Understanding effect of network structure of PolyEthylene Glycol Diacrylate hydrogels on cellular activities using bovine chondrocytes as a model system

In this dissertation, we investigated the effect of poly(ethylene glycol) hydrogel network structures on various cellular activities such as proliferation and matrix production using chondrocytes as a cell source. We have used various methods to alter the network structure, which involves utilizing precursors with varying molecular weight and changing the degree of polymerization. The latter was achieved by varying initiator concentrations, and duration of polymerization time. PEG hydrogels constituting PEG precursors of molecular weights 3.4kDa, 6kDa, 10kDa and 20KDa were synthesized, characterized and subsequently tested for their effects on chondrocytes biosynthetic activity. The structural parameters of the hydrogel network that varied due to our experimental differences were determined. Specifically, the hydrogel structures were characterized in terms of swelling ratio, the molecular weight between crosslinks, the mesh size, and the pore size. Swelling ratio of the hydrogels decreased while gel content increased with longer UV exposure and higher photo-initiator concentration. High compressive modulus was observed with PEG 3.4kDa whereas PEG 20kDa exhibited high ultimate stress, which was 37 times higher than the other hydrogels. Cells in hydrogels with a lower MW (PEG3.4) showed higher proliferation and more ECM synthesis in comparison with high molecular weight PEG hydrogels. This could be attributed to the bigger pore size of the high molecular weight PEG hydrogels, which could result in loss of cells. However, the amount of DNA observed after week 4 normalized to day 1 revealed that cells in hydrogels with a higher MW showed a higher proliferation and the amount of ECM synthesized per cell was larger. Histology revealed that proteoglycans were more diffused in the matrix with higher MW, indicating the presence of higher pore size. Cellular aggregation was observed after 2 weeks in all the hydrogels except for the 3.4kDa hydrogels; the aggregation of cells in 20kDa was also very minimal. A low degree of polymerization led to a high proliferation rate during the first week followed by enhanced ECM production. Conversely, a high degree of polymerization led the cells to a greater amount of protein synthesis during the first week, followed by proliferation. While the cells behaved in different ways at early time points of the culture, at the end of 4 weeks, there was no significant difference between the number of cells, nor ECM production in different hydrogels. This study clearly shows that the network structure has a temporal effect on chondrocyte functions

Degradable Polymer Brushes Prepared via Surface-Initiated Controlled Radical Polymerization

Harm-Anton Klok, Carl Riachi, Nicolas Bertrand Schüwer

Hydrolytically degradable polymer brushes would represent an interesting platform for the development of functional coatings for various biomaterials applications In this manuscript, the surface- initiated atom transfer radical copolymerization of 5.6-benzo-2-methylene1,3-dioxepane (BMDO) and poly(ethylene glycol) methacrylate (PEGMA) has been investigated to prepare degradable po;ymer brushes. The copolymerization of BMDO and PEGMA results in brushes that contain hydrolytically labile ester linkages in the polymer backbone. The thickness of these polymer brushes can be controlled via the polymerization time and the BMDO content by adjusting the monomer feed composition The degradation of the brushes was investigated by ellipsometry and atomic force microscopy (AFM) experiments While the brushes were relatively stable under neutral and mild-basic conditions (pH 9), degradation was significantly enhanced under acidic conditions Degradation was found to be accelerated at low pH values (pH 3-5) and with increasing BMDO content

2009

Shape-Persistent, Thermoresponsive Polypeptide Brushes Prepared by Vapor Deposition Surface-Initiated Ring-Opening Polymerization of alpha-Amino Acid N-Carboxyanhydrides

Solenne Desseaux, Harm-Anton Klok

Surface-grafting thermoresponsive polymers allows the preparation of thin polymer brush coatings with surface properties that can be manipulated by variation of temperature. In most instances, thermoresponsive polymer brushes are produced using polymers that dehydrate and collapse above a certain temperature. This report presents the preparation and properties of polymer brushes that show thermoresponsive surface properties, yet are shape-persistent in that they do not undergo main chain collapse. The polymer brushes presented here are obtained via vapor deposition surface-initiated ring-opening polymerization (SI-ROP) of gamma-di- or tri(ethylene glycol)-modified glutamic acid N-carboxyanhydrides. Vapor deposition SI-ROP of gamma-di- or tri(ethylene glycol)-modified l- or d-glutamic acid N-carboxyanhydrides affords helical surface-tethered polymer chains that do not show any changes in secondary structure between 10 and 70 degrees C. QCM-D experiments, however, revealed significant dehydration of poly(gamma-(2-(2-methoxyethoxy)ethyl)-l-glutamate) (poly(L-EG(2)-Glu)) brushes upon heating from 10 to 40 degrees C. At the same time, AFM and ellipsometry studies did not reveal significant variations in film thickness over this temperature range, which is consistent with the shape-persistent nature of these polypeptide brushes and indicates that the thermoresponsiveness of the films is primarily due to hydration and dehydration of the oligo(ethylene glycol) side chains. The results presented here illustrate the potential of surface-initiated NCA ring-opening polymerization to generate densely grafted assemblies of polymer chains that possess well-defined secondary structures and tunable surface properties. These polypeptide brushes complement their conformationally unordered counterparts that can be generated via surface-initiated polymerization of vinyl-type monomers and represent another step forward to biomimetic surfaces and interfaces.

Amer Chemical Soc2015

Mechanochemically Responsive Surfaces and Soft Tissue Adhesive Interfaces Generated via Controlled Radical Polymerization

Nariye Cavusoglu Ataman

Thin coatings applied to surfaces dramatically improve the properties of materials. Thus, they have found application in a broad variety of areas including biomedicine, nanotechnology, bioelectronics and optics, amongst others. Polymer brushes prepared via surface-initiated controlled radical polymerization (SI-CRP) techniques represent a versatile class of thin polymer coatings that provide manifold opportunities to engineer surface and interface properties. Densely grafted polymer brushes prepared via SI-CRP possess several attractive material properties such as low friction coefficients and the ability to prevent biofouling, which are related to the stretched conformation of the surface tethered polymer chains. Those distinctive characteristics of polymer brushes make them highly appealing to engineer the properties of functional interfaces. A first aim of this Thesis is to investigate the mechanochemistry of hydrophilic polymer brushes prepared via surface-initiated atom transfer radical polymerization (SI-ATRP) from silica substrates. The second aim of the Thesis is to explore poly(2-hydroxyethyl methacrylate) (PHEMA) based polymer brushes as soft-tissue adhesives in wet tissue environment. This Thesis is divided into four chapters, which are briefly summarized below. Chapter 1 provides a summary of recent literature that has reported detachment or degrafting of densely grafted hydrophilic polymer brushes from substrates upon exposure to aqueous media. The collective results from these reports suggest that swelling-induced stretching of hydrophilic polymer brushes lead to mechanochemical activation of the bonds near the brush-substrate interface and subsequently facilitates hydrolytic chain cleavage. Chapter 2 aims to shed light on the influence of two fundamental structural polymer brush parameters on the degrafting of surface-tethered, densely grafted polymer chains, viz. (i) the chemical composition of the ATRP initiator that is used to grow the polymer brushes and (ii) the effect of surface curvature. To this end, a series of poly(poly(ethylene glycol) methacrylate) (PPEGMA) and poly(poly(ethylene glycol) methyl ether methacrylate) (PPEGMEMA) brushes were grafted from both flat silica substrates as well as silica nanoparticles. Subsequently, the degrafting of the surface-tethered polymer chains was investigated in cell culture medium and phosphate buffered saline (PBS) using various techniques. The aim of the final two Chapters of this Thesis is less on exploring the chain conformation of polymer chains grafted by SI-CRP, but more to use the ability of SI-CRP to prepare thin polymer films that present a high surface concentration of functional groups to mediate soft tissue adhesion. Chapter 3 describes the preparation of photo-active poly(2-hydroxyethyl methacrylate) (PHEMA) brushes as soft tissue bioadhesives. Tissue adhesion in this Chapter is induced as UV-irradiation of PHEMA brushes converts the initially bio-inert side chain hydroxyl groups of these brushes into (putatively) aldehyde groups, which can react with amine groups present in the tissue. The prepared PHEMA brush coated substrates are biocompatible, which avoids fibrous capsule formation and presents functionalities that can be transformed, on-demand, from a non-reactive, non-adhesive state to a reactive, adhesive state in wet tissue environment. While the photoactivation of the PHEMA side chain functional groups is very efficient, it is not very well defined from a chemistry point of view. Chapter 4 therefore, explores an alternative photoactivation chemistry, which is based on the use of the diazirine motif. These diazirine motifs were incorporated in poly(2-hydroxyethyl methacrylate-co-poly(ethylene glycol) methyl ether methacrylate) (P(HEMA-co-PEGMEMA)) copolymer brushes via post-polymerization modification. These diazirine modified brushes are attractive as they may not only be activated by UV-light but also by electrochemical means.