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  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.
31

An In Vitro Evaluation Of Chitosan As A Biomaterial Focusing On The Effects Of The Degree Of Deacetylation

Hamilton, Virginia 11 December 2004 (has links)
The material characteristics play a role in the suitability of chitosan for biomedical applications. This is not surprising since the degree of deacetylation of chitosan influences antimicrobial activity, degradation rate, immune reaction and mechanical properties such as strength and elongation. This study examines chitosans of variable material characteristics for wound and bone healing applications. Chitosan films of 76, 78, 80, 87, 91, 92, and 95% degree of deacetylation were tested in vitro for cellular responses by fibroblast and bone cell lines. The in vitro responses were compared to the material characteristics of molecular weight, degree of deacetylation, swelling index, and ash content.
32

Hydrogels as Biofunctional Coatings and Thiol-Ene Clickable Bioinks for Biofabrication / Hydrogele als biofunktionale Beschichtungen und Thiol-Ene-clickbare Biotinten für die Biofabrikation

Bertlein, Sarah January 2019 (has links) (PDF)
Ziel dieser Arbeit war die Entwicklung von funktionalisierbaren Hydrogel Beschichtungen für Schmelz-elektrogeschriebene PCL Gerüste und von Bio-druckbaren Hydrogelen für die Biofabrikation. Hydrogel Beschichtungen von Schmelz-elektrogeschriebenen Konstrukten ermöglichten die Kontrolle der Oberflächen-Hydrophilie und damit Zell-Material Interaktionsstudien in minimal Protein-adhäsiven Umgebungen. Zu diesem Zweck wurde ein hydrophiles sternförmiges vernetzbares Polymer verwendet und eine Optimierung der Beschichtungsbedingungen durchgeführt. Außerdem boten neu entwickelte photosensitive Konstrukte eine Zeit- und pH-unabhängige Biofunktionalisierung. Bio-druckbare Hydrogele für die Biofabrikation basierten auf der Allyl-Funktionalisierung von Gelatine (GelAGE) und modifizierten Hyaluronsäure-Produkten, die das Hydrogel-Vernetzen mittels Thiol-En Click Chemie ermöglichen. Die Optimierung der GelAGE Hydrogel-Eigenschaften wurde durch eine detaillierte Analyse der Syntheseparameter, variierender En:SH Verhältnisse, unterschiedlicher Vernetzungsmoleküle und Photoinitiatoren erreicht. Die Homogenität der Thiol-En Netzwerke wurde mit denen der freien radikalischen Polymerisation verglichen und die Verwendbarkeit von GelAGE als Bio-Tinte für den Extrusions-basierten Bio-Druck wurde untersucht. Es wurde angenommen, dass reine Hyaluronsäure-basierte Bio-Tinten eine Beibehaltung der mechanischen und rheologischen Eigenschaften, der Zellviabilität und der Prozessierbarkeit ermöglichen trotz geringerem Polymer- und Thiol-Anteil der Hydrogele. Hydrogel-Beschichtungen: Hoch definierte PCL Gerüste wurden mittels MEW hergestellt und anschließend mit sechs armigen sternförmigen vernetzbaren Polymeren (sP(EO-stat-PO)) beschichtet. Die Vernetzung wird durch die wässrig-induzierte Hydrolyse reaktiver Isocyanatgruppen (NCO) von sP(EO-stat-PO) bedingt. Diese Beschichtung erhöhte die Oberflächen-Hydrophilie und stellte eine Plattform für weitere Biofunktionalisierungen, in minimal Protein-adhäsiven Umgebungen, dar. Nicht nur das Beschichtungsprotokoll wurde hinsichtlich der sP(EO-stat-PO) Konzentrationen und der Beschichtungsdauern optimiert, sondern auch Vorbehandlungen der Gerüste wurden entwickelt. Diese waren essentiell um die finale Hydrophilie von sP(EO-stat-PO) beschichteten Gerüste so zu erhöhen, dass unspezifische Protein-Adhäsionen vollständig unterbunden wurden. Die sP(EO-stat-PO) Schichtdicke, von ungefähr 100 nm, ermöglicht generell in vitro Studien nicht nur in Abhängigkeit der Gerüst-Biofunktionalisierung, sondern auch in Abhängigkeit der Gerüst-Architektur durchzuführen. Das Ausmaß der Hydrogel-Beschichtung wurde mittels einer indirekten Quantifizierung der NCO-Hydrolyse-Produkte ermittelt. Kenntnis über die NCO-Hydrolyse-Kinetik ermöglichte ein Gleichgewicht zwischen ausreichend beschichteten Gerüsten und der Präsenz der NCO-Gruppen herzustellen, welche für die anschließenden Biofunktionalisierungen genutzt wurden. Diese Zeit- und pH-abhängige Biofunktionalisierung war jedoch nur für kleine Biomoleküle möglich. Um diese Beschränkung zu umgehen und auch hochmolekulare Biomoleküle kovalent anzubinden, wurde ein anderer Reaktionsweg entwickelt. Dieser basierte auf der Photolyse von Diazirin-Gruppen und ermöglichte eine Zeit- und pH-unabhängige Biofunktionalisierung der Gerüste mit Streptavidin und Kollagen Typ I. Die Fibrillen bildende Eigenschaft von Kollagen wurde genutzt um auf den Gerüsten verschiedene Kollagen-Konformationen zu erhalten und eine erste in vitro Studie bestätigte die Anwendbarkeit für Zell-Material Interaktionsstudien. Die hier entwickelten Gerüste könnten verwendet werden um tiefere Einblicke in die Grundlagen der zellulären Wahrnehmung zu erhalten. Insbesondere die Komplexität mit der Zellen z.B. Kollagen wahrnehmen bleibt weiterhin klärungsbedürftig. Hierfür könnten diverse Hierarchien von Kollagen-ähnlichen Konformationen an die Gerüste gebunden werden, z.B. Gelatine oder Kollagen-abgeleitete Peptidsequenzen. Dann könnte die Aktivierung der DDR-Rezeptoren in Abhängigkeit der Komplexität der angebundenen Substanzen bestimmt werden. Aufgrund der starken Streptavidin-Biotin Bindung könnten Streptavidin funktionalisierte Gerüste eine vielseitige Plattform für die Immobilisierung von jeglichen biotinylierten Molekülen darstellen. Gelatine-basierte Bio-Tinten: Zuerst wurden die GelAGE-Produkte hinsichtlich der Molekulargewichts-Verteilung und der Integrität der Aminosäuren-Zusammensetzung synthetisiert. Eine detailliert Studie, mit variierenden molaren Edukt-Verhältnissen und Synthese-Zeitspannen, wurde durchgeführt und implizierte, dass der Gelatine Abbau am deutlichsten für stark alkalische Synthesebedingungen mit langen Reaktionszeiten war. Gelatine beinhaltet mehrere funktionalisierbare Gruppen und anhand diverser Model-Substanzen und Analysen wurde die vorrangige Amingruppen-Funktionalisierung ermittelt. Die Homogenität des GelAGE-Polymernetzwerkes, im Vergleich zu frei radikalisch polymerisierten GelMA-Hydrogelen, wurde bestätigt. Eine ausführliche Analyse der Hydrogel-Zusammensetzungen mit variierenden funktionellen Gruppen Verhältnissen und UV- oder Vis-Licht induzierbaren Photoinitiatoren wurde durchgeführt. Die UV-Initiator Konzentration ist aufgrund der Zell-Toxizität und der potenziellen zellulären DNA-Beschädigung durch UV-Bestrahlung eingeschränkt. Das Zell-kompatiblere Vis-Initiator System hingegen ermöglichte, durch die kontrollierte Photoinitiator-Konzentration bei konstanten En:SH Verhältnissen und Polymeranteilen, die Einstellung der mechanischen Eigenschaften über eine große Spanne hinweg. Die Flexibilität der GelAGE Bio-Tinte für unterschiedliche additive Fertigungstechniken konnte, durch Ausnutzung des temperaturabhängigen Gelierungsverhaltens unterschiedlich stark degradierter GelAGE Produkte, für Stereolithographie und Extrusions-basiertem Druck bewiesen werden. Außerdem wurde die Viabilität zellbeladener GelAGE Konstrukte bewiesen, die mittels Extrusions-basiertem Bio-Druck erhalten wurden. Die Verwendung diverser multifunktioneller und makromolekularer Thiol-Vernetzungsmoleküle ermöglichte eine Verbesserung der mechanischen und rheologischen Eigenschaften und ebenso der Prozessierbarkeit. Verglichen mit dem kleinen bis-Thiol-funktionellen Vernetzungsmolekül waren geringere Thiol-Vernetzer-Konzentrationen notwendig um bessere mechanische Festigkeiten und physikochemische Eigenschaften der Hydrogele zu erhalten. Der Extrusions-basierte Bio-Druck unterschiedlicher eingekapselter Zellen verdeutlichte die Notwendigkeit der individuellen Optimierung von Zell-beladenen Hydrogel-Formulierungen. Nicht nur die Zellviabilität von eingekapselten Zellen in Extrusions-basierten biogedruckten Konstrukten sollte bewertet werden, sondern auch andere Parameter wie die Zellmorphologie oder die Kollagen- oder Glykosaminoglykan-Produktion, da diese einige der essentiellen Voraussetzungen für die Verwendung in Knorpel Tissue Engineering Konzepten darstellen. Außerdem sollten diese Studien auf die stereolithographischen Ansätze erweitert werden und letztlich wäre die Flexibilität und Zellkompatibilität der Formulierungen mit makromolekularen Vernetzern von Interesse. Makromolekulare Vernetzer ermöglichten die Reduktion des Polymeranteils und des Thiol-Gehalts und können, insbesondere in Kombination mit dem Zell-kompatibleren Vis-Initiator-System, voraussichtlich zu einer gesteigerten Zellkompatibilität beitragen, was zu klären bleibt. Hyaluronsäure-basierte Bio-Tinten: Unterschiedliche Hyaluronsäure-Produkte (HA) wurden synthetisiert, sodass diese En- (HAPA) oder Thiol-Funktionalitäten (LHASH) beinhalteten, um reine HA Thiol-En vernetzte Hydrogele zu erhalten. In Abhängigkeit des Molekulargewichts der HA-Produkte, der Polymeranteile und des En:SH Verhältnisses, konnte eine große Spanne an mechanischen Festigkeiten abgedeckt werden. Aufgrund der hohen Viskosität war allerdings im Falle von hochmolekularen HA (HHAPA) Produkt-Lösungen (HHAPA + LHASH) die Handhabbarkeit auf 5.0 wt.-% beschränkt. Die Verwendung der gleichen HA Thiol-Komponenten (LHASH) ermöglichte Hybrid-Hydrogele, mit HA und GelAGE, mit reinen HA-Hydrogelen zu vergleichen. Obwohl der Polymeranteil von HHAPA + LHASH Hydrogelen signifikant geringer war, als im Vergleich zu Hybrid-Hydrogelen (GelAGE + LHASH), wurden für gleiche En:SH Verhältnisse ähnliche mechanische und physikochemische Eigenschaften reiner HA-Hydrogele bestimmt. Aufgrund der geringen Viskosität niedermolekularer HA Lösungen (LHAPA + LHASH) konnten diese nicht für den Extrusions-basierten Druck verwendet werden. Das nicht temperaturabhängige HHAPA + LHASH System hingegen konnte mit nur einem Viertel des Polymeranteils der Hybrid Formulierungen gedruckt werden. Im Vergleich zu der Hybrid Bio-Tinte wurde angenommen, dass das hoch viskose Verhalten von HHAPA + LHASH Lösungen, der geringere Polymeranteil, der geringere Druck für das Drucken und eine demzufolge geringere Scherspannung, maßgeblich zu der hohen Zellviabilität in Extrusions-basiert-biogedruckten Konstrukten beisteuerten. Die niedrigmolekulare HA Formulierung (LHAPA + LHASH) konnte zwar nicht für den Extrusions-basierten Druck verwendet werden, allerdings besitzt dieses System Potential für andere additive Fertigungstechniken wie z.B. der Stereolithographie. Um dieses System weiterzuentwickeln wäre, analog zu dem GelAGE System, eine detailliertere Studie zu den Funktionen eingekapselter Zellen hilfreich. Außerdem sollte die Initiierung dieses Systems mit dem Vis-Initiator untersucht werden. / Aim of this thesis was the development of functionalizable hydrogel coatings for melt electrowritten PCL scaffolds and of bioprintable hydrogels for biofabrication. Hydrogel coatings of melt electrowritten scaffolds enabled to control the surface hydrophilicity, thereby allowing cell-material interaction studies of biofunctionalized scaffolds in minimal protein adhesive environments. For this purpose, a hydrophilic star- shaped crosslinkable polymer was used and the coating conditions were optimized. Moreover, newly developed photosensitive scaffolds facilitated a time and pH independent biofunctionalization. Bioprintable hydrogels for biofabrication were based on the allyl-functionalization of gelatin (GelAGE) and modified hyaluronic acid-products, to enable hydrogel crosslinking by means of the thiol-ene click chemistry. Optimization of GelAGE hydrogel properties was achieved through an in-depth analysis of the synthesis parameters, varying Ene:SH ratios, different crosslinking molecules and photoinitiators. Homogeneity of thiol-ene crosslinked networks was compared to free radical polymerized hydrogels and the applicability of GelAGE as bioink for extrusion-based bioprinting was investigated. Purely hyaluronic acid-based bioinks were hypothesized to maintain mechanical- and rheological properties, cell viabilities and the processability, upon further decreasing the overall hydrogel polymer and thiol content. Hydrogel coatings: Highly structured PCL scaffolds were fabricated with MEW and subjected to coatings with six-armed star-shaped crosslinkable polymers (sP(EO-stat-PO)). Crosslinking results from the aqueous induced hydrolysis of reactive isocyanate groups (NCO) of sP(EO-stat-PO) and increased the surface hydrophilicity and provided a platform for biofunctionalizations in minimal protein adhesive environments. Not only the coating procedure was optimized with respect to sP(EO-stat-PO) concentrations and coating durations, instead scaffold pre-treatments were developed, which were fundamental to enhance the final hydrophilicity to completely avoid unspecific protein adsorption on sP(EO-stat-PO) coated scaffolds. The sP(EO-stat-PO) layer thickness of around 100 nm generally allows in vitro studies not only in dependence on the scaffold biofunctionalization but also on the scaffold architecture. The hydrogel coating extent was assessed via an indirect quantification of the NCO-hydrolysis products. Knowledge of NCO-hydrolysis kinetics enabled to achieve a balance of sufficiently coated scaffolds while maintaining the presence of NCO-groups that were exploited for subsequent biofunctionalizations. However, this time and pH dependent biofunctionalization was restricted to small biomolecules. In order to overcome this limitation and to couple high molecular weight biomolecules another reaction route was developed. This route was based on the photolysis of diazirine moieties and enabled a time and pH independent scaffold biofunctionalization with streptavidin and collagen type I. The fibril formation ability of collagen was used to obtain different collagen conformations on the scaffolds and a preliminary in vitro study demonstrated the applicability to investigate cell-material interactions. The herein developed scaffolds could be applied to gain deeper insights into the fundamentals of cellular sensing. Especially the complexity by which cells sense e.g. collagen remain to be further elucidated. Therefore, different hierarchies of collagen-like conformations could be coupled to the scaffolds, e.g. gelatin or collagen-derived peptide sequences, and the activation of DDR receptors in dependence on the complexity of the coupled substances could be determined. Due to the strong streptavidin-biotin bond, streptavidin functionalized scaffolds could be applied as a versatile platform to allow immobilization of any biotinylated molecules. Gelatin-based bioinks: First the GelAGE products were synthesized with respect to molecular weight distributions and amino acid composition integrity. A detailed study was conducted with varying molar ratios of reactants and synthesis durations and implied that gelatin degradation was most dominant for high alkaline synthesis conditions with long reaction times. Gelatin possesses multiple functionalizable groups and the predominant functionalization of amine groups was confirmed via different model substances and analyses. Polymer network homogeneity was proven for the GelAGE system compared to free radical polymerized hydrogels with GelMA. A detailed analysis of hydrogel compositions with varying functional group ratios and UV- or Vis-light photoinitiators was executed. The UV-initiator concentration is restricted due to cytotoxicity and potential cellular DNA damages upon UV-irradiation, whereas the more cytocompatible Vis- initiator system enabled mechanical stiffness tuning over a wide range by controlling the photoinitiator concentration at constant Ene:SH ratios and polymer weight percentages. Versatility of the GelAGE bioink for different AM techniques was proved by exploiting the thermo-gelling behavior of differently degraded GelAGE products for stereolithography and extrusion-based printing. Moreover, the viability of cell-laden GelAGE constructs was demonstrated for extrusion-based bioprinting. By applying different multifunctional thiol-macromolecular crosslinkers the mechanical and rheological properties improved concurrently to the processability. Importantly, lower thiol-crosslinker concentrations were required to yield superior mechanical strengths and physico-chemical properties of the hydrogels as compared to the small bis-thiol-crosslinker. Extrusion-based bioprinting with distinct encapsulated cells underlined the need for individual optimization of cell-laden hydrogel formulations. Not only the viability of encapsulated cells in extrusion-based bioprinted constructs should be assessed, instead other parameters such as cell morphology or production of collagen or glycosaminoglycans should be considered as these represent some of the crucial prerequisites for cartilage Tissue Engineering applications. Moreover, these studies should be expanded to the stereolithographic approach and ultimately the versatility and cytocompatibility of formulations with macromolecular crosslinkers would be of interest. Macromolecular crosslinkers allowed reducing polymer weight percentages and amounts of thiol groups and are thus expected to contribute to increased cytocompatibility, especially in combination with the more cytocompatible Vis-initiator system, which remains to be elucidated. Hyaluronic acid-based bioinks: Different molecular weight hyaluronic acid (HA) products were synthesized to bear ene- (HAPA) or thiol-functionalities (LHASH) to enable pure HA thiol-ene crosslinked hydrogels. Depending on the molecular weight of modified HA products, polymer weight percentages and Ene:SH ratios, a wide range of mechanical stiffness was covered. However, the manageability of high molecular weight HA (HHAPA) product solutions (HHAPA + LHASH) was restricted to 5.0 wt.-% as a consequence of the high viscosity. Based on the same HA thiol component (LHASH), hybrid hydrogels of HA with GelAGE were compared to pure HA hydrogels. Although the overall polymer weight percentage of HHAPA + LHASH hydrogels was significantly lowered compared to hybrid hydrogels (GelAGE + LHASH), similar mechanical and physico-chemical properties of pure HA hydrogels were determined with maintained Ene:SH ratios. Low viscous low molecular weight HA precursor solutions (LHAPA + LHASH) prevented the applicability for extrusion-based bioprinting, whereas the non-thermoresponsive HHAPA + LHASH system could be bioprinted with only one-fourth of the polymer content of hybrid formulations. The high viscous behavior of HHAPA + LHASH solutions, lower polymer weight percentages, decreased printing pressures and consequently declined shear stress during printing, were hypothesized to contribute to high cell viabilities in extrusion-based bioprinted constructs compared to the hybrid bioink. The low molecular weight HA precursor formulation (LHAPA + LHASH) was not applicable for extrusion-based printing, but this system has potential for other AM techniques such as stereolithography. Similar to the GelAGE system a more detailed study on the functions of encapsulated cells would be useful to further develop this system. Moreover, the initiation with the Vis-initiator should be conducted.
33

Cellulose Biomaterials for Bone Tissue Engineering

Leblanc Latour, Maxime 03 February 2023 (has links)
Designing artificial tissue is an essential part of modern-day medicine. This is also true for bone tissue repair. Work presented in this thesis shows the steps and development of novel cellulose-based biomaterials for bone tissue engineering (BTE). Cellulose is used as the core component in these biomaterials. This work begins with an overview of the thesis, followed by a background review of the relevant biological and physical concepts. Thereafter, original research on the biomechanical properties of apple-derived cellulose are carried out in vitro and in vivo. Afterward, relevant physical forces are applied to the same type of material, to investigate the osteogenic response. Finally, cellulose nanofibrils were chemically modified to create scaffolds through UV crosslinking. These were mechanically characterized and used as scaffolds for osteogenic cell culture. As demonstrated in this work, the use of cellulose-sourced biomaterials is certainly a promising alternative compared to the industry standard. Numerous studies have demonstrated how cellulose-based biomaterials can be employed in several branches of reconstructive medicine. However, uncertainties still exist in the application of these materials for bone tissue reconstruction such as their performance under physical stress, and in their scalability. The research presented in this thesis attempts to address these gaps in knowledge. Specifically, the results presented here show how these materials can be promising candidates for low-load BTE applications. Furthermore, it is also demonstrates that mechanosensitive pathways that regulate osteogenesis remain functional on these materials. Finally, UV-curable cellulose-derived scaffolds create a more scalable and controllable biomaterial for BTE implants, notably using light-based three-dimensional printing technologies.
34

Properties of solid wood : responses to drying and heat treatment

Sehlstedt-Persson, Margot January 2005 (has links)
The hygro-thermal processes that wood is exposed to during drying and heat treatment lead to different reactions or responses in the wood material. The objective of this thesis has been to study the impact of different drying and heat-treatment strategies on various responses, such as strength, sorption/desorption behaviour, dimensional stability and colour changes. A decrease of shear strength along grain direction was found for high- temperature dried pine that was dried at temperatures exceeding 100°C, compared to boards dried at lower temperatures. No unambiguous decrease of surface hardness, cleavage strength or toughness was found for clear wood samples when high-temperature dried material was compared to material dried at lower temperatures. A decrease of hygroscopicity for wood exposed to increased temperatures was found. The higher the temperature, the greater was the decrease in equilibrium moisture content, EMC. The desorption isotherm of dried pine wood compared to initial desorption isotherm for fresh, green wood also showed lower EMC. Noticeable colour-change responses to heat treatment were found when different wood constituents such as pine and spruce sap and extractives from pine heartwood were heat-treated separately. Colour changes increased with time and temperature. An accelerated colour change was found for pine sap and extractives at temperatures exceeding 70°C. Studies of material properties such as extractive content and its influence on diffusivity show that density has greater influence than extractive content on diffusivity in pine and spruce. Pine shoved lower diffusivity than spruce, but when extractives were removed from pine heartwood, no difference was found in diffusivity compared to pine sapwood or spruce heartwood in a comparison of average levels between unpaired groups. The relation between diffusion coefficients in tangential, radial and axial direction in solid pine sapwood were found to be 1:1.8:7 respectively. Phenomena within the area of process dynamics were also studied. Calculations of thickness of a thin, dry outer shell formed in pine sapwood boards early in the capillary phase of drying were done based on temperature and mass flux measurements. Comparison with dry shell thickness analysed in a computer tomography scanner showed fairly good agreement. / Godkänd; 2005; 20061214 (haneit)
35

Porous structures based on nanopolysaccharides for medical applications

Naseri, Narges January 2014 (has links)
Recently, the use of bio-based nanomaterials has attracted much interest in medical applications due to their cytocompatibility, good moisture stability, good mechanical properties, hydrophilic surfaces and their ability to form porous structures. The aim of this work is to develop porous nanocomposites based on nanochitin and nanocellulose with controlled pore size and porosity in order to facilitate cell growth and interconnectivity and to investigate their potential in wound dressing and cartilage regeneration. The first and second study focus on the electrospinning of chitosan-based nanocomposite mats reinforced with chitin nanocrystals (ChNC) and cellulose nanocrystals (CNC) with different surface characteristics for wound dressing. Electrospinning processing resulted in porous mats of fibers with diameters in the range of 223 to 1240 nm. The microscopy studies showed that diameter of the electrospun fibers decreased with the inclusion of both types of nanocrystals. The addition of nanocrystals as well as crosslinking had a positive impact on the mechanical performances of the mats. The tensile strength and tensile modulus of the mats were the highest with the addition of ChNC due to better compatibility with the matrix and increased further (tensile strength of 64.9 MPa and the modulus of 10.2 GPa) after crosslinking. Furthermore, surface charges of cellulose nanocrystals isolated with different hydrolysis process had a significant impact on the electrospinning solution properties as well as properties of the resulting fibers. The water vapor transmission rate and O2/CO2 permeability of the electrospun mats as well as cytocompatibility towards adipose derived stem cells were considered favorable for wound dressing.Investigated in the third study were nanofibrous porous scaffolds created via freeze-drying for use in cartilage repair. Cellulose nanofibers were used as reinforcement in a matrix of gelatin and chitosan and crosslinked using genipin. The scaffolds showed interconnected pores up to 250 µm and the pore walls had nanoscaled roughness. Compression modulus of the scaffolds was in the range of 1-3 MPa, which decreased significantly when tested in phosphate buffered saline (PBS) at 37°C. The moisture uptake was in the range of 1000 - 3000 wt %, due to moisture trapped in the pores. These scaffolds showed potential in cartilage repair because their high porosity (≈ 95%) and mechanical performance is favorable for cell attachment and extracellular matrix (ECM) production as well as its cytocompatibility towards chondrocytes.The work in all three studies showed that fully bio-based porous nanocomposites tailored using polysaccharide nanoparticles as reinforcements in biopolymer matrices have excellent potential in biomedical products and implants.
36

Extrusion processing of wood raw materials for use in wood-polymer composites

Hietala, Maiju January 2011 (has links)
The interest in wood-polymer composites and their use in different applications has been growing over the last 10-15 years. Environmental issues and demands on lower material costs are the driving forces behind the increasing use of renewable materials such as wood and other natural fibres as reinforcement in polymer composites. Wood flour consisting of small wood particles is currently used as the main wood raw material in commercial wood-polymer composites. However, the reinforcing potential of wood flour is limited. A better reinforcement could be achieved by using wood fibres with a higher aspect ratio as raw material, but individual fibres are seldom used in composite manufacturing due to processing problems and high cost. Therefore, the objective of the work was to study the possibility to use wood chips as raw material and separate individual fibres with higher aspect ratios from the wood chips during the composite manufacturing process. First, the effect of the extrusion process only on wood raw material was studied without a matrix polymer, and then composites using polypropylene as matrix were made. The main goal was to produce wood particles/fibres with high aspect ratio during the manufacturing of wood polymer composites. The effects of extrusion parameters, different screw configurations, raw materials and raw material pre-treatments were evaluated. The size of the separated wood particles and fibres was measured using optical fibre analysis methods. Microstructure of wood particles as well as the fractured surfaces of prepared composites were examined using electron microscopy. The mechanical properties of the composites were measured using flexural and impact testing. The results showed that wood chips can be used as raw material in a one-step manufacturing process of wood-polymer composites. Also, individual fibres with a higher aspect ratio than wood flour were separated from the wood chips with suitable processing conditions
37

Nanostructured materials isolated from bio-residues, and their characterization

Rodríguez, Martha Herrera January 2012 (has links)
The use of natural components in nanocomposites has continuously increased, due to environmental problems that are growing day by day. The use of bio-residues from forest industries to develop new materials will not only alleviate ecological problems but also affect the economy of forest industries positively.The aim of this work was to characterize cellulose nanowhiskers isolated from two different industrial bio-residues, one from bioethanol production and another from specialty cellulose production. Furthermore, the structure and permeability of thin films made of these nanowhiskers were studied. In the first study, the characteristics of nanowhiskers isolated from bioethanol residue were compared with nanowhiskers from microcrystalline cellulose (MCC). The nanowhiskers from ethanol residue had lower surface charge compared with whiskers obtained from MCC when analyzed by conductometric titration. The AFM microscopy showed that both cellulose nanowhisker suspensions presented individualized whiskers with diameters less than 10 nm. Nanowhiskers from ethanol residue showed higher relative crystallinity than the nanowhiskers from MCC, and the films made from both whiskers showed transparency in visual light. In addition, the nanowhiskers extracted from bio-residue were more thermally stable than the whiskers extracted from MCC, having a higher degradation onset temperature and maximum degradation temperature.In the second study, nanowhiskers isolated from two different bioresidues were compared. It was seen that both nanowhiskers suspensions (reject cellulose and ethanol residue) exhibited flow birefringence. Transmission electron microscopy study showed that the nanowhiskers extracted from the reject cellulose were slightly longer (377 nm) than the ones extracted from the ethanol residue (301 nm). The casted films of nanowhiskers from reject cellulose showed a stronger interference in the UV and visible region, compared with the other films. The comparative crystallinity was higher for reject cellulose nanowhiskers than for ethanol residue whiskers. Moreover, the thermal stability was slightly higher for the ethanol residue whiskers than for the reject cellulose whiskers. In the last study, cellulose nanowhiskers were isolated from the reject cellulose using hydrochloric and sulphuric acid hydrolysis processes with a aim to obtain different surface characteristics. Sulfuric acid whiskers had higher surface charge than the hydrochloric acid whiskers. Thin spin-coated films with two different configurations were prepared; one with alternate layers of poly(allylamine hydrochloride) (PAHCl) and cellulose nanowhiskers, and the second one with a single layer of PAHCl coated with 25 layers of whiskers. In addition, the film roughness, and surface charge of the whiskers was shown to increase the hydrophilic behavior of the films, being highest for a single layer of PAHCl coated with cellulose nanowhiskers. The gas permeability was measured and the coefficient was highest for hydrogen (H2) followed by helium (He), oxygen, (O2) and carbon dioxide (CO2) and nitrogen (N2). It was observed that the surface charge did not affect the gas permeability of the films and did not display selective gas barrier. The results showed that CNW can be extracted from ethanol residue and reject cellulose, and that these whiskers had similar characteristics as nanowhiskers obtained from other non-residual sources. This work has demonstrated that bio-residues can potentially be used as a source of new nanosize materials, thereby increasing the value of the forest resources.
38

Process and properties of continuous fibers based on cellulose nanocrystals and nanofibers

Hooshmand, Saleh January 2014 (has links)
In recent years, composites made from natural fibers based on cellulose have received increasing attention since they have a low environmental impact and good mechanical properties. However, these fibers are short and discontinuous and the conventional spinning techniques used for these fibers results in continuous yarns with mechanical properties considerably lower than that of the single fibers. The aim of this work was to prepare continuous fibers where nano-sized cellulose crystals and cellulose nanofibers were used to improve the fiber properties. Two different strategies have been used to reach this aim. In the first study, bio-based fibers of cellulose acetate butyrate (CAB) and cellulose nanocrystals (CNC) using triethyl citrate (TEC) as plasticizer were prepared by melt spinning. Two different dispersion techniques were studied. In the first technique, the water content of the CNC suspension was reduced and exchanged to ethanol using centrifugation. In the second, the water in the CNC suspension was completely exchanged to ethanol by a sol-gel process. Results showed that tensile modulus and tensile strength of the nanocomposite fibers produced with the first technique were lower than CAB-TEC fibers, but the fibers produced by the sol-gel process showed an increase in the tensile modulus and had no decrease in the strength. Optical microscopy of the fibers indicated less aggregations in the sol-gel prepared materials. The results indicate that the sol-gel process is enhancing the dispersion of cellulose nanocrystals and can be a suitable way to prepare nanocomposite fibers. The second study is an extension of the first study. Here the effect of weight concentration of CNC and fiber drawing was studied. The microscopy studies showed that the addition of CNC in CAB resulted in defect-free and smooth fiber surfaces. An addition of 10 wt% CNC enhanced the storage modulus and increased the tensile strength and Young’s modulus. Fiber drawing improved the mechanical properties further. In addition, a micromechanical model of the composite material was used to estimate the stiffness and showed that theoretical values were exceeded for the lower concentration of CNC but not reached for the higher concentration. In conclusion, this dispersion technique combined with melt spinning can be used to produce all-cellulose nanocomposites fibers and that both the increase in CNC volume fraction and the fiber drawing increased the mechanical performance. In the third study a different strategy was used. Here low cost and environmentally friendly continuous fibers of native cellulose were prepared by dry spinning an aqueous suspension of cellulose nanofibers (CNF). The CNF were extracted from banana rachis, a bio-residue from banana cultivation in Columbia. The effect of spinning rate and CNF concentration on the mechanical properties of the fibers were investigated. The results showed that there was a relationship between the spinning rate and concentration. The modulus of the fibers was increased from 7.7 to 12.6 GPa and the strength increased from 131 to 222 MPa when the lowest concentration and highest speed was used. This improvement is believed to be due to an increased orientation of the CNF in the fiber. A minimum concentration of 6.5 wt% was required for continuous fiber spinning. However, this relatively high concentration is thought to limit the orientation of the CNF in the fiber. The process used in this last study has a good potential for up-scaling providing a continuous fiber production with well-controlled speed but further work is required to increase the orientation and subsequently the mechanical properties.The results from these three studies shows that it is possible to spin continuous fibers where nanocellulose is used as a reinforcing agent. It is also shown that the dispersion and alignment of the nanocellulose plays a key role in improving the mechanical properties.
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Processing and Properties of Nanostructured Biocomposites

Vargas, Natalia Herrera January 2014 (has links)
In this work, nanostructured biocomposite fibers and films with cellulose nanofibers (CNF), cellulose nanocrystals (CNC) and chitin nanocrystals (ChNC) were prepared using solutions mixing followed by electrospinning and melt compounding. The main processing challenges for these materials were to find parameters for: 1) fiber alignment in electrospinning, 2) feeding the nanomaterials into the extruder and 3) dispersion and distribution of the nanomaterials in the polymeric matrix. This thesis consists of three publications, which are summarized below.The first study was about random and aligned cellulose fibers prepared by electrospinning. Cellulose acetate (CA) was used as a matrix and a mixture of acetic acid and acetone (1:1) was used as a solvent. CNC with different concentrations (0–5 wt-%) were used as reinforcement. Microscopy studies showed fibers with smooth surfaces, different morphologies and diameters ranging between 200 and 3300 nm. It was found that the fiber diameters decreased with increased CNC contents. The microscopy studies also indicated well-aligned fibers. Results from dynamic mechanical thermal analysis indicated improved mechanical properties with the addition of CNC. The storage modulus of electrospun CA fibers increased from 81 to 825 MPa for fibers with 1 wt% CNC at room temperature. X-ray analysis showed that the electrospun CA fibers had a crystalline nature and that there was no significant change in crystallinity with the addition of CNC.In the second study, polylactic acid (PLA) and its nanocomposite based on CNF and glycerol triacetate (GTA) were prepared using a co-rotating twin-screw extruder. GTA was used as a plasticizer, a processing aid to facilitate nanofiber dispersion and as a liquid medium for feeding. The optical, thermal and mechanical properties were characterized and the toughening mechanism was studied. The addition of GTA (20%) and CNF (1%) resulted in increased degree of crystallinity and thus decreased optical transparency. Furthermore, these additives showed a positive effect on the elongation at break and toughness, which increased from 2 to 31% and from 1 to 8 MJ/m3, respectively. A combination of nanofiber-matrix interfacial slippage and a massive crazing effect is suggested for PLA toughening. CNF were expected to restrict the spherulite growth and therefore enhance the craze nucleation. In the third study, triacetate citrate plasticized poly lactic acid and its nanocomposites based on cellulose nanocrystals (CNC) and chitin nanocrystals (ChNC) were prepared using a co-rotating twin-screw extruder. The materials were compression molded to films using two different cooling rates. The cooling rates and the addition of nanocrystals (1 wt%) had an impact on the crystallinity as well as the optical, thermal and mechanical properties of the films. The fast cooling resulted in more amorphous materials, increased transparency and elongation to break, (approx. 300%) when compared with slow cooling. Chitin nanocomposites were more transparent than cellulose nanocomposites; however, microscopy study showed presence of agglomerations in both materials. The mechanical properties of the plasticized PLA were improved with the addition of a small amount of nanocrystals resulting in PLA nanocomposites suitable for use in film blowing and thus packaging applications. Summing up, this thesis shows that solution mixing followed by electrospinning can be used to produce reinforced green nanocomposite fibers with random or aligned orientation with, probably, potential to be used in membranes, filters or even in medical applications. It was also shown that PLA-CNF nanocomposites can be prepared using extrusion and liquid feeding and that small amounts of CNF changed the fracture mechanism, resulting in increased toughness. In addition, the cooling rate of the plasticized PLA and its nanocomposite films was found to significantly impact the film properties.
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Processing and characterization of membranes based on cellulose nanocrystals for water purification : Nanocellulose as functional entity

Karim, Zoheb January 2014 (has links)
Membrane technology is being extensively used in water purification as an energy efficient and low cost process. Nanostructured (NSM) and nanoenabled (NEM) membranes are favored in this context as nanoscaled entities are expected to provide high surface area, high mechanical properties and versatile surface chemistry as well as provide better control on the pore size and distribution, flux and selectivity of the membrane. Biobased nanoparticles as nanocrystals are expected to have a significant advantage in this context. Thus, the main aim of this work was to explore the use of cellulose nanocrystals as functional entities for the fabrication of nanoenabled composite membranes and apply these fabricated membranes for the removal of dyes and metal ions from polluted water. The first study deals with the isolation of cellulose nanocrystals (CNCBE) from wood using the bioethanol pilot scale setup. Cellulose was prepared from wood by diluted acid treatment in the bioethanol plant followed by dewaxing and bleaching. The cellulose was converted into cellulose nanocrystals by mechanical grinding using lab scale homogenizer. The isolated nanoparticles had a diameter of 5-15 nm and formed a thick gel at 2 wt%. X-ray photoelectron spectroscopy illustrated the presence of O=C-O surface functional groups, directly related to the negative zeta-potential values. Fabricated films of CNCBE denoted good mechanical properties, optical properties and cytocompatibility. Thus, a new isolation route that can be followed to produce nanocrystals in large quantities (600 g/ day) has been developed. In a second study, fully biobased nanocomposite membranes of cellulose nanocrystals and chitosan have been fabricated by freeze-drying and crosslinking with gluteraldehyde in vapor phase. The chitosan bound the CNCSL in a stable and nanoporous membrane network with thickness of 250-270 μm. Homogenous dispersion of CNCSL within chitosan matrix was reported based on scanning electron microscopy (SEM). The Brunauer Emmett and Teller (BET) studies showed a decrease in surface area (3.1 to 2.9 m2/g) and average pore size (17 to 13 nm) after crosslinking. The mechanical performance of composite membranes was low, being 0.98 ± 0.4 and 1.1 ± 0.3 MPa of tensile strength for uncross-linked and cross-linked membranes, respectively. In spite of low water flux (64 L m−2 h−1), the composite membranes successfully removed 98%, 84% and 70% respectively of positively charged dyes like Victoria Blue 2B, Methyl Violet 2B and Rhodamine 6G, from a model wastewater after a contact time of 24 h. In the third study layered membranes containing a highly porous support layer and a dense functional layer has been fabricated following a filtration and hot pressing method. Microsized cellulose fibers from sludge bioresidues was used as the support layer to provide mechanical stability and allow water flow without any hindrance. A nanocomposite system of nanocrystals (CNCSL, CNCBE and PCNCSL) with gelatin as matrix was used as the functional layer. Bubble point measurement confirmed the membrane pore sizes (5-6 m), in microfiltration range, which resulted in high water permeability < 4000 Lh-1m-2 at 1.5 bars. Efficient removal of Ag+, Cu2+ and Fe3+ from industrial wastewater was achieved using these membranes. The removal of metal ions was expected to be driven by the electrostatic attraction between negatively charged nanocellulose and the positively charged metal ions. The work has demonstrated that highly efficient water treatment membranes can be fabricated from nanocellulose via tailoring their ability to interact and selectively adsorb heavy metal ions and dyes.

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