Doubly cross-linked gels were fabricated based on tetra-poly(ethylene glycol) (Tetra-PEG) by shear strain. These are gels with two network structures present in the same polymeric network. The second network structure is introduced by applying a mechanical field to the first natural network structure. These doubly cross-linked gels indicated a negative energy elasticity supporting earlier findings where the energy elasticity was found significantly negative for Tetra-PEG gel. Acquired results indicate implications for past research on the elasticity of polymer gels where the energy contribution was approximated to zero. Obtained results also indicated that the modulus of rigidity for the doubly cross-linked gels is constant regardless of applied shear strain during fabrication. This would indicate that the same second network structure is formed for the interval of 25-800% applied shear strain. The residual strain for the fabricated gels can be well-described using an exponential fitting of the apparent shear modulus of the first network structure and an expression derived from the two-network theory and classic rubber theory. These theories also seem to predict the experimental residual strains for lower strain regions (<100%) quite well. However for larger strain regions (>100%) non-linear effects seem to affect the results causing a deviation. A slight increased modulus of rigidity was noted for the doubly cross-linked gels compared to the regular Tetra-PEG gel. However as the reproducibility of the concluded measurements could not be confirmed during this thesis the results are not conclusive and only indicate the conclusions mentioned above.

The diploma thesis is focused on the study of interactions between biopolymers and model probes in a hydrogel media utilizing diffusion processes. The main aim was to test and to optimize selected combination of diffusion methods (diffusion cells and unsteady diffusion) in study of interactions between biopolymers and organic dyes. Anionic (humic acids) and cationic (chitosan) representatives of biopolymers were utilized and methylene blue and methylene orange were used as a reactivity probe in the case of humic acids and chitosan, respectively. Another appreciable part of the work is represented by in-depth characterization of applied biopolymers and hydrogel samples by a wide range of analytical and physico-chemical methods (rheometry, UV-VIS, FT-IR etc.). Besides particular experimental results on studied systems, the diploma thesis provides both theoretical and practical background for subsequent applications of the innovative and full-automated method for reactivity mapping of biopolymers.

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Ziel der vorliegenden Arbeit ist es, das Verhalten polyelektrolytischer Gele und Hydrogelschichtsysteme auf Basis der Kontinuumsmechanik zu modellieren. Die Untersuchung des Materialverhaltens gegenüber externer Stimulation erfolgt anhand numerischer Simulationen, wodurch Einblicke in den komplexen Quellprozess und darin auftretende Phänomene gewährt werden. Die vorgenommene Modellierung und Simulation gestattet dabei eine Optimierung der Systemeigenschaften für den Anwendungsfall. Anwendungsfehler und die Anzahl nötiger Versionen zur Systemgestaltung können hierdurch effektiv verringert werden. Hydrogele stellen wichtige Vertreter aus der Klasse intelligenter Materialien dar, d.h. sie sind in der Lage auf Umwelteinflüsse durch eine reversible Änderung ihrer Materialeigenschaften zu reagieren. Sie bestehen aus einer mit Wasser gefüllten Polymermatrix, in welcher ionische Ladungsträger vorliegen. Dabei sind mobile Ladungsträger im Wasser enthalten. Die Materialeigenschaften von Hydrogelen beruhen auf ihrer chemischen Zusammensetzung und können jeweils spezifisch für ihren Anwendungsfall angepasst werden. So reagieren polyelektrolytische Hydrogele mit einem reversiblen Quell- bzw. Schrumpfungsprozess auf externe elektrische Felder und die Änderung der chemischen Zusammensetzung im umgebenden Lösungsmittelbad. Die reversible Volumenänderung gegenüber externer Stimuli eröffnet Hydrogelen ein breites Anwendungsfeld. Insbesondere sind sie für die Entwicklung neuer Messsysteme relevant, da über klassische Messgrößen hinaus auch sehr spezifische chemische Größen untersucht werden können. Auch eignen sich Hydrogele als aktive Komponenten mikrofluidischer Ventile, welche auf die Zusammensetzung des Fluides reagieren. Da sie keine externe Ansteuerung bzw. Energieversorgung benötigen, sind sie leichter miniaturisierbar als klassische Ventile. Aufgrund ihrer hohen Leistungsdichte sind Hydrogele prinzipiell auch für die Entwicklung leichter und energieeffizienter Aktoren geeignet. Hierbei stellt die von der Größe des Hydrogels abhängige Reaktionszeit bislang jedoch eine Hürde dar, wodurch ihr Einsatz in kleinskaligen Anwendungen wahrscheinlicher ist. Die Kombination mehrerer Schichten aus Hydrogelen ermöglicht hierbei die Anwendung als Biegeaktoren bzw. Messsysteme mit erhöhter Genauigkeit. Da ein fundiertes Wissen über die komplexen Vorgänge im Hydrogel eine große Rolle bei der Weiterentwicklung neuartiger Anwendungen spielt, wird in dieser Arbeit das komplexe Materialverhalten numerisch abgebildet. Hierbei wird insbesondere Wert auf die Interaktion der chemischen, elektrischen sowie der mechanischen Domäne gelegt. Dabei wird a priori angenommen, dass die komplizierte Mikrostruktur des porösen Hydrogels als Kontinuum darstellbar ist und die relevanten Phänomene über Feldgleichungen abgebildet werden können. Es wird eine Einführung über polyelektrolytische Hydrogele und Hydrogelschichtsysteme gegeben, wobei insbesondere auf deren mikrostrukturellen Aufbau und daraus ableitbare Einsatzgebiete eingegangen wird. Mittels eines Überblicks über Modellierungsansätze in der publizierten Literatur werden die hier verwendeten Modellierungsansätze motiviert und in das Umfeld bestehender Vorarbeiten eingeordnet. Nach der Einführung notwendiger Grundlagen der physikalischen Chemie wird die Modellbildung mit der gewählten Kinematik, den Bilanzgleichungen und den Materialgleichungen zur Beschreibung von Hydrogelen und Hydrogelschichtsystemen vorgestellt. Die Bilanzgleichungen umfassen hierbei die Massenerhaltung des Polymers und der ionischen Spezies, die Impuls- und Drehimpulsbilanz sowie die Maxwell-Gleichungen. Nach der Beschreibung geeigneter Materialgleichungen folgt eine Zusammenstellung des gekoppelten chemo-elektro-mechanischen Feldproblems. Zur Lösung des gekoppelten Feldproblems wird die Finite-Elemente-Methode (FEM) genutzt. Die Validierung des erstellten Modells wird anhand eines Quellexperiments von Frijns et al. durchgeführt. Durch den Vergleich mit einem auf der Theorie Poröser Medien (TPM) basierenden Modells kann das hier verwendete Modell abgeglichen werden, wobei eine gute Übereinstimmung zwischen den Ergebnissen beider Modelle herrscht. Basierend auf den Vorarbeiten von Wallmersperger et al. und Attaran et al. wurde das hier genutzte Modell um einen zeitlichen Term in der Beschreibung der Referenzkonzentration erweitert. Hieraus resultiert eine qualitative Verbesserung in der Darstellung des zeitlichen Quellverlaufes bei chemischer Stimulation. In Anlehnung an vorhergehende Arbeiten auf dem Gebiet erfolgt die Kopplung von der mechanischen Domäne zur chemischen Domäne über eine vom Verzerrungszustand abhängige Konzentration gebundener Ladungsträger. Hier durchgeführte Untersuchungen zeigen, dass diese Rückkopplung auch unter der Annahme kleiner Deformationen Relevanz besitzt und nicht vernachlässigt werden sollte. Anders als z.B. mit der TPM oder mit dem Flory-Rehner Modell ist es unter Verwendung des hier entwickelten Modells möglich, Grenzschichtphänomene zwischen Gel und Lösungsmittelbad aufzulösen. Durch die Untersuchung der Grenzschicht zeigt sich eine annähernd lineare Abhängigkeit der Grenzschichtdicke von der relativen Permittivität des Hydrogels. Auch lässt sich ein Zusammenhang zwischen der Konzentration gebundener Ladungsträger und der Grenzschichtdicke identifizieren. Um das Potential der gewählten Methode zu demonstrieren, wird anhand des Beispiels eines einfachen zweischichtigen Hydrogel-Biegebalkens eine Untersuchung über die inneren Vorgänge im Schichtsystem durchgeführt. Die Untersuchung gewährt hierbei Einblicke z.B. in den zeitlichen Verlauf des elektrischen Potentials, der Konzentration mobiler Ionen sowie der resultierenden Verzerrungen. In einem abschließenden numerischen Beispiel wird die Kontaktkraft sowie die auftretenden mechanischen Spannungen in einem Greifersystem, bestehend aus zwei hydrogelbasierten Biegeaktoren, ausgewertet. / Aim of this work is to model the behavior of polyelectrolyte gels and hydrogel-layer systems based on continuum mechanics. For this, numerical simulations are conducted to gain an insight into the complex phenomena occurring during the swelling process of hydrogels. The process of modeling and simulation is important to enhance the understanding of these systems and to improve the properties of hydrogel-based applications. Also, errors resulting from a lack of knowledge as well as from the number of required experimental works can be reduced significantly. Hydrogels belong to the class of smart materials. Therefore, they show a response to changes in their environment by a reversible change of material properties. Polyelectrolyte hydrogels consist of a polymer network with fixed ionic groups. Within the void of the polymer matrix, interstitial water containing mobile ions is present. The properties of hydrogels depend on their chemical composition and can be adjusted for the specified application. They exhibit a reversible swelling or deswelling process when subjected to electric fields or changes of the chemical composition of the surrounding solution bath. This reversible volume change opens a wide field of potential applications. The most promising field of application seems to be in the realms of measurement systems, since hydrogels can be used for the investigation of very specific chemical or biochemical measures. In addition, hydrogels can be used as active components in microfluidic devices, which open and close depending on the chemical composition of the bypassing fluid. Without a need for external control and energy supply, they are suitable for miniaturisation. As a consequence of their high energy density, hydrogels are also relevant candidates for the development of lightweight and energy efficient actuator systems. Due to their size-dependent reaction time, their use in small-scale actuatoric devices is the most promising in this field. the combination of multiple hydrogel layers can be used to increase the sensitivity of hydrogel-based measurement devices. Hydrogel-layer systems can also be used for actuatoric devices, such as gripper devices or microfluidic valves. In the presented model, the interaction between the electrical, chemical and the mechanical fields are respected. It is assumed, that the complex microstructure of hydrogels may be treated as a continuum and that the relevant phenomena can be described using field equations. A brief introduction on polyelectrolyte gels and hydrogel-layer systems is given. Here, their microstructure is described and relevant applications are summarized. By giving a brief literature review, the modeling approaches of this thesis are motivated and arranged within the field of existing models. After an introduction of relevant basics of physical chemistry, the model formulation is presented. Here, the chosen kinematics as well as the used balance equations and material laws are given. They include the conservation of mass for the polymer and the ionic species, the conservation of linear momentum and angular momentum as well as the Maxwell equations. After presenting suitable material laws, the field equations of the fully coupled chemo-electro-mechanical field problem are summarized. The numerical solution of the field problem is obtained by applying the Finite-Element-Method (FEM). The validation of the presented model is conducted by a comparison with experimental results of Frijns et al. After this, the used model is compared with the Theory of Porous Media (TPM), which results in a good agreement of both approaches. Since the used model represents an extension of the preliminary work of Wallmersperger et al. and Attaran et al., differences in the models are discussed. By extending the previous models by an additional temporal term in the description of the reference concentration, a qualitative improvement of the prediction of the time behavior under chemical stimulation is achieved. As done in preliminary works, the mechanical and the chemical domains are coupled by a strain dependent concentration of fixed charges. It can be shown, that this backcoupling is of significance also for small deformations. In contrast to the TPM and the Flory-Rehner model, phenomena occurring in the interface layer between adjacent domains can be investigated using the presented model. An almost linear dependence between the relative permittivity of the hydrogel and the thickness of the boundary layer is observed. It also depends on the concentration of fixed charges in the polymer network. To demonstrate the potential of the applied method, a numerical investigation of a two-layered hydrogel bending actuator is investigated. The results give an insight for example into the time-dependent distribution of the electric potential, the concentration of mobile ions as well as the resulting strains. In another numerical example, the contact force in a gripper consisting of two hydrogel-based bending actuators is evaluated.

<p>Superabsorbent polymer (SAP) hydrogel particles have been used as internal curing agents in concrete mixes as they are capable of absorbing and subsequently releasing large amounts of water. This reduces autogenous shrinkage during early stages of hydration. The size, shape, and composition of the hydrogel particles can be controlled during the synthesis, hence providing the opportunity to custom synthesize these internal curing agents to elicit desired structure-property relationships. Utilization of optimized dosage and formulation of SAP has the potential to improve the microstructure, durability, and strength of internally cured concrete. </p> <p>The first study focuses on the synthesis and application of novel composite hydrogel particles as internal curing agents in cementitious mixes. Composite polyacrylamide hydrogel particles containing two different amorphous silica–either nanosilica or silica fume–were used to investigate whether the internal curing performance of hydrogel particles could be enhanced. The dosage and type of silica, crosslinker amount were varied to identify the composite polyacrylamide hydrogel particle composition that provides optimum benefits to internally cured cementitious systems. The synthesized hydrogels were characterized by means of absorption capacity tests, compositional and size analysis. The beneficial impacts of the addition of composite hydrogels on cement paste microstructure are highlighted, including the preferential formation of cement hydration products (such as portlandite) within the hydrogel-induced voids that appeared to be influenced by the composition of the hydrogel particles. The interrelationship between extent of hydration, size of hydrogel voids, and void-filling with hydration products was found to strongly influence mechanical strength and is thus an important structure-property relationship to consider when selecting hydrogels for internal curing purposes. This study informs the design of composite hydrogel particles to optimize performance in cementitious mixes. Additionally, it provides a novel means of incorporating other commonly used admixtures in concrete without facing common challenges related to dispersion and health hazards.</p> <p>The second study focuses on the utilization of two retarding admixture-citric acid and sucrose-to custom synthesize composite polyacrylamides to investigate whether the composite hydrogels could delay hydration of cement paste. Isothermal calorimetry analysis results showed that composite sucrose-containing polyacrylamide hydrogel particles were successfully able to retard main hydration peak of cement paste, beyond the retardation capabilities of the pure polyacrylamide hydrogels. Thus, this study provides avenues of exploring the utilization of common admixtures to formulate novel composite hydrogels that imparts specific properties to cementitious systems.</p> <p>In another study, SAP formulated by admixture industries were used to investigate the feasibility of internal curing of bridge decks and pavement patches with SAP particles. The microstructure and early age hydration properties of SAP-cured cementitious systems were studied. Mitigation of microcracks in the matrix, along with portlandite growth in SAP voids, were observed in SAP-cured mortars. Presence of SAP also mitigated autogenous shrinkage and improved early age hydration as observed by isothermal calorimetry analysis. This thesis highlights some of the beneficial impacts of SAP-cured cementitious systems, and the potential to harness those benefits in large-scale applications of SAP-cured concrete.</p> <br>

No description available.

Understanding cell migration is a key step in unraveling many physiological phenomena and predicting several pathologies, such as cancer metastasis. In particular, confinement has been proven to be a key factor in the cellular migration strategy choice. As our insight in the field improves, new tools are needed in order to empower biologists’ analysis capabilities. In this framework, microfluidic devices have been used to engineer the mechanical and spatial stimuli and to investigate cellular migration response in a more controlled way. In this work, we will review the existing technologies employed in the realization of microfluidic cellular migration assays, namely the soft lithography of PDMS and hydrogels and femtosecond laser micromachining. We will give an overview of the state of the art of these devices, focusing on the different geometrical configurations that have been exploited to study specific aspects of cellular migration. Our scope is to highlight the advantages and possibilities given by each approach and to envisage the future developments in in vitro migration studies under spatial confinement in microfluidic device

Biological hydrogels are highly promising materials for bone tissue engineering (BTE) due to their high biocompatibility and biomimetic characteristics. However, for advanced and customized BTE, precise tools for material stabilization and tuning material properties are desired while optimal mineralisation must be ensured. Therefore, reagent-free crosslinking techniques such as high energy electron beam treatment promise effective material modifications without formation of cytotoxic by-products. In the case of the hydrogel gelatin, electron beam crosslinking further induces thermal stability enabling biomedical application at physiological temperatures. In the case of enzymatic mineralisation, induced by Alkaline Phosphatase (ALP) and mediated by Calcium Glycerophosphate (CaGP), it is necessary to investigate if electron beam treatment before mineralisation has an influence on the enzymatic activity and thus affects the mineralisation process. The presented study investigates electron beam-treated gelatin hydrogels with previously incorporated ALP and successive mineralisation via incubation in a medium containing CaGP. It could be shown that electron beam treatment optimally maintains enzymatic activity of ALP which allows mineralisation. Furthermore, the precise tuning of material properties such as increasing compressive modulus is possible. This study characterizes the mineralised hydrogels in terms of mineral formation and demonstrates the formation of CaP in dependence of ALP concentration and electron dose. Furthermore, investigations of uniaxial compression stability indicate increased compression moduli for mineralised electron beam-treated gelatin hydrogels. In summary, electron beam-treated mineralized gelatin hydrogels reveal good cytocompatibility for MG-63 osteoblast like cells indicating a high potential for BTE applications.

Biological materials such as extracellular matrix scaffolds, cancer cells, and tissues are often assumed to respond elastically for simplicity; the viscoelastic response is quite commonly ignored. Extracellular matrix mechanics including the viscoelasticity has turned out to be a key feature of cellular behavior and the entire shape and function of healthy and diseased tissues, such as cancer. The interference of cells with their local microenvironment and the interaction among different cell types relies both on the mechanical phenotype of each involved element. However, there is still not yet clearly understood how viscoelasticity alters the functional phenotype of the tumor extracellular matrix environment. Especially the biophysical technologies are still under ongoing improvement and further development. In addition, the effect of matrix mechanics in the progression of cancer is the subject of discussion. Hence, the topic of this review is especially attractive to collect the existing endeavors to characterize the viscoelastic features of tumor extracellular matrices and to briefly highlight the present frontiers in cancer progression and escape of cancers from therapy. Finally, this review article illustrates the importance of the tumor extracellular matrix mechano-phenotype, including the phenomenon viscoelasticity in identifying, characterizing, and treating specific cancer types.

No description available.