München, Techn. Univ., Diss., 2002. / Computerdatei im Fernzugriff.

München, Techn. Univ., Diss., 2002. / Computerdatei im Fernzugriff.

München, Techn. University, Diss., 2002.

This dissertation is aimed towards using stimuli-responsive pNIPAm-co-AAc microgels synthesized via free-radical precipitation polymerization to prepare stimuli-responsive hydrogel microlenses. Chapter 1 gives a detailed background of hydrogels, and their applications using responsive hydrogels. Chapter 2 describes the use of colloidal hydrogel microparticles as microlens elements and the fabrication method to form the hydrogel microlens arrays via Coulombic interactions. Chapter 3 shows the demonstration of tunable microlenses prepared by the method used in Chapter 2. In this chapter the microlenses are subjected to various pH and temperature in aqueous solutions. Chapter 4 describes that the microlens arrays constructed on Au nanoparticle-functionalized glass substrates by self-assembly display dramatic changes in lensing power in response to an impingent frequency-doubled Nd:YAG laser. The microlens photoswitching is highly reversible, with sub-millisecond lens switching times. Chapter 5 describes the development of bioresponsive hydrogel microlenses as a new protein detection technology. The microlens method is shown to be very specific for the target protein, with no detectable interference from nonspecific protein binding. Chapter 6 describes the use of bioresponsive hydrogel microlenses as a label-free biosensing scaffolding. These microstructures simultaneously act as the biosensors scaffolding/immobilization architecture, transducer, amplifier, and also allow for broad tunability of the analyte concentration to which the microlens is sensitive.

No / There is a lack of hydrogel materials whose properties can be tuned at the point of use. Biological hydrogels, such as collagen, gelate at physiological temperatures; however, they are not always ideal as scaffolds because of their low mechanical strength. Their mechanics can be improved through cross-linking and chemical modification, but these methods still require further synthesis. We have demonstrated that by combining collagen with a thermoresponsive polymer, poly(N-isopropylacrylamide) (PNIPAM), the mechanical properties can be improved while maintaining cytocompatibility. Furthermore, different concentrations of this polymer led to a range of hydrogels with shear moduli ranging from 105 Pa down to less than 102 Pa, similar to the soft tissues in the body. In addition to variable mechanical properties, the hydrogel blends have a range of micron-scale structures and porosities, which caused adipose-derived stromal cells (ADSCs) to adopt different morphologies when encapsulated within and may therefore be able to direct cell fate. / EPSRC

Biodegradable, pH-responsive hydrogel networks composed of poly(methacrylic acid) crosslinked with varying mol percentages of polycaprolactone diacrylate were synthesized. These materials were characterized using NMR and FTIR. The equilibrium and dynamic swelling properties of these pH-responsive materials were studied. Also, the materials’ degradation was characterized using swelling studies and gel permeation chromatography. Methods were developed to incorporate these novel hydrogels as sensing components in silicon-based microsensors. Extremely thin layers of hydrogels were prepared by photopolymerizion atop silicon microcantilever arrays that served to transduce the pH-responsive volume change of the material into an optical signal. Organosilane chemistry allowed covalent adhesion of the hydrogel to the silicon beam. As the hydrogel swelled, the stress generated at the surface between the hydrogel and the silicon caused a beam deflection downward. The resulting sensor demonstrated a maximum sensitivity of 1nm/4.5E-5 pH unit. Sensors were tested in protein-rich solutions to mimic biological conditions and found to retain their high sensitivity. The existing theory was evaluated and developed to predict deflection of these composite cantilever beams. Another type of hydrogel-based microsensor was fabricated utilizing porous silicon rugate filters as transducers. Porous silicon rugate filters are garnering increased attention as components of in vivo biosensors due to their ability for remote readout through tissue. Here, the biodegradable, pH-responsive hydrogel was polymerized within the pores of a porous silicon rugate filter to generate a novel, completely degradable sensor. Silicon was electrochemically etched in hydrofluoric acid to generate the porous silicon rugate filter with its reflectance peak in the near infrared region. Poly(methacrylic acid) crosslinked with polycaprolactone diacrylate was polymerized within the pores using UV free radical photopolymerization. The reflectance peak of this sensor varied linearly with pH in the region pH 2.2 to 8.8. This work shows promise towards utilizing porous silicon rugate filters as transducers for environmentally responsive hydrogels for biosensing applications. / text

En ingénierie tissulaire, la survie in vivo de cellules souches implantées au sein d’un biomatériau est limitée par les conditions d’un environnement ischémique qui se caractérise par un déficit en oxygène et en nutriments. Récemment, dans le cadre d’un projet de recherche dédié au développement d’un hydrogel composite à base de fibrine, biomatériau conçu pour améliorer la survie de cellules souches post-implantation, il a été mis en évidence la nécessité de contrôler dans le temps et l’espace la disponibilité du glucose au sein de ce matériau. Cet apport in situ de glucose est réalisé par dégradation contrôlée de l’amidon, un polymère de glucose. Cette production est assurée par action enzymatique d’un catalyseur spécifique de l’hydrolyse de l’amidon, l’amyloglucosidase (AMG).Toutefois, il convient de maitriser différents paramètres tels que la fuite de l’AMG en dehors de l’hydrogel ou encore sa perte d’activité au cours du temps. Dans ce contexte, l’encapsulation de l’AMG dans des nanoparticules d’un polymère biodégradable et biocompatible, ici l’acide poly(lactique-co-glycolique) (PLGA), devrait permettre le contrôle des paramètres susmentionnés.Des nanoparticules de type core-shell contenant l’AMG (NPe) ont été synthétisés par l’adaptation d’un protocole de double émulsion (water-oil-water). Différentes méthodes ont été développées pour déterminer les propriétés physico-chimiques et biochimiques des nanoparticules produites. Le protocole de synthèse a été optimisé afin de produire des nanoparticules reproductibles et stériles utilisables dans des hydrogels implantables in vivo.Le cahier des charges de l’hydrogel enrichi en amidon et en NPe impose un apport continu du glucose pendant 1 mois. La stabilité des nanoparticules a été étudiée en solution et dans les hydrogels. La production de glucose grâce à ces NPe a été investiguée en solution et en hydrogel mettant en avant l’intérêt de ces nanoparticules au sein du dispositif. / In tissue engineering, the in vivo survival of stem cells located within a biomaterial is limited by an ischemic environment characterized by a low supply of oxygen and nutrients. Recent studies on fibrin based hydrogels (designed to improve stem cells survival after implantation) have highlighted the need to control the spatiotemporal availability of glucose within a biomaterial scaffold. Glucose release occurs through the degradation of starch, a glucose polymer, at a rate controlled by the action of the enzyme amyloglucosidase (AMG), a specific catalyst for the hydrolysis of starch.In order to eventually be of clinical impact, critical parameters must be tuned, such as the AMG leakage outside the hydrogel and its loss of activity over time. In this context, AMG encapsulation within nanoparticles of a biodegradable and biocompatible polymer, here poly(lactic-co-glycolic acid) (PLGA), is a promising means toward controlling the above parameters.The AMG-containing core-shell type nanoparticles (NPe) were synthesized by an adaptation of the double emulsion technique (water-oil-water). Different methods have been developed to determine the physicochemical and biochemical properties of the resulting nanoparticles. The synthesis was optimized to produce sterile and reproducible nanoparticles appropriate for in vivo implantable hydrogels.Nanoparticle stability and glucose release were investigated in solution and in hydrogels. A key specification of the hydrogel system, enriched in starch and NPe, is the continuous supply of glucose over 1 month. Glucose production was observed to meet this specification, highlighting the potential advantages of this approach.

A hydrogel consists of a cross-linked polymer network and solvent molecules, capable of large, reversible deformation in response to a variety of external stimuli. In particular, diverse instability patterns have been observed experimentally in swelling hydrogels under mechanical constraints. The present study develops a general theoretical framework based on a variational approach, which leads to a set of governing equations coupling mechanical and chemical equilibrium conditions for swelling deformation of hydrogels, along with proper boundary conditions. A specific material model is employed for analytical and numerical studies, for which the nonlinear constitutive behavior of the hydrogel is derived from a free energy function combining rubber elasticity with a polymer solution theory. A finite element method is then developed and implemented as a user-defined material (UMAT) in the commercial package, ABAQUS. By numerical simulations, the effect of constraint on inhomogeneous swelling of substrate-attached hydrogel lines is elucidated. It is found that crease-like surface instability occurs when the width-to-height aspect ratio of the hydrogel line exceeds a critical value. Next, by considering a hydrogel layer on a rigid substrate, swell-induced surface instability is studied in details. A linear perturbation analysis is performed to predict the critical condition for onset of the surface instability. In contrast to previously suggested critical conditions, the present study predicts a range of critical swelling ratios, from about 2.5 to 3.4, depending on the material properties of the hydrogel system. A stability diagram is constructed with two distinct regions for stable and unstable hydrogels with respect to two dimensionless material parameters. Numerical simulations are presented to show the swelling process, with evolution of initial surface perturbations followed by formation of crease-like surface patterns. Furthermore, with combined swelling and mechanical compression, the stability analysis is extended to predict a general critical condition that unifies the swell-induced surface instability of hydrogels with mechanically induced surface instability of rubbers. The effect of surface tension is found to be critical in suppressing short-wavelength modes of surface instability, while the substrate confinement suppresses long-wavelength modes. With both surface tension and substrate confinement, an intermediate wavelength is selected at a critical swelling ratio for onset of surface instability. Both the critical swelling ratio and the characteristic wavelength depend on the initial thickness of the hydrogel layer as well as other material properties of the hydrogel. It is found that the hydrogel layer becomes increasingly stable as the initial layer thickness decreases. A critical thickness is predicted, below which the hydrogel layer swells homogeneously and remains stable at the equilibrium state. Finally, three-dimensional finite element models are developed to simulate swelling deformation of hydrogel lines. Depending on the aspect ratio of the cross section as well as the material properties of the hydrogel, two types of swell-induced instability patterns are envisaged, i.e., localized surface instability versus global buckling. / text

Peptides and peptide derivatives have shown great scope as biomaterials and for biomedicaltherapy application. It has been demonstrated that classes of these peptides can form fibrillar hydrogels making them a good candidate for ECM mimics. In particular, the ionic complementary peptides, composed of alternating hydrophobic and hydrophilic amino acidshave been reported as successful cell scaffolds. The simple structure of such ionic complementary peptides is generally seen to spontaneously self-assemble into β-sheet richfibrils in the presence of water. The highly aqueous environment, along with the inter meshing of fibres, results in an architecture akin to the natural ECM of the body, making peptide hydrogels highly suitable as cell culture scaffolds. The structure of such hydrogels, usually comprising 8-32 amino acids, has been widely reported as easily modifiable, thus, allowing for control of the final material properties. This study explores the potential use of a range of ionic-complementary peptides for the culture of primary bovine chondrocytes. Modifications and additions to peptide sequence, such as charge and amino acid substitution, were investigated. In all studies only 1 design parameter (sequence, charge etc.) was varied, to allow for better understanding of the effect of materials properties upon cell response. The encapsulation of primary bovine chondrocytes was undertaken, with the aim of providing a suitable cell scaffold capable of maintaining chondrocyte viability and function in vitro. Despite in vivo work being beyond the scope of this thesis, the properties of the hydrogel scaffold were designed with final aim of being suitable for use with matrix associated autologous chondrocyte implantation (MACI) in clinical therapy.

acase@tulane.edu / Bringing a newly formulated drug used for neurological applications to the market is a highly time and labor-intensive process. The current pathway of bringing a drug to market requires extensive drug testing on animal models at the pre-clinical stage. Live animal models are expensive, low-throughput and increasingly recognized to be poor predictors of clinical outcomes in the process of drug development. An in vitro testing platform would address these above stated problems by providing a pre-screening process that could improve the high attrition rates of novel pharmaceutical compounds and also reduce the demand for the number of animals used for testing. This dissertation presents the progress of studies that were conducted to create a three-dimensional myelinated in vitro peripheral nerve-on-a-chip model that could be subject to electrophysiological and histological testing to be used as a tool for drug screening. In the first study, our model utilized an ultra-violet light cured methacrylated heparin hydrogel as the growth permissive substrate and a polyethylene glycol gel as a growth restrictive boundary that contained three-dimensional neural growth from an embryonic rat’s dorsal root ganglion explant. The model enabled electrophysiological field recording testing to measure metrics such as compound action potential amplitude and nerve conduction velocity. However, the heparin hydrogel presented issues with immunohistochemistry and histological studies leading us to recreate the model with a different growth permissive substrate. The second study utilized a methacrylated gelatin hydrogel in place of the heparin as the growth substrate. The dense neural growth was rapider than heparin while the gel allowed electrophysiological and histological testing to conclusively show the presence of myelin. Data from the histological testing was used to tabulate structural measurement such as percentage of myelinated axons and g-ratios which were then correlated with the electrophysiological data. This study paved way to use this model to simulate a demyelinating physiology and assess the effectiveness of a possible neuroprotective agent. The third and final study investigated the usage of the peripheral nerve-on-a-chip as model of demyelination by using forskolin and twitcher mouse serum, adapted from the established in vivo model of Krabbe’s disease. The effects of demyelination were observed using electrophysiological, immunohistochemistry, and histological studies. The corticosteroid dexamethasone was also included in the demyelination models to assess its extent of neuroprotection against the demyelinating agents. The results established a novel myelinated peripheral nerve-on-a-chip model which could be subject to electrophysiological, immunohistochemistry, and histological studies. The model has the potential to be used to simulate various pathologies and evaluate the efficacy of drugs before animal testing could be conducted. / 1 / Ashwin Sivakumar