LEE- Polimer Bilim ve Teknolojisi Lisansüstü Programı

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http://hdl.handle.net/11527/19428

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Yazar "Baş, Yahya" ile LEE- Polimer Bilim ve Teknolojisi Lisansüstü Programı'a göz atma
  • Öge
    Silk fibroin cryogel-based shape memory organohydrogels
    (Graduate School, 2022-08-19) Baş, Yahya ; Okay, Oğuz ; 515191025 ; Polymer Science and Technology
    From textile fibers to biotechnology, silk has been a pivotal material for humanity for centuries. Fibroin, one of the two proteins that consist of silk, facilitates the structural integrity and physical endurance of the silk fibers and has been isolated and widely used for several purposes with its features like hydrophilicity, biocompatibility, and mechanical robustness. Polymeric gels are polymer-based, crosslinked wet and soft materials, including pore walls spaced by solvents that dissolve the polymer. Regarding the solvent-separator type that they include, the polymeric gels are mainly categorized into three types: hydrogels, organogels, and aerogels and they include water, organic solvents, and air in their pores, respectively. Hydrogels consist of over 90% water in their structure, most of all, possess unparalleled biomimicry due to their hydrophilic nature. Cryogels are a subclass of hydrogels that are synthesized under 'cryogenic' conditions, meaning under the freezing temperature of the water. Contrary to the conventional hydrogel case, the reason that gelation reaction occurs here is not the heat given to the system, rather it is the increased concentration of gel components in the unfrozen high concentration channels in the reaction system. This influence is called the 'cryo-concentration effect'. This effect provides the cryogels with more reliable mechanical performance and a strictly ordered porous structure that conventional hydrogels lack. Also, the porous morphology of the cryogels mimics the natural extracellular matrix structure of the animals perfectly, according to the scanning electron microscopy (SEM) image comparison of acellularized tissue scaffold and cryogels. Like there are conventional covalent crosslinks between the polymer chains of polymeric gels, there can also be some secondary interactions whose impact becomes significant upon an increased number of them. They regulate the chain conformation with various bindings they form out of many electrostatic interactions. These switchable crosslinks give the materials adjustable mechanical properties, fixing temporary shapes and healing the defects in their structure. The materials having such properties are called 'smart materials'. Smart materials have a variety of application areas such as sustained drug release systems, nanorobotics, biosensors, and tissue engineering. In the scope of this thesis, it was aimed to form an interconnected micro-organogel structure made of n-Octadecyl acrylate (C18A) and acrylic acid (AAc) monomers, within the pores of silk fibroin cryogels which are freeze-dried and swollen in an organogel precursor solution. The effect provides the OHGs their smart material abilities are the reversible dissipation and assembling of crystalline domains formed by the long alkyl tails of C18A monomers, with the repelling influence of water surrounding them and the hydrophobic interactions between them. These weak bonds would disappear upon heat exposure, over the melting temperature of poly(C18A) crystallites, around 50 oC, and would be reformed after the materials cooled down. These switchable crosslinks should provide the OHGs with their altering mechanical (softening and reformability) and shape memory properties. To confirm these assumptions, two sets of samples were prepared and tested. One set of OHG was made of constant fibroin concentration (5 w/v %) cryogel templates and embedded with micro-organogel structures including 4 different C18A monomer mole fractions: 10, 20, 25, and 30 %. The other set of OHG consisted of 4 different fibroin concentration cryogels: 5, 10, 15, and 20 w/v% scaffolds, and each one was filled with the organogel that has a 30% mole fraction C18A. The questioned parameters in each OHG set were the influence of the C18A mole fraction of organogel and fibroin concentration of cryogel, respectively. Initially, cryogel templates having 4 different fibroin concentrations were characterized regarding their swelling properties, pore morphologies and sizes, and uniaxial compression mechanical behavior. It was observed that cryogel porosity has decreased from 92 % to 74% with increasing fibroin concentration from 5 w/v% to 20 w/v%. SEM visualization showed that the average pore diameter decreased from 25.7 μm to 16.8 μm. Fourier Transform Infrared Spectrophotometry (FT-IR) results indicated a 10 % increase in the β-sheet conformation ratio. After the freeze-drying and organogel synthesis within, the OHG porosity was decreased to 45-8 %. The calculated specific pore volume was 17.3 – 2.9 mL/g for cryogels and it was also decreased to a minimum of 0.16 mL/g for OHGs. While Young's modulus was calculated as 0.04 MPa for 5 w/v% cryogels, regarding the increased fibroin concentration and C18A mole fraction, it was raised to a maximum of 17.7 MPa for OHGs with fibroin concentration of 20 w/v%, and C18A mole fraction of 30%. Fracture strains were observed as 10 % lower for the cryogels that became OHGs, while the fracture stresses increased 10 times their initial values. OHGs were subjected to differential scanning calorimetry (DSC) and rheometer tests to investigate this affiliation between temperature and material properties. In DSC testing, the materials were heated up from 0 to 80 oC in two cycles. The melting peaks observed around 54 oC during the heating stage of the test and the crystallization peak at 43 oC during the cooling stage were attributed to the reversible dissipation and regeneration of poly(C18A) crystallites. The first series of OHGs indicated that the increased mole fraction of C18A has increased the number of secondary interactions. This effect was comparatively shown between OHGs with the aid of a parameter called crystallinity index (fcry) which was calculated from the area below the melting peaks of crystallites in the heating stage of DSC tests. fcry describes the fraction of C18A monomers that contributed to the formation of poly(C18A) crystallites. The fcry values that were calculated regarding the melting enthalpies have shown a drop from 15.7 to 2 % when the C18A mole fractions were decreased from 30 to 10%. As a consequence of the bending shape memory tests, also, the OHGs with maximum C18A mole fractions have been seen to be perfectly capable of keeping the shape dictated to them, whereas OHGs with minimum C18A mole fractions were not. The second series of OHGs indicated that, with increasing fibroin concentration and cryogel pores getting narrower, the amount of secondary interactions within the micro-organogel structure gets lower. The calculated fcry values got decreased from 15.7 to 2.9 % with increasing fibroin concentration, from 5 to 20 w/v%. The morphology of cryogels and OHGs was investigated with X-Ray Diffraction (XRD) tests. Growth and sharpening of the peaks corresponding to 2θ = 9.5o, 20o, 24.6o angles with increasing fibroin concentration in cryogels revealed an increase in regular formations such as β-sheets. In the XRD patterns of OHGs, 2θ = 21.6o peaks, which are not seen in cryogels and correspond to the d = 0.41 nm range, revealed that C18A crystallites were formed in the structure. It is thought that the reduction of these peaks with increasing fibroin concentration is due to the difficulty of the orientation required for the formation of crystalline domains in the narrowing pores. Rheometer measurements done with OHGs with 5 w/v% fibroin and 30% C18A mole fraction have exhibited a significant drop in the elastic modulus (G') of the material, from 1.5 MPa to 220 kPA between room temperature and heated to 65 oC state. Once the material cooled down to room temperature again, it recovered its initial G' value, which indicated the existence of the reversibly dissipated and reformed crystalline domains. These results indicated the OHG's reversibly adjustable viscoelastic properties with the influence of heat. Also, all OHG's synthesized have exhibited shape memory capability with around 95% shape recovery ratio each.