Reinforcing hydrogels with in situ formed amorphous CaCO3

Hydrogels are often employed for tissue engineering and moistening applications. However, they are rarely used for load-bearing purposes because of their limited stiffness and the stiffness-toughness compromise inherent to them. By contrast, nature uses hydrogel-based materials as scaffolds for load-bearing and protecting materials by mineralizing them. Inspired by nature, the stiffness or toughness of synthetic hydrogels has been increased by forming minerals, such as CaCO3, within them. However, the degree of hydrogel reinforcement achieved with CaCO3 remains limited. To address this limitation, we form CaCO3 biominerals in situ within a model hydrogel, poly(acrylamide) (PAM), and systematically investigate the influence of the size, structure, and morphology of the reinforcing CaCO3 on the mechanical properties of the resulting hydrogels. We demonstrate that especially the structure of CaCO3 and its affinity to the hydrogel matrix strongly influence the mechanical properties of mineralized hydrogels. For example, while the fracture energy of PAM hydrogels is increased 3-fold if reinforced with individual micro-sized CaCO3 crystals, it increases by a factor of 13 if reinforced with a percolating amorphous calcium carbonate (ACC) nano-structure that forms in the presence of a sufficient quantity of Mg2+. If PAM is further functionalized with acrylic acid (AA) that possesses a high affinity towards ACC, the stiffness of the hydrogel increases by a factor 50. These fundamental insights on the structure-mechanical property relationship of hydrogels that have been functionalized with in situ formed minerals has the potential to enable tuning the mechanical properties of mineralized hydrogels over a much wider range than what is currently possible.

Approche expérimentale du comportement mécanique des polymères en sollicitation rapide

In the more recent applications of technical polymers, they are frequently submitted to high strain rates. Being viscoelastic by nature, their mechanical properties are rate dependent. It is therefore important to investigate their properties over a wide range of strain rates. The aims of this study are twofold: i) to propose an experimental technique applicable to a large domain of strain rates, included those covering impact tests, and ii) to apply this technique to the study of the rate dependence of the mechanical properties of some technical polymers. In the first section, the deformation mechanisms of polymers, and the way in which they are influenced by strain rate, are reviewed. Some basics of fracture mechanics are also briefly presented. A detailed study of our original approach to the high rates tests then follows. This technique is based on the reduction of dynamic effects which often affect the measurements at high loading rates. Rather than impact tests, our technique is a quasi-static tests at high rates. This results from the use of a damper as coupling element between the servohydraulic piston and the specimen, together with the specific design of load and displacement detectors. The transient acceleration, responsible for the dynamic effects is decreased, but the high loading rates are maintained. The second section is devoted to the investigation of the rate effect on the tensile and rupture properties of polyoxymethylene (POM) and impact modified polymethylmethacrylate (PMMA). It is shown that high molecular weight POM is less sensitive to the increase of the loading rates than for lower molecular weights. The physical structure, which depends on the processing conditions, plays an important role as well. Results show greater embrittlement by increasing the loading rate than by lowering the temperature. Finally, a molecular model of fracture is proposed. PMMA modified by different fractions and morphologies of the rubbery phase are submitted to tensile and fracture tests at room temperature. Several stages in the micromechanisms of deformation are identified by transmission electron microscopy. The progressive disappearance of some of these stages leads to ductile to brittle transitions in some of the materials studied. The velocity at which these transitions occur depends on the fraction and on the morphology of the modifying phase. It is also shown that a spherical morphology of the rubbery phase is not necessarily required to improve the toughness, but that this latter depends on the interactions between the phases.

EPFL1996

Functional polymer brushes via surface-initiated controlled radical polymerizations

Stefano Tugulu

The term polymer brush refers to a well defined arrangement of polymer chains, which are tethered with one end to an interface and usually the surface of a solid substrate. Due to steric repulsion the polymer chains furthermore adopt a defined, stretched chain conformation, which significantly differs from the random walk conformation of free polymer chains in solution or in conventionally solution casted polymer coatings. From a structural point of view polymer brushes can therefore be regarded as the most defined type of polymer coating that can currently be realized. Especially the combination of surface-initiated polymerizations (SIP) with modern, controlled polymerization methods thereby allows tailoring the structure of the polymer brushes almost on the molecular level. Specifically precise control over the thickness, composition, chain architecture, grafting density and via the use of lithographic techniques also over the topography of the brushes can be achieved. This high degree of control over the structural and implicated (physico-)chemical properties of the brush allows at the same time tailoring and fine tuning the interfacial properties of the brush. This thesis describes the systematic exploitation of well-defined polymer brushes as functional coatings with fine-tuned physicochemical properties to direct and control the interaction with and the response of specific biological, bioanalytical and even reactive chemical environments. In detail the use of surface-initiated atom transfer radical polymerization (SI-ATRP) for the fabrication of functional polymer brushes will be presented. These brushes were used as highly defined coatings in various applications ranging from biomedical and bioanalytical applications to biomimetic materials fabrication (Figure 1). Figure 1 Numerous synthetic approaches for the preparation of polymer brushes have been described in the literature. Chapter 2 will therefore present a summary of the most important types of surface-initiated polymerizations, focusing especially on controlled SIP methods and SI-ATRP. Chapter 3 describes a chemoselective and specific immobilization strategy to decorate intrinsically bioinert polymer brushes with proteins of interest in a defined orientation and density. Specifically the resulting substrates represent attractive candidates for the realization of protein microarrays. In view of this application protein-small molecule and protein-protein interactions as well as posttranslational modifications were analyzed within a feasibility study. Due to their well-defined structure polymer brushes represent also attractive candidates for the realization of molecularly defined cell adhesion substrates for tissue engineering or as highly defined biophysical model system to analyze cell adhesion and mechanics. Chapter 4 describes the decoration of intrinsically bioinert polymer brushes with small peptide ligands with defined density to induce integrin specific cell adhesion on the brushes. Peptide functionalized polymer brushes could be readily endothelialized and adhering cell layers have shown to preserve their homeostatic ability to respond to an applied mechanical stimulus in a physiological manner. In Chapter 5 the limitations of high density polymer brushes in biomedical applications were assessed by analyzing their stability under in vitro cell culture conditions. Intrinsically bioinert high density polymer brushes were found to degraft from glass or silicon substrates upon swelling in good solvents, which lead to deterioration of the bioinert properties of the substrate. The degrafting of the brushes might be connected to swelling induced mechanical activation of chemical bonds, which link the brushes to the substrate. Chapter 6 describes the transfer of the developed SIP approach onto flexible PDMS substrates by exploiting the formation of interpenetrating networks of ATRP-initiator siloxane and PDMS. The modification of the PDMS substrates with hydrophilic brushes effectively reduces the unspecific protein adsorption on the substrates. This approach might therefore represent an attractive and facile route for the surface modification of miniaturized PDMS devices for biomedical or bioanalytical applications. In Chapter 7 the systematic development of a reliable and robust protocol for the aqueous SI-ATRP of sodium methacrylate is presented, which gives access to poly(methacrylic acid) (PMAA) brushes with defined molecular weight and grafting density. The resulting brushes were assessed for their ability to direct the mineralization of calcium carbonate. Chapter 8 describes the use of photolithographically patterned poly(methacrylic acid) brushes as biomimetic, acidic macromolecular matrix to fabricate microstructured calcite thin films that are an exact 3D replica of the PMAA brush. The presented strategy relies on three key elements: (i) the use of photolithographic techniques to prepare microstructured PMAA brushes; (ii) the ability of PMAA brushes to stabilize amorphous calcium carbonate (ACC) and (iii) the possibility to convert the metastable ACC phase into a polycrystalline calcite film via a thermal treatment.

EPFL2007

Solution behavior of peptide and peptide-hybrid materials

Harald Nuhn

Peptide based and peptide hybrid materials represent two interesting and challenging classes of biomaterials. Both classes consist of and contain, respectively, one or more polypeptide blocks, endowing the resulting polymers with enhanced self-assembly properties, biocompatibility or novel characteristics. Peptide based materials, solely comprised of amino acids, benefit from their advanced self-assembly properties. In the case of peptide hybrid materials, being conjugates of peptides and synthetic polymers, either the polypeptide or the synthetic polymer block can drive the self-assembly process. Furthermore, the latter class benefits from combining the advantages of peptides and synthetic polymers, thus overcoming limitations related to the use of the individual components. The application of these materials is still challenging because it is not possible to predict their final self-assembly behavior based on their design. In order to facilitate a target-oriented design of novel materials for application such as drug delivery, tissue engineering, or self-healing materials, a detailed understanding of the complex interactions within and between these molecules is essential. The work presented in this thesis addresses these fundamental questions. Over the last decades, a plethora of peptide and peptide hybrid materials has been reported. Chapter 1 gives an overview of selected articles describing different synthesis routes for peptide as well as peptide hybrid materials, showing examples of molecules and their self-assembly into superstructures, and highlighting recent applications. Chapter 2 describes the synthesis and characterization of different peptide hybrid diblock oligomers composed of two classes of peptide sequences covalently attached to a poly(ethylene glycol) (PEG) block. The first class of peptide sequences contains the intrinsic propensity to form coiled coils, whereas the second class is composed of "switch" peptides and is able to adopt both amphiphilic α-helical and β-strand conformations. Upon dissolution, the first class yielded superstructures comprised of dimeric and tetrameric coiled coil aggregates, whereas the second class formed fibrillar assemblies. In both cases, the self-assembly properties were driven by the peptide block, and the final superstructure consisted of a peptidic core wrapped by the synthetic polymer block. Chapter 3 presents a detailed study on the aqueous solution self-assembly of two poly(styrene)n-b-poly(L-lysine)m peptide hybrid oligomers. The aim was to clarify whether the observed rod-like micelles, obtained by direct dissolution of the diblock oligomers in aqueous solution, reflect intrinsic self-assembly properties of the hybrid diblock oligomers or whether they present structures that are energetically trapped due to the glassy nature of the polystyrene block. To address this question, the influence of a variety of sample preparation techniques, including the use of organic solvents, dialysis from an organic solvent and several thermal treatments, was investigated. All treatments resulted in solution aggregates that differed in size and shape from samples prepared by direct dissolution in aqueous solution. Thus, it seems likely that direct dissolution of polystyrene-b-poly(L-lysine) diblock oligomers in aqueous solution results in kinetically trapped, rod-like aggregates, which can be transformed into non-spherical micelles by the appropriate treatment. Chapter 4 contains a comprehensive study on peptide-based materials, investigating four different series of short elastin-like peptides (ELPs) based on the GVGVP motif. The objective was to test whether and how defined changes in the primary structure of short ELPs affect the secondary structure and the lower critical solution temperature (LCST) behavior. Therefore, the number of repeats was altered, and within one peptide of constant length, all valines were successively substituted by the more hydrophobic amino acids isoleucine, leucine and phenylalanine, respectively. In parallel, a theoretical approach based on the partition coefficient log P was used to predict the shift of the LCST as function of chain length and composition. For short ELPs, the LCST is strongly affected by changes in the primary structure. This has also been predicted by calculating log P. An increase in the hydrophobicity resulted in a shift of the LCST towards lower temperatures. Furthermore, it was shown for the first time that the sensitivity of short ELPs towards external factors, such as salt concentration, is determined by the intrinsic propensity of the incorporated amino acids to form either α-helices or β-strands. In summary, the obtained results on peptide based and peptide-hybrid materials provides deeper insights into their self-assembly characteristics and highlights the potential of these materials to be used for technical and medical applications.

EPFL2008