The tissue microenvironment is a complex living system containing heterogeneous mechanical and biophysical cues. Cellular components are surrounded by extracellular matrix and interstitial fluid, while transport of nutrients and biochemical factors is achieved via the vasculature. Each constituent of the tissue microenvironment can play a significant role in its ability to function normally. Many diseases including cancer have been linked with dysfunction in the tissue microenvironment; therefore an improved understanding of interaction between components of this complex system is needed. In vitro platforms mimicking the tissue microenvironment appear to provide the most promising avenue for studies of cell-cell and cell-matrix interactions as well as elucidation of the mechanisms leading to disease phenomena such as tumor metastasis. However, successful recapitulation of all three primary components of the tissue microenvironment in three dimensions has remained challenging. In particular, matching mechanical cues and biochemical transport to in vivo conditions is difficult because of lack of quantitative characterization of the physical properties and parameters of such platforms. In this work, extensive characterization of collagen I hydrogels, popular for use as extracellular matrix mimics, was performed in order to enable tuning to specific in vivo conditions. Additionally, perfusion of blood in a 3D tissue microenvironment platform fabricated using collagen hydrogels was characterized to enable future advances in in vitro modeling of the in vivo microenvironment. Finally, the tissue microenvironment platform is modified to enable biochemical gradients within the hydrogel and used to examine directed migration (chemotaxis) of human breast cancer cells in response to gradients in growth factor combined with varied stiffness and pore diameter of the extracellular matrix. / Ph. D.

This work investigates the use of irreversible electroporation (IRE) for tissue engineering applications and as a cancer ablation therapy. IRE uses short, high-intensity electric pulses to create pores in a cell's membrane and disrupt its stability. At a certain energy level, damage to the cell becomes too great and it leads to cell death. The particular mechanisms that drive this response are still not completely understood. Thus, further characterization of this behavior for cell death induced by pulsed electric fields (PEFs) will advance the understanding of these types of therapies and encourage their use to treat unresectable tumors that can benefit from the non-thermal mechanism of action which spares critical blood vessels and nerves in the surrounding area. We evaluate the response to PEFs by different cell types through experimental testing combined with computer simulations of these treatments. We show that IRE can be used to kill a specific type of bacteria that produce cellulose which can be used as an implantable material to repair damaged tissues. By killing these bacteria at particular times and locations during their cellulose production, we can create conduits in the overall structure of this material for the transport of oxygen and nutrients to the cells within the area after implantation. The use of tissue models also plays a key role in the investigation of various cancer treatments by providing a controlled environment which can mimic the state of cells within a tumor. We use tumor models comprised of a mix of collagen and cancer cells to evaluate their response to IRE based on the parameters that induce cell death and the time it takes for this process to occur. The treatment of prostate and pancreatic cancer cells with standard monopolar (only positive polarity) IRE pulses resulted in different time points for a full lesion (area of cell death) to develop for each cell type. These results indicate the presence of secondary processes within a cell that induce further cell death in the border of the lesion and cause the lesion to increase in size several hours after treatment. The use of high-frequency irreversible electroporation (H-FIRE)--comprised of short bursts of high-intensity, bipolar (both positive and negative polarity) pulses--can selectively treat cancer cells while keeping healthy cells in the neighboring areas alive. We show that H-FIRE pulses can target tumor-initiating cells (TICs) and late-stage, malignant cancer cells over non-malignant cells using a mouse ovarian cancer model representative of different stages of disease progression. To further explore the mechanisms that drive this difference in response to IRE and H-FIRE, we used more complex tumor models. Spheroids are a type of 3D cell culture model characterized by the aggregation of one or more types of cells within a single compact structure; when embedded in collagen gels, these provide cell-to-cell contact and cell-to-matrix adhesion by interactions of cells with the collagen fibers (closely mimicking the tumor microenvironment). The parameters for successful ablation with IRE and H-FIRE can be further optimized with the use of these models and the underlying mechanisms driving the response to PEFs at the cellular level can be revealed. / Ph. D. / This work investigates the use of irreversible electroporation (IRE) for tissue engineering applications and as a cancer ablation therapy. IRE uses short, high-intensity electric pulses to create pores in a cell’s membrane and disrupt its stability. At a certain energy level, damage to the cell becomes too great and it leads to cell death. The particular mechanisms that drive this response are still not completely understood. Thus, further characterization of this behavior for cell death induced by pulsed electric fields (PEFs) will advance the understanding of these types of therapies and encourage their use to treat unresectable tumors that can benefit from the non-thermal mechanism of action which spares critical blood vessels and nerves in the surrounding area. We evaluate the response to PEFs by different cell types through experimental testing combined with computer simulations of these treatments. We show that IRE can be used to kill a specific type of bacteria that produce cellulose which can be used as an implantable material to repair damaged tissues. By killing these bacteria at particular times and locations during their cellulose production, we can create conduits in the overall structure of this material for the transport of oxygen and nutrients to the cells within the area after implantation. The use of tissue models also plays a key role in the investigation of various cancer treatments by providing a controlled environment which can mimic the state of cells within a tumor. We use tumor models comprised of a mix of collagen and cancer cells to evaluate their response to IRE based on the parameters that induce cell death and the time it takes for this process to occur. The treatment of prostate and pancreatic cancer cells with standard monopolar (only positive polarity) IRE pulses resulted in different time points for a full lesion (area of cell death) to develop for each cell type. These results indicate the presence of secondary processes within a cell that induce further cell death in the border of the lesion and cause the lesion to increase in size several hours after treatment. The use of high-frequency irreversible electroporation (H-FIRE)—comprised of short bursts of high-intensity, bipolar (both positive and negative polarity) pulses—can selectively treat cancer cells while keeping healthy cells in the neighboring areas alive. We show that H-FIRE pulses can target tumor-initiating cells (TICs) and late-stage, malignant cancer cells over non-malignant cells using a mouse ovarian cancer model representative of different stages of disease progression. To further explore the mechanisms that drive this difference in response to IRE and H-FIRE, we used more complex tumor models. Spheroids are a type of 3D cell culture model characterized by the aggregation of one or more types of cells within a single compact structure; when embedded in collagen gels, these provide cell-to-cell contact and cell-to-matrix adhesion by interactions of cells with the collagen fibers (closely mimicking the tumor microenvironment). The parameters for successful ablation with IRE and H-FIRE can be further optimized with the use of these models and the underlying mechanisms driving the response to PEFs at the cellular level can be revealed.

The unique compressive, optical, and biocompatible properties of silicone hydrogels allow them to be used in a wide variety of applications in the biomedical field. However, the relatively weak mechanical behavior, as well as the highly deformable nature of these elastomeric materials, presents a myriad of challenges when attempting to understand their constitutive and fracture properties in order to improve hydrogel manufacturing and performance in applications. In this thesis, a series of experimental techniques were developed or adapted from common engineering approaches in order to investigate the effects of rate and temperature on the viscoelastic constitutive and fracture behavior of two solvated semi-interpenetrating polymer network silicone hydrogel systems. Viscoelastic characterization of these material systems was performed by implementing a series of uniaxial tension and dynamic mechanical analysis shear tests in order to generate relevant master curves and corresponding thermal shift factors of such properties as shear relaxation modulus, dynamic moduli, and the loss factor. Concurrently, the cohesive fracture properties were studied by utilizing a "semi-infinite" strip geometry under constrained tension in which thin pre-cracked sheets of these cured hydrogels were exposed to several different loading conditions. Fracture tests were performed over a relevant range of temperatures and crosshead rates to determine and generate a master curve of the subcritical strain energy release rate. Experimental methods utilizing high-speed camera images and digital image correlation to monitor viscoelastic strain recovery in the wake of a propagating crack were explored. The results from this thesis may prove useful in an investigation of the interfacial fracture of these hydrogel systems on several different polymer substrates associated with an industrial manufacturing problem. / Master of Science

Due to the limited regenerative capacity of the central nervous system (CNS) upon injury, regenerative medicine and tissue engineering strategies show great promise for treatment. These aim to restore tissue functions by combining principles of cell biology and engineering, using biomaterial scaffolds which can help in recapitulating the 3D environment of the brain and improving cell survival after grafting. Stroke and TBI are severe forms of disruptions of brain architecture, and two of the leading causes of mortality and morbidity worldwide, as no effective treatments are available. Several studies report how neural stem cells (NSCs) are able to improve functional recovery upon transplantation. However, the efficacy of these treatments is limited because of the mortality these cells are subject to after transplantation. In this context, the transplantation of mesenchymal cells (MSCs) has shown beneficial effects by secreting molecules and factors that help in the healing process. In this study, we tested alginate-based hydrogels as candidates to support human NSCs and MSCs transplantation into the brain, in the view of exploiting the beneficial effects of both and analyzing whether their combined use could have a synergistic effect. In the first part, we studied the suitability of alginate-based scaffolds for the three-dimensional encapsulation and culture of hNSCs and hMSCs. We analyzed their ability to support cell survival, and we evaluated whether changes in their concentration or modifications with ECM molecules could influence cell viability. We showed that the best survival conditions are found when using an RGDs-functionalized alginate scaffold at a low concentration (0.5% w/v). We then worked on the identification of the best conditions for MSCs culture and the definition of coculture conditions. Since serum is necessary for MSCs, but it is reported to induce glial differentiation of NSCs, we explored two different experimental setups. On one hand, we investigated the feasibility to exploit biomaterials to create "compartmentalized" cocultures that would at least partially retain serum. In parallel, we positively observed that MSCs can survive, proliferate and maintain their stemness even in absence of serum, supporting the hypothesis that the use of “compartmentalized” coculture systems would likely be exploitable for MSCs culture. Finally, we tested the reported beneficial effects of MSCs in our 3D culture system, in which NSCs do not show a great viability. Encapsulated NSCs were cultured on an MSCs monolayer, and we analyzed cell survival, proliferation, differentiation and stemness retention. Gene expression analyses highlighted that NSCs maintain stemness characteristics, but we were not able to observe any improvement in NSCs survival in coculture, with respect to standard culture. In the last part of the project we decided to test our system for tissue engineering approaches, exploiting axotomized brain organotypic slices (OSCs). We evaluated the presence of cells 7 days after transplantation, their integration in the OSCs and glial response. Preliminary results suggest that the biomaterial does not cause activation of glial cells, although stem cells do not seem to migrate out of scaffold and integrate into the brain slice.

Many grand societal challenges are rooted in the need for new materials, such as those related to energy, health, and the environment. However, the traditional way of discovering new materials is basically trial and error. This time-consuming and expensive method can't meet the quickly growing requirements for material discovery. To meet this challenge, the government of the United States started the Materials Genome Initiative (MGI) in 2011. MGI aims at accelerating the pace and reducing the cost of discovering new materials. The success of MGI needs materials innovation infrastructure including data tools, computation tools, and experiment tools. The last decade has witnessed significant progress for MGI, especially with respect to hard materials. However, relatively less attention has been paid to soft materials. One important reason is the lack of experimental tools, especially characterization tools for high-throughput experimentation. This dissertation aims to enrich the toolbox by trying new sensor tools for high-throughput characterization of hydrogels. Piezoelectric-excited millimeter-sized cantilever (PEMC) sensors were used in this dissertation to characterize hydrogels. Their capability to investigate hydrogels was first demonstrated by monitoring the synthesis and stimuli-response of composite hydrogels. The PEMC sensors enabled in-situ study of how the manufacturing process, i.e. bulk vs. layer-by-layer, affects the structure and properties of hydrogels. Afterwards, the PEMC sensors were integrated with robots to develop a method of high-throughput experimentation. Various hydrogels were formulated in a well-plate format and characterized by the sensor tools in an automated manner. High-throughput characterization, especially multi-property characterization enabled optimizing the formulation to achieve tradeoff between different properties. Finally, the sensor-based high-throughput experimentation was combined with active learning for accelerated material discovery. A collaborative learning was used to guide the high-throughput formulation and characterization of hydrogels, which demonstrated rapid discovery of mechanically optimized composite glycogels. Through this dissertation, we hope to provide a new tool for high-throughput characterization of soft materials to accelerate the discovery and optimization of materials. / Doctor of Philosophy / Many grand societal challenges, including those associated with energy and healthcare, are driven by the need for new materials. However, the traditional way of discovering new materials is based on trial and error using low throughput computational and experimental methods. For example, it often takes several years, even decades, to discover and commercialize new materials. The lithium-ion battery is a good example. Traditional time-consuming and expensive methods cannot meet the fast-growing requirements of modern material discovery. With the development of computer science and automation, the idea of using data, artificial intelligence, and robots for accelerated materials discovery has attracted more and more attention. Significant progress has been made in metals and inorganic non-metal materials (e.g., semiconductors) in the past decade under the guidance of machine learning and the assistance of automated robots. However, relatively less progress has been made in materials having complex structures and dynamic properties, such as hydrogels. Hydrogels have wide applications in our daily lives, such as drugs and biomedical devices. One significant barrier to accelerated discovery and engineering of hydrogels is the lack of tools that can rapidly characterize the material's properties. In this dissertation, a sensor-based approach was created to characterize the mechanical properties and stimuli-response of soft materials using low sample volumes. The sensor was integrated with a robot to test materials in high-throughput formats in a rapid and automated measurement format. In combination with machine learning, the high-throughput characterization method was demonstrated to accelerate the engineering and optimization of several hydrogels. Through this dissertation, we hope to provide new tools and methods for rapid engineering of soft materials.

<p dir="ltr">Tissue engineering utilizes polymers, cells, and other bioactive factors to promote regeneration within damaged tissue. The main works in this thesis employ naturally derived polymers for use in tissue engineering and explores ways to recapitulate native environments <i>in vitro</i>.</p><p dir="ltr">Collagen (col) is the most prevalent protein in the body. Col type I, II, and III are all fibril-forming collagens that provide structure to tissues. All three types polymerize <i>in vitro</i> to form hydrogels, and these hydrogels have often been studied for use in tissue engineering. Other applications include <i>in vitro </i>tissue models for studies on drug diffusion and drug delivery. Blending collagen types is of particular interest as col I is easier to source and is therefore cheaper than other collagen types. However, to confer biological signals to tissues where col II or III are more abundant (e.g., cartilage or cardiac tissue, respectively), col II or III can be added to col I to form col I/II or col I/III gels, respectively. Additionally, adding multiple types of col to hydrogel models better recapitulates the native environment and can better capture effects on drug diffusion. In this work, compared to col I alone, col I/II hydrogels polymerize more slowly, form more fibril bundles, result in softer hydrogels, and impede transport of larger macromolecules. On the other hand, col I/III gels polymerize at a similar rate to col I, create heterogenous fibril structures, are oftentimes stiffer than col I, and also impede transport of larger macromolecules. Additionally, this work explored the effect of polymerization temperature on blended gel polymerization and properties.</p><p dir="ltr">The second work evaluates col I/II hydrogels for a specific application: cartilage tissue engineering for osteoarthritic applications. Col II is the primary protein found in cartilage. Other components include: glycosaminoglycans, such as hyaluronic acid (HA) and chondroitin sulfate, chondrocytes (cartilage cells), and other small signaling molecules. Building on prior work in the group, high molecular weight hyaluronic acid (HA) was added to col I/II hydrogels, and cartilage differentiation of mesenchymal stem cells (MSCs) was assessed under ideal laboratory conditions and under pro-inflammatory, osteoarthritic conditions (i.e., cytokine-supplemented media of oncostatin M (OSM) at 10 ng/mL and tumor necrosis factor-α (TNF-α) at 20 ng/mL). The addition of HA did not dramatically impact cartilage differentiation of MSCs, however, HA did mitigate the effect of inflammation via downregulation of a degradative enzyme. HA had little impact on inflammatory cytokine production of interleukin (IL)-6 or IL-8, both of which are upregulated during osteoarthritis. However, a linear model suggests that HA and IL-8 are strongly correlated. Thus, this system should be explored further with different HA concentrations or presentations (e.g., chemically modified).</p><p dir="ltr">The last primary chapter of this thesis provides depth to the pro-inflammatory, osteoarthritic model used in the previous chapter. Different pro-inflammatory environments are studied using cytokines found in OA. MSC pellets (used in literature as controls to confirm chondrogenic potential of MSCs) were used to evaluate these inflammatory environments since MSCs are commonly used in tissue engineering. Six treatments were studied: negative control (without the chondrogenic growth factor TGF-β3), positive control (with the chondrogenic growth factor TGF-β3), and four cytokine treatments all with TGF-β3. First, IL-1β at 10 ng/mL was utilized as a comparison to literature. The other three cytokine groups used TNF-α at 20 ng/mL and OSM at 10 ng/mL individually or combined to form the main experimental group, OSM+TNF-α. All cytokine treatment groups limited cartilage production, but OSM decreased production to a statistically lesser extent than other cytokine groups. This trend was similar to observations made via immunostaining of cartilage matrix and gene expression analysis of aggrecan. Furthermore, OSM+TNF-α statistically lowered aggrecan gene expression. In terms of degradation, when compared to all other groups, OSM dramatically increased the protein expression of the degradative enzyme matrix metalloproteinase-13 (MMP-13). Evaluation of inflammatory markers (IL-6 and IL-8) revealed no signal for OSM-treated pellets. TNF-α yielded some signal after 1 week in culture but no signal after two weeks. IL-1β and OSM+TNF-α both resulted in sustained IL-6 and IL-8 expression, however, IL-1β exhibited large variance. Thus, each cytokine contributes to various pathways that are present in OA. Since the combination of OSM and TNF-α appeared to lower cartilage gene expression and resulted in sustained and reproducible IL-6 and IL-8 production, it may serve as a better model of OA than a single cytokine such as IL-1β.</p>

Glioblastoma multiforme (GBM) is the most common and lethal primary brain cancer with an average survival time of 15 months. GBM is considered incurable with even the most aggressive multimodal therapies and is characterized by near universal recurrence. Irreversible electroporation (IRE) is a cellular ablation method currently being investigated as a therapy for a variety of cancers. Application of IRE involves insertion of electrodes into tissue to deliver pulsed electric fields (PEFs), which destabilize the cell membrane past the point of recovery, thereby inducing cell death. While this treatment modality has numerous advantages, the lack of selectivity for malignant cells limits its application in the brain where damage to healthy tissue is especially deleterious. In this dissertation we hypothesize that a form of IRE therapy, high-frequency IRE (H-FIRE), may be able to act as a selective targeted therapy for GBM due to its ability to create an electric field inside a cell to interact with altered inner organelles. Through a comprehensive investigation involving experimental testing combined with numerical modeling, we have attained results in strong support of this hypothesis. Using tissue engineered hydrogels as our platform for therapy testing, we demonstrate selective ablation of GBM cells. We develop mathematical models that predict the majority of the electric field produced by H-FIRE pulses reach the inside of the cell. We demonstrate that the increased nuclear to cytoplasm ratio (NCR) of malignant GBM cells compared to healthy brain—evidenced in vivo and in in vitro tissue mimics—is correlated with greater ablation volumes and thus lower electric field thresholds for cell death when treated with H-FIRE. We enhance the selectivity achieved with H-FIRE using a molecularly targeted drug that induces an increase in NCR. We tune the treatment pulse parameters to increase selective malignant cell killing. Finally, we demonstrate the ability of H-FIRE to ablate therapy-resistant GBM cells which are a focus of many next-generation GBM therapies. We believe the evidence presented in this dissertation represents the beginning stages in the development of H-FIRE as a selective therapy to be used for treatment of human brain cancer. / Ph. D.

Les hydrogels moléculaires avec un réseau de fibres auto-assembles sont utilisés dans différents domaines dont le relargage de médicaments, les senseurs, l’ingénierie tissulaire et la nano-modélisation. Les hydrogels moléculaires à base d’acides biliaires, qui sont une classe de biocomposés d’origine naturelle, montrent une biocompatibilité améliorée et sont de bons candidats pour des applications dans le domaine biomédical. Ces hydrogels présentent une bonne bio-dégradabilité et une diversité fonctionnelle grâce aux faibles interactions supramoléculaires et aux structures chimiques précisément contrôlées. Dans cette thèse, des nouveaux hydrogels moléculaires à base des acides biliaires et leurs dérivés ont été étudiés pour mieux comprendre la relation entre la structure chimique du gélifiant et la formation de gels moléculaires. Un dimère de l'acide cholique avec un groupe diéthylènetriamine est insoluble dans l'eau. Par contre, il peut former des hydrogels grâce à un réseau tri-dimensionnel de fibres en présence de certains acides carboxyliques. L'addition d'acide carboxylique peut protoner le groupe amine secondaire et défaire les interactions intermoléculaires entre les dimères et favoriser la formation des liaisons hydrogènes acide-dimère. Seuls les acides carboxyliques faibles et hydrophiles causent la gélation des dimères. La résistance mécanique des hydrogels formés peut être modifiée par un choix judicieux d'acides. Les interactions hydrophobes et les liaisons hydrogènes entre les chaînes latérales d'acides carboxyliques peuvent améliorer les propriétés mécaniques des hydrogels. La solubilité marginale du complexe acide-dimère a été considérée comme un facteur critique pour la formation d'hydrogels. Un autre système d’hydrogélation à base d’acides biliaires a été développé par l’introduction de dioxyde de carbone (CO2) dans des solutions aqueuses de certains sels d’acides biliaires, qui donne un hydrogel composé de molécules biologiques entièrement naturelles et fournit un réservoir commode du CO2 dans l’eau. Le groupement carboxylate des sels d’acides biliaires peut être partiellement protoné dans les solutions aqueuses, ce qui amène la dissolution marginale dans l’eau et la formation d’hydrogels avec une structure fibreuse. L’aspect et les propriétés mécaniques des hydrogels dépendent de la concentration de CO2. Le bullage avec CO2 pendant une ou deux secondes génère un hydrogel transparent avec des nanofibres. Le bullage supplémentaire forme des hydrogels plus forts. Mais réduit la transparence et la force mécanique des hydrogels. D’ailleurs, les hydrogels transparents ou opaques redeviennent des solutions transparentes quand ils sont chauffés avec bullage de N2. La transition sol-gel est réversible et reproductible. La force mécanique et la transparence des hydrogels peuvent être améliorées par l’addition de sels inorganiques comme NaCl par l’effet de relargage. Toutes les composantes de ces hydrogels sont naturelles, donnant des hydrogels biocompatibles et potentiellement utiles pour des applications dans le domaine biomédical. Le dimère mentionné ci-dessus possède des propriétés d’auto-assemblage dépendamment de sa concentration. Ceci a été étudié en utilisant un sel organique de dimère/acide formique avec un rapport molaire 1/1. Le sel du dimère s’auto-assemble dans l’eau et ainsi forme des nanofibres isolées et mono-dispersées à des concentrations faibles. Les fibres enchevêtrées donnent des réseaux fibreux 3D bien dispersés de façon aléatoire à des concentrations plus élevées. Quand la concentration du sel du dimère est supérieure à la concentration critique de gélation, le réseau fibreux est assez fort pour immobiliser la solution, qui provoque la formation d’un hydrogel isotrope. L’augmentation supplémentaire de la concentration du sel du dimère peut augmenter l’anisotropie de l’hydrogel et former ainsi un hydrogel nématique. La formation de domaines ordonnés des nanofibres alignées donne ces propriétés optiques à l’hydrogel. L’agitation de systèmes aqueux du sel de dimère favorise aussi la formation de nanofibres alignées. / Molecular hydrogels are soft materials formed by the self-assembly of small molecules in aqueous solutions via supramolecular interactions. Although much effort has been made in the past several decades in the study of these hydrogels, the mechanism of their formation remains to be understood and the prediction of their formation is a challenge. The main purpose of this thesis is to develop novel molecular hydrogels derived from bile acids, which are naturally occurring biocompounds, and to find the relationship between the gelator structure and the gelation ability. Two new molecular gelation systems based on bile acids and their derivatives have been developed, which may be useful in biomedical applications. The marginal solubility of the solute in water has been found to be a prerequisite for the formation of such molecular hydrogels. The alignment of the nanofibers in the gels leads to the formation of nematic hydrogels. The first gelation system is based on a cholic acid dimer as a gelator, which has two cholic acid molecules covalently linked by a diethylenetriamine spacer. This dimer is insoluble in water, but it forms hydrogels with 3-D fibrous networks in the presence of selected carboxylic acids. The carboxylic acids protonate the dimer, making it marginally soluble in water to yield hydrogels. Only weak and hydrophilic carboxylic acids were capable of inducing the gelation of the dimer and the mechanical strength of the hydrogels could be varied by judicious choice of the acids. Hydrophobic interactions and hydrogen bonding between the side chains of carboxylic acids improve the mechanical properties of hydrogels. The marginal solubility of the acid-dimer complex is regarded to be the critical factor for the formation of hydrogels. Another hydrogelation system was developed by purging to the aqueous solutions of a series of bile salts with carbon dioxide (CO2), yielding hydrogels made of entire natural biological molecules and providing a convenient storage reservoir of CO2 in water. Bile salts are well dissolved in water, while the solubility of bile acids is limited. The carboxylate group of bile salts may be partially protonated in aqueous solutions by bubbling CO2, making them only marginally soluble in water. This forms fibrous structures. Both the appearance and mechanical properties of the hydrogels depend on the amount of CO2 purged. Bubbling CO2 initially induced the formation of transparent hydrogels with nanofibers. Continued purging with CO2 strengthened the hydrogel mechanically, while further addition of CO2 reduced the transparency and mechanical strength of the hydrogel. Both the transparent and opaque hydrogels reverted to transparent solutions when heated and bubbling N2. The sol-gel transition process was reversible and repeatable. The mechanical strength and transparency of the hydrogels could be improved by adding inorganic salts such as NaCl via a salting-out effect. All the hydrogel components are naturally biological compounds, making such hydrogels biocompatible and potentially useful in biomedical applications. The cholic acid dimer linked with a diethylenetriamine spacer was able to assemble in water and form isolated nanofibers in the presence of certain carboxylic acids at a much lower concentration than the CMC of sodium cholate. These nanofibers entangle with each other to yield well-dispersed and randomly-directed 3-D fibrous networks at higher concentrations. When the concentration of dimer salt is above the minimum gelation concentration, the fibrous network is strong enough to immobilize the solution, leading to the formation of an isotropic hydrogel. Further increase of the dimer salt concentration may transit the hydrogels to be anisotropic, thus the formation of nematic hydrogels. The formation of ordered domains of the aligned nanofibers led to anisotropic optical properties of the hydrogels. Stirring the aqueous systems of dimer salt also promoted the alignment of the nanofibers. These molecular hydrogels with ordered aggregates may be useful in applications such as cell culture and mechano-optical sensing.

Le but de ma thèse a été de concevoir des matériaux chimio-mécano répondants, des matériaux capables de permettre une transformation chimique réversible lorsqu’ils sont soumis à un stress mécanique. Tous les systèmes conçus ont été développés sur des substrats en silicone. Une première approche a consisté à créer des surfaces à sites cryptiques où une biotine est enfouie dans des brosses de chaines de poly(éthylène glycol). Le système streptavidine/biotine a été utilisé comme modèle. Ces surfaces sont anti-adsorbantes à la streptavidine sauf lorsqu’elles sont étirées à 50% où la biotine est reconnue mais les surfaces sont non réversibles. Dans une seconde approche, nous avons modifiés la surface du silicone par adsorption d’une multicouche de polyélectrolytes. Cette stratégie est basée sur la réticulation covalente du film par l’enzyme β-galactosidase modifiée. Nous sommes ainsi parvenus à créer une surface présentant une activité catalytique modulable par l’étirement mécanique, et ce, d’une façon partiellement réversible. Ce travail représente le premier exemple d’un système où une contrainte mécanique imposée à un matériau permet la déformation conformationnelle d’une enzyme et ainsi la diminution de l’activité catalytique. Dans une dernière approche, nous avons conçu un système mixte composé d’un substrat de silicone sur lequel un gel de polyacrylamide est greffée de façon covalente. Des enzymes ou des mécanophores pourront ainsi être inclus dans le réseau polymérique du gel de polyacrylamide et être étirés. Nous sommes parvenus à préparer de tels systèmes où l’hydrogel reste solidaire du film de silicone, sans apparition de craquelures jusqu’à 50%d’étirement. / The goal of my PhD was to develop new routes to design chemo-mechanoresponsive materials, materials that respond chemically to a mechanical stress, in a reversible way. All the systems designed during my PhD thesis were based on the functionalization of silicone sheets. First we created cryptic site surfaces by embedding biotin ligands into PEG brushes. The couple streptavidin/biotin was used as a model system. At rest, the surface so-prepared was antifouling and biotin ligands were specifically recognized by the streptavidin when the surface was stretched at 50%. Unfortunately, in this first approach, the mechanosensitive surface did not lead to a reversible process. In a second approach, we modified the silicone surface by using the polyelectrolyte multilayer (PEM) film deposition. This strategy was based on the covalent cross-linking of modified enzyme, the β-galactosidase, into the PEM. We succeeded in modulating the enzyme activity in the film under stretching and this approach appears as partially reversible under stretching/unstretching cycles. This work represents the first reported system where enzymatic activity can be modulated by stretching due to modulation of the enzyme conformation. In a last approach, we also designed a mixed system consisting of a silicone sheet onto which a polyacrylamide hydrogel is covalentlyattached with the goal to create a stretchable gel into which one can covalently attach enzymes or chemical mechanophores. These enzymes or mechanophores can thus be put under mechanical stress. We succeeded in creating a system that can be stretched up to 50% without detachment of the gel from the silicone and without inducing cracks in the gel.

Les biomatériaux de type hydrogel ou organogel obtenus à partir de molécules de faible masse molaire suscitent de plus en plus un vif intérêt autant sur le plan fondamental que sur le plan applicatif. Le travail, exposé ici, a pour objectif de synthétiser des composés amphiphiles présentant des variations de structure afin de pouvoir établir des relations entre la structure de ces composés et leurs propriétés gélifiantes. Nous avons été amenés à synthétiser des composés (bimodulaires et trimodulaires) avec et sans motif de jonction de type polyoxyéthylénique entre la partie peptidique et le module hydrophobe. Dans un deuxième temps, d?autres molécules amphiphiles gélifiantes ont été préparées, en substituant le motif polyoxyéthylénique par une entité dérivée du glycérol, pour permettre d'obtenir des composés d'origine renouvelable mais également pour diversifier la source de matières premières. Les composés visés ont été obtenus et leur comportement en solution a été étudié par différentes techniques (tensiométrie, conductimétrie, infrarouge,...). Certains d'entre eux, présentant des propriétés gélifiantes, associés aux sels de bore, ont été utilisés pour préserver le matériau bois (Pin Sylvestre). Les essais effectués se sont révélés concluant en termes de réduction du lessivage du bore car les tests avec Poria Placenta (champignon) cultivé sur un milieu gélosé ou avec les Réticulitermes Flavipès (termites) ont montré qu'un bois traité par l'association « borax-hydrogel » reste protégé même après lessivage / Biomaterials hydrogel or organogel obtained from low molecular weight molecules give rise to an increasing interest both in terms on the fundamental and application domain. The work outlined here, aims to synthesize amphiphilic compounds with structural changes in order to establish relationships between the structure of these compounds and their gelling properties. We were asked to synthesize compounds (bimodular and trimodular) with and without cause polyoxyethylene type junction between the peptide and the hydrophobic unit. In a second step, other gelling amphiphilic molecules were prepared by substituting the polyoxyethylene pattern derived by an entity glycerol, allowing to obtain compounds from renewable sources but also to diversify the source of raw materials. The target compounds were obtained and their behavior in solution has been studied by different techniques (tensiometry, conductivity, infrared,...). Some of them, with gelling properties, associated with boron salts have been used to preserve the wood material (Scots pine). The tests proved conclusive in terms of reducing the leaching of boron as tests with Poria placenta (mushroom) grown on an agar medium or with Reticulitermes flavipes (termite) showed that wood treated with the association "borax-hydrogel" is protected even after leaching