Tailoring the (bio)activity of polymeric and metal oxide nano- and microparticles in biotic and abiotic environments

Tailoring the (bio)activity of polymeric and metal oxide nanoand microparticles in biotic and abiotic environments Sathish Ponnurangam Polymeric and metal oxide microand nanoparticles are being increasingly introduced into biomedical applications such as tissue engineering as well as in consumer products, which has boosted extensive research towards developing predictive paradigms of their (bio-)activity. The core hypotheses which are tested in the four interrelated studies of this work is that the (bio-)activity of the particles is defined not only by their intrinsic properties such as the composition/structure, functional groups, surface charge, and size/morphology, but also on the concentration of particles which in turn is determined by specific applications. In addition, the (bio)activity of the particles can be controlled by the applicationspecific biomolecules or surfactants. These hypotheses are tested on polymeric and metal oxide particles from the perspective of their application in tissue engineering of articular cartilage and consumer products (antioxidant additives and dyes), respectively. The modeling of the transport properties of biomaterials, as well as of the adsorption properties of metal oxide nanoparticles can help to determine or interpret the observed relationships. To test this hypothesis, the first study addresses the effect of the concentration of the polysebacic anhydride microparticles on the cellular viability of chondrocytes (cartilage cells) and on the properties of in vitro grown tissue. The polysebacic anhydride based microparticles, which are one most common biocompatible and biodegradable polymer, is used in this study for short term delivery of bioactive molecules such as growth factors and Vitamin C in articular cartilage tissue engineering. An in situ short term delivery of nutrients and growth factors is hypothesized to overcome the problem of the decrease in the diffusion caused by the progressive elaboration of extracellular matrix in 3D agarose scaffolds with time. I found that, contrary to the reported safety of polyanhydrides, the polysebacic anhydride -based microparticles are cytotoxic to chondrocytes (cartilage cells) at concentrations relevant to delivery in articular cartilage engineering. The cytotoxicity is explained by the lipotoxicity from the polysebacic anhydride degradation products. The available data allow suggesting that the bioactivity of polysebacic anhydride polymer is collectively and uniquely determined by its degradation rate and hydrophobicity which requires further verification. It is found that the cytotoxicity of polysebacic anhydride can be mitigated significantly by administering of bovine serum albumin. The bovine serum albumin-based mitigation strategy makes polysebacic anhydride microparticles effective in the short-term delivery of growth factors in articular cartilage engineering. With this bovine serum albumin-based mitigation strategy, the loading concentrations of fast degrading polyanhydride-based particles can be increased to levels relevant to in situ delivery requirements for articular cartilage tissue engineering. This opens up possibility of obtaining tissues with spatially homogenous properties. Another critical factor that arises due to the presence of the polymeric particles is the modification of transport properties of agarose hydrogel scaffold. To address this issue, a model is developed to extract the pore/void size as a function of agarose concentration from the turbidity measurements. The second study verifies the importance of functional groups of polymer particles in the model system of polyacrylate-based nanoparticles and chondrocytes in a 3D agarose hydrogel scaffold. From this study, the charge on the functional groups of the polymers is found to have a significant effect on their bioactivity. Specifically, the polyacrylate-based nanoparticles are shown to be biocompatible (at 0.2% w/v) to chondrocytes in terms of cellular viability. However, they exhibit bioactivity which is detrimental to the synthesis of extracellular matrix even at such low concentrations. An inverse correlation is established between mechanical strength and the negative surface charge of these nanoparticles. The cause for poor matrix synthesis is suggested to be the disruption of the inter-cellular signaling process by higher anionicity. The main impact of this study is that the utility of anionically charged polymeric particles in articular cartilage engineering should be scrutinized carefully, since it is shown that for a clinically relevant scaffold system such as agarose hydrogels, these particles may act detrimentally. Study 3 focuses on the (bio-)activity of redox-active metal oxide nanoparticles toward chondrocytes. For the first time, nanoceria is shown to greatly improve the biochemical as well as mechanical properties of the cartilage tissue grown in vitro. The effect is explained by the unique redox properties of the nanoparticles. Specifically, nanoceria is biocompatible toward chondrocytes at concentrations at least up to 1000 μg/mL, while the constructs cultured with embedded nanoceria at 1000 μg/mL have mechanical and biochemical properties significantly better than those of the control. Interestingly, the bioactivity of nanoceria in combating inflammatory chemical such as interleukin-1α depends on the mode of its administration. In particular, when nanoceria is suspended in the growth medium, it offered significant chondroprotectivity against interleukin-1α and partial prevention of the matrix degradation. However, when nanoceria is directly embedded into the constructs, it does not offer any protection against interleukin-1α. These results show the great potential of nanoceria in improving tissue properties and combating the arthritic inflammation. If the mode of nanoceria administration and cellular uptake are further optimized, the long term protection of cartilage can be achieved. In contrast to nanoceria, ferric (hydr)oxide nanoparticles, which are also redox active, detrimentally affect the growth of cartilage tissue. The adverse effect increases with increasing nanoparticle size and confirms the initial hypothesis of the size dependence of bioactivity of ferric (hydr)oxide nanoparticles, which is based on earlier observation from our group of the catalytic activity of these nanoparticles. Specifically, the mechanical properties of the tissue constructs embedded with hematite and ferrihydrite nanoparticles degrade with increasing nanoparticle size. On the other hand, the cellular viability remains similar among all these sample sets. These two results together imply that the bioactivity is directly related to the redox potential and the catalytic redox mechanism (including either the deactivation or generation of reactive oxygen species) of the metal oxide nanoparticles toward redox active constituents at the biointerface. Study 4 addresses the effects of nanoparticle size and morphology on the adsorption and dispersion of the ferric (hydr)oxide nanoparticles in the presence of surfactants, which is relevant to the dispersion of the nanoparticles in their application in image and contrast agents, cancer therapy, ferrofluids, paints, and cosmetics. The first experimental evidence is obtained for the dependence of the surface density, speciation, and packing order of adsorbed fatty acids on the nanoparticle size and morphology. The conditions under which fatty acids form self-assembled monolayers and bilayers on such nanoparticles in water are distinguished and the electric polarization of the nanoparticles is demonstrated to be a powerful tool for manipulating the interfacial properties of the nanoparticles. Specifically, an increase in nanosize improves the adsorption capacity and affinity of hematite nanoparticles, in agreement with the nanosize-induced changes in the structural and electronic properties of the nanoparticles. Consequently, an increase in nanosize of hematite nanoparticles improves the packing order of laurate and hence hydrophobicity of the nanoparticles provided a similar hexagonal habit of these nanoparticles. It was also observed that, independent of morphology, the dispersibility of the nanoparticles by fatty acids in water degrades as nanoparticle size decreases. This effect is attributed to the decrease in the adsorption capacity which ends up in the formation of a loosely packed bilayer. This implies that the surface electronic and structural properties, which can be controlled by size for metal oxide nanoparticles, offer important ways to manipulate adsorption properties. Morphology of the nanoparticles is critical for the self-assembly if (i) the majority of the molecules are chemisorbed and (ii) the morphology-determined surface arrangement of the adsorption sites is geometrically incompatible with a surfactant ordering. The surface ratio of the physisorbed/chemisorbed surfactant depends on the surface charge of nanoparticles. Negative polarization suppresses chemisorption. This effect can be used to remove the geometrical constraints for self-assembly. The dominance of the chemisorbed (inner-sphere) complexes results in the isotropic surface filling by fatty acids, while the prevalence of outer-sphere complexes favors the cluster-like filling pattern. A new approach to density functional theory modeling of the in situ FTIR spectra of adsorbed species is developed to solve the technically challenging problem of determining the adsorbed structures especially at the magnetic oxide-water interfaces. This new approach constitutes searching of the adsorbed structure that provides the best correspondence of the trends of the theoretical and experimental spectra upon variations of the solution pH, ionic strength and protonation-deprotonation of the carboxylate group, instead of conventional approach of searching for the structure that provides the best one-to-one correspondence between the theoreti