thick 900 nm reference AZO device, an improved charge carrier collection in the near-infrared range is observed. Regardless of the AZO thickness, the presence of Ag mesh slows down cell degradation upon mechanical tensile stress, which could be interesting for implementation into flexible thin film CIGS modules.
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http://dx.doi.org/10.1080/14686996.2018.1552480DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6346713PMC
November 2018

Piezoresistive Soft Condensed Matter Sensor for Body-Mounted Vital Function Applications.

Sensors (Basel) 2016 Mar 4;16(3). Epub 2016 Mar 4.

Laboratory for High Performance Ceramics, Empa, Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf 8600, Switzerland.

A soft condensed matter sensor (SCMS) designed to measure strains on the human body is presented. The hybrid material based on carbon black (CB) and a thermoplastic elastomer (TPE) was bonded to a textile elastic band and used as a sensor on the human wrist to measure hand motion by detecting the movement of tendons in the wrist. Additionally it was able to track the blood pulse wave of a person, allowing for the determination of pulse wave peaks corresponding to the systole and diastole blood pressures in order to calculate the heart rate. Sensor characterization was done using mechanical cycle testing, and the band sensor achieved a gauge factor of 4-6.3 while displaying low signal relaxation when held at a strain levels. Near-linear signal performance was displayed when loading to successively higher strain levels up to 50% strain.
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http://dx.doi.org/10.3390/s16030326DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4813901PMC
March 2016

Spatial heterogeneity of malaria in Ghana: a cross-sectional study on the association between urbanicity and the acquisition of immunity.

Malar J 2016 Feb 11;15:84. Epub 2016 Feb 11.

Research Group Infectious Disease Epidemiology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany.

Background: Malaria incidence has declined considerably over the last decade. This is partly due to a scale-up of control measures but is also attributed to increasing urbanization. This study aimed to analyse the association between malaria and urbanization and the effect of urbanicity on the acquisition of semi-immunity.

Methods: In 2012, children with fever presenting to St Michael's Hospital Pramso/Ghana were recruited. The malaria-positive-fraction (MPF) of fever cases was calculated on community-level to approximate the malaria risk. The mean age of malaria cases was calculated for each community to estimate the acquisition of semi-immunity. The level of urbanicity for the communities was calculated and associations between MPF, urbanicity and immunity were modelled using linear regression.

Results: Twenty-six villages were included into the study with a mean MPF of 35 %. A linear decrease of 5 % (95 % CI: 4-6 %) in MPF with every ten-point increase in urbanicity was identified. The mean age of malaria patients increased by 2.9 months (95 % CI: 1.0-4.8) with every ten-point increase in urbanicity.

Discussion: The results confirm an association between an increase in urbanicity and declining malaria risk and demonstrate that the acquisition of semi-immunity is heterogeneous on a micro-epidemiological scale and is associated with urbanicity.
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http://dx.doi.org/10.1186/s12936-016-1138-4DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4751679PMC
February 2016

Urbanicity and Paediatric Bacteraemia in Ghana-A Case-Control Study within a Rural-Urban Transition Zone.

PLoS One 2015 29;10(9):e0139433. Epub 2015 Sep 29.

Research Group Infectious Disease Epidemiology, Bernhard Nocht Institute for Tropical Medicine, Hamburg, Germany; German Centre for Infection Research (DZIF), partner site Hamburg-Borstel-Lübeck, Hamburg, Germany.

Background: Systemic bacterial infections are a major cause of paediatric febrile illness in sub-Saharan Africa. Aim of this study was to assess the effects of social and geographical determinants on the risk of bacteraemia in a rural-urban transition zone in Ghana.

Methods: Children below 15 years of age with fever were recruited at an outpatient department in the suburban belt of Kumasi, Ghana's second largest city. Blood was taken for bacterial culture and malaria diagnostics. The socio-economic status of participants was calculated using Principle Component Analysis. A scale, based on key urban characteristics, was established to quantify urbanicity for all communities in the hospital catchment area. A case-control analysis was conducted, where children with and without bacteraemia were cases and controls, respectively.

Results: Bacteraemia was detected in 72 (3.1%) of 2,306 hospital visits. Non-typhoidal Salmonella (NTS; n = 24; 33.3%) and Salmonella typhi (n = 18; 25.0%) were the most common isolates. Logistic regression analysis showed that bacteraemia was negatively associated with urbanicity (odds ratio [OR] = 0.8; 95% confidence interval [CI]: 0.7-1.0) and socio-economic status (OR = 0.8; 95% CI: 0.6-0.9). Both associations were stronger if only NTS infections were used as cases (OR = 0.5; 95% CI: 0.3-0.8 and OR = 0.6; 95% CI: 0.4-1.0, respectively).

Conclusions: The results of this study highlight the importance of individual as well as community factors as independent risk factors for invasive bacterial infection (IBI) and especially NTS. Epidemiological data support physicians, public health experts and policy makers to identify disease prevention and treatment needs in order to secure public health in the transitional societies of developing countries.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0139433PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4587855PMC
May 2016

SVAS3: Strain Vector Aided Sensorization of Soft Structures.

Sensors (Basel) 2014 Jul 17;14(7):12748-70. Epub 2014 Jul 17.

Bio-Inspired Robotics Lab, Department of Mechanical and Process Engineering, ETH Zürich, 8092 Zurich, Switzerland.

Soft material structures exhibit high deformability and conformability which can be useful for many engineering applications such as robots adapting to unstructured and dynamic environments. However, the fact that they have almost infinite degrees of freedom challenges conventional sensory systems and sensorization approaches due to the difficulties in adapting to soft structure deformations. In this paper, we address this challenge by proposing a novel method which designs flexible sensor morphologies to sense soft material deformations by using a functional material called conductive thermoplastic elastomer (CTPE). This model-based design method, called Strain Vector Aided Sensorization of Soft Structures (SVAS3), provides a simulation platform which analyzes soft body deformations and automatically finds suitable locations for CTPE-based strain gauge sensors to gather strain information which best characterizes the deformation. Our chosen sensor material CTPE exhibits a set of unique behaviors in terms of strain length electrical conductivity, elasticity, and shape adaptability, allowing us to flexibly design sensor morphology that can best capture strain distributions in a given soft structure. We evaluate the performance of our approach by both simulated and real-world experiments and discuss the potential and limitations.
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http://dx.doi.org/10.3390/s140712748DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4168483PMC
July 2014

Comparison of piezoresistive monofilament polymer sensors.

Sensors (Basel) 2014 Jan 13;14(1):1278-94. Epub 2014 Jan 13.

EMPA Dübendorf, Überlandstrasse 129, Dübendorf 8600, Switzerland.

The development of flexible polymer monofilament fiber strain sensors have many applications in both wearable computing (clothing, gloves, etc.) and robotics design (large deformation control). For example, a high-stretch monofilament sensor could be integrated into robotic arm design, easily stretching over joints or along curved surfaces. As a monofilament, the sensor can be woven into or integrated with textiles for position or physiological monitoring, computer interface control, etc. Commercially available conductive polymer monofilament sensors were tested alongside monofilaments produced from carbon black (CB) mixed with a thermo-plastic elastomer (TPE) and extruded in different diameters. It was found that signal strength, drift, and precision characteristics were better with a 0.3 mm diameter CB/TPE monofilament than thick (~2 mm diameter) based on the same material or commercial monofilaments based on natural rubber or silicone elastomer (SE) matrices.
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http://dx.doi.org/10.3390/s140101278DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3926615PMC
January 2014

Light-intensity-induced characterization of elastic constants and d33 piezoelectric coefficient of PLZT single fiber based transducers.

Sensors (Basel) 2013 Feb 12;13(2):2419-29. Epub 2013 Feb 12.

Department of Materials Science, University of Silesia, 41-200 Sosnowiec, Poland.

Enhanced functionality of electro-optic devices by implementing piezoelectric micro fibers into their construction is proposed. Lanthanum-modified lead zirconate titanate (PLZT) ceramics are known to exhibit high light transparency, desirable electro-optic properties and fast response. In this study PLZT fibers with a diameter of around 300 microns were produced by a thermoplastic processing method and their light-induced impedance and piezoelectric coefficient were investigated at relatively low light intensity (below 50 mW/cm2). The authors experimentally proved higher performance of light controlled microfiber transducers in comparison to their bulk form. The advantage of the high surface area to volume ratio is shown to be an excellent technique to design high quality light sensors by using fibrous materials. The UV absorption induced change in elastic constants of 3% and 4% for the piezoelectric coefficient d(33).
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http://dx.doi.org/10.3390/s130202419DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3649393PMC
February 2013

Debinding mechanisms in thermoplastic processing of a Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ)- stearic acid-polystyrene mixture.

ChemSusChem 2013 Feb 23;6(2):336-44. Epub 2012 Dec 23.

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for High Performance Ceramics, Überlandstrasse 129, 8600 Dübendorf, Switzerland.

In the present study, we address the interaction between a thermoplastic binder system and Ba(0.5)Sr(0.5)Co(0.8)Fe(0.2)O(3-δ) (BSCF) during thermal treatment of the thermoplastic feedstock. BSCF powder was coated with different amounts of stearic acid (SA) acting as a surfactant. Oxygen release from the uncoated BSCF surface changes the decomposition of polystyrene (PS) in inert atmospheres from a pyrolytic to a thermoxidative mechanism, thereby decreasing the break-down temperature and the activation energy. In mixtures with coated BSCF powder, the decomposition products of SA carbonatize the BSCF surface, which inhibits oxygen release. Mass spectrometry of the breakdown products indicates that the decomposition of SA in the presence of BSCF also modifies the decomposition pathway of PS. The influence of BSCF on the polymer decomposition reaction in air is not as strong. Oxygen diffusion seems to be responsible for the differences to pure PS in reaction rates and the activation energies.
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http://dx.doi.org/10.1002/cssc.201200540DOI Listing
February 2013

Textile Pressure Sensor Made of Flexible Plastic Optical Fibers.

Sensors (Basel) 2008 Jul 25;8(7):4318-4329. Epub 2008 Jul 25.

Empa, Swiss Federal Laboratories for Materials Testing and Research, Laboratory for High Performance Ceramics, Ueberlandstrasse 129, 8600 Duebendorf, Switzerland.

In this paper we report the successful development of pressure sensitive textile prototypes based on flexible optical fibers technology. Our approach is based on thermoplastic silicone fibers, which can be integrated into woven textiles. As soon as pressure at a certain area of the textile is applied to these fibers they change their cross section reversibly, due to their elastomeric character, and a simultaneous change in transmitted light intensity can be detected. We have successfully manufactured two different woven samples with fibers of 0.51 and 0.98 mm diameter in warp and weft direction, forming a pressure sensitive matrix. Determining their physical behavior when a force is applied shows that pressure measurements are feasible. Their usable working range is between 0 and 30 N. Small drifts in the range of 0.2 to 4.6%, over 25 load cycles, could be measured. Finally, a sensor array of 2 x 2 optical fibers was tested for sensitivity, spatial resolution and light coupling between fibers at intersections.
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http://dx.doi.org/10.3390/s8074318DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3697177PMC
July 2008

Sensor for Measuring Strain in Textile.

Sensors (Basel) 2008 Jun 3;8(6):3719-3732. Epub 2008 Jun 3.

Wearable Computing Lab, ETH Zürich, Gloriastrasse 35, 8092 Zürich, Switzerland.

In this paper a stain sensor to measure large strain (80%) in textiles is presented. It consists of a mixture of 50wt-% thermoplastic elastomer (TPE) and 50wt-% carbon black particles and is fiber-shaped with a diameter of 0.315mm. The attachment of the sensor to the textile is realized using a silicone film. This sensor configuration was characterized using a strain tester and measuring the resistance (extension-retraction cycles): It showed a linear resistance response to strain, a small hysteresis, no ageing effects and a small dependance on the strain velocity. The total mean error caused by all these effects was +/-5.5% in strain. Washing several times in a conventional washing machine did not influence the sensor properties. The paper finishes by showing an example application where 21 strain sensors were integrated into a catsuit. With this garment, 27 upper body postures could be recognized with an accuracy of 97%.
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http://dx.doi.org/10.3390/s8063719DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3714661PMC
June 2008