Publications by authors named "Hans M Wyss"

30 Publications

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An integrated system for automated measurement of airborne pollen based on electrostatic enrichment and image analysis with machine vision.

Talanta 2022 Jan 28;237:122908. Epub 2021 Sep 28.

Institute of Microanalytical Systems, Department of Chemistry, Zhejiang University, Hangzhou, 310058, China. Electronic address:

Here we describe an automated and compact pollen detection system that integrates enrichment, in-situ detection and self-cleaning modules. The system can achieve continuous capture and enrichment of pollen grains in air samples by electrostatic adsorption. The captured pollen grains are imaged with a digital camera, and an automated image analysis based on machine vision is performed, which enables a quantification of the number of pollen particles as well as a preliminary classification into two types of pollen grains. In order to optimize and evaluate the system performance, we developed a testing approach that utilizes an airflow containing a precisely metered amount of pollen particles surrounded by a sheath flow to achieve the generation and lossless transmission of standard gas samples. We studied various factors affecting the pollen capture efficiency, including the applied voltage, air flow rate and humidity. Under optimized conditions, the system was successfully used in the measurement of airborne pollen particles within a wide range of concentrations, spanning 3 orders of magnitude.
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http://dx.doi.org/10.1016/j.talanta.2021.122908DOI Listing
January 2022

Conventional glaucoma implants and the new MIGS devices: a comprehensive review of current options and future directions.

Eye (Lond) 2021 12 14;35(12):3202-3221. Epub 2021 Jun 14.

Microsystems Research Section, Department of Mechanical Engineering, Eindhoven University of Technology, Eindhoven, The Netherlands.

Glaucoma is a progressive optic neuropathy that is the second leading cause of preventable blindness worldwide, after cataract formation. A rise in the intraocular pressure (IOP) is considered to be a major risk factor for glaucoma and is associated with an abnormal increase of resistance to aqueous humour outflow from the anterior chamber. Glaucoma drainage devices have been developed to provide an alternative pathway through which aqueous humour can effectively exit the anterior chamber, thereby reducing IOP. These devices include the traditional aqueous shunts with tube-plate design, as well as more recent implants, such as the trabeculectomy-modifying EX-PRESS implant and the new minimally invasive glaucoma surgery (MIGS) devices. In this review, we will describe each implant in detail, focusing on their efficacy in reducing IOP and safety profile. Additionally, a critical and evidence-based comparison between these implants will be provided. Finally, we will propose potential developments that may help to improve the performance of current devices.
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http://dx.doi.org/10.1038/s41433-021-01595-xDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8602385PMC
December 2021

Nonmonotonic swelling and compression dynamics of hydrogels in polymer solutions.

Phys Rev E 2020 Dec;102(6-1):062606

Department of Mechanical Engineering, Materials Technology, Eindhoven University of Technology, 5600MB Eindhoven, Netherlands.

Hydrogels are sponge-like materials that can absorb or expel significant amounts of water. Swelling up from a dried state, they can swell up more than a hundredfold in volume, with the kinetics and the degree of swelling depending sensitively on the physicochemical properties of both the polymer network and the aqueous solvent. In particular, the presence of dissolved macromolecules in the background liquid can have a significant effect, as the macromolecules can exert an additional external osmotic pressure on the hydrogel material, thereby reducing the degree of swelling. In this paper, we have submerged dry hydrogel particles in polymer solutions containing large and small macromolecules. Interestingly, for swelling in the presence of large macromolecules we observe a concentration-dependent overshoot behavior, where the particle volume first continuously increases toward a maximum, after which it decreases again, reaching a lower, equilibrium value. In the presence of smaller macromolecules we do not observe this intriguing overshoot behavior, but instead observe a rapid growth followed by a slowed-down growth. To account for the observed overshoot behavior, we realize that the macromolecules entering the hydrogel network not only lead to a reduction of the osmotic pressure difference, but their presence within the network also affects the swelling behavior through a modification of the solvent-polymer interactions. In this physical picture of the swelling process, the net amount of volume change should thus depend on the magnitudes of both the reduction in osmotic pressure and the change in effective solvent quality associated with the macromolecules entering the pores of the hydrogel network. We develop a phenomenological model that incorporates both of these effects. Using this model we are able to account for both the swelling and compression kinetics of hydrogels within aqueous polymer solutions, as a function of the size of the dissolved macromolecules and of their effect on the effective solvent quality.
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http://dx.doi.org/10.1103/PhysRevE.102.062606DOI Listing
December 2020

Compression and swelling of hydrogels in polymer solutions: A dominant-mode model.

Phys Rev E 2020 Dec;102(6-1):062607

Institute AMOLF, Theory of Biomolecular Matter, Science Park 104, 1098XG Amsterdam, The Netherlands.

The swelling and compression of hydrogels in polymer solutions can be understood by considering hydrogel-osmolyte-solvent interactions which determine the osmotic pressure difference between the inside and the outside of a hydrogel particle and the changes in effective solvent quality for the hydrogel network. Using the theory of poroelasticity, we find the exact solution to hydrogel dynamics in a dilute polymer solution, which quantifies the effect of diffusion and partitioning of osmolyte and the related solvent quality change to the volumetric changes of the hydrogel network. By making a dominant-mode assumption, we propose a model for the swelling and compression dynamics of (spherical) hydrogels in concentrated polymer solutions. Osmolyte diffusion induces a biexponential response in the size of the hydrogel radius, whereas osmolyte partitioning and solvent quality effects induce monoexponential responses. Comparison of the dominant-mode model to experiments provides reasonable values for the compressive bulk modulus of a hydrogel particle, the permeability of the hydrogel network, and the diffusion constant of osmolyte molecules inside the hydrogel network. Our model shows that hydrogel-osmolyte interactions can be described in a conceptually simple manner, while still capturing the rich (de)swelling behaviors observed in experiments. We expect our approach to provide a roadmap for further research into and applications of hydrogel dynamics induced by, for example, changes in the temperature and the pH.
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http://dx.doi.org/10.1103/PhysRevE.102.062607DOI Listing
December 2020

Effects of Surfactant and Urea on Dynamics and Viscoelastic Properties of Hydrophobically Assembled Supramolecular Hydrogel.

Macromolecules 2018 Jul 25;51(13):4813-4820. Epub 2018 Jun 25.

Laboratory of Macromolecular and Organic Chemistry, Department of Chemical Engineering and Chemistry, Institute for Complex Molecular Systems, and Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

Physically associated hydrogels based on strong hydrophobic interactions often have attractive mechanical properties that combine processability with elasticity. However, there is a need to study such interactions and understand their relation to the macroscopic hydrogel properties. Therefore, we use the surfactant sodium dodecyl sulfate (SDS) and urea as reagents that disrupt hydrophobic interactions. The model hydrogel is based on a segmented copolymer between poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA). We show that both agents influence viscoelastic properties, dynamics, and relaxation processes of the model hydrogel. In particular, the relaxation time is significantly reduced by urea, as compared to SDS, whereas the surfactant causes a decrease of the modulus of the hydrogel more efficiently. The reversibility of the effects of SDS and urea can be exploited, for instance, by using an injectable sol that solidifies when the SDS or urea diffuses out of the sample. Surfactant-induced processability may be advantageous in future applications of hydrophobically assembled physical hydrogels.
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http://dx.doi.org/10.1021/acs.macromol.8b00892DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6041769PMC
July 2018

Diffusing-wave spectroscopy in a standard dynamic light scattering setup.

Phys Rev E 2017 Dec 18;96(6-1):062611. Epub 2017 Dec 18.

Department of Mechanical Engineering, Eindhoven University of Technology, 5600MB Eindhoven, The Netherlands.

Diffusing-wave spectroscopy (DWS) extends dynamic light scattering measurements to samples with strong multiple scattering. DWS treats the transport of photons through turbid samples as a diffusion process, thereby making it possible to extract the dynamics of scatterers from measured correlation functions. The analysis of DWS data requires knowledge of the path length distribution of photons traveling through the sample. While for flat sample cells this path length distribution can be readily calculated and expressed in analytical form; no such expression is available for cylindrical sample cells. DWS measurements have therefore typically relied on dedicated setups that use flat sample cells. Here we show how DWS measurements, in particular DWS-based microrheology measurements, can be performed in standard dynamic light scattering setups that use cylindrical sample cells. To do so we perform simple random-walk simulations that yield numerical predictions of the path length distribution as a function of both the transport mean free path and the detection angle. This information is used in experiments to extract the mean-square displacement of tracer particles in the material, as well as the corresponding frequency-dependent viscoelastic response. An important advantage of our approach is that by performing measurements at different detection angles, the average path length through the sample can be varied. For measurements performed on a single sample cell, this gives access to a wider range of length and time scales than obtained in a conventional DWS setup. Such angle-dependent measurements also offer an important consistency check, as for all detection angles the DWS analysis should yield the same tracer dynamics, even though the respective path length distributions are very different. We validate our approach by performing measurements both on aqueous suspensions of tracer particles and on solidlike gelatin samples, for which we find our DWS-based microrheology data to be in good agreement with rheological measurements performed on the same samples.
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http://dx.doi.org/10.1103/PhysRevE.96.062611DOI Listing
December 2017

Monodisperse Microshell Structured Gelatin Microparticles for Temporary Chemoembolization.

Biomacromolecules 2018 02 16;19(2):386-391. Epub 2018 Jan 16.

Eindhoven University of Technology, WTB/MaTe and ICMS , Eindhoven, The Netherlands.

Embolization is a nonsurgical, minimally invasive procedure that deliberately blocks a blood vessel. Although several embolic particles have been commercialized, their much wider applications have been hampered owing mainly to particle size variation and uncontrollable degradation kinetics. Herein we introduce a microfluidic approach to fabricate highly monodisperse gelatin microparticles (GMPs) with a microshell structure. For this purpose, we fabricate uniform gelatin emulsion precursors using a microfluidic technique and consecutively cross-link them by inbound diffusion of glutaraldehyde from the oil continuous phase to the suspending gelatin precursor droplets. A model micromechanic study, carried out in an artificial blood vessel, demonstrates that the extraordinary degradation kinetics of the GMPs, which stems from the microshell structure, enables controlled rupturing while exhibiting drug release under temporary chemoembolic conditions.
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http://dx.doi.org/10.1021/acs.biomac.7b01479DOI Listing
February 2018

Compression and Reswelling of Microgel Particles after an Osmotic Shock.

Phys Rev Lett 2017 Sep 31;119(9):098001. Epub 2017 Aug 31.

Institute for Complex Molecular Systems, Eindhoven University of Technology, 5600MB Eindhoven, The Netherlands.

We use dedicated microfluidic devices to expose soft hydrogel particles to a rapid change in the externally applied osmotic pressure and observe a surprising, nonmonotonic response: After an initial rapid compression, the particle slowly reswells to approximately its original size. We theoretically account for this behavior, enabling us to extract important material properties from a single microfluidic experiment, including the compressive modulus, the gel permeability, and the diffusivity of the osmolyte inside the gel. We expect our approach to be relevant to applications such as controlled release, chromatography, and responsive materials.
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http://dx.doi.org/10.1103/PhysRevLett.119.098001DOI Listing
September 2017

Tough Supramolecular Hydrogel Based on Strong Hydrophobic Interactions in a Multiblock Segmented Copolymer.

Macromolecules 2017 Apr 5;50(8):3333-3346. Epub 2017 Apr 5.

Laboratory of Macromolecular and Organic Chemistry, Department of Chemical Engineering and Chemistry, Institute for Complex Molecular Systems, and Department of Mechanical Engineering, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

We report the preparation and structural and mechanical characterization of a tough supramolecular hydrogel, based exclusively on hydrophobic association. The system consists of a multiblock, segmented copolymer of hydrophilic poly(ethylene glycol) (PEG) and hydrophobic dimer fatty acid (DFA) building blocks. A series of copolymers containing 2K, 4K, and 8K PEG were prepared. Upon swelling in water, a network is formed by self-assembly of hydrophobic DFA units in micellar domains, which act as stable physical cross-link points. The resulting hydrogels are noneroding and contain 75-92 wt % of water at swelling equilibrium. Small-angle neutron scattering (SANS) measurements showed that the aggregation number of micelles ranges from 2 × 10 to 6 × 10 DFA units, increasing with PEG molecular weight. Mechanical characterization indicated that the hydrogel containing PEG 2000 is mechanically very stable and tough, possessing a tensile toughness of 4.12 MJ/m. The high toughness, processability, and ease of preparation make these hydrogels very attractive for applications where mechanical stability and load bearing features of soft materials are required.
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http://dx.doi.org/10.1021/acs.macromol.7b00319DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5406785PMC
April 2017

Convection associated with exclusion zone formation in colloidal suspensions.

Soft Matter 2016 Jan;12(4):1127-32

Department of Mechanical Engineering, Materials Technology, Eindhoven University of Technology, Eindhoven, The Netherlands. and Bernal Institute, University of Limerick, Limerick, Ireland.

The long-range repulsion of colloids from various interfaces has been observed in a wide range of studies from different research disciplines. This so-called exclusion zone (EZ) formation occurs near surfaces such as hydrogels, polymers, or biological tissues. It was recently shown that the underlying physical mechanism leading to this long-range repulsion is a combination of ion-exchange at the interface, diffusion of ions, and diffusiophoresis of colloids in the resulting ion concentration gradients. In this paper, we show that the same ion concentration gradients that lead to exclusion zone formation also imply that diffusioosmosis near the walls of the sample cell must occur. This should lead to convective flow patterns that are directly associated with exclusion zone formation. We use multi-particle tracking to study the dynamics of particles during exclusion zone formation in detail, confirming that indeed two pronounced vortex-like convection rolls occur near the cell walls. These dramatic flow patterns persist for more than 4 hours, with the typical velocity decreasing as a function of time. We find that the flow velocity depends strongly on the surface properties of the sample cell walls, consistent with diffusioosmosis being the main physical mechanism that governs these convective flows.
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http://dx.doi.org/10.1039/c5sm01502bDOI Listing
January 2016

Cell Mechanics: Combining Speed with Precision.

Authors:
Hans M Wyss

Biophys J 2015 Nov;109(10):1997-8

Mechanical Engineering and Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, The Netherlands. Electronic address:

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http://dx.doi.org/10.1016/j.bpj.2015.10.019DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4656880PMC
November 2015

Topochemical polymerization in self-assembled rodlike micelles of bisurea bolaamphiphiles.

Soft Matter 2014 Feb;10(7):952-6

Rod-like micelles, formed from bolaamphiphiles with oligo(ethylene oxide) hydrophilic outer segments and a hydrophobic segment with diacetylene flanked by two urea moieties, were covalently fixated by topochemical photopolymerization to high degrees of polymerization by optimizing the hydrophobic core and the hydrophilic periphery of the bolaamphiphiles. Analysis of the polymerized product with dynamic light scattering in chloroform showed degrees of polymerization of approximately 250. Cryo-TEMof bolaamphiphiles before and after UV irradiation showed that the morphology of the rods was conserved upon topochemical polymerization.
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http://dx.doi.org/10.1039/c3sm52605dDOI Listing
February 2014

Tough stimuli-responsive supramolecular hydrogels with hydrogen-bonding network junctions.

J Am Chem Soc 2014 May 6;136(19):6969-77. Epub 2014 May 6.

Institute for Complex Molecular Systems, ‡Laboratory of Chemical Biology, §Department of Chemistry and Chemical Engineering, ∥Department of Mechanical Engineering, Eindhoven University of Technology , P.O. Box 513, NL 5600 MB Eindhoven, The Netherlands.

Hydrogels were prepared with physical cross-links comprising 2-ureido-4[1H]-pyrimidinone (UPy) hydrogen-bonding units within the backbone of segmented amphiphilic macromolecules having hydrophilic poly(ethylene glycol) (PEG). The bulk materials adopt nanoscopic physical cross-links composed of UPy-UPy dimers embedded in segregated hydrophobic domains dispersed within the PEG matrix as comfirmed by cryo-electron microscopy. The amphiphilic network was swollen with high weight fractions of water (w(H2O) ≈ 0.8) owing to the high PEG weight fraction within the pristine polymers (w(PEG) ≈ 0.9). Two different PEG chain lengths were investigated and illustrate the corresponding consequences of cross-link density on mechanical properties. The resulting hydrogels exhibited high strength and resilience upon deformation, consistent with a microphase separated network, in which the UPy-UPy interactions were adequately shielded within hydrophobic nanoscale pockets that maintain the network despite extensive water content. The cumulative result is a series of tough hydrogels with tunable mechanical properties and tractable synthetic preparation and processing. Furthermore, the melting transition of PEG in the dry polymer was shown to be an effective stimulus for shape memory behavior.
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http://dx.doi.org/10.1021/ja500205vDOI Listing
May 2014

Long-range repulsion of colloids driven by ion exchange and diffusiophoresis.

Proc Natl Acad Sci U S A 2014 May 18;111(18):6554-9. Epub 2014 Apr 18.

Department of Mechanical Engineering and Institute for Complex Molecular Systems , Eindhoven University of Technology, 5612 AZ, Eindhoven, The Netherlands.

Interactions between surfaces and particles in aqueous suspension are usually limited to distances smaller than 1 μm. However, in a range of studies from different disciplines, repulsion of particles has been observed over distances of up to hundreds of micrometers, in the absence of any additional external fields. Although a range of hypotheses have been suggested to account for such behavior, the physical mechanisms responsible for the phenomenon still remain unclear. To identify and isolate these mechanisms, we perform detailed experiments on a well-defined experimental system, using a setup that minimizes the effects of gravity and convection. Our experiments clearly indicate that the observed long-range repulsion is driven by a combination of ion exchange, ion diffusion, and diffusiophoresis. We develop a simple model that accounts for our data; this description is expected to be directly applicable to a wide range of systems exhibiting similar long-range forces.
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http://dx.doi.org/10.1073/pnas.1322857111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4020040PMC
May 2014

Monocytic cells become less compressible but more deformable upon activation.

PLoS One 2014 27;9(3):e92814. Epub 2014 Mar 27.

Department of Biomedical Engineering, Eindhoven University of Technology, Eindhoven, the Netherlands; Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, the Netherlands.

Aims: Monocytes play a significant role in the development of atherosclerosis. During the process of inflammation, circulating monocytes become activated in the blood stream. The consequent interactions of the activated monocytes with the blood flow and endothelial cells result in reorganization of cytoskeletal proteins, in particular of the microfilament structure, and concomitant changes in cell shape and mechanical behavior. Here we investigate the full elastic behavior of activated monocytes in relation to their cytoskeletal structure to obtain a better understanding of cell behavior during the progression of inflammatory diseases such as atherosclerosis.

Methods And Results: The recently developed Capillary Micromechanics technique, based on exposing a cell to a pressure difference in a tapered glass microcapillary, was used to measure the deformation of activated and non-activated monocytic cells. Monitoring the elastic response of individual cells up to large deformations allowed us to obtain both the compressive and the shear modulus of a cell from a single experiment. Activation by inflammatory chemokines affected the cytoskeletal organization and increased the elastic compressive modulus of monocytes with 73-340%, while their resistance to shape deformation decreased, as indicated by a 25-88% drop in the cell's shear modulus. This decrease in deformability is particularly pronounced at high strains, such as those that occur during diapedesis through the vascular wall.

Conclusion: Overall, monocytic cells become less compressible but more deformable upon activation. This change in mechanical response under different modes of deformation could be important in understanding the interplay between the mechanics and function of these cells. In addition, our data are of direct relevance for computational modeling and analysis of the distinct monocytic behavior in the circulation and the extravascular space. Lastly, an understanding of the changes of monocyte mechanical properties will be important in the development of diagnostic tools and therapies concentrating on circulating cells.
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http://journals.plos.org/plosone/article?id=10.1371/journal.pone.0092814PLOS
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3968036PMC
December 2015

Capillary micromechanics for core-shell particles.

Soft Matter 2014 May 13;10(18):3271-6. Epub 2014 Mar 13.

Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong.

In this work, we have developed a facile, economical microfluidic approach as well as a simple model description to measure and predict the mechanical properties of composite core-shell microparticles made from materials with dramatically different elastic properties. By forcing the particles through a tapered capillary and analyzing their deformation, the shear and compressive moduli can be measured in one single experiment. We have also formulated theoretical models that accurately capture the moduli of the microparticles in both the elastic and the non-linear deformation regimes. Our results show how the moduli of these core-shell structures depend on the material composition of the core-shell microparticles, as well as on their microstructures. The proposed technique and the understanding enabled by it also provide valuable insights into the mechanical behavior of analogous biomaterials, such as liposomes and cells.
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http://dx.doi.org/10.1039/c3sm53066cDOI Listing
May 2014

Mesoscale characterization of supramolecular transient networks using SAXS and rheology.

Int J Mol Sci 2014 Jan 16;15(1):1096-111. Epub 2014 Jan 16.

Institute for Complex Molecular Systems, Eindhoven University of Technology, PO Box 513, 5600 MB, Eindhoven, The Netherlands.

Hydrogels and, in particular, supramolecular hydrogels show promising properties for application in regenerative medicine because of their ability to adapt to the natural environment these materials are brought into. However, only few studies focus on the structure-property relationships in supramolecular hydrogels. Here, we study in detail both the structure and the mechanical properties of such a network, composed of poly(ethylene glycol), end-functionalized with ureido-pyrimidinone fourfold hydrogen bonding units. This network is responsive to triggers such as concentration, temperature and pH. To obtain more insight into the sol-gel transition of the system, both rheology and small-angle X-ray scattering (SAXS) are used. We show that the sol-gel transitions based on these three triggers, as measured by rheology, coincide with the appearance of a structural feature in SAXS. We attribute this feature to the presence of hydrophobic domains where cross-links are formed. These results provide more insight into the mechanism of network formation in these materials, which can be exploited for tailoring their behavior for biomedical applications, where one of the triggers discussed might be used.
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http://dx.doi.org/10.3390/ijms15011096DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3907858PMC
January 2014

Towards the self-assembly of anisotropic colloids: monodisperse oblate ellipsoids.

J Colloid Interface Sci 2014 Feb 28;416:30-7. Epub 2013 Oct 28.

Eindhoven University of Technology, Materials Technology, Eindhoven, Netherlands; Institute for Complex Molecular Systems, Eindhoven University of Technology, Eindhoven, Netherlands. Electronic address:

We present a robust and straightforward method for producing colloidal particles of oblate ellipsoidal shape via thermo/mechanical stretching of elastomeric films with embedded spherical particles. Our method produces uniformly sized and shaped colloidal particles. The method can be used for producing biaxially stretched particles of different aspect ratios and volumes; moreover, the method has a higher yield and batch size than previously reported methods for producing non-spherical particles via film stretching. These particles are ideal model systems for studying the self-assembly and gel formation for systems with anisotropic shapes and interactions. We illustrate this by adding of a non-adsorbing polymer to the solvent, thereby inducing directional depletion interactions between the particles.
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http://dx.doi.org/10.1016/j.jcis.2013.10.027DOI Listing
February 2014

Kinetic model for the mechanical response of suspensions of sponge-like particles.

Faraday Discuss 2012 ;158:407-24; discussion 493-522

Eindhoven University of Technology, Mechanical Engineering, Materials Technology (MaTe), P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

A dynamic two-scale model is developed that describes the stationary and transient mechanical behavior of concentrated suspensions made of highly porous particles. Particularly, we are interested in particles that not only deform elastically, but also can swell or shrink by taking up or expelling the viscous solvent from their interior, leading to rate-dependent deformability of the particles. The fine level of the model describes the evolution of particle centers and their current sizes, while the shapes are at present not taken into account. The versatility of the model permits inclusion of density- and temperature-dependent particle interactions, and hydrodynamic interactions, as well as to implement insight into the mechanism of swelling and shrinking. The coarse level of the model is given in terms of macroscopic hydrodynamics. The two levels are mutually coupled, since the flow changes the particle configuration, while in turn the configuration gives rise to stress contributions, that eventually determine the macroscopic mechanical properties of the suspension. Using a thermodynamic procedure for the model development, it is demonstrated that the driving forces for position change and for size change are derived from the same potential energy. The model is translated into a form that is suitable for particle-based Brownian dynamics simulations for performing rheological tests. Various possibilities for connection with experiments, e.g. rheological and structural, are discussed.
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http://dx.doi.org/10.1039/c2fd20025bDOI Listing
December 2012

Injectable hydrogels from segmented PEG-bisurea copolymers.

Biomacromolecules 2012 Dec 28;13(12):3966-76. Epub 2012 Nov 28.

Laboratory for Macromolecular and Organic Chemistry, Department of Mechanical Engineering, Institute for Complex Molecular Systems, Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, The Netherlands.

We describe the preparation of an injectable, biocompatible, and elastic segmented copolymer hydrogel for biomedical applications, with segmented hydrophobic bisurea hard segments and hydrophilic PEG segments. The segmented copolymers were obtained by the step growth polymerization of amino-terminated PEG and aliphatic diisocyanate. Due to their capacity for multiple hydrogen bonding within the hydrophobic segments, these copolymers can form highly stable gels in water at low concentrations. Moreover, the gels show shear thinning by a factor of 40 at large strain, which allows injection through narrow gauge needles. Hydrogel moduli are highly tunable via the physical cross-link density and the length of the hydrophilic segments. In particular, the mechanical properties can be optimized to match the properties of biological host tissues such as muscle tissue and the extracellular matrix.
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http://dx.doi.org/10.1021/bm301242vDOI Listing
December 2012

Biophysical properties of normal and diseased renal glomeruli.

Am J Physiol Cell Physiol 2011 Mar 1;300(3):C397-405. Epub 2010 Dec 1.

Physics, Harvard University, Cambridge, Massachusetts, USA.

The mechanical properties of tissues and cells including renal glomeruli are important determinants of their differentiated state, function, and responses to injury but are not well characterized or understood. Understanding glomerular mechanics is important for understanding renal diseases attributable to abnormal expression or assembly of structural proteins and abnormal hemodynamics. We use atomic force microscopy (AFM) and a new technique, capillary micromechanics, to measure the elastic properties of rat glomeruli. The Young's modulus of glomeruli was 2,500 Pa, and it was reduced to 1,100 Pa by cytochalasin and latunculin, and to 1,400 Pa by blebbistatin. Cytochalasin or latrunculin reduced the F/G actin ratios of glomeruli but did not disrupt their architecture. To assess glomerular biomechanics in disease, we measured the Young's moduli of glomeruli from two mouse models of primary glomerular disease, Col4a3(-/-) mice (Alport model) and Tg26(HIV/nl) mice (HIV-associated nephropathy model), at stages where glomerular injury was minimal by histopathology. Col4a3(-/-) mice express abnormal glomerular basement membrane proteins, and Tg26(HIV/nl) mouse podocytes have multiple abnormalities in morphology, adhesion, and cytoskeletal structure. In both models, the Young's modulus of the glomeruli was reduced by 30%. We find that glomeruli have specific and quantifiable biomechanical properties that are dependent on the state of the actin cytoskeleton and nonmuscle myosins. These properties may be altered early in disease and represent an important early component of disease. This increased deformability of glomeruli could directly contribute to disease by permitting increased distension with hemodynamic force or represent a mechanically inhospitable environment for glomerular cells.
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http://dx.doi.org/10.1152/ajpcell.00438.2010DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3063968PMC
March 2011

Temperature-controlled transitions between glass, liquid, and gel states in dense p-NIPA suspensions.

Adv Mater 2010 Aug;22(31):3441-5

Department of Physics & SEAS, Harvard University, Cambridge, MA 02138, USA.

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http://dx.doi.org/10.1002/adma.200904189DOI Listing
August 2010

Soft colloids make strong glasses.

Nature 2009 Nov;462(7269):83-6

Department of Physics and Harvard School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA.

Glass formation in colloidal suspensions has many of the hallmarks of glass formation in molecular materials. For hard-sphere colloids, which interact only as a result of excluded volume, phase behaviour is controlled by volume fraction, phi; an increase in phi drives the system towards its glassy state, analogously to a decrease in temperature, T, in molecular systems. When phi increases above phi* approximately 0.53, the viscosity starts to increase significantly, and the system eventually moves out of equilibrium at the glass transition, phi(g) approximately 0.58, where particle crowding greatly restricts structural relaxation. The large particle size makes it possible to study both structure and dynamics with light scattering and imaging; colloidal suspensions have therefore provided considerable insight into the glass transition. However, hard-sphere colloidal suspensions do not exhibit the same diversity of behaviour as molecular glasses. This is highlighted by the wide variation in behaviour observed for the viscosity or structural relaxation time, tau(alpha), when the glassy state is approached in supercooled molecular liquids. This variation is characterized by the unifying concept of fragility, which has spurred the search for a 'universal' description of dynamic arrest in glass-forming liquids. For 'fragile' liquids, tau(alpha) is highly sensitive to changes in T, whereas non-fragile, or 'strong', liquids show a much lower T sensitivity. In contrast, hard-sphere colloidal suspensions are restricted to fragile behaviour, as determined by their phi dependence, ultimately limiting their utility in the study of the glass transition. Here we show that deformable colloidal particles, when studied through their concentration dependence at fixed temperature, do exhibit the same variation in fragility as that observed in the T dependence of molecular liquids at fixed volume. Their fragility is dictated by elastic properties on the scale of individual colloidal particles. Furthermore, we find an equivalent effect in molecular systems, where elasticity directly reflects fragility. Colloidal suspensions may thus provide new insight into glass formation in molecular systems.
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http://dx.doi.org/10.1038/nature08457DOI Listing
November 2009

Strain-rate frequency superposition: a rheological probe of structural relaxation in soft materials.

Phys Rev Lett 2007 Jun 7;98(23):238303. Epub 2007 Jun 7.

Department of Physics & HSEAS, Harvard University, Cambridge, Massachusetts 02138, USA.

The rheological properties of soft materials often exhibit surprisingly universal linear and nonlinear features. Here we show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, strain-rate frequency superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.
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http://dx.doi.org/10.1103/PhysRevLett.98.238303DOI Listing
June 2007

Mechanism for clogging of microchannels.

Phys Rev E Stat Nonlin Soft Matter Phys 2006 Dec 11;74(6 Pt 1):061402. Epub 2006 Dec 11.

Department of Physics and DEAS, Harvard University, Cambridge, Massachusetts 02138, USA.

We investigate clogging of microchannels at the single-pore level using microfluidic devices as model porous media. The process of clogging is studied at low volume fractions and high flow rates, a technologically important regime. We show that clogging is independent of particle flow rate and volume fraction, indicating that collective effects do not play an important role. Instead, the average number of particles that can pass through a pore before it clogs scales with the ratio of pore to particle size. We present a simple model that accounts for the data.
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http://dx.doi.org/10.1103/PhysRevE.74.061402DOI Listing
December 2006

Fluids of clusters in attractive colloids.

Phys Rev Lett 2006 Jan 18;96(2):028306. Epub 2006 Jan 18.

Department of Physics, Harvard University, Cambridge, Massachusetts 02138, USA.

We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact.
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http://dx.doi.org/10.1103/PhysRevLett.96.028306DOI Listing
January 2006

Quantification of microstructures in stable and gelated suspensions from Cryo-SEM.

J Colloid Interface Sci 2002 Apr;248(2):340-6

Nonmetallic Materials, Department of Materials, ETH Zurich, Switzerland.

It is shown that the microstructures of concentrated suspensions can be analyzed in a quantitative way from cryo-SEM images of high-pressure frozen samples, both in the electrostatically stabilized and in the flocculated state. Suspensions of spherical silica particles (40 vol%) in an aqueous solution were used. The average particle radius was 525 nm with a polydispersity of below 7%. The suspensions were high-pressure frozen, which resulted in a quenching of the configuration without apparent change in volume or crack formation. After fracturing the samples at liquid nitrogen temperature, the fracture surface was etched by controlled sublimation of the frozen aqueous phase, coated with 8 nm of platinum, and examined by stereo-cryo-SEM. The 3-dimensional positions of all the visible particles were determined from the SEM images. Assuming an isotropic particle configuration in the sample before cracking, it is possible to extract the 3-dimensional pair correlation function from the particle positions on the fracture surface. A comparison to recent results from Brownian Dynamics simulations shows good agreement between our experiments and the simulations.
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http://dx.doi.org/10.1006/jcis.2001.8220DOI Listing
April 2002

Relation between microstructure and mechanical behavior of concentrated silica gels.

J Colloid Interface Sci 2004 May;273(2):455-62

Nonmetallic Materials, Department of Materials, ETH Zurich, Zurich, CH-8092, Switzerland.

We produce concentrated (40 vol%) gels of uniformly sized silica particles by an in situ process, based on the enzyme-catalyzed hydrolysis of urea in the liquid phase of electrostatically stabilized suspensions. Two different methods are used: Either the pH of the suspensions is shifted toward the isoelectric point of the particles (delta pH method), or the ionic strength is continuously increased at constant pH (deltaI method). We compare the two kinds of gels in terms of elastic and yield behavior as well as microstructure by using rheological measurements in oscillation and high-pressure freezing in combination with cryo-SEM, respectively. Results suggest a strong increase of elastic and yield properties in concentrated particle gels with decreasing homogeneity of their microstructures.
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http://dx.doi.org/10.1016/j.jcis.2004.01.065DOI Listing
May 2004

Small-angle static light scattering of concentrated silica suspensions during in situ destabilization.

J Colloid Interface Sci 2004 Mar;271(2):388-99

Nonmetallic Materials, Department of Materials, ETH Zurich, Zurich 8092, Switzerland.

The aggregation of concentrated aqueous silica suspensions is characterized by means of static light scattering. We use an in situ destabilization mechanism based on the enzyme-catalyzed hydrolysis of urea. This method enables us to continuously and homogeneously change the interparticle potential from repulsive to attractive without disturbing the aggregation process. Moreover, our electrostatically stabilized suspensions can be destabilized by two different methods. In the first method, the pH is shifted toward the isoelectric point of the particles ( Delta pH method), thereby leading to a decrease of their surface charge. In the second method, the ionic strength is continuously increased at constant pH ( Delta I method), leading to a compression of the electrical double layer around the charged particles. A laboratory-built flat-cell light-scattering instrument is used, which allows fast data acquisition and an adjustment of the sample cell thickness. To circumvent multiple scattering effects, we use a very small sample thickness ( approximately 13 microm). In addition, the refractive index difference between the aqueous phase and the particles is reduced by adding sucrose to the liquid phase of our suspensions. We are able to characterize the structural changes at the very early stages of the destabilization process, where no significant effects are yet detected in macroscopic rheological measurements. While during the Delta pH destabilization, the scattering curve shows significant changes only after some characteristic delay time, it changes continuously during the Delta I destabilization. The latter is attributed to the formation of a weak pre-gel structure in the suspensions, as a shallow secondary minimum appears in the interparticle potential. Data are evaluated by using a HMSA square-well structure factor model. Results are in good agreement with those predicted from DLVO theory.
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http://dx.doi.org/10.1016/j.jcis.2003.09.051DOI Listing
March 2004

Diffusing-Wave Spectroscopy of Concentrated Alumina Suspensions during Gelation.

J Colloid Interface Sci 2001 Sep;241(1):89-97

Nonmetallic Materials, Department of Materials, ETH Zurich, Zurich, CH-8092, Switzerland

Using diffusing-wave spectroscopy, we followed the aggregation and gelation of concentrated (30 vol%) alumina suspensions. The suspensions were destabilized by either shifting the pH to the isoelectric point or by increasing the ionic strength. Both effects can be achieved continuously and homogeneously by using an enzyme-catalyzed internal chemical reaction. Based on the light-scattering data, we could derive quantitative information about the sol-gel transition and the viscoelastic properties of the gels, as well as a characterization of changes in the microstructure. The elastic moduli determined from light scattering are found to be in good agreement with rheological measurements. Copyright 2001 Academic Press.
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http://dx.doi.org/10.1006/jcis.2001.7668DOI Listing
September 2001
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